“三江”北段昌都陆块晚三叠世钾质-超钾质火山岩成因及地质意义  

Petrogenesis and geological implications of the Late Triassic potassic-ultrapotassic rocks in Changdu Block,northern segment of the Sanjiang area

作　　者：于峻川[1,2] 莫宣学[2] 喻学惠[2] 朱弟成[2] 李逸川[2,3] 黄雄飞[2] 

机构地区：[1]中国国土资源航空物探遥感中心,北京100083 [2]地质过程与矿产资源国家重点实验室中国地质大学地球科学与资源学院,北京100083 [3]中国地质图书馆,北京100083

出　　处：《岩石学报》2014年第11期3334-3344,共11页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2009CB421002);111计划项目(B07011);长江学者和创新团队发展计划(IRT1083);中国地质调查局研究项目(1212011220920)联合资助

摘　　要：昌都微陆块内部发育一套晚三叠世粗安岩。本文基于系统的野外踏勘,对具有代表性的火山岩进行岩石学、地球化学等研究。结果显示该区火山岩的Si O2含量为51.36%~58.04%,全碱含量(Na2O+K2O)为5.03%~7.84%,根据岩石K2O,Mg O及K2O/Na2O比值判断,属钾质-超钾质火山岩,具高K、高Al、低Ti的特征。本区岩石强烈富集大离子亲石元素(LILE)和轻稀土元素(LREE),Nb、Ta、Ti的负异常明显,整体特征类似于弧火山岩。其稀土总量较高(270×10-6~960×10-6),轻重稀土分馏明显,重稀土分布形态较平缓。岩石具有较高的初始锶同位素比值(87Sr/86Sr)i=0.7150~0.7176和较低的初始钕同位素比值(143Nd/144Nd)i=0.51180~0.51184,εNd(t)=-10.9^-10.2,二段Nd模式年龄为t2DM=1.82~1.88Ga。本区岩石的地球化学特征显示其源区是由俯冲板片所产生的流体/熔体交代的岩石圈地幔,源区矿物组成分析结果显示,其源区以尖晶石二辉橄榄岩为主,含少量石榴石二辉橄榄岩及金云母。本区钾质-超钾质岩石虽然与西藏南部的钾质-超钾质岩石有相似的地球化学特征,但却产于碰撞后的构造背景之下。结合其所具有的弧火山岩特征及时代构造背景判断其为一种"滞后型"弧火山岩,其形成过程可以概括为三个阶段。The Late Triassic trachyandesite crops out in the Changdu Block,northern segment of the Sanjiang area. Based on systematic field investigations,we carry out geochemical studies on representative volcanic rocks. The results show that the Si O2 contents of samples range from 51. 36% to 58. 04%,and the total alkalis is relatively high（ 5. 03% ~ 7. 84%）,with high content of K2 O,Mg O and K2 O / Na2 O, belonging to potassic-ultrapotassic series. These rocks have some common characteristics, which enrichment in K,Al,LILE,LREE and depletion in Ti,Nb and Ta,are similar to arc-like rocks. The total concentration of rare earth elements is relatively high while the REE pattern is nearly flat,indicating LREE enrichment. The initial87 Sr /86 Sr range from 0. 7150 to0. 7176,with negative εNd（ t）（- 10. 9 ^- 10. 2） and the second modal ages（ t2DM） varying from 1. 82 Ga to 1. 88 Ga,collectively revealing that the source region of these rocks is probably associated with the mantle metasomatism induced by the ancient subduction fluids / melt. The analysis results of the mineral constituent show that the potassic-ultrapotassic rocks originate from the low degree of partial melting of a mixture of spinel lherzolite and garget lherzolite. Geochemical characteristics of these rocks show many similarities with the potassic-ultrapotassic rocks in southern Tibet,but the former arc volcanism took place after continent-continent collision.Comprehensively considering the obvious arc-type characteristics and tectonic environment,we suggest that the potassic-ultrapotassic rocks in this research are typical lagged arc volcanic rocks,and the genetic models can be summarized into three stages.

关 键 词：晚三叠世 钾质-超钾质火山岩 滞后型弧火山岩 昌都微陆块 古特提斯 

分 类 号：P588.144[天文地球—岩石学]