Petrogenesis of the Mesozoic Zijinguan mafic pluton from the Taihang Mountains, North China Craton: Petrological and Os-Nd-Sr isotopic constraints

摘要

Mantle peridotites show low Re/Os ratios and sub-chondritic ~(187)Os/~(188)Os ratios (<0.13), which contrast sharply with the radiogenic Os isotopic composition of continental crust (e.g., ~(187)Os/~(188)Os = 6 ～ 80 for the Archean rocks). This provides a potential to use the Re-Os isotopes of mantle-derived magmas to trace the processes of mantle-crust interaction. We report petrological and Os isotopic data for the Zijinguan Cretaceous mafic pluton from the North China Craton to place constraints on their petrogenesis, mantle source characteristics, and the role of crustal contamination. The mafic pluton consists of hornblende pyroxenites, hornblende gabbros and gabbrodiorites. These rocks are enriched in LILE (e.g., Ba and Sr) and LREE, depleted in HFSE (e.g., Nb, Zr). They possess radiogenic Os isotopic compositions with ~(187)Os/~(188)Os = 0.187-0.603, and EM1-like Sr-Nd isotopic compositions (I_(Sr) = 0.7058-0.7066, ε_('Nd)(t) = -13.8 to -18.2). Petrography and elemental data suggest a significant fractionation and accumulation of olivine + pyroxene + hornblende in forming the mafic pluton. Clinopyroxene and plagioclase from the gabbrodiorites show complicated compositional zoning, suggesting a process of magma recharge in the late-stage of magma evolution. This is supported by their higher ε_(Nd)(t) values (-14) than the other two rock types (-15 to -18). The parental magmas to the mafic pluton should be hydrous, which originated from an old, enriched lithospheric mantle, and were significantly contaminated by Pre-cambrian mafic lower crust and TTG gneisses during magma evolution, as is suggested by the highly radiogenic Os- and unradiogenic Nd isotopic compositions of the mafic pluton.