THE CENTRAL ASIAN OROGENIC BELT AND GROWTH OF THE CONTINENTAL CRUST IN THE PHANEROZOIC

Abstract

Asia is the world's largest composite continent, comprising numerous old cratonic blocks and young mobile belts. During the Phanerozoic it was enlarged by successive accretion of dispersed Gondwana-derived terranes. The opening and closing of palaeo-oceans would have inevitably produced a certain amount of fresh mantle-derived juvenile crust. The Central Asian Orogenic Belt (CAOB), otherwise known as the Altaid tectonic collage, is now celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic. It is composed of a variety of tectonic units, including Precambrian microcontinental blocks, ancient island arcs, ocean island, accretionary complexes, ophiolites and passive continental margins. Yet, the most outstanding feature is the vast expanse of granitic intrusions and their volcanic equivalents. Since granitoids are generated in lower-to-middle crustal conditions, they are used to probe the nature of their crustal sources, and to evaluate the relative contribution of juvenile v. recycled crust in the orogenic belts. Using the Nd-Sr isotope tracer technique, the majority of granitoids from the CAOB can be shown to contain high proportions (60 to 100%) of the mantle component in their generation. This implies an important crustal growth in continental scale during the period of 500-100 Ma. The evolution of the CAOB undoubtedly involved both lateral and vertical accretion of juvenile material. The lateral accretion implies stacking of arc complexes, accompanied by amalgamation of old microcontinental blocks. Parts of the accreted arc assemblages were later converted into granitoids via underplating of basaltic magmas. The emplacement of large volumes of post-accretionary alkaline and peralkaline granites was most likely achieved by vertical accretion through a series of processes, including underplating of basaltic magma, mixing of basaltic liquid with lower-crustal rocks, partial melting of the mixed lithologies leading to generation of granitic liquids, and followed by fractional crystallization. The recognition of vast juvenile terranes in the Canadian Cordillera, the western US, the Appalachians and the Central Asian Orogenic Belt has considerably changed our view on the growth rate of the continental crust in the Phanerozoic.