SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet

Abstract

We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174-190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent, concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks; Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).