Nd- and Sr-isotope systematics for the Kamiskotia-Montcalm area: implications for the formation of late Archean crust in the western Abitibi Subprovince, Canada

Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, U.S.A.

Received May 8, 1990

Revision accepted September 12, 1990

Geochemistry and Nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic, calc-alkalic, and lamprophyric suites in the Kamiskotia-Montcalm area. The Kamiskotia gabbroic complex (KGC) and cogenetic, bimodal volcanic rocks have eNd(t) = +2.2 to +2.6, consistent with a direct derivation from a long-term, light rare-earth element (LREE)-depleted mantle. The Montcalm gabbroic complex has decreasing eNd(t) upsection from +2.8 to + 1 .O, consistent with contamination by long-term, LREE-enriched (with respect to the long-term, LREE-depleted Abitibi mantle) crust during fractionation. Two calc-alkalic lamprophyre samples, characterized by large-ion lithophile element (LILE) and LREE enrichment and high MgO, Ni, and Cr contents, have eNd(t) of +2.5 and +2.8, indicating a derivation from a depleted mantle source that had undergone recent trace-element enrichment. A different lamprophyre suite is extremely LILE and LREE enriched and has an eNd(t) of + 1 .O, indicating a derivation from a slightly different source that had earlier LREE enrichment. Granitoid rocks internal and external to greenstone belt rocks have eNd(t) = +2.5 to +3.8 and +0.6 to -0.4, respectively. The lower values provide additional evidence for the existence of LREE-enriched crust in this area.

Considering these data along with other radiogenic isotope studies, a petrogenetic and tectonic model is suggested for the crustal development of the southern Abitibi Subprovince. From >2740 to 2698 Ma-the major period of volcanic activity- komatiitic and tholeiitic suites and one lamprophyre suite were derived from a uniformly LREE-depleted mantle reservoir with eNd(t) = +2 to +3. Calc-alkalic granitoids were emplaced generally after 2700 Ma. Their long-term, LREE-depleted and LREE-enriched Nd isotopic signatures are similar to signatures in continental-arc settings (e.g., the Coastal Batholith of Peru). Form 2690 to 2670 Ma, when transpressional tectonism prevailed, mantle-derived magmatism was represented by long-term, LREE-enriched (eNd(t) = + 1 to +2) lamprophyric and alkalic volcanic suites.

The Kamiskotia suite has a seven-point, whole-rock - mineral isochron Sm-Nd age of 2710 2 30 Ma, identical to U-Pb zircon ages for the suite, indicating closed-system behavior. An Rb-Sr mineral - whole-rock isochron age from one KGC sample is 2450 2 30 Ma, identical to U-Pb ages for the Hearst-Matachewan dike swarm, a prominent feature in the KGC area. Regression of whole-rock and mineral-isotope data for one granitoid sample with a U-Pb zircon age of 2696 * 1.5 Ma gives identical ages of 2530 2 30 Ma in the Sm-Nd and Rb-Sr systems. The latter data add to an increasing body of evidence for cryptic, late thermal events after granitoid-greenstone belt development in the southern Abitibi Subprovince.

La gCochimie et les compositions isotopiques du Nd servent a caractkriser les magmas d'origine crustale ou mantellique, et pour ttablir les bornes dans les modeles p6trogCnetiques des suites tholCiitiques, calco-alcalines et lamprophyriques dans la rCgion de Kamiskotia-Montcalm. Le complex gabbroique de Kamiskotia (CGK) et les roches volcaniques bimodales, cogCnCtiques, fournissent les valeurs de eNd(t) = +2,2 a +2,6 qui sont en accord avec une dCrivation directe de magma mantellique appauvri, a long terme, en terres rares 1Cgeres. Dans le complexe gabbroique de Montcalm les valeurs de eNd(t) diminuent du bas vers le haut de la coupe de +2,8 a + 1,0, ce qui tCmoigne d'une contamination par une crotite enrichie, a long terme, en terres rares ICgeres (relativement au manteau d9Abitibi appauvri long terme en t e r i s rares 1Cgkres) durant le fractionnement. Deux Cchantillons de lamprophyre calco-alcalin, caractCrisCs par un enrichissement des ClCments lithophiles a grands rayons ioniques (LILE) et des terres rares ICgeres, et des teneurs ClevCes en MgO, Ni et Cr, possedent des valeurs de eNd(t) de +2,5 et +2,8 qui indiquent une origine d'une source de magma mantellique appauvrie, laquelle aurait subi un enrichissement rtcent en ClCments traces. Une suite diffbrente de lamprophyre est trks fortement enrichie en LILE et terres rares Itgbres, son eNd(t) ttant + 1,O rCvkle une source magmatique 1Cgbrement diffkrente, anterieurement enrichie en terres rares ltgbres. Les roches granitoides qui se trouvent B I'intCrieur et 21 I'extCrieur de la ceinture de roches vertes fournissent eNd(t) = +2,5 a +3,8 et +0,6 a -0,4, respectivement. Les valeurs les plus faibles corroborent l'existence d'une croQte enrichie en terres rares ltgbres dans cette rCgion.

Ces donnees jointes aux rtsultats de d'autres Ctudes d'isotopes radiogkniques, suggkrent un modkle pCtrogCnCtique et tectonique pour expliquer le dCveloppement de la croQte de la sous-province d'Abitibi sud. De >2740 Ma a 2698 Ma-la ptriode d'activite volcanique la plus importante-les suites komatiitiques et tholtiitiques et une suite de lamprophyre furent dkrivtes d'un rkservoir de magma mantellique appauvri uniformkment en terres rares ICgkres, avec eNd(t) = +2 a +3. Les granitoides calco-alcalins furent mis en place gCnCralement apres 2700 Ma. Leurs signatures de Nd isotopique, identifiant un magma appauvri a long terme en terres rares 1Cgeres ou un magma enrichi a long terme en terres rares lCgbes, sont analogues aux signatures dCvoilCes dans des contextes d'arc continental (ex., le batholite CBtier du PCrou). De 2690 a 2670 Ma, pCriode dominCe par un tectonisme de transpression, le magmatisme dCrivC du manteau Ctait reprCsentC par des suites lamprophyritiques et de volcanites alcalines enrichies, a long terme, en terres rares lCgirres avec eNd(t) = + I a +2.

Les analyses gCochronologiques de la suite de Kamiskotia, sur roche totale et sur minCraux, ont permis de tracer avec sept points une isochrone Sm-Nd fournissant un dge de 2710 2 30 Ma, identique aux dges U-Pb sur zircon de la mCme suite, ce qui indique un comportement en systeme fermt. Une isochrone Rb-Sr sur minCraux - roche totale d'un Cchantillon du CGK a fourni un dge de 2450 -+ 30 Ma, identique aux dges U-Pb obtenus pour I'essaim de dykes d'Hearst-Matachewan, une particularit6 gtologique importante dans la rCgion du CGK. Une courbe de rkgression des donnees isotopiques sur roche totale et sur

Can. J. Earth Sci. 28, 58-76 (1991)

'Present address: BP Resources Canada Limited, 890 West Pender Street, Suite 700, Vancouver, B.C., Canada V6C 1K5. Printed in Canada 1 Imprim6 au Canada

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BARRIE AND SHIREY 59

minCraux d'un Cchantillon de granitoi'de caractCrisC par un bge U-Pb sur zircon de 2696 -C 1,5 Ma a fourni des bges identiques de 2530 ? 30 Ma pour les systkmes Sm-Nd et Rb-Sr. Ces derniers rbsultats s'ajoutant aux nornbreuses donnCes, tendent a dkmontrer I'existence d'bvknements thermiques tardifs, cryptiques, posterieurs au dCveloppement de la ceinture de granitoides - roches vertes de la sous-province d'Abitibi sud.

[Traduit par la rkdaction]

Introduction This study investigates the geochemistry and Nd-Sr-isotope

systematics of late Archean supracrustal rocks in the Kamis- kotia-Montcalm area of the Abitibi Subprovince. The approach is to analyze a variety of igneous rock suites and mineral separates from a geographically restricted area where there is good geologic, structural, and geochronologic control. The objectives are twofold: first, to identify and characterize potentially distinct mantle and crustal sources for the supracrus- tal rocks using whole-rock geochemistry and Nd isotopic compositions; and second, to determine mineral-isochron ages in the Nd and Sr systems to examine postcrystallization metamorphic events. Previous trace-element and radiogenic isotope studies for Abitibi mantle-derived rocks have indicated that the mantle was variably depleted in NdISm, RbISr, and U/Pb with respect to a reservoir with chondritic ratios for these elements. When considered on a subprovince scale, mantle- derived Abitibi tholeiitic and komatiitic intrusions appear uniformly depleted in their Nd and Sr isotopic compositions (e.g., Machado et al. 1986). However, other investigations have detected different isotopic signatures (Basu et al. 1984), even within "cogenetic" suites (Cattell et al. 1986); and mantle heterogeneities have been found in Archean rocks elsewhere in the southern Superior Province (Shirey and Hanson 1986). Crustally derived Abitibi rocks show primitive and slightly enriched Pb isotopic signatures (GariCpy and AlEgre 1985; Deloule et al. 1989), and isotopic heterogeneities have been found in crustally derived rocks in northwestern Ontario (e.g., Morrison et al. 1985; Shirey and Hanson 1986; Smith et al. 1987). Recent U-Pb studies (e. g., Mortensen 1987; Corfu et al. 1989; Barrie and Davis 1990) provide a precise chronologic framework, which is essential for unravelling the magmatic and tectonic events of the Abitibi Subprovince. We reappraise previous Nd isotopic studies in light of the new U-Pb ages and compare them with our results, to produce a more accurate portrayal of the Nd isotopic signatures for Abitibi mantle and crustal sources.

The first ages from Nd-isotope mineral - whole rock regres- sions for Superior Province granitoid intrusions are reported here, including analyses on zircon, titanite, clinopyroxene, plagioclase, and apatite. Ages in the Nd and Sr systems correspond to postcrystallization thermal events that are not recorded by the development of new metamorphic mineral assemblages.

Geological and geochronological background The Kamiskotia-Montcalm area is located in the western-

most part of the Abitibi Subprovince of the Superior Province (Fig. 1). It can be divided into three regions: the Kamiskotia area, with the Karniskotia gabbroic complex (KGC) and related volcanic rocks to the east; a central granitoid terrane; and the Montcalm area, with the Montcalm gabbroic complex (MGC) to the west (Fig. 2). The area has been subjected to regional metamorphism to the lower greenschist facies, and locally to the lower amphibolite facies proximal to the margins of large granitoid intrusions.

The KGC is a large synvolcanic intrusion that is divided

into three zones of mafic cumulates and a fourth zone consisting of quartz diorite - tonalite - granite granophyric rocks that overlie and are partly along strike from the upper zone. The lower zone is composed of locally layered troctolite, olivine gabbro, and magnesian gabbronorite; the middle zone is massive gabbro and anorthositic gabbro; and the upper zone is locally layered anorthositic gabbro containing abundant cumu- late Fe-Ti oxides. The KGC intrudes into and is overlain by Kamiskotia rhyolites and basalts, which host significant mas- sive sulfide mineralization (Pyke and Middleton 1970); it is underlain by mafic and intermediate "lower mafic volcanic rocks" and a 2 m thick cherty oxide-sulfide iron formation. This stratigraphic succession is generally near vertical and faces to the north and east. Several granitoid masses composed of hornblende ? biotite tonalite to granite, and locally rimmed with contact intrusive breccia, have intruded the stratigraphy in the Kamiskotia area. These include the Turnbull Township tonalite, which exhibits mixed magma texture with fine-grained and locally pillowed KGC rocks, and the Cote Township tonalite, which has a well-developed foliation parallel to its margin. The westernmost extent of the Destor-Porcupine fault zone, a major brittle-ductile shear zone that extends hundreds of kilometres across the Abitibi Subprovince, is present in the lower mafic volcanic rocks to the south. Alkaline rocks, represented by an unusual suite of lamprophyre dikes, and mesothermal lode-gold mineralization are associated with the fault zone in this region. The north-northwest-trending Matach- ewan mafic dike swarm cuts all stratigraphy and the Destor- Porcupine fault zone.

The centrally located granitoid rocks are typical of regional granitoid rocks that constitute a major of granitoid- greenstone terranes of the southern Superior Province. They are composed of several discrete plutons that range in composition from trondhjemite-tonalite to granodiorite-granite, including the Fortune Township granodiorite and the Groundhog River tonalite. Each pluton is defined by concentric foliation patterns near their margins which may be the product of diapir-like emplacement and subsequent in situ ballooning (Barrie and Davis 1990).

In the Montcalm area, the MGC is in contact with pre- dominantly mafic volcanic rocks to the south and undifferen- tiated volcanic and sedimentary rocks to the north. The MGC forms a crescent s h a ~ e within the volcanic rocks and can be divided into three zones: a pyroxenite zone in the extreme northwestern part, a gabbro - anorthositic gabbro zone in the north-central part, and a ferroan gabbro zone to the south and west. The Montcalm Ni-Cu deposit (3.6 Mt; 1.4% Ni, 0.7% Cu) is located in the northernmost part within cumulate gabbroic rocks and is cut by numerous dikes that range in composition from lamprophyric to pyroxenitic to granodioritic. The geology and tectonic setting of the MGC and Ni-Cu deposit have been described in detail in Barrie and Naldrett (1989) and Barrie et al. (1990).

A chronologic summary of the magmatic and tectonic activity in the Kamiskotia-Montcalm area is presented in Fig. 3. U-Pb zircon ages indicate that tholeiitic magmatic activity occurred from 2707 to 2702 Ma, coincident with or followed by granitoid

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CAN. J. EARTH SCI. VOL, 28, 1991

Granitoid - Greenstone

Metasedimentary

High-grade Gneiss

I+ Plutonic

1. Location of the study area within

/ \ w the Superior Province (modified after Card and Ceisielski 1986).

- 4 ~ ~ 4 0 '

Volcanic rocks, + + + + + undifferentiated

FIG. 2. Simplified geology of the Kamiskotia-Montcalm area, with sample locations. KGC, Kamiskotia gabbroic complex; MGC, Montcalm gabbroic complex; Kb, Karniskotia basalts; Kr, Kamiskotia rhyolites; LMV, lower mafic volcanics; CTT, Cote Township tonalite; TTT, Tumbull Township tonalite; GRT, Groundhog River tonalite; FTG, Fortune Township granite.

emplacement at 2707-2694 Ma. The Cote Township tonalite has a discordant zircon population with a 2 0 7 ~ b / 2 0 6 ~ b age of 2926 Ma, which has been interpreted as representing an inherited component from an older sialic component. The timing of deformational events, constrained by geologic rela- tionships along with U-Pb ages, is coincident with and in part related to the magmatism. Bulk crustal rotation responsible for the north- and northeast-facing stratigraphy in the Kamiskotia

area occurred between 2705 and about 2694 Ma, followed by contact-strain development in aureoles or zones surrounding the granitoid plutons at 2696-2692 Ma. North-south compression postdated contact-strain development for the Groundhog River tonalite and is possibly dated by closure of the U-Pb system in titanite from this intrusion at 2692 Ma. A minimum age for regional north-south compression and transpressive movement along the westernmost Destor-Porcupine fault zone (Fig. 2)

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BARRIE AND SHIREY

Age Magmatic events Deformation events

2 9 2 6 + ~ a Older sialic crust (inherited zircon in CTT)

b2707 Ma Lower mafic volcanic rocks

2 7 0 7 - Kamiskotia gabbroic complex

2 7 0 5 Ma Kamiskotia rhyolite Turnbull Township tonalite Onset of regional crustal warping

2 7 0 2 Ma Montcalm gabbroic complex T

2 6 9 6 Ma Cote Township tonalite crr contact-strain development

2 6 9 4 Ma Groundhog River tonalite GRT contact-strain development

(zircon) 'I

cooling(?J

t North-south compression

2 6 9 2 Ma (titanitel

Transpression: development of shear zones

along the western Destor-Porcupine fault zone

2 6 8 7 M a Brlstol Township lamprophyre suite

(garnet, titanite)

FIG. 3. Summary of U-Pb ages for magmatic and tectonic events in the Kamiskotia-Montcalm area (modified after Barrie and Davis 1990). CTT, Cote Township tonalite; GRT, Groundhog River tonalite.

may be constrained by a U-Pb age for a mildly deformed, lamprophyre-related garnetite dike within the fault zone, at 2687 * 3 Ma (U-Pb garnet and titanite, Barrie 1990b).

Results Samples were selected to represent the compositional range

of igneous rock types from the study area; their locations are shown in Fig. 2. Their major- and trace-element compositions are presented in Table 1, and rare-earth element (REE) plots in Fig. 4. Sm-Nd and Rb-Sr isotope results for whole rocks are given in Table 2, and for mineral separates in Table 3. Element abundances determined by isotope dilution are included in Table 1 for the whole rocks and in Table 3 for the mineral separates. Petrological descriptions and more precise sample locations are given in the Appendix, and a detailed discussion of the geochemical and isotopic analytical techniques is given in Barrie (1990a).

Whole-rock geochemistry and isotope data For this study, the "epsilon" notation is used in the presenta-

tion of the Nd-isotope data. The initial 1 4 3 ~ d / 1 4 4 ~ d values are expressed as eNd(t) values, or the deviation in parts per 10 000 from a chondritic uniform reservoir (CHUR) of the same age. They are calculated as 104[(143Nd/144~d)(,)/(143~d/ 144~d)CHUR(,, - 11, using U-Pb ages, and CHUR values of 0.1967 and 0.51 2638 for 147Sm/144~d and 1 4 3 ~ d / 1 4 4 ~ d , respectively (Faure 1986). Depleted mantle (DM) model ages (tDM) are the age of separation from a maritle source with a 147Sm/144Nd = 0.220, and an eNd(t) value of +2.7 at 2.7 Ga, that has evolved from a chondritic source since 4.5 Ga. This is based on a corrected value for Alexo komatiites after Dupre et

al. (1984), using 2714 Ma, a U-Pb age for adjacent rhyolites from Corfu et al. (1989). The calculated values for this model do not deviate significantly from the depleted mid-ocean-ridge basalt (MORB) mantle model of De Paolo (1980). Model ages for samples with '47Sm/144Nd within 15% of the depleted mantle value of 0.220 have greater errors (shown within paren- theses in Table 2). The eNd(t) values are considered to reflect a light rare-earth element (LREE)-depleted mantle source if they are within 2u analytical error (generally 0.5 epsilon units) of +2.7. They are considered to reflect a long-term, LREE- enriched source with respect to the LREE-depleted mantle source if they have values lower and outside of the 2u analytical error for the depleted mantle model. For the Sr-isotope data, initial 87Sr/86Sr values are expressed without reference to the chondritic or isotopically depleted mantle reservoirs and are calculated using U-Pb ages.

The Kamiskotia suite is represented by two ferroan gabbros, a dacitic granophyre, a rhyolite, and a basalt (samples 1-5, respectively). The suite is characterized by relatively flat REE patterns (Fig. 4a) and a restricted range of eNd(t) values from +2.2 to + 2.6 (Table 2). Ferroan gabbroic rocks range from 8 to 12 x chondritic values, with positive Eu anomalies that decrease upsection. They are cumulates of predominantly clinopyroxene, plagioclase, and Fe-Ti oxides. The Kamiskotia basalt sample is slightly enriched in the REE with respect to Kamiskotia chill-margin compositions (Fig. 4a) and the cumulate ferroan gabbros, and has no Eu anomaly. The Kamiskotia rhyolite sample has REE pattern typical of high-silica rhyolites found in bimodal volcanic suits of the Basin and Range (Crecraft et al. 1981; Bacon and Metz 1984) and elsewhere in the Superior Province (e.g., MacGeehan and MacLean 1980; Ashwal et al. 1983; Lesher et al. 1986). It is characterized by elevated

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62 CAN. J , EARTH SCI. VOL. 28, 1991

TABLE 1 . Major- and trace-element

Sample no. : 1 2 3 4 5 6 7 8 9 Field no.: 87- 12g 84-402 88-1 GF-2 88-2 84-86 86-7 86-6 &6-427

-

Si02 (wt. %) 44.3 Ti02 1.98 A1203 17.4 Fez03 17.9 MnO 0.17 MgO 2.72 CaO 10.60 NazO 2.12 K20 0.22 p205 0.18 LO1 2.39

Total 100.0 100.4 99.8

NOTES: Major elements were determined by X-ray fluorescence spectrometry (XRF) at X-Ray Assay Laboratories Limited, Toronto. Trace elements by instrumental neutron-activation analysis and XRF, and by isotope dilution for Sm and Nd; Rb and Sr by isotope dilution or by XRF. Mg' = mol% MgO x 100/(Mg0 + FeO), with FeO/FeO ,,,,, = 0.85. LOI, loss on ignition.

high-field-strength (HFS) incompatible-element contents and high RbISr and K20/Na20 ratios. The dacitic granophyre sample has a similar but lower REE pattern, lower HFS contents, and lower Rb/Sr and K20/Na20 ratios in comparison with the rhyolite.

Lower mafic volcanic rocks have flat to slightly LREE- enriched REE patterns, with Ce/Yb ranging from 2.3 to 10.6 (Fig. 4b) and eNd(t) values ranging from + 1.7 to + 3.0. Sample 6 has higher Cr and Ni contents and a higher Mg number despite its higher SO2, Zr, and Y contents and CeIYb ratio in comparison with samples 7 and 8. This would suggest that it was derived from a chemically distinct source, or possibly under different melting conditions.

MGC samples were taken from the gabbro - anorthositic gabbro zone immediately stratigraphically above the Ni-Cu deposit (samples 9 and 10) and from an outcrop with a fine-grained chill-like texture in the ferroan gabbro zone (sample 11). Petrographic observations indicate that samples 9

and 10 are adcurnulates, reflected in their low REE and HFS element abundances, whereas sample 1 1 is an orthocumulate (Fig. 4c; Table 1). The MGC has steeper REE patterns than the KGC, with CeIYb ranging from 9.3 to 28.6, and it has more variable eNd(t) values that range from +2.5 and +2.8 in the gabbro - anothositic gabbro zone to + 1.0 in the ferroan gabbro zone.

Granitoid rocks represented by samples 14-18 plot in the granite, granodiorite, and tonalite fields on modal Q-A-P and normative Or-Ab-An diagrams. They can be divided into two groups on the basis of their location and their ~ ~ ~ ( t ) values (Fig. 2; Table 2). One group, which includes samples 14, 15, and 18, represents smaller p n i t o i d bodies internal to the boundaries of the greenstone rocks. These granitoids have high ~ ~ ~ ( t ) values from -t 2.7 to + 3.8. The other group includes samples 16 and 1 7 and is located in the central granitoid region. These granitoids have low eNd(t) values of +0.6 and -0.4, respectively. Initial strontium isotopic ratios (Table 2) are lower than estimates for

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I

BARRIE AND SHIREY

data for samples 1- 19

10 11 12 13 14 15 16 17 18 19 86-426 87-42 86-418 86-392 87-12t 86-101 TTG- I 86-220 86-222 87-2

46.9 49.3 49.2 50.2 76.2 73.9 71.4 71.9 68.0 36.10 0.16 1.07 0.81 0.83 0.18 0.35 0.17 0.15 0.56 2.27

21.3 16.8 11.7 12.7 13.1 12.5 15.6 15.7 14.4 7.68 5.4 12.3 8.0 7.9 1.3 3.3 1.8 0.8 4.3 20.30 0.11 0.19 0.13 0.12 0.03 0.06 0.03 0.02 0.08 0.46 8.46 6.5 11.40 9.46 0.22 0.78 0.70 0.41 1.24 1.78

11.55 10.60 9.55 7.83 1.21 2.64 2.82 2.00 4.29 25.7 1.48 2.28 3.30 4.31 4.89 4.00 5.25 6.48 4.38 1.44 0.34 0.16 0.44 1.38 2.41 1.24 1.75 1.91 1.17 1.16 0.02 0.22 0.42 0.54 0.05 0.07 0.06 0.06 0.14 0.46 2.98 0.62 3.93 3.31 0.62 1 .OO 0.39 0.54 1.47 2.30

98.2 100.1 98.9 98.6 100.2 99.8 99.9 100.0 100.0 99.75

the depleted MORB-like mantle (e .g., 0.70 105, Machado et al. 1986). Whole-rock Rb-Sr isotope data for granitoids of the Superior Province in general exhibit open-system behavior or have been reset by later thermal events (e.g, Birk and McNutt 1981; Turek et al. 1982; Beakhouse et al. 1988). Therefore, little emphasis is given to the petrogenetic significance of this system.

Lamprophyric rocks in the Kamiskotia-Montcalm area are

1 represented by samples 12 and 13, from lamprophyre dikes in the Montcalm deposit area, and sample 19, a sample of a garnetite dike from the Bristol Township lamprophyre suite.

, The Montcalm area lamprophyre samples are characterized by

I high Cr, Ni, Th, LREE, and P2O5 values and steep REE patterns (Fig. 4e). Although samples 12 and 13 were taken from separate dikes more than 40 m apart, they have a similar geochemistry and probably represent the same intrusive event. Their major-element contents are similar to the means for "calc-alkalic" lamprophyres (Rock 1987), except for their MgO

contents, which are significantly higher than the mean (1 1.4 and 9.5%, respectively, compared with 6.9 + 2.6%, n = 754; Rock 1987). The samples have nearly identical eNd(t) values of + 2.8 and +2.5, respectively. Sample 12 has an Isr(t) value of 0.701 5. Sample 19 contains abundant Zr-rich melanite garnet, titanite, apatite, and epidote. It is extremely enriched in the LREE, Zr, Y, Hf, and Th, significantly enriched in the heavy REE, Nb, and U, but not particularly enriched in Cr or Ni. The whole-rock eNd(t) and Isr(t) values for this sample are + 1.0 and 0.7013, respectively (Table 2).

Isochrons and regression ages for whole rocks and mineral separates

Mineral separates for four whole-rock samples were analyzed for Sm-Nd and Rb-Sr isotopes to produce mineral - whole- rock isochrons (Figs. 5-8). The McIntyre 1 regression model (McIntyre et al. 1966) has been used to calculate regressions for these data. If the regression has a mean square of the

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50 Internal

10

5

L - I I I 1 1

'- ( e l

Garnetife

50

10 11 I I l l I 1 I 1 1 1 I

La Ce NdSmEu Tb Y bLu La Ce Nd SmEuTb YbLu

FIG. 4. Chondrite-normalized REE plots using the values of Evensen et al. (1978). The REE are spaced proportional to their ionic radii for a + 3 valence. Open circles represent Tb values from extrapolation between Sm and Yb. For the Kamiskotia suite, the shaded region represents the range for Kamiskotia gabbroic complex chill-zone compositions (n = 5; Barrie 1990a). Granitoid samples internal (14, 15, 18) and external (16, 17) to the boundaries of greenstone rocks are labelled. ( a ) Kamiskotia suite; (b ) lower mafic volcanic rocks; ( c ) Montcalm gabbroic complex; (d) granitoids; ( e ) lamprophyres.

weighted deviates (MSWD) of less than 3, the scatter in the data can be accommodated by analytical uncertainty, and the regression line is termed an isochron. Ages and initial ratios for the regression lines are given in Table 4.

A seven-point Sm-Nd isochron for the Kamiskotia suite, including plagioclase and clinopyroxene analyses, has an age of 2710 2 30 Ma (2-u error; Fig. 5a), in complete agreement with the U-Pb ages of 2707 t 2 Ma for a KGC pegmatitic gabbro and 2705 k 2 Ma for a Kamiskotia rhyolite (Barrie and Davis 1990). This isochron has an INd value of 0.50925 * 4, which corresponds to an eNd(t) value of 2.5 * 0.8, representing the primary igneous value for the entire Kamiskotia suite. The zircon fraction (sample 22) was not included in the regression. It plots just outside of analytical error of the isochron. It has very low Sm and Nd concentrations of 2.1 and 1.1 ppm, respectively (SrnlNd = 1.88), and an extremely high 1 4 7 S d ' 4 4 ~ d ratio of 1.135. In felsic and intermediate rocks, zircon has 5- 16 pprn Sm, 15-90 pprn Nd, and S d N d ratios of 0.18-0.36 (e.g., Gromet and Silver 1983; Fujimaki 1986). The low concentra- tions and extreme LREE depletion may reflect local fraction- ation of a LREE-enriched phase such as allanite within the subpegmatitic pocket in the KGC upper zone. An Rb-Sr isochron for a ferroan gabbro sample and clinopyroxene and zircon frac- tions has an age of 2450 t 35 Ma (Fig. 5 b). This age is identical within error to three U-Pb discordia ages from regressions of baddeleyite and zircon fractions, all at 2452'; Ma, from the Hearst - Matachewan mafic dike swarm (Hearnan 1989) and one baddeleyite fraction (207Pb/206Pb minimum age of 2460 Ma: L. Heaman, personal communication, 1989) taken from a Matachewan dike 12 km from this location.

The regression of isotope data for mineral separates from

samples 14 and 15 produces ages younger than their zircon and titanite U-Pb ages. For the Cote Township tonalite, a whole- rock - piagioclase - apatitel - apatite2 Sm-Nd isochron has an age of 2615 k 15 Ma (Fig. 6), significantly younger than the U-Pb age for the same sample of 2696 k 4 Ma (Barrie and Davis 1990). This age represents essentially a two-point isochron, controlled by the apatite fractions with their similar isotopic ratios, and the precision of the age error should be considered with some caution. The zircon fraction does not fall on the isochron. This fraction was hand culled but still contained metamict zircons with minor inclusions. The metamict zircons are believed to be responsible for the fraction's nonsystematic behaviour. This fraction has 79 pprn Sm and 159 pprn Nd, significantly higher than zircon separates from other intermedi- ate and felsic rocks (Gromet and Silver 1983; Fujimaki 1986). The Rb-Sr system does not yield meaningful results, indicating that Rb or Sr or both were open to remobilization after crystallization in this sample.

For the Groundhog river tonalite, regressions of the Sm-Nd and Rb-Sr data give ages that are identical at 2530 t 35 Ma (Fig. 7), younger than the U-Pb zircon age of 2696 5 2 Ma, and a U-Pb titanite age of 2692 2 5 Ma (Barrie and Davis 1990). The agreement between the Sm-Nd and Rb-Sr isotope systems is strong evidence that this age has geological significance. Similar ages of 2530,2508, and 2504 Ma have been reported for individual, concordant metamorphic titanite grains in granulite- and upper-amphibolite-grade rocks of the nearby Kapuskasing Structural Zone 40 km to the west (Krogh et al. 1988). The titanite fraction has 357 pprn Sm, 964 pprn Nd, 2.6 pprn Rb, and 596 pprn Sr. The Sm and Nd concentrations are about half those of Gromet and Silver's (1983) granodiorite titanite fraction

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BARRIE AND SHIREY 65

1

TABLE 2. Whole-rock Nd and Sr isotopic data

Sample U-Pb ageb

No. Field no. Rock unit 147Sm/144~da 1 4 3 ~ d / 1 4 4 ~ d a 2 20 (Ma) eNd(t)' tDMd 8 7 ~ b / 8 6 ~ ~ 8 7 ~ r / 8 6 ~ P 2 20 Isr(t)'

1 87-128 Kfgb, 0.1643 0.512186? 19 2707 2.47 2750 2 84-402 Kfgbz 0.1927 0.512684221 2707 2.32 (2880) 0.0321 0.701981?64 0.700722

Replicate 0.5126802 18 2707 3 88-1 Kgphyre 0.1348 0.511647?14 2707 2.24 2760 4 GF-2 Kr 0.1332 0.511639218 2705 2.65 2710 5 88-2 Kb 0.1624 0.512160223 2705 2.62 2720 6 84-86 LMV 0.1385 0.511684219 2720* 1.79 2820 7 86-7 LMV 0.1993 0.512770220 2720* 1.69 (3390) 8 86-6 LMV 0.1927 0.512719+-18 2720* 3.01 (2600) 9 86-427 Mgb 0.1248 0.511498231 2702 2.79 2700

10 86-426 Mgb 0.1061 0.511 143?24 2702 2.35 2730 11 87-42 Mfgb 0.1339 0.511567237 2702 0.97 2880 12 86-418 MLD 0.1024 0.5110985 17 2700 2.75 2700 0.0535 0.70355029 0.701458 13 86-392 MLD 0.1006 0.511053218 2700 2.51 2720 14 87-12t 'MT 0.1092 0.5112262 18 2707 2.95 2690 15 86-101 CTT 0.1202 0.5114732 19 2694 3.83 2600 0.5228 0.720285214 0.699899 16 TTG-1 GRT 0.1884 0.512520? 15 2896 0.60 (3430) 0.0755 0.7040232 15 0.701075

1 17 86-220 FTG 0.1351 0.511526217 2690 -0.37 3010 18 86-222 MGD 0.1079 0.511195?20 2700 2.73 2700 19 87-2 GD 0.1379 0.511648k 13 2687 1.02 2870 0.00365 0.701401 2 9 0.701259

NOTES: Samples 1-5: Kamiskotia ferroan gabbro,, ferroan gabbro,, granophyre, rhyolite, basalt; 6-8: lower mafic volcanic rocks; 9-1 1: Montcalm gabbros, ferroan gabbro; 12, 13: Montcalm calc-alkalic lamprophyre dikes; 14: Turnbull Township tonalite; 15: Cote Township tonalite; 16: Groundhog River tonalite; 17: Fortune Township granodiorite; 18: Montcalm granodiorite dike; 19: garnetite dike.

"Measured values. b U - ~ b ages from Banie and Davis (1990) and Banie (1990b) with 2u errors of 2 1.5 to 4 Ma. Asterisk indicates >2707 Ma, inferred to be 2720 Ma

considering other volcanic rocks in the region. 'Calculated as 104[('43~d/144~d)(,)/(143Nd/144~d)CH~~~, - 11 using U-Pb ages and CHUR values of 0.1967 and 0.512638 for '47Sm/'44Nd and 143Nd/1ezNd,

respectively. dDepleted mantle model ages: see text for discussion. 'Calculated using U-Pb ages.

(655 ppm Sm, 2680 ppm Nd), and Rb and Sr are 7-10 times Discussion higher than those for a titanite from an Archean felsic intrusion Petrogenesis (0.3 ppm Rb and 82 ppm Sr; Hanson et al. 1971). Interestingly, Karniskotia trace-element and isotope data are consistent the bulk zircon fraction plots well below the Sm-Nd isochron, with direct derivation from a primitive mantle source that has yet falls on the isochron in the Rb-Sr system. The extreme undergone long-term Nd/Sm depletion. Barrie (1990~) noted disturbance found in the zircon's Sm-Nd system may indicate that the primitive magmas parental to the KGC can be repre- LREE depletion and (or) heavy REE enrichment of metamict sented by a 1:1 mixture of olivine-rich adcumulates at the base zircon grains after crystallization, whereas the Rb-Sr system of the intrusion and chilled liquid compositions from the mixed was reset at 2530 Ma and remained unaffected by any subsequent magma outcrops: this primitive magma is consistent with a 34% metamorphic, meteoric, or hydrothermal activity. partial melt of a chemically primitive, lherzolitic mantle. The

A three-point whole-rock - melanite - epidote Sm-Nd chill-zone samples and indeed the entire Kamiskotia suite have isochron for the garnetite dike is calculated at 2500 2 160 Ma primitive mantle-like immobile, incompatible-element ratios (MSWD = 0.37), significantly younger then the U-Pb garnet- (Fig. 9, 10). These ratios should not vary appreciably during titanite age of 2687 2 3 Ma (Barrie 1990b). The higher error in closed-system fractionation or the addition of co-genetic low- age is due to a restricted range in 147Sm/144Nd. The plagioclase pressure basaltic melts. This indicates that throughout its fraction does not plot on the isochron. Almost all of the crystallization history, the Karniskotia suite has not undergone plagioclase grains included in the hand-culled fraction were significant contamination by crustal material. Previous studies opaque, and many contained visible inclusions of alteration of Abitibi ultramafic rocks that have not assimilated significant products. If the plagioclase data are included, the regression crustal material have found long-term, LREE-depleted eNd(t)

1 line gives an age of 2270 k 50 Ma (MSWD = 6.03). The values of +2.5 2 0.3 (using 2702 Ma; Machado et al. 1986) high-purity melanite fraction has unusually high Sm and Nd and +2.4 + 0.5 (using 2752 Ma; Dupre etal. 1984), interpreted

I concentrations of 217 and 918 ppm, respectively, with a as reflecting Abitibi mantle compositions. These are identical to 147Sm/144~d ratio of 0.143, lower than the chondritic value of those of the Kamiskotia suite. 0.1967. Generally, such LREE enrichment in garnet is not The Nd isotopic data for the Kamiskotia suite support found in siliceous systems (e.g., Schnetzler and Philpotts 1970; geological and geochemical evidence for a cogenetic relation- Shimuzu and Kushiro 1975; Nicholls and Harris 1980). Rb- and ship between the coeval KGC and Kamiskotia volcanic rocks. Sr-isotope data for the garnitite dike do not exhibit closed- Previous studies have noted that volcanogenic massive sulfide system behavior (Tables 2, 3). deposits within the Kamiskotia basalts and rhyolites are likely

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BARRIE

FIG. 5. Isochrons for the Kamiskotia suite. Closed circles are data used for regression calculations; open circle not included in regres- sions. Analytical errors are given in Tables 2 and 3 and fall within the circles. (a) Sm-Nd whole-rock (1-5) and mineral isochron; mineral analyses (20, clinopyroxene; 21, plagioclase) from whole-rock sample 2; zircon (22) not included in regression. (b) Rb-Sr whole-rock (2) and mineral (20, clinopyroxene; 22, zircon) isochron for a ferroan gabbro sample; plagioclase (21) not included in regression.

FIG. 6. Sm-Nd whole-rock (15) and mineral (23, plagioclase; 24,, apatite,; apatite2) isochron for the Cote Township tonalite. Zircon (25) not included in regression.

the product of hydrothermal convection driven by heat produced by the underlying KGC during crystallization (e.g . , Campbell et al. 1982; Hart 1984). Kamiskotia basalts may represent fractionation products of a primitive tholeiitic parental liquid similar to KGC chill-zone compositions, with the residual phases represented in the KGC (Barrie 1 9 9 0 ~ ) . The Kamiskotia rhyolites and the underlying felsic granophyres have nearly

AND SHIREY

(b) 0 . 7 0 6 -

0.705 - 2530 -L 35 Ma

FIG. 7 . Regression lines for the Groundhog River tonalite. (a) Sm-Nd whole-rock (16) and mineral (26, plagioclase; 27, titanite; 28, apatite) data; zircon (29) not included in regression. (b) Rb-Sr whole-rock (16) and mineral (26, plagioclase; 27, titanite; 28, apatite; 29, zircon) data.

identical REE abundances, suggesting that the rhyolites are eruptive equivalents of the granophyre (Campbell et al. 1982). Both have distinctive, high, flat chondrite-normalized patterns with strong negative Eu anomalies. The rhyolites have been modeled by either 10% partial melting of the primitive basalts or by 30% partial melting of evolved Kamiskotia basalts by Hart (1984). Alternatively, they may represent residual liquids after fractionation of highly evolved basaltic andesites that are present in the volcanic sequence, similar to a model proposed for the felsic Tinden Sill and the adjacent Skaergaard Intrusion of East Greenland (Hunter and Sparks 1987).

In contrast with the restricted Nd isotopic compositions and trace-element ratios for the Kamiskotia suite, the Montcalm data generally decrease upsection and suggest that contamina- tion by an older crustal component occurred during fraction- ation. The eNd(t) values decrease upsection and are accom- panied by decreasing Ni and Cr contents and Mg numbers which mark the fractionation of mafic phases, sulfide, and chromite (Fig. 8; also Banie et al. 1990). Contamination by crustal material is consistent with models for the genesis of the Montcalm Ni-Cu deposit. Magmatic sulfide deposits may form due to silification of a mafic magma prior to, during, or after emplacement at supracrustal levels (Irvine 1975; Mainwaring and Naldrett 1977).

The major- and trace-element geochemistry of the granitoid suite is typical of Archean trondhjemite-tonalite-granodiorite ( n G ) suites found worldwide (e.g., Jahn etal. 1981; Gower et al. 1983; Martin 1987). Martin (1987) suggested a three-stage

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CAN. 1. EARTH SCI. VOL. 28, 1991

FIG. 8. Trace elements and trace-element ratios vs. eNd(t) . ( a ) Ni vs. e N d ( t ) (b ) Cr vs. e N d ( t ) (c) CeIYb vs. eNd(t) . (d) TWTa vs. ~ ~ , j ( t ) . Error bar represents typical 2 a analytical error in ' 4 3 ~ d / 1 4 4 ~ d . 0, Kamiskotia suite; A lower mafic volcanic suite; M, Montcalm suite; 0, granitoid suite; e, Montcalm lamprophyre dikes; *, Bristol township lamprophyre garnetite dike. Broken lines encircle Kamiskotia, Montcalm, and granitoid suites. T, average of Timiskaming alkalic volcanic rocks ( n = 12), from Basu et al. (1984) and A. M. Goodwin (unpublished data), using 2689 Ma; A, average of Alexo komatiites, with Nd data ( n = 11) from DuprC et al. (1984) and Ce and Yb data ( n = 7 ) from Whitford and Amdt (1978), using 2714 Ma. Alexo ThITa average of 2.3 estimated using primitive-mantle values in Hofmann (1988), in light of similar NbITh values between Alexo and primitive mantle (Jochum et al. 1988).

TABLE 4 . Summary of ages from Sm-Nd and Rb-Sr isotope data

Initial ratio for regression

Samples System Age (Ma) MSWDa or isochron

Kamiskotia suite

1-5, 20, 21 Sm-Nd 2710230 2.77 0 .50925t4 2 ,20 , 22 Rb-Sr 2450235 0.29 0.7008524

Cote Township tonalite

15,23,241,242 Sm-Nd 2615215 2.24 0.5094022

Groundhog River tonalite

16,26-28 Sm-Nd 2530235 14.0 0.5094124 16, 26-28 Rb-Sr 2530k35 8.7 0.7012422

Bristol Township lamprophyre suite

19, 31, 32 Sm-Nd 2500k 160 0.37 0.50959*7

NOTES: Ages and initial ratios with 2u errors calculated using a McIntyre 1 model least-squares cubic regression program (McIntyre et al. 1966), with 2u errors for '43Nd/1"Nd and 87Sr/s6Sr = 0.000040, 147Sm/144Nd = 0.3%, 87Rb/86Sr = 2.0%.

"Ages with MSWD values (3 indicate geological scatter inside of analytical error and represent isochrons. See text for discussion.

process for derivation of eastern Finland TTG parental magmas, which may apply to the granitoid suite here: first, melting of the upper mantle to produce a tholeiitic crust; second, transforma- tion of this tholeiitic material to a garnet-bearing amphibolite; and third, partial melting of the garnet-bearing amphibolite to produce the TTG parental magmas. The final TTG composition may reflect fractionation from, and (or) the addition of, cumulate phases to the parental magma during crystallization.

Addition of cumulus feldspar adequately explains the inverse relationship between Al2O3 and HFS elements and REE contents for the granitoid suite.

The central granitoids have low eNd(t) values and tDM ages greater than 3.0 Ga, implying that they were derived from, or contaminated by, a source relatively enriched in Nd that evolved separately from a long-term, LREE-depleted mantle for >300 Ma. This indirect evidence for the presence of an older crustal component in the Kamiskotia-Montcalm area is sup- ported by the presence of a 207~b*/206~b* (minimum) age of 2926 Ma found in one zircon fraction from the Cote Township tonalite (Barrie and Davis 1990). Elsewhere in the Abitibi Subprovince, there is scant evidence for the presence of older crust. Three individual zircon grains in the Pontiac Group metasedimentary rocks south of Noranda, Quebec, have slight- ly discordant 207~b/206Pb minimum ages between 2900 and 2950 Ma (GariCpy et al. 1984), and Pb-isotope data for granitoid rocks in the northeastern Abitibi Subprovince are consistent with derivation from 3.4 to 3.0 Ga sialic crust (Gariepy and Allbgre 1985).

The Montcalm lamprophyre dikes are similar in petrology and geochemistry to many lamprophyres found in the southern Superior Subprovince (McNeil and Kerrich 1986; Wyman and Kerrich 1989). Rocks of this composition, with high MgO, Ni, Cr, Th, and LREE abundances and high Ce/Yb ratios (Fig. 8), are generally considered to be the products of partial melting of a metasomatically enriched mantle, with or without crustal contamination (e.g., Bachinski and Scott 1979; Rock 1987). The eNd(t) values for the Montcalm dikes are identical to the values for the depleted Abitibi mantle. This indicates that they were derived directly from a portion of the depleted Abitibi mantle that was subjected to metasomatic enrichment shortly before lamprophyre extraction. This is clearly represented in Figs. 8c and 8d, where the dikes plot with high Ce/Yb and

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0.1 1 10

Ta/Yb FIG. 9. ThIYb vs. TaIYb. Vertical field represents Kamiskotia chill

zone and primitive mafic volcanic rocks (n = 11) and closely corresponds to N-type MORB field (Pearce 1983). Horizontal field represents Timiskaming alkalic volcanic rocks (A. M. Goodwin, unpublished data) and closely corresponds to Vulsini alkalic volcanics (Rogers et al. 1985). Diagonal array corresponds to mid-ocean-ridge and intraplate basalts and alkalic volcanic rocks (e.g., West African rift-related potassic lavas; Mitchell and Bell 1976). Vectors shown for subduction-zone enrichment (S), crustal contamination (C), within- plate enrichment (W), and fractional crystallization (F) in basaltic systems (from Pearce 1983).

ThITa ratios directly above the Abitibi depleted mantle repre- sented by the Alexo komatiites. In many respects, the Montcalm dikes are similar to more siliceous, monzodiorite-trachyandes- ite suites found in northwestern Ontario. In the Rainy lake area, Shirey and Hanson (1984, 1986) considered monzodiorites and trachyandesites together as a sanukitoid suite (named after enriched magnesian andesites in Japan), with relatively high MgO, Ni, and Cr contents for a given Si02 content, accom- panied by Sr, Nb, Zr, and LREE enrichment. Lower eNd(t) values for the sanukitoid suite ranging from + 1.0 to + 1.6 were interpreted as reflecting a direct derivation from a slightly less depleted mantle composition under this region of the Superior Province, with no significant crustal interaction (Shirey and Hanson 1986).

Geological relationships and geochemical data suggest that the Bristol Township lamprophyre garnetite dike is a product of fractionation under volatile-rich conditions, from a parental magma represented by adjacent, biotite lamprophyre rocks. The biotite lamprophyres are rich in apatite and carbonate and have low Si02 contents (32.6-38.0 wt.%) and high MgO (9.7-12.0 wt.%), K20 (0.6-4.5 wt.%), P2O5 (1.1-5.8 wt.%), and C 0 2 (4.0-9.0 wt.%) contents (n = 4; C. T. Barrie and D. Wark, unpublished data). Using the classification of Rock (1 986), the Bristol Township lamprophyre suite is most similar to aillikitic ultramafic lamprophyres, distinguished from carbonatites by a relatively low carbonate content, and from kimberlites by a high CaO content, and the association with high-Ti - high-Zr garnet (>5.0 wt.% TiOz, >3500 ppm Zr; C. T. Banie and D. Wark, unpublished data). There is no consensus over the origin of such carbonate-rich lamprophyric magmas and their fractionated derivatives, although direct melting of a metasomatically enriched mantle is gaining increasing acceptance (see Rock 1986 for review). For the garnetite dike sample, the low eNd(t) value of + I .O would suggest that it was derived from a source with some previous LREE enrichment relative to the depleted

AND SHIREY 69

Age (Ma) FIG. 10. Sm-Nd isotope data vs. age for Abitibi suites. (a)

'47Sm/'44Nd VS. age. (b ) eNd(t) VS. age. Symbols as in Fig. 8, with Kamiskotia, Montcalm, and Montcalm calc-alkalic lamprophyre dike suites averaged. Ages from stratigraphic relationships and U-Pb data, from Cattell et al. (1984), Corfu et al. (1989), and Banie and Davis (1990). D, pyroxene separates from Dundonald ultramafic sill; HM, Hunter Mine Group pyroxenite; SR, Stroughton Roquemaure Group ultramafic rocks (Machado et al. 1986); A, Alexo komatiites, n = 11 (Duprk et al. 1984); MI, Mumo Township komatiites, n = 9 (Zindler 1982); M2, Pyke Hill komatiite, MUNO Township, n = 6 (Walker etal. 1988); N, Newton Township komatiite, n = 12 (Cattell et al. 1984); T, Tirniskaming alkalic volcanic rocks, n = 12 (Basu et al. 1984).

Abitibi mantle. The tDM of 2.9 Ga suggest one scenario where LREE enrichment of the depleted mantle occurred approxim- ately 200 Ma prior to extraction of the garnetite dike.

Tectonic setting and magma generation from mantle and crustal reservoirs

The trace-element and isotopic signatures of the KGC and related volcanic rocks are consistent with geological and U-Pb geochronological evidence for their formation in a continental- arc-like regime. The Kamiskotia bimodal volcanic assemblage with its massive sulfide mineralization is typical of settings generally considered to represent a rifted-arc or back-arc tectonic regime (e.g., Cathles et al. 1983). The presence of older crustal rocks nearby (described above) would suggest that rifting must have been within or proximal to an ensialic crust, possibly a continental margin. One notable feature about the geochemistry of the KGC chill margins and basalt is their MORB-like trace-element signatures, exemplified by their ThIYb and TaIYb ratios (Fig. 9). The trace-element contents of KGC chill-margin rocks and Kamiskotia basalts are consistent with low-Pressure tholeiitic fractionation from a partial melt, derived from a chemically primitive mantle peridotite at shallow depths, with little or no contamination by crustal components (Barrie 1 9 9 0 ~ ) . Similarly, the Nd isotopic signature for the entire Kamiskotia suite is consistent with a direct derivation from a long-term, LREE-depleted Abitibi mantle with little or no interaction from long-term, LREE-enriched crustal material. In modem continental-arc settings, such as in the central

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Peruvian Andes and the Peninsular Ranges Batholith of Baja California, there is isotopic evidence that mantle-derived tholeiitic magmas rose through the lithosphere with little or no interaction from crustal material. Large gabbroic intrusions representing the earliest intrusive rocks of the Coastal Batholith were emplaced in a tectonically thinned, extensional marginal basin (Atherton et al. 1983). Pb- and Sr-isotope studies indicate minimal interaction with older crustal material for Coastal Batholith rocks (Barreiro and Clark 1984; Pitcher et al. 1985). Similarly, large gathroic intrusions in the Peninsular Ranges are sightly older and have depleted isotopic signatures (higher eNd(t) values and lower Is,( t), 2 0 6 ~ b / 2 0 4 ~ b ,207~b/204~b , and 2 0 8 ~ b / 2 0 4 ~ b values) relative to nearby granitoid plutons (Wala- wender 1976; Walawender and Smith 1980; DePaolo 1981 b).

The tectonic setting of the MGC is less well understood due to limited exposure. It was emplaced into a crustal substrate in a dynamic tectonic environment during the waning stages of volcanic activity in the Abitibi and Wawa subprovinces. Shortly after formation it was subjected to deformation from the central granitoid plutons (Barrie and Naldrett 1989). Relatively high CeIYb ratios throughout the intrusion indicate that parental magmas were enriched in LREE, and high eNd(t) values in the gabbro - anorthositic gabbro zone suggest a derivation from an isotopically depleted, MORB-like mantle. Contamination by an older, LREE-enriched crust during fractionation is inferred from decreasing Nd signatures with decreasing MgO, Ni, and Cr contents upsection during emplacement. Similar trace- element and isotopic variations seen in the Usushwana mafic- ultramafic intrusive suite of the Kapvaal Craton, South Africa and Swaziland, known to have been emplaced in a dynamic tectonic environment, have been explained by contamination by upper-crustal material (Hegner et a / . 1984). Increasing Isr(t) and decreasing eNd(t) values upsection is a common feature in large mafic intrusions emplaced in stable continental settings and has been interpreted as reflecting the addition of batches of more radiogenic magma from lower-crustal magma chambers (Sharpe 1985; Lambert et a/ . 1989).

Like the Montcalm gabbroic complex, the Montcalm dikes were emplaced in a dynamic tectonic setting, synchronous with a transition from extensional to compressional tectonism ob- served in the Kamiskotia area to the east (Fig. 3). In comparison with modem alkalic rocks, their trace-element contents and ratios are similar to primitive-alkalic magmas associated with convergent plate boundaries (Pearce 1983; Rogers et al. 1985). This is shown on the Th/Yb - TdYb plot (Fig. 9), where high ThIYb ratios are generally accompanied by large-ion lithophile element (LILE) and LREE enrichment. For modem settings, high ThIYb ratios for a given TdYb ratio are considered to be the result of enrichment of an upper-mantle source by a siliceous melt or fluid derived from subducted sedimentary material (Pearce 1983). Partial melting of this LILE- and LREE-enriched source would then produce lamprophyres like the Montcalm lamprophyre dikes. Other Abitibi alkalic suites have similar chemical characteristics (high MgO, Ni, Cr, LREE; low Nb, Ta), such as lamprophyre dikes spatially and temporally associated with gold deposits along the Destor-Porcupine and Larder Lake fault zones (Wyman and Kerrich 1989) and the Timiskaming Group basanites and trachytes near Kirkland Lake, Ontario (A. M. Goodwin, unpublished data) (Fig. 9).

The granitoid suite was emplaced during times of extensional tectonics associated with the KGC, and later during a more compressional regime (Barrie and Davis 1990). In this respect

the granitoid suite is similar to the Cretaceous Peninsular Ranges batholith of Baja California, the product of arc magmatism formed during subduction of the Pacific plate under the North American plate. Gromet and Silver (1987) found that tonalites and low-K granodiorites exhibit a systematic increase in CeIYb from west to east, generally accompanied by an increase in Is, values. These trends cannot be explained by high-level fractionation of major- and trace-silicate phases. They are consistent with partial melting of arc basalt under increasingly high pressures, leaving behind gabbroic or am- phibolitic residue at low pressures to the west and an eclogitic residue to the east (Gromet and Silver 1987). Contamination by older crustal material may account for the increase in Is, values to the east. The Kamiskotia-Montcalm granitoids have Ce/Yb ratios most similar to those of the central and eastern granitoids of the Peninsular Ranges, consistent with their derivation from a basaltic source at high pressures. Long-term, LREE-enriched Nd isotopic signatures and generally higher CeIYb ratios for the central granitoids, which represent a regional granitoid terrane that encompasses the greenstone belts of the southern Superior Province, are consistent with derivation from an older basaltic source at greater depths in this area. Further Nd isotopic studies of the regional granitoids are necessary to test whether there are areally distinct domains of older, long-term, LREE-enriched crust in the vicinity of the southern Abitibi Subprovince and elsewhere in the southern Superior Province.

Timing of extraction from mantle and crustal sources and implications for tectonic evolution in the southern Abitibi Subprovince

U-Pb geochronology indicates that perhaps half of the volcanic stratigraphy presently exposed in the southern Abitibi Subprovince predates the development of the KGC, with ages extending back to 2747 Ma (Mortensen 1987). From 2707 to 2698 Ma, extensional tectonism prevailed at least in the Kamiskotia and Noranda, Quebec, areas where bimodal volcan- ism was accompanied by significant massive sulfide forma- tion. At 2697 Ma, thoeliitic and komatiitic volcanism ceased in the southern Abitibi Subprovince, and indeed there are no known mafic or ultramafic volcanic rocks in the entire Superior Province younger than 2697 Ma. At this time, extensional tec- tonism gave way to north-south-directed compression (Barrie and Davis 1990). A late dextral sense of displacement has been documented at the boundaries of the Quetico Subprovince in northwestern Ontario (Corfu and Stott 1986; Poulsen 1984; Davis et al. 1989), and elsewhere in the Abitibi Subprovince 1 (Hubert et al. 1984; Dimroth et al. 1986), suggesting a common tectonic event across 1200 km of the southern Superior Province

I from 2690 to 2685 Ma. Late transpression was at least in part i responsible for the development of late deformation fabrics within the Destor-Porcupine fault zone and was generally coincident with alkalic magmatism and Au mineralization (Corfu and Stott 1986; Barrie and Shirey 1989; Corfu et al. 1989; Davis et al. 1989). The close spatial association between major fault structures and mantle-derived alkalic rocks in the southern Abitibi Subprovince implies that, at least locally, these faults served as conduits for alkalic magmas that extended to mantle depths (Kerrich et al. 1987; Wyman and Kerrich 1988).

Southern Abitibi magmatism and magma source characteris- tics are considered in a chronologic framework in Fig. 10. Abitibi komatiitic and tholeiitic suites with higher S d N d ratios and eNd(t) values from + 2 to +3 were emplaced from 2720 to

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BARRIE Ah ID SHIREY 71

2697 Ma. Their high eNd(t) values are consistent with minimal interaction with a long-term, LREE-enriched crustal compon- ent, typical of arc-related extensional regimes. Abitibi alkalic rocks with the lowest S d N d ratios and a range of eNd(t) values from + 1 to +2.6 were emplaced from 2700 to 2673 Ma and mark a major transition for mantle-derived magmas across the southern Superior Province, from long-term, LREE-depleted komatiites and thoeliites in extensional regimes to variably enriched basanites, trachytes, and lamprophyres in a transpres- sional regime (Barrie and Shirey 1989). One possible explana- tion is that these alkalic suites may represent LILE and LREE enrichment in the mantle related to fluids derived from a subducted sedimentary component (Pearce 1983). Granitoid rocks were emplaced in a predominantly compressional regime from 2707 to 2690 Ma, characterized by lower S d N d ratios but with a wide range of eNd(t) values from -0.4 to +3.8 that reflect derivation from both older and coeval basaltic material.

Evidence for late thermal events in the Kamiskotia-Montcalm area

Thermal events after crystallization have affected rocks in the Kamiskotia-Montcalm area and reset both the Nd and Sr mineral systematics locally. As previously mentioned, a Rb-Sr isochron for the KGC ferroan gabbro whole-rock and mineral separates (Fig. 5b) agrees with U-Pb ages for the Hearst- Matachewan dike swarm (Heaman 1989). This sample was taken 7 m away from a 6 m thick, vertical Matachewan dike in an area where the dikes are particularly densely spaced at 200 m intervals on average. The Rb-Sr isochron age is interpreted as the age of thermal-hydrothermal metamorphism due to the emplacement of the neighbouring dike. Delaney (1987) has reviewed heat-transfer processes by conduction between host rocks and adjacent cooling vertical mafic dikes. For cases where the thermal properties of the dike and host rock are identical, such as for Kamiskotia gabbroic rock and diabase dikes, the ambient temperature at 1.2 dike widths would rise approximately 200°C. This would be in addition to the temperature due to the geothermal gradient and to heating from other Matachewan dikes and sills in the vicinity. The presence of intergranular fluid would allow for slightly more rapid heat transfer in crystalline igneous rocks and would aid Rb and Sr diffusion. Resetting of the K-Ar system in hydrous phases up to 15 m from a 10 m thick dike has been clearly documented (Hanes et al. 1988) and modeled by convection of fluids adjacent to mafic dikes (York and Hyodo 1988). The Sm-Nd system, generally considered more resistant to resetting than the Rb-Sr and K-Ar systems, has not been disturbed for this sample. A whole-rock - plagioclase - clinopyroxene Sm-Nd regression line has an age of 2700 -+ 30 Ma (MSWD = 8.0), in agreement with the Kamiskotia suite Sm-Nd isochron (Fig. 5a) and the U-Pb age for the KGC.

The emplacement of the Hearst-Matachewan dike swarm represents a major magmatic event after the stabilization of the Superior Province. It extends over greater than 250 000 km2 of the Superior Province, the second largest dike swarm of the Canadian Shield and one of the largest on Earth (Fahrig 1987). The studies of Phinney and Morrison (1988) provide evidence for extensive Matachewan-related thoeliitic magma chambers at upper- to mid-crustal levels, which may have been capable of resetting the Rb-Sr system regionally. Their phase-equilibria studies on theoliitic matrices of megacrystic basalts similar to Matachewan dike matrix material indicated that Matachewan

plagioclase compositions can be reproduced only at pressures corresponding to depths of about 10-12 km. The thermal effects of Hearst-Matachewan magmatism may have been responsible for partially or completely resetting Rb-Sr and K-Ar ages for Archean rocks in a cryptic fashion, with no detectable change in mineral assemblages. For example, Rb-Sr isochron ages for many granitoids across the southern Superior Province are 2400-2460 Ma (e.g . , Frith and Doig 1975; Clark and Cheung 1980; Birk and McNutt 1981), and three alteration zones along the Destor-Porcupine and Larder Lake fault zones are 2390, 2410, and 2440 Ma (Kemch et al. 1987).

Younger regression ages for two granitoid rocks (Figs. 6, 7) in the Nd and Sr systems are older than the emplacement age for the Hearst-Matachewan dike swarm. Particularly noteworthy are the identical ages of 2530 Ma in the Nd and Sr systems for sample 16 from the Groundhog River tonalite, coeval with U-Pb titanite ages at the base of the Kapuskasing Structural Zone (Krogh et al. 1988) and more than 160 Ma younger than U-Pb zircon and titanite ages from the same sample (Fig. 3). In the southern Abitibi Subprovince, radiogenic isotopic studies have investigated mineralization and alteration associated with mesothermal Au systems (e.g., Bell et al. 1989; Wong et al. 1989) and volcanogenic sulfide deposits (e.g., Maas et al. 1986; Shandl and Davis 1989). These studies have documented posttectonic hydrothermal fluid migration related to late thermal events from 2650 to 2400 Ma (see Bell et al. 1989 for a recent review).

The young Sm-Nd and Rb-Sr ages in the Kamiskotia area reflect subgreenschist-facies thermal events in the southern Superior Province after the vast majority of magmatic activity in the late Archean. One mechanism than can adequately explain the late addition of heat to this region is crustal underplating of hot, mantle-derived magmas and their intrusion into lower- and mid-crustal levels. Crustal underplating by mafic-ultramafic magmas explains the seismic velocity versus depth profiles in many crustal terranes (Furlong and Fountain 1986), and it can explain the anticlockwise P-T paths common to many granu- lite terranes interpreted as the roots of continental arcs (Bohlen 1987). Thermal modelling of P-T paths for the nearby Kapuskasing Structural Zone requires elevated heat input from the mantle between 2680 and 2500 Ma (Percival et al. 1988). This modelling is consistent with magmatic underplating and thermal conduction to the upper crust, resulting in the younger Nd and Sr ages in the Karniskotia-Montcalm area. Alternatively, overthrusting of granulite-facies rocks of the Kapuskasing Structural Zone (located 5 km west of Fig. 2) from the west at 2.67-2.45 Ga (Percival et al. 1989; West 1989) and related fluid migration could reset the Nd and Sr systems. Systematic studies using Nd and Sr isochron systematics along with U-Pb and 4 0 ~ r - 3 9 ~ r mineral chronology are needed to properly understand the late thermal events from 2.67 to 2.45 Ga in the southern Abitibi Subprovince.

Conclusions A geochemical and Nd isotopic study of the Kamiskotia-

Montcalm area has characterized tholeiitic, calc-alkalic, and lamprophyre rock suites and their mantle and crustal sources. An Sm-Nd isochron at 2710 + 30 Ma for the Kamiskotia gabbroic complex and the bimodal Kamiskotia volcanic rocks provides further evidence that they are cogenetic and ultimately derived from the Abitibi Subprovince depleted (eNd(t) = +2 to +3) mantle. In contrast, the Montcalm gabbroic complex

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72 CAN. J. EARTH SCI. VOL. 28. 1991

exhibits Nd isotopic enrichment upsection from a depleted mantle signature near its base, consistent with progressive contamination by a long-term, LREE-enriched crustal compon- ent. Montcalm lamprophyre dikes are LILE and LREE en- riched, and they have high MgO, Cr, and Ni contents and Nd- isotope values like the Abitibi depleted mantle. These data indicate that trace-element enrichment occurred in the mantle shortly before emplacement. The Bristol Township lampro- phyre suite, like the Timiskaming alkalic volcanic suite (Basu et al. 1984; A. M. Goodwin, unpublished data), has enriched LILE and LREE contents and an enriched Nd isotopic signature (eNd(t) = + 1 .O) and indicates a source with LREE enrichment for 100-200 Ma prior to eruption relative to a depleted mantle.

Granitoid rocks represent crustal sources, with eNd(t) values from -0.4 to +3.8. These data indicate that the crustal sources were derived from both enriched older crustal material and recent additions of late Archean crust derived form the Abitibi depleted mantle. The distribution of the enriched granitoids and the enriched part of the Montcalm gabbroic complex may indicate a distinct provinciality for the enriched crustal source. REE patterns for the granitoid rocks are consistent with their derivation from a basaltic amphibolite at moderate to high pressures.

Considering other mantle-derived suites and recent U-Pb geochronology, a two-stage petrotectonic evolution is proposed for southern Abitibi mantle-derived rocks. Firstly, komatiitic, tholeiitic, and minor lamprophyre suites derived from a long- term, LREE-depleted mantle reservoir were emplaced from >2740 to 2697 Ma. This was followed by alkalic magmatism derived from LILE-enriched mantle reservoirs, emplaced in a predominantly transpressional tectonic setting from 2700 to 2673 Ma. Granitoid intrusions were derived from both older, long-term, LREE-enriched and LREE-depleted crustal sources and were emplaced in predominantly continental-arc- like settings.

Sm-Nd and Rb-Sr whole-rock and mineral regression and isochron ages document thermal events that postdate most late Archean magmatism and deformation in the Kamiskotia- Montcalm area. A Rb-Sr whole-rock and mineral isochron age of 2450 * 35 Ma for a Kamiskotia gabbro sample is identical to U-Pb ages for the voluminous Hearst-Matachewan dike swarm (Heaman 1989) and similar to many Rb-Sr ages from across the southern Superior Province. Both the Sm-Nd and Rb-Sr systems were reset at 2530 * 35 Ma for the Groundhog River tonalite, much younger than its U-Pb zircon age of 2696 k 2 Ma, and a 2 0 7 ~ b / 2 0 6 ~ b titanite minimum age of 2692 * 5 Ma (Barrie and Davis 1990), but coeval with titanites from the basal Kapuskasing Structural Zone (Krogh et al. 1988). These ages may reflect the cryptic, late thermal events, possibly related to magmatic underplating during the stabilization of the southern Superior Province.

Acknowledgments This study has been funded by grants from the Geological

Society of America, the Sigma Xi Research Society, and by a predoctoral fellowship to CTB at the Department of Terrestrial Magnetism, Carnegie Institution of Washington. These organi- zations are thanked for their support. We are grateful to D. Davis, J. Ludden, F. Corfu, N. Amdt, and E. Spooner for comments on the first draft; D. Gerlach, P. Castillo, R. Carlson, and M. Gorton for helpful comments concerning chemical and mass spectrometric procedures; and A. J. Naldrett for support and supervision to CTB .

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Appendix: Sample descriptions and locations Rock unit names and abbreviations based on field observations and lithologic names and descriptions are given

for each sample. Locations are by township, by universal transverse Mercator grid northing and easting coordinates, and for several samples by drill-core number and interval (from Teck Corporation drilling, 1976-1977, in Montcalm Township). Samples used for U-Pb analyses (Barrie 1990a; Barrie and Davis 1990) are noted with an asterisk.

Sample 87-128; Turnbull Tp.; UTM 5267810N, 446910E. Kamiskotia ferroan gabbro (Kfgb,): medium-grained mesocumulate ferroan gabbro, with 45% clinopyroxene, 43% plagioclase, 8% hornblende-actinolite, 3% Ti- magnetite, 1% apatite, and trace titanite and epidote.

Sample 84-402; Robb Tp.; UTM 5377760N, 451380E. Kamiskotia ferroan gabbro (Kfgb2): subpegmatitic orthocumulate ferroan gabbro, with 45% plagioclase (An60-64), 34% clinopyroxene, 10% orthopyroxene, 5% hornblende, 3% Ti-magnetite and ilmenite, 2% chlorite, 1% quartz, and trace biotite and apatite.

Sample 88-1; Godfrey Tp.; UTM 5370900N, 456971E. Kamiskotia granophyre (Kgphyre): medium- to fine- grained, moderately altered massive quartz diorite, with 60% actinolite and chlorite after pyroxene and hornblende, 20% feldspar, 15% myrmekitic quartz, 2% epidote, and 1% Fe-Ti oxides.

Sample GF-2"; Godfrey Tp.; UTM 5368630N, 457900E. Kamiskotia rhyolite (Kr): flow-banded quartz- and feldspar-phyric rhyolite, 85% fine-grained quartz-sericite matrix, 7% quartz, 7% K-feldspar, and trace apatite and zircon.

Sample 88-2; Godfrey Tp.; UTM 53669610N, 459225E. Kamiskota basalt (Kb): massive, aphyric pillowed basalt.

Sample 84-86; Enid Tp.; UTM 5376450N, 434740E. Lower mafic volcanic rock (LMV): foliated, fine-grained plagioclase-phyric pillowed basalt.

Sample 86-7; Carscallen Tp.; UTM 5360005N, 447920E. Lower mafic volcanic rock (LMV): foliated, fine- grained pillowed basalt.

Sample 86-6; Carscallen Tp.; UTM 5359450N, 448450E. Lower mafic volcanic rock (LMV): foliated, fine- grained pillowed basalt.

Sample 86-427; Montcalm Tp.; UTM 5391480N, 418950E; DDH no. EE25, 145-146 m. Montcalm gabbro (Mgb): medium-grained, massive adcumulate gabbro, with 59% plagioclase, 40% clinopyroxene, and trace magnetite.

Sample 86-426; Montcalm Tp. ; UTM 5391460N, 418990E; DDH no. EE-25,89-91 m. Montcalm gabbro (Mgb): medium-grained, subporphyritic mesocumulate anorthositic gabbro with plagioclase phenocrysts; 62% plagioclase and 38% clinopyroxene.

Sample 87-42; Montcalm Tp.; UTM 5386550N, 417750E. Montcalm ferroan gabbro (Mfgb): fine-grained massive orthocumulate gabbro, with 46% plagioclase, 50% clinopyroxene, 2% amphibole, and 2% Fe-Ti oxides.

Sample 86-418; Montcalm Tp.; UTM 5391500N, 418925E; DDH no. EE-35, 118-1 19 m. Montcalm lampro- phyre dike (MLD): medium-grained lamprophyre dike, with 56% sodic amphibole, 35% feldspar, 5% biotite, 2% titanite, 1% magnetite, 1% apatite, and trace sulfide.

Sample 86-392; Montcalm Tp.; UTM 5391490N, 418900E; DDH no. EE-56, 514-515 m. Montcalm lamprophyre dike (MLD): medium-grained lamprophyre dike, with 52% sodic amphibole, 37% feldspar, 6% biotite, 2% titanite, 1 % magnetite, 1.5% apatite, and trace sulfide.

Sample 87-12t; Turnbull Tp.; UTM 5367810N; 446915E. Turnbull Township tonalite (TTT): medium-grained massive leucogranite, with 54% alkali feldspar, 25% quartz, 20% plagioclase, and trace apatite, titanite, and zircon.

Sample 86-101*; Cote Tp.; UTM 5378860N, 438620E. Cote Township tonalite (CTT): medium- to coarse- grained tonalite, with 50% plagioclase (An35), 35% quart, 10% hornblende, 1% biotite, 1% chlorite, and trace zircon, apatite, titanite, epidote, and sericite.

Sample TTG-l*; Enid Tp.; UTM 5373290N, 433390E. Groundhog River tonalite (GRT): foliated medium- to coarse-grained tonalite, with 45% plagioclase (An3o), 35% quartz, 10% orthoclase, 5% biotite, 3% muscovite, 1% epidote (primary?), and trace zircon, apatite, titanite, and hematite.

Sample 86-220; Fortune Tp.; UTM 5386300N, 424450E. Fortune Township granodiorite (FTG): coarse-grained

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porphyritic granodiorite, with 60% alkali feldspar, 22% quartz, 14% plagioclase, 4% biotite, and trace apatite, titanite, epidote, and zircon.

Sample 86-22"; Montcalm Tp. ; UTM 539 1500N, 41 8920E. Montcalm granodiorite dike (MGD): medium- grained massive granite, with 50% alkali feldspar, 27% plagioclase, 15% quartz, 5% hornblende, 2% biotite, 0.7% titanite, and trace epidote, apatite, and zircon.

Sample 87-2"; Bristol Tp.; UTM 5358450N, 458350E. Bristol Township lamprophyre suite garnetite dike (GD): coarse-grained garnetite, with 75% melanite garnet, 8% biotite, 4% plagioclase, 4% epidote, 1.5% apatite, 1.5% quartz, 1.5% pyrite-chalcopyrite-pyrrhotite, 1.5% carbonate, 1 % magnetite, 1 % allanite, 0.7% chlorite, and 0.2% titanite.

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