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《Earth Science Frontiers》 2003-04
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SHRIMP DATING OF OPHIOLITE AND LEUCOCRATIC ROCKS WITHIN OPHIOLITE

JIAN Ping, LIU dun-yi, ZHANG Qi, ZHANG Fu-qin, SHI Yu-ruo, SHI Guang-hai, ZHANG Lu-qiao, TAO Hua Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China; Institute of Geology and Geophysics , Chinese Academy of Science , Beijing 100029, China; Institut of Regional Geology of the Nei Monggol Autonomous Region , Huhhot 010055, China  
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.
【Fund】: 国家自然科学基金(40234045);; 国家自然科学基金(49902005 4072043);; 国土资源部国际合作与科技司重点项目(2000206)
【CateGory Index】: P588.3;P597
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