3430 to 3417 Ma calc-alkaline volcanism in the McPhee Dome and Kelly Belt, and growth of the eastern Pilbara Craton

Abstract

The McPhee Dome and Kelly Belt in the eastern Pilbara Craton contain well-preserved, proximal, shallow-marine, dominantly-intermediate, volcanic successions. These have SHRIMP UPb zircon ages of 3430 ± 3 and 3417 ± 9 Ma respectively, which are younger than geochemically similar ∼3.52 and ∼3.47 to 3.45 Ga suites in other eastern Pilbara greenstone belts. Basalts, andesites and dacites from the McPhee Dome are similar in composition to modern arc rocks, whereas dacites and rhyodacites from the Kelly Belt have geochemical characteristics of Archaean tonalite-trondhjemite-granodiorite (TTG) magmas. The geochemistry of the McPhee Dome basalts, basaltic andesites and dacites most likely reflects infracrustal fractional crystallization of a mantle derived magma combined with assimilation of older crust, or a silicic melt. In contrast the Kelly Belt dacites and rhyodacites were most likely derived by melting of subducted mafic crust. The extensional submarine environment of pre-3.4 Ga, dominantly mafic and intermediate volcanism in the eastern Pilbara differs from that of approximately coeval granitoid magmatism. The pre-3.4 Ga granitoids have the major and trace element features of TTG suites and appear to have been emplaced into already thick crust culminating in core complex formation. Available data are most consistent with growth of the eastern Pilbara Craton in convergent tectonic settings, involving pulses of extensional shallow-marine, arc-like volcanism between 3.52 and 3.42 Ga, overlapping with, and/or punctuated by, periods of crustal thickening and the intrusion of complex TTG granitoid batholiths.