博格达上大河沿岩体铷-锶同位素年龄及其地质意义

Rb-Sr Isotope Age of the Shangdaheyan Intrusion in the Bogda Orogenic Belt and its Geological Implications

博格达造山带是上古生代裂谷带，于石炭纪末闭合。该带的侵入活动主要表现为大量辉绿岩岩床、岩墙和岩株，并有零星分布的闪长岩和花岗岩岩体。上大河沿侵入体一组矿物岩石样品的ＲｂＳｒ 等时线测定获得其年龄为２９８．４±０．７６ Ｍａ 和８７ Ｓｒ８６ Ｓｒ 初始比０．７０４１。结合野外证据，表明博格达山的大部分侵入岩定位于海西旋回，且是裂谷闭合后初始拉张阶段的产物，同时表明该带海西期以后的侵入活动并不重要。

The Bogda region is interpreted to be an upper Palaeozoic continental rift, which was closed towards the end of the Carboniferous period. Intrusive activities in that region are represented by a large number of diabasic sills, dykes and stocks with a few dioritic and granitic bodies. No date of age determination have so far been reported about these intrusive rocks.The Shangdaheyan intrusion is composed of a spectrum from dioritic to granitic rocks. These rocks are characterized mineralogically by the existence of augite with or without other pyroxene, and are thus considered to represent a series of differentiates of a basaltic magma. A group of samples from this intrusion were determined in this paper, which yielded a mineral-rock Rb Sr isochron with an age of 298.4±0.76 Ma and an initial 87 Sr/ 86 Sr ratio of 0.7041. With the support from field evidence, these results indicate that most of , if not all, the intrusions of the Bogda region were emplaced during the Hercynian cycle as a result of initial extension following rift closure.