作者：周喜文 耿元生贺兰山孔兹岩系变质时代华北克拉通演化regionkhondaliteseriesmetamorphicagenorthchinacraton黑云斜长片麻岩碎屑锆石年龄岩浆活动石榴magmaticorigin鄂尔多斯地块timeshrimpagesordosblock富铝片麻岩
摘要：贺兰山孔兹岩系作为华北克拉通西部孔兹岩带的重要组成部分,其变质时代问题一直悬而未决.利用SHRIMP锆石U-Pb定年技术,对贺兰山孔兹岩系中3个代表性富铝片麻岩(石榴堇青钾长片麻岩、石榴堇青二长片麻岩与石榴黑云斜长片麻岩)样品进行了精确定年.发现这3种岩石虽处不同层位,但其碎屑锆石年龄却非常集中,各测点~(207)Pb/~(206)Pb年龄总体变化在2.0～2.1Ga之间,加权平均年龄则在2017～2040Ma之间.这些碎屑锆石都具有岩浆结构特征,反映当时曾存在大规模花岗质岩浆活动,所成岩体为孔兹岩系沉积提供了充足物源.另有少量大于2.5Ga的碎屑锆石(2520～2949Ma),表明本区存在太古代岩浆活动记录.本区石榴堇青二长片麻岩中发育典型的变质增生锆石,其成因很可能与黑云母的脱水熔融反应有关.利用该锆石确定贺兰山孔兹岩系的变质时代为1950±8Ma.该时代与东部大青山、乌拉山孔兹岩系变质时代相同,表明华北克拉通西部的阴山地块与鄂尔多斯地块大体是以平行的方式正面拼贴到一起的,形成了目前的孔兹岩带. Abstract： The khondalite series in the Helanshan region is a main part of the Khondalite Belt in the western block of the North China Craton, whose metamorphic time is still unknown. Using SHRIMP zircon U-Pb dating technique, three representative Al-rich gneiss samples from the Helanshan khondalite series were analyzed, including garnet-cordierite-K-feldspar gneiss, garnet-cordierite monzo-gneiss and garnet-biotite-plagioclase gneiss. The results show that the three rocks have nearly same detrital zircon age though they were collected from different strata. Apparent ~(207)Tpb/~(206)Pb ages of detrital zircons range from 2. 0 to 2. 1Ga and weighted mean ages are from 2017Ma to 2040Ma. According to CL image and Th/U value, all the detrital zircons are magmatic origin, indicating a large scale of granitic intrusion at that time, which could provide en
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