Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province

Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province

RICHARD A. STERN AND GILBERT N. HANSON Department of Earth and Space Sciences, SUNY at Stony Brook, Stony Brook, NY 11974, U.S.A.

AND

STEVEN B. SHIREY Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd.

Washington, DC 20015, U. S. A.

Received July 14, 1988

Revision accepted February 13, 1989

In southwestern Superior Province, diorite, monzodiorite, and trachyandesite ("sanukitoids") occurring within syn- to post-tectonic intrusive complexes and within greenstone belts have the following chemical characteristics: 55-60 wt. % SO2, MgO > 6 wt. %, Mg# > 0.60, Ni and Cr both > 100 ppm, Na20 + K20 = 6 wt. %, Sr and Ba both 600- 1800 ppm, and rare-earth-element (REE) patterns that are strongly light rare-earth-element (LREE) enriched (Ce, = 80-250, Yb, = 4- 10) and show no Eu anomalies. Sanukitoids and their granodioritic derivatives constitute at least 5 % of the exposed crust in the study area. The sanukitoids cannot be derived by melting, fractionation, or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids. Crustal contamination of komatiites fails to explain the high large-ion-lithophile-element (LILE) contents of the sanukitoids. Rather, we suggest that the sanukitoids were derived by hydrous melting of LILE-enriched mantle peridotite at pressures between 10 and 15 kbar. The sanukitoids with steepest REE

patterns have the lowest FeO contents, indicating that the part of the mantle source with the highest Mg# had the most fractionated REE pattern prior to melting. Mantle source regions to the sanukitoids had different Mg#'s and were enriched in LILE'S

(metasomatized) to varying extents by fluids of crustal or mantle origin prior to melting.

Dans le sud-ouest de la province du lac SupCrieur, la diorite, la monzonite et la trachyandesite (<< sanukitoides *) qui affleurent dans les complexes intrusifs syn- et post-tectoniques, ainsi que dans les ceintures de roches vertes, prksentent les caractCristiques chimiques suivants : 55 -60% par poids de SO2, MgO > 6 % par poids, valeurs Mg# > 0,60 Ni et Cr > 100 pprn chacun, Na20 + K 2 0 = 6 % par poids, Sr et Ba 600- 1800 pprn chacun et les courbes des terres rares fortement enrichies en terres rares l tgkes (Ce, = 80-250, Yb, = 4- 10) et sans anomalies apparentes en Eu. Les sanukitoides et leurs dCrivCs granodioritiques foment au moins 5 % de la crotite exposee dans la rtgion CtudiCe. Les sanukitoides ne peuvent pas originer de la fusion, du fractionnement ou de la contamination crustale des basaltes ou des lamprophyres contemporains des sanukitoides. La contamination crustale des komatiites est insuffisante pour expliquer les teneurs en ClCments lithophiles B grand rayon fonique (LILE) dans les sanukitoides. Nous suggkrons plut6t que les sanukitoides dkrivaient de la fusion hydratke d'une pkrioditite du manteau enrichie en ClCments lithophiles B grand rayon ionique, B des pressions entre 10 et 15 kbar. Les sanukitoides qui prksentent les courbes les plus abruptes de distribution des terres rares renferment plus les faibles teneurs en FeO, attestant que la portion du magma parent dans le manteau ayant les valeurs Mg# les plus fortes corresponait au stade de fractionnement maximum des terres rares prkc6dant la fusion. Les rBgions du manteau d'oh dCrivent les sanukitoides possB daient des valeurs Mg# diffkrentes et furent enrichies en ClCments lithophiles B grand rayon fonique (mCtasomatisme) L des degrCs variables par des fluides d'origine crustale ou mantellique avant le stade de fusion.

[Traduit par la revue]

Can. J. Earth Sci. 26, 1688-1712 (1989)

Introduction Monzodiorites and trachyandesites with very distinctive

chemical characteristics are present within the Superior Province, and their origin may shed light on how material was transferred between the mantle and crust during the Archean. The outstanding chemical characteristics of the monzo- dioritesl and trachyandesites are as follows: 55-60 wt.% SO2, Mg#'s2 greater than 0.60, Ni and Cr abundances over 100 ppm, and strong enrichment in certain large-ion-lithophile elements (LILE'S), such as the light rare-earth elements (LREE'S) (Ce, = 80-250x chondrites3), and in Sr and Ba (both about 1000 pprn). The rocks have the primitive ferromagnesian-element characteristics of high-Mg andesites that

'Terminology follows Streckeisen (1976). 'Mg# = cation mole fraction Mg2+/(Mg2+ + F%). 3Chondrite-normalizing values for the rare-earth elements (REE'S)

from Masuda et al. (1973) divided by 1.20 (see Table 1 in Hanson 1980).

Printed in Canada 1 Imprim6 au Canada

occur in modern arc environments and also the high LILE contents of certain high-K andesites, shoshonites, and appinites. The monzodiorites are closely associated with larger volumes of evolved grandodiorite.

The rock association consisting of the primitive monzodiorites and their derivatives has been termed the "sanukitoid suite" (Shirey and Hanson 1984). The term was introduced because the monzodiorites and trachyandesites have Mg#'s and transition-metal contents similar to those of high-Mg andesites occurring within the Setouchi volcanic belt of Japan, locally referred to as "sanukitoids" (Koto 1916; Tatsumi and Ishizaka 1982a, 1982b). The Japanese sanukitoids are glassy volcanic rocks that have chemical characteristics and experimental phase equilibria consistent with an origin by partial melting of mantle peridotite under hydrous conditions (Tatsumi 1981, 1982; Tatsumi and Ishizaka 1982a, 19826). For exarn- ple, the Japanese sanukitoids have 55 -60 wt. % Si02, Mg#'s near 0.70, Ni contents near 200 ppm, and olivine of F o ~ ~ - ~ ~ composition on their liquidi.

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STERN

Shirey and Hanson (1984) proposed a mantle origin for the Archean sanukitoids. They also brought attention to the similarity of the major- and trace-element compositions of sanukitoids to estimated compositions of the total Archean crust. Thus, an intriquing possibility emerged for explaining the origin of parts of the Archean crust, that is, by direct extraction from the mantle of melts of high-Mg andesite composition. This was a significant departure from established concepts about the origin of sialic components of the Archean crust, which are commonly held to have formed by melting or fractionation of basaltic precursors (e.g., Barker and Arth 1976; Jahn et al. 1981).

In this study, we examine in greater detail the origin of the Archean sanukitoid suite. Data are presented that summarize the geology and areal extent of the sanukitoid suite in southwestern Superior Province (Fig. I), a region in which high- quality geological maps are readily available. Chemical data from the literature and previously unpublished data upon an intrusion of the sanukitoid suite, the Roaring River Complex, are used to further test the proposed mantle origin for these rocks and to examine more closely the character of the source rocks, the melting process, and the significance of the associated evolved granodiorite.

Geological characteristics of the sanukitoid suite Within the southwestern part of the Superior Province, the

sanukitoid suite occurs within synkinematic to early postkine- matic intrusive complexes consisting of monzodiorite and spatially and temporally related diorite, monzonite, and granodiorite. The monzodioritic intrusive complexes appear to be members of a more widespread and compositionally diverse rock association that also includes nepheline syenites, gabbro - pyroxenite, and lamprophyre (e.g . , Percival 1983; Percival and Stern 1984: R. A. Stern 1985). The common features of all these rocks are their high abuninces of LILE'S compared with their country rocks. Additionally, there is partial lithological overlap between the sanukitoid suite and these other rock types. For example, the monzodioritic complexes may include syenitic phases, most contain inclusions of gabbro and pyroxenite, and they are commonly cross-cut by lamprophyre dykes. Accordingly, we refer to these rocks collectively as "~ l~~-en r i ched intrusions." The LILE-enriched intrusions in southwestern Superior Province, which have been dated by radiometric techniques, have similar crystallization ages, around 2700 Ma (e.g., Catanzaro and Hanson 1971 ; Prince and Hanson 1972; Hanson et al. 1971; Goldich and Fischer 1986).

Figure 2 shows the location of the recognized LILE-enriched intrusions within southwestern Superior Province. The monzodioritic intrusions, which possibly contain members of the sanukitoid suite (on the basis of geochemical data) are indicated with a star beside their names. Also shown in Fig. 2 are two locations where trachyandesites with the chemistry of sanukitoids are exposed.

Monzodioritic intrusive complexes are found within the Wabigoon and Wawa volcanic -plutonic belts, and monzodioritic rocks occur as inclusions within metesedimentary rocks of the Quetico belt (Smith and Williams 1980; Day and Weiblen 1986). Examples of monzodioritic intrusions containing the sanukitoid suite include the Roaring River Complex described in more detail below, the Jackfish - Weller lakes, Ottertail Lake, Icarus, Eye-Dashwa lakes, Perching Gull

Lakes, and Ryckman Lake plutons and parts of the Giants Range batholith (references in Fig. 2).

Intrusion of monzodioritic complexes containing the sanukitoid suite postdated major deformation in their host rocks, which consist of tonalite-granodiorite gneiss and metavol- canic rocks. The monzodioritic intrusive complexes typically have steeply inclined linear and planar mineral fabrics that are concordant with the margins of the intrusion. These fabrics are interpreted as having formed after major deformation of the host rocks, during intrusion as steeply dipping arcuate sheets of magma along gneiss - supracrustal contacts (Schwerdtner et al. 1979, 1983; Schwerdtner 1986; Davidson 1980). Schwerdtner et al. (1979) called such bodies "crescentic granitoid plutons" because they show arcuate outcrop patterns, a reflection of their intrusion along preexisting contacts (Fig. 2). There is also some evidence that intrusion was concurrent with faulting (Thurston and Davis 1985), and feniti- zation of wall rocks has been observed adjacent to some monzodioritic intrusions (e.g., Longstaffe et al. 1980; Birk and McNutt 1981). The monzodioritic complexes are also cut by post-tectonic biotite granite, marking the close of Archean intrusive activity in this part of the Superior Province (Percival 1983).

Some of the monzodioritic intrusive complexes are concen- trically zoned from diorite or monzodiorite to granodiorite, suggestive of high-level magma chambers; examples include the Jackfish-Weller lakes pluton (Longstaffe et al. 1980, p. 1049), the Giants Range batholith (J. C. Green, 1970, p. 53), the Ottertail Lake pluton (Shirey 1984, p. 265 -268), the Eye-Dashwa lakes pluton (Brown et al. 1980), and the Ryckman Lake pluton (Birk et al. 1979). The Roaring River Complex differs, appearing to consist of mutually intruding batches of contemporaneous magma of varying composition (see below).

Modal abundances of quartz, alkali feldspar, and plagioclase of the sanukitoid suite from southwestern Superior Province are included in Fig. 3. Most of the rocks of the sanukitoid suite are diorites, monzodiorites, quartz monzodiorites, monzonites, and granodiorites, according to modal classification. These rocks all have a very distinctive mafic mineral assemblage consisting of clinopyroxene (salite to augite), amphibole (edenite, magnesio-hornblende, actinolitic hornblende), and biotite, with accessory sphene, apatite, and epidote. Orthopyroxene (hypersthene) occurs within the Jackfish - Weller lakes pluton (Sutcliffe 1980). Pyroxenite and gabbro occur as centimetre- to kilometre-sized inclusions within the monzodioritic complexes. Kersantite or spessartite lamprophyre dykes commonly cross-cut the monzodioritic complexes.

The Roaring River Complex (Fig. 4) displays many of the rock types and intrusive relations that typify the sanulutoid suite in southwestern Superior Province. The Roaring River Complex measures about 70 km in overall length and varies between 1 and 15 km wide, forming a crescentic outline (Fig. 4). All rocks enclosed by a heavy line in Fig. 4 are considered part of the Roaring River Complex. The Roaring River Complex is a new name that emphasizes the lithological complexity and contemporaneous nature of intrusive phases. Previously, portions of the Roaring River Complex had been called the Colwill Lake pluton and Damon Lake syenite (Percival 1983; R. A. Stern 1985).

Like other monzodioritic complexes containing the sanukitoid suite, the Roaring River Complex is lithologically heter-

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1690 CAN. J . EARTH SCI. VOL. 26, 1989

FIG. 1. Generalized geological map of part of the Archean Superior Province showing the locations of Figs. 2 and 4.

ogeneous from outcrop to map scale. The major components of the complex include diorite through granodiorite of the sanukitoid suite, inclusions of gabbro, and cross-cutting lamprophyre dykes and biotite granite plugs.

The wall rocks to the Roaring River Complex include coarse-grained, foliated to gneissic, hornblende - biotite tonalite, quartz diorite, and granodiorite. Where exposed, the contacts are intrusive, with dykes of massive, hornblende diorite intruding the gneisic fabric parallel to the layering. In several places, gneisses adjacent to the complex are highly sheared.

Massive to layered gabbro and clinopyroxenite occur mainly as inclusions within diorite through granodiorite. The gabbroic rocks are coarse grained to pegmatitic and consist of plagioclase (An55 -70), clinopyroxene, edenitic hornblende, biotite, magnetite, ilmenite, apatite, and pyrite. Local clinopyroxenite contains minor plagioclase and amphibole.

Clinopyroxene - hornblende monzodiorite, diorite, monzonite, and quartz monzodiorite to granodiorite constitute the sanukitoid suite in the Roaring River Complex. Both gradational and intrusive boundaries are present among these phases, and current sampling shows lithological and chemical continuity between diorite and granodiorite. Diorite through granodiorite phases occur as mutually intruding batches of magma with dimensions of the order of metres to hundreds of metres (Fig. 5).

Diorite and monzodiorite are typically salmon coloured and medium to coarse grained, with ragged phenocrysts of amphibole and biotite. Magnesian clinopyroxene occurs as discrete crystals or as cores to amphibole: Amphibole shows a range of compositions, including actinolite, magnesio-hornblende, and edenite (nomenclature after Leake 1978). Edenite is the common amphibole visible in hand specimens of diorite to monzonite. Some monzodiorite samples contain biotite and clinopyroxene without amphibole. The ferromagnesian minerals of the monzodioritesare enclosed in a matrix of anti- perthitic plagioclase (less than An3o), rnicrocline, and quartz. Quartz and feldspars occur as anhedral, interlocking grains, resulting in a sugary texture in hand specimen. Abundant

accessory minerals include apatite, Fe - Ti oxides, epidote, and sphene. All of the minerals present within monzodiorite are present within hornblende granodiorite, including remnant cores of clinopyroxene within hornblende phenocrysts. Micro- cline occurs as centimetre-sized phenocrysts that produce a porphyritic texture in granodiorite.

Hornblende and biotite locally define a weak foliation within diorite to granodiorite. Foliations strike parallel to the margins of the intrusion and are steeply dipping. The diorite to granodiorite phases locally contain mafic inclusions of hornblende, clinopyroxene, biotite, and plagioclase, varying from centi- metres to metres in size.

Small plugs and dykes of biotite leucogranite locally intrude diorite - granodiorite and gabbro - pyroxenite. Dykes and pods of quartz-feldspar pegmatite and aplite in turn cut biotite granite. Biotite granite is coarse grained and K-feldspar porphyritic, with abundant quartz (25-35%) occurring as bluish grey phenocrysts. The major ferromagnesian phases are biotite and opaques. Traces of hornblende and sphene are present locally.

Mesocratic to melanocratic dykes between 1 cm and 1 m wide cut all other rocks of the Roaring River Complex. The essential mineralogy includes plagioclase (about An50), hornblende, biotite, Fe - Ti oxides, clinopy roxene, and apatite, from which they are best classified as kersantitic (plagioclase, biotite, augite) to spessartitic (plagioclase, hornblende, augite) lamprophyres (Streckeisen 1979). The dykes are fine to medium grained and are commonly hornblende porphyritic. The dykes are commonly backveined by the host diorite to granodiorite, and they are also commonly broken up and rein- truded by their host rocks.

Chemical characteristics of the sanukitoid suite Table 1 includes chemical analyses of intrusive and volcanic

rocks characteristic of the Archean sanukitoid suite within southwestern Superior Province. "Sanukitoid" as used in this paper refers to primitive, high-Mg, intermediate-silica rocks

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STERN ET AL. 1691

FIG. 2. Distribution of syntectonic to post-tectonic intrusions (shown in black) that are substantially enriched in LILE'S in comparison with their wall rocks. The main rock types are diorite, monzodiorite, granodiorite, and syenite, with lesser gabbro, pyroxenite, and lamprophyre. Intrusions that may contain the sanukitoid suite are indicated by a star beside the name of the intrusion. Two locations where there are primitive trachyandesites of the sanukitoid suite are indicated by the large stars. The Roaring River Complex is located in the upper right portion of the map. Partial list of references by intrusion: Barnum Lake (Kehlenbeck 1977); Bell Lake (Rogers 1964; Trowel1 19836); Blalock (Williams 1978); Burchell-Moss lakes (Giblin 1964; Harris, 1970; Smith 1978; Smith and Williams 1980); Entwine Lake (Davies 1965; Sage et al. 1975); Eye-Dashwa lakes (Goodwin 1978; Brown et a1 1980; Kamineni et al. 1988); Flora (Birk and McNutt 1977; Birk et al. 1979); Giants Range batholith (Clear and Farm Lake facies, J. C. Green 1970; Sims and Viswanathan 1972; Arth and Hanson 1975); Greenwater Lake (Stott 1985; Thurston 1985; Smith and Williams 1980); Hood Lake (Thurston 1985; Schwerdtner 1986); Icarus (Kavanaugh 1969; Goldich et a1 1972; Hanson and Goldich 1972; Arth and Hanson 1975); Jackfish-Weller lakes (Sutcliffe and Fawcett 1979, 1980; Sutcliffe 1980; Longstaffe et al. 1980, 1982); Kekekabic (Stark 1927; Sims and Mudrey 1972); Kekekaub Lake (Percival 1983; R. A. Stern 1985); Linden (Sims et al. 1972; Arth and Hanson 1975); Norway Lake (Schwerdtner et al. 1979; Sage et al. 1975); Ottertail Lake (Goldich and Peterman 1980; Shirey 1984); Pakashkan-Loganberry lakes (Sage et al. 1974); Pekagoning Lake (Sage et al. 1975); Penassen Lakes (Scott 1985); Perching Gull Lakes (R. A. Stern 1984; Percival and Stern 1984); Poohbah Lake alkalic complex (Mitchell 1976; Mitchell and Platt 1979; Sage et al. 1979; Sage 1983~); Ryckman (Birk et al. 1979); Saganaga (Harris 1968; Hanson 1972; Arth and Hanson 1975; Evans 1987); Sleigh Lake (Percival 1983; R. A. Stern 1985); Smirch Lake (Sage et al. 1975); Snowbank (Hanson 1964; Sims and Mudrey 1972); Sturgeon Narrows and Squaw Lake alkalic complexes (Rogers 1964; Sage 1983b; Trowel1 1983a, 1983b); Valora Lake (Rogers 1964; Trowel1 1983b); Vista-Sesganaga lakes (Rogers 1964; Trowell 1983b).

(intrusive or extrusive) that show very high LILE contents. In The chemical criteria that we suggest are typical of sanuki- lithological terms, the sanukitoids are diorites, monzodiorites, toids are intermediate silica contents (55 -60 wt. % SOz), monzonites, or trachyandesites. The sanukitoid suite includes Mg#'s of 0.6 or greater, Ni > 100 ppm and Cr > 200 ppm, these rocks plus related quartz monzodiorite through grano- K20 greater than 1 wt. %, Sr and Ba greater than 500 ppm, diorite. RbISr ratios less than 0.1, and REE patterns that are strongly

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/ ', MODES ,/ \, Q

/ \:s( "'. NORMS : \ \ , /,

o RELATED ROCKS

FIG. 3. Summary of modal (Streckeisen 1976) and cation- normative compositions of the sanukitoid suite (sanukitoids plus related rocks) from intrusions throughout southwestern Superior Province. A, alkali feldspar or orthoclase component; P, plagioclase or anorthite + albite component; Q, quartz. Data from intrusions referenced in caption to Fig. 2 and new data from the Roaring River Complex.

enriched in LEE'S and have little or no Eu anomalies (Fig. 6). The Archean sanukitoids may be silica oversaturated, saturated, or undersaturatd on a normative basis. Because there are very few experimental data on sanukitoids and their potential source rocks (see below), we do not know the compositional range of primary (unfractionated) sanukitoids.

The word "sanukitoid" originated in the Sanuki region of southwest Japan, where glassy andesites with bronzite needles were first referred to as "sanukites" by Weinschenk (1891). Later, Koto (1916) introduced the term "sanukitoid" to describe "all textural modifications of. . . sanukite . . . ," apparently for the purpose of including rocks associated with sanukite but with different phenocryst assemblages, such as clinopyroxene and magnesian olivine. More recently, Tatsumi and Ishizaka (1982a, 198221) described the sanukitoids as relatively aphyric, plagioclase-free andesites and basalts. Shirey and Hanson (1984) adopted the term sanukitoid to emphasize the andesitic composition and primitive ferromagnesian character of the Archean rocks and the potential mantle origin for the rocks and because it was a term that had not been exten- sively used in the literature. Thus, we use "sanukitoid" as a chemical term, not a field classification. The rocks may be intrusive or extrusive.

In the Superior Province, the sanukitoids are closely associated with progressively more siliceous and chemically evolved quartz monzodiorite, quartz monzonite, and granodiorite (Table 1; Figs. 3, 7). The evolved rocks have chemical characteristics consistent with their derivation from sanukitoid parents, such as their relatively high Mg#'s, high abundances of Sr and Ba, and steeply LEE-enriched patterns, which are subparallel to those of the sanukitoids.

Figure 3 shows the cation-normative compositions of the sanukitoid suite in southwestern Ontario, where the trend towards silica oversaturation in the suite is well illustrated. Figure 7 shows the chemical continuity between 55 and 70 wt. % Si02 and the characteristically high LILE contents throughout the suite.

Oxygen-isotopic data of sanukitoids from the Jackfish- Weller lakes, Ottertail Lake, Ryckrnan Lake, Flora Lake, and Taylor Lake plutons range from 6.7 to 7.7%,, extending to about 9%, for more-siliceous rocks (Longstaffe 1979; Longstaffe and Birk 1981). Initial 87Sr/86 Sr ratios for the Jackfish-Weller lakes, Ottertail Lake, Ryckman Lake, Flora Lake, Taylor Lake, Icarus, and Giants Range plutons are approximately 0.701 (Hanson et al. 1971; Catanzaro and Hanson 1971; Prince and Hanson 1972; Birk and McNutt 1981). EN^ values at 2700 Ma, the age of crystallization of most of the analyzed sanukitoids in southwestern Superior Province, fall between + 1 and +2.5 (Shirey 1984; Shirey and Carlson 1985a, 1985b, 1986, 1988; Shirey and Hanson 1986). Pb-isotope data indicate low initial 238U/204Pb (u, at 2700 Ma = 7.4-8.0; Arth and Hanson 1975; Oversby 1978; Shirey and Carlson 1986, 1988). These data suggest negligible contributions from substantially older continental crust.

Table 2 compares the chemistry of the average Archean sanutikoid (including intrusive and extrusive rocks) from southwestern Superior Province (column 1) with volcanic and intrusive rocks of more recent origin. The average sanukitoid has 56% Si02, 6.8 wt.% MgO, an Mg# of 0.63, Ni = 150 ppm, Cr = 350 ppm, Na20 = 3.9%, K20 = 2.1%, Sr and Ba both = 1200 ppm, Ce about 120 x chondrites, and a RbISr ratio of 0.05. Compared with the Miocene sanukitoids of Japan (column 2), the Archean rocks have lower MgO, Ni, Cr, and RbISr ratios and much higher abundances of Na20, K20, P205, Sr, and Ce and higher CeIYb ratios. The Archean sanukitoids have higher Fe, MgO, and Ba contents and lower Ce/Yb and KIRb ratios than a high-Mg andesite sample from the Aleutians (column 3). The Archean rocks have higher Si02 and LILE'S and lower Ti02 and MgO than the high-Mg basaltic andesites from Peru (column 4). The Cascade Range of western United States contains a high proportion of high-K andesites (Ewart 1982) with many of the characteristics of the Archean rocks; however, the Archean rocks tend to have slightly higher abundances of ferromagnesian elements and lower Ti02. The high LILE content, espe- cially the elevated K20 content of the Archean sanukitoids, is similar to that of the shoshonite association. Included in Table 2 is an average shoshonitic basaltic andesite (Morrison 1980). The shoshonitic basaltic andesites tend to have lower ferro- magnesia-element abundances and higher K20/Na20 ratios than the Archean sanukitoids. The shoshonite association also tends to be dominated by basalts to basaltic andesites, with only minor siliceous derivatives, unlike the Archean sanukitoid suite.

The Archean sanukitoid suite and the commonly associated gabbroic, pyroxenitic, and larnprophyric rocks have chemical and lithological similarities to the (intrusive) Caledonian appinite suite of the British Isles and the Appalachians (Hall 1967; Wright and Bowes 1979; Halliday and Stephens 1984; Bender et al. 1984; Fowler 1988). The appinite suite consists largely of pyroxenite, gabbro, and lamprophyre but also includes syenitic, dioritic, and granodioritic rocks. Most rock types have relatively high LILE contents. Included in Table 2 is an analysis of average diorite from the compositionally

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STERN ET AL. 1693

FIG. 4. Geological map of the Roaring River Complex (bounded by heavy line) and surrounding rocks. Compiled from the following sources: Milne (1964), Rogers (1964), Sage et al. (1974), Percival (1983, 1987a, 1987b), R. A. Stern (1985), and our subsequent mapping.

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FIG. 5. Typical field occurrence of the sanukitoid suite. Outcrop shows intrusion of discrete batches of diorite to meladiorite, monzonite, and quartz monzonite from the Roaring River Complex.

heterogeneous appinite suite. The diorite has lower LILE'S than the Archean sanukitoids but is quite similar in other respects.

The Roaring River Complex includes diorite to granodiorite, which are characteristic of the sanukitoid suite. Major- and trace-element compositions of representative rock types from the Roaring River Complex, including diorite to granodiorite, gabbro -pyroxenite, biotite granite, and lamprophyre dykes, are presented in Table 3. Typical REE patterns are shown in Fig. 8.

Some of the important aspects of the chemistry of the sanukitoid suite from the Roaring River Complex include (i) con- tinuous variation in Si02 (from 55 to 73 wt. %) and variation from near silica saturation to silica oversaturation on a normative basis; (ii) relatively high and decreasing Mg#'s (from 0.59 to 0.43), concurrent with decreasing MgO, Fe203, N2o3, CaO, and Ti02; (iii) Ni contents of about 80 pprn and Cr about 135 pprn in diorite to monzodiorite; (iv) high and decreasing contents of Na20 (from 5.5 wt. % to 4.3 wt. %), Sr (from 2032 pprn to 537 pprn), Ce (from 350 pprn to 52 ppm), and P2O5 (from 0.69 wt. % to 0.08 wt. %); (v) high K20 (0.76 -4.1 wt. %); and (vi) subparallel, LREE-enriched patterns in which Eu anomalies are small or absent (Fig. 8).

Gabbro and clinopyroxenite are enriched in FeO, MgO, and CaO and depleted in Na20 and K20 in comparison with diorite and monzodiorite, and they are moderately to strongly nepheline normative. Clinopyroxenite has about 1500 pprn Cr and a high Mg# of 0.76. Gabbro and clinopvroxenite are LREE-

enriched but are concave downward fromkm to Ce (Fig. 8), suggestive of clinopyroxene or hornblende accumulation. Their concave-downward LREE patterns are not those expected for the residues required to relate diorite through granodiorite of the sanukitoid suite, since the REE patterns for the sanukitoid suite show constant LREE enrichment.

Late biotite granite is enriched in K and depleted in Sr and Ba in comparison with the diorite-granodiorite series. REE

patterns have CeIYb ratios similar to those of the sanukitoid suite but show significant negative Eu anomalies (Fig. 8), sug- gesting that these rocks may not be on the same liquid line of descent as the sanukitoid suite.

The lamprophyre dykes generally have less than 50% Si02 and are silica saturated to strongly silica undersaturated. Com- pared with diorite and monzodiorite, lamprophyre dykes have either higher or lower Ni and Cr abundances and Mg#'s. The lamprophyre dykes have a range in Sr abundances similar to that of the sanukitoid suite but have lower Ba abundances. The lamprophyre dykes have REE patterns that are strongly LREE

enriched and are subparallel over a large range in overall abundance (Fig. 8). The CeINd ratios of the lamprophyres are generally lower than those of the sanukitoid suite.

Petrogenesis of the Archean sanukitoids The distinctive chemical features of the Archean sanukitoids

are their intermediate silica contents, relatively primitive ferromagnesian-element abundances, and very high Sr, Ba, alkali-element, and LREE contents. In the following discussion, specific petrogenetic models are examined that might explain this unusual chemistry. The modelling utilizes the data set from throughout southwestern Superior Province (Table 1, samples 1-9) and also the least-evolved rocks from the Roar- ing River Complex. The petrogenetic models that we will con- sider for the origin of the most primitive sanukitoids are (1) melting, fractionation, or crustal contamination of basalt; (2) fractionation from coeval lamprophyres; (3) combined assimilation - fractional crystallization (AFC) of komatitic liquids; and (4) melting of LILE-enriched mantle peridotite.

Melting, fractionation, and contamination of Archean basalt It is very difficult to explain the sanukitoids by partial melt-

ing or fractional crystallization of typical basalts of the Superior Province. The difficulties can be appreciated by considering the MgO-FeO systematics of the sanukitoids shown in Fig. 9a. In mafic or ultrarnafic systems, liquids derived by partial melting or fractional crystallization leaving amphibolite, granulite, or eclogite residues will generally have lower MgO contents and lower Mg#'s than the parent melt or source rock (e.g., T. H. Green and Ringwood 1968; Helz 1976; C. R. Stern and Wyllie 1978). Thus, parental melts or source rocks to the sanukitoids must lie at higher MgO contents than

Can

. J. E

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TA

BL

E 1. C

hem

ical

ana

lyse

s of

the

san

ukito

id s

uite

fro

m v

ario

us p

arts

of

sout

hwes

tern

Sup

erio

r Pr

ovin

ce

I 2

3 4

5 6

7 8

9 10

11

12

13

14

15

16

Sin

, (w

t. %

)0

56.3

8 54

.77

55.9

2 54

.21

56.3

5 58

.80

55.8

9 55

.93

55.3

2 56

.19

58.9

5 61

.29

63.5

7 67

.55

68.9

7 70

.59

Ti%

0.

59

0.72

0.

43

1.13

0.

67

0.72

0.

76

0.81

0.

74

0.80

0.

54

0.32

0.

73

0.52

0.

24

0.13

A

12%

12

.67

14.4

4 14

.55

16.1

4 15

.93

14.5

0 13

.9

14.7

8 15

.30

16.9

3 17

.79

14.1

9 14

.74

16.1

5 15

.50

16.0

4 F

W,b

2.

14

8.59

6.

48

2.19

3.

48

8.73

3.

08

2.26

1.

97

5.12

1.

64

1.28

1.

30

Fe

d

4.17

8.

33

6.74

5.

09

2.99

4.

46

4.28

3.

47

2.76

1.

84

2.21

M

nO

0.

11

0.12

0.

12

0.12

0.

12

0.11

0.

10

0.14

0.

14

0.09

0.

10

0.09

0.

08

0.07

0.

06

0.03

M

gO

8.27

8.

01

7.40

6.

68

6.29

6.

07

6.06

6.

72

6.48

4.

75

4.58

5.

53

2.54

1.

40

1.40

0.

53

CaO

8.

76

8.74

6.

85

7.75

7.

97

6.04

7.

60

7.57

7.

52

5.97

4.

94

5.14

5.

07

2.91

2.

62

1.27

N

a,O

3.84

2.

88

4.97

3.

80

3.82

3.

66

4.56

3.

38

4.33

5.

37

4.21

4.

84

4.21

5.

09

5.92

5.

43

K2°

2.

75

1.35

2.

87

1.85

1.

74

2.46

4.

07

1.73

2.

25

2.75

3.

10

3.21

4.

17

3.00

2.

97

4.62

p

24

0.

33

0.36

0.

42

0.37

0.

36

0.59

0.

30

0.34

0.

60

0.30

0.

27

0.49

0.

18

0.11

0.

05

Rb

(~

m)

46

67

77

34

62

.5

102

81.5

89

74

.1

97

125

Sr

1276

62

7 16

88

910

1413

11

64

1871

65

0 12

37

2502

99

1

1010

21

00

1051

12

43

570

Ba

1833

61

2 85

1

1725

10

48

1881

10

17

1260

17

00

1159

16

93

1157

Z

r 56

13

8 12

2 18

9 53

14

0 16

7 27

2 11

2 19

0 10

3 14

2 Ce

64.7

64

82

.6

83

84.2

19

2 72

.7

94.6

84

.0

56

200

44

67

Yb

2.31

0.

57

1.65

1.

64

1.12

1.

61

1.92

0.

913

0.61

1.

1 0.

96

Ni

188

150

108

159

163

103

10

16

Cr

469

218

29 1

42

9 14

6 16

Mg#

(all

FeZ

t)

0.71

0.

65

0.69

0.

59

0.62

0.

61

0.64

0.

60

0.62

0.

57

0.61

0.

68

0.52

0.

46

0.53

0.

45

(Cel

Yb)

, 7.

2 28

.7

12.8

13

.5

43.9

11

.6

12.6

23

.5

23.5

46

.5

17.8

KtRb

496

356

199

425

327

331

229

257

34 1

27

5 46

2 29

0 34

2 30

7 R

blSr

0.

04

0.04

0.

08

0.02

0.

05

0.05

0.

07

0.04

0.

08

0.10

0.

04

0.08

0.

06

0.22

N

orm

, q

uar

kc

0.0

1.09

0.

0 0.

25

3.87

1.

43

0.0

2.22

1.

98

8.86

15

.35

13.8

8 15

.66

Nor

m. ne~h.~

0.0

3.94

0.

0 6.

28

0.0

1.80

SAMPLE ID

E~

MC

AT

ION

: (I

) Bio

tite-

pyro

xene

dio

rite

, Jac

M~

sh- W

elle

r la

kes

plut

on (

sam

ple

S7&

10. S

ntcl

iffe

198

0); (

2) m

onzn

dior

ite, m

argi

nal p

hase

of

Res

t Is

lad

plu

ton

(sam

ple 5

-80:

Shi

rey

and Ha

nson

19

86);

(3) p

yrox

ene-

horn

blen

de

mon

zodi

orite

from

Ryc

kman

Lak

e pl

uton

(sam

ple

RY

92, B

irk e

t 01.

1979

); (4

) mon

zdio

rite

occ

umng

as m

etre

-siz

ed in

clus

ion

with

in LC La

Cro

ix G

rani

te. V

erm

illirm

Gra

nitic

C

ompl

ex o

f Q

uetic

o su

bpro

vinc

e (s

ampl

e 5,

Day

and

Wei

blcn

198

6); (

5) d

iori

le d

yke,

Jac

kfis

h-W

elle

r la

kes

plut

on (

sam

ple

E12

6, L

ongs

taff

e et

al.

198

0); (

6) m

onzo

dior

irc,

Otte

rtai

l Lak

e pl

uton

(sa

mpl

e 41

-66,

Shi

rey

and

Han

son

1986

); (7

) sye

nndi

orite

, Ica

rus p

luto

n (s

ampl

e 34.

Arth

and

Han

son

1975

); (8

) tra

chya

ndes

ite o

f the

Rocky I

slet

Bay

Com

plex

(sam

ple 5

3-80

. Shi

rey

and

Han

son

1986

); (9

) tra

chya

ndes

ite

From

the

New

ton

Lak

e Fo

rmat

ion.

Min

neso

ta (s

ampl

e 6.

Art

h an

d H

anso

n 19

75):

(10)

mon

zodi

orite

occ

urri

ng a

s ki

lom

etre

-siz

ed in

clus

ion

with

in l

euco

gran

ite o

f Q

uetic

o su

bpro

vinc

e. s

outh

of

Bla

lock

plu

ton

(sam

ple

29, S

mith

and

Will

iam

s 19

80: W

illia

ms

1978

); (1

1) m

onzo

dior

ite f

rom

Ica

rus

plut

nn (

sam

ple

23, A

rth a

nd H

anso

n 19

75):

(12)

bio

tite-

horn

blen

de

mon

zadi

orite

, Ryc

kman

Lak

e pl

uton

(sa

mpl

e R

Y21

. B

irk e

t 01

. 19

79);

(13

) m

onzo

dior

ite,

Eye

-Das

hwa

lake

s pl

uton

(sa

mpl

e 16

2, K

amin

eni

el a

l. 1

988)

: (1

4) g

rano

dior

ite, B

umhe

ll L

ake

plut

on (

sam

ple

18.

Smith

and

Will

iam

s 19

80);

(15

) gr

anod

iori

te,

JacM

~sh-

Wel

ler l

akes

plu

ton

(sam

ple

F118

, L

ongs

taff

e er

01.

19

80):

(16

) gr

anod

iori

te fr

om F

lora

Lak

e pl

uton

(sa

mpl

e FL

5, B

irk

er a

l. 1

979)

. "E

leve

n ox

ides

nor

mal

ized

to 1

00%

. 'T

otal

Fe

as F

+03

or

FeO

whe

re o

nly

one

oxid

e sh

own.

'C

atio

n m

olec

ular

nor

m (

tota

l Fe as ~

e'+

).

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1696 CAN. I. EARTH SCI . VOL. 26, 1989

2 C e N d S m E u G d Dy E r Yb

FIG. 6. REE analyses of sanukitoids from southwestern Superior Province. All these data were collected at SUNY at Stony Brook by isotope-dilution mass spectrometry. (1) Monzodiorite from Rest Island pluton (sample 5-80; Shirey and Hanson 1986); (2) trachyandesite from the Rocky Islet Bay Complex (sample 53-80, Shirey and Hanson 1986); (3) monzodiorite from Ottertail Lake pluton (sample 41-66; Shirey and Hanson 1986); (4) trachyandesite from the Newton Lake Formation, Minnesota (sample 6, Arth and Hanson 1975); (5) monzodiorite from Roaring River Complex (sample 86-92a, this study); (6) syenodiorite from Icarus pluton (sample 34, Arth and Hanson 1975). See Fig. 2 for location of intrusions. Nor- malizing values reported in Hanson (1980, Table 1, column 1).

the sanukitoids and at higher MgIFe ratios. The field of basalts from the Superior Province is drawn in

Fig. 9a along with the average basalt from the Superior Province (Goodwin 1977). The basalts cannot be parental to the sanukitoids because they have MgO contents and MgIFe ratios similar to or lower than those of the sanukitoids. For example, Fig. 9a shows a calculated path of fractional crystallization ("FC") for average Superior Province basalt using the modelling approaches of Nielsen (1985, 1988). The differ- entiated melts have much lower MgO and MgIFe ratios than the sanukitoids. A qualitatively similar result would be expected for partial melting of basalts.

The Ni and Cr contents of average Superior Province basalts, 161 pprn and 249 ppm, respectively (Table 2), are similar to or lower than those of the sanukitoids, which average 350 pprn Cr and 150 pprn Ni, and are therefore insufficient to be considered parental to the sanukitoids. Since partial melting or fractional crystallization of basalt will leave phases such as amphibole, pyroxene, or garnet in the residue, the bulk distribution coefficients for Ni and Cr would probably be much greater than 1 (Gill 1974; Gill 198 1, Table 6.3). If the modelling calculations of Nielsen (1985, 1988) are used, the liquid after 40% fractional crystallization of average basalt has 54% Si02 but only 5 pprn Ni and 14 pprn Cr. The parental rocks to the sanukitoids would need much more than 350 pprn Cr and 150 pprn Ni.

The high LILE abundances of the sanukitoids also cannot be generated by melting Archean basalts over the range of extents of melting necessary to generate the sanukitoids. Mass- balance considerations and a number of melting experiments under a variety of P, T, and XHI0 conditions have shown that

to reach the relatively high feromagnesian-element and silica abundances in the sanukitoids requires 25 -50 % melting of a basalt (T. H. Green and Ringwood 1968; Holloway and Burnham 1972; Gill 1974; Helz 1976; C. R. Stern and Wyllie 1978). Average Superior Province basalt has 177 pprn Sr and 143 pprn Ba (Goodwin 1977), whereas average sanukitoids have about 1200 pprn Sr or Ba. To produce these levels of Sr and Ba in a melt of an average basalt source limits the degree of melting to less than 15 % . If plagioclase were present in the residue, significant enrichment in Sr would be unlikely. Experimental studies show that 5-20% melting of basalt at arnphibolite, granulite, or eclogite grade produces silica-oversaturated trondhjemites, tonalites, granodiorites, or quartz diorites, not near-silica-saturated diorites or monzodiorites (e.g., T. H. Green 1982; Rapp and Watson 1988).

Indirect evidence from melting experiments suggests that melts near the solidus of basalt for dry, eclogite-grade conditions, where alkalis, Sr, and Ba may be incompatible, would probably start out syenitic and, at higher extents of melting, become silica oversaturated as the alkalis are diluted in the melt (T. H. Green and Ringwood 1968, p. 132; C. R. Stern and Wyllie 1978; T. H. Green 1982). Syenites generated under these conditions would be expected to have very low abundances of Fe, Mg, Cr, or Ni, unlike the parental sanukitoids. Low extents of partial melting required to explain the enrichments in incompatible elements are not consistent with the major-element requirements of the sanukitoids.

It is not possible to explain the LILE enrichment of the sanukitoids by crustal contamination of average Archean basalts. Since basalts and older granitoids in the southwestern Superior Province have lower abundances of Sr, Ba, Ce, and other incompatible elements than the sanukitoids, contamination of basalt melts by granitoids could not lead to the high LILE abundances. Table 4 includes a calculated liquid composition resulting from incremental assimilation and fractional crystallization of average Superior Province basalt. The assimilant used was the present upper crust (Taylor and McLennan 1981), which is more LILE enriched than average Archean crust (Taylor and McLennan 1985). The calculated liquid has about the same silica content as the sanukitoids, but Sr (193 ppm) and Ba (305 ppm) are all well below the expected abundances in the sanukitoids. Contamination by partial melts of typical crustal granitoids would not alleviate the problem because such melts would have even lower Sr than the parent granitoids because of feldspar in the residue.

Figure 10 shows the REE pattern of the liquid resulting from crustal contamination of a basalt with a flat, lox-chondritic REE pattern. The contaminant used was the average felsic component of the Archean crust of Taylor and McLennan (1985, Table 7.8). The contaminated liquid has a flatter REE pattern than the sanukitoids, with much lower LREE concentrations. On the FeO -MgO diagram (Fig. 9a), incremental fractional crystallization and crustal assimilation generates a path (AFC)

that fails to intersect the sanukitoid field. In summary, the sanukitoids cannot be derived by melting

or fractionation of typical basalts of the Superior Province. The products of melting or fractionation of basalts would have MgO abundances, Mg#'s, and Ni and Cr contents much lower than those of the sanukitoids. Neither low extents of melting of basalt nor crustal contamination of basalt can explain the high LILE contents, steep REE patterns, and primitive ferromagnesian-element character of the sanukitoids.

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STERN ET AL.

LEGEND Sanukitoids from SW Superior Province

0 Potential derivatives of sanukitoids A Sanukitoid suite from the Roaring River

Complex

MgO, Na20, K20. P205 in w t . X oxide ST. Ba. Ni. Y in ppm FeT is total Fe in cation mol% FeO

2000 ' A 0 @ o * A M A A @o 15

1500 - OgaB A BA

0 a*. W~&i 0% B8 10 1000 :

0 0 0

0. 0 0 0 , : & % O 500 A 6' .A

5

FIG. 7. Compilation of chemical data for the sanukitoid suite from intrusions throughout southwestern Superior Province and from the Roaring River Complex. References in caption to Fig. 2.

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1698 CAN. J. EARTH SCI. VOL. 26, 1989

TABLE 2. Comparison of Archean sanukitoids with other rocks

Archean sanukitoid Japan Aleutians Andes Cascades Appinite Shoshonite Melt Crust Basalt Mantle

1 2 3 4 5 6 7 8 9 10 11

SiO, (wt.%) 56.07 57.35 58.5 52.6 54.06 55.36 55.46 55.5 57.0 51.50 44.20 TiO, 0.71 0.66 0.94 1.16 1.43 0.92 0.93 2.5 1 .O 1.15 0.13 A1z03 14.88 15.56 15.2 16.6 16.66 15.75 16.75 12.3 15.2 15.40 2.05 Fe-203 2.4 3.63 1.77 2.55 0.5 2.81 FeO 7.08 6.27 1.5 7.59 4.70 5.48 4.01 6.8 9.6 9.14 8.29 MnO 0.12 0.13 0.06 0.12 0.13 0.15 0.11 0.1 0.22 0.13 MgO 6.85 8.59 5.0 8.16 5.42 5.87 4.81 9.3 5.9 6.50 42.21 CaO 7.65 6.80 7.6 7.66 7.64 7.15 6.71 10.4 7.3 9.32 1.92 Na,O 4.04 2.81 3.7 3.3 3.65 2.81 2.94 2.0 3.0 2.60 0.27 K20 2.23 1.68 2.2 1.72 2.15 2.25 3.66 0.5 0.9 0.49 0.06 pzo, 0.36 0.14 0.54 0.54 0.21 0.60 0.19 0.03 Total 99.99 99.99 97.64 98.91 100.01 97.72 98.53 99.9 99.9 99.32 99.09

Rb ( P P ~ ) 60 78 17 49 46.7 64 63 28 10 1 Sr 1229 276 2600 650 1194 433 956 215 177 27.5 Ba 1214 343 482 1015 529 567 220 143 8.75 Zr 111 170 189 173 121 100 106 15.5 C e 97.0 24.65 88.5 47 107 29 31 9.6 Yb 1.6 1.57 0.633 1.83 3.5 2.2 2.5 Ni 154 203 150 139 83.2 71 53 130 161 2200 Cr 352 45 1 395 189 229 14 1 230 249 3010

Mg# 0.63 0.71 0.71 0.66 0.55 0.60 0.58 0.70 0.52 0.50 0.90 (CelYb), 15.5 4.02 35.8 25.7 7.8 3.6 0.98 KIRb 309 179 1074 29 1 382 292 482 267 407 498 RbISr 0.05 0.28 0.01 0.08 0.04 0.15 0.07 0.13 0.06 0.04

SAMPLE IDENTIFICATION: (1) Average of 11 sanukitoids from southwestern Superior Province - 9 samples from Table 1 and samples S76-34 and S78-56 from Sutcliffe (1980); (2) average high-magnesian andesite from the Setouchi volcanic belt, southwest Japan - major elements, Ce, Yb, Ni, and Cr from Tatsumi and Ishizaka (1982b), N = 19; Rb, Sr, and Th from Ishizaka and Carlson (1983), N = 9; (3) high-Mg andesite 70-B49 from western Aleutian Ridge (Kay 1978); (4) Low-Si Cenozoic Peruvian andesite NPA-22 from Noble et al. (1975); (5) average high-K basaltic andesite from Tertiary -Recent rocks of western U.S.A. - Western Zone (Ewart 1982, Appendix 3); (6) average pyroxene mica diorite to hornblende diorite from the Caledonian appinite suite of the Loch Lomond - Loch Fyne region, Scotland (Wright and Bowes 1979, Table 1). (7) average shoshonitic basaltic andesite (Morrison 1980); (8) melt composition presumed to be in equilibrium with pyrolite - 40% olivine composition at 10 kbar, 1000°C for water-saturated conditions (D. H. Green 1976); 28% liquid is present with residual phases olivine, orthopyroxene, and clinopyroxene; (9) composition of total Archean crust from Taylor and McLennan (1985, Table 7.10).; (10) weighted average basalt from the Superior Province (Goodwin 1977, Table IV). Ce, Yb, U, Th, and Rb abundances from Taylor and McLennan (1985, Table 7.8); (11) Average major-element composition of 384 spinel lherzolites (Maaloe and Aoki 1977); Rb, Sr, Ba, Zr, U, and Th are mantle values reported by Frey et al. (1978).

Fractionation and contamination of lamprophyres Lamprophyres from the Roaring River Complex and other

locations in southwestern Superior Province are shown as a field in Fig. 9b. The lamprophyres, like the greenstone-belt basalts, are unsuitable as parents to the sanukitoids because they have MgO contents similar to or lower than those of the sanukitoids and also have similar or much lower Mg#'s. Table 3 includes analyses of typical lamprophyres intruding the sanukitoid suite of the Roaring River Complex. The lamprophyres have 45 - 50 % Si02 and are silica undersaturated on a normative basis. Considerable fractionation of a silica-undersaturated mineral, such as amphibole or biotite, would be required to raise their silica contents, and this would necessarily lead to lower transition-metal abundances. The lamprophyres are inappropriate as parental rocks to the sanukitoids because they have similar or lower Ni and Cr contents (Table 3). From the MgO - FeO diagram (Fig. 9b) it can be seen that mixing or contamination of lamprophyres by the upper crust would lead to magmas with lower Mg#'s than the sanukitoids.

Assimilation -contamination of komatiite Komatiites are better candidates than basalts as source rocks

to the sanukitoids because they can have higher MgO abundances and higher Mg/Fe ratios than the sanukitoids (Fig. 9c). Sparks (1986) suggested that the evolved silica content and primitive ferromagnesian character of sanukitoids might be explained by crustal contamination of komatiitic melts. The sanukitoids do lie between komatiite and a crustal contaminant on the FeO-MgO diagram (Fig. 9c), and it has been shown that crystallizing peridotitic komatiites could assimilate up to three times their mass of felsic crust (Sparks 1986). Crustal contamination of komatiites might explain the elevated silica and LILE contents of the sanukitoids. The possible effects of contamination have been tested by considering interaction of komatiitic liquids with the present upper-crust composition of Taylor and McLennan (1981; Table 4) and, for the REE'S, the average Archean felsic rock (Taylor and McLennan 1985, Table 7.8). These crustal comvositions were chosen because they are the most LILE-enriched estimates of the crust, thus presenting the most favourable conditions for contamination.

Two types of modelling calculations are considered (Table 4). First, a single komatiite composition was simply mixed with a melt of the same composition as the upper crust. This type of calculation was used by Sparks (1986), and it assumes that the heat required for melting of the crust was derived from

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TA

BLE

3. C

hem

ical

ana

lyse

s of

rep

rese

ntat

ive

sam

ples

fro

m t

he R

oari

ng R

iver

Com

plex

Sanu

kito

id s

uite

L

ampr

ophy

re

Qua

rtz

Bio

tite

gra

nite

dy

kes

Clin

opyr

oxen

ite

Gab

bro

Dio

rite

M

onzo

dior

ite

mon

zodi

orit

e G

rano

dior

ite

Gra

nodi

orit

e G

rano

dior

ite

85-0

7a

85-0

9 84

-31

86-9

2a

85-1

5 86

-75a

86

-74

86-9

4 86

-95

86-7

8 86

-62

86-5

0

SiO

z (w

t. %

) 49

.27

40.8

5 55

.09

57.2

3 62

.18

65.2

8 68

.17

72.7

4 71

.23

74.9

7 44

.85

49.9

1 T

i02

0.46

1.

27

0.95

0.

68

0.58

0.

47

0.36

3 0.

202

0.36

3 0.

144

1.04

0.

78

A1z

03

7.74

14

.13

17.2

4 16

.63

15.9

2 16

.51

16.1

2 14

.86

15.0

0 14

.05

16.8

6 14

.62

FeZ

O3O

8.

94

22.0

8 6.

36

5.74

4.

52

3.70

2.

86

1.64

2.

38

1.11

14

.58

9.86

M

nO

0.20

2 0.

149

0.11

5 0.

090

0.07

1 0.

074

0.05

6 0.

039

0.50

0.

03

0.23

6 0.

156

MgO

13

.87

6.84

4.

49

4.20

2.

98

1.80

1.

33

0.62

0.

67

0.20

2 4.

91

9.75

C

aO

18.1

8 12

.22

8.48

6.

21

4.50

3.

48

3.12

1.

76

1.57

0.

72

11.2

0 8.

74

Naz

O

1.10

1.

83

5.48

5.

17

4.87

4.

60

5.09

4.

34

4.38

3.

73

3.43

2.

97

K20

0.

401

0.46

6 0.

76

3.22

3.

79

3.79

2.

85

4.11

4.

44

5.14

1.

00

1.91

p2

05

0.14

6 0.

055

0.69

0.

44

0.36

0.

24

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5 0.

083

0.11

0 0.

041

1.34

0.

435

Tot

al

100.

29

99.8

9 99

.64

99.6

1 99

.77

99.9

5 10

0.12

10

0.39

10

0.19

10

0.14

99

.45

99.1

3

Rb

(PP

~)

16

17

4 3

1 55

92

58

86

12

0 18

5 23

4

1 Sr

23

8 95

8 20

32

1867

13

09

726

892

537

238

86

1816

86

6 8

Ba

116

212

450

2575

16

17

1596

10

22

974

792

305

500

646

B Z

r 59

68

37

0 20

9 21

9 20

5 13

7 10

3 24

0 82

12

0 20

5 3

Ni

178

30

80

79

52

28

22

7 5

3 43

21

9 $

36

13

9 4

22

37 1

C

r 14

57

26

138

137

84

52

Ce

38.3

6 20

.29

351

135.

9 13

4 12

5.1

97.1

9 52

.49

1 60

77.3

25

6 10

1 .O

Nd

31

.46

16.1

6 13

0.8

67.4

1 59

.31

54.7

4 39

.86

18.5

8 63

.90

23.5

9 14

9.3

50.4

4 S

m

7.49

3.

97

21.0

3 1 1

.SO

9.69

8.

57

5.71

2.

76

10.6

7 3.

33

28.8

7 8.

60

Eu

1.93

1.

27

4.92

2.

99

2.23

1.

79

1.37

0.

680

1.43

0.

428

7.92

2.

25

Gd

5.41

2.

95

11.9

0 6.

87

5.68

5.

01

3.18

1.

61

6.44

1.

98

17.9

5 5.

48

DY

3.

24

1.81

5.

60

3.42

2.

89

3.06

1.

73

1.02

4.

45

1.51

8.

97

3.26

E

r 1.

31

0.72

6 2.

09

1.26

1.

10

1.40

0.

765

0.53

1 2.

06

0.87

1 3.

32

1.42

Y

b 0.

973

0.52

5 1.

52

0.93

4 0.

831

1.23

0.

678

0.56

4 1.

62

0.96

7 2.

22

1.18

Mg#

(al

l F

ez+

) 0.

76

0.38

0.

58

0.59

0.

57

0.49

0.

48

0.43

0.

36

0.27

0.

40

0.66

R

blSr

0.

07

0.02

0.

002

0.02

0.

04

0.13

0.

07

0.16

0.

50

2.2

0.01

0.

05

KlR

b 20

8 22

8 15

77

862

572

342

408

397

307

23 1

36

1 38

7 N

orm

. qu

artz

b 0.

0 4.

80

11.6

4 16

.47

24.2

0 21

.44

28.9

8 N

orm

. ne

vh.

3.87

10

.37

0.0

0.90

8.

46

1.13

NOTES: M

ajor

and

trac

e el

emen

ts (e

xcep

t REE'S)

wen

: an

alyz

ed u

sing

dir

ect-c

urre

nt p

lasm

a em

issi

on s

pect

rom

etry

(DC

P)

at L

amon

t-Doh

erty

Geo

logi

cal O

bser

vato

~y. M

axim

um d

evia

tion

of r

eplic

ate

anal

yses

: Si

O,, f 0.

5%; T

iO,, AI,O,, F

qO,,

MnO

. MgO

. C

aO.

N+O

, Sr

, and

Ba,

+2%

; K,O

and

P,O

,, +

4%

; R

b, N

i, an

d C

r, f

10%

; Zr,

f 12

%. T

he REE w

ere

anal

yzed

at

Ston

y B

rook

by

isot

ope

dilu

tion

usin

g an

NB

S de

sign

6 in

. (1

5.24

cm

) ra

dius

mas

s sp

eclr

ornc

ter.

Sam

ple

repl

icat

ion

is b

ette

r th

an 2

% fo

r th

e R

EE

, an

d an

alyt

ical

pre

cisi

on is

bet

ter

than

1%

. "T

oral

Fe as Fe@,,.

BC

atio

n mol

ecul

ar n

orm

(al

l R a

s FG

'). Can

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( a ) S a n u k i t o i d S u i t e - .-. diorite 84-3 1

A-~mzdi 86-92a *-Oqtz mzdi 85- 15 -

v-vgranodiorite 86-74 0-ogranodiorite 86-94

I l l l l I I f I I ~ ~ J

Ce Nd SmEuGd Dy E r Yb

. o L a m p r o p h y r e Dykes

0-086-62 ~ - - ~ 8 5 - 1 8 a

A-~gabbro 85-01 .-mgabbro 85-09 1 1 1 1 1 1 1 1 1 1 1 1 1

Ce Nd SmEuGd Dy E r Yb

I Biotite Gran i te

0-086-95 A-A86-73 0-086-78

Ce N d SmEuGd Dy E r Yb

FIG. 8. Chondrite-normalized REE patterns of representative samples from the Roaring River Complex. (a) The sanukitoid suite; (b) lamprophyre dykes and gabbro-pyroxenite; and (c) biotite granite. Analyses by isotope dilution at SUNY at Stony Brook. The REE sample replication is better than 2 % , and analytical precision is better than 1 % . Normalizing values reported in Hanson (1980, Table 1, column I). mzdi, monzodiorite; qtz mzdi, quartz monzodiorite.

previous extensive crystallization of olivine from a more primitive komatiitic liquid and that, subsequently, the two liquids were mixed. The major-element composition of the fractionated komatiite before assimilation in Table 4 is the same analysis used in the calculations of Sparks (1986); the trace-element abundances were estimated from the literature. A melt derived from equal parts of komatiite and crust is shown in Table 4. We concur with Sparks (1986) that it is possible to explain some of the ferromagnesian major- and trace-element characteristics of sanukitoids by crustal contamination of komatiitic liquids. However, the abundances of Na20, K20, Sr, and Ba are far too low in the contaminated melt to match their abundances in the sanukitoids (Table 4). For example, Sr is too low by a factor of six. Appealing to a melt of the felsic rocks to increase the LILE'S would lead to an even lower Sr content because feldspar would be in the residue.

The second type of calculations involved incremental assimilation and fractional crystallization (AFC) of a peridotitic komatiite, following the methods of Nielsen (1985, 1988). In the calculations, we varied the ratio of the mass of crust assimilated to the mass of crystals removed during each fraction step between 0 (no assimilation) and 3 (high extents of assimilation). Sparks (1986) calculated that a mass ratio of "assimilant" I "crystals removed" = 3 is probably a maximum for a komatiite that assimilates hot, felsic crust. For the modelling calculations, we selected a peridotitic komatiite (sample 422195) from Munro Township for which high- quality major- and trace-element data are available (Nesbitt and Sun 1976; Sun and Nesbitt 1978). In Fig. 9c are plotted the liquid lines of descent for perfect fractional crystallization ("assimilant" / "crystals removed" = 0), moderate extents of assimilation (0.5 and 1.5), and the maximum extent of assimilation (3). Symbols are placed at intervals of 5% fractional crystallization. The arrows indicate the point at which the liquid has reached 56% Si02, the same silica content as the sanukitoids. Table 4 includes an analysis of a liquid

composition resulting from 30% fractional crystallization using an "assimilant" 1 "crystals removed" ratio of 1.5.

The results of the detailed AFC modelling calculations shown in Fig. 9c and Table 4 confirm that it is possible to produce liquids with MgO, FeO, Ni, and Cr contents similar to those of the sanukitoids. The AFC paths at high extents of assirnila- tion do intersect the sanukitoid field (Fig. 9c), and the liquid composition in Table 4 is a high-Mg andesite composition. A problem with advocating contamination is that it might be expected that the sanukitoids would be distributed vertically on the FeO - MgO diagram (Fig. 9c). The sanukitoids, however, form a subhorizontal trend on the FeO-MgO diagram, which is not consistent with AFC controlling their distribution.

The liquids resulting from the AFC process also have Na20 (2.2 wt.%), K20 (1.45 wt.%), Sr (171 ppm), andBa (289 ppm) abundances that are far lower than those of the sanukitoids. Sr is too low by a factor of seven. Figure 10 shows the results of modelling calculations for the REE'S during the AFC process if a komatiite with a flat, 5 X chondritic REE pattern is assumed. The contaminated melt has a much lower LREE

content and a flatter REE pattern than the sanukitoids. These detailed AFC modelling calculations confirm the

previous bulk mixing calculations, which suggested that although it may be possible to explain some of the major- element and ferromagnesian trace-element characteristics of the sanukitoids by crustal contamination of komatiitic liquids, it is not possible to explain their LILE abundances. The problem of LILE enrichment cannot be solved by appealing to a melt of felsic crustal compositions used in the calculations. Such melts could have REE patterns up to a factor of two higher than the parent, but they would have significant negative Eu anomalies that would be imparted on the contaminated melt. Secondly, feldspar would remain in the residue, and thus the Sr content of the partial melt would be similar to or lower than the felsic rock. It is obvious that the crustal contaminant would have to have higher incompatible-element enrichment and a

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STERN ET AL.

: (a) BASALTS

- ARCHEAN : SANUKlTOlDS

F

' UPPER CRUST

: (b)

- ARCHEAN LAMPROPHYRES

. SANUKlTOlDS

i UPPER CRUST

FeO (cation mol%) FeO (cation mol%) FIG. 9. FeO vs. MgO for the Archean sanukitoids. "Upper crust'' is the present upper-crust composition from Taylor and McLennan (1981).

(a) A field of basalts from the Superior Province. The closed diamond is the average basalt from the Superior Province (Goodwin 1977). Paths of fractional crystallization (FC) and incremental assimilation of upper crust and fractional crystallization (AFC) (Nielsen 1985, 1988) are shown with open diamonds at intervals of 5% fractional crystallization of olivine, pyroxenes, plagioclase, and spinel. The AFC path assumes that for each one part mass of crystals removed from basalt, 0.5 parts of upper crust are assimilated. (b) The field of lamprophyres from the Roaring River Complex and the Lake Vermilion area (Geldon 1972; Arth and Hanson 1975). (c) A field of komatiites with AFC modelling calculations (Nielsen 1985, 1988) upon a peridotitic komatiite parent (closed diamond), sample 422195 from Munro Township (Nesbitt and Sun 1976; Sun and Nesbitt 1978). FC refers to perfect fractional crystallization; 0.5, 1.5, and 3 refer to the mass proportion of upper crust assimilated to crystals fractionated during incremental AFC calculations. Open diamonds are shown at intervals of 5% fractional crystallization. Arrows refer to the point at which the rock has reached a silica content of 56%, equivalent to that of the average Archean sanukitoid. (d) The olivine saturation surface (Roeder and Emslie 1970; Hanson and Langmuir 1978; Langmuir and Hanson 1980) if a pyrolite source is assumed and K, = (Fe01Mg0),,1(Fe0/Mg0),, = 0.30. FC refers to the anhydrous fractionation path of a high-Mg andesite, calculated from Nielsen (1985, 1988). Also shown is the field of Miocene high-Mg andesites of the Setouchi volcanic belt (Tatsumi and Ishizaka 1982a, 1982b). E refers to an experimental melt of pyrolite - 40% olivine at 10 kbar and 1100°C for water-saturated conditions (D. H. Green 1976).

steeper REE pattern than the sanukitoids; such rocks are not common.

Partial melting of mantle peridotite We suggest that the sanukitoids were possibly derived by

shallow melting of LILE-enriched mantle peridotite. Experi- mental and geochemical evidence supporting this hypothesis is summarized below.

Experimental evidence The possibility of generating andesite or basaltic andesite

from peridotite under hydrous conditions was vigorously debated in the early 1970's (e.g., D. H. Green 1973, 1976; Mysen et al. 1974; Nicholls 1974; Mysen and Boettcher 1975). There was general agreement that quartz-normative melts could be produced from peridotite, but the composition of the melts and the maximum pressures of melting were, and still are, areas of disagreement (e.g., Mysen 1982). For water-

saturated conditions, estimates of the maximum pressure for which intermediate silica melts can be formed from peridotite range between 25 kbar (Mysen 1982) and 10 kbar (Nicholls 1974; D. H. Green 1976). For dry conditions, the pressure would be less than 5 kbar (1 kbar = 100 MPa) (Jacques and Green 1980; Takahashi and Kushiro 1983). Melting experiments upon natural high-magnesium andesites of the Setouchi volcanic belt (Tatsumi 1981, 1982) demonstrate their potential to equilibrate with the mantle. These rocks become multiply saturated in olivine and two pyroxenes between 10 and 15 kbar under water-undersaturated to -saturated conditions.

In Table 2 we include one of the most conservative estimates (D. H. Green 1976) of the composition of a melt presumed to be in equilibrium with peridotite at 10 kbar for water-saturated conditions. The melts derived from natural peridotite may be somewhat different in chemistry because natural aqueous fluids would contain significant quantities (tens of weight percents) of silica, alkalis, and other LILE'S at high pressures

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1702 CAN. 1. EARTH SCI. VOL. 26, 1989

TABLE 4. Modelling calculations of crustal contamination of komatiites and basalt

Basalt contamination Komatiite contamination

AFC (Nielsen 1985, 1988) Bulk mixing (Sparks 1986) AFC (Nielsen 1985, 1988) crust Average Archean

contaminanta Basaltb Meltc Komatiited Melte Komatiitef Meltg sanukitoidh

SiO, (wt.%) 66.0 52.22 56.53 48.2 57.1 46.47 55.59 56.35 TiO, 0.6 1.17 1.49 0.5 0.55 0.35 0.53 0.71 A1203 16.0 15.61 12.70 12.3 14.15 8.46 13.32 14.95 FeO, 4.5 11.83 12.94 11.7 8.1 11.10 9.03 7.11 MgO 2.3 6.59 4.51 15.7 9.0 23.77 9.13 6.88 CaO 3.5 9.45 7.78 10.9 7.2 8.86 8.72 7.69 Na20 3.8 2.64 2.77 0.6 2.2 0.84 2.22 4.06 K20 3.3 0.50 1.28 0.2 1.75 0.15 1.45 2.24 Total 100.0 100.01 100.0 100.1 100.05 100.0 99.99 100.00

Mg# 0.48 0.50 0.38 0.71 0.66 0.79 0.64 0.63 Ni @pm) 20 161 2 1 500 260 1049 160 154 Cr 35 249 4 1 1500 768 3250 443 352 Sr 350 177 193 50 200 38 171 1229 Ba 700 143 305 50 375 13 289 1214

"Present upper-crust composition (Taylor and McLennan 1981) used as the crustal contaminant. This is the same composition used by Sparks (1986). bAverage basalt of the Superior Province (Goodwin 1977) recalculated to 100%. 'Melt resulting from incremental assimilation of upper crust and fractional crystallization of olivine, orthopyroxene, clinopyroxene, plagioclase, and spinel

by average Superior Province basalt (Neilsen 1985, 1988). For each mass unit of basalt removed as crystals, 0.5 mass units of crust were assimilated. This composition is the result of 35% fractional crystallization and consists of 79% by mass original basalt and 21% crust.

dKomatiite from Sparks (1986, Table 3). Ni, Cr, Sr, and Ba are estimated average values (Basaltic Volcanism Study Project 1981). 'Result of 50% bulk mixing of komatiite to the left with upper crust. fKomatiite 422195 from Munro Township, Ontario (Nesbitt and Sun 1976; Sun and Nesbitt 1978). gMelt resulting from incremental assimilation of upper crust and fractional crystallization of olivine, orthopyroxene, clinopyroxene, plagioclase, and spinel

by komatiite 422195 (Nielsen 1985, 1988). For each mass unit of komatiite removed as crystals, 1.5 mass units of crust were assimilated. This composition is the result of 30% fractional crystallization and consists of 60% by mass original komatiite and 40% crust.

hAverage of 11 sanukitoids from southwestern Superior Province recalculated to 100% anhydrous. See Table 2 for a more complete analysis.

1 C e Nd S m E u G d Dy E r Yb

FIG. 10. REE modelling calculations (Nielsen 1985, 1988) of incremental assimilation and fractional crystallization of a hypothetical basalt with a flat, 1 0 ~ chondritic REE pattern, and a peridotitic komatiite with a flat, 5 x chondritic REE pattern. The REE composition of the assimilated material ("felsic contaminant") is that of the Archean felsic end member used by Taylor and McLeman (1985, Table 7.8); the major-element composition was taken as the present upper-crust composition of Taylor and McLennan (1981) in order to be consistent with the modelling calculations of Sparks (1986). The field of Archean sanukitoids is also shown. The contaminated melts

(Schneider and Eggler 1986; Eggler 1987), and the fluid would dissolve in the first-formed melts. Natural aqueous fluids with their load of dissolved components would enhance the formation of intermediate-silica melts from peridotite compared with the melting experiments where pure H20 is generally used. The peridotite solidus temperatures would be lowered relative to those obtained in melting experiments using pure H20 because a low-melting fraction is provided to the source. This would extend the temperature range over which intermediate liquids could form (Wyllie 1982) and result in greater diversity of magmas than the experimental data show.

Melting of a peridotite to form a sanukitoid could have pro- ceeded from a liquid in equilibrium with olivine, two pyroxenes, spinel, and amphibole at low percentages of melting, to olivine and one or two pyroxenes at higher temperatures (D. H. Green 1976). An important observation from the experimental data is that garnet is not stable in peridotite residues under the low-pressure conditions necessary to form the sanukitoids. Garnet would be stable only at pressures greater than 15 -20 kbar (D. H. Green and Ringwood 1970; D. H. Green 1973; Olafsson and Eggler 1983). Therefore, garnet would not be expected in the residue for the sanukitoids melting between 10 and 15 kbar.

From the experimental data it appears that moderate degrees of melting (about 25%) are required to generate high-Mg, quartz-normative basaltic andesites from peridotite under water-saturated conditions (D. H. Green 1976). There are no data on the composition of melts closer to the solidus of peridotite under hydrous conditions. Green suggested that

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finally nepheline-normative, intermediate-silica melts near the solidus. He reasoned that the alkalis would probably behave as incompatible elements and that their increasing concentration in low-degree melts would eventually yield nepheline in the norm. Olafsson and Eggler (1983) commented that melts were "nepheline normative" in fluid-absent melting of amphibole peridotite, although no compositions were reported. As evi- dent from Table l , the sanukitoids vary from silica undersaturated to oversaturated on a normative basis, and the sanukitoids that are nepheline normative are also most enriched in the alkalis.

This discussion simply emphasizes that we do not know the full range of magmas that could be derived from hydrous melting of the mantle and that we need not expect only one magma type. Apparently, a range of melts of intermediate silica may be possible, including high-Mg andesites.

MgO -FeO systematics The mantle origin of the Archean sanukitoids is supported

by the similarity of their FeO and MgO contents to those of the Japanese high-Mg andesites and to those of experimentally determined melts of peridotite under hydrous conditions (Fig. 9d). For reference, Fig. 9d also includes a field of possible melts of pyrolite leaving olivine of composition FoS9 in the residue. The melt field was calculated from mass- balance considerations and use of the relation Kd = (FeO/ MgO)o,/(FeO/MgO),l, = 0.30, suitable for 1 atm (1 atm = 101.325 kPa) conditions (Roeder and Emslie 1970; Hanson and Langmuir 1978; Langmuir and Hanson 1980).

The Archean sanukitoids lie below and to the right of the pyrolite melt field, as do some of the Japanese high-Mg andesites and the experimentally determined liquid composition. The presence of Fe3+ in the sanukitoid melts would increase their Mg#'s and move them closer to the pyrolite melt field. For example, an average sanukitoid with Mg/(Mg + FeT) = 0.63 would have Mg/(Mg + Fe2+) = 0.66 at an oxygen fugacity buffered by wustite-magnetite at 1200°C (Sack et al. 1980; Kilinc et al. 1983). This is equivalent to an Fe3+/Fe, ratio of 0.10, which would move the sanukitoids about 0.5 cation mol% FeO to the left on the FeO -MgO diagram. This correction would be cancelled, howevear, by the effect of higher pressures on the position of the melt field, which would move away from the sanukitoids (see Rajamani et al. 1985). One possible explanation for the position of the sanukitoids relative to the pyrolite melt field is that their parent melts originated in the melt field and had higher MgO contents but subsequently fractionated olivine and pyroxene (Fig. 9d). Another possibility is that the source region had a lower Mg# than pyrolite, resulting in melts with lower Mg#'s. For example, if the solidus olivine is Fogs, the solidus melts are more FeO rich at a given MgO content than melts in equilibrium with FoS9 (Fig. 9d). The experimental determination of the melt composition by Green (1976) shown in Fig. 9d lies slightly below the pyrolite solidus because the liquidus olivine composition is Fogs.

The Archean sanukitoids have FeO contents that vary by about 2 mol%. This variation cannot be a result of fractionation from a single parental magma. Included in Fig. 9d is a line showing the fractionation path of a sanukitoid crystallizing olivine and pyroxene. Fractionation of olivine and pyroxene results in a large decrease in MgO (and magma liquidus temperature) but almost no change in total FeO content. The variation in FeO cannot be explained by plagioclase fractionation

because the sanukitoids have high Sr contents and no Eu anomalies, indicating that plagioclase was not fractionating. Thus, even if the sanukitoid samples were derivatives of more- magnesian parental magmas, the variation in FeO should reflect differences in the FeO content of the primary melts (Langmuir and Hanson 1980).

Processes that could produce differences in the FeO content of mantle melts were considered by Langmuir and Hanson (1980) and included (i) differences in the pressures of melting, (ii) differences in the temperatures and extents of melting, and (iii) differences in the Mg# of the source.

It is very difficult to explain the variations in FeO content of the sanukitoids by similar extents of melting at different pressures. Although the effect of increasing pressure is to increase the FeO content of melts at the solidus of peridotite (e.g., Langmuir and Hanson 1980), this effect is offset by increasing Kd's at higher pressure (e.g., Ford et al. 1983; Takahashi and Kushiro 1983). A difference of about 2 mol% FeO in melts at the solidus of pyrolite under dry conditions requires a difference in pressure of melting of about 35- 40 kbar (Fig. 7 in Rajamani et al. 1985). It is very unlikely that the differences in FeO content of the sanukitoids could be explained by such large differences in pressure because, based on experimental data, sanukitoids may be produced from peridotite only within a narrow pressure range, 10- 15 kbar.

The variations in FeO content of the sanukitoids are also difficult to explain by different extents of melting of a similar source. The variation in FeO content produced by different extents of melting would depend on how melting occurred. Melts produced during adiabatic rise of a mantle diapir could have significant differences in their FeO content, with melts at higher pressures and lower extents of melting having the highest FeO contents (Langmuir and Hanson 1980; Rajamani et al. 1985). Liquids formed by adiabatic melting over a pressure interval of 10- 15 kbar would have about 2 mol% variation in their FeO content (Rajamani et al. 1985). For adiabatic melting, the melts with highest FeO content should have the most-fractionated REE patterns and greatest LILE enrichment, since they would be produced at the lowest extents of melting. The sanukitoids with the higher FeO content, however, have the least-fractionated REE patterns and show the least enrichment in LILE'S (Table 1; Fig. 6), characteristics that are not consistent with adiabatic melting. Liquids resulting from different extents of melting under isobaric conditions would probably show little variation in FeO content. For example, the experiments of Jaques and Green (1980), at 10 kbar under dry conditions, show variations of only about 0.5% FeO (cation mole percent) for melts produced by 16-35 % melting of a source with pyrolite minus 40% olivine composition.

A possible explanation for the differences in FeO content for the sanukitoids is that the source was heterogeneous in its Mg#. In Fig. 9d it can be seen that a difference of about 4 mol% in the forsterite composition of the olivine (FoS9 to

in the source could explain the differences in the FeO content of the sanukitoids. The sanukitoids with higher FeO could be derived from that part of the source with a lower Mg#. The differences in Mg# of the peridotite source could be a result of heterogeneous distribution of basic components (e.g., Horan et al. 1987). For example, a source consisting of interlayered peridotitic mantle and komatiite would have a lower Mg# than a peridotitic mantle alone.

To summarize, the MgO-FeO relations of the sanukitoids are compatible with a mantle origin. The sanukitoids could

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have been derived from a source with an Mg# similar to pyro- pareit results in abundances of Nd and Sm that are similar to lite and subsequently fractionated olivine, or they could have those of sanukitoids, but the path of melting is curved, reflect- been derived from a source that had a lower Mg# than pyro- ing the slightly higher bulk-distribution coefficient for Sm. We lite. Furthermore, there is enough variability in the FeO conclude that the source rocks to the sanukitoids had signifi- content among the sanukitoids to suggest that there was hetero- cantly higher Nd and Sm abundances and higher NdISm ratios geneity in the Mg# of the source region. than chondrites. Furthermore. the linearitv of the data suggest -

Cr and Ni contents The Cr and Ni contents of the sanukitoids (averaging 350

and 150 ppm, respectively) are similar to those of young volcanic rocks suspected of having a mantle origin. For example, high-Mg andesites from the Setouchi volcanic belt average 450 ppm Cr and 200 ppm Ni (Table 2), and a high-Mg basaltic andesite from Peru has 395 ppm Cr and 139 ppm Ni (Noble et al. 1975). If the Archean sanukitoids fractionated even a small amount of olivine, then Ni contents would have been higher in the primary melts.

Isotope data Initial 87Sr/86Sr ratios of 0.701, EN^ (2700 Ma) values of

+ 1 to +2.5, and single-stage 238U/204Pb values of 7.4 - 8.0 are all consistent with derivation of the sanukitoids from a mantle source with little or no contribution from significantly older continental crust (see references above). The 6180 values of +6.9 to + 8.1%, for the sanukitoids are, however, higher than values for modern mantle-derived melts.

LILE enrichment in the source of the sanukitoids The high contents of alkalis, Sr, Ba, and LREE'S relative to

primitive mantle compositions (Table 2) are the most notable characteristics of the Archean sanukitoids. Was LILE enrichment a result of the extent of melting and the residual mineralogy of the mantle, or was it a characteristic of the source at the time of melting? For example, perhaps the strong LREE

enrichment and variable heavy-rare-earth-element (HREE) depletion of the sanukitoids (Fig. 6) could be explained by melting of a source leaving garnet in the residue (e.g., Arth and Hanson 1972).

Figure 11 shows a plot of Sm, versus Nd, for the sanukitoids and for the least-evolved members of the sanukitoid suite from the Roaring River Complex. The data are collinear about a regression line that intersects the Sm axis close to the origin. All the sanukitoids have Ndn/Smn ratios and Nd and Sm abundances that are significantly greater than those of chondrites. The distribution of data is difficult to explain by low extents of melting of a source with chondritic NdISm ratios. Figure 11 shows a path for 0% to 5 % batch melting of peridotite with 10% garnet and with 2X chondritic abundances of Nd and Sm (GP1). Garnet peridotite is used as an example of a residue mineralogy that might allow the greatest enrichment in LREE'S. Sources that do not leave garnet in the residue will show lower relative enrichments. The KA's "

(mineral-melt distribution coefficients) used in the calculations were those for mantle melting systems compiled by Han- son (1980), which are probably lower estimates of the Kd's in a sanukitoid (andesite) system. Larger Kd9s would result in less enrichment. These calculations show that it would be very difficult to generate magmas with the abundances of Nd and Sm and the high NdISm ratios of the sanukitoids from a parent with 2 x chondritic REE abundances even with garnet in the residue. Also shown in Fig. 11 is the path for 1 % to 15 % batch melting of a garnet peridotite with Nd = 10 x chondrites and a Nd,,/Smn ratio of 1.7 (GP2). Melting of this LREE-enriched

that the sanukitoids are not related by different extents of melting of a similar source.

Figure 12a is a plot of Ndn/Sm, versus Smn/Yb, for the sanukitoids, illustrating small variations in NdISm ratios but wide variation in SmIYb ratios. The variation in SmIYb describes the crossing HREE patterns seen in Fig. 6. Curves for low extents of melting of peridotite (P) and amphibole peridotite (AP) and three melting curves for garnet peridotite (GP) are also shown in Fig. 12a. The curves marked P, AP, and GP1 are melting curves for source 1 with chondritic NdISm and SmIYb ratios. Regardless of the residue mineralogy or extent of melting, it isimpossible to explain the high ~ d / ~ m and SmIYb ratios of the sanukitoids from a source with chondritic ratios. The curve marked GP2 refers to melting of garnet peridotite source 2, which has a chondritic SmIYb ratio but a Ndn/Sm, ratio of 1.5. It is still very difficult to explain the steep SmIYb ratios of some of the sanukitoids using a source with chondritic SmIYb without appealing to very high abundances of garnet in the residue (10% used in calculations). The sanukitoids with highest SrnIYb ratios are best explained by melting sources with fractionated HREE patterns, such as source 3, which produces the melting curve GP3. Thus, although the HREE depletion may have been accentuated by the presence of garnet in the residue, at least some of the sanukitoids were derived from sources with fractionated HREE

abundances. Since none of the model peridotitic sources alone can

explain the large variation in SmIYb ratios for the sanukitoids, their source region must have had different SmIYb ratios. The heterogeneity in SmIYb of the source must be significant when one considers that the experimental data prohibit garnet in the residue. At 10 - 15 kbar under hydrous conditions, olivine, clinopyroxene, orthopyroxene, and amphibole would be present at low extents of melting, with olivine and pyroxene remaining to higher extents (D . H . Green 1976). Garnet would only be stable at pressures greater than 15 -20 kbar (D. H. Green and Ringwood 1970; D. H. Green 1973; Olafsson and Eggler 1983), which is beyond the pressure stability of andesite in peridotite. From Fig. 12a it can be seen that if the residue is garnet free, the variation in the SmIYb ratio of the source must have been similar to that of the sanukitoids. Figure 12b summarizes the likely range of SmIYb and NdISm ratios that may characterize the source region of the sanukitoids. The source is inferred to be LREE enriched and variably HREE depleted compared with chondrites.

Discussion on the origin of the sanukitoids The strong enrichment in the sanukitoids of the LREE'S,

alkalis, Sr, Ba, and P2O5 is consistent with melting of an enriched mantle-source region. Enrichment of the mantle source could have resulted from metasomatism of a depleted source by fluids, melts, or both (Lloyd and Bailey 1975; Bailey 1982, 1987; Eggler 1987; Erlank et al. 1987). Meta- somatized mantle nodules with high LILE abundances are a well-known feature of alkalic lavas (Frey and Green 1974; Lloyd and Bailey 1975; Wass and Rogers 1980; Bailey 1982).

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STERN ET AL. 1705

FIG. 11. Chondrite-normalized Nd vs. Sm for Archean sanukitoids from various parts of southwestern Superior Province and the least- evolved rocks of the Roaring River Complex. A linear least-squares regression line is drawn through the sanukitoid data set. "Upper crust" is present upper-crust composition of Taylor and McLennan (1981). GP1 is a path of 0% to 5% nonmodal melting of a garnet peridotite source with Sm and Nd = 2 x chondrites. The starting composition contained 60% olivine, 20% orthopyroxene, 10% clinopyroxene, and 10% garnet. GP2 is a path of 0% to 15 % n o d a melting of garnet peridotite with Nd = 10X chondrites and Nd,/ Sm,, = 1.7. Melting proportions were calculated using eq. [9] of Mysen (1982). Kd7s used were those compiled by Hanson (1980) for mantle melting.

The Nd-isotope data for the sanukitoids limit the interval between metasomatism and melting to less than about 100 Ma (Shirey and Carlson 1985a, 1985b, 1986, 1988; Shirey and Hanson 1986).

As discussed previously, the crossing REE patterns of the sanukitoids (i.e., variable SmIYb ratios) suggested that the source region was variably HREE depleted. Figure 13 shows calculated REE patterns of two source regions for the sanukitoids. The inferred REE patterns are similar to those of metasomatized mantle nodules (e.g . , Frey and Green 1974; Erlank et al. 1987). Ten percent batch melting of the proposed sources, leaving olivine, orthopyroxene, and clinopyroxene in the residue, could explain the sanukitoids. The actual extent of melting cannot be determined without prior knowledge of the source composition. The percentage of melting could be higher or lower than lo%, probably up to 30% (e.g., D. H. Green 1976); however, the shape of the REE patterns of the melts would not be greatly affected.

There is an interesting correlation between the slope of the REE patterns and FeO content of the sanukitoids. Figure 14 shows a negative correlation between FeO and SmIYb, and FeO is also negatively correlated with Sr and K20 (Table 1). This distribution is opposite to what would be expected if LREE enrichment was a result of metasomatism by a basic, LREE-

enriched melt. In such a case, the melt-enriched portions of the source would develop lower Mg#'s, higher Sm/Yb ratios, and higher Sr and K20 abundances, and the sanukitoids derived from such a source would have higher FeO, Sm/Yb, Sr, and K20.

One possible explanation is that the variation in FeO content could be due to variability in the Mg# of the source, such that the part of the source with low Mg# generated the melts with highest FeO content (see MgO - FeO section). In this case, the negative correlation between FeO and Sm/Yb suggests that the

FIG. 12. Chondrite-normalized Nd/Sm vs. SdYb for Archean sanukitoids from various parts of southwestern Superior Province and the least-evolved rocks of the Roaring River Complex. (a) Melting curves for various peridotitic assemblages. P, AP, and GP1 are curves for 0% to 5% batch melting of source 1 with chondritic Nd/Sm and Sm/Yb leaving peridotite, amphibole peridotite, and garnet peridotite residues, respectively. GP2 and GP3 refer to 0% to 15% batch melting of garnet peridotite starting compositions 2 and 3. Peridotite starting material consisted of 55 % olivine, 25 % clinopyroxene, and 20 % orthopyroxene, melting according to eq. [lo] in Mysen (1982). Gar- net peridotite mineralogy and melting proportions are described in the caption to Fig. 11. Amphibole peridotite consisted of 55 % olivine, 20% clinopyroxene, 15% orthopyroxene, and 10% amphibole, melting according to eq. 191 of Mysen (1982) for garnet peridotite but with amphibole substituted for garnet. K,'s used were those compiled by Hanson (1980) for mantle melting. (b) Approximate range of source compositions that could explain the sanukitoid data.

part of the mantle source with a lower Mg# had a flatter REE

pattern than the part of the source with a higher Mg#. This is precisely what has been observed in mantle nodule suites (e.g., Frey and Green 1974). The nodules that show the greatest LREE enrichment and HREE depletion are also those with the highest Mg#. The negative correlation between FeO and Sm/Yb, Sr, Ba, and K20 can then be explained by the action of metasomatizing fluids upon a mantle that was already heterogeneous in its Mg#. In this case, the data suggest that the part of the mantle source with the higher Mg# was most affected by metasomatism prior to melting.

The ultimate source of the LILE'S is unclear. The high abundances of K20, Na20, Sr, Ba, LREE'S, and P205 and the absence of Eu anomalies suggest that the enriching component also had these characteristics. The primitive isotopic character of the sanukitoids does not permit the LILE'S to be derived from much older silicic crust. If fluids were the source of LILE'S, they could have been derived from dehydration of

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1706 CAN. 3. EARTH SCI. VOL. 26, 1989

o . l l l ~ I r l l l l l l l l l I Ce N d S m E u G d Dy E r Y b

FIG. 13. Explanation for the crossing REE patterns and correlation of SmIYb with FeO for the sanukitoids (see also Fig. 14). The slopes of the REE patterns for the sanukitoids are controlled by the extent of e~ichInent in the mantle source, which is related to Mg# of the source. Patterned field shows a possible range of mantle composition prior to enrichment, such that the part of the source with the lower Mg# has a flatter REE pattern with higher overall REE'S than the part with LREE depletion and high Mg#. These source rocks undergo different extents of metasomatism, such that the high-Mg# source achieves the greatest LREE enrichment. Melts derived by 10% batch melting of two possible source regions are shown. Starting compositions consisted of 55% olivine, 25% clinopyroxene, and 20% orthopyroxene, melting according to eq. [lo] of Mysen (1982). Kd's used were those compiled by Hanson (1980) for mantle melting.

altered mafic crust during subduction (e.g., Tatsumi et al. 1986), from rapidly recycled and subducted sediments, from hybridization of siliceous melts (derived from melting of subducted material) with overlying mantle peridotite (e. g . , Wyllie and Sekine 1982), or from juvenile mantle sources, such as crystallizing mantle melts at greater depths. The lack of significant negative Eu anomalies in the sanukitoids cannot be used to eliminate a sedimentary component (e.g., McLennan and Taylor 1981) because Archean sediments generally do not have significant negative Eu anomalies (Taylor and McLennan 1985). The high Ba/Ce ratios (average = 12.5) are consistent with subducted sediments (e.g., Kay 1980).

Fractionation of sanukitoids and generation of the sanukitoid suite

It is very difficult to determine which members of the sanukitoid suite are unfractionated, primary melts of the mantle and which are chemically fractionated. The experimental melting data on hydrous peridotite are few, and the major-element compositions of the experimental liquids are controversial (see above). There are also uncertainties about the trace-element composition of primary andesite. For example, estimated Ni contents of such melts based on mineral-melt Kd7s range between 100 ppm (Gill 1981, Fig. 5.17d) and 700 ppm (Mysen 1982). Until better experimental data are obtained for andesite in equilibrium with peridotite, we suggest that the natural andesites are our best guide to the composition of such

h : A . A

.m 0 m

A ROARING RIVER COMPLD(

0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

FeO (cation mol%)

FIG. 14. Chondrite-normalized SmIYb vs. FeO for Archean sanukitoids from various parts of southwestern Superior Province and the least-evolved rocks of the Roaring River Complex. Olivine fractionation will produce very little movement on this diagram.

melts. The high-Mg andesites of Japan have Ni contents between 100 and 300 ppm and Mg#'s from 0.65 to 0.77 (Tatsumi and Ishizaka 1982a, 1982b). Thus, sanukitoids with less than about 100 ppm Ni and Mg#'s less than 0.65 should be suspected of having undergone some degree of fractionation, if they were derived from sources similar to pyrolite. The primary Ni contents and Mg#'s could be lower if the source rocks had lower Ni contents and Mg#'s.

The lithological and chemical continuity between primitive sanukitoids and evolved granodiorites (Fig. 7 ) suggests a possible origin for the granodiorites by fractionation of sanukitoid parental magmas. The characteristics of the granodiorites include very high Sr and Ba abundances, high Mg#'s, and relatively high Ni and Cr abundances for their silica content (Table 1). Examples of intrusions where sanukitoids are demonstrably gradational with evolved quartz monzodiorites and granodiorites are the Jackfish-Weller lakes pluton (Sutcliffe 1980), the Icarus pluton (Arth and Hanson 1975), and the Ryckrnan Lake stock (Birk et al. 1979).

Experimental phase equilibria for high-Mg andesites suggest that olivine and one or two pyroxenes would be the liquidus phases at high temperature (Tatsumi 1981, 1982; Tatsumi and Ishizaka 1982a, 1982b), with amphibole and spinel at lower temperatures (e.g., D. H. Green 1976). Separation of olivine and pyroxene would lower the MgO, Ni, and Cr contents of the derivative magmas (e.g., see Fig. 9d). The appar- ent absence of olivine in the Archean sanukitoids may be explained if olivine is in reaction relation with the sanukitoid liquid, i.e., olivine + liquid = amphibole + pyroxene (Mysen 1982). Since the amphibole + pyroxene assemblage is stable at lower temperatures, a drop in temperature would cause olivine to be resorbed by reaction with the liquid. Enrichment in silica could occur when amphibole or biotite became fractionating phases. The absence of plagioclase on the liquidus is consistent with the high Sr contents and lack of negative Eu anomalies for the most primitive sanukitoids.

Ongoing studies of the Roaring River Complex show that some of the chemical variation within the sanukitoid suite was probably a result of fractionation of clinopyroxene, feldspar, amphibole, biotite, and trace minerals. Previous studies of fractionation within monzodioritic complexes from other areas confirm the importance of crystal -liquid fractionation in con-

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ET AL. 1707

trolling chemical variation. For example, Longstaffe et al. (1980, 1982) suggested that fractional crystallization of hornblende and plagioclase were important in the evolution of the Jackfish - Weller lakes pluton. Sutcliffe (1980), studying the same intrusion, suggested that fractionation occurred by restite unmixing of dioritic residuum from a granodioritic matrix. In some cases, fractionation of sanukitoids appears to have involved separation of immiscible silicate liquids (Evans 1987).

Summary and implications We have shown that the Archean monzodiorites and

trachyandesites are consistent with an origin by direct partial melting of LIL~-emiched mantle peridotite at shallow depths under hydrous conditions. We infer that the source became enriched as a result of interaction with a metasomatizing agent, such as a fluid. Furthermore, the peridotite source appears to have been heterogeneous in its Mg#, LREE enrichment, and HREE depletion. The inferred heterogeneities in the REE'S of the source can be attributed to variable extents of metasomatism prior to melting, but the heterogeneities in Mg# appear to have existed prior to metasomatism. Perhaps the inferred heterogeneities in Mg# of the source were a result of previous episodes of addition or removal of basic melts. The chemical heterogeneity of the sanukitoids suggests that they sampled only small portions of their source region. This would be consistent with melting occurring on a very local scale in the mantle, probably depending on the availability and local concentration of fluids.

Water-rich fluids derived at depth would contain large quantities (tens of weight percents) of silica, alkalis, and other LILE'S at mantle pressures (Schneider and Eggler 1986; Eggler 1987). If such fluids migrate through mantle peridotite below its vapour-saturated solidus, they may cause subsolidus (metasomatic) LILE enrichment. Since mantle -H20-fluid bulk-distribution coefficients for the REE'S increase with decreasing pressure (Mysen 1979), mantle at shallower depths may show the greatest relative enrichments. Spera (1981) suggested that metasomatizing fluids may rise isothermally and could intro- duce sufficient heat into the upper mantle to eventually cause melting. Melting would occur when the temperature of the mantle invaded by the fluid was at the solidus for the invading vapour. Any additional fluids would dissolve in the first- formed melts. The intermediate silica and high LILE contents of the sanukitoids are therefore not fortuitous; they are a logical consequence of introducing fluids into the upper mantle. For the sanukitoids, the processes of source enrichment and melting should be highly correlated, and this is consistent with the Nd-isotope data, which suggest that the sources of the LREE-enriched sanukitoids had a long-term history of LREE

depletion (Shirey 1984; Shirey and Carlson 1985a, 1985b, 1986, 1988).

If fluids were the source of LILE enrichment, an obvious question is, what was the source of such fluids? The fluids could be derived from the mantle, or they could have resulted from crustal recycling. In the Archean Wyoming craton, Pb-isotope data suggest that LILE enrichment of andesites with many of the characteristics of sanukitoids was related to recycling of older crust (Mueller and Wooden 1988). In modern tectonic settings, high-Mg andesites are associated with subduction of oceanic lithosphere in both continental (e.g., Peru) and island-arc settings (e.g., Aleutians; see Table 2). A geological setting involving subduction of oceanic lithosphere

could provide the necessary framework to return H20 and LILE'S to the mantle, ingredients that seem essential for the generation of the Archean sanukitoids. The presence of sanukitoids may indicate that subduction and recycling of juvenile crustal material were important processes during the Archean.

An intriguing aspect of the sanukitoids is their similarity in major-element composition to estimates of the total Archean crust (Table 2). Addition of sanukitoids to the Archean crust would therefore not change current estimates of its major- element composition (Taylor and McLennan 1985) but would have increased its LILE content. It thus seems possible that portions of the Archean crust were created by direct extraction of silica-oversaturated, LILE-enriched melts from the mantle.

Although members of the sanukitoid suite appear to constitute only a small portion of the Archean crust (about 5%), it should be emphasized that we selected for study only the most easily recognized and least-deformed intrusions. It will be important to look carefully at other rock types and other ter- ranes to determine the full extent of the sanukitoid suite. The sanukitoid suite may occur within monzodioritic complexes in other areas of the Superior Province, such as the Abitibi and Pontiac (Racicot et al. 1984; Rive et al. 1988; E. H. Chown, personal communication, 1988) and Sachigo subprovinces (Smith and Longstaffe 1974). The sanukitoid suite also appears to be present within other Archean cratons (see Shirey and Hanson 1984), such as the Beartooth Mountains of Montana and Wyoming (Mueller et al. 1983), the North Atlantic Craton (e.g., Table 7 of Collerson and Bridgewater 1979), and the Dharwar Craton, India (Balakrishnan and Rajamani 1987).

In the southwestern Superior Province, the syn- to post- tectonic sanukitoid suite seems to be related, temporally and chemically, to lamprophyres and syenites (e.g., Schulz et al. 1979; Schulz 1982). These rocks have in common high LILE

abundances and similar isotopic characteristics (e.g., McCall et al. 1987; Bell and Blenkinsop 1987). It is possible that these widely variable rock types were derived from source rocks in the mantle and lower crust that were similarly enriched in incompatible elements. The high Mg#'s of the sanukitoids relative to those of the coeval lamprophyres (Fig. 9b) suggest that the latter were derived from a source that had a lower Mg# than the source for the sanukitoids. For example, the lamprophyres could be derived from portions of the mantle that were enriched in basic components (e.g., Horan et al. 1987). The syenite bodies could have diverse origins, including derivation from mantle sources at greater depths than the sanukitoids, melting of peridotite in the presence of C02 instead of H20 (Wyllie 1979), or perhaps by melting of granitoids in the lower crust. Perhaps all these melting processes were a result of introduction of fluids into heterogeneous sources in the mantle and lower crust.

The data for the sanukitoids and perhaps these other LILE- enriched rocks strongly suggest that LILE enrichment in their source regions preceded melting and crust formation (e.g., O'Nions and McKenzie 1988). We need to examine other rock types in the Archean crust, such as tonalites and granodiorites, to test whether their source rocks or parental melts were also enriched. If so, it could indicate that source enrichment was an important process associated with formation of the Archean continental crust.

Acknowledgments R. Stern was supported by a 1967 science and engineering

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scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC) and by a SUNY at Stony Brook dissertation fellowship. We thank the Geological Sur- vey of Canada for field support (1984, 1985) and funds for some of the REE analyses. Field and laboratory support came from the National Science Foundation (EAR8607973). We are grateful to John Percival (Geological Survey of Canada) for fruitful discussions and thorough reviews of earlier manu- scripts. Comments by I. Annesley, B. McNutt, and an anony- mous Journal reviewer greatly improved the manuscript. S. McLennan, W. Sharp, J . Whalen, and members of the Stony Brook isotope lab also provided constructive criticism of the manuscript. S. Van Sickle, P. Bartholomew, and V. Haniford Stern provided field assistance.

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Documents

Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province