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Tectonic Juxtaposition of the Archean Abitibi Greenstone Belt and Pontiac Subprovince Evidence from Geobarometry, Geochemistry, and Ar-Ar Geochronology of Metasedimentary Rocks and Granitoids




Feng, Rui

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University of Saskachewan




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The Southern Volcanic Zone (SVZ) of the Abitibi greenstone belt and the Pontiac Subprovince (PS) in the Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes respectively. This study attempts to evaluate the relationship between the SVZ and PS, and the possible ,existence of an older basement to' the SVZ, based on petrology, geochemistry, and Ar-Ar geochronology. SVZ and PS sediments have similar major element compositions, but the PS sediments are depleted in 180, and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, HE, Zr and total REE, and have greater Rb/K, Cs/Rb, Ta/Nb, Th/La, and Ba/La ratios than their SVZ counterparts. Two sub-types of REE patterns in PS sediments have moderately fractionated distributions with La/Yb, =3.9-17, and 7.8-13.6 respectively, but one has slight Eu anomalies (Eu/Eu * = 0.89-1.16), whereas the second has pronounced negative anomalies (Eu/Eu" =0.52-0.81) coupled with enrichment of incompatible elements, e.g. K, Rb, Th, U, Nb, and Ta. The SVZ sediments have €Ni2.7Ga) values of -0.71 to 1.83, corresponding to model TCHUR =2.82-2.55 Ga, with most around 2.65 Ga, close to the depositional age. The PS sediments have €Ni2.7Ga) =0.95 to -3.51 (TCHUR =2.7 to -3.1 Ga), consistent with detrital zircon ages of -2.69 Ga to -3.1 Ga. These data suggest that the SVZ sediments formed at oceanic arc-settings from juvenile rocks, whereas the PS sediments were deposited in a continental-arc setting where older, evolved crustal rocks, older arc, and juvenile arc rocks all contributed. In the SVZ, minor synvolcanic tonalite-trondhjemite-granodiorite series (TTG; -2700 Ma) resemble Barker's low-AI type TTG, having extremely low K20 « 1.5 wt%), Rb «50 ppm), and Sr «200 ppm) concentrations, enhanced Ti02 (0,5 wt%), Fe203 (4.4 wt%), Nb, Y, and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. They probably originated by extreme fractionation of basaltic magma from partial melting of a depleted mantle source in a suprasubduction zone environment. Syntectonic tonalite-granodiorite-granite and quartz monzonite series (TGG M; -2690 Ma) resemble Barker's high-AI type TTG, having low K20 (1-3 wt%); Rb (10- 50 ppm), Ba « 1000 ppm), and U contents, enhanced Sr, low Ba/Sr (0.5-1.25) ratios, depletion of Ta, Nb and Ti, and strongly fractionated REE patterns (La/Yb; = 49 to 21) with minor Eu anomalies. The TGGM formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting slab. Late tectonic quartz syenite-quartz monzonite-granite series (SMG; 2681 to 2676 Ma), are similar to the TGGM compositionally, but display lower CaO/K20 + Na20, and greater Rb, Ba, Th and U contents. These rocks may represent mixtures of magmas derived from a "metasomatised" mantle and TGGM-type sources. A late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite series (SS; 2680-2670 Ma) are shoshonitic-type magmatism distributed along regional strike-slip faults. The primitive magmas (Si02:s:65 wt.%) have extreme co-enrichment of LILE and Cr, Co, and Ni, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti; the evolved phases (Si02 65 wt%) have lower compatible and incompatible element contents stemming from crystal fractionation. The primitive magmas may have been derived from low degrees of partial melting of a depleted mantle wedge that was metasomatized by alkali-rich fluids during subduction. The PS and the Lacorne block within the SVZ have a syn-, to late-tectonic monzodiorite-monzonite-granodiorite-syenite series (MMGS; 2690-2670 Ma), and a garnet-muscovite granite series (GMG; -2644 Ma). The MMGS resemble the Abitibi SMG and SS series, but are distinguished by strong correlations of U, Th, Zr and Hf with Si02, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co, and Ni with LREE, Li, and Cs in some MMGS phases. The GMG have K20/Na20􁪽 1, restricted Si02 range (69-75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's (La/Yb, = 16 to 0.9), with pronounced negative Eu anomalies. They are similar to Phanerozoic collisionalleucogranites and formed by partial melting of PS metasediments; . 4°Ar/9Ar mineral ages and existing conventional zircon geochronology, indicate an earlier thermal activity (2690-2670 Ma) that separately caused low pressure, greenschist facies metamorphism in the SVZ and intermediate pressure metamorphism in the PS, as well as a second thermal event (2660-2630 Ma) that was coeval with the generation of the GMG series in the PS and the Lacorne block. Collectively, these features may indicate an oceanic subduction episode, during which syn-volcanic TTG, syn-tectonic and syn-metamorphic to late- and post- tectonic volcanic-arc type granites (TGGM, SMG, SS, and MMOS) were produced, and a later arc-arc, or arc-continental collisional episode, during which the PS collided with and underthrust the SVZ at -2670 Ma with partial melting of thickened crust to yield GMG magmas. Accordingly, the SVZ was not developed on basement, and the SVZ and PS represent terranes formed separately, that were tectonically juxtaposed later at about 2680 to 2670 Ma.










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