| 其他摘要 | Panzhihua syenitic stock in the Pan-Xi area, spatially and temporally associated with the giant Fe-Ti-V oxide deposit-bearing Panzhihua gabbroic inrrusion, is genetically related to the Late Paleozoic mantle plume magmatic activity. This syenitic intrusion consists of intermediate intrusive and felsic rocks, and related felsic extrusive rocks, and syenite porphyries, exhibiting continuous variations in chemical composition. In combination with previous research results, a systematic study on mineral chemistry, major element, trace element and Sr-Nd isotopic characteristics of the Panzhihua syenitic pluton and related felsic rocks was conducted. This paper aims to discuss magma evolution and petrogenesis of the Panzhihua syenitic intrusion, and assess the mechanism of crustal evolution during mantle plume upwelling. This work has allowed us to reach the following conclusions:
(1) The diorite in the Panzhihua syenitic intrusion consists predominantly of oligoclase and andesine, and subordinate augite and fayalite. The principal minerals in syenite are perthites and albites, and the subordinate minerals are augite, ferro-barroisite, ferro-richterite. The quartz syenite comprises mainly perthites and quartzs. The alkaline granite consists predominantly of perthites and quartz, with subordinate augite, and arfvedsonite. The syenite porphyry is composed mainly of phenocrysts of microcline and albite, and groundmass of microcline, albite, quart, biotite, and apatite. The trachyte displays porphyritic textures with phenocrysts of albite, quartz, and augite. The rhyolite comprises phenocrysts of albite and quart, and groundmass made of albite, K-feldspar, arfvedsonite and riebeckite.
(2) Mineral chemistry study indicates that, Na and K contents in feldspars increase from intermediate to felsic rocks, whereas Ca contents decrease. The Fe,Na, and Ca concentrations in augites increase with decreasing Mg contents. In contrast, amphiboles in felsic rocks have higher Fe and Na and lower Ca and Mg contents than those in intermediate rocks. The characteristics suggest that the chemical variations of minerals in different rock types are mostly controlled by fractional crystallization of parental magmas.
(3) Trace-element study implies that the Panzhihua syenitic intrusion has distinctive A-type chemistry characterized by elevated high-field-strength element (HFSE) contents and high Ga/Al ratios. Nb/Ta and Zr/Hf ratios of the syenitic intrusion are similar to those of the typical oceanic island basalt (OIB), indicating that they share a common source.
(4) The similarity of Sr and Nd isotopic compositions for all the rocks in the Panzhihua syenitic stock suggests that their parental magmas were derived from a common reservoir. Diorite, syenite, syenite porphyry, quartz syenite and alkaline granite predominantly originated by two distinct syenitic melt from differentiation of the underplated, Emeishan plume-related, mildly alkaline basaltic magmas at or near the base of the crust, and subsequent fractional crystallization of individual melts in the upper crust. Some of these melts erupted to the surface, thereafter formed trachyte and rhyolite.
(5) The present study reveals that the underplating of huge basalt magmas near or at the base of lower crust is necessary for the formation of the A-type granites in Emeishan large igneous province. The Late Paleozoic mantle plume activity plays an important role in the growth of juvenile crust in the inner zone of the Emeishan large igneous province. |
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