藏南冈底斯带中段始新世岩浆作用的厘定及其大地构造意义

本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICPMS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(～50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。

The identification of the Eocene magmatism and tectonic significance in the middle Gangdese magmatic belt, southern Tibet

We conducted systematically zircon LA-ICP-MS U-Pb dating and the whole-rock geochemical analyses for the granitic plutons and hornblende gabbro pluton in the middle Gangdese magmatic belt, southern Tibet. Based on studied results, we elucidate their petrogenesis, evolutionary geodynamic processes and tectonic setting. The zircon U-Pb analyses demonstrate that crystallization and emplacement ages of the plutons range from 41Ma to 55Ma, belonging to Early-Middle Eocene magmatic activities. Geochemically, the granitic samples lie in calc-alkaline and high K calc-alkaline fields, integrated with enrichments of LREE and LILE (Rb, Ba and K) and depletions of Nb, Ta and P (HFSE), suggesting arc-related geochemical affinities. Moreover, the Aluminium saturation index (A/CNK) of the granitic samples show the metaluminous and weak peraluminous features characterized by I-type granitic rocks. Hornblende gabbro was the product of partial melting of garnet peridotite and was contaminated by crustal material during ascending. Based on petrology, geochemistry, geochronology and regional setting, we address that the granitoids and related igneous rocks were formed in a transitional tectonic setting from initional collision to syn-collisional setting. In the early stage of Eocene, the breakoff of Neo-Tethys oceanic slab triggered asthenosphere upwelling, resulting in partial melting of lithospheric mantle forming basic magmas. Subsequently, the basic magmas were emplaced to lower crust and resulted in partial melting of lower crustal materials forming granitic magma through magma mixing in the Gangdese belt.