Survived and disappeared intra-oceanic arcs of the Paleo-Asian Ocean: evidence from Kazakhstan.

Abstract

This paper reviews published and presents new data on U-Pb detrital zircon ages, and petrographic, geochemical and isotope (Sm-Nd, Lu-Hf) compositions obtained from greywacke sandstones of Kazakhstan in order to reconstruct fossil intra-oceanic arcs that once existed at Pacific-type convergent margins of the Paleo-Asian Ocean (PAO) in Paleozoic time. We focus on orogenic belts of central Kazakhstan (Itmurundy and Tekturmas) and eastern Kazakhstan (Zharma and Char) in the western Central Asian Orogenic belt. These orogenic belts host accretionary complexes with greywacke sandstones of early Paleozoic (central Kazakhstan) and middle-late Paleozoic (eastern Kazakhstan) ages. First, we evaluate general perspectives for studying sandstones to reconstruct survived and disappeared magmatic arcs, taking into account episodes of subduction erosion. Then we discuss the analytical data from sandstones to make conclusions about the ages and formation settings of their igneous protoliths and define maximum deposition ages. Finally, we discuss the role of serpentinite mélanges in tectonic reconstructions. We argue that sandstones hosted by accretionary complexes are typically greywackes deposited close to their igneous sources and buried rapidly. The provenances of the studied greywacke sandstones of central and eastern Kazakhstan were dominated by mafic to andesitic igneous protoliths derived from juvenile mantle sources. The igneous rocks in the provenances were emplaced in an intra-oceanic arc setting. The sandstones were deposited in fore-arc/trench basins or, to a lesser degree, in back-arc basins. The data from both sandstones and serpentinite mélanges reconstruct middle-late-Cambrian, Ordovician, late-Devonian and Carboniferous arcs of the western PAO. The middle-late Cambrian arcs were fully destroyed by subduction erosion, whereas the Ordovician and Carboniferous arcs survived. The late-Devonian arcs were also eroded, but partly. Both the early and late Paleozoic active margins of the PAO were characterized by alternating periods of accretionary growth and subduction erosion.