Petrogenesis of Early-Middle Jurassic gabbros in southern Tibet with implications for crustal growth in the southern Lhasa subterrane

Abstract

ABSTRACT As a natural laboratory for the study of crustal growth and accretionary tectonics, the Lhasa terrane was once a microcontinent with a Precambrian basement, which was transformed into a juvenile terrane by remarkable Phanerozoic crustal reworking and growth. Here, we report the geochronological, mineralogical, and geochemical data of the Numa hornblende gabbros and Dongga cumulate hornblende gabbros in the southern Lhasa subterrane (SLS). Zircon U-Pb dating yields ages of 163.7 ± 2.2 Ma and 182.8 ± 2.2 Ma for the Numa hornblende gabbros and Dongga cumulate hornblende gabbros, respectively, which suggests generation in the Early-Middle Jurassic. Both the Numa hornblende gabbros and Dongga cumulate hornblende gabbros are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and they have positive εHf(t) (+11.1 to +17.3) and εNd(t) (+3.6 to +6.0) values. Whole-rock geochemical and isotopic data indicate that their primary magma was derived by the partial melting of a depleted mantle wedge metasomatized by hydrous fluids. Our data, combined with previous research results, indicate that the Numa-Dongga gabbros formed in a continental margin arc setting related to the northward subduction of the Neo-Tethys oceanic slab. In addition, comprehensive studies reveal that the Early Jurassic to Late Cretaceous were the most important periods for crustal growth in the SLS and crustal thickening in the Mesozoic was concentrated mainly in the Early Jurassic (ca. 190–170 Ma) and early Late Cretaceous (ca. 100–80 Ma), which is related to the magmatic flare-up induced by slab roll-back. The recycling of subducted oceanic crust and the underplating of mantle-derived magmas play important roles in the growth and evolution of the continental crust in the SLS.