EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE - PEGMATITE SUITE AT RED CROSS LAKE, MANITOBA, CANADA. I. GEOLOGICAL SETTING

1793

§ E-mail address: [email protected] Present address: Manitoba Geological Survey, 360–1395 Ellice Avenue, Winnipeg, Manitoba R3G 3P2, Canada.2 Present address: Geochemical Laboratory, Geological Survey of Alabama, Tuscaloosa, Alabama 35486-9780, U.S.A.3 Present address: 5360 Bunting Avenue, Richmond, British Columbia V7E 5W1, Canada.

The Canadian Mineralogist Vol. 50, pp. 1793-1806 (2012) DOI : 10.3749/canmin.50.6.1793

EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE – PEGMATITE SUITE AT RED CROSS LAKE,

MANITOBA, CANADA. I. GEOLOGICAL SETTING

Petr ČERNÝ§

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

M. tiMothy CorKery

Manitoba Geological Survey, 360–1395 Ellice Avenue, Winnipeg, Manitoba R3G 3P2, Canada

NorMaN M. haLDeN, KareN Ferreira, WiLLiaM C. BriSBiN, LeoNarD e. ChaCKoWSKy1 aND roBert e. MeiNtZer2

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

FreD J. LoNGStaFFe

Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

DaviD L. trUeMaN3

Tantalum Mining Corporation of Canada Ltd., Lac du Bonnet, Manitoba ROE OGO, Canada

aBStraCt

Three types of subvertical leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in the Red Cross Lake greenstone belt of the Sachigo Subprovince, in the northwestern part of the Archean Superior Province of the Canadian Shield. The greenstone belt is situated in the northern margin of the Oxford–Stull domain of the North Caribou terrane, in tectonic contact with the Hudson Bay terrane. The tholeiitic greenstone belt, flanked by plutonic masses of tonalitic gneiss to the north and tonalite–granodiorite gneiss to the south, consists of a bimodal assemblage of metabasalt + subvolcanic metagabbro and felsic lapilli tuff of andesitic to rhyolitic compositions, mafic mylonite with a mylonitized granitoid, and volcaniclastic metagreywacke. Regional context and petrochemical features of the south-flanking tonalitic-granodioritic masses and of the greenstone-belt lithologies indicate a largely juvenile, deep-oceanic origin. To the north, the isotopic signatures of tonalitic-granodioritic masses indicate an older continental association. The leucogranites and pegmatites and related intrusions were subvertically emplaced into, or alongside, the predominantly mylonitized metavolcanic rocks of the WNW–ESE-trending North Kenyon Shear Zone. Some of the leucogranites are geochemically rather primitive, but they grade into more evolved pegmatite bodies and ultimately into extremely fractionated lepidolite-subtype pegmatites. Investigation of the origin of the rare-element enrichment in these pegmatites, within the limits of our reconnaissance sampling, did not show above-average enrichment of rare lithophile elements in any of the main regional map units.

Keywords: leucogranite, granitic pegmatite, greenstone belt, granodiorite, metatonalite, Red Cross Lake, Manitoba.

1794 the CaNaDiaN MiNeraLoGiSt

iNtroDUCtioN

Granitic pegmatites of the rare-element class represent the ultimate product of igneous fractionation of diverse felsic magmas. The geochemically most evolved pegmatites of the lithium – cesium – tantalum (LCT) family (see Černý & Ercit 2005 for pegmatite classification) are derived from peraluminous S-type or mixed S- and I-type plutons (less commonly I-types alone). The most fractionated granitic pegmatites of this category consolidate from residual magmas, generated by differentiation, that are strongly enriched in volatile components (H2O, boron, fluorine), phosphorus, rare alkalis, and commonly also in several incompatible rare elements such as Be, Sn, Nb and Ta. Extreme cases of fractionation are found among complex pegmatites with spodumene, petalite and lepidolite, such as the Tanco pegmatite in southeastern Manitoba (e.g., Černý 2005, Stilling et al. 2006), the Volta Grande pegmatites in Brazil (Lagache & Quéméneur 1997), the Koktokay pegmatite #3 in Altai, northwest China (e.g., Wang et al. 2006), and the Wodgina main-lode pegmatite in Australia (Sweetapple & Collins 2002).

In the 1960s, lepidolite-rich pegmatites were discovered at Red Cross Lake in northeastern Mani-toba (Potter 1962, Jambor & Potter 1967). We started examining these pegmatites and associated intrusive rocks in 1981, and confirmed quite early the exceptional caliber of their geochemical signature, and recognized their potential to fundamentally contribute to the under-standing of mineral behavior in a pegmatite subjected to shearing stress.

This paper is the first in a four-part series that presents the results of our multifaceted study of these dikes, which are, to the best of our knowledge, the most fractionated pegmatites so far encountered on a global scale. We present here the regional and detailed geological setting of the lepidolite-bearing dikes and related pegmatites and leucogranites. Part II (Černý et al. 2012a) is devoted to the petrochemistry of the leucogranite – pegmatite suite, its petrological links and the extreme geochemical evolution of the lepido-lite pegmatites. Part III (Brisbin et al. 2012) presents the response of the lepidolite pegmatites to intensive shearing, which has not been analyzed elsewhere. Part IV (Černý et al. 2012b) deals with the mineralogy of the suite and elucidates the mode of incorporation and the abundance of the rare elements in the lepidolite-bearing granitic pegmatites.

PrevioUS aND CUrreNt WorK

Potter (1962) published the first account of the discovery of sheared lepidolite-bearing pegmatites at Red Cross Lake in northeastern Manitoba, within the framework of regional large-scale geological mapping. Jambor & Potter (1967) reported unusually high Rb and

Cs contents in micas of the lepidolite pegmatites and in bulk samples of the dikes. These authors suspected the presence of pollucite, confirmed later by Bannatyne (1985). Sopuck (1971) remapped the immediate area around Red Cross Lake, and Elbers (1976) commented on the pegmatite occurrence. Trueman (1982) summa-rized the results of exploration mapping and drilling of the lepidolite-bearing dikes. Our present paper is based on 31 years of intermittent study of the leucogranites, pegmatites and their host rocks in the Red Cross Lake area at the University of Manitoba (Chackowsky et al. 1985, Černý et al. 1985, 1994, Eby 1986, Wang et al. 1986, 1988, Teertstra & Černý 1994, Selway et al. 1998, 1999), and on the first results of a reconnais-sance examination of the greenstone belt and flanking batholithic bodies by the Manitoba Geological Survey.

The work in the field was hindered by limited funding, remote location of the area, transportation difficulties, and extensive cover of glacial till, which leaves good outcrops restricted to narrow shorelines. Despite our efforts to sample all available outcrops, our observations could cover only segments of irregular linear traverses as dictated by the limited exposure.

exPeriMeNtaL

Whole-rock chemical analyses were performed at the XRF, ICP–MS and AA laboratories of the Department of Earth Sciences, Memorial University of Newfound-land, at Activation Laboratories Ltd. in Ancaster, Ontario, and in part at the Geochemistry Facility of the Department of Geological Sciences, University of Manitoba. Standard analytical techniques were followed for sample preparation and analysis, following Jenner et al. (1990), Longerich (1995), and Hoffman (1992). Whole-rock compositional data were processed with the IGPET program; chondrite-normalization values are those of Taylor & McLennan (1985).

The oxygen-isotope measurements were performed at the Department of Earth Sciences, University of Western Ontario, London, Canada, on 15 to 30 mg of rock powder using the ClF3 method of Borthwick & Harmon (1982), as modified after Clayton & Mayeda (1963). The O2 was converted to CO2 by reaction with a hot graphite rod. A CO2–H2O oxygen-fractionation factor of 1.0142 was used to calibrate the reference gas for the mass spectrometers. All values are reported in parts per thousand (‰) using the normal delta nota-tion relative to Vienna Standard Mean Ocean Water (VSMOW). Replicate analyses of NBS–28 gave values of +9.70 ± 0.10‰. Reproducibility of duplicate analyses of rock powders was normally better than ±0.2‰.

reGioNaL GeoLoGy

The Red Cross Lake leucogranite + pegmatite suite is located in northeastern Manitoba, in the Sachigo

the LeUCoGraNite – PeGMatite SUite at reD CroSS LaKe, MaNitoBa 1795

Subprovince of the Archean Superior Province of the Canadian Shield (Fig. 1). The suite is hosted by the Red Cross Lake greenstone belt, situated in the Oxford–Stull domain (Stott et al. 2011) at its boundary with the Hudson Bay terrane to the north (Fig. 1).

The Red Cross Lake greenstone belt

The Red Cross Lake greenstone belt is the northern branch of the two easternmost extremities of the Oxford Lake – Knee Lake greenstone belt, the main portion of which lies farther to the southwest. The part of the Red Cross Lake belt that hosts the leucogranite – pegmatite suite consists of five units: metabasalt, metagabbro, felsic lapilli metatuff, an assemblage of granitic rocks, and minor metasedimentary rocks (Fig. 2). The granitic rocks and parts of the supracrustal metabasaltic-

gabbroic sequence that are located within the North Kenyon Shear Zone (Fig. 2) are extremely mylonitized.

Sopuck (1971) mapped tight, near-isoclinal folds with subvertical axial planes, defining a syncline flanked by anticlines parallel to the WNW–ESE strike of the greenstone belt. This deformation was overprinted by mylonitization along one of the branches of the North Kenyon Shear Zone, a major composite structural break that extends from the western margin of the Superior Province in Manitoba across the eastern provincial boundary for hundreds of km into northwestern Ontario (Osmani & Stott 1988, Skulski et al. 2000, Stone et al. 2004).

In the map area shown in Figure 2, the shear zone passes through parts of both the metabasaltic-gabbroic sequence and the granitic rocks. Moderate to extensive shearing also affected the metavolcanic and metasedi-

FiG. 1. Location of the Red Cross Lake pegmatite field at the boundary between the Hudson Bay terrane and Oxford–Stull domain in northeast Manitoba. The green area represents a spur of the Oxford Lake – Knee Lake greenstone belt. The area of Figure 2 is shown by the yellow rectangle.


mentary units and tonalite – trondhjemite – granodiorite (TTG) rocks of the Hudson Bay terrane in the vicinity of the shear zone proper.

The metabasalt, metagabbro and metasedimen-tary units are the only lithologies in the area with the compositional potential to have recorded varia-tions in metamorphic grade. The observed mineral assemblages are compatible with lower-amphibolite to upper-greenschist facies over most of the map area shown in Figure 2. We do recognize that shearing and movement along the North Kenyon Shear Zone have likely occurred more than once (cf. Powell & Hodgson 1992), but the details and sequence of events remain to be worked out in the Red Cross Lake area. It is difficult to determine the metamorphic grade of the mylonite, as comminution has destroyed most primary minerals in the metavolcanic rocks. For simplicity, the prefix “meta” is omitted from figures, tables, and in details in the following descriptions.

Metavolcanic rocks

The volcanic constituents of the greenstone belt span a broad spectrum of geochemical signatures, from tholeiitic through calc-alkaline, with a distinct break between ocean floor and arc tholeiite to calc-alkaline compositions (Fig. 3, Table 1). MORB-like basalt or back-arc basalt (or both) occur north of the North Kenyon Shear Zone (Figs. 4a, b). South of the shear zone, the supracrustal rocks are more diverse, comprising a wide range of lithologies including mafic and felsic volcanic types (Fig. 4c). This sequence ranges from basalt through andesite in the mafic members and dacite through rhyolite in the felsic members, and also contains numerous volcaniclastic sequences. However, thick sequences of basalt dominate the greenstone belt south of the North Kenyon Shear Zone, and structural imbrication of oceanic and island-arc packages cannot be ruled out. This split is also apparent in the abun-dances and patterns of the rare-earth elements (REE) in extended element plots: metabasalt and gabbro

FiG. 2. Local geology of the Red Cross Lake pegmatite field. NGR, CGR, WGR: Northern, central, and western leucogranites, respectively; SPG: spodumene pegmatites, LPG: lepidolite pegmatites.


show consistently flat profiles with REE contents at ~103 chondrite values, whereas intermediate to felsic metavolcanic rocks display patterns that are distinctly inclined and average about 803 chondritic for the light REE and 53 chondritic for the heavy REE (Fig. 4). The oxygen isotope signatures are mostly typical of basaltic to intermediate whole-rock compositions (5.2 to 7.5‰ d18O), with a small break between basalt and andesite. Distinctly higher values in some samples (7.7 up to 9.4‰ d18O; Table 2) suggest that they have been contaminated, possibly with metasedimentary units. Indeed, arc-related andesite and basalt south of the North Kenyon Shear Zone are extensively intruded by gabbro and interlayered with mostly mafic as well as minor sediments derived from felsic volcanic units. However, the diverse arc-related rocks in the south half of the Red Cross Lake greenstone belt are distinct from the rift-related basalt north of the North Kenyon Shear Zone; in the latter, no metasediments were observed.

Metabasalt is largely pillowed but intercalated with subordinate massive layers. Pillowed basalt ranges from aphyric to porphyritic with plagioclase phenocrysts, and from massive to highly vesicular. Pillows have selvages and are locally separated by interpillow hyaloclastite. Massive basalt layers are homogeneous, equigranular, with a very fine salt-and-pepper structure. Both the pillowed and massive basalt show local gradation to a coarser grain-size and texture. Compositionally, the metabasalts cover a considerable range in silica content from picrobasalt to basaltic andesite (Fig. 3), but this variation does not correlate with variations

in other components or textural features of the rocks (Table 1). The REE abundances are quite uniform from the light to heavy end of their spectrum, with flat patterns throughout (Fig. 4). In terms of modern basaltic analogues, they correspond to back-arc ridge basalt or MORB, as characterized by Rollinson (1993).

Metagabbro, in part leucocratic, is considerably variable in mineral mode, composition and grain size. Some outcrops suggest that the metagabbro varies from a dominant mesocratic rock along the northern side of the body to coarsely glomeroporphyritic in the central parts, and to a fine-grained melanocratic phase in the south. The mesocratic gabbro retains an igneous texture, with pseudomorphic hornblende replacing equant grains of pyroxene in a plagioclase-rich matrix, preserving evidence of the highest metamorphic condi-tions reached. The chemical composition of metagabbro closely corresponds to that of some of the surrounding metabasalt; the metagabbro can be considered a subvol-canic intrusive analogue of the metabasaltic rocks (Table 1, Fig. 3).

Felsic lapilli metatuff (2833 ± 2 Ma, U–Pb on zircon; Stone et al. 2004) ranges from ash flows with lapilli-size fragments to crystal tuff, and locally to volcanic breccia with deformed clasts attaining 1 3 20 cm in size. The metatuff is generally layered (5 to 50 cm), and aphyric to coarse grained, with abundant feldspar or quartz frag-ments (~2 mm). Its composition varies from andesitic to rhyodacitic–rhyolitic (Table 1, Fig. 3).

FiG. 3. Plot of SiO2 versus alkalis for the greenstone belt lithologies (after Le Maitre et al. 2002). Dark pink diamond: syntectonic gabbro, open green triangle: mafic mylonite, X: (basaltic) greywacke, +: (rhyodacitic) greywacke; the other symbols are self-explanatory. Note the tholeiitic trend of the metavolcanic assemblage and the gap between gabbros and intermediate metavolcanic rocks.


Metasedimentary rocks

Metasedimentary rocks underlie a sizeable area to the west–northwest of the map area, continuous with the small segment at the northwest shore of Red Cross Lake (Fig. 2). These rocks seem to consist of metagreywackes only; no pelitic components were observed. Representa-tive compositions of metagreywackes sampled within the area of our map suggest monolithologic sources characteristic of restricted source-areas of either basaltic or rhyodacitic composition (Table 3). Although obvi-ously physically reworked, their chemical compositions suggest little, if any, compositional change during weathering and transport, which were likely of limited duration.

The greywacke unit (unit 4 in Fig. 2) mapped by Sopuck (1971) along the northern margin of the green-stone belt could not be confirmed. However, abundant metagreywacke and minor metapelite are known ~35 km west–northwest of the map area (Fig. 2; H.P. Gilbert, pers. commun., 2011). The tectonic style affecting the greenstone belt suggests that slices of these and related metasedimentary rocks could have been introduced eastward into deeper parts of the greenstone belt that are not currently exposed at surface.

Mylonitized rocks

Mylonitized granitic rocks extend in a narrow zone west–northwest from Red Cross Lake (North Kenyon Shear Zone, unit 10 in Fig. 2). Mineral modes indicate

TABLE 1. COMPOSITION OF REPRESENTATIVE METAVOLCANIC

AND GABBROIC ROCKS AT RED CROSS LAKE

____________________________________________________________

Gb Gb Bas Bas Bas And Dc Rdc

30-1G 48-2G 8-3 1-42' 45-1G 58-2G 17-1G 34-1G

___________________________________________________________

2SiO wt.% 49.78 50.43 44.73 49.23 51.86 60.40 67.76 71.49

2TiO 1.30 0.62 1.14 0.93 1.33 0.49 0.43 0.29

2 3Al O 13.98 14.60 15.68 16.26 14.37 18.47 15.54 15.17

2 3Fe O 15.24 11.69 14.54 13.95 15.16 5.42 4.28 2.33T

MnO 0.23 0.19 0.23 0.20 0.23 0.09 0.06 0.01

MgO 6.51 8.74 5.43 9.02 4.76 2.27 1.04 0.77

CaO 9.80 9.98 15.40 8.36 9.58 5.40 3.61 2.73

2Na O 2.46 2.61 1.10 1.06 2.05 4.35 4.78 5.05

2K O 0.30 0.26 0.18 0.55 0.13 1.62 1.34 1.29

2 5P O 0.11 0.05 0.08 0.07 0.11 0.11 0.17 0.09

LOI 0.77 1.25 n.a. n.a. 0.77 0.65 1.18 0.96

Total 100.48 100.42 98.51 99.63 100.34 99.35 100.19 100.18

Li ppm n.d. n.d. n.d. n.d. n.d. 27 29 61

Rb 2 5 70 154 2 28 24 25

Cs 0.1 0.9 b.d.l. b.d.l. 0.1 3.2 0.7 0.7

Sr 227 99 128 101 77 484 428 393

Ba 40 31 48 90 45 437 341 263

Pb b.d.l. 5 b.d.l. n.d. 8 6 7 b.d.l.

Zn 109 71 101 84 113 38 81 b.d.l.

Cu 168 115 124 124 149 15 30 68

Ni 78 102 130 162 98 22 b.d.l. b.d.l.

V 355 260 372 294 314 73 37 25

Cr 200 321 291 279 96 30 b.d.l. b.d.l.

Ga 18 14 22 17 19 24 22 21

Sc 45 52 54 40 45 14 5 3

U 0.13 0.05 2.00 n.d. 0.15 0.32 0.80 0.82

Th 0.82 0.17 0.30 0.48 1.19 1.00 2.95 3.32

Zr 67 29 63 46 70 113 129 137

Sn b.d.l. b.d.l. n.d. 70 b.d.l. n.d. 1 1

Nb 3.2 1.1 3.0 2.1 2.3 5.8 5.3 3.4

S n.a. n.a. 1530 220 n.a. 199 n.a. n.a.

Cl n.a. n.a. 83 58 n.a. 155 n.a. n.a.

Y 26 16 22 16 29 9 10 8

La 5.60 0.93 3.75 2.33 3.33 8.06 24.80 17.00

Ce 14.00 2.87 9.83 5.96 9.09 17.09 47.20 31.60

Pr 1.92 0.47 1.48 0.91 1.35 2.39 4.82 3.16

N.d. 9.19 2.62 7.34 4.77 6.79 10.58 16.80 10.90

Sm 2.76 1.10 2.39 1.73 2.49 2.39 2.82 1.85

Eu 1.06 0.49 0.81 0.56 1.04 0.71 0.91 0.63

Gd 3.52 1.72 3.25 2.49 3.70 2.25 2.35 1.53

Tb 0.68 0.37 0.58 0.43 0.78 0.32 0.36 0.24

Dy 4.55 2.54 3.98 3.13 5.00 1.84 1.84 1.32

Ho 1.00 0.60 0.82 0.63 1.10 0.36 0.36 0.26

Er 2.940 1.760 2.466 1.950 3.190 1.013 0.980 0.720

Tm 0.442 0.290 0.364 0.280 0.494 0.146 0.143 0.113

Yb 2.850 1.870 2.329 1.900 3.030 0.917 0.910 0.760

Lu 0.424 0.282 0.380 0.280 0.456 0.140 0.137 0.115

K/Rb 1555 476 21 30 534 482 456 430

K/Ba 62.3 69.6 31.1 50.7 24.0 30.8 32.6 40.7

Ba/Rb 25.0 7.0 0.7 0.6 22.3 15.7 14.0 10.6

Rb/Sr 0.0 0.0 0.55 1.52 0.03 0.06 0.06 0.06

Al/Ga 4089 5563 3771 5061 4047 4072 3727 3829

____________________________________________________________

Symbols used in column headings: Gb gabbro, Bas basalt, And andesite,

Dc dacite, Rdc: rhyodacite. Abbreviations: b.d.l.: below detection limit, n.a.:

2 3not analyzed, n.d.: not detected. Total Fe as Fe O . For convenience, rockT

names refer to the unmetamorphosed equivalent.

TABLE 2. OXYGEN ISOTOPE COMPOSITION OF

RED CROSS LAKE ROCKS

____________________________________________________________

Rock type Sample ä O Rock type Sample ä O18 18

____________________________________________________________

Metavolcanic rocks Metagreywacke

_________________________ ________________________

mafic mylonite 56-1G 5.2 basaltic 32-117 7.8

px-phyric gabbro 30-1G 5.6 basaltic 35-1A 7.7

gabbro 48-2G 6.7 basaltic 56-1A 6.7

basalt 48-1G 6.6 rhyodacitic 17-1A 6.8

basalt 32-1G 6.7 rhyodacitic 29-1A 8.0

basalt 35-1G 6.7

basalt 46-1G 7.7 Southern TTG suite

basalt 15-1G 9.2 ________________________

basalt 45-G 9.4

andesite tuff 50-1G 7.4 tonalite S-12-1 7.5

andesite 26-1G 8.2 tonalite S12-1(?) 7.2

dacite 17-1G 6.5 trondhjemite 44-1G 5.9

dacite tu ff 29-1G 7.1 trondhjemite 57-1G 6.9

rhyodacite 34-1G 7.6

____________________________________________________________

Values of ä O are quoted in ‰ relative to the VSMOW standard. For18

convenience, rock names refer to the unmetamorphosed equivalent.

Average standard error: ±0.1‰.


FiG. 4. Chondrite-normalized REE patterns of the meta volcanic rocks: (a) basalt, (b) gabbro (black circles) and mafic mylonite (green triangles), (c) intermediate to felsic rocks (andesite: black triangles, dacite: green circles, rhyodacite: red cross), and (d) greywackes of basaltic (X) and rhyodacitic (+) composition. Note the perfect overlap of the flat patterns of the samples of basalt, gabbro and mafic mylonite (a and b) and the negatively sloping (i.e., LREE-enriched but HREE-depleted) patterns of the samples of intermediate to felsic rocks and metagreywackes (c and d). The REE abundances in the metagreywackes closely mimic those of their metavolcanic sources.


a range from dominant granodiorite to minor granite, locally with mafic domains, and to homogeneous and equigranular or plagioclase-phyric tonalite. The tonalite contains gabbroic xenoliths.

The North Kenyon Shear Zone also contains strongly sheared mafic gneiss and mylonite, derived from basalt and related gabbro (mylonite, North Kenyon Shear Zone in Fig. 2). All rock types are strongly sheared and laminated. Recrystallization of metaba-salt to a hornblende-dominant rock, and of gabbro to feldspathic amphibolite (rarely with relict garnet) is widespread. Remnants of pillow selvages in basalt and of plagioclase-phyric texture in augen gabbro are very rare. Felsic blastomylonite of possible volcanic origin is very rare and thoroughly deformed and recrystallized; the protolith is not recognizable.

The Oxford–Stull domain

The Oxford–Stull domain comprises plutonic and extrusive rocks that seem to have formed in a juvenile oceanic environment between 2830 and 2710 Ma and are apparently not contaminated by early Mesoarchean crust (Syme et al. 1998). The dominant tonalite–grano-diorite gneiss (2812 ± 2.5 Ma, U–Pb on zircon; Stone et al. 2004) on the southern flank of the greenstone belt has a seriate porphyritic texture in the less deformed parts. It represents the igneous protolith of the otherwise strongly foliated augen gneiss. Typically, plagioclase augen are hosted by a quartz + plagioclase + biotite matrix. However, K-feldspar was introduced into the gneiss along numerous deformed granodiorite and pegmatite dikes. An older leucocratic biotite tonalite augen gneiss is locally observed in the dominant gneissic rock, as well as late undeformed dikes of leucocratic tonalite that crosscut the gneiss.

The only type of granitic rock readily distinguish-able from the tonalite gneiss is the biotite trondhjemite, which forms a wedge-shaped body adjacent to the tonalite–granodiorite gneiss in the western part of the map area (Fig. 2). The biotite trondhjemite is a grey, medium-grained, strongly foliated rock, locally crosscut by veins of pink pegmatite. Biotite tends to be clustered, possibly replacing hornblende. Biotite trondhjemite contains rafts of gabbro up to 15 m long.

Compositionally, the entire tonalite gneiss assembly is quite diverse, but all of its compositions cluster rela-tively tightly in the region of classic TTG lithologies (Figs. 5a, 6a, Table 4). All the rocks fall within the field of volcanic-arc granite of Pearce et al. (1984) (Fig. 7). With the exception of the most felsic member of the TTG suite, the REE patterns of the tonalites and trond-hjemites are quite uniform and monotonous (Fig. 8). The random selection of samples analyzed for oxygen isotopes shows a range of low d18O values overlapping those of the metavolcanic rocks (5.9–7.5‰ d18O, Table 2). This is consistent with the deep-seated suboceanic origin of the TTG suite with only negligible, if any,

assimilation of pre-existing continental crust (Skulski et al. 2000).

TABLE 3. COMPOSITION OF REPRESENTATIVE TYPES

OF METAGREYW ACKE, RED CROSS LAKE, MANITOBA

____________________________________________________________

Mafic Felsic

_______________________ ______________

35-1A 32-117 56-1A 17-1A 29-1A

____________________________________________________________

2SiO wt.% 49.06 50.41 47.75 68.43 69.29

2TiO 0.79 0.91 0.97 0.42 0.30

2 3Al O 14.73 14.52 15.97 15.73 16.15

2 3Fe O 11.26 12.03 10.60 4.21 3.21T

MnO 0.20 0.20 0.18 0.06 0.05

MgO 7.71 7.05 4.81 1.07 0.85

CaO 11.56 10.58 12.09 3.30 2.94

2Na O 2.54 2.06 1.85 4.83 5.45

2K O 0.22 0.23 0.36 1.41 1.29

2 5P O 0.09 0.08 0.07 0.16 0.07

LOI 2.71 1.25 5.75 1.35 0.89

Total 100.87 99.32 100.40 100.97 100.49

2 2Na O/K O 11.6 9.0 5.1 3.4 4.2

2 3 2Al O /Na O 5.8 7.0 8.6 3.3 3.0

Rb ppm 10 b.d.l. b.d.l. 20 30

Cs 0.6 0.5 3.0 1.1 1.4

Sr 178 110 138 418 420

Ba 41 51 79 337 290

Pb b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Zn 68 86 78 77 46

Cu 70 117 113 13 7

Ni 108 97 111 9 8

Co 46 43 40 8 6

V 242 282 278 42 35

Cr 273 209 215 11 8

Sc 34 38 33 5 4

U b.d.l. b.d.l. 0.4 0.4 b.d.l.

Th 0.8 1.2 0.3 2.7 2.7

Zr 65 67 58 135 112

Hf 1.7 1.9 1.4 3.1 2.7

Sb 0.5 0.9 3.8 b.d.l. b.d.l.

As b.d.l. 5 24 b.d.l. b.d.l.

Ta b.d.l. b.d.l. 0.4 0.6 0.7

S 470 78 680 15 8

Br 1.1 b.d.l. b.d.l. b.d.l. 0.6

Y 20 21 21 8 5

La 7.37 8.46 4.82 23.10 13.10

Ce 18 18 12 41 23

Pr n.a. n.a. n.a. n.a. n.a.

Nd 5 9 10 14 9

Sm 2.47 2.65 1.94 2.31 1.47

Eu 0.77 0.86 0.79 0.85 0.54

Gd n.a. n.a. n.a. n.a. n.a.

Tb 0.4 0.6 0.3 b.d.l. b.d.l.

Dy n.a. n.a. n.a. n.a. n.a.

Ho n.a. n.a. n.a. n.a. n.a.

Er n.a. n.a. n.a. n.a. n.a.

Tm n.a. n.a. n.a. n.a. n.a.

Yb 2.13 2.46 2.06 0.80 0.56

Lu 0.30 0.31 0.29 0.13 0.08

____________________________________________________________

2 3b.d.l.: below detection limit, n.a.: not analyzed. Total Fe as Fe O .T


FiG. 5. Albite – anorthite – orthoclase (Ab–An–Or) plot of the TTG plutonic rocks in the (a) southern and (b) northern parts of the map area. The field boundaries are after Barker (1979).

FiG. 6. Ocean-ridge-granite-normalized spider diagrams of TTG rocks of the (a) southern and (b) northern parts of the map area (normalization values from Pearce et al. 1984). Note the close similarity of the patterns in the two graphs, but the restriction of the northern TTG data to the higher compositional range of values. Solid symbols: southern, open symbols: northern. Symbol: ORG: ocean- ridge granite.


The Hudson Bay terrane

In contrast to the Oxford–Stull domain, where the plutonic rocks are apparently juvenile in origin, the Hudson Bay terrane in the Red Cross Lake area consists of plutonic TTG rocks that have undergone significant recycling. These rocks, which range in age from 2.84 to 2.71 Ga (with a metamorphic overgrowth on zircon at 2.74 to 2.72) (Skulski et al. 2000), contain inherited zircon as old as 3.57 Ga, and are characterized by 2.9 to 3.6 Ga Sm–Nd model ages. These data indicate a substantial contribution of older crust, interpreted to be an early continental fragment at the northern margin of the Superior Province (Skulski et al. 2000).

Tonalite gneiss (2785 ± 1 Ma, U–Pb zircon; Stone et al. 2004) is the predominant rock-type; it is widely variable in texture, modal composition and geochemical signature. The rock is typically well foliated, with strings of ribbon quartz and rotated plagioclase porphyroclasts in a schistose, medium-grained, biotite-bearing matrix. This medium-grey augen orthogneiss is locally injected by leucocratic tonalite and dikes of two-mica pegmatite that are also strongly deformed and in part boudinaged. However, some outcrops in the northern part of the map area (Fig. 2) consist of relatively undeformed, seriate to porphyritic tonalite and younger granodiorite, with minor granite. Younger aplite and pegmatite dikes show

only moderate deformation. The granodiorite is locally distinctly intrusive into the tonalite.

The tonalite gneiss is relatively heterogeneous (Fig. 6b), as expected from its modal and textural vari-ability. The tonalitic gneiss in the northern part of the map area is compositionally similar to that in the TTG suite to the south, but in general the rocks in the north are relatively more fractionated (Figs. 5, 7, 8, Table 4).

Leucogranites and pegmatites

Three categories of leucogranites and rare-element pegmatites constitute the population of the Red Cross Lake pegmatite field proper, from relatively simple intrusions to highly fractionated and rare-element-enriched derivatives. These rocks will be described in detail in Part II (Černý et al. 2012a).

SUMMary

Within the limits of the present sampling and analyt-ical work, the rocks of the Red Cross Lake greenstone belt and adjacent plutonic suites closely correspond to analogous rock formations in the Superior Province and similar Archean and Proterozoic terranes elsewhere, as reviewed by Černý & Meintzer (1988) and Breaks et al. (2005), among others. Although these pegmatite-

FiG. 7. (a) Rb versus Y+Nb and (b) Nb versus Y diagrams of TTG rocks of the southern and northern parts of the map area. Field boundaries from Pearce et al. (1984). Note in (a) that the rocks in the northern suite plot in the upper part of the field for VAG, close to the syn-COLG field. VAG: volcanic-arc granite, WPG: within-plate granite, ORG: ocean-ridge granite, syn-COLG: syn-collisional granite. Symbols as in Figures 5 and 6.


FiG. 8. Chondrite-normalized REE abundances in the samples of (a) tonalite, (b) trondhjemite and (c) granitic rock of the southern and northern plutonic suites. Note the perfect overlap of the tonalite and trondhjemite patterns (b and c), but the broader spread and minor Eu anomalies in the granodiorite (blue squares) and granite (red circles) patterns. Shaded field in the trondhjemite and granite diagrams marks the field of the tonalite plots (a). Solid symbols: southern, open symbols: northern.

bearing fields are numerous, data on the whole-rock composition of the host metamorphic and plutonic lithologies are scarce (e.g., Černý et al. 1981, Breaks & Moore 1992). This is very unfortunate, as the host rocks in their downward extension and their basement generate, on partial to complete melting, the felsic magmas that evolve into the rare-element pegmatites. Thus the potential of pegmatite mineralization is ulti-mately rooted in the rare-element abundances in the regional protolith. Nevertheless, the data presented here do not indicate any unusual enrichment of the intermediate to felsic metavolcanic or metasedimentary lithologies in elements that are notably concentrated in granitic pegmatites, such as the rare alkalis, Nb, Ta, Sn,

Be, and Ga. The extreme fractionation attained by the Red Cross Lake lepidolite pegmatites, to be described in the Part II of this series of papers, was apparently attained by partial melting and differentiation of granitic magmas from ordinary metamorphic protoliths with no anomalous pre-enrichment of the rare elements in the source rocks, syngenetic or overprinted.

aCKNoWLeDGeMeNtS

Research leading to this paper was supported by NSERC Operating, Research, Equipment and Major Installation grants to PČ, and Research Associate support to REM. Further financing was provided by


TABLE 4. COMPOSITIONS OF REPRESENTATIVE TTG PLUTONIC ROCKS,

RED CROSS LAKE, MANITOBA

___________________________________________________________________________________

Southern area Northern area

______________________________ ______________________________

Tonalite Trondhjemite Granodiorite Tonalite Trondhjemite Granite

Sample 67-1G 58-1G 60-1G 59-1G 66-2G 9-1H 10-1H 65-1G 44-1G 11-1H

___________________________________________________________________________________

2SiO wt.% 69.25 73.42 69.09 67.91 73.72 72.20 72.65 70.92 70.61 75.80

2TiO 0.27 0.17 0.31 0.36 0.08 0.34 0.28 0.25 0.27 0.16

2 3Al O 15.80 13.88 15.49 15.40 13.94 15.20 14.90 15.31 15.65 12.80

2 3Fe O 3.03 2.15 3.16 3.28 1.11 2.60 1.95 2.57 2.40 1.30T

MnO 0.04 0.03 0.03 0.06 0.01 0.04 0.02 0.04 0.05 0.03

MgO 0.97 0.42 0.74 1.33 0.28 0.77 0.69 0.62 0.80 0.31

CaO 3.03 2.71 2.75 2.90 2.26 3.04 2.88 2.51 2.79 0.94

2Na O 4.26 4.00 4.94 4.19 3.67 4.40 4.35 4.23 4.66 3.10

2K O 1.59 1.08 1.63 3.05 2.50 1.66 2.13 1.73 1.56 4.84

2 5P O 0.08 0.02 0.08 0.12 0.02 0.09 0.06 0.08 0.10 0.03

2H O n.d. n.d. n.d. n.d. n.d. 0.30 0.50 n.d. n.d. 0.30+

Total 98.32 97.88 98.22 98.57 97.59 100.64 100.41 98.26 98.89 99.61

Li ppm 33 17 27 15 16 n.d. n.d. 392 122 n.d.

Rb 29 17 37 89 32 110 65 150 138 160

Cs 0.6 0.3 2.2 8.0 0.4 40.0 0.9 16.3 9.3 0.8

Sr 470 367 596 632 299 360 285 371 359 80

Ba 565 293 588 1296 850 300 405 698 338 360

Pb b.d.l. b.d.l. b.d.l. 15 8 12 16 10 16 20

Zn 23 16 16 22 8 62 41 23 60 23

Cu 4 4 13 5 b.d.l. 24 b.d.l. 14 16 b.d.l.

Ni 8 b.d.l. b.d.l. 16 b.d.l. b.d.l. b.d.l. 6 28 b.d.l.

V 22 17 32 44 7 24 17 23 18 7

Cr 25 13 23 35 b.d.l. b.d.l. b.d.l. 26 b.d.l. b.d.l.

Ga 21 19 21 19 15 19 17 22 23 18

Sc b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 4 3 b.d.l. 4 3

U 0.14 0.39 0.70 2.33 0.25 1.40 0.84 1.09 1.93 10.00

Th 3.27 5.60 2.07 7.68 2.27 6.40 6.20 6.78 9.23 30.00

Zr 144 167 132 151 32 150 130 130 138 120

Hf 3.0 4.4 3.3 3.8 0.9 3.7 3.0 3.1 3.9 4.3

Sn n.d. n.d. n.d. n.d. n.d. 6.5 0.1 n.d. 2 1.6

Ta 0.09 0.18 0.50 0.38 0.07 1.80 0.86 0.61 2.12 2.60

Nb 2.86 2.48 5.96 4.30 1.47 7.20 5.20 6.18 9.20 22.0

S 76 83 249 119 62 n.d. n.d. 104 n.d. n.d.

Cl 75 70 b.d.l. 116 b.d.l. n.d. n.d. 51 n.d. n.d.

Y 1.85 1.77 5.92 7.23 1.42 10 7 6 10 36

La 21.09 13.42 17.47 33.19 8.78 23.00 17.00 23.51 29.20 35.00

Ce 40.13 25.08 32.36 61.41 16.78 42.00 27.50 42.22 49.20 68.00

Pr 4.27 2.70 3.75 6.81 1.95 4.50 2.75 4.46 4.61 7.90

Nd 14.52 9.15 13.99 24.34 7.21 15.00 9.50 15.42 15.10 27.00

Sm 1.78 1.33 2.27 3.49 1.09 2.70 1.65 2.38 2.49 5.70

Eu 0.51 0.49 0.66 0.90 0.44 0.68 0.55 0.58 0.66 0.56

Gd 1.03 0.86 1.78 2.43 0.73 2.00 1.40 1.76 2.07 5.50

Tb 0.10 0.10 0.23 0.30 0.08 0.31 0.21 0.23 0.32 0.97

Dy 0.44 0.43 1.24 1.55 0.34 1.60 1.15 1.24 1.64 5.80

Ho 0.08 0.08 0.23 0.29 0.06 0.30 0.22 0.23 0.30 1.10

Er 0.178 0.203 0.642 0.763 0.131 0.780 0.540 0.618 0.830 2.900

Tm 0.024 0.030 0.091 0.115 0.015 0.120 0.080 0.089 0.126 0.470

Yb 0.143 0.221 0.590 0.766 0.105 0.700 0.520 0.568 0.830 3.000

Lu 0.023 0.034 0.090 0.119 0.017 0.110 0.080 0.077 0.123 0.440

K/Rb 457 530 364 286 657 125 272 96 94 251

K/Ba 23.4 30.6 23.0 19.5 24.4 45.9 43.7 20.6 38.3 111.6

Ba/Rb 19.5 17.3 15.8 14.6 26.9 2.7 6.2 4.7 2.5 2.3

Rb/Sr 0.06 0.05 0.06 0.14 0.11 0.31 0.23 0.4 0.38 2.00

Al/Ga 3981 3865 3903 4289 4917 4233 4638 3682 3549 3763

___________________________________________________________________________________

2 3b.d.l.: below detection limit, n.d.: not detected. Total iron as Fe O . Loss on ignition is not included.T


the Canada – Manitoba Mineral Development Agree-ment (1982–1989). Extensive logistical support was provided by Tantalum Mining Corporation of Canada, Ltd. and Manitoba Department of Energy and Mines. P.B. Tomascak, then at University of Maryland, and now the State University of New York at Oswego, tested the behavior of the Rb–Sr isotopes. H.P. Gilbert, Manitoba Geological Survey, provided some information and rock samples from the greenstone belt. Meticulous reviews of the manuscript by G. Beakhouse and H.P. Gilbert, and the editorial clean-up by R.F. Martin, considerably improved this publication.

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Received July 17, 2012, revised manuscript accepted October 25, 2012.

Documents

EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE - PEGMATITE SUITE AT RED CROSS LAKE, MANITOBA, CANADA. I. GEOLOGICAL SETTING