青藏高原北部可可西里狮头山含硬玉岩类的基本特征及地质意义
Basic characteristics and geological significance of jadeite-bearing rocks at the Shitoushan,Hoh Xil Mountains ,Qinghai
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摘要: 描述了可可西里山南狮头山含硬玉岩类的岩石学、矿物学特征。含硬玉岩类的原岩为辉长岩，围岩为石炭—二叠系。由于变质作用不彻底，保留有原岩的矿物组合。典型的高压变质矿物组合为：钠长石 硬玉 霓石 蓝闪石 绿帘石 绿泥石。硬玉SiO2、FeO偏低，Al2O3、Na2O明显偏高，变质矿物组合中以富钠矿物为主，缺少石英，与国内外含硬玉岩类的原岩成分、变质矿物组合均不相同，是一种新的高压低温硬玉岩类。狮头山高压变质带向西延至若拉岗日、大横山一带，与北侧的西金乌兰—金沙江板块缝合带(西段)平行产出，以此推测缝合带南侧的羌北—昌都板块向北消减于巴颜喀拉板块之下。Abstract: The paper describes the petrology,mineralogy and mineral composition of jadeite-bearing rocks at South Shitoushan,Hoh Xil Mountains ,northern Qinghai-Tibet Plateau.The protolith of jadeite-bear-ing rocks is gabbro,with Permo-Carboniferous country rocks.As metamorphism did not proceed thor-oughly,the mineral assemblage of the protolith have been preserved.The typical high-pressure metamor-phic mineral assemblage is albite+jadeite+aegirine+glaucophane+epi dote+chlorite.Jadeite has lower SiO 2 and FeO contents and notably higher Al 2 O 3 and Na 2 O contents.The metamorphic mineral assemblage consists dominantly of Na-rich minerals and lacks quartz,which is different from the protolith composition and metamorphic mineral assemblage of jadeite-bearing rocks in China and other countries.So it is considered to be a new type of high-pressure and low-temperature jadeite rock.The Shitoushan high-pressure meta-morphic belt extends westward to Rola Kangri and Dahengshan and parallels the Xijinwulan-Jinsha suture zone(west segment )to the north.Thus it is inferred that the North Qiangtang-Qamdo plate on the southern side of the suture zone has been subducted northward beneath the Bayan Har plate.
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