蛇绿岩及蛇绿岩中浅色岩的SHRIMP U-Pb测年

SHRIMP DATING OF OPHIOLITE AND LEUCOCRATIC ROCKS WITHIN OPHIOLITE

文中简要评述了蛇绿岩的层状辉长岩,斜长岩和斜长花岗岩,以橄榄岩为主岩的花岗岩和蛇绿岩中的埃达克岩的锆石SHRIMP U-Pb年龄的地质意义。层状辉长岩(或堆晶层状辉长岩)通常起源于洋脊下的岩浆房,因而它的形成年龄代表洋壳形成的时代。斜长岩与层状辉长岩的时代相近或略晚。斜长花岗岩年龄的解释极其依赖锆石组成和地球化学证据。橄榄岩为主岩的花岗岩,可能记录蛇绿岩的侵位时代。蛇绿岩中的埃达克岩是消减洋壳在深部的部分熔融的产物。文中发表了新疆扎河坝蛇绿岩SHRIMP定年的中间成果,并简略地介绍了滇川西部金沙江和内蒙古图林凯等地的研究实例。根据层状辉长岩的测定结果,扎河坝蛇绿岩形成于(489±4)Ma,金沙江蛇绿岩形成于(328±8)Ma。内蒙古图林凯蛇绿岩中埃达克岩形成于(467±13)Ma～(429±7)Ma。块状辉长岩、斜长花岗岩和橄榄岩为主岩的花岗质岩石记录了蛇绿岩的复杂演化。新疆扎河坝蛇绿岩中的块状辉长岩中存在多组锆石年龄值。较老的一组为468～511 Ma,与层状辉长岩和斜长岩相似,记录了蛇绿岩或洋壳的形成时代,但是,岩石中的大部分锆石年龄为396～419 Ma,加权平均年龄为(406±4)Ma,可能反映了一次部分熔融事件。滇川西部金沙江蛇绿岩中的斜长花岗岩的形成年龄为约300～285Ma,晚于层状辉长岩和?

This paper is a brief overview on the geochronological significance of SHRIMP zircon dating of layered gabbro, anorthosite and plagiogranite, as well as adakite and peridotite-hosted granite that occur within ophiolites. Layered gabbro (layered cumulate gabbro) was derived from magma chamber under ridge, thus its formation age records the generation of oceanic crust. Anorthosite was coeval with or slightly younger than the hosting layered gabbro. For plagiogranite, the interpretation of zircon ages strongly depends on the population and geochemical evidence. The peridotite-hosted granite may record the timing of emplacement of ophiolite. The adakite associated with ophiolite resulted from partial melting of the subducted oceanic crust. The paper presents the ongoing SHRIMP data of the Zhaheba ophiolite, Xinjiang. Also, SHRIMP dating of the ophiolites along the Jinshajiang, Western Yunnan and Si-chun Provinces, and the adakites in the Tulingkai ophiolite, Inner Mongonia is briefly introduced. According to the ages obtained from the layered gabbros, the Zhaheba ophiolite was formed at 489 + 4 Ma, and the Jinshajiang, (328+8) Ma. The adakites are dated at (467±13) Ma-(429±7) Ma. Massive gabbro, plagiogranite and peridotite-hosted granite record the complicated geological evolution of ophiolite. There are two more age groups obtained in the Zhaheba massive gabbro. The older group ranges from 468-511 Ma, which is consistent with the ages of layered gabbro and anorthosite. However, the majority of ages are 396-419 Ma,with weighted mean of (406±4) Ma,probably reflects a later partial melting. In the Jinshajiang Ophiolite, the plagiogranites were formed at ca. 300~285 Ma, which is significantly younger than the layered gabbro and anorthosite. The studied plagiogranites are high in total REE content, enriched in LREE, with high initial 87Sr/86Sr. Inherited zircons also indicate the presence of an older crust component. This evidence suggests that continental contamination was involved in the magma generation and evolution, probably related to the subduction of the ocean crust. Zircons from the Jiyidu peridotite-hosted granite vein yield the age of (263 ±6) Ma, that is in good agreement with the age of the granodiorite intruding the ophiolite. The peridotite-hosted granite is much younger than the formation of ophiolite, but it is significantly older than the Indosinain (ca. 220 Ma) collisional granitoids in the region. Therefore it may reflect the emplacement of the ophiolite.