Island arc-type bimodal magmatism in the eastern Tianshan Belt, Northwest China: Geochemistry, zircon U–Pb geochronology and implications for the Paleozoic crustal evolution in Central Asia

Abstract

The bimodal association of basalts and rhyolites in the Hongshankou area of the East Tianshan area (NW China) provides an important suite to investigate magma petrogenesis and tectonics evolution in a convergent margin setting. In this study we present petrological, geochemical and zircon U–Pb geochronological data from the basalts and rhyolites. Zircons from the basalts yield crystallization ages of 347.1±3.9Ma whereas those from the rhyolites are dated as 344.4±1.9 and 345.0±2.5Ma. The major and trace element signatures of basalts suggest high alumina basalt (HAB) as the source magma which was generated from metasomatised asthenosphere mantle. Both the mafic and felsic rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and are depleted in high field strength elements (HFSE). Broadly homogenous Rb–Sr and Sm–Nd isotopic compositions of mafic and felsic rocks are also characteristic. The basalts display a distinct enrichment in incompatible elements and positive εNd(345Ma) (from +5.87 to +8.25). We envisage a subduction-related origin to account for the LILE enrichment with arc-like melts (enriched in Th and LREE and depleted in Nb, with εNd(345Ma)>+6) produced through the partial melting of a depleted-mantle source, and the involvement of minor crustal component. The rhyolites also display high εNd(345Ma) (from +6.35 to +8.53). Consequently, we suggest a mixing process between a source similar to mid-ocean ridge basalt (depleted end-member mantle) and arc-like magmas for the petrogenesis of the mafic rocks, whereas the rhyolites are best interpreted to have formed by fractional crystallization from the parental magma of the basalts. Several lines of evidence from elemental and isotopic geochemistry suggest a close genetic relationship between the mafic and felsic rocks in the Hongshankou bimodal suites. We propose that the essentially bimodal character of this complex reflects the features of back-arc extensional magmatism, induced by the subduction of the Junggar plate during the Late Paleozoic.