GEOCHEMICAL EVOLUTION OF GRANITOIDS FROM THE ARCHEAN ABITIBI SOUTHERN VOLCANIC ZONE AND THE PONTIAC SUBPROVINCE, SUPERIOR PROVINCE, CANADA - IMPLICATIONS FOR TECTONIC HISTORY AND SOURCE REGIONS

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada ( approximately 2.75-2.67 Ga). Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at approximately 2700 Ma at pressures of approximately 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O ( < 1.5 wt%), Rb ( < 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment. Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at approximately 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1-3 wt%), Rb (10-50 ppm), Ba ( < 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5-1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(La/Yb)n=49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab. Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Wate-beag) to stocks (e.g., Garrison), and were emplaced at shallow levels ( less-than-or-equal-to 1 kbar) between 2681 to 26 76 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO/(K2O+Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a "metasomatised" mantle and TGGM-type sources. Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 less-than-or-equal-to 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 greater-than-or-equal-to 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series ( approximately 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2O/Na2O greater-than-or-equal-to 1, restricted SiO2 range (69-75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [ (La/Yb)n=0.9-16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone. Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of approximately 70 Ma.