3430 TO 3417 MA CALC-ALKALINE VOLCANISM IN THE MCPHEE DOME AND KELLY BELT, AND GROWTH OF THE EASTERN PILBARA CRATON

The McPhee Dome and Kelly Belt in the eastern Pilbara Craton contain w ell-preserved, proximal, shallow-marine, dominantly-intermediate, volc anic successions. These have SHRIMP U-Pb zircon ages of 3430 +/- 3 and 3417 +/- 9 Ma respectively, which are younger than geochemically simi lar similar to 3.52 and similar to 3.47 to 3.45 Ga suites in other eas tern Pilbara greenstone belts. Basalts, andesites and dacites from the McPhee Dome are similar in composition to modern are rocks, whereas d acites and rhyodacites from the Kelly Belt have geochemical characteri stics of Archaean tonalite-trondhjemite-granodiorite (TTG) magmas. The geochemistry of the McPhee Dome basalts, basaltic andesites and dacit es most likely reflects infracrustal fractional crystallization of a m antle derived magma combined with assimilation of older crust, or a si licic melt. In contrast the Kelly Belt dacites and rhyodacites were mo st likely derived by melting of subducted mafic crust. The extensional submarine environment of pre-3.4 Ga, dominantly mafic and intermediat e volcanism in the eastern Pilbara differs from that of approximately coeval granitoid magmatism. The pre-3.4 Ga granitoids have the major a nd trace element features of TTG suites and appear to have been emplac ed into already thick crust culminating in core complex formation. Ava ilable data are most consistent with growth of the eastern Pilbara Cra ton in convergent tectonic settings, involving pulses of extensional s hallow-marine, are-like volcanism between 3.52 and 3.42 Ga, overlappin g with, and/or punctuated by, periods of crustal thickening and the in trusion of complex TTG granitoid batholiths. (C) 1998 Elsevier Science B.V.