西昆仑大同岩体岩浆成因绿帘石矿物学研究及其对岩体形成构造环境的制约

MINERALOGICAL STUDY ON MAGMATIC EPIDOTE FROM DATONG PLUTON,AND ITS IMPLICATION FOR TECTONIC SETTING OF WESTERN KUNLUN REGION

大同岩体位于西昆仑北带,形成于早奥陶世,对于研究西昆仑造山带早古生代构造演化具有重要意义,但对其形成的构造环境的认识还具有很大的不确定性。本文尝试通过岩浆成因绿帘石的研究,为该岩体形成的构造环境提供新的制约。大同岩体中岩浆成因绿帘石呈自形-半自形,具有明显的成分环带,并发育褐帘石核,同时绿帘石熔蚀现象明显,常为新鲜黑云母包裹,或与新鲜的钾长石呈港湾状或不规则形态接触。绿帘石的Ps值(绿帘石端员组分的摩尔分数)介于0.26～0.31之间,并具有较低的TiO2含量,w(TiO2)<0.2%。绿帘石结晶于深部岩浆房(>1.0GPa),并在岩浆上升过程中发生熔蚀。绿帘石熔蚀速率计算结果显示,大同岩体岩浆上升速率不小于900m/a,从而表明岩体是快速上升侵位的产物,形成于拉张环境。结合区域地质资料,我们认为西昆仑地区早古生代早期存在一期俯冲挤压背景下的间歇拉张作用。

The Datong pluton, intruded in the West-Kunlun region during the Ordovician , plays an important role in exploring the tectonic evolution history of the West-Kunlun region during Paleozoic time, which is still under debate. The authors tried to put some new constraints on the tectonic evolution of this region via mineralogy and mineral chemistry studies of magmatic epidote. Magmatic epidote in the Datong pluton is present as subhedral to euhedral crystals included in biotite, hornblende, or, embaye2 or in vermicular contact with unaltered K-feldspar. Magmatic epidote with melting corrosion structure, zonation and allanite core can also be found in thin sections. The Ps （Ca2Al2Fe3＋ Si3O12 （OH） end member） of magmatic epidote lies between 0. 26- 0. 31, and TiO2 is 〈 0. 2% by weight. Magmatic epidote crystallized in a deep magma chamber with pressure of at least 1.0 GPa, and underwent corrosion by melt on the way of ascending. The rate of magma transport was calculated to be higher than 900 m/a, which can only be explained by fracture-transportation rather than diapir. The Datong pluton was emplaced rapidly in an extensional environment. Considering the regional geological conditions, the authors inferred that there existed an intermittent extension under a compression background in the West Kunlun region.