Northward Subduction Of Bangong-Nujiang Tethys: Insight From Late Jurassic Magmatic Rocks In Western Qiangtang Terrane

New zircon LA-ICPMS U–Pb age data, whole-rock major and trace elements, and zircon Hf isotopic data of intrusive rocks from Larelaxin and Caima plutons in the southern margin of the Western Qiangtang subterrane, dacite from Renacuo, basalt and diabasic samples from Duobuza, provide important evidence of northward subduction of the Bangong–Nujiang Tethyan Ocean lithosphere.Host granitoids including quartz diorites, granodiorites, and syenogranites and associated mafic varieties including gabbroic enclaves, dioritic dikes and enclaves, and quartz dioritic enclaves are investigated in this study. Five host granitoid samples are dated at 163–160 Ma, and one dioritic dike and three dioritic enclave samples are dated at 162–158 Ma, which indicates that these rocks were contemporaneous. The quartz diorites and granodiorites are normal calc-alkaline I-type granitoids. The syenogranites are characterized by high Si O2(74–77 wt.%) and differentiation index(DI= 92–97) and marked depletion in Ba, Nb, Ta, Sr, P, Ti, and Eu, thus indicating that they are highly fractionated I-type granitoids. The host granitoids exhibit uniform zircon εHf(t) values(-1.4 to +1.9). Considering the current data and their negative whole-rock εNd(t) values(-4.7 to-3.5) reported in recent studies, these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle-derived or juvenile crust-derived components. One gabbroic enclave sample exhibits high Al2O3(18.3 wt.%), low Mg O(3.3 wt.%), high Ti O2(1.4 wt.%), and high Nb(24 ppm) and is geochemically similar to high-alumina basalt(HAB) and high-Nb basalt(HNB). This indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene. The dioritic dike and enclave samples yield εHf(t) values(-1.6 to +3.1) similar to those of the host granitoids. They most likely originated from magma mixing between mantle-derived and crust-derived melts at or close to the Moho. The intrusive rocks with geochemical diversity of the Larelaxin and Caima plutons from Bangong Tso in western Tibet can be interpreted to have resulted from melting, assimilation, storage, and homogenization(a MASH process) above subduction zones, thus indicating that the Bangong–Nujiang Tethyan Ocean lithosphere was subducted northward beneath the Western Qiangtang subterrane during the Late Jurassic.One dacitic sample is dated at 154 Ma, one diabasic samples from Duobuza is dated at 157 Ma, which indicates that these rocks were contemporaneous. The dacites(Si O2 = 66-69 wt.%) are characterized by high Sr(520–1083 ppm), low Yb(0.75–0.92 ppm), low Y(9.8–10.5 ppm), thus indicating that they are adakites. They are medium-K calc-alkalic series(K2O/Na2 O = 0.53-0.68), and have positive εNd(t) values(+1.0 ~ +3.4) and εHf(t) values(+6.4 ~ +7.0). These rocks can be interpreted to have resulted from the partial melting of subducted oceanic crust. The diabases are characterized by high Ti O2(3.37–3.85 wt.%). These samples are similar to ocean island basalts(OIB)(e.g., [La/Yb]N = 7.75-10.06; Nb/Y = 1.77-2.31). They have positive εNd(t) values(+3.3 ~ +3.7), positive εHf(t) values(+4.7 ~ +5.7) and negative to positive zircon εHf(t) values(–1.5 ~ +5.2). Adakites and OIB-type rocks indicate that the Bangong–Nujiang Tethyan Ocean ridge was subducted northward beneath the Western Qiangtang subterrane during 156±2 Ma.