Magma Replenishment as Revealed by Textural and Geochemical Features of Plagioclase Phenocrysts in Subduction-Related Lavas  

作　　者：LIU Xiujin LIU Wei XIAO Wenjiao WAN Bo 

机构地区：[1]Key Laboratory of Geochemical Cycling of Carbon and Mercury in the Earth's Critical Zone, Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences [2]Geochemical Research Center of Soil Quality,China Geological Survey [3]Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences [4]Institutions of Earth Science, Chinese Academy of Sciences [5]State Key Laboratory of Lithospheric evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences [6]Xinjiang Research Center fo r Mineral Resources, Xinjiang Institute of Geography, Chinese Academy of Sciences

出　　处：《Acta Geologica Sinica(English Edition)》2019年第2期464-476,共13页地质学报（英文版）

基　　金：financial support by the Natural Science Foundation of China (41573030, 41503024);the Geological Survey Program from China Geological Survey (No. DD20190518)

摘　　要：Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.

关 键 词：MAGMA REPLENISHMENT resorption REE pattern PLAGIOCLASE phenocryst East JUNGGAR 

分 类 号：P588.1[天文地球—岩石学]