AGE AND PETROGENESIS OF 2 LATE ARCHEAN MAGMATIC SUITES, NORTHWESTERN SUPERIOR PROVINCE, CANADA - ZIRCON U-PB AND LU-HF ISOTOPIC RELATIONS

The paper presents U-Pb ages for zircon, titanite, and monazite, and Hf isotopic data for zircon, from the rocks of two magmatic suites occurring mostly in the Archean Uchi Subprovince and partly in the neighbouring Berens River and English River subprovinces of the northwestern Superior Province, Ontario. These data, together with observations on the morphologies and, where evident, the inheritance of the zircon crystals, constrain the nature of the sources of the magmas and provide a record of various crustal processes in their evolution. The older of the two magmatic suites formed at 2744-2740 Ma along segments of a common arc system. The suite consists of (1) several trondhjemitic to granodioritic plutons, with epsilon(Hf) values of 6.1, intruded into older crust and possibly formed from magma produced by partial melting of subducted, juvenile oceanic crust; (2) an assemblage of dacitic pyroclastic volcanic rocks, with epsilon(Hf) values of 3.2-4.0, associated with tholeiitic basalts and probably derived from magma melted from arc mantle; and (3)a bimodal assemblage of tholeiitic basalts, rhyolites, and porphyries, also with epsilon(Hf) values of 6.1, associated with a volcanogenic massive sulphide deposit and apparently formed by differentiation of mantle-derived basaltic melts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma, comprises late orogenic plutons and batholiths of dioritic to dominantly granodioritic composition. The characteristics of these intrusions are consistent with a process combining melting of a metasomatized mantle source and subsequent fractional crystallization of the derived magmas at shallow depths. However, most of the studied occurrences show evidence of crustal contamination through various combinations of assimilation of lower-crustal material, assimilation of underthrust sedimentary rocks, and contamination by wall rock materials during the latest stages in the emplacement of the plutons. The involvement of crustal material is indicated by the presence of zircon xenocrysts and by epsilon(Hf) values ranging from 1.4 to 4.4. Only one intrusion, with an epsilon(Hf) value of 5.0 and no xenocrystic zircon, appears to have escaped widespread contamination, perhaps because the ascent of its magma was facilitated by a crustal-scale fracture system.