Petrogenesis and tectonic implications of the Triassic rhyolites in the East Kunlun Orogenic Belt, northern Tibetan Plateau

The East Kunlun Orogenic Belt(EKOB),which is in the northern part of the Greater Tibetan Plateau,contains voluminous Late Triassic intermediate-felsic volcanic rocks.In the east end of the EKOB,we identified highly differentiated peralkaline-like Xiangride rhyolites(~209 Ma)that differ from the widespread andesitic-rhyolitic Elashan volcanics(~232–225 Ma)in terms of their field occurrences and mineral assemblages.The older,more common calc-alkaline felsic Elashan volcanics may have originated from partial melting of the underthrust Paleo-Tethys oceanic crust under amphibolite facies conditions associated with continental collision.The felsic Elashan volcanics and syn-collisional granitoids of the EKOB are different products of the same magmatic event related to continental collision.The Xiangride rhyolites are characterized by elevated abundances of high field strength elements,especially the very high Nb and Ta contents,the very low Ba,Sr,Eu,P,and Ti contents;and the variably high ^(87)Sr/^(86)Sr ratios(up to 0.96),exhibiting remarkable similarities to the characteristic peralkaline rhyolites.The primitive magmas parental to the Xiangride rhyolites were most likely alkali basaltic magmas that underwent protracted fractional crystallization with continental crust contamination.The rock associations from the early granitoids and calc-alkaline volcanic rocks to the late alkaline basaltic dikes and peralkaline-like rhyolites in the Triassic provide important information about the tectonic evolution of the EKOB from syn-collisional to post-collisional.We infer that the transition from collisional compression to postcollisional extension occurred at about 220 Ma.

Exotic origin of the Chinese continental shelf: new insights into the tectonic evolution of the western Pacific and eastern China since the Mesozoic

The effect of paleo-Pacific subduction on the geological evolution of the western Pacific and continental China is likely complex. Nevertheless, our analysis of the distribution of Mesozoic granitoids in the eastern continental China in space and time has led us to an interesting conclusion： The basement of the continental shelf beneath East and South China Seas may actually be of exotic origin geologically unrelated to the continental lithosphere of eastern China. By accepting the notion that the Jurassic- Cretaceous granitoids in the region are genetically associated with western Pacific subduction and the concept that subduction may cease to continue only if the trench is being jammed, then the termination of the granitoid magmatism throughout the vast region at -88±2 Ma manifests the likelihood of ＂sudden＂, or shortly beforehand （- 100 Ma）, trench jam of the Mesozoic western Pacific subduction. Trench jam happens if the incoming ＂plate＂ or portion of the plate contains a sizeable mass that is too buoyant to subduct. The best candidate for such a buoyant and unsubductable mass is either an oceanic plateau or a micro-continent. We hypothesize that the basement of the Chinese continental shelf represents such an exotic, buoyant and unsubductable mass, rather than seaward extension of the continental lithosphere of eastern China. The locus of the jammed trench （i.e., the suture） is predictably located on the shelf in the vicinity of, and parallel to, the arc-curved coastal line of the southeast continental China. It is not straightforward to locate the locus in the northern section of the East China Sea shelf because of the more recent （〈20 Ma） tectonic re-organization associated with the opening of the Sea of Japan. We predict that the trench jam at - 100 Ma led to the re-orientation of the Pacific plate motion in the course of NNW direction as inferred from the age-progressive Emperor Seamount Chain of Hawaiian hotspot origin （its oldest unsubdued Meiji and Detroit seamounts are -82 Ma）, making the boundary between the Pacific plate and the newly accreted plate of eastern Asia transform fault at the location east of the continental shelf of exotic origin. This explains the apparent-40 Myr magmatic gap from - 88 to - 50 Ma prior to present-day western Pacific subduction initiation. We propose that basement penetration drilling on well-chosen sites is needed to test the hypothesis in order to reveal the true nature of the Chinese continental shelf basement. This testing becomes critical and cannot longer be neglected in order to genuinely understand the tectonic evolution of the western Pacific and its effect on the geology of eastern China since the Mesozoic, including the cratonic lithosphere thinning, related magmatism/mineralization, and the mechanism of the subsequent South China Sea opening, while also offering novel perspectives on aspects of the plate tectonics theory. We also suggest the importance of future plate tectonic reconstruction of the western Pacific to consider the nature and histories of the Chinese continental shelf of exotic origin as well as the probable transform plate boundary from - 100 to -50 Ma. Effort is needed to reveal the true nature and origin of the - 88 ± 2 Ma granitic gneisses in Taiwan and the 110-88 Ma granitoids on the Hainan Island.