The Bangxi-Chenxing tectonic zone in Hainan Island (South China) as the eastern extension of the Song Ma-Ailaoshan zone: Evidence of late Paleozoic and Triassic igneous rocks

Abstract

Mafic-intermediate igneous rocks in the Bangxi-Chenxing tectonic zone of Hainan Island, South China, can be used to constrain the Paleotethyan tectonic evolution in Southeast Asia. This paper presents petrologic, geochronological, elemental and Sr–Nd–Hf–O isotopic data for Carboniferous basalts, Permian andesites and Triassic gabbro-dolerites in this zone. Zircon grains from these rocks have U-Pb ages of ∼330 Ma, ∼257 Ma and ∼240 Ma, respectively. The ∼330 Ma basalts contain 1400 Ma captured zircon grains, and display MORB-like geochemical characteristics with positive εNd (t) values of +5.86–+9.85, reflective of a MORB source with the input of a slab-derived component in a continental back-arc basin setting. The ∼257 Ma andesites contain older captured zircon grains, and have arc-like geochemical signature with weakly positive εNd (t) values of +0.26–+0.38, zircon εHf (t) values of −0.5–+1.4 and δ18O values of 6.35–7.16‰, which were derived from a metasomatized mantle wedge modified by the sediment-derived component in a continental arc setting. The ∼240 Ma gabbro-dolerites exhibit enrichment in LILEs, depletion in HFSEs, negative εNd (t)-εHf (t) values (−8.45 to −1.05 and −5.9 to −2.7, respectively) and crustal-like δ18O values (7.26–8.70‰), indicative of a post-collisional environment in response to the asthenosphere upwelling shortly after the closure of the Bangxi-Chenxing back-arc basin. The Carboniferous-Triassic mafic-intermediate igneous rocks thus record the Carboniferous-Permian arc-basin system and its Triassic closure. The Bangxi–Chenxing tectonic zone separated South Hainan from North Hainan and westerly extended to the Song Ma–Ailaoshan suture zone.