U-Pb zircon geochronology and geochemistry from NE Vietnam: A ‘tectonically disputed’ territory between the Indochina and South China blocks

Volcanoplutonic complexes in NE Vietnam have recently been interpreted as intraplate products of the Emeishan plume. Alternatively, mafic–ultramafic rocks have been considered as dismembered Palaeotethyan ophiolites juxtaposed along a tectonic mélange zone. New U–Pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the Indochina–South China blocks. Mafic–ultramafic rocks exposed within a tectonic mélange (Song Hien Tectonic Zone) include sub-alkaline pillow basalts that define two geochemically distinct ophiolitic suites (SH-1: N-MORB-like, SH-2: transitional E-MORB-like). Both suites have geochemical signatures suggestive of crustal contamination, compatible with a volcanic passive margin/rift setting. We suggest that SH-1 basalts may correlate with the Devonian–Carboniferous Jinshajiang–Ailaoshan–Song Ma branch of the Palaeotethys and form part of the associated Dian–Qiong belt, whereas SH-2 basalts are co-magmatic with Middle–Late Permian mafic–ultramafic intrusive rocks (dolerites, gabbros, peridotites) that developed in a rift basin, most likely on the margin of the down-going South China plate during west-vergent subduction beneath Indochina. During continental orogenesis and thrust stacking, these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the Song Hien Tectonic Zone. Post-collisional relaxation led to the development of a rift basin (Song Hien rift) comprising Late Permian–Triassic volcano-sedimentary strata including < 270–265 Ma terrigenous sandstones, < 252 Ma mudstones, and c. 254–248 Ma felsic effusives. Granites and granodiorites were emplaced across NE Vietnam between c. 252 and 245 Ma in a syn- to post-collisional setting. The Late Permian–Early Triassic felsic magmatic rocks best correlate with coeval rocks in SW Guangxi and the Central and Western Ailaoshan fold belts (China) and the Truong Son fold belt (Vietnam); together they signal the final to post-collisional stages of Indochina–South China collision. We demonstrate that the analysed magmatic rocks in the Lo-Gam–Song Hien domains of NE Vietnam are not genetically linked to the Emeishan Large Igneous Province in the Yangtze block of South China, as has been previously widely proposed.