HR: 14:48h
AN: GP13B-05 INVITED     [Abstracts]
TI: Accretionary Growth in the Central Asian Orogenic Belt of Southern Siberia, Mongolia and Kasakhsthan
AU: * Kroner, A
EM: kroener@mail.uni-mainz.de
AF: Institut fuer Geowissenschaften, Universitaet Mainz, Saarstrasse 21, Mainz, 55099 Germany
AU: Windley, B F
EM: brian.windley@btinternet.com
AF: Department of Geology, Leicester University, University Road, Leicester, LE1 7RH United Kingdom
AU: Badarch, G
EM: gbadarch@magicnet.mn
AF: Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, 63, Peace Avenue, Ulaanbaatar, 210351 Mongolia
AU: Tomurtogoo, O
EM: igmr@magicnet.mn
AF: Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, 63, Peace Avenue, Ulaanbaatar, 210351 Mongolia
AU: Hegner, E
EM: hegner@min.uni-muenchen.de
AF: Institut fuer Geo- und Umweltwissenschaften, Universitaet Muenchen, Theresienstrasse 41, Muenchen, 80333 Germany
AU: Demoux, A
EM: demoux@uni-mainz.de
AF: Institut fuer Geowissenschaften, Universitaet Mainz, Saarstrasse 21, Mainz, 55099 Germany
AU: Wingate, M T
EM: mwingate@tsrc.uwa.edu.au
AF: Tectonics Special Research Centre, University of Western Australia, 35, Stirling Highway, Crawley, WA 6009 Australia
AB: The Central Asian Orogenic Belt (CAOB) records a ca. 800 Ma history of arc and microcontinent accretion, from S to N, during evolution and closure of the SW Pacific-type Paleo-Asian ocean in the period ~1000 to ~300 Ma. We contest previous evolutionary models in terms of a single, large island arc. The earliest history of ocean opening is recorded by ~1020-1050 Ma ophiolites in southern Siberia. The next younger events farther S are exemplified by the evolution of ~850-570 Ma arc-ophiolite terrains, including boninites, followed by earliest Paleozoic subduction/accretion, suturing and oblique collision during which some of the arc terrains were metamorphosed to granulite-facies. Precambrian microcontinental fragments are documented by Archean to Mesoproterozoic ages, and we also found Archean to Neoproterozoic detrital zircons in Mongolian arc-derived sediments, suggesting derivation from continental sources. Voluminous intermediate to felsic island-arc magmatic rocks in Mongolia were generated at 460-417 Ma with-in a broad belt extending from northern Mongolia to the Gobi Desert. Zircon xenocrysts and Nd model ages for some of these rocks are between 600 and >1300 Ma, suggesting that older material was involved in their generation. SHRIMP detrital zircon ages for arc-derived sandstones document continental input dating back to the late Archean. Several microcontinental fragments have been identified in southern Kazakhstan with zircon ages between 2.0 and 2.7 Ga. Extensive rifting with formation of bimodal volcanic sequences occurred at ~775-820 Ma. Precambrian detrital zircons from clastic sediments resting on ocean floor supports the view that the oceanic domains between the microconti-nents and early Paleozoic arc complexes were narrow, probably marginal basins. A metadacite and granodiorite from an arc complex near Lake Balkash provided identical zircon ages of 478-480 Ma and contain zircon xenocrysts as old as 2290 Ma. One 480 Ma diorite sample from central Kazakhstan contains a 3.88 Ga zircon xenocryst. These rocks are unlikely to have formed in an intraoceanic environment, and we favour an Andean- or Japan-type setting. Overall the CAOB records the formation of SW-Pacific style small forearc and back-arc ocean basins that probably evolved between island arcs and microcontinents and were closed during continuous accretion between the Neoproterozoic and Paleozoic. Final closure of the Paleo-Asian ocean probably occurred in the late Permian when the North China craton was attached to the CAOB. The presence of large volumes of felsic volcanic rocks in the Mongolian arcs, together with ubiquitous Precambrian xenocrystic and detrital zircons, argue for significant involvement of older material in the production of CAOB crust, and previous crustal growth models require substantial revision. Similar inheritance has recently been reported from seemingly juvenile Neoproterozoic felsic rocks of the Arabian-Nubian shield where a purely intra-oceanic development is now also in doubt.
DE: 1115 Radioisotope geochronology
DE: 8038 Regional crustal structure
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 9320 Asia
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005