Petrology, geochemistry, and origin of metamorphosed mafic rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

Abstract

The Trans Vietnam Orogenic Belt (TVOB) of Southeast Asia is thought to have formed by a continent -continent collision between the South China and Indochina cratons in Permian -Triassic times. Here we focus on the nature and origin of metamorphosed mafic rocks that are widely distributed throughout the TVOB, including rocks from the Cangshan Mountains (northern TVOB), the Red River Shear Zone and Song Ma Suture Zone (central TVOB), and the Kontum Massif (southern TVOB). Amphibolite facies metamorphosed mafic rocks are widely distributed throughout all of these areas, while granulite facies and eclogite facies metamorphosed mafic rocks occur only in the Kontum Massif and Song Ma Suture Zone, respectively. Major and trace element compositions of these TVOB metamorphosed mafic rocks indicate an overall tholeiitic affinity, but suggest a wide array of tectonic settings for the precursor mafic magmas, including volcanic -arc basalt (VAB), mid -ocean ridge basalt (MORB), and within -plate basalt (WPB). In addition, Fe -rich gabbro -derived mafic metamorphic rocks occur in the southern part of the TVOB (in the Kontum Massif). On the basis of the distributions of each type of mafic metamorphic rocks and their metamorphic grades, there were arc and oceanic crust between the South China Craton and the Indochina Craton.Keywords: Collision zone, Metamorphosed mafic rocks, Trans Vietnam Orogenic Belt, Indochina Craton, South China Craton INTRODUCTIONContinental -continent collision zones are characterized by a variety of crustal materials including metamorphic rocks, granitoids, and accretionary prisms, in which deciphering the metamorphic evolution and history of magma generation are critical in unraveling the continental collision process. By evaluating the tectonic settings of the igneous protoliths of metamorphic rocks caught up between two collided continents, or even metamorphic rocks of the collided continental fragments themselves, we can place important constraints on the processes involved in continental collision, and this technique works especially well for mafic metamorphic rocks. Such rocks are typically considered to be derived from igneous rock precursors (e.g., Bucher and Frey, 1994), and the original tectonic setting of the source magma and process of igneous rock formation (i.e., basaltic lava flow versus gabbroic intrusion) of these rocks is generally determined based on petrography, geochemistry, and geological relationships with other lithologies observed in the field (e.g., Pearce and Cann, 1973). Asian continent is considered to be a composite of a number of smaller aggregated microcontinents (Metcalfe, 1999), and in Southeast Asia alone, a total of five microcratons have been identified, including the South China craton, Indochina craton, Shan -Thai craton, Sibumasu craton, and West Burma craton. Metamorphic rocks related to the continental collisions in Southeast Asia are exposed along the boundaries or sutures between these microcontinents ( Fig. 1), and undoubtedly preserve important information p...

Early Paleozoic or Early-Middle Triassic collision between the South China and Indochina Blocks: The controversy resolved? Structural insights from the Kon Tum massif (Central Vietnam)

“…Duc complex indicate that these rocks underwent a polymetamorphic evolution (e.g. Nakano 387 et al, 2007Nakano 387 et al, , 2009Nakano 387 et al, , 2013Osanai et al, 2008;Usuki, 2009;Yonemura et al, 2013). The main 388 metamorphic assemblage in metapelite is characterized by biotite-garnet-sillimanite, yielding 389 P and T conditions of 0.7 to 0.8 GPa, and 720-750°C, respectively.…”

“…Mafic rocks, such as garnet 157 amphibolite, eclogites, HP granulite, and biotite-hornblende orthogneiss, are widespread in 158 the complex (Fig. 4C;Osanai et al, 2008;Yonemura et al, 2013). These 159 rocks are interpreted as refractory restites enclosed in the migmatite.…”

Evolution of supracrustal rocks of the Indochina Block: Evidence from new detrital zircon U–Pb ages of the Kontum Massif, Central Vietnam

“…Regarding this, unraveling the Precambrian evolutionary histories of the Indochina Block as a part of the Precambrian continental blocks of Asia is important to reconstruct the break-up and amalgamation process of the supercontinent. In addition, the Indochina Block collided with the South China Block on its north along the Song Ma suture (e.g., Cai and Zhang, 2009;Nakano et al, 2010Nakano et al, , 2013Yonemura et al, 2013;Faure et al, 2014;Hieu et al, , 2017Thanh et al, 2019;Hieu et al, 2020a) (Fig. 1a).…”