Geochemistry of Permo-Triassic mudstone of the Satpura Gondwana basin, central India: Clues for provenance

摘要

Major and trace element data of the Permo-Triassic mudstones from the intracratonic Satpura Gondwana basin, central India have been used to investigate provenance. The Satpura succession (~ 5 km thick) unconformably overlies the Precambrian basement. Sediment dispersal pattern suggests that the Precambrian rocks straddling the southern margin of the Satpura basin were the most probable source for the sediments. The lowermost unit of the Satpura succession comprises glacio-marine and glacio-fluvial deposits. The rest of the succession largely represents a variety of fluvial depositional systems with some records of fluvio-deltaic and fluvio-lacustrine sedimentation under a climatic spectrum of temperate, humid to warm, semi-arid. The present study is confined to the lower six formations namely Talchir, Barakar, Motur, Bijori, Pachmarhi and Denwa, arranged in that order from bottom to top.The oldest rocks of this sequence, the Talchir Formation, are enriched in mobile elements such as Na, Ca and Mg, depleted in alumina and have high ICV (0.93-1.29) and lower CIA values (61.82-74.89) compared to those of the younger mudstones. K_2O/Al_2O_3 ratios of the Talchir mudstones (0.24-0.28) are higher than the younger mudstones in this sequence. These results suggest that the Talchir mudstones were weathered less intensely and are thus more immature than the younger mudstones. This is consistent with the prevailing cold climate and relatively higher rate of basin subsidence. In contrast, the majority of the mudstones from the younger formations are depleted in mobile elements, have relatively low ICV (0.18-0.87) and K_2O/Al_2O_3 values (0.07-0.26), higher CIA values (76.99-92.86), and appear compositionally more mature reflecting moderate to high intensity source rock weathering. Among them, the Barakar mudstones likely have been more intensely weathered than mudstones from any of the other formations.The trace element compositions and ratios are consistent with most mudstones being derived from a felsic source with some mudstones being derived from a more mafic source. For example, La/Sc (0.805 and 0.114), La/Th (6.245, 5.072, 7.418, 6.245, and 5.072) and Eu/Eu* (1.431, 0.868, and 0.805) of some mudstones indicate mafic source. Bivariate plot of Th/Sc and Zr/Sc indicates considerable enrichment of zircon that suggests recycling from meta-sedimentary rocks.Prominent negative Eu anomaly, high LREE/HREE, and flat HREE pattern reflect sediment derivation from predominantly felsic rocks of the old upper continental crust. SiO_2/Al_2O_3 versus K_2O/Na_2O plot for the mudstones suggests that they formed in a passive margin. Also, total REE content of most of the mudstones is conformable with passive margin tectonic setting and craton interior type of provenance. Plotting in La-Th-Sc, Th-Sc-Zr/10 and La/Sc versus Ti/Zr diagrams largely indicates continental margin settings for majority of the mudstones but fails to differentiate between active and passive continental margins.