Petrological and geochemical characteristics of a Paleoproterozoic magmatic arc (Narsajuaq terrane, Ungava Orogen, Canada) and comparisons to Superior Province granitoids

Abstract

The Narsajuaq terrane is a major component of the Paleoproterozoic Ungava (Trans-Hudson) Orogen of northern Quebec and is a well-exposed mid-crustal block interpreted to be the plutonic roots of a magmatic arc. The Narsajuaq terrane is comprised of a series of hornblende ± pyroxene-bearing plutons and supracrustal rocks which were accreted to the Archean 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 groups: (1) the Cape Smith suite (1898-1839 Ma) consists of various deformed and undeformed plutons that are found intrusive into the Cape Smith Belt lithologies; (2) the Gneissic suite, which ranges in age from 1863-1844 Ma and consists of a well-layered sequence of diorite and tonalite, intruded by and interlayered with variably deformed and metamorphosed granitic veins; (3) the Younger suite, consisting of discrete, kilometre-size bodies of metamorphosed plutons (diorite-monzodiorite-tonalite-granite) intrusive into the Gneissic suite, ranging in age from 1836-1821 Ma; and (4) granitic sheets of the Anatectic suite which intruded the Narsajuaq terrane ca 1800 Ma. A series of metamorphosed sedimentary rocks (predominantly semi-pelites) of the Sugluk Group are also found in the Narsajuaq terrane, and contain zircons of both Archean and Paleoproterozoic age. Plutons of the Gneissic and Younger suites are predominantly calc-alkaline and metaluminous to mildly peraluminous and contain variable but generally moderate to high concentrations of the large-ion lithophile elements and light rare earth elements (REEs), pronounced negative Nb and Ti anomalies, and fractionated REEs, comparable to modern arc granites. A range in isotopic compositions is noted for the suites, with the Gneissic and Younger suites having initial εNd and 87Sr/86Sr(i) values ranging from juvenile (+4.0 and 0.7020, respectively) to enriched (−10.7 and 0.7048). The Anatectic suite granites range to extremely enriched isotopic compositions with initial εNd up to −18.5 and 87Sr/86Sr(i) = 0.7064. The Cape Smith suite has a much more restricted isotopic composition which is predominantly juvenile (initial εNd = +3.2to+1.5 and 87Sr/86Sr(i) = 0.7020 to 0.7024), and suggests a minor role for crustal contamination, primarily via sediment subduction. The isotopic signature of the Narsajuaq terrane plutons indicate that at least three sources 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 terrane suites define a similar chronology and evolution, indicating a commonality and dominance of arc-accretionary processes for crustal formation in the Precambrian Shield of Canada. Higher trace element abundances in the Narsajuaq terrane plutons compared to the Archean samples may be related to different subduction zone fluxes which may reflect different bulk compositions of subducted oceanic sediments during the Proterozoic.