Characteristics and Petrogenesis of the Zoned Plagioclase Intrachyandesite from the Shangshu Formation, Jiangshan, Zhejiang Province
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摘要: 火成岩中斜长石的环带结构和成分特征与岩石成因密切相关，可以反映岩浆的压力、温度、熔体成分和水含量。本文对浙江江山新元古代上墅组粗安岩中具环带结构的斜长石开展了详细的矿物学研究，以揭示蕴含的成因信息，为区域岩浆演化过程提供约束。显微镜下观察斜长石可分为斑晶和基质两种，其中斑晶可见明显环带特征，背散射电子图像显示核部多呈熔蚀的板状且较宽较暗，边部较窄较明亮，在核部与边部边界处有较多的钛铁矿微晶。电子探针分析表明斜长石核部相对富K、Na、Si，斜长石An（[100×Ca/(Ca+Na)]）=44~52，属于拉长石—中长石；边部相对富Ca、Al、Fe、Mg，An=54~66，属于拉长石，呈现反环带结构。显微镜下观察发现斜长石边部较窄且含有大量钛铁矿包体，反映结晶环境突变，同时斜长石核部多呈现熔蚀的板状，指示除了结晶环境的突变外，岩浆体系中还有外来富钙质岩浆的加入，因此反环带结构可能是岩浆混合的结果。区域资料显示浙江境内的上墅组主要形成于新元古代华夏与扬子两大陆块碰撞拼贴之后的陆内拉张构造背景，为一套双峰式火山岩。因此，本文研究的斜长石反环带结构可能是幔源的基性岩浆上升注入到经下地壳重熔形成的偏酸性浅部岩浆房后发生岩浆混合作用形成的。Abstract: The oscillatory zone and compositional characteristics of the plagioclase is closely related to petrogenesis, which can reflect the pressure, temperature, melt ingredients, and water content of the magma. In this paper, a detailed mineralogical study of the plagioclase with oscillatory zone in the Neoproterozoic Shangshu Formation has been carried out in order to reveal the petrogenesis and regional magmatic evolution. The plagioclase can be divided into phenocrysts and substrates under the microscope, and the phenocrysts have an obvious oscillatory zone, the core of which shows a corroded tabular with dark color in the backscattered electron image, but the edge is narrow and bright. The boundary between the core and the edge contains a large number of ilmenite microcrystals. Electron probed analysis results show that the plagioclase has a reverse zoning structure, of which the core is relatively rich in K, Na, and Si, with An ([100*Ca/(Ca+Na)]) =44~52, belonging to labradorite-andesine, while the edge is relatively rich in Ca, Al, Fe, and Mg, with An=54~66, belonging to labradorite.The reverse zoning structure of plagioclase is probably caused by magma mixing and temperature and pressure rapidly changing as the plagioclase core has a corroded tabular, but the edge is narrow and contains a high number of ilmenite inclusions, which indicate there exists the sudden crystalline environment change and external magma injection. Combined with the regional data, the Shangshu Formation is a set of bimodal volcanic rocks formed at the intracontinental tensional tectonic setting following the collision of the Cathaysia and Yangtze continents in the Neoproterozoic. It is suggested that the plagioclase with oscillatory zone is the result of magmatic mixing after the mantle-derived basic magma rises and injects into the slightly acidic shallow magma chamber, which is formed after the remelting of the lower crust.
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