亞洲大地構造與造山作用研究:東南亞實驗
摘要
Asia that comprises numerous ancient cratonic blocks and young mobile belts is the largest composite continent on Earth today. During the Phanerozoic, it was enlarged by successive accretion of dispersed terranes that, associated with opening and closure of the Paleo-Asian and Tethyan oceans, had produced a significant amount of juvenile or mantle-newly-derived continental crust. The Central Asian orogenic belt (CAOB), for instance, is celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic or, more specifically, in the Paleozoic. Zircon U-Pb and Hf isotope data of "representative" magmatic rocks we have obtained/synthesized from West and South Asia, in particular from Iran and Tibet, suggest that, before continental collisions by Arabia and India, respectively, the eastern Tethyan orogenic belt (ETOB) was characterized not only by Tethyan subductions but also by accretionary orogenic processes that eventually led to significant juvenile crustal production from the Jurassic to Eocene or, in places, to the Oligocene. Taking together, both CAOB and ETOB appear to have evolved over time from an accretionary into a collisional system. The zircon Hf isotope data further reveal that in contrast to generating vast portions of juvenile crust in the early, accretionary stages of orogenic development, crustal recycling plays a more substantial role in the later, collisional stages. The latter involves addition of older continental crust into the mantle, which subsequently melted and caused compositional transformation of the juvenile crust that formed in the accretionary stages. Similar features are observed in young volcanic rocks from the Coastal Range of eastern Taiwan, i.e., the northern Luzon arc or part of the complex subduction/ accretionary system in South East Asia (SEA) that would evolve one day to resemble the CAOB and ETOB by collision with the advancing Australian continent, and consequently become a key component of the Earth's future supercontinent "Amasia". SEA is therefore a natural laboratory or modern example that provides us the best albeit rare opportunity to investigate in greater detail the ACT (Accretionary and Collisional Tectonics) processes responsible not only for the mountain building and crustal formation in Asia during the Phanerozoic, but also for the continental orogenesis worldwide through the geologic time. In this talk, a newly proposed research project entitled "Accrerionary& Collisional Tectonics in Asia: Studying Continental Orogeny from Magmatic Perspectives through South East Asia Experiments" will be briefed, with emphasis on our ongoing studies from Burma and Sumatra and in the hope of attracting furthermore collaborations.