2. Faculty of Earth Resources, Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, China
Southeast Asia is geographically located south and southwest of China, east of India, and north of Australia, and plays an important role in "the Belt and Road Initiative" of China. Southeast Asia lies on the intersection of the Eurasian, IndianAustralian and Philippine plates, and is located in the easternmost segment of the Tethyan tectonic domain and much of the region can now be regarded as part of the Eurasian Plate (Metcalfe, 2011; Simons et al., 2007). It consists of a collage of continental blocks or fragments, together with accreted volcanic arcs/back-arc basins and suture zones that have recorded a long-term convergence history between multiple tectonic domains induced by subduction (Fig. 1). The long and complex tectonic evolution history, a diversity of plutonic magmatism, volcanic eruption, and a wealth of mineral deposits are developed in Southeast Asia (Hall and Spakman, 2015; Zaw et al., 2014; Metcalfe, 2011), in particular the Sn-W deposits occurring in this area constitute one of most important Sn-W metallogenic belts in the world (Schwartz et al., 1995; Liu and Ma, 1993), which last~2 800 km long and~400 km wide and have a close association with three granite belts, namely the eastern, main range and western granitoid belts (Fig. 2).
The main constituents of present-day Southeast Asia are a collage of allothigenous continental blocks mainly including the South China, Indochina-East Malaya, Sibumasu, West Burma, West Sumatra and SW Borneo blocks. These blocks were separated and rifted from the margin of eastern Gondwana and drifted north to progressively assemble during the Late Paleozoic to Cenozoic (Metcalfe, 2013, 2011; Barber and Crow, 2003). Following the separation of the South China Block and Indochina Block from Gondwana, the Paleotethys Ocean opened in the Middle Devonian, in response to a period of Early Devonian rifting on the NE margin of Gondwana (Metcalfe, 2013, 2011). The continental rifting and subsequent Paleotethys Ocean Basin development led to the formation of sedimenthosted/orogenic gold deposits in Peninsular Malaysia (Makoundi et al., 2014; Zaw et al, 2014). Subduction of the Paleotethys beneath the Indochina Block triggered formation of two phases of arc magmatism with associated porphyry-skarn Cu-Au and epithermal Au-Ag deposits during the Early Permian and Early Triassic, respectively (Lai et al., 2014; Zaw et al., 2014). The Sibumasu Block was rifted from Gondwana in the Early Permian and collided with Indochina Block by Late Triassic (Metcalfe, 2013). Continuous subduction of the Paleotethys beneath the Indochina Block resulted in the Indosinian orogeny in SE Asia during the Triassic and Jurassic, which in turn closed the Paleotethys Ocean (Metcalfe, 2013, 2011; Sengör, 1987). Many sediment-hosted/orogenic gold deposits formed throughout this period along or adjacent to major suture zones (Zaw et al., 2014).
The Neotethys Ocean opened in the Mid-Late Permian as a result of rifting of the Cimmerian continental fragments from Gondwana (Hou and Zhang, 2015). The West Burma Block and other small continental fragments now located in SW Sumatra were developed within the Neotethys Ocean Basin during the Late Triassic to Late Jurassic (Metcalfe, 2013, 2011). The northward shift of the Lhasa Block and West Burma Block led to the formation of some branches of the Neotethys Ocean, as represented by eastern ophiolite belt in northern Myanmar (Liu et al., 2016). The amalgamation of the West Burma Block with the Sibumasu Block to form the Shan Boundary suture in Myanmar and the Woyla suture in Indonesia took place during the Jurassic and Early Cretaceous (Metcalfe, 2013, 2011; Morley, 2012). Subduction of the Neotethys Ocean Plate beneath the Cimmerian continent not only led to the formation of typical subduction-related porphyry Cu-Au deposits in western Myanmar (Gardiner et al., 2015), but also accounted for the formation of many magmatic-hydrothermal W-Sn deposits in hinterland, in particular in central and southern Myanmar (Jiang et al., 2017; Gardiner et al., 2015; Zaw et al., 2014). The collision of Arabian and Indian continents with the Eurasian Continent led to the closure of the main branch of the Neotethys Ocean during the Late Cretaceous to Paleogene (Hou and Zhang, 2015). The Neogene epithermal deposits in central Myanmar and northwestern Sumatra formed as a result of the oblique subduction of the Sunda Trench (Zaw et al., 2014).
This special issue assembles 10 papers that focus on the latest developments in understanding the magmatism, tectonics and metallogeny of Southeast Asia. This volume is a beneficial attempt to integrate tectonic, magmatic, and ore-forming processes in Southeast Asia. It should be noted that most areas of Southeast Asia still remain relatively understudied, and much more studies are necessary for the detailed description of the long-term and complicated magmatic, tectonic and metallogenic history of Southeast Asia.
The papers in this volume are organized roughly in order from tectonic and magmatism to metallogeny, and start with an overview paper by Zhang et al. (2019). This review paper synthesizes the information from many published papers in terms of individual components of Southeast Asia and its associated ore deposits and attempts to summarize this information into a comprehensive and connected account. The authors suggest that the main metallogenic combinations reflect the attribute of the main tectonic events and that the tectonic-metallogenic belts in Southeast Asia can be divided into 24 categories formed in 7 tectonic units:(1) island arc settings; (2) arccontinental collision zones; (3) back-arc basins; (4) continentcontinent collision structural-magmatic belts; (5) marginal fold belts; (6) continent-continent collisional orogenic belts; (7) continental nuclei.
Two papers discuss the Mesozoic magmatic activities related to Tethyan events in Myanmar and Laos. Li et al. (2019) report zircon U-Pb ages, geochemical and Sr-Nd-Hf isotopic data of quartz diorite and granodiorite from the Payangazu complex in central Myanmar. Unlike the Wuntho-Popa arc in western Myanmar related to the Neo-Tethyan subduction, these Early Cretaceous intrusions are interpreted to have formed as a result of the southward subduction of an ocean slab that was possibly an extension of the Bangong-Nujiang Ocean. Shi et al. (2019) focus their study on volcanic rocks in the XaignabouliLuang Prabang volcanic belt, NW Laos. The authors report zircon U-Pb ages of 235-232 and 278 Ma for volcanic rocks from the Nam Hang Formation and Muang-Nan Formation, respectively. Geochemical and isotopic compositions of these volcanic rocks indicate that they may have formed in a conti-nental margin arc setting possibly induced by the Nan back-arc basin eastward subduction.
Two papers discuss the Late Mesozoic magmatic and associated W-Sn mineralization event in Southeast Asia. Nguyen et al. (2019) present new zircon U-Pb ages, geochemical and Hf isotopic data as well as mineral chemistry for the Paioar granites, which are thought to be associated with W-Sn mineralization in northern Vietanm. The ore-related granites are interpreted as S-type granites formed in response to large-scale crustal extension during the Late Cretaceous (~82 Ma); and the reducing condition and crystal fractionation are interpreted as key factors for the tin-tungsten deposition by facilitating the accumulation and transportation of W-Sn in fluids. Jiang et al. (2019) describe the geology of the Hermyingyi W-Sn deposit in southern Myanmar and report new results of molybdenite Re-Os age and sulfur isotopic compositions. By combining with previously published geochronological and geochemical data for the Hermyingyi monzogranite (Jiang et al., 2017), the authors provide a precise age constraint on the mineralization event and discuss the genetic relationship between the granitic magmatism and W-Sn mineralization in the deposit.
Four papers discuss orogenic Au, sandstone-type Cu, skarn Fe, and lateritic-type bauxite deposits in Laos. Guo et al. (2019) provide a new description of the geology of the Phapon gold deposit in the Luang Prabang-Loei metallogenic belt of NW Laos. In the paper, they describe the alteration geology, and report fluid inclusion and H-O isotope data. By using these data, the authors conclude that this structurally-controlled deposit represents an orogenic style gold deposit that was formed by hydrothermal deposition from a fluid of a metamorphicdominant source due to fluid-rock interaction. Huang J G et al. (2019) describe the Xinzhai sandstone-type copper deposit located in the Jiangcheng-Phongsaly-Phrae Mesozoic basin of northern Laos. The authors identify two types of orebodies in the deposit, namely the lameller copper orebodies hosted in sandstones and vein-type copper orebodies along the faults. They interpret sulfur isotopic compositions to reflect derivation of sulfur from bacterial sulfate reduction. Oxygen and hydrogen isotopic compositions indicate that the ore-forming fluid was derived from a basin fluid. A far-source accumulation and lake facies sedimentary environment is suggested by the high mineral grain maturity and the structural maturity of sandstones from the Jurassic Huakaizuo Formation. Taken together, they propose a sulfate reduction model for copper deposition in the deposit. Hou et al. (2019) report the results of zircon U-Pb and pyrite Re-Os dating as well as zircon Hf isotopic composition of the Pha Lek skarn Fe deposit in the Truong Son Fe-polymetallic belt, northern Laos. On the basis of these new data, the authors discuss the genetic relationship between granitic magmatism and Fe-dominant mineralization and propose a Late Carboniferous-Early Permian subduction-related skarntype Fe mineralization model for the deposit. Long et al. (2019) report their study on the lateritic-type bauxite deposit in the Boloven Plateau, southern Laos. The authors propose that the bauxite deposit overlying the sandstone was derived from weathering of the alkali basalt.
Finally, one paper discusses a Pb-Zn deposit of the Tethyan belt in northern Sumatra (Indonesia). Huang C W et al. (2019) conclude from fluid inclusion and carbon and oxygen isotopic evidence that the Anjing Hitam Pb-Zn deposit, mainly hosted by the middle member of the Carboniferous-Permian Kluet Formation of the Tapanuli Group, formed in two stage ore-forming processes, namely an early stage of Carboniferous sedimentary-exhalative-type (SEDEX) Pb-Zn mineralization, overprinted by a late stage of Pleistocene hydrothermal vein-type Pb-Zn mineralization.ACKNOWLEDGMENTS
We are very grateful to all the authors for their efforts in preparing the papers for this special issue within a tight time frame. We also thank all the reviewers who provide insightful and constructive comments and suggestions that helped the authors to improve their papers significantly. This study is financially supported by the National Key R & D Program of China (No. 2017YFC0602405), the National Natural Science Foundation of China (Nos. 41503043, 91755208). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0871-8.
Barber, A.J., Crow, M.J., 2003. An Evaluation of Plate Tectonic Models for the Development of Sumatra. Gondwana Research, 6(1): 1-28. DOI:10.1016/s1342-937x(05)70642-0
Chen, X.F., Ye, J.H., Xiang, Y.C., et al., 2017. Geological Characteristics and Exploration Potential of the Important Deposits in Southeast Asia. Geological Bulletin of China, 36(1): 50-65. DOI:10.3969/j.issn.1671-2552.2017.01.005(inChinesewithEnglishAbstract)
Gardiner, N.J., Searle, M.P., Robb, L.J., et al., 2015. Neo-Tethyan Magmatism and Metallogeny in Myanmar-An Andean Analogue?. Journal of Asian Earth Sciences, 106: 197-215. DOI:10.1016/j.jseaes.2015.03.015
Hall, R., Spakman, W., 2015. Mantle Structure and Tectonic History of SE Asia. Tectonophysics, 658: 14-45. DOI:10.1016/j.tecto.2015.07.003
Hou, L., Liu, S.S., Guo, L.N., et al., 2019. Geology, Geochronology, and Hf Isotopic Composition of the Pha Lek Fe Deposit, Northern Laos:Implications for Early Permian Subduction-Related Skarn Fe Mineralization in the Truong Son Belt. Journal of Earth Science, 30(1): 109-120. DOI:10.1007/s12583-018-0864-7
Hou, Z.Q., Zhang, H.R., 2015. Geodynamics and Metallogeny of the Eastern Tethyan Metallogenic Domain. Ore Geology Reviews, 70: 346-384. DOI:10.1016/j.oregeorev.2014.10.026
Jiang, H., Li, W.Q., Jiang, S.-Y., et al., 2017. Geochronological, Geochemical and Sr-Nd-Hf Isotopic Constraints on the Petrogenesis of Late Cretaceous A-Type Granites from the Sibumasu Block, Southern Myanmar, SE Asia. Lithos, 268. DOI:10.1016/j.lithos.2016.11.005
Lai, C.K., Meffre, S., Crawford, A.J., et al., 2014. The Western Ailaoshan Volcanic Belts and Their SE Asia Connection:A New Tectonic Model for the Eastern Indochina Block. Gondwana Research, 26(1): 52-74. DOI:10.1016/j.gr.2013.03.003
Liu, C.Z., Chung, S.L., Wu, F.Y., et al., 2016. Tethyan Suturing in Southeast Asia:Zircon U-Pb and Hf-O Isotopic Constraints from Myanmar Ophiolites. Geology, 44(4): 311-314. DOI:10.1130/g37342.1
Liu, S.S., Yang, Y.F., Guo, L.N., et al., 2018. Tectonic Characteristics and Metallogeny in Southeast Asia. Geology in China, 45(5): 863-889. DOI:10.12029/gc20180501
Liu, Y.J., Ma, D.S., 1993. Vein-Type Tungsten Deposits of China and Adjoining Regions. Ore Geology Reviews, 8: 233-246. DOI:10.1016/0169-1368(93)90018-T
Makoundi, C., Zaw, K., Large, R.R., et al., 2014. Geology, Geochemistry and Metallogenesis of the Selinsing Gold Deposit, Central Malaysia. Gondwana Research, 26(1): 241-261. DOI:10.1016/j.gr.2013.08.023
Metcalfe, I., 2011. Tectonic Framework and Phanerozoic Evolution of Sundaland. Gondwana Research, 19(1): 3-21. DOI:10.1016/j.gr.2010.02.016
Metcalfe, I., 2013. Gondwana Dispersion and Asian Accretion:Tectonic and Palaeogeographic Evolution of Eastern Tethys. Journal of Asian Earth Sciences, 66: 1-33. DOI:10.1016/j.jseaes.2012.12.020
Morley, C.K., 2012. Late Cretaceous-Early Palaeogene Tectonic Development of SE Asia. Earth-Science Reviews, 115(1/2): 37-75. DOI:10.1016/j.earscirev.2012.08.002
Schwartz, M.O., Rajah, S.S., Askury, A.K., et al., 1995. The Southeast Asian Tin Belt. Earth-Science Reviews, 38: 95-293. DOI:10.1016/0012-8252(95)00004-T
Sengör, A.M.C., 1987. Tectonics of the Tethysides:Orogenic Collage Development in a Collisional Setting. Annual Review of Earth and Planetary Sciences, 15(1): 213-244. DOI:10.1146/annurev.ea.15.050187.001241
Simons, W.J.F., Socquet, A., Vigny, C., et al., 2007. A Decade of GPS in Southeast Asia:Resolving Sundaland Motion and Boundaries. Journal of Geophysical Research, 112(B6): B06420. DOI:10.1029/2005jb003868
Zaw, K., Meffre, S., Lai, C.K., et al., 2014. Tectonics and Metallogeny of Mainland Southeast Asia-A Review and Contribution. Gondwana Research, 26(1): 5-30. DOI:10.1016/j.gr.2013.10.010