HR: 14:10h
AN: T23D-03    [Abstracts]
TI: Accretionary Orogen and Phanerozoic Continental Growth in Central Asia
AU: * Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No. 128, Academia Road, Section 2, Taipei, 11529 Taiwan
AB: The Central Asian Orogenic Belt (CAOB) that welded together the Siberian and Sino-Korean cratons is characterized by vast expanse of granitic rocks of Paleozoic to Mesozoic ages. The most striking feature of these granitoids is their dominantly positive eNd(T) values, low ISr ratios (ca. 0.705) and young TDM model ages (1500 Ma and younger). This indicates that the Phanerozoic crustal evolution in Central Asia is characterized by massive generation of `juvenile' granitoids, and much of the crust is made of young mantle-derived material. Considering the immense geographic coverage, the CAOB represents undoubtedly the world's most important site of crustal growth in the Phanerozoic. The tectonic evolution of the CAOB is mainly related to accretion of arc complexes as suggested by Seng”r et al. (1993). This idea is generally supported by the available Sr-Nd-O isotopic data in the literature. However, the granites with negative eNd(T) values require the existence and participation of old Precambrian blocks in magma generation, thus accretion of old terranes is also confirmed. The conversion of arc assemblages (turbidites included) into granitoids (= intracrustal differentiation) was most likely triggered by underplating of basaltic magma, which provided not only heat source but also the added juvenile material. The process involved mixing of underplated magma and lower crustal rocks, melting of the mixed sources, and followed by fractional crystallisation. In conclusion, the syn-orogenic granitoids were generated in two stages: first during the building of individual arcs via subduction zone magmatism, and then conversion of arc complexes plus underplated basaltic liquids within accreted arc complexes. Melting of accreted arc complexes may be further promoted by water-saturated sediments. The syn-orogenic granitoids appear to dominate the felsic magmatism of the early Paleozoic (300 Ma and older) when arc complexes were being built. The origin of A-type granites has long been controversial, but most post-accretionary A-type granites from the CAOB are demonstrably of mantle origin (sensu lato). The generation of voluminous Permian to Cretaceous alkaline and peralkaline granites could be related to the Siberian superplume or to the lithospheric delamination. In this connection, intraplate magmatism (vertical accretion) was an important process, in addition to the lateral accretion of arc complexes, of crustal growth in the Phanerozoic. The rate of apparent crustal growth for the entire period of the Phanerozoic may be estimated, but the growth rate at any given time interval of 50-100 Ma is not possible due to insufficient amount of reliable age data. For the entire Altaid Collage, Seng”r et al. (1993) estimated that during the 350 Ma of crustal evolution, a total area of about 2.5 million km2 of juvenile crust was added to Asia. This is translated into a growth rate of about 0.3 km3/a. If this is combined with that of the Canadian Cordillera and western United States, the rate would be at least 50% higher than the global growth rate of ca. 1.1 km3/a deduced from arc magmatism by Reymer and Schubert (1984, 1986). (Supported by NSC-Taiwan, NSC93-2116-M-001-125, NSC94-2116-M-001-021)
DE: 1020 Composition of the continental crust
DE: 1040 Radiogenic isotope geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: Fall Meeting 2005