Abstract:

The initial Neo-Tethyan evolution in the Lhasa block is an important part for the reconstruction of the Tethys space-time framework. Due to the lack of research objective, its understanding needs to be deepened. The Nanmulin granodiorite and quartz diorite are located in the central Lhasa block. We conducted systematic zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for determining the petrogenesis and source region of these rocks. The results provide new constraints for the early Neo-Tethyan evolution. Zircon U-Pb dating shows that both lithologies were formed in the Late Triassic, with the granodiorite emplaced at (214.3±1.1) Ma and the quartz diorite at (206.1±1.3) Ma. Geochemically, these rocks have Rittmann index (σ) of 1.79-4.71 and are calc-alkaline. They are peraluminous with Al₂O₃ varying from 15.01% to 17.57%,and A/CNK=0.76-1.12. Moreover, the granodiorite has high SiO₂(61.87%-73.72%, average 65.76%) and high K (up to 4.25%) and low Mg^#(31-46, average 41). Rb, U, K, Pb and LREEs are enriched and Nb, Ta, Ti and HREEs are depleted, suggesting arc-related or crust-like geochemical affinities. Rb/Sr (0.05-5.50, average 2.54) values suggest that their parental magmas were crustal sourced. The granodiorite and quartz diorite have high (La/Yb)_N values (average 12.05), with no clearly negative Eu anomalies (δEu=0.57-1.01, average 0.73). In summary, these data indicate that the Nanmulin intermediate-felsic rocks are I-type formed by the partial melting of juvenile crust. We propose that the northward subduction of the Neo-Tethyan was initiated before Late Triassic, which was before the formation of the newly-discovered Late Triassic Be-RB±(Nb-Ta) rare metal mineralization.

Key words: Lhasa block, Nanmulin, Late Triassic, zircon U-Pb geochronology, petrogenesis