Age, Hf isotopes and geochemistry of Early Cretaceous granodiorite-porphyry in the middle segment of Bangong Co-Nujiang suture zone of Tibet

Early Cretaceous igneous rocks are widely distributed in the middle segment of Bangong Co-Nujiang suture zone, and the nature of magma source and petrogenesis of these rocks are of great significance for the closure of the Bangong Co-Nujiang Ocean. The authors carried out systematic petrographic, geochemical and isotopic studies of the granodiorite-porphyries from Dongkaco microcontinent, middle segment of the Bangong Co-Nujiang suture zone.The results show that the granodiorite-porphyries have the age of 109.4±1.9 Ma by zircon U-Pb dating method, and granodiorite-porphyries are characterized by high K calc-alkaline series, enrichment of SiO₂, Al₂O₃ and depletion of Mg^#.The content of ∑REE is relatively low(196.45×10^-6~207.6×10^-6)and the rocks are relatively enriched in LREEs and depleted in HREEs. In the spider diagram the trace elements are inclined to the right in zigzag form and display enrichment of large-ion lithophile elements and relative depletion of high field strength elements.The granodiorite-porphyries have negative zircon ε_Hf(t) values(-4.21~-10.59), corresponding to crustal Hf model ages of 1438~1842 Ma and showing the characteristics of the ancient crust.Combining the above characteristics with regional information, it is believed that the granodiorite-porphyries were found in the Dongkaco microcontinent and belonged to I-type granite, which may be the product of magmatic activity caused by the oceanic fragmentation during the collision between the Dongkaco micro-continent and the Central Lhasa or Nie Rong terrane.