Abstract
The subduction and close of the Mesozoic Bangong–Nujiang Ocean (BNO) led to a collision of the Lhasa and Qiangtang blocks, which formed the backbone of the Tibetan Plateau (the largest and highest plateau on Earth). However, the detailed subduction processes (in particular, the oceanic subduction processes) within the BNO are still not clear. Here, we focus on the plutonic complex of the oceanic arc in the Bangong–Nujiang suture (BNS) and report field observations on zircon U–Pb ages, Lu–Hf isotopes, and the Al-in-hornblende barometry of quartz diorites from the Lameila pluton in western Tibet. Zircon from the quartz diorites yielded a LA-ICP-MS U–Pb age of 164 Ma. The zircon showed very positive εHf(t) values from 10.5 to 13.9, suggesting the Lameila pluton was likely sourced from the depleted-mantle wedge, which is in contrast with contemporary (164–161 Ma) volcanic rocks in the region that had negative εHf(t) values of −7.4 to −16.2 and a magma source from partial melting of subducted sediments. The Lameila pluton showed a temperature-corrected Al-in-hornblende pressure of 3.9 ± 0.8 kbar, corresponding to an emplacement depth of 13 ± 3 km. Therefore, the thickness of the Jurassic oceanic arc crust must have doubled since the initial growth of the oceanic arc on the BNO crust, with a crustal thickness of 6.5 km during the Middle Jurassic. In combination with previous works on volcanic rocks, this study further supports a two-subduction zone model in association with the BNO during the Middle Jurassic, namely, a north-dipping BNO–Qiangtang subduction zone and an oceanic subduction zone within the BNO. The latter oceanic subduction zone produced the depleted-mantle-derived Lameila pluton and the subducted sediment-derived volcanic rocks in the fore arc.
Highlights
The Tibetan Plateau is the largest plateau on Earth, with an average elevation above 4 km [1] and crustal thickness of 60–80 km [2] over an area of 2.5 million km2
Lhasa–Qiangtang collision that followed the closure of the Bangong–Nujiang Ocean (BNO), are critical in understanding the formation processes of the Tibetan Plateau
The oscillatory zoning patterns, Th and U concentrations, and typical magmatic zircon rare earth element (REE) patterns (Figure 6c) imply that this age represents the time of magma emplacement
Summary
The Tibetan Plateau is the largest plateau on Earth, with an average elevation above 4 km [1] and crustal thickness of 60–80 km [2] over an area of 2.5 million km2 It was formed by the sequential amalgamation of continental or oceanic arc blocks (Figure 1) over several orogenic cycles since the Paleozoic, and further enhanced by the collision and continuing convergence of the India and. Recent studies on the 164–161 Ma high-Mg andesites from Shiquanhe [19] and Daruco [20] have suggested a Jurassic oceanic arc developed within the BNO Apart from these significant findings, the configuration of this arc, such as its crustal thickness and the source(s) for the arc magmatism are still unclear
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