Geochronology, geochemistry and significance of the Early Paleozoic back-arc type ophiolite in Lvliangshan area, North Qaidam
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摘要: 岩石学、地球化学、年代学及Lu-Hf同位素综合研究表明在柴北缘西段绿梁山大平沟地区出露一套弧后盆地型蛇绿岩，岩石类型主要包括变质橄榄岩、变火山岩、变辉长岩及斜长花岗岩。其中变火山岩具有LREE亏损，类似N-MORB的稀土配分模式，同时又具有富集大离子亲石元素，亏损Nb、Ta等高场强元素的岛弧火山岩的地球化学特征，应形成在弧后盆地环境。斜长花岗岩为低钾准铝质花岗岩，具有LREE略微富集，HREE平坦的稀土配分型式，显示强烈Eu正异常，其εHf（t）值介于13.7~15.3之间，为变辉长岩部分熔融的产物，熔融温压条件可能为P=0.8~0.9GPa和T=~800℃。年代学研究结果表明变辉长岩的形成时代为535±2Ma，斜长花岗岩的形成时代为493±3Ma，指示本地区弧后盆地拉张时限至少介于493~535Ma之间，而柴北缘地区古大洋俯冲消减作用应早于535Ma。
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关键词:
- 地球化学 /
- 锆石U-Pb年龄和Hf同位素特征 /
- 弧后盆地型蛇绿岩 /
- 柴达木盆地北缘
Abstract: Petrological, geochemical, geochronological and Lu-Hf isotopic studies indicate there is an ophiolite mélange in the Lvliangshan area, west segment of the North Qaidam of China. This ophiolite mélange mainly consists of metaperidotite, metavolcanic, metagabbro and plagiogranite. The geochemical characteristics of metavolcanic show LREE depletion, similar to N-MORB, and its spider diagram shows the enrichment of the LILE and depletion of HFSE which formed in a back-arc basin setting. The plagiogranite is low-potassium metaluminous granite with the characteristics of relative enrichment of LREE and positive Eu anomaly. Zircons from plagiogranite have εHf(t)=13.7~15.3, indicating the plagiogranite could have originated by partial melting of metagabbro under the conditions of P=0.8~0.9GPa and T=~800℃. Zircon U-Pb geochronological studies suggest that the formation ages of metagabbro and plagiogranite are 535±2Ma and 493±3Ma, respectively. In this study, we proposed the existence of back-arc basin in Lvliangshan area between 493~535Ma, and the oceanic slab subduction occurred before 535Ma in the North Qaidam. -
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