福建漳浦复式花岗岩体的成因：锆石U-Pb年代学、元素地球化学及Nd-Hf同位素制约

漳浦复式花岗岩体位于福建东南沿海,为一由多期岩浆作用形成的大岩基,出露面积大于1500km2。按岩性特征,该复式岩体可肢解为长桥黑云母花岗岩、程溪碱长花岗岩和湖西花岗闪长岩3个单元。锆石LA-ICP-MS U-Pb定年结果表明,3个单元岩石的成岩年龄分别为119Ma、101Ma和96Ma。化学组成上,各单元岩石均具有亚碱、准铝或弱过铝、贫磷特征,它们的A/NKC值均在1.10以下,P2O5含量均低于0.20%,均可归为钙碱性的I型花岗岩。各单元岩石均富Cs、Rb、Th、U、Pb和轻稀土,贫Ba、Sr、P、Ti,但长桥和程溪单元较之湖西单元岩石Rb/Sr、Rb/Ba比值高,K/Rb比值低,并表现出更显著的铕负异常,指示各单元岩石的分异演化程度各不相同。3单元岩石具有较均一的Nd同位素组成,εNd(t)=-2.43～-3.24,tDM2=1.11～1.16Ga,但锆石Hf同位素组成普遍具有较大的变化范围,长桥、程溪和湖西3单元岩石的εHf(t)值分别为-8.3～+3.0、+1.7～+10.2和-2.5～+3.5,变化幅度均在6个εHf单位以上,反映岩体的形成存在不同来源物质的贡献。综合分析表明,各单元岩石的形成均经历了幔源岩浆与其诱发地壳物质熔融产生的长英质岩浆在地壳深部混合,随后又经不同程度结晶分异的二阶段成岩过程。各单元岩石之间显著的成岩时差及成分变异趋势指示复式岩体不可能为同一原始岩浆分异演化的产物,而最可能为演化程度各异的壳幔混源岩浆叠次侵位复合的结果。

Petrogenesis of the Zhangpu Composite Granite Pluton in Fujian Province: Constraints from Zircon U-Pb Ages, Elemental Geochemistry and Nd-Hf Isotopes

The Zhangpu composite granite pluton,with an outcrop area over 1500 km2,is located in the coastal region of SE Fujian Province.It is a big batholith formed by multi-satge magmatisms.According to the lithological characteristics,this pluton can be divided into three intrusions: the Changqiao(CQ) biotite granite,the Chengxi(CX) alkali-feldspar granite and the Huxi(HX) granodiorite.Zircon U-Pb ages obtained by LA-ICP-MS for the three intrusions are 119 Ma,101 Ma and 96 Ma,respectively.All three intrusions are subalkaline,metaluminous to weakly peraluminous and poor in phosphorus,with A/NKC values less than 1.10 and P2O5 contents lower than 0.20%,thus can be grouped into calc-alkaline I-type granites.They are enriched in Cs,Rb,Th,U,Pb and LREE,and depleted in Ba,Sr,P,Ti,but the CQ and CX granites show higher Rb/Sr and Rb/Ba,and lower K/Rb ratios,and display more pronounced negative europium anomalies relative to the granites in the HX intrusion,indicating that the three intrusions had underwent different degrees of differentiation.All of the analyzed rocks show homogeneous Nd isotopic compositions,with εNd(t) values ranging from-2.43 to-3.24,and tDM2 values,from 1.11 to 14.16 Ga.However,they display highly variable zircon Hf isotopic compositions with εHf(t) values spreading over 6 εHf units(specifically,-8.3 ~ +3.0 for the CQ intrusion,+1.7 ~ +10.2 for the CX intrusion,and-2.5 ~ +3.5 for the HX intrusion,respectively).Such a large variation in zircon Hf isotopic compositions argues strongly that different source components have contributed to magma genesis.Based on a synthesis of petrography,geochronology,elemental and isotopic geochemistry,it is suggested that the granites in all three intrusions were most likely generated via a two-stage process including formation of parental magma by mixing of mantle-derived magma and induced crustal-melted felsic magma in the deep crust,and then underwent variable degrees of differentiation during magma ascent.The distinct differences in rock-forming ages and chemical variation tendency among the three intrusions indicate that the Zhangpu composite pluton could not be generated by fractional crystallization from a common primary magma,rather it was most likely produced by multiple-stage intrusions of crust-mantle mixing magma with variable degrees of differentiation.