勉县—略阳蛇绿混杂岩带镁铁质—安山质火山岩块年龄和该带构造演化的复杂性  

Ages of Metamorphic Mafic—Andesitic Volcanic Rock Blocks and Tectonic Evolution Complexity of Mianxian—Lueyang Ophiolitic Mélange Belt

作　　者：张宗清[1] 唐索寒[1] 张国伟[2] 杨永成 王进辉[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]西北大学地质系,西安710069 [3]陕西区域地质调查研究院,咸阳712000

出　　处：《地质学报》2005年第4期531-539,共9页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号40072071;49773182;49732080);地质调查项目(编号200313000057)资助的成果。

摘　　要：位于南秦岭略阳—勉县—镇巴高川一线的勉略蛇绿混杂岩带一般认为形成于印支期,但笔者研究表明,除存在印支期的岩块外,也存在大量晋宁期的变质镁铁质火山岩块和岛弧火山岩块:1略阳三岔具岛弧火山岩性质的变质安山质火山岩的Sm-Nd等时年龄为873±71(2σ)Ma,INd=0.511796±67(2σ),MSWD0.73;2勉县安子山类似N-MORB洋壳碎片的变质镁铁质火山岩块的Sm-Nd等时年龄为877±78(2σ)Ma,INd=0.51190±10(2σ),MSWD0.93;3出露于略阳庄科轻稀土强烈亏损蛇绿岩块,虽然其Nd同位素组成分散,不形成等时线,但是其高场强大离子不相容微量元素与勉县安子山变质镁铁质火山岩具有十分类似地球化学特征,它们可能是大致同时,由类似源区形成的。这些事实表明,勉略蛇绿构造混杂岩带是复杂的,可能是多期构造事件叠加复合构造活动带。The Mianxian-Lueyang ophiolitic melange belt along Lueyang county-Mianxian county-Gaochuan,Zhenba county, Shaanxi Province, has been recently found and defined in the Qinling orogen. It is deemed to be a major suture zone resulting from collision and final amalgamation of the Sino-Korean （North China） and Yangtze craton blocks, two major tectonic units of eastern Asia, in the Indosinian. However, the new obtained isotopic geochronological data of the metamorphic mafic-andesitic volcanic rock blocks of mid-ocean ridge and island-arc ophiolite or island-arc volcanic rock characteristics in the zone indicate that the tectonic suture zone is very complex. Besides the reported ages of Indosinian, it contains large quantities of metamorphic mafic-andesitic rock blocks formed in the Jinlinian period （Neoproterozoic） . The metamorphic andesitic volcanic rocks in the Pianqiaogou valley, Sancha area, Lueyang county have island-arc volcanic rock features and its Sm-Nd isochron age is 873±71（2σ）Ma, corresponding to INd=0. 511796±67（2σ）,MSWD 0.73. The zircon U-Pb age of the plagiogranites in the rock block is 890±17Ma. The metamorphic mafic volcanic rocks in the Anzishan area, Mianxian county and the Zhuanke area, Lueyang county are characterized by mid-ocean ridge and island-arc ophiolites and the former give a Sm-Nd isochron age of t=877±78（2σ）Ma, with INd=0. 51190±10（2σ）,MSWD 0.93. Although the Sm-Nd isotopic data of the latter are scattered, its high field large ion lithophile incompatible trace elements are related close to the ones in the Anzishan area. They are formed from an analogous source in the roughly same period. The data display the Mianxian-Lueyang ophiolite tectonic mélange belt is complex, that is a multiple superimposed tectonic active belt possibly.

关 键 词：同位素年代学 勉略蛇绿构造混杂岩带 秦岭造山带 蛇绿混杂岩带 岛弧火山岩 等时年龄 镁铁质 岩块 构造演化 复杂性 

分 类 号：P588.14[天文地球—岩石学]