Geochronology, geochemistry and tectonic setting of the Dianzhong Formation volcanic rocks in the Riduo area, Eastern Gangdese belt, Tibet

The Linzizong Group is a typical Paleogene volcanic assemblage in the Gangdise belt, which records the orogenic history of subduction and collision between India and Eurasia.Zircon U-Pb chronology and Lu-Hf isotope studies were conducted on the Dianzhong Formation basic and intermediate volcanic rocks at the bottom of the Linzizong Group in the Riduo region of the Eastern Gangdese belt.LA-ICP-MS zircon U-Pb dating results show that ages of the Dianzhong Formation basalt and andesite crystalline tuff are 57.9±1.2 Ma and 57.4±1.2 Ma respectively, indicating that they were formed in the Late Paleozoic.Geochemically, the volcanic rocks generally belong to high-K(calc-alkaline) series.The basic volcanic rocks have low SiO₂(48.67%~49.34%), high Al₂O₃(15.25%~18.59%) and high MgO(3.76%~8.69%).They are characterized by LREE enrichment and positive Eu anomalies(δEu=1.15~1.37).The intermediate volcanic rocks have high SiO₂(54.92%~64.16%), high Al₂O₃(15.85%~16.72%), high K₂O(0.65%~2.14%) and low MgO(1.34%~3.67%).They are relatively enriched in LREE and weak negative Eu anomalies(δEu=0.77~0.92).Both volcanic rocks exhibit fractionated REE and rightward patterns, strong enrichment of LILE(Rb, Ba, Sr), and depletion of HFSE(Nb, Ta, Ti).The zircon ε_Hf(t) values of the basic volcanic rocks range from 4.86 to 8.97, and the ε_Hf(t) values of the intermediate volcanic rocks are from 0.26 to 6.37.Based on previous results, it is suggested that the Dianzhong Formation volcanic rocks in Riduo area were formed in the syn-collision tectonic setting of the collision between India and Eurasia.The basic volcanic rocks were mainly derived from the partial melting of overlying mantle wedge metasomatized by the fluids of subducting plate, while the intermediate volcanic rocks were originated from partial melting of the juvenile crust materials, both of which were influenced by the contamination of old crustal materials to various degrees.