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.

Recent geological and petrological investigations of the metamorphic rocks from Vietnam revealed the following new evidences to understand the tectonic evolution of Southeast Asia; 1) findings of ultrahigh -T (~ 1000 °C) pelitic granulites, high -to ultrahigh -P (~ 40 kbar) mafic metamorphic rocks and high -P/medium -T gneisses from the Kontum Massif, 2) eclogite and high -P granulite from the Song Ma suture zone, and 3) ultrahigh -T aluminous granulite from the Red River zone. These lines of evidence are strongly indicative of the highestmetamorphic conditions in each metamorphic terrane. Estimated P -T conditions and reaction textures from these rocks delineate a characteristic clockwise pressure -temperature -time (P -T -t) path for each other. Based on the combination of P -T paths from these complexes, two -stages of metamorphic field gradient are identified. An earlier M0 -stage of high -P/T gradient is recognized, based on the peak -P conditions from the Kontum Massif and Song Ma suture zone. A later M1 -stage of low -P/T gradient is also identified by linking the peak metamorphic conditions from the Kontum Massif, the Song Ma suture zone and the Red River zone. The former metamorphic field gradient could represent an early continental collision event and the latter would indicate a peak metamorphic stage caused by very high -T magmatic intrusion (asthenosphere upwelling) as a heat source of ultrahigh -T metamorphism. A simultaneous collision metamorphism throughout Vietnam should have taken place during the continental collision between Indochina and South China cratons, which led to the formation of Trans Vietnam Orogenic Belt.

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Latest Mesozoic to earliest Cenozoic deformation affected SE Asia's Sundaland core. The deformation event bridges the Mesozoic SE Asian fusion with the Cenozoic era of rifting, translation, basin formation, and the creation of modern SE Asian oceans. Southern Cambodia and Vietnam are central to this shift, but geological investigations of the region are in their infancy. Based on apatite and zircon fission track analyses (AFTA and ZFTA), stratigraphic and structural observations, seismic data, thermal maturity, and igneous rock dating, the geological evolution of southern Cambodia and Vietnam is investigated. Diverse depositional styles, igneous activity, structural deformation and subsurface unconformities testify to a highly variable Phanerozoic tectonic setting. Major latest Cretaceous to Paleocene thrusting and uplift affected the Kampot Fold Belt and surrounding regions and the associated up to ~11 km exhumation probably exceeds earlier denudation events since at least Permian time. The present relief of the Bokor Mountains rising high above the Kampot Fold Belt represents an artifact after differential erosion and only 2.5–4.5 km of erosion affected this area. The latest Cretaceous to Paleocene orogenesis affected much of greater Indochina probably owing to plate collision along eastern Sundaland or a combination of collisions along both east and west Sundaland. AFTA and ZFTA data document protracted cooling of Cretaceous granites and locally elevated thermal gradients persisting a few tens of million years after their emplacement. The thermal gradient had stabilized by early Miocene time, and Miocene cooling probably reflects a renewed denudation pulse driven by either regional tectonism or climate‐enhanced erosion.