International Geologiical Congress - Oslo 2008

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ASI-06 Pre-Mesozoic accretionary tectonics in Central Asia

 

Tectonic evolution and continental growth in Altai orogen, China: Constrained by geochronology and Nd isotopic mapping of intrusions

 

Tao Wang, Institute of Geology (China)
Ying Tong, Institute of Geology (China)
Dawei Hong, Institute of Geology (China)
Bor-ming Jahn, Institute of Earth Sciences (Taiwan)
Kovach Victor P., Institute of Precambrian Geology and Geochronology (Russian Federation)
Baofu Han, Key Laboratory of Orogenic Belt and Crustal Evolution (China)
 

 

The Central Asia Orogenic Belt is the world's largest Phanerozoic accretionary orogen. The Altai orogen was key to understand the orogeny. Voluminous intrusions occur in the Chinese Altai orogen. New zircon U-Pb dating revealed the plutons were mainly emplaced in early- and middle-Paleozoic (ca. 460 Ma and 408 Ma), some in late Paleozoic (270-290 Ma), and a few in Mesozoic (200-220 Ma and 155 Ma). The early- and middle-Paleozoic granitoids are mostly of I-type. Their sources contain both old continental and younger juvenile mantle-derived rocks. The structural patterns of these plutons suggest that the regional deformation took place during 460 - 410 Ma and 410 - 370 Ma. All these suggest that the plutons were mainly formed in continental arc settings. The late Paleozoic post-orogenic intrusions consist bimodal felsic and mafic plutons. The felsic intrusions are of A-type and highly differentiation. We suggested this magmatism belongs to post-orogenic (accretional). The Mesozoic granites show features calc-alkaline and highly differentiation, which are believed being emplaced in a non-orogenic setting.

More than 70 Sr-Nd isotopic data for granitoid intrusions were used for Nd isotopic mapping. The map indicates that εNd(T) values range from -4 to +2 in the central Altai, and from +1.4 to + 6 in the southern Altai. Correspondingly, TDM values decrease from 1.6 - 1.1 Ga in the central Altai to 1.0 - 0.5 Ga in the southern Altai, except for a continental fragment. These results demonstrate that the central Altai contains widespread old continental basement, whereas the southern Altai comprises significant proportion of juvenile crust, suggesting that the crust grew southward. The early-middle Paleozoic syn- and late Paleozoic and Mesozoic post/non-orogenic granitoids contain ca. 50 - 80% and 70 - 90% mantle-derived juvenile materials, respectively. The syn- and post/non-orogenic granitoid intrusions represent horizontal (lateral) and vertical crustal growth, which contributed ca. 18 - 28 % and 7 - 8%, respectively, to the area of the Chinese Altai. This paper provides an example demonstrating that isotopic mapping is an effective approach to elucidate terrane compositions of accretionary orogens and continental growth.
With these new constraints, we present a model to account for the tectonic evolution in the Chinese Alai. This model illustrates that the early-middle Paleozoic Altai orogenic process experienced formation of an active continental margin, splitting of the margin to form a back-arc ocean and the final closing of the back-arc ocean. Consequently, back-arc opening and closure of microcontinent margins is probably a common process in the central Asian accretionary orogen.

 

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