山东昌乐新生代碱性玄武岩中的巨晶单斜辉石成因研究

Translated title of the contribution: Study on the genesis of clinopyroxene megacrysts in the Cenozoic alkali basalt at Changle, Shandong Province

Xiao Ming Chen, Li Hui Chen, Xi Sheng Xu

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

There are four types of pyroxenes in the Cenozoic alkali basalt at Changle, Shandong Province; phenocryst clinopyroxenes, metrix clinopyroxenes, clinopyroxene megacrysts and homologous orthopyroxene xenocrysts. The coronas, between the clinopyroxene megacrysts and alkali basalt, could be divided into complex coronas and simple coronas. The complex coronas appeared in the outside of larger clinopyroxene megacrysts and could be divided into four zones based on their characteristics of textures and components. The zones of interior, transitional and exterior are composed of clinopyroxenes with different textures and a few melts and metal oxides. The edge zone is composed of clean clinopyroxene without melt and metal oxide. The clinopyroxenes at the coronas display comb texture in the interior zone, sieve texture in the transitional zone and both similar sector-zoned and sieve texture in the exterior. The compositions of the clinopyroxenes, melts and metal oxides at the coronas are controled by the clinopyroxene megacrysts only in the interior zone, and by basaltic magma in other zones. The simple coronas appeare in the outside of samller clinopyroxene megacrysts and are composed of whole clinopyroxene without melt and metal oxide. The coronas of clinopyroxene megacrysts are resulted from the temperature difference between clinopyroxene megacrysts and basaltic magma. If the grain of clinopyroxene megacryst is larger, the time taken to adjust the difference of temperature is longer, then the complex coronas with four zones will form; if the grain is smaller, the simple coronas with edge zone will form as a result. The compositions of homologous orthopyroxene xenocrysts are within the range of compositions of the orthopyroxenes in the lherzolite xenoliths. The clinopyroxene rim or reaction rim surrounding the orthopyroxene xenocrysts are the results of the compositional differnece between orthopyroxene and basaltic magma, similar to the reaction rim texture described by Bowen (1956). All clinopyroxene megacrysts, clinopyroxenes in the lherzolite xenoliths and part of the core of clinopyroxene phenocrysts are located in the "granulites and the inclusions in basalts" field of Aoki and Shiba (1973) in the A1 IV-A1VI diagram, suggesting a relatively high-pressure crystallization. It suggests that part of the core of clinopyroxene phenocrysts come from mantle lherzolite, and they belong to xenocrysts. Clinopyroxenes in the coronas, metrix clinopyroxenes and a majority of clinopyroxene phenocrysts are plotted in the "igneous rocks" field. Based on the estimated crystallization pressure for clinopyroxene megacrysts and clinopyroxenes in the lherzolite xenoliths, we suggest a formation mechanism for the clinopyroxene megacrysts as following: first, the asthenosphere-derived alkali basaltic melts invaded into the lithospheric mantle near the lithosphere asthenosphere boundary and produced clinopyroxene megacrysts, then the melts with a great lot of clinopyroxene megacrysts mixed with the existing magmatic system from mantle lithosphere entraining an abundance of the xenocrysts of clinopyroxenes and orthopyroxenes and lherzolite xenoliths. The mixed alkali basaltic magmas with clinopyroxene megacrysts, xenocrysts and xenoliths ascended from the crustal magma chambers, and erupted at the surface.

Original languageChinese
Pages (from-to)1105-1116
Number of pages12
JournalYanshi Xuebao/Acta Petrologica Sinica
Volume25
Issue number5
Publication statusPublished - 2009
Externally publishedYes

Keywords

  • Alkali basalt
  • Changle
  • Clinopyroxene megacryst
  • Forming model
  • Pyroxene xenocryst
  • Shandong province

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