THE SHOLL SHEAR ZONE, WEST PILBARA - EVIDENCE FOR A DOMAIN BOUNDARY STRUCTURE FROM INTEGRATED TECTONOSTRATIGRAPHIC ANALYSES, SHRIMP U-PB DATING AND ISOTOPIC AND GEOCHEMICAL DATA OF GRANITOIDS

The Pilbara Block provides a record of Archaean continental growth inv olving the tectonic accretion of outboard island-arcs and collisions w ith other continental-scale fragments. This record of continental grow th is balanced by breakup and strike-slip dismemberment of the contine nt. New SHRIMP U-Pb in zircon ages and Sm-Nd data provide evidence in the West Pilbara which demonstrates that subduction-related and tecton ic-accretion processes at the western margin of that ancestral contine nt between 3.15-2.78 Ga were coeval with, and genetically related to, crustal-scale tectonics and basin formation inboard of that margin. Th e tectonic division of the West Pilbara is defined by integrated tecto nic analyses, geochronology, geochemistry and isotopic analyses. Geoch ronological studies clearly indicate that the western Pilbara comprise s two domains with different recorded geohistories, whereas geochemist ry and isotopic systematics reflect the changing tectonic regimes thro ugh time. In combination, these studies allow the development of a rec onstruction of the relative positions of the domains through time on t he western margin of the Pilbara Block. The supracrustal rocks of the northern Roebourne Lithotectonic Complex (Domain 6 in a Pilbara-wide s cheme) were formed in an island are setting, facing an ocean to the no rth-west, prior to 3260 Ma; the time of emplacement of voluminous gran itoids into the complex. In contrast, the supracrustal rocks of the so uthern Sholl Belt (Pilbara Domain 5) were formed in a back-are setting behind a north-west-facing are between 3125 and 3112 Ma, with more-or -less synchronous granite emplacement at about 3115 Ma. The two domain s were tectonically juxtaposed, between 2991 and 2925 Ma, by the Shell Shear Zone, a largely sinistral shear zone, with subsequent volcanism in both domains to about 2925 Ma. The Roebourne Lithotectonic Complex (Domain 6) is interpreted to be an allochthonous terrane, which forme d north-east relative to its present position, but indigenous to the P ilbara Block rather than an exotic terrane. The East Pilbara is interp reted to have acted as a cratonic hinterland during the convergent mar gin tectonics that affected the two West Pilbara domains. (C) 1998 Els evier Science B.V.