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

Abstract

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 garnet-phengite-quartz schist in the ÔNam Co antiformÕ, 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 P-T conditions of the three stages are of 14-16 kbar and 520-550°C (I), 24-27 kbar and 650-750°C (II), and 3-7 kbar and 430-510°C (III), and show a clockwise P-T path based on their mineral assemblages and stability fields in the P-T pseudosection. Thermobarometric results yield similar peak pressure and temperature (26-28 kbar and 650-710°C). These data suggest that the Song Ma eclogite underwent high-pressure metamorphism in subduction zone with a low thermal gradient 8°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 0.01-0.05, indicating a metamorphic origin. SHRIMP U-Pb isotopic analyses of this zircon yield a 206 Pb ⁄ 238 U 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.

The volcaniclastic series from the Luang Prabang Basin, Laos: A witness of a triassic magmatic arc?

“…1), represented by a ribbon of ultramafic and mafic rocks displaying a MORB affinity. As for the Ailaoshan Suture, these rocks are Devonian to Carboniferous in age (e.g., Trung et al, 2006;Vuong et al, 2013;Zhang R.Y. et al, 2013Zhang R.Y.…”

“…Recent investigations Faure et al, 2014 and references therein) evidenced a high-pressure, low-temperature metamorphic event during the Middle Triassic (Fig. 1B), which is interpreted as the result of the collision between the South China and Indochina blocks, following a south-(e.g., Zhang R.Y. et al, 2013;Faure et al, 2014) or a north-(e.g., Nakano et al, 2010) dipping subduction.…”

“…230 Ma (Liu J. et al, 2012), most authors proposed an Early to Middle Triassic collision (e.g., Zhang, R.Y. et al, 2013 andFaure et al, 2014 for a recent review).…”

Mineral phase equilibria and zircon geochronology constrain multiple metamorphic events of high‐pressure pelitic granulites in south‐easternTibetan Plateau

“…The U–Pb ages of zircons from these rocks changed from 249 to 230 Ma, and they marked a HP metamorphism (>14 kbar) during the Early–Middle Triassic. Moreover, in the Song Ma Suture, which is the southern extension of the Jinshajiang–ALS Suture, HP granulites and eclogites occur in the Dien Bien Phu area of north‐western Vietnam (Nakano et al, , ; Zhang et al, ). SHRIMP U–Pb analyses of zircons from these rocks yielded a metamorphic age of 230.5 ± 8.2 Ma (Zhang et al, ).…”

PGE geochemical constraints on the origin of the Ni-Cu-PGE sulfide mineralization in the Suoi Cun intrusion, Cao Bang province, Northeastern Vietnam

“…1). The Jinshajiang -Ailao Shan Suture is the result of amalgamation of the South China and Indochina blocks at~230 Ma (Nam 1998;Carter et al 2001;Zhang et al 2013). This age is interpreted as the closure age of the Paleotethys Ocean, and the collision of the two blocks in the Indosinian Orogeny (Krobicki et al 2008;Lepvrier et al 2008;Hoa et al 2008a, b).…”