摘要
南帕米尔发育早白垩世大陆弧岩浆活动,岩浆产物从初始S型花岗岩向后期I型花岗岩的转变记录了弧演化过程。本文对南帕米尔穆尔加布岩体中二云母花岗岩和黑云母花岗岩进行锆石U-Pb年龄和Hf-O同位素、全岩主量、微量元素和Sr-Nd同位素分析。结果表明,二云母花岗岩形成于115.2 Ma,具有高SiO_(2)、Al_(2)O_(3)含量和低MgO含量;属于高钾钙碱性系列,具有过铝质特征,为S型花岗岩;富集轻稀土元素(LREEs)和大离子亲石元素(LILEs),亏损重稀土元素(HREEs)和高场强元素(HFSEs);具有富集的Sr-Nd同位素组成((^(87)Sr/^(86)Sr)i=0.710543~0.712025,ε_(Nd)(t)=-9.4)、变化较大的锆石ε_(Hf)(t)值(-10.0~1.8)和δ^(18)O值(6.2‰~9.3‰),与变杂砂岩和变泥质岩的白云母脱水熔融的熔体特征一致;源区由70%沉积岩、20%下地壳基底和10%幔源岩浆组成,沉积源中加入了富石榴子石的深部地壳组分而导致Nd-Hf同位素解耦。黑云母花岗岩形成于104.8 Ma,具有高SiO_(2)、K_(2)O含量和低MgO含量;富集LREEs和LILEs,亏损HFSEs,具有Eu负异常(Eu/Eu*=0.43~0.76);具有富集的全岩Sr-Nd同位素((^(87)Sr/^(86)Sr)i=0.710781~0.711090,ε_(Nd)(t)=-10.8~-10.7)和锆石Hf同位素(ε_(Hf)(t)=-11.9~-4.1)组成,以及变化较大的锆石δ^(18)O值(6.9‰~9.7‰);来源于混合源区的高程度(15%~50%)部分熔融,包含65%帕米尔下地壳、15%变沉积岩和20%软流圈地幔。古特提斯洋闭合期间,Karakul-Mazar三叠纪沉积岩底垫到中南帕米尔的下地壳基底。早白垩世低角度俯冲的新特提斯大洋板块发生脱水反应,驱动Karakul-Mazar沉积岩部分熔融,并伴有深部地壳组分的加入,形成了穆尔加布二云母花岗岩。随后,新特提斯大洋板块发生板块沉降,大量软流圈地幔上涌触发岩浆爆发,造成下地壳部分熔融,并伴有少量底垫沉积物的贡献,形成穆尔加布高δ^(18)O值黑云母花岗岩。岩浆源区中沉积岩与下地壳的主次关系控制了南帕米尔早白垩世大陆弧演化,沉积岩的底垫与耗尽使初期S型花岗岩向后期Ⅰ型花岗岩转变。
Early Cretaceous continental arc magmatism was developed in South Pamir,with the transition of magmatic products from initial S-type granites to later I-type granites recording the arc evolution.This study presents new zircon U-Pb ages and Hf-O isotopes,whole-rock elements and Sr-Nd isotopes for the Murgob two-mica granites and biotite granites to constrain their petrogenesis and geodynamic processes.Zircon U-Pb dating indicates that the two-mica granites formed at 115.2 Ma and the biotite granites formed at 104.8 Ma.The geochemical data show that the two-mica granites have high SiO_(2),Al_(2)O_(3),and low MgO contents.They belong to the high-K calc-alkaline series with peraluminous features,and belong to S-type granites.They are enriched in LREEs and LILEs,depleted in HREEs and HFSEs,and have enriched Sr-Nd isotopic compositions((^(87)Sr/^(86)Sr)i=0.710543-0.712025,ε_(Nd)(t)=-9.4),variable zirconε_(Hf)(t)(-10.0 to 1.8)andδ^(18)O values(6.2‰-9.3‰).The geochemical characteristics of the samples are consistent with muscovite dehydration melting of the metagreywacke and metapelite.Their source was 70%metasedimentary rocks,20%lower crustal basement,and 10%mantle-derived magma.The incorporation of garnet-rich deep crustal components to their sedimentary source results in the Nd-Hf isotopic decoupling.The biotite granites have high SiO_(2),high K_(2)O,and low MgO contents.They are enriched in LREEs and LILEs and depleted in HFSEs,with negative Eu anomalies(Eu/Eu*=0.43-0.76).They have enriched whole-rock Sr-Nd isotopic compositions((^(87)Sr/^(86)Sr)i=0.710781-0.711090,ε_(Nd)(t)=-10.8 to-10.7)and enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-11.9 to-4.1)and variable zirconδ^(18)O values(6.9‰-9.7‰).They were formed by high-degree partial melting(15%-50%)of a mixed source comprising 65%lower crust,15%metasedimentary rocks,and 20%asthenospheric mantle.The Karakul-Mazar Triassic sedimentary rocks were relaminated in the Central-South Pamir lower crust during the closure of the Paleo-Tethys Ocean.The dehydration reaction of the Early Cretaceous low-angle subduction of the Neo-Tethys oceanic slab triggered partial melting of the Karakul-Mazar sedimentary rocks,with the addition of deep crustal components to form the Murgab two-mica granites.Subsequently,as the Neo-Tethys oceanic slab underwent slab foundering,the upwelling of the asthenospheric mantle triggered magmatic flare-up,leading to partial melting of the lower crust with the subordinate contribution of relaminated sediments,forming the Murgob highδ^(18)O biotite granites.The predominant and subordinate status between the sediments and the lower crust in the magmatic source controlled the evolution of the South Pamir Early Cretaceous continental arc,and the relamination and consumption of sediments caused the transition from the initial S-type granite to late I-type granite.
作者
马鑫
王健
但卫
王强
唐国荣
唐功建
MA Xin;WANG Jian;DAN Wei;WANG Qiang;TANG Guorong;TANG Gongjian(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China;CAS Center for Excellence in Deep Earth Sciences,Guangzhou 510640,Guangdong,China)
出处
《地球化学》
北大核心
2025年第1期39-58,共20页
Geochimica
基金
第二次青藏高原综合科学考察研究(STEP)(2019QZKK0702)
新疆维吾尔自治区重大科技专项(2020A03005-1)
国家自然科学基金项目(41722205、41673033)联合资助。
