西藏林周地区始新世托龙Ⅰ型花岗岩对印度-欧亚板块碰撞时限的制约  

Eocene Tuolong Ⅰ-type granite in Linzhou area, Tibet:Constraints on the timing of the collision between India and Eurasian plates

作　　者：杨成业[1] 冯佳佳[1] 李玉彬 张金树[1] 张根[1] 夏洋洋[1] YANG Cheng-ye;FENG Jia-jia;LI Yu-bin;ZHANG Jin-shu;ZHANG Gen;XIA Yang-yang(College of Engineering,Tibet University,Lhasa 850000,China;College of Earth Science,Chengdu University of Technology,Chengdu 610059,China;Tibet Institute of Geological Survey,Lhasa 850000,China)

机构地区：[1]西藏大学工学院,西藏拉萨850000 [2]成都理工大学地球科学学院,四川成都610059 [3]西藏自治区地质调查院,西藏拉萨850000

出　　处：《岩石矿物学杂志》2022年第6期1080-1096,共17页Acta Petrologica et Mineralogica

基　　金：青藏高原第二次科学考察项目(2019QZKK0806);西藏大学地质资源与地质工程特色学科建设项目(Zf22002004);西藏自治区科技计划项目(XZ202201ZY0020G)。

摘　　要：目前已有研究对印度-欧亚板块碰撞时限的认识存在诸多分歧。本文对西藏林周县中部出露的托龙岩体开展了锆石U-Pb年代学、全岩地球化学和锆石Hf同位素分析,以期进一步约束印度-欧亚板块的碰撞时间。托龙岩体内出露的石英二长斑岩呈岩株状侵入到帕那组中,锆石U-Pb定年结果显示该石英二长斑岩形成于始新世(50.5±0.6 Ma),锆石εHf (t)值为-0.6~-0.5。石英二长斑岩具有较低的A/CNK值、较高的K_(2)O含量和K_(2)O/Na_(2)O值,属于准铝质和钾玄岩系列。岩石普遍亏损高场强元素Nb、Ta、Ti和P,富集大离子亲石元素Th、U和Pb,轻、重稀土元素分异明显,(La/Yb)N=15.3~16.3,Eu负异常较弱,δEu=0.56~0.69。托龙岩体较低的A/CNK值、P_(2)O_(5)与SiO_(2)的负相关性以及Y含量与Rb含量之间的正相关性,表明托龙石英二长斑岩为Ⅰ型花岗岩。综合岩体的地球化学特征、同位素特征和前人研究成果,认为托龙石英二长斑岩可能是相对古老的变质中基性岩部分熔融的产物,形成于印度-欧亚板块碰撞引起的俯冲板片断离环境,指示了印度-亚洲大陆的碰撞应发生于50 Ma之前。Timing of the collision between India and Eurasian plate is still debated. In this paper, zircon U-Pb geochronology, whole rock geochemistry and zircon Hf isotope analysis have been carried out for the Tuolong intrusion exposed in the middle of Linzhou County, Tibet, in order to further constrain the collision time of the India-Eurasian plate. The quartz monzonite porphyry outcropped in Tuolong pluton intruded into the Pana Formation as stock. Zircon U-Pb dating results show that the quartz monzonite porphyry was formed in Eocene(50.5±0.6 Ma) with εHf(t) value of-0.6 to-0.5. Quartz monzonite porphyries display metaluminous and shoshonitic characteristics with low A/CNK and high K_(2)O/Na_(2)O value and high K_(2)O contents. They are generally depleted in high field strength elements(Nb, Ta, Ti and P) and enriched in large ion lithophile elements(Th, U and Pb). Chondrite-normalized REE patterns are characterized by high( La/Yb)Nvalues( 15.3~16.3) with weak negative Eu anomalies( 0.56~0.69), the differentiation of light and heavy rare earth elements is obvious. The low A/CNK value, the negative correlation between P_(2)O_(5)and SiO_(2), and the positive correlation between Y content and Rb content indicate that the quartz monzonite porphyry is I-type granite. Combined with results of previous studies, it is considered that the Tuolong quartz monzonite porphyry may be the product of partial melting of the relatively ancient metamorphic intermediate-basic rock, and was formed as the consequences of subduction and break-off caused by the IndiaEurasian plate collision, indicating that the India-Asia continent collision should have occurred before 50 Ma.

关 键 词：西藏 地球化学 锆石U-PB年代学 HF同位素 印度-亚洲大陆碰撞 

分 类 号：P588.121[天文地球—岩石学] P597[天文地球—地质学]