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Abstract
Identification of arc magmatic rock associations in a subduction zone has important implications for specifically revealing the geodynamic evolution of the subduction system. The closure time of the Bangong-Nujiang Tethyan Ocean and the detailed subduction processes have been hotly debated, hindering our understanding of the tectonic evolution of central Tibet. Here, we investigated the ca. 110-104 Ma Gerze lavas (basalts, basaltic andesites, andesites, dacites, and rhyolites) in southern Qiangtang. Fusion of slab fluid-metasomatized mantle wedge could yield the basalts, and such basaltic magmas, if contaminated with ancient basement orthogneisses, could have formed the andesites. The basaltic andesites with high Nb and Nb/La are similar to the Nb-enriched arc basalts and probably originated from slab melt-metasomatized mantle. The dacites were generated by fractional crystallization of the subducted melange-derived intermediate magmas. The rhyolites have geochemical characteristics (high SiO2 and La/Yb; low MgO and Sr/Y) similar to those of Jamaican-type adakites and were possibly sourced from the subducted oceanic plateau at low pressures. The Gerze Jamaican-type adakites and Nb-enriched basalt association could imply intense slab-mantle interactions. The Gerze lava suites show clear arc affinities, indicating that oceanic subduction may have lasted until 100 Ma. Based on previous studies and a noticeable ca. 145-125 Ma magmatic lull in southern Qiangtang, we suggest that the Bangong-Nujiang oceanic subduction geodynamics involved normal subduction (170-145 Ma), flat subduction (145-125 Ma), and slab roll-back (125-101 Ma). Moreover, the flat subduction was most likely caused by subduction of the oceanic plateau. Therefore, we propose, for the first time, that Tethyan oceanic plateau subduction during the Early Cretaceous could explain the tectonic evolution of the Bangong-Nujiang Ocean and distinctive magmatism in southern Qiangtang, central Tibet.
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