Provenance and tectonic setting of the Carboniferous sedimentary rocks of the East Junggar Basin, China: Evidence from geochemistry and U-Pb zircon geochronology

摘要

The modern Junggar Basin was located in the Central Asian Orogenic Belt (CAOB) during the Devonian and Carboniferous. Few data concerning the weathering, provenance and tectonic setting of the Carboniferous sedimentary rocks in the Junggar Basin have been published. The recent discovery of an oil-gas field in the East Junggar Basin provides the impetus for this study. Twenty-eight sediment samples were recovered from eight boreholes, and all were analysed for major and trace element geochemistry. Two samples of sedimentary rocks were selected for zircon U-Pb dating. Low chemical index of alteration (CIA) values (44-71) and relatively high index of compositional variability (ICV) values (0.72-1.83) suggested weak chemical weathering and an immature source for these samples. The relatively low Th/U (2.16-10.55) and Rb/Sr (0.03-5.44) ratios of the drill core samples reflect a simple recycling history. Two samples indicated upper Carboniferous ages (314.6. Ma) and a lower-middle Devonian age (397.9. Ma). Geochemistry and detrital zircon geochronology from the East Junggar Basin reveal a distinct difference in the composition and source rock ages of clastic rocks from the lower and the upper Carboniferous. Major and trace element compositions of the lower Carboniferous sedimentary rocks (LCRs) were derived from typical mafic rocks and an intermediate ocean island arc of early to middle Devonian age, whereas the upper Carboniferous sedimentary rocks (UCRs) were derived from a continental island arc or active continental margin setting with a dominantly felsic to intermediate late Carboniferous age. The current data provide little support for tectonic models that show a passive continental margin of a Precambrian crystalline basement; our study suggests that a volcanic arc accretion or archipelago (early Carboniferous to early-middle Devonian) model for the East Junggar Basin is more viable.