GEOCHEMICAL CHARACTERISTICS OF GRANITOID ROCKS FROM WITHIN THE ARCHEAN MICHIPICOTEN GREENSTONE-BELT, WAWA-SUBPROVINCE, SUPERIOR-PROVINCE, CANADA - IMPLICATIONS FOR SOURCE REGIONS AND TECTONIC EVOLUTION

Three distinct tonalite-trondhjemite-granodiorite (?TG) magma series o ccur syngenetically with bimodal mafic-felsic cyclical volcanics in th e middle to late Archean Michipicoten Greenstone Belt (MGB),Canada. Be tween 2.72 and 2.60 Ga, two additional series are recognised as pre-te ctonic,late tectonic, and post-tectonic syenite-quartz monzonite grani toid stocks (SMG series) and an alkali feldspar syenite-alkali feldspa r quartz syenite stock (SS series). The synvolcanic TTG series, such a s the Hawk Lake Granitic Complex and the Jubilee Lake Stock, are chara cterised by moderate to low K2O ( < 2.5 wt.%), Rb ( < 60 ppm), and Sr concentrations ( < 400 ppm), moderate TiO2 levels (0.1-1.0 wt.%), La/S m (2-12), and Gd/Yb (0.8-2.9), moderate to large negative Eu-anomalies , and small negative Ta + Nb troughs on ocean ridge granite (ORG)-norm alised plots. The TTG series are compositionally similar to synvolcani c MGB felsic volcanics of cycles 1 to 3. The high La/Sm, La/Ta and low Zr/Th are features of sialic crust; a contribution of ancient sialic crust in the genesis of the felsic volcanics and TTG-series granitoids of all three felsic volcanic cycles and synvolcanic TTG stocks is req uired in the MGB. The parental magma, like that of the TTG series of t he Abitibi Greenstone belt (AGB) may originate in oceanic mantle. The SMG series have elevated K2O (2.0-6.3 wt.%), Rb (40-280 ppm), Ba (10-2 800 ppm), and Sr (250-1350 ppm), steep REE patterns reflected in eleva ted La/Sm (3-12) and Gd/Yb (1.8-5.0) without Eu-anomalies, a moderate to large negative Ta+Nb trough on ocean ridge granite-normalised plots , and low TiO2 ( < 0.5 wt.%) and CaO/(Na2O + K2O). The SMG series is c ompositionally different to the MGB felsic volcanic rocks and to the T TG-series granitoids. The source is tentatively placed in the mantle w edge overlying a southward-dipping subducted plate of oceanic crust, a nd the elevated LILE signature is attributed to metasomatism of this s ource by fluids derived from the subducted slab of oceanic crust. The SMG-series granitoids, like those of the Abitibi, are different when c ompared to the sanukitoids in that they are compositionally more evolv ed; however, they may be related to these rocks by low-pressure fracti onation, and may be derivatives of sanukitoid-like magmas. The alkali feldspar syenite-alkali feldspar quartz syenite, represented by the He rman Lake Stock of the SS series, has a primitive composition ( < 60 w t.% SiO2) with low REE abundances and relatively flat LREE and HREE, b ut steep MREE profiles. These rocks are strongly LILE-enriched, but do not develop pronounced Ta+Nb troughs on ORG-normalised profiles. Thes e rocks may be derived by low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. The elevated Ti/Y and Zr/Y of these rocks app ear to suggest that small amounts of partial melting were responsible for the genesis of these rocks. The granitoid rocks record an overall progression from contributions made from the continental crust in a co ntinental margin environment to magmas dominated by contributions from subducted crust or the overlying variably metasomatised mantle wedge during the terminal phase of are-continent collision.