PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF A PALEOPROTEROZOIC MAGMATIC ARC (NARSAJUAQ TERRANE, UNGAVA OROGEN, CANADA) AND COMPARISONS TO SUPERIOR PROVINCE GRANITOIDS

The Narsajuaq terrane is a major component of the Paleoproterozoic Ung ava (Trans-Hudson) Orogen of northern Quebec and is a well-exposed mid -crustal block interpreted to be the plutonic roots of a magmatic are. The Narsajuaq terrane is comprised of a series of hornblende+/-pyroxe ne-bearing plutons and supracrustal rocks which were accreted to the A rchean Superior Province basement during the Paleoproterozoic. On the basis of geological field relationships and geochronological data the plutonic rocks of the Narsajuaq terrane are divided into four main gro ups: (1) the Cape Smith suite (1898-1839 Ma) consists of various defor med and undeformed plutons that are found intrusive into the Cape Smit h Belt lithologies; (2) the Gneissic suite, which ranges in age from 1 863-1844 Ma and consists of a well-layered sequence of diorite and ton alite, intruded by and interlayered with variably deformed and metamor phosed granitic veins; (3) the Younger suite, consisting of discrete, kilometre-size bodies of metamorphosed plutons (diorite-monzodiorite-t onalite-granite) intrusive into the Gneissic suite, ranging in age fro m 1836-1821 Ma; and (4) granitic sheets of the Anatectic suite which i ntruded the Narsajuaq terrane ca 1800 Ma. A series of metamorphosed se dimentary rocks (predominantly semi-pelites) of the Sugluk Group are a lso found in the Narsajuaq terrane, and contain zircons of both Archea n and Paleoproterozoic age. Plutons of the Gneissic and Younger suites are predominantly calc-alkaline and metaluminous to mildly peralumino us and contain variable but generally moderate to high concentrations of the large-ion lithophile elements and light rare earth elements (RE Es), pronounced negative Nb and Ti anomalies, and fractionated REEs, c omparable to modern are granites. A range in isotopic compositions is noted for the suites, with the Gneissic and Younger suites having init ial epsilon(Nd) and Sr-87/Sr-86((i)) values ranging from juvenile (+4. 0 and 0.7020, respectively) to enriched (-10.7 and 0.7048). The Anatec tic suite granites range to extremely enriched isotopic compositions w ith initial epsilon(Nd) up to -18.5 and Sr-87/Sr-86((i))=0.7064. The C ape Smith suite has a much more restricted isotopic composition which is predominantly juvenile (initial epsilon(Nd)= +3.2 to +1.5 and Sr-87 /Sr-86((i)) = 0.7020 to 0.7024), and suggests a minor role for crustal contamination, primarily via sediment subduction. The isotopic signat ure of the Narsajuaq terrane plutons indicate that at least three sour ces were involved in their petrogenesis: depleted mantle, average 2.8 Ga Superior Province crust and older (3.1 Ga) Superior Province crust. When compared to plutonic suites from the NE and SE Superior Province (Minto block,Abitibi, Pontiac and Opatica belts), the Narsajuaq terra ne suites define a similar chronology and evolution. indicating a comm onality and dominance of arc-accretionary processes for crustal format ion in the Precambrian Shield of Canada. Higher trace element abundanc es in the Narsajuaq terrane plutons compared to the Archean samples ma y be related to different subduction zone fluxes which mag reflect dif ferent bulk compositions of subducted oceanic sediments during the Pro terozoic. (C) 1998 Elsevier Science B.V.