U-Pb dating and tectonic implication of ophiolite and metabasite from the Song Ma suture zone, northern Vietnam

Abstract

The Song-Ma suture zone including the “classical Song Ma suture” and the Nam Co antiform between Indochina and South China blocks in northern Vietnam is a crucial area for understanding the tectonic evolution of Southeast Asia. To explore their natures and evolutionary history, this paper reports new petrological studies and SIMS zircon U-Pb age data for the Song Ma ophiolites, metabasites and fine-grained quartzite. The Song Ma ophiolites consist mainly of peridotite (Fo90-91, En92), basalt and gabbro, and experienced greenschist- to lower amphibolite-facies ocean-floor metamorphism. Lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite. We identified two types of zircon in the altered pyroxenite: I uniform metamorphic (70 vol.%), and II, zircon having inherited magmatic cores and metamorphic rims (30 vol.%). All U-Pb analyses of both zircon types yield an upper intercept age of 340 ± 29 Ma that we interpret as the protolith age. Type I zircon and Type II metamorphic rims together yield a mean 206Pb/238U age of 280 ± 2 Ma, with rodingite forming broadly coevally at 283 to 282 Ma. Metabasalt (plagioclase-amphibole schist) has a MORB-type geochemical affinity. Magmatic zircon from a plagioclase-amphibole schist yields a protolith age of 315 ± 4 Ma. High-pressure eclogite, garnet-phengite-quartz schist, and garnet-hornblende rock are distributed in the northwestern Nam Co antiform. Zircon crystals from both eclogite and garnet hornblende rock have low Th/U ratios of 0.01 to 0.07 and 0.02 to 0.06, and yield metamorphic ages of 231 ± 8 Ma and 228 ± 3 Ma, respectively. The ages of detrital zircon grains from fine-grained, banded quartzite from the Nam Co antiform range from Paleozoic to Proterozoic with a peak at 1.7 to 1.8 Ga. All available data indicate a three-stage tectonic evolution: (1) 340 to 280 Ma: the Paleotethys that separated the Indochina and South China blocks was characterized by oceanic crust formation at 340 to 315 Ma and ocean-floor metamorphism at 283 to 280 Ma, (2)