新疆尼勒克县布谷拉超钾质岩浆岩的~(40)Ar-~(39)Ar年龄和地球化学特征

本文在西天山尼勒克县布谷拉地区厘定出了具有构造指示意义的超钾质岩浆岩,它侵位在石炭纪大哈拉军山组地层中,出露面积约为0.02km2,在空间上该类岩石与铜的矿化异常带或矿化带紧密共生。布谷拉超钾质岩浆岩具有典型的斑状结构,斑晶以钾长石、橄榄石、辉石为主,偶见白榴石残留体或假象。造岩矿物及岩石化学组成特征显示,布谷拉超钾质岩浆岩为斜长白榴岩类超钾质岩浆岩。布谷拉超钾质岩浆岩微量元素蛛网图表现为Nb-Ta、Zr-Hf、Ti的负异常和Pb的正异常,暗示该超钾质岩浆岩形成于造山阶段。微量元素地球化学特征显示,布谷拉超钾质岩浆岩是含金云母的石榴石尖晶石二辉橄榄岩部分熔融的产物,其形成深度介于70～80km狭窄的范围内。布谷拉超钾质岩浆岩的40Ar-39Ar有效坪年龄为274.2±1.6Ma,等时线年龄为274.3±4.4Ma,表明该类岩石形成于早二叠世。布谷拉超钾质岩浆岩的地球化学特征及40Ar-39Ar定年研究结果显示,在早二叠世至少阿吾拉勒山的东段仍处于造山演化阶段。

40Ar/39Ar age and geochemistry of the ultrapotassic magmatic rocks from Bugula in Nileke, Xinjiang and its tectonic implication.

The ultrapotassic magmatic rocks with tectonic significance have been confirmed at Bugula in Nileke, West Tianshan. It intruded in the Carboniferous Dahalajunshan Formation outcrop area is about 0.02km², associated with the copper anomalous or mineralized zones. The ultrapotassic magmatic rocks show typical porphyritic texture, and the dominated phenocrysts are K-feldspar, olivine and pyroxene with some residual leucite or its pseudomorph occasionally. The Bugula ultrapotassic magmatic rocks are defined as the plagioleucitites type of ultrapotassic rocks by means of their rock-forming minerals and geochemical characteristics. Their negative Nb, Ta, Zr, Hf, Ti anomalies and positive Pb anomaly in the trace element spider diagram suggest that the Bugula ultrapotassic magmatic rocks were formed in the orogenic stage. The trace element geochemical study indicaes that the ultrapotassic magma might be the products of partial melting of the phologopite-bearing garnet-spinel lherzolite occurred in the depth of a narrow range of 70km to 80km. A plateau age of 274.2±1.6Ma of a flat ⁴⁰Ar-³⁹Ar spectrum was obtained for the ultrapotassic magmatic rocks by laser stepwise heating method. Its inverse isochron age of 274.3±4.4Ma is agreed with the plateau age within analytical error. So, we suggested that the eastern Awulale Mountains were within the stage of orogenic evolution in Middle Permian at least on the basis of the geochemical and geochronological studies of the ultrapotassic magmatic rocks.