滇西兰坪盆地莲花山岩体成因与构造意义:岩石地球化学、锆石U-Pb年代学及Hf同位素约束

发布时间：2018-05-28 13:45

本文选题：莲花山岩体 + 钾质岩浆岩　；参考：《岩石学报》2017年07期

【摘要】：沿金沙江-哀牢山古特提斯缝合带两侧发育的巨型新生代陆内钾质岩浆岩带一直是地质学界关注的热点,其西侧的印支地块钾质岩浆岩尚缺乏深入探讨,制约着对钾质岩浆岩带成因的理解以及对印支地块深部壳幔物质结构的认识。本研究选取的莲花山岩体位于印支地块北部的兰坪盆地东南端,依次对其开展系统的岩相学、元素地球化学、锆石U-Pb年代学以及Hf同位素研究。结果显示,岩体主要由石英二长斑岩组成,LA-ICPMS锆石U-Pb定年结果制约岩体侵位时代在~34Ma。岩石样品具有富碱(Na_2O+K_2O=9.0%~9.2%)、高K(K_2O/Na_2O=1.0~1.2)特征,属钾玄岩浆系列;富集Th和U等大离子亲石元素,亏损Nb和Ta等高场强元素,轻稀土元素富集((La/Yb)N=22~24)。综合反映莲花山钾质岩浆岩系由大洋板片俯冲作用造成的富集源区的部分熔融作用而产生。锆石εHf(t)在+1.4~+4.6,集中在+2.8~+4.0,对应的Hf同位素地壳模式年龄为1027~815Ma,集中在934~860Ma。岩体集中的锆石εHf(t)值和缺乏继承锆石的特征,说明岩浆岩上侵过程中未受到围岩的混染。目前已知的印支地块东部自新元古代以来(1000Ma)经历的俯冲作用仅有金沙江-哀牢山古特提斯洋二叠纪(ca.290~250Ma)西向俯冲,莲花山岩体锆石具有的新元古代Hf同位素地壳模式年龄反映古特提斯洋板片俯冲过程有陆源沉积物质加入地幔中,该认识与岩石Nb/U比值(1.3~4.7)所反映的加入地幔的俯冲物质属性一致。莲花山岩体和其西侧同期卓潘钾质杂岩体与扬子西缘钾质岩体群同属金沙江-哀牢山钾质岩浆岩带的组成部分,它们由始新世时期古特提斯缝合带加厚岩石圈拆沉作用导致的软流圈热上涌而诱发。该构造-热事件影响的空间范围可能较传统认为的更大,其向西远涉至印支地块内部,从而导致了莲花山和卓潘等岩体的形成。
[Abstract]:The giant Cenozoic intracontinental potash magmatic belt developed along the sides of the Jinshajiang-Ailaoshan paleoTethys suture zone has always been a hot topic in the geological field, but the Indosinian block potash magma in the west side of the belt has not been thoroughly discussed. It restricts the understanding of the genesis of the potash magmatic belt and the material structure of the deep crust and mantle of the Indosinian block. The Lianhuashan rock body selected in this study is located at the southeast end of the Lanping basin in the northern part of the Indosinian block. The lithology, element geochemistry, zircon U-Pb chronology and HF isotopic studies have been carried out in turn. The results show that the rock mass is mainly composed of quartz monzonitic porphyry, and the results of U-Pb dating of zircon restrict the emplacement age of the intrusive body at 34 Ma. The rock samples have the characteristics of rich alkali-rich Na2OK2O-9.0 and 9.2, and high KKK _ 2O / Na _ 2O _ (1.0 ~ 1.2), which belong to the kaliacite magma series, and are rich in large ion lithophile elements such as Th and U, depleted in high field strength elements such as NB and Ta, and enriched in La-r-Yb / N _ (22) O _ (24) ~ (2 +). It is shown that the Lianhuashan potash magmatic series is caused by partial melting of the enriched source area caused by subduction of oceanic plate. The zircon 蔚 Hfct) ranges from 1.4 to 4.6, mainly in the range of 2.8 ~ 4.0, corresponding to the crustal model age of 1027 ~ 815 Ma and 934 ~ 860 Ma. The concentration of zircon 蔚 Hftand and the lack of inherited zircon indicate that the magmatic rocks are not mixed with each other during the intrusion. The known subduction in the eastern part of the Indosinian block since the Neoproterozoic is only the Jinsha River and Ailao PaleoTethys Permian Permian Permian with westward subduction. The age of neoproterozoic HF isotopic crustal model of zircon from Lianhuashan pluton reflects the subduction process of Paleotertes oceanic plate in which continental sedimentary material is added to the mantle. This understanding is consistent with the subduction properties of the mantle as reflected by the rock Nb/U ratio of 1.3 ~ 4.7. The Lianhuashan rock body and the Zhuopan potash complex in the western side of Lianhuashan rock body and the potash rock mass group in the western margin of Yangtze are both part of the Jinshajiang-Ailaoshan potash magmatic belt. They were induced by the thermal upwelling of the asthenosphere caused by the thickening lithosphere in the Paleocene suture zone during the Eocene. The influence of the tectonic-thermal event on the spatial range may be larger than the traditional thought, which extends westward to the interior of the Indosinian block, resulting in the formation of the Lianhuashan and Zhuopan rock bodies.
【作者单位】：中国地质大学地质过程与矿产资源国家重点实验室;中国地质科学院地质力学研究所;云南省地质矿产勘查院;
【基金】：国家重点基础研究发展计划“973”项目(2015CB452606、2009CB421008) 国家重点研发计划(2016YFC0600307) 国家自然科学基金青年项目(41602090)联合资助
【分类号】：P584;P597

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