MELTING ACCRETIONARY WEDGE AND BUILDING MATURE CONTINENTAL CRUST IN THE ALTAI OROGENIC BELT, CENTRAL ASIA

The question about how accreted crustal segments evolved into a mature and vertically stratified continental crust in the Central Asian Orogenic Belt (CAOB) has not yet been satisfactorily answered. Granitoid magmas emplaced in the accretionary wedges can potentially provide crucial clues in this context. Here, we present a case study from the Chinese Altai orogenic belt, a central part of the CAOB, using combined approach to characterize the granitoid genesis in the framework of the crustal evolution in the region. The studied area is characterized by voluminous Silurian-Devonian granitoids intruding a greywacke-dominated Ordovician flysch sequence that is interpreted as a giant accretionary complex. Based on geochemical data, these intrusions were previously interpreted as volcanic-arc granitoids with 60‒90% juvenile (depleted mantle-derived) contributions. However, their geochemical signatures, and in particular Nd isotopic compositions, are comparable to the Ordovician flysch rocks that are dominated by youthful and geochemically rather unevolved components. This allows us to examine the possibility that the granitoids might have been derived mainly from the flysch sequence. Pseudosection modeling shows that the flysch is fertile and may produce large volumes of granitic–granodioritic melt at attainable P‒Tconditions, leaving a high-density residue such as garnet- and/or garnet‒pyroxene-rich granulite in the deep crust. In addition, the isostatic residual Bouguer anomaly map and the forward gravity modeling document that a major gravity high over the Chinese Altai coincides with the regional extent of the Silurian-Devonian granitoids. This implies a likely presence of a high density lower crust, comparable to the modeled granulitic residue. Taken together, we suggest that the granitoids in the Altai orogen were produced by melting of the Ordovician flysch sequence. This process led to crustal differentiation and formation of mature continental crust in the accretionary wedge. Our study therefore contributes to a better understanding of crustal evolution in the accretionary orogens in general.

This study is supported by NSF of China (41273048), HKRGC grants (HKU705311P and HKU704712P) , and 100 Talents Program of the Chinese Academy of Sciences to Dr Y.D. JIANG.