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

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 lithological 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 dioritic 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 metaluminbus 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 calcakaline 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 epsilon(Hf)(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 epsilon(Hf)(t) values (= -0.5 similar to +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.