摘要
柴北缘构造带内发育大规模的印支期花岗岩,其成因过程和构造背景仍存在争议。为确定该地区花岗岩的形成时代、成因和岩浆活动的构造背景,本文选取柴北缘构造带东段乌兰地区花岗岩和花岗闪长岩进行了岩相学、全岩地球化学、锆石U-Pb年代学和Lu-Hf同位素研究。它们以高SiO_(2)、K_(2)O含量,低MgO、CaO含量和Mg^(#)为特征,属于弱过铝质高钾钙碱性Ⅰ型花岗岩(A/CNK=0.99~1.08),相对富集轻稀土元素(LREE)和Rb、K、Cs等大离子亲石元素,相对亏损重稀土元素(HREE)和Nb、Ta、Ti等高场强元素。LA-ICP-MS锆石U-Pb定年显示花岗岩和花岗闪长岩侵位年龄分别为256±3 Ma和243±1 Ma,均为印支期岩浆活动的产物。Lu-Hf同位素组成显示花岗岩的ε_(Hf)(t)值为-6.2~-2.7,对应的二阶段模式年龄(T_(DM2))为1676~1452 Ma,花岗闪长岩的ε_(Hf)(t)值为-4.8~-0.2,对应的二阶段模式年龄(T_(DM2))为1577~1280 Ma,富集的Hf同位素组成反映其原始岩浆起源于中元古代地壳物质的部分熔融。然而,花岗闪长岩具有较高的Mg^(#)值和Cr、Ni含量,结合其较高的ε_(Hf)(t)值暗示花岗闪长岩形成过程中可能有幔源物质的贡献。此外,花岗岩和花岗闪长岩中均发育暗色镁铁质包体,包体和寄主岩中斜长石具有环带结构和筛状结构,并发育刀刃状黑云母和针状磷灰石,暗示岩浆演化过程中存在岩浆混合作用。从空间上看,乌兰地区三叠纪花岗岩体与西秦岭同期岩浆岩共同构成了青海南山岩浆岩带,结合前人对该区域地质背景分析,本文认为这一时期花岗质岩体的形成与古特提斯洋向北俯冲密切相关,大洋俯冲诱发幔源岩浆底侵到下地壳,并使下地壳发生部分熔融产生了区域上广泛分布的花岗质岩浆。
The large-scale Indosinian magmatic rocks are developed in the North Qaidam Tectonic Belt(NQB),whose genetic process and tectonic background are still controversial.In order to find out the formation age,origin and tectonic of intrusive rocks in this area.Here we present an integration of methods including petrology,mineralogy,geochemistry,zircon U-Pb geochronology and in situ Hf isotopic compositions for the granite and granodiorite in the eastern segment of the NQB.The granite and granodiorite are characterized by relatively high contents of SiO_(2) and K_(2)O,but low contents of MgO,CaO,and Mg^(#),belonging to slightly peraluminous I-type granite and accompanied by A/CNK values varying from 0.99 to 1.08.Both of them show enrichment of LREEs and LILEs(Rb,K,Cs)and depletion of HREEs and HFSEs(Nb,Ta,Ti),which are consistent with subduction-related arc-magmatic.LA-ICP-MS zircon U-Pb dating suggests that granite and granodiorite were emplaced at Indosinian(256~243 Ma).The zirconε_(Hf)(t)values of granites range from-6.2~-2.7 with T_(DM2)=1676~1452 Ma and the zirconε_(Hf)(t)values of granodiorites range from-4.8~-0.2 with T_(DM2)=1577~1280 Ma,suggesting that the processor magmas generateds mainly by partial melting of the Mesoproterozoic crustal material.The granodiorite has high Mg^(#)value and Cr,Ni contents,combined with the higher ofε_(Hf)(t)value,it is suggested that the precursor magmas of the granodiorite were formed with additional input of a mafic component derived from mantle materials.Mafic microgranular enclaves(MMEs)are common in granites and granodiorites,most of them show disequilibrium textures,such as the occurrence of disequilibrium textured plagioclase,blade-shaped biotite and acicular apatite.It is inferred that the MMEs probably generated by the magma mixing.The spreading eastward Wulan granites and West Qinling Triassic magmatic rocks constitute the Qinghai Nanshan magmatic belt.Combined with the previous analysis of the coeval magmatic events,we propose that these granitic rock masses might be related to partial melting of the crust because of underplating of mantle-derived magma,during the northward subduction of the Paleo-Tethys ocean.
作者
张金鹏
牛漫兰
李晨
李秀财
孙毅
苑潇宇
王磊
Zhang Jinpeng;Niu Manlan;Li Chen;Li Xiucai;Sun Yi;Yuan Xiaoyu;Wang Lei(School of Resources and Environmental Engineering,Hefei University of Technology,Hefei 230009)
出处
《地质科学》
CAS
CSCD
北大核心
2022年第4期1103-1129,共27页
Chinese Journal of Geology(Scientia Geologica Sinica)
基金
国家自然科学基金项目(编号:42272246,41272221,41902235)资助。
关键词
印支期
花岗岩
柴北缘构造带
古特提斯洋
锆石U-Pb定年
LU-HF同位素
Indosinian
Granite
The North Qaidam tectonic belt
The Paleo-Tethys ocean
Zircon U-Pb geochronology
Lu-Hf isotopic compositions
