Petrogenesis of the Mesozoic intrusive complexes from the southern Taihang Orogen, North China Craton: Elemental and Sr-Nd-Pb isotopic constraints

Abstract

A geochemical and isotopic study was carried out for three Mesozoic intrusive suites (the Xishu, Wu'an and Hongshan suites) from the North China Craton (NCC) to understand their genesis and geodynamic implications. The Xishu and Wu'an suites are gabbroic to monzonitic in composition. They share many common geochemical features like high Mg# and minor to positive Eu anomalies in REE patterns. Initial Nd-Sr isotopic compositions for Xishu suite are ENd (135 Ma)=-12.3 to -16.9 and mostly ISr = 0.7056-0.7071; whereas those for Wu'an suite are slightly different. Pb isotopic ratios for Xishu suite are (206Pb/204Pb)i = 16.92-17.3, (207Pb/204Pb)i = 15.32-15.42, (208Pb/204Pb)i=37.16-37.63, which are slightly higher than for Wu'an suite. The Xishu-Wu'an complexes are considered to originate from partial melting of an EM1-type mantle source, followed by significant contamination of lower crustal components. The Hongshan suite (mainly syenite and granite) shows distinctly higher ENd(135 Ma) values (-8 to -11) and slightly higher Pb isotopic ratios than the Xishu-Wu'an suites. It was formed via fractionation of a separate parental magma that also originated from the EM1-type mantle source, with incorporation of a small amount of lower crustal components. Partial melting of the mantle sources took place in a back-arc extensional regime that is related to the subduction of the paleo-Pacific slab Beneath the NCC. © Springer-Verlag 2004.