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洞错地幔橄榄岩、均质辉长岩矿物化学特征及其构造意义

, PP. 308-319

Keywords: 洞错蛇绿岩,矿物学特征,尖晶石,橄榄石,辉石

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Abstract:

洞错蛇绿岩出露于改则县洞错北约20km处,去申拉以南,地表形态为楔状体,大致沿日土-改则-丁青断裂带分布;侵位于侏罗系木嘎岗日群中,并被下白垩统朗山组不整合覆盖。蛇绿岩剖面各岩石单元发育齐全:主要由地幔橄榄岩(方辉橄榄岩、二辉橄榄岩)、堆晶岩、辉绿岩墙和壳层熔岩组成。对洞错蛇绿岩地幔橄榄岩和均质辉长岩中尖晶石和辉石化学成分测定和构造环境判别分析显示:洞错地区尖晶石二辉橄榄岩(XDC28)投点于深海橄榄岩区域,指示尖晶石二辉橄榄岩经历了少于10%的熔体提取,而方辉橄榄岩分别经历了13%~30%的亏损,代表了亏损程度较弱的地幔橄榄岩残留(<30%),明显不同于岛弧环境地幔橄榄岩强烈亏损的特征。辉石化学成分研究确认了尖晶石二辉橄榄岩属于深海橄榄岩。斜方辉石和单斜辉石具有不同的形态和化学成分,反映了不同的结晶温度(分别为900~1000℃,<500~600℃)。前者为岩石的形成温度,而后者则为冷却变质或重结晶过程中达到再平衡时的温度。

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