今天是2018年12月11日

拉萨地体北部永珠地区早白垩世岩浆岩地球化学、锆石U-Pb年代学、Hf同位素组成及其地质意义

DOI:10.3799/dqkx.2018.711

作者:张诗启,戚学祥,韦诚,陈松永

联系地址:中国地质科学院地质研究所,北京 100037

摘要:拉萨地体北部出露大面积早白垩世岩浆岩,对它们的成因和形成机制的研究,有助于揭示拉萨地块白垩纪时期的岩浆作用过程及动力学背景.通过岩石学、地球化学和同位素地质学方法对拉萨地体北带永珠地区早白垩世中-酸性岩浆岩进行了研究.结果显示黑云母二长花岗岩、流纹岩和安山岩的锆石LA-ICP-MS U-Pb年龄分别为118±1.0 Ma、121±0.8 Ma和 115± 0.8 Ma,代表了其侵入和喷出时代.黑云母二长花岗岩、花岗斑岩和流纹岩为高钾钙碱性过铝质-强过铝质岩浆岩 (A/CNK=1.01~1.35),亏损高场强元素Nb、P、Ti和大离子亲石元素Ba、Sr,富集大离子亲石元素Rb、K和放射性元素U、Th;稀土配分图显示LREE富集,HREE近平坦,Eu明显负异常,为形成于大陆边缘的岛弧岩浆岩特征.黑云母二长花岗岩和流纹岩的锆石Hf初始比值ε Hf( t )分别为-1.21~3.01和-0.68~5.35,对应的两阶段模式年龄分别为0.99~1.26 Ga和 0.84~ 1.22 Ga,为壳幔混源岩浆.安山岩为高钾钙碱性,亏损Nb、Ta、P、Ti、U和Sr,富集Rb、K和Th,稀土配分图显示LREE富集,HREE近平坦,Eu轻微负异常,为形成于大陆边缘弧的岩浆岩.结合前人研究成果,分析认为永珠地区早白垩世岩浆岩形成于班公湖-怒江特提斯洋壳南向俯冲作用下的大陆边缘弧环境,由俯冲的班公湖-怒江中特提斯洋板片在深部脱水熔融,进而诱发上覆地幔楔部分熔融形成基性岩浆上涌,导致下地壳物质发生部分熔融形成酸性岩浆,它们在上升过程中按不同比例混合,形成中性和酸性岩浆侵入到地下或喷出地表,形成侵入岩和火山岩.

关键字:早白垩世岩浆岩;地球化学;锆石U-Pb年龄;永珠地区

Geochemistry, Zircon U-Pb Dating and Hf Isotope Compositions of Early Cretaceous Magmatic Rocks in Yongzhu Area, Northern Lhasa Terrane, Tibet, and Its Geological Significance

Author:Zhang Shiqi, Qi Xuexiang, Wei Cheng, Chen Songyong

Author Address:Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

Abstract:The study on the petrogenesis and tectonic setting of the Early Cretaceous magmatic rocks in the northern Lhasa is important to define the geodynamic evolution for the Lhasa terrane. In this paper, it is reported of petrology, petrogeochemistry, zircon U-Pb ages and zircon Hf isotopic comopositions of Early Cretaceous magmatic rocks from Yongzhu area in the northern Lhasa terrane. Zircon U-Pb ages for biotite-monzonitic granite, rhyolite and andesite are 118±1.0 Ma, 121±0.8 Ma and 115±0.8 Ma respectively, representing their intrusion and eruption period. Biotite monzonitic granite, granite porphyry and rhyrolite show similar geochemical characteristics. They are high K calc-alkaline and weakly peraluminous-strongly peraluminous granites (A/CNK=1.01-1.35). In primitive mantle-normalized spider diagrams, these rocks are characterized by enriched large ion lithophile elements Rb, K and radioactive elements U, Th, and negative anomalies in Nb, P, Ti, Ba and Sr. Chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE and negative Eu anomalies. Above chemical natures suggest that they are island arc igneous rocks and formed in continental margin arc setting. The Hf isotopic compositions in the biotite monzonitic granite and rhyolite are -1.21 to 3.01 and -0.68 to 5.35, respectively, and two stage model ages are 0.99-1.26 Ga and 0.84-1.22 Ga, respectively, which suggests mixed source of crust and mantle. In contrast, the andesite shows slightly different geochemical characteristics. They are characterized by (1) high K calc-alkaline; (2) negative anomalies in Nb, Ta , P, Ti, U and Sr,and enrichment of Rb, K and Th in primitive mantle-normalized spider diagrams; (3) chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE, and slight negative Eu anomalies; (4) formation in the continental margin arc setting. It is proposed that the Early Cretaceous magmatic rocks in Yongzhu were formed in the continental margin arc setting. During southern subduction of Bangonghu-Nujiang Tethyan oceanic basin, dehydration melting of the subduction oceanic plate produced the high thermal molten mass, which induced partial melting of the mantle wedge and formation of mafic magma. Then upwelling of mafic magma induced partial melting of the lower crust material and formation of acidic magma. During ascent process of the mafic magma and acidic magma, the two types of magma mixed in different proportion, and formed volcanic and plutonic rocks.

Keyword:Early Cretaceous magmatic rock; geochemistry; zircon U-Pb age; Yongzhu area

 

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作者简介:张诗启(1982-),男,博士研究生,主要从事构造地质学、岩石学专业研究.ORCID: 0000-0002-2394-6988. E-mail: zhangshiqi0396@163.com

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