秦岭造山带早古生代蛇绿岩的多阶段演化: 从岛弧到弧间盆地

秦岭商-丹缝合带是分隔北秦岭早古生代造山带和南秦岭晚古生代造山带的地质界线,其中的丹凤蛇绿岩被认为代表了秦岭地区早古生代的洋壳残片。迄今,前人已经提出多种模式来解释丹凤蛇绿岩成因和构造背景(如:岛弧、洋岛和成熟的大洋等)。然而,这些单一的构造演化模式却很难解释两个基本事实:(1)不同类型镁铁质岩(如N-MORB、E-MORB和IAT等)的穿时性分布;(2)几乎所有的早古生代镁铁质岩都显示出多种构造环境的叠加。对陕西太白鹦鸽嘴地区一条具有较完整层序的蛇绿岩剖面研究发现,剖面中存在HTI型(TiO2:1.21%～1.56%)和LTI(TiO2:0.09%～0.35%)两种类型的镁铁质岩(包括玄武岩和辉长岩),HTI型镁铁质岩具有LREE亏损,没有Nb、Ta负异常等的E-MORB特征;LTI具有LREE富集,Nb、Ta负异常的IAT特征。地球化学显示二者的源区均为北秦岭岩石圈地幔楔。本文获得鹦哥嘴蛇绿岩两个LTI型辉长岩锆石U-Pb年龄分别为523.8±1.3Ma和474.3±1.4Ma。认为秦岭早古生代蛇绿岩应是SSZ环境下多阶段演化的结果:第一阶段:约524Ma,秦岭洋盆向北俯冲开始。俯冲板片的脱水作用使熔融温度降低,形成的流体交代地幔楔,在北秦岭南缘产生了一个不成熟的岛弧;第二阶段:先存岛弧裂开阶段,约524～474Ma。秦岭洋壳的持续俯冲,在先形成的岛弧上拉张出了弧间盆地,形成了主要由轻稀土亏损、高Ti拉斑玄武岩和辉长岩组成的E-MORB型岩石组合;第三阶段:弧前盆地闭合阶段,474Ma之后。在这个阶段新生的弧间盆地闭合,俯冲洋壳携带的深海沉积物与北秦岭岩石圈地幔楔相互作用形成了北秦岭李子园的玻安岩。秦岭早古生代蛇绿岩的多阶段成因是典型特提斯构造域演化特征在秦岭地区的重现。

A model for multi-stage of the Early Palaeozoic Danfeng ophiolite in Qinling orogen belt: From arc to inter-arc basin

The origin of the Early Palaeozoic Danfeng ophiolite in Qinling orogen is the key for understanding the tectonic evolutionary processes of proto-Tethyan Ocean in China.Several models have been presented to clarify the evolutionary of the Early Palaeozoic Danfeng ophiolite,e.g.the model of arc,ocean island and mature oceans,etc.But these simple model fail to explain two facts:(1) the diachronism of different mafic-rock type(N-MORB and IAT) ;(2) almost every mafic-rock type shows the characteristic of the superposition of different tectonic setting(arc,BABB and ocean ridge).After studying the Yingezui ophiolite of the Early Palaeozoic Danfeng ophiolite in Shaanxi Province,we discovered that the mafic-rocks(including basalts and gabbros) can be divided into two geochemical group:HTI(TiO2:1.21% ～ 1.56%) and LTI(TiO2:0.09% ～ 1.07%).The two groups cover nearly all of the mafic-rock types in the Early Palaeozoic Danfeng ophiolite.The HTI group with E-MORB affinity is characterized by depletion LREE without negative Nb-Ta anomaly.The LTI group with IAT affinity is characterized by slight enrichment of LREE and with negative Nb-Ta.We propose a multi-stage modal for the origin of the Early Palaeozoic Qinling ophiolite.StageⅠ:ca.524Ma,the ocean possibly started to subduct towards the north.With the dehydration of the subduction slab,an immature arc system being composed of LTI type mafic-rock was formed in the south rim of the North Qinling terrane.Stage Ⅱ:ca.524 ～ 474Ma,the immature arc extended to an arc basin,leading to HTI type mafic-rock(E-MORB).Stage Ⅲ:after 474Ma,the arc basin started to close and plus to the immature arc system,leading to the MORB affinity with IAT.A multi-stage evolution characterize of Tethyan Tectonic Region appear once more in the early Palaeozoic ophiolite in Qinling orogen.