Zircon U-Pb ages, geochemical characteristics and constraints on the Bangong-Nujiang Tethys Ocean subduction of Late Jurassic-Early Cretaceous intrusive rocks from Mami area, Tibet

Large-scale Mesozoic igneous rocks distributed on the north and south sides of the Bangong - Nujiang suture zone are important in constraining the evolution of the Tethyan Ocean.This paper reports the zircon U-Pb age, major and trace geochemical characteristics of the felsic intrusive rocks in the Mami area, south of the Bangong-Nujiang suture zone.The ²⁰⁶Pb/²³⁸U weighted average ages of 146~145 Ma for the granitic porphyry and 140 Ma for the granitic amphibolite indicate that the intrusive rocks were emplaced during the Late Jurassic to Early Cretaceous.Both granitic porphyry and granodiorite show high content of SiO₂(64.67%~77.45%), Al₂O₃ (12.59%~16.24%)and low MgO(0.19%~0.89%), which are weakly - strongly peraluminous calc-alkaline granite.These rocks are also enriched in large-ion lithophile elements(LILE)of Rb, K, and depletion in high field strength elements(HFSE)of Nb, Ta, Ti.The total amount of rare earth elements(REE)is low(∑REE=129 ×10^-6~201×10^-6), and the fractionation degree is high(LREE/HREE= 2.06~9.18).The REE patterns show enrichment in light rare earth elements(LREEs)with negative Eu anomalies, which consistent with the REE distribution patterns of felsic magmatic rocks formed on the active continental margin.Comprehensive analysis shows that the Late Jurassic-Early Cretaceous felsic intrusive rocks in Mami mainly originated from the partial melting of ancient crustal materialand and underwent fractional crystallization during the magmatic evolutionary process.Theses intrusive rocks show the characteristics of volcanic arc-type, suggesting that they formed in the background of ocean subduction.Previous studies, in combination with regional data, show that Bangong - Nujiang Ocean underwent southward subduction during the Late Jurassic to Early Cretaceous, forming the Cuoqin-Shenzha tectonic-magmatic arc.It further confirmed the bidirectional subduction of the Bangong - Nujiang Tethyan Ocean during the Late Jurassic - Early Cretaceous period, which provides new lithological evidence for the tectonic evolution of the Tethyan Ocean.