新疆库米什变质地体研究

THE STUDY OF KUMISHI METAMORPHIC TERRANE IN XINJIANG

新疆库米什地区的变质杂岩,过去曾被认为是泥盆系地层和海西期花岗岩体。本文研究表明,该杂岩体的诸方面特征与其周围古生代岩块存在明显差异,而与塔里木克拉通北缘库鲁克塔格地区的托格拉克布拉克群可以对比。实际上,库米什变质杂岩并非泥盆系地层和海西期花岗岩体。而是天山造山带中的前寒武纪变质地体,是因弧后扩张从塔里木克拉通北缘裂解出来的大陆碎块。

Kumishi metamorphic terrane is a Precambrian complex in the Tianshan orogenic belt. The Aketaxi mafic-ultramafic rocks and the Southern Tianshan tectonic melange demark the northern and southern boundaries of the terrane, respectively, and separate the terrane from an adjacent Paleozoic block by showing the fault contact between Kumishi terrane and Paleozoic rocks. Although the Kumishi terrane was considered to be composed of Devonian strata and Hercynian granite bodies, the gneisses of the terrane are dated at 2600-2800 Ma by a single zircon grain. The U-Pb evaporation method indicates that it is a metamorphic complex formed during the Precambrian. The study of metamorphic characteristies, rock-mineral assemblages, and the structural deformation style for the terrane suggests the terrane differ in many aspects from its adjacent Paleozoic block and, instead, is more similar to the Tuogelakebulake complex of the Kuruketage region in the northern margin of Tarimu craton. These findings imply that the Kumishi terrane is a fragment of continental crust derived from the Tarimu craton.The Kumishi terrane accreted to its present position from back-arc spreading of the south Tianshan area. The subduction of Paleo-Jungger-Tianshan oceanic plate toward the Tarimu plate 1ed to the development of grabens in the northern margin of the Tarimu craton. A Rb-Sr whole-rock isochron age recorded tectonothermal event affected all of northern Tarimu before the Kumishi terrane separated from the Tarimu craton in the Middle-Late Ordovician. During the Silurian, the oceanic ophiolites and radiolarian siliccous rocks were generated during the formation of the south Tianshan marginal sea. At the same time, Kumishi terrene was pulled into the marginal sea basin. In the Late Devonian, the south Tianshan marginal sea began to close and the Kumishi terrane accreted to the northern margin of Tarimu craton which forms its present tectonic assemblage.