THE DE-PAS BATHOLITH IN NEW QUEBEC - COMP OSITE INFRASTRUCTURE WITH PROTEROZOIC MAGMATIC ARC

The De Pas Batholith, in New Quebec, is a mass of granitoids elongated in a N-S direction, covering 7000 km(2) in the southeastern part of t he Archean Rae province. It is located between the Paleoproterozoic Ne w Quebec Orogen in the West and the Torngat Orogen in the East. The ba tholith is composed, in its south half, of two parallel plutonic units : a chamockitic intrusion in the West, enclosed in granulitic gneisses , and a granitic unit in the East intruding an Archean orthogneiss com plex migmatized at amphibolite facies grade. Field evidence indicates that the chamockitic unit is younger than the granitic unit. Both plut onic units are differentiated. The granitic unit ranges in composition from pyroxenite and diorite to the dominant porphyritic granodiorite. The chamockitic unit varies from norite to the dominant porphyritic o pdalite. Late intrusive granites are present. The granitic unit and ch arnockitic unit show calc-alkalic evolutionary trends, granodioritic f or the the granitic unit and monzonitic for the chamockitic unit. Simi lar major elements patterns suggest a common source area. Compatible t race element and heavy rare earth abundances are similar in both units . However, incompatible elements (Rb, Th, and U) and light rare earth are less abundant in the chamockitic unit. This poverty in certain lar ge-ion lithophile elements and high field strength elements is a prima ry characteristic of the charnockitic magma. The overall geochemical c haracteristics of the De Pas Batholith are similar to those of a pluto nic are in a subduction setting. Only the late granites show evidence of a collisional setting. The differences in trace elements of the gra nitic unit and the charnockitic unit might be explained by contrasting styles of contamination of the magmas by continental materials in dif ferent intrusive conditions and geological settings. The granitic magm a was intruded into Archean (?) crust of amphibolite facies, with whic h the granitic unit was in physical equilibrium. The charnockitic magm a crystallised in the deeper granulitic crust, poor in highly lithophi le elements. The De Pas Batholith occurs in the hinterland of the New Quebec Orogen, and represents an early subductional magmatic are forme d in two stages separated by a period of crustal thickening in the are zone.