Contributions of mantle pyroxenite to the early Permian mantle magmatism in the southern Central Asian Orogenic Belt

Abstract

ABSTRACT Mantle magmatism is active in the southern Central Asian Orogenic Belt (CAOB) in the early Permian. This paper discusses contributions from mantle pyroxenite and the onset of the early Permian mantle magmatism based on the whole-rock major elements and Sr-Nd isotopes of the early Permian basalts and mafic gabbroic rocks as well as olivine chemistries. The clinopyroxene-orthopyroxene barometer yields 1.0–3.7 kbar of emplacement pressure for these mafic-ultramafic intrusions. The Ca-in-olivine hydrometer using compositions of the most primitive olivine (Fo≥86 mol.%) yields 2.7–4.1 wt.% H2O for the parental magmas. Thus, the early Permian mafic-ultramafic intrusions formed from hydrous magmas at shallow crusts. The bulk-rock evolution of most of these mafic gabbroic rocks is consistent with the liquid lines of descent of basaltic magmas in the Hark diagrams of MgO, CaO, Al2O3, and Na2O+K2O versus SiO2, suggesting that these gabbroic rocks have melt-like compositions. The gabbroic rocks and associated basalts are characterized by low CaO, high Fe/Mn ratios, and high FC3MS values (FeO/CaO-3*MgO/SiO2), and the primitive olivines (Fo >80 mol.%) have low Ca and Mn contents with expected high Ni contents without sulphide segregation, implying that pyroxenite-derived melts (46–71 wt.%) contributed to the early Permian mantle magmatism. In addition, Sr-Nd isotopic compositions of these early Permian intrusions suggest a mixing mantle source of the enriched mantle I (EMI) and high U/Pb mantle (HIMU) components, recording geochemical signatures of the previously subducted slabs. Mantle pyroxenite formed from reactions between slab-derived (eclogite-derived) melts and peridotite at high pressure. The onset of the early Permian mantle magmatism is possibly related to the lower solidus temperature and higher melt production of mantle pyroxenite (forming a mantle plume) relative to peridotite at high pressure.