Late Paleozoic- Early Mesozoic tectonics of Hainan Island: Key to understanding Paleotethyan geology

In Southeast Asia, establishing the origin and associated tectonic setting of Late Paleozoic-Early Mesozoic igneous rocks is complicated by structural overprinting and the complex tectonic evolution of the Paleotethyan regime. Hainan Island, located at the south-eastern margin of the Paleotethys, and lacking significant tectonic overprints is a key to understand amalgamation history of the Indochina and South China blocks and to constraining the tectonic evolution of Paleotethys ocean in southeast Asia.

The Late Paleozoic-Early Mesozoic record of igneous rocks on Hainan Island includes the following. 1) ca. 350 Ma island arc andesites and ca. 330 Ma metabasites, the latter with both MORB- and arc-like geochemical affinities, positive ε_Nd(t) values of +5.86 – +9.85 and rare inherited zircons with a zircon age of 1400 Ma inferred to be derived from a MORB source with the input of a slab-derived component. Together with the ~350 Ma island arc andesites, the Carboniferous tectonic environment is supposed to be a continental back-arc basin setting. 2) Late Permian gneiss granitoids (272-252 Ma) characterized by a gneissic foliation and calc-alkaline I-type geochemical affinities with negative Nb-Ta and Ti anomalies, related to metasomatized mantle wedge modified by the sediment-derived component in a continental arc setting. 3) ca. 257 Ma arc-like andesites, which further validate a subduction-related setting. 4) Peraluminious Early-Middle Triassic massive granitoids (251–243 Ma) with slightly high A/CNK ratios, δ¹⁸O values (up to 11.75 ‰) and Sr/Y ratios, inferred to have formed in a compressive regime from a mixed source of greywacke and metabasite. 5) Middle-Late Triassic (242–225 Ma) high-K calc-alkaline granitoids with high zircon temperatures (842–867°C) and geochemical signatures of A-type granites. They show slightly low whole-rock ε_Nd(t) and zircon ε_Hf(t) values, suggestive of the derivation from a metabasite–greywacke source in an extensional setting. 6) ca. 240 Ma gabbro-dolerites showing enrichment in LILEs, depletion in HFSEs, negative ε_Nd (t)-ε_Hf (t) values (−8.45 to −1.05 and −5.9 to −2.7, respectively) and crustal-like δ¹⁸O values (7.26–8.70‰), it is implied that the Hainan Island entered into post-collisional environment in response to the asthenosphere upwelling shortly after the closure of back-arc basin.

Thus, Hainan Island provides a record of Carboniferous back-arc basin opening, followed by an extended Permian–Triassic history of subduction-related consumption leading to orogenic assembly and extensional collapse between the South China and Indochina blocks. Such a tempo-spatial pattern is consistent with that along the Song Ma–Ailaoshan suture zone rather than the magmatic history of eastern South China and indicates that the Paleotethys extended west to at least Hainan Island in the Late Paleozoic-Early Mesozoic.