二叠纪峨眉山大火成岩省正长岩和过碱性花岗岩成因的矿物微量元素和同位素约束

Syenite and peralkaline granite plutons of the Permian Emeishan large igneous province mainly occur in the Pan-xi region and are associated spatially and temporally with mafic-ultramafic intrusions that host large Fe-Ti-V oxide deposits. It remains unclear if there is a genetic link between the formation of syenite and peralkaline granite plutons and Fe-Ti-V oxide bearing layered intrusions. We are going to carry on a project that is aimed to in situ analyze trace elements and Sr, Pb isotopes for primary minerals including clinopyroxene, hornblende, plagioclase and K-feldspar in the different types of the rcoks in these intrusions and plutons and to obtain partition coefficients of REE, LILE and HSFE between Na-rich clinopyroxene and melt using a Piston Sylinder apparatus. The new data set will enable us to calculate the trace element compositions of melts that are equilibrium with different types of clinopyroxene in syenite and peralkaline granite plutons. The results will help to decipher the nature of source rocks and magma differentiation in the formation of syenite and peralkaline granite and to discuss the origin of syenite and peralkaline granite and their relationships with layered intrusions. The final goal is to provide more information on mantle source, magma diversity and differentiation and related mineralization for better understanding the whole magmatic system of the mantle plume regime in the formation of the Emeishan large igneous province.

二叠纪峨眉山大火成岩省正长岩和过碱性花岗岩主要发育在攀西地区，并与含钒钛磁铁矿床的镁铁-超镁铁质层状岩体具有较好的时空耦合关系，正长岩和过碱性花岗岩的成因及与层状岩体和大型钒钛磁铁矿床是否存在成因联系一直备受关注。本次研究拟开展正长岩、过碱性花岗岩和层状岩体中主要组成矿物的原位微量元素和Sr、Pb同位素分析工作、并通过活塞圆筒装置测定碱性岩浆体系中富Na辉石/熔体的分配系数。利用前人和本项目获得的分配系数估算与不同类型单斜辉石平衡的熔体微量元素组成，查明正长岩和过碱性花岗岩的源岩/源区性质、成分和岩浆演化过程以及岩浆侵位过程中地壳组分加入程度；探讨正长岩和过碱性花岗岩的成因，揭示其与镁铁-超镁铁质层状岩体和大型钒钛磁铁矿床的成因联系，为探讨峨眉山大火成岩省巨量岩浆的源区特征、多样性、演化过程和成矿作用提供重要信息，从而更加全面地了解峨眉山大火成岩省的地幔柱岩浆体系。

二叠纪峨眉山大火成岩省正长岩和过碱性花岗岩主要发育在攀西地区，并与含钒钛磁铁矿床的镁铁-超镁铁质层状岩体具有较好的时空耦合关系，正长岩和过碱性花岗岩的成因及与层状岩体和大型钒钛磁铁矿床是否存在成因联系一直备受关注。本次研究针对正长岩、过碱性花岗岩和层状岩体中主要组成矿物进行了原位微量元素和Sr、Pb同位素分析，并通过活塞圆筒装置测定碱性岩浆体系中富Na辉石/熔体的分配系数。研究结果表明，层状岩体中与单斜辉石平衡的熔体成分与峨眉山高钛玄武岩成分类似，单斜辉石的Zr-Hf负异常与Zr和Hf在单斜辉石和熔体之间的分配系数较低有关。因此，高钛玄武岩和层状岩体分别代表深部岩浆房高钛苦橄质岩浆经分离结晶作用产生的高钛玄武质岩浆的喷出和侵入产物。层状岩体和偏铝质正长岩中与单斜辉石平衡的熔体成分比较之后显示，偏铝质正长岩是底侵的玄武质岩浆分离结晶的产物。层状岩体中主晶矿物环带的结构和成分结构表明，在其形成过程中岩浆房经历了粒间熔体的岩浆不混熔作用，造成岩浆房下部聚集大量富铁熔体形成钒钛磁铁矿矿床，由于重力作用向上运移的富硅熔体一方面形成淡色辉长岩，另一方面可能演化成过碱性花岗岩。但这一点需要我们获得准确的富Na辉石/熔体的分配系数才能确定。综上，本项目通过详细的矿物学研究，揭示了攀西层状岩体与时空上紧密共生的高钛玄武岩、正长岩和过碱性花岗岩之间的关系，为理解峨眉山大火成岩省巨量岩浆的源区特征、多样性、演化过程和成矿作用提供重要信息。相关研究已发表英文SCI论文4篇，中文核心期刊论文4篇、以及1篇硕士论文。1名成员入选中科院青促会，1名成员入选湖北省楚天学子计划。