New sedimentary constraints for the Late Devonian north-dipping Paleo-Tethys subduction and its eastern continuation on Hainan Island, South China

Abstract

As a major boundary between the Simao-Indochina and South China blocks, the Jinshajiang–Ailaoshan–Song Ma suture zone preserves the remnants of a back-arc basin or a branch of the Paleo-Tethys Ocean. However, whether this suture zone extends eastward to the Hainan Island is still controversial. Here, we use detrital zircon U–Pb geochronology to explore the source-to-sink characteristics of the Lower Carboniferous-Lower Permian strata in Hainan Island and mainland southeast China (SE China). The results, combined with published data for the Devonian-Triassic sandstones from Hainan and SE China, show that the Devonian-Lower Permian samples display similar detrital zircon age populations at ∼430 Ma, ∼930–980 Ma, ∼1600–1800 Ma and ∼2400 Ma. However, the Upper Permian-Triassic sandstones have a predominant zircon population at ∼370 Ma, indicating a dramatic change in the detritus provenance for Hainan-SE China since the early Late Permian. The presence of similar 400-350 Ma detrital zircons peaked at ∼370 Ma with ϵHf(t) values from −12 to +3 in the sedimentary rocks from both Hainan-SE China and Simao-Indochina blocks indicates that these two terranes should have a similar detritus source with abundant Late Devonian magmatic rock exposure during the Late Permian-Triassic. These Late Devonian magmatic rocks were most likely formed by the north-dipping Paleotethyan subduction. This conclusion is consistent with the recent discovery of Upper Devonian arc-related tuffs, volcaniclastics, tuffaceous chert and rhyolite in the North Qiangtang, Simao-Indochina and East Malaya terranes, and possibly also in the southern Hainan. These Upper Devonian rocks could have acted as detritus source for the basins in SE China-Hainan, as well as the Youjiang Basin in SW China, after the collision between the Indochina and South China blocks. Combined with the transition from a carbonate platform to clastic facies in the early Late Permian in SE China, the Indochina-South China collision on Hainan Island may have occurred at ∼266-260 Ma, earlier than that (∼250-230 Ma) along the Jinshajiang–Ailaoshan–Song Ma suture zone. Such a temporal–spatial source-to-sink relationship between the Late Devonian arc and basins on the southern margin of South China suggests that SE China-Hainan should be located within the Paleo-Tethys rather than the Paleo-Pacific tectonic domain during the Early Carboniferous-early Late Permian.