冈底斯中段达居复式花岗岩体成因:锆石U-Pb年代学、元素地球化学与Hf同位素制约

Petrogenesis of the Daju composite granite pluton in the middle segment of the Gangdese belt:Constraints from zircon U-Pb ages,elemental geochemistry and Hf isotopes

本文报道了冈底斯中段达居复式岩体的锆石U-Pb年龄与Hf同位素组成,以及全岩元素地球化学组成,据此探讨了岩浆的形成机制与演化过程,进而全面揭示了岩体的成因。根据岩性和岩相特征,达居复式岩体可区分为昌那和桑嘎两个单元,前者岩性主要为花岗闪长岩,岩体内含丰富的闪长质包体;后者岩性主要为中细粒二长花岗岩,岩性总体较均匀。锆石LA-ICP-MS U-Pb定年结果表明,昌那单元花岗闪长岩和其中的闪长质包体具有一致的成岩年龄,分别为52.7±0.7Ma和52.3±0.6Ma,桑嘎单元二长花岗岩的成岩年龄为45.3±0.7Ma。地球化学组成上,二单元岩石均具有亚碱、准铝的特征,碱铝指数(AKI值)多低于0.9,铝饱和指数(A/NKC值)均小于1.0,属于准铝质钙碱性I型花岗岩。它们均富集轻稀土和大离子亲石元素(如Rb,Pb),亏损高场强元素(如Nb,Ta,Ti),具有弧型岩浆岩的地球化学组成特征。与昌那单元岩石相比,桑嘎单元岩石富硅、富碱,具有较高的Rb/Sr、Rb/Ba比值,并表现出较显著的Eu负异常,指示其经历了较高程度的分异演化。昌那单元花岗闪长岩和其中的闪长质包体具有一致亏损的Hf同位素组成,二者的ε_(Hf)(t)值分别为+4.6^+11.5和+5.0^+11.1,桑嘎单元总体也具有亏损的Hf同位素组成,但其ε_(Hf)(t)值具有较大的变化范围(-0.5^+14.7)。综合分析表明,昌那单元岩石应为底侵的亏损地幔岩浆与其诱发初生地壳熔融产生的岩浆经混合作用形成,桑嘎单元岩石为这一混源岩浆经进一步分异演化后侵位结晶的产物,且成岩过程中极可能有古老的印度地壳物质参与。

This paper presents comprehensive zircon U-Pb ages and Hf isotopic compositions, and whole-rock geochemistry for the Daju composite granite pluton in the middle segment of the Gangdese belt, with the aims of elucidating the formation mechanism and evolution process of their magmas, and gaining new insights into their petrogenesis. According to the lithologieal and petrographic characteristics, this pluton can be divided into two units, i. e. , the Changna （CN） unit and the Sangga （SG） unit. The CN unit consists mainly of granodiorites with abundant dioritic enclaves, while the SG unit is composed of medium to fine-grained monzogranites with generally homogenerous lithology. Zircon LA-ICP-MS U-Pb dating yields coeval ages of 52.7 ±0. 7Ma for the granodiorites and of 52. 3 ±0. 6Ma for the enclosed dioritie enclaves in the CN unit, while the monzogranites of the SG unit were emplaced at 45.3 ± 0. 7Ma. Geochemically, all rocks of the two units show sub-alkaline and metaluminous signatures, with agpaitic index （AKI） values mostly less than 0. 9 and aluminum saturation index （ A/NKC ） values less than 1.0, thus can be grouped into metaluminous calc- akaline I-type granites. They are also enriched in LREEs and LILEs （e. g. , Rb, Pb）, and depleted in HFSEs （e. g. , Nb, Ta, Ti）, showing the geochemical characteristics of arc magma rocks. Compared with granodiorites in the CN unit, the monzogranites in SG unit are rich in silicon and alkali, and have higher Rb/Sr and Rb/Ba ratios, and display more pronounced negative europium anomalies, indicating that the SG unit had experienced higher degrees of differentiation. The granodiorites and the enclosed dioritic enclaves in CN unit show accordantly depleted Hf isotopic compositions with 6He（t） values ranging from ＋4. 6 to ＋ 11.5, and from ＋5.0 to ＋ 11.1, respectively. The monzogranites in SG unit also show depeleted Hf isotopic compositions but with relatively wide variation range for εHf（t） values （ = -0. 5 - ＋ 14. 7）. Based on a synthesis of petrography, geochronology, elemental and isotopic geochemistry and tectonics, we suggest that the granodiorites and the enclosed dioritic enclaves in CN unit were generated by mixing of depleted mantle- derived magma and its induced juvenile crustal-melting magma. The monzogranites of the SG unit were emplaced after further differentiation of the mixed magma, and the ancient India crustal materials were most likely involved in their petrogenesis.