Late Paleozoic- Early Mesozoic tectonics of Hainan Island: Key to understanding Paleotethyan geology
Authors: He Huiying , Cawood Peter , Wang Yuejun 
Published: March 2020

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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 &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt;(t) values of +5.86 &amp;#8211; +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&amp;#8211;243 Ma) with slightly high A/CNK ratios, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values (up to 11.75 &amp;#8240;) 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&amp;#8211;225 Ma) high-K calc-alkaline granitoids with high zircon temperatures (842&amp;#8211;867&amp;#176;C) and geochemical signatures of A-type granites. They show slightly low whole-rock &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt;(t) and zircon &amp;#949;&lt;sub&gt;Hf&lt;/sub&gt;(t) values, suggestive of the derivation from a metabasite&amp;#8211;greywacke source in an extensional setting. 6) ca. 240 Ma gabbro-dolerites showing enrichment in LILEs, depletion in HFSEs, negative &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt; (t)-&amp;#949;&lt;sub&gt;Hf&lt;/sub&gt; (t) values (&amp;#8722;8.45 to &amp;#8722;1.05 and &amp;#8722;5.9 to &amp;#8722;2.7, respectively) and crustal-like &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values (7.26&amp;#8211;8.70&amp;#8240;), 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.&lt;/p&gt;&lt;p&gt;Thus, Hainan Island provides a record of Carboniferous back-arc basin opening, followed by an extended Permian&amp;#8211;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&amp;#8211;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.&lt;/p&gt;