Origin and Tectonic Implication of Ophiolite and Eclogite in the Song Ma Suture Zone between the South China and Indochina Blocks

Southeast Asia consists of several microcontinents that detached from the northeastern margin of Gondwanaland. The Song Ma belt in northern Vietnam consists of ophiolite, metabasite, metasedimentary rocks and eclogite, and it is thought to be a suture zone between the Indochina and South China blocks. However, the nature and boundaries of the Song Ma belt and the collision age of the two blocks have long been debated. In this article, petrological and geochemical studies on the Song Ma ophiolite and eclogite and first sensitive high-resolution ion microprobe (SHRIMP) age dating of eclogite provide new light to resolve such debate. Eclogite consisting of garnet, omphacite, phengite, quartz, barroisite and rutile is closely associated with garnetphengitequartz schist in the Nam Co antiform, a northern subunit of the Song Ma belt. The eclogite experienced a three-stage metamorphic evolution: (I) pre-eclogite stage (amphibolite facies) defined by inclusions of taramite, barroisite, quartz, zoisite/epidote, mica, rutile & rare chlorite in garnet, (II) eclogite stage and (III) retrograde stage of amphibolite to greenschist facies. The PT conditions of the three stages are of 1416 kbar and 520550 degrees C (I), 2427 kbar and 650750 degrees C (II), and 37 kbar and 430510 degrees C (III), and show a clockwise PT path based on their mineral assemblages and stability fields in the PT pseudosection. Thermobarometric results yield similar peak pressure and temperature (2628 kbar and 650710 degrees C). These data suggest that the Song Ma eclogite underwent high-pressure metamorphism in subduction zone with a low thermal gradient 8 degrees C km-1. The Song Ma ophiolite is composed of serpentinized peridotite, gabbro, basalt, mafic dyke and chert, and experienced ocean-floor metamorphism. Metabasalt and gabbro of ophiolite suite and eclogite all have MORB-type geochemical affinities. Zircon separates from eclogite have very low Th/U ratios of 0.010.05, indicating a metamorphic origin. SHRIMP UPb isotopic analyses of this zircon yield a 206Pb/238U weighted mean age of 230.5 +/- 8.2 Ma. This age is interpreted as the closure age of the Paleotethys Ocean that separated the South China and Indochina blocks, and the subsequent collision of the two blocks that took place at the Middle Triassic corresponding to the major episode of the Indosinian Orogeny.