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

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.

早二叠世，中亚造山带（Central Asian Orogenic Belt, CAOB）南部的地幔岩浆作用活跃。本文基于早二叠世玄武岩、镁铁质辉长岩的全岩主量元素、Sr-Nd同位素数据以及橄榄石化学成分，探讨了地幔辉石岩对该期地幔岩浆作用的贡献，以及早二叠世地幔岩浆作用的起始机制。利用单斜辉石-斜方辉石压力计计算得到，该类镁铁质-超镁铁质侵入岩的侵位压力为1.0~3.7 kbar。基于最原始橄榄石（Fo≥86 mol.%）成分的橄榄石钙含量水含量计结果显示，母岩浆的水含量为2.7~4.1 wt.% H₂O。据此认为，早二叠世镁铁质-超镁铁质侵入岩形成于浅地壳环境的含水岩浆体系中。多数镁铁质辉长岩的全岩演化趋势与玄武质岩浆的结晶分异轨迹一致，在以MgO、CaO、Al₂O₃、Na₂O+K₂O与SiO₂为变量的哈克（Hark）图解中呈现出良好的线性关系，表明该类辉长岩具有类熔体的成分特征。该类辉长岩与伴生玄武岩具有低CaO、高Fe/Mn比值以及高FC3MS值（FeO/CaO-3*MgO/SiO₂）的特征；且原始橄榄石（Fo>80 mol.%）具有低Ca、Mn含量与高Ni含量，未发生硫化物熔离，暗示地幔辉石岩来源的熔体（占比46~71 wt.%）参与了早二叠世的地幔岩浆作用。此外，该类早二叠世侵入岩的Sr-Nd同位素组成显示其源区为富集型地幔I型（enriched mantle I, EMI）与高U/Pb地幔（high U/Pb mantle, HIMU）的混合地幔源，记录了早期俯冲板片的地球化学印记。地幔辉石岩由俯冲板片来源（榴辉岩源）熔体与橄榄岩在高压下反应形成。相较于高压下的橄榄岩，早二叠世地幔岩浆作用的起始可能与地幔辉石岩更低的固相线温度与更高的熔体产出量（形成地幔柱）相关。