【作者】 刘伟； 潘小菲； 谢烈文； 李禾；

【Author】 LIU Wei,PAN XiaoFei,XIE LieWen and LI He Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China

【机构】 中国科学院地质与地球物理研究所中国科学院矿产资源研究重点实验室；

【摘要】 本文选择大兴安岭南段林西地区的5个典型花岗岩体,在岩相学、全岩主微量元素和Nd-Sr同位素组成研究的基础上,对5个岩体的继承锆石/前锆石和岩浆锆石进行了系统的SHRIMP U-Pb年龄测定和LA-MC-ICPMS Hf同位素组成测定,试图阐明林西花岗岩源岩的组成和性质。锆石SHRIMP U-Pb定年表明:大部分林西花岗岩侵位于早白垩世（135～125Ma）,它们的源岩的年龄为～146Ma。一部分花岗岩类是在早三叠世（241Ma）和晚侏罗世末（146Ma）侵位的,它们的源岩的年龄分别是263Ma和165Ma。测定了100个锆石²⁰⁶Pb/²³⁸U年龄,都年轻于300Ma,反映在下地壳源区不存在前寒武纪岩石。做了175个锆石Hf同位素组成测定,均给出高正值ε_Hf（t）,说明源岩具有初生地壳的性质。在相同的ε_Nd（t）值下,林西花岗岩的锆石ε_Hf（t）值显著高于地球阵列和夏威夷洋岛玄武岩,这种ε_Hf～ε_Nd脱耦性指示源岩中含有远洋沉积物即古生代俯冲增生杂岩的组分。²⁰⁶Pb/²³⁸U年龄t=263～165Ma的锆石的ε_Hf（t）值构成近乎平行于亏损地幔Hf同位素演化线的趋势列,说明源岩基本为俯冲洋壳镁铁-超镁铁岩。t=146～125Ma的锆石的ε_Hf（t）值大幅度降低;同时,从晚侏罗世末到早白垩世,发生了强烈的花岗质岩浆活动。地幔上隆和岩浆底侵以及俯冲洋壳的折返,是造成下地壳源岩组成急剧变化和热梯度上升的原因。以底侵镁铁质岩石为主、以古生代俯冲增生杂岩为次的源岩的熔融,产生了马鞍子、夜来改和龙头山2花岗岩（岩套2）。林西镇南西的小城子岩体的源岩则以古生代俯冲增生杂岩为主,并含一定量的底侵镁铁质岩石。5个岩体的岩浆锆石的¹⁷⁶Hf/¹⁷⁷Hf值系统低于继承锆石/前锆石者,t=146～125Ma的锆石从中心到边缘¹⁷⁶Hf/¹⁷⁷Hf值呈现降低的趋势或者系统的变化。上述特征反映从源岩的初始熔融直到最终产生花岗岩浆的全过程中,下地壳的熔融区间逐渐扩张、卷入熔融的组分不断增多的过程。岩套1花岗岩类是镁质或Ⅰ型花岗岩,岩套2则表现出A型花岗岩以及从典型到不典型的铁质花岗岩的特征。岩套1和岩套2花岗岩类的岩相学和地球化学特征取决于源岩的性质。岩套1的源岩是相对氧化和含水的洋壳镁铁-超镁铁岩或俯冲增生杂岩;岩套2的源岩则由相对还原和贫水的底侵拉斑玄武岩以及不同分数的俯冲增生杂岩构成。更多还原

【Abstract】 Sensitive high-resolution ion microprobe（SHRIMP） U-Pb dating and laser-ablation multi-collector ICPMS Hf isotope analyses are conducted on inherited / antecrystal and magmatic zircons from the Linxi granitoids,in the southern segment of the Da Hinggan Mts.of China.SHRIMP dating establishes an Early Cretaceous（135～125Ma） age for most Linxi granitoids and the age of their source rocks at～146Ma.However,some granitoids were intruded in the Early Triassic（241 Ma） and the terminal Late Jurassic （146Ma）,whilst their respective source rocks were generated at 263Ma and 165Ma.A total of 100 SHRIMP analyses yield ²⁰⁶Pb/²³⁸U ages younger than 300Ma,whereas all 175 Hf isotope analyses give high positiveε_Hf（t） values.These results indicate juvenile source materials in the lower crust with a notable absence of Precambrian basement.At a givenε_Nd（t）,Linxi granitoids exhibit distinctly higher zirconε_Hf（t） than both the terrestrial array and the Hawaiian basalt array.This Hf-Nd isotopic decoupling suggests a source component of pelagic sediments,i.e.Paleozoic subduction accretion complexes.Zirconε_Hf（t） values（t =263～165Ma） form a trend sub-parallel to the depleted mantle Hf isotope evolution curve,but those（t = 146～125Ma） markedly fall below the latter.The first trend indicates a provenance from essentially the MORBs.However,the abruptε_Hf（t） decrease,together with extensive Early Cretaceous magmatism,is interpreted as reflecting mantle upwelling and resultant underplating,and exhumation of subducted oceanic crust.Quantitative simulation shows that Suite 2 granitoids derive from underplated mafic rock,Paleozoic subduction accretion complex,and subducted MORB slices in descending order of mass fractions.However,the Xiaochengzi intrusion（Suite 1） occurring southwest of Linxi derives mainly from Paleozoic subduction accretion complex with a quantity of underplated mafic rocks.Among each of these 5 intrusions,magmatic zircons exhibit systematically decreased ¹⁷⁶Hf/¹⁷⁷ Hf as compared with that of inherited/antecrystal zircons.Growth stages of zircons with t=146～125Ma tend to decrease in ¹⁷⁶Hf/¹⁷⁷ Hf from center to margin,or show systematic variations.The above results suggest that expansion of the melting region and widening of compositional range occurred in the lower crust over the period between incipient source rock melting and the terminal melting episode during production of Linxi granitoids.The magnesian orⅠ-type geochemical characteristics of Suite 1 granitoids is attributed to a source reservoir of relatively oxidized and hydrous oceanic crust.However,a heterogeneous source that consists of relatively reduced,anhydrous underplated tholeiites and hydrous Paleozoic subduction-accretion complex causes geochemical signatures of Suite 2 granitoids that exhibit affinity to A-type granites but range from typical to less pronounced ferroan.更多还原