Genesis of oceanic island basalt in ophiolitic mélange from core area of Central Asian Orogenic Belt

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the largest orogens in the world and comprises island arcs, seamounts, accretionary wedges, oceanic plateaux and microcontinents accreted during the closure of the Paleo-Asian Ocean which is a long-lived ocean that existed for ~800 Ma from the late Mesoproterozoic to the Mesozoic. The CAOB extends from the Urals in the west, through Kazakhstan, northern China, and Mongolia to the eastern Russian coast. The core area, which extends from central Kazakhstan to Xinjiang (northwestern China), are divided into three main geological domains. To the northeast, (1) the Altai range is composed by Early and Late Palaeozoic units that were accreted and docked to the Siberian margin and affected by high-grade metamorphism. To the south, the convergence between the Tarim Block and several micro continents such as Yili and Central Tianshan formed the (2) Palaeozoic Tianshan Orogen. The central and northwestern parts of Central Asia display a horseshoe shape that can be followed from North Tianshan to West Junggar around the Balkash Lake area. This megastructure is termed the (3) Kazakh Orocline. Until now so many studies focused on this area. However, the architecture and orogenic processes of the CAOB are still hotly debated. Remnants of oceanic island basalts (OIB) in association with ophiolites have been identified in several accretionary complexes in core area of CAOB. The nature of occurrence, geochemical characteristics of these OIBs and ophiolites has been taken to indicate extensive crust-mantle cycling magmatism. Therefore, in this paper, we provide an overview of the salient features of twelve OIB-hosting with age of Late Neoproterozoic to Early Carboniferous in core area of CAOB, and distinguish rock association of typical seamount from ophiolitic melanges, including pillow basalt, volcanic breccia, reef limestone, olistostromes rocks, and terrestrial clastic rocks. Combined with unique structural features of seamount, we preliminary reconstruct stratigraphic framework of paleoseamount. Geochemically, the volcanic rocks from paleoseamount are mainly alkaline basalts with litte tholeiitic basalts. The alkaline basalts are characterized by LREE enrichment and HREE depletion, and no obvious Nb, Ta and Ti negative anomalies, suggesting typical OIB affinity. The Nd, Sr and Pb isotopic features indicate that the magma source have enriched components and show fluid metasomatism. In short, this are consistent with geochemical features of Louisville, Ontong Java and Hawaiian seamounts which are related to mantle plume, but not develop high magnesium picrite and komatitic basalt. The OIB in core area of CAOB are at least two possible sources: (1) seamount fragments were scraped and retained in the accretionary wedge; (2) partial melting of mantle metasomatism rocks. However, we can not rule out the possibility of partial melting of subducted oceanic crust and terrigenous sediments. Therefore, the source of OIB in core area of CAOB are very complex and show crust-mantle interaction. This is similar to mantle plume activity and seamounts evolution in modern ocean.