Using clinopyroxene to decode the origin and evolution of 773 Ma alkaline mafic sill in Helanshan, northwestern China: Missing information about primary melts in bulk rocks

Abstract

An integrated study involving both bulk rock and mineral compositions was conducted on a newly identified mafic sill in Helanshan area from northwest of China. Apatite U-Pb dating constrained its emplacement age at 772.6 ± 3.4 Ma (MSWD = 1.2). Its primary melts estimated based on clinopyroxene composition are enriched in LREEs and Sr, and depleted in HREEs, HSFEs, and Pb. Moreover, they have high Nb/U (33–99) and Zr/Hf (32–41) ratios. Combined with their depleted (87Sr/86Sr)i ratios (0.704146–0.705100), the primary melts of Helanshan sill were most probably generated by low degree melting of carbonated asthenosphere mantle in the transitional depths of garnet- to spinel stabilities, rather than from metasomatized lithospheric mantle as indicated by whole-rock compositions. Striking compositional discrepancy exists between the primary melts and bulk rocks, and were principally caused by significant crustal assimilation during their ascent to emplacement level. As a result, some pivotal information about primary melts including less radiogenic (87Sr/86Sr)i ratios, Zr, Hf and Ti depletions, superchondritic Nb/U ratio, was erased during this process. The Helanshan mafic sill represents a new pulse of Neoproterozoic mafic magmatism in northwestern North China Craton, and formed as result of regional continental extension, which was probably related to the breakup of Rodinia supercontinent.