Friday, August 29, 2014

Evidence of Rapid Sediment Recycling From PaleoProterozoic

Paleoproterozoic S-type granites in the Helanshan Complex, Khondalite Belt, North China Craton: Implications for rapid sediment recycling during slab break-off


Dan et al


S-type granites, typically derived from the rapid recycling of sedimentary rocks, are sometimes accompanied by contemporary mafic magmatism and granulite metamorphism. However, the geodynamic context for such rock suites is often highly disputed, with various model proposed, including back-arc basin opening, lithospheric delamination, mantle plume and continental rifting. The Paleoproterozoic Khondalite Belt in the North China Craton provides an example of synchronous mafic and felsic magmatism that was accompanied by granulite-facies metamorphic events for which the tectonic affinities of these rocks remains unclear. This study integrates in situ zircon Hf–O isotope analyses, whole-rock geochemistry and Nd isotope results for the earliest two-mica granites (ca. 1.95 Ga) in order to provide constraints on the above issues. The granites are strongly peraluminous (A/CNK value greater than 1.1), and characterized by high zircon δ18O values of 7.3 to 10.6‰, corresponding to calculated magmatic δ18O values of 9.1 to 12.3‰, similar to those of typical S-type granites. They have relatively high and homogeneous ɛNd(t) values of -1.1 to +0.9 and highly variable zircon ɛHf(t) values ranging from -1.0 to +8.3. In situ zircon Hf-O isotopic compositions indicate that the S-type granites may contain some mantle or juvenile crustal components in addition to a sediment component. Based on the new results and published data, a slab break-off model is proposed to explain the rapid recycling of sedimentary precursors and the generation of the ca. 1.95 Ga S-type granites

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