Remnants of a Middle Triassic island arc on western margin of South China Block: Evidence for bipolar subduction of the Paleotethyan Ailaoshan Ocean

Abstract

Identifying ancient magmatic arcs in collisional orogens is important for regional tectonic reconstruction. Here, we first identified Middle Triassic (ca. 235–237 Ma) arc-type hornblende diorite and granodiorite in the Ailaoshan high-grade metamorphic belt in SW China, east of the Paleotethyan Ailaoshan-Song Ma suture. These rocks show arc-type geochemical affinities, such as large-ion lithophile element (LILE; e.g., Rb and Ba) enrichments and high-field-strength element (HFSE; e.g., Nb, Ta and Ti) depletions. They are characterized by positive εNd(t) (+2.90 to +4.63), εHf(t) (+9.9 to +14.1), and higher δ18O (mean 6.35–6.65‰) than typical mantle-derived magmas. Whole-rock geochemical and Sr-Nd-Hf-O isotope data demonstrate that the hornblende diorite was derived from depleted MORB-like mantle metasomatized by subducted oceanic sediments, and the granodiorite was formed by fractional crystallization from mafic magmas. These two rock types likely represent remnants of a continental arc, whose upper arc architecture may have been mostly-/completely-eroded by the subsequent Indosinian (Triassic) and Himalayan (Cenozoic) orogenic events. Tectonically, the presence of Triassic arc remnants east of the Paleotethyan Ailaoshan-Song Ma suture suggests the occurrence of east-dipping (present orientation) subduction of the Paleotethys beneath the South China Block, a hypothesis discarded by most previous studies due to the lack of arc magmatic evidence. Combined with previous studies in the Ailaoshan tectonic zone and its southern extension (Song Ma tectonic zone), we propose that the Paleotethyan Ailaoshan Ocean may have been bipolar-subducted westward and eastward beneath the Indochina and South China blocks, respectively.