U-Pb DATING AND TECTONIC IMPLICATION OF OPHIOLITE AND METABASITE FROM THE SONG MA SUTURE ZONE, NORTHERN VIETNAM

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 (Fo(90-91), En(92)), 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 Pb-206/U-238 age of 280 +/- 2 Ma, with rodingite forming broadly coevally at 283 to 282 Ma. Metabasalt (plagioclase-amphibole schist) has a MORE-type geochemical affinity. Magmatic zircon from a plagioclase-amphibole schist yields a protolith age of 315 +/- 4 Ma. High-pressure edogite, 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) <280 to 230 Ma: the Paleotethyan oceanic lithosphere was subducted and subjected to HP metamorphism at ca. 230 Ma; closure of the Paleotethys led to collision between the Indochina and south China blocks in the Middle Triassic; and (3) <230 Ma: breakoff of the Paleotethyan oceanic lithosphere with subsequent exhumation of the subducted slabs. The HP metamorphic rocks were tectonically juxtaposed against the Nam Co complex. However the subduction polarity (northward versus southward subduction) is problematic, and requires more investigation.