Abstract:

The Erdene granite in Southern Mongolia is located on the western margin of the massive Cretaceous magmatic granite in the whole Northeast Asia. The zircon U-Pb geochronology, rock geochemistry and zircon Hf isotopes in the Erdene region were studied to determine the formation age, petrogenesis, and tectonic setting.The main rock type is biotite granite. The zircon LA-ICP-MS U-Pb dating yields  a weighted mean age of (123.4±1.4) Ma, indicating that it formed in the Early Cretaceous. The granite has high silicon(w(SiO₂）=71.01%-72.66%), high alkali(w(Na₂O+K₂O)=8.52%-8.90%), poor phosphorus(w(P₂O₅)=0.06%-0.11%), low magnesium and calcium（w(MgO)=0.27%-0.39%, w(CaO)=1.10%-1.27%)，and A/CNK=(1.05-1.07），indicating that it belongs to the metaluminous-peraluminous rock. It is characterized by enrichment of LILE(such as Rb, U,Th), depletion of HFSE(such as Ta, Zr, Hf) and depletion of Nb,Sr,Ba elements, with obvious negative Eu anomalies(δEu=0.11-0.15).The average magmatic temperature is 816  ℃. It belongs to A1 type granite. The εHf(t) of the rocks ranges from 2.54-7.92, and the TDM2 varies from 1 016-673 Ma, indicating that a little young materials might have been  added in the source region. Based on the regional extensional tectonics, it is believed that the large Erdene granite formed in an extensional thinning environment, which is related  to the decompression and melting of the middle and lower crust.