摘要:
蛇绿岩是造山带研究的关键问题之一,文章在野外地质调查的基础上,通过岩石学、同位素年代学、地球化学等综合研究,认为东秦岭丹凤地区分水岭蛇绿岩是与俯冲带相关的SSZ型蛇绿岩。其中的变玄武岩是产于弧前构造环境的玻玄岩,它具有较玻安岩稍低的SiO2含量,同时具有高MgO(9.24%~18.74%)和 Mg#(74~85)、低 TiO2(0.15%~0.38%)和球粒陨石标准化稀土元素配分曲线呈“U”型及相容元素 Ni、Cr丰度较高,高场强元素(HFSE)Y、V、Zr、Yb 丰度较低等地球化学特征。变辉长岩的 LA-ICP-MS 锆石 U-Pb年龄表明分水岭蛇绿岩形成于(499.7±6.1) Ma,玻玄岩的存在暗示着此时的古商丹洋强烈向北俯冲,在洋内俯冲带和北秦岭南缘之间形成了与俯冲带相关的SSZ型蛇绿岩,进一步补充完善了秦岭造山带的地质演化历史。
关键词:
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蛇绿岩
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玻玄岩
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弧前
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东秦岭
Abstract:
Ophiolite is one of the key topics in orogenic belt research. In this paper, based on field survey, the authors studied petrology, isotopic age and geochemistry comprehensively, and reached the conclusion that Fenshuiling ophiolite in Danfeng area of Eastern Qinling Mountains is SSZ type ophiolite related to the subduction zone. Metabasalt of ophiolite is a kind of hyalobasalt formed in the fore-arc setting and possesses the following geochemical features in comparison with bonite: its SiO2content is a little lower, MgO (9.24%~18.74%) and Mg#(74~85) values are higher, and TiO2(0.15%~0.38%) values are much lower. Its chondrite-normalized REE patterns are of “U” type, compatible elements Ni, Cr are higher, and HFSE Y, V, Zr, Yb values are relatively lower. The LA-ICP-MS zircon U-Pb age of metagabbro ndicates that Fenshuiling ophiolite was formed at (499.7±6.1) Ma or so. The existence of hyalobasalt suggests that the paleo-Shangdan oceanic crust subducted northwards strongly, and SSZ type ophiolite was formed in the fore-arc setting between the subduction zone and the southern edge of the Northern Qinling block. The results achieved by the authors further complete the understanding of the evolution of the Qinling orogenic belt.
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