藏北若拉岗日岩群变质火山岩的基本特征及构造属性

General characteristics and tectonic attributes of the metamorphic volcanic rocks from the Rola Kangri Group Complex,northern Xizang

关键词:  若拉岗日岩群  变质火山岩  藏北

Key words:  Rola Kangri Group Complex  metamorphic volcanic rock  northern Xizang

摘要:

藏北的若拉岗日岩群,因处于拉竹龙-金沙江缝合带南缘,其成生背景及构造属性历来倍受关注。白头山、岗盖日、狮头山等地的若拉岗日岩群主要为一套变质中基性火山岩系,不同地段夹杂大理岩、变质砂板岩、硅质岩、超基性岩等小岩片或块体,分别具有洋岛型、洋脊型及岛弧型火山岩的基本特征,并构成白头山西以岛弧火山岩为主的弧火山岩片,岗盖日以洋脊型变玄武岩为主夹杂硅质岩、超基性岩小岩片而共同组成的蛇绿混杂岩片及白头山、狮头山以洋岛型火山岩为主夹杂碳酸盐岩块体的大洋海山岩片,从而指出该区若拉岗日岩群为一套不同构造属性的火山岩与大理岩、硅质岩、超基性岩等相伴的构造混杂岩,其中既有某大洋的产物,又有某洋岛及陆缘岛弧的产物。

Abstract:

Much attention has long been drawn to the Rola Kangri Group Complex in northern Xizang for its tectonic setting in the southern part of the Lazhuglung-Jinshajiang suture zone.The 1∶250 000 geological survey in Baitoushan,Ganggairi and Shitoushan within the study area in 2004 has disclosed that the Rola Kangri Group Complex is composed of a suite of metamorphic intermediate-basic volcanic rocks intercalated with slices and/or masses of marble,metamorphic sandstone and slate,siliceous rocks and ultrabasic rocks.These rocks are all characteristic of oceanic-island,oceanic-ridge and island-arc volcanic rocks,including arc volcanic slices dominated by island-arc volcanic rocks in western Baitoushan;ophiolitic mélange slices dominated by oceanic-ridge metamorphic basalt intercalated with siliceous rocks and ultrabasic rocks in Ganggairi,and oceanic seamount slices dominated by oceanic-island volcanic rocks intercalated with carbonate rocks in Baitoushan and Shitoushan.It can be seen that the Rola Kangri Group Complex should be considered as the tectonic mélanges dominated by the volcanic rocks associated with marble,siliceous rocks and ultrabasic rocks formed in the oceanic,oceanic-island and continental-margin island-arc environments,and totally in the archipelagic arc-basin systems.