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(Na2O +K2O =9.0％～9.2％)、高K(K2O/Na2O=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(＜ l000Ma)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.%The Cenozoic intra-continental potassic magmatic belt along the Jinshajiang-Ailaoshan Paleo-Tethyan suture zone has been a hot issue in geological research,whereas the igneous rocks in the Indo-China Block,west to Jinshajiang-Ailaoshan potassic magmatic belt have been rarely investigated,restricting a deeper insight into the formation of this potassic magmatic belt and the architecture of crust-mantle underneath the Indo-China Block.In this paper,we carry out a systematic study of the petrography,whole rock geochemistry,zircon U-Pb dating and Hf isotope for the Lianhuashan intrusion,which is located in the southeast of the Lanping Basin,the northern part of the Indo-China Block.Our results demonstrate that the Lianhuashan intrusion is mainly composed of quartz monzonite porphyries.LA-ICPMS zircon U-Pb dating shows that the Lianhuashan intrusion was emplaced at ～34Ma.The rocks show shoshonitic affinity,with high total alkali (Na2O + K2O =9.0％ ～ 9.2％),and potassium (K2O/Na2 O =1.0 ～ 1.2).They are also enriched in LILEs (Th and U) and LREEs ((La/Yb) N =22 ～ 24),but relatively depleted in HFSEs (Nb and Ta).Based on the above-mentioned data,it is suggested that the magmas for the Lianhuashan intrusion were derived from an enriched source region ever metasomatized by oceanic slab subduction.Zircon εHf (t) values range from + 1.4 to + 4.6,mostly clustering between + 2.8 and + 4.0,corresponding to Hf crust model ages (tDM2) ranging from 1027Ma to 815Ma and mainly clustering between 934Ma and 860Ma.The narrow range of zircon εHf (t) values and lack of inherited zircons preclude assimilation of wall rocks during the magma ascent.As currently known,since Neoproterozoic (＜ 1000Ma),the Indo-China Block had been merely subducted by the Jinshajiang-Ailaoshan Paleo-Tethyan ocean during Permian (ca.290 ～ 250Ma).Neoproterozoic Hf crust model ages demonstrate an input of terrigenous sediments into mantle during the Paleo-Tethyan slab subduction,which is consistent with Nb/U ratio (1.3 ～ 4.7) of the Lianhuashan intrusion.The Lianhuashan intrusion,the Zhuopan potassic maric complex to the west of the Lianhuashan intrusion,and potassic rocks in the western Yangtze block,together constitute a part of the Jinshajiang-Ailaoshan potassic magmatic belt,which was triggered by upwelling asthenosphere after delamination of lithosphere underneath the Paleo-Tethyan suture.This tectonic-thermal event,which gave rise to the formation of the Lianhuashan and the Zhuopan intrusion,is suggested to have influenced westwards to the interior of the Indo-China Block,spatially more broadly than previously believed.
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