Isotope study of alkali-feldspar granite zircon in the middle Gangdise batholith and its geological significance

Abstract: In this paper, the authors conducted zircon LA-ICP-MS U-Pb dating and Lu-Hf analysis of the alkali-feldspar granite in Tama of Xietongmen region along the middle Gangdise magmatic belt. The results suggest that the weighted mean ages of two samples are (40.02±0.39)Ma and (40.65±0.32)Ma for the pluton, respectively. In essence, the two ages are of the same geological significance in that they both represent the crystallization age or emplacement age of the alkali-feldspar granite. Zircon Lu-Hf analytical data reveal that values of 176Hf/177Hf vary from 0.282633 to 0.282878 (averaging 0.282765), and the values of εHf（t） range from -4.08 to 4.15 (0.28 on average) , with the peak values of εHf（t）varying from -1 to 1. TDMC model ages are from 822 Ma to 1373 Ma (averaging 1075 Ma) with peak model ages being ca.1000-1200 Ma. The obvious feature is that the values of the εHf（t）show alternating positive and negative values, corresponding to a small value and indicating the crustal-mantle contamination process, which resulted from partial melting of the ancient crust. Combined with geological setting and study results, the authors consider that the alkali-feldspar granite might have generated from the slab break off of the Neo-Tethys oceanic crust, which led to upwelling of the asthenosphere and further induced the partial melting and recycling of the ancient crust. During the process, the mantle materials played a vital role in forming the alkali-feldspar granite in the study area.