南天山榆树沟高压麻粒岩地体锆石U—Pb定年及其地质意义

南天山榆树沟高压麻粒岩地体的主体为一套蛇绿岩。从该地体顶部单元的岩石中分离出的锆石绝大部分为浑圆形或椭球形，少量为两端略圆化的四方柱状晶体；镜下粒度分析结果显示分选良好的典型沉积特征；其ＺｒＯ２／ＨｆＯ２比值在４５－５７之间；结合岩石的岩石学和地球化学特征，推测斜长石榴苏辉岩的原岩可能主要由来自洋岛的基性火山激浪沉积碎屑和洋底风化产生的粘土组成。

ZIRCON U-Pb AGE AND ITS GEOLOGICAL SIGNIFICANCE OF HIGH-PRESSURE TERRANE OF GRANULITE FACIES IN YUSHUGOU AREA, SOUTHERN TIANSHAN MOUNTAIN

The Yushugou high-pressure terrain of granulite facies in southern Tianshan Mountain is basically composed of ophiolite rock. Zircon grains are selected from the rock in the upper part of the terrain, and most of them are round or elliptical, some are tetragonal prism with round edges. The granulometric analyses show that they are well-sorted in sedimentation. The ZrO2/ HfO2 ratios range from 45 to 57. These characteristics mentioned above, together with the petrological and geochemical ones of plagioclase-garnet-orthopyroxenite, indicate that the protolith of the rock bearing the zircons may be mainly derived from volcanic surf sedimentary debris in oceanic island and from clay formed by weathered seafloor, and the zircons are simply volcanic debris. The ophiolite age is about 440Ma and the metamorphic age of the first stage (amphibolite facies or granulite facies) is about 364Ma obtained with the method of concordia diagram. The ages provide an important evidence for determining the space-time extension of Paleozoic southern Tianshan ocean, calculating the subduction rate of oceanic crust and studying the formation mechanism of ophiolite of granulite facies.