Study of Permo-carboniferous paleoclimatic event in Eastern Himalaya

Abstract

The history of Late Paleozoic cryospheric event of Permo-Carboniferous Period is widely spread and preserved globally in the form of sedimentary facies. The Permo-Carbonifero us sedimentary sequences of Peninsular India have been well studied and categorized mainly into three major basins, i.e., Son-Mahanadi basin, Pranhita-Godavari basin and Damodar basin. Their equivalent sedimentary sequences from the Permo-Carboniferous were also deposited simultaneously, around the northern margin of peninsular India which is also known as peri-Gondwana sequences. These peri-Gondwana sequences are well preserved and exposed in the Himalaya i.e., Kashmir, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh. The sedimentary sequences of Sikkim-Darjeeling Himalaya have witnessed a wide span of climate change during the Permo-Carboniferous period. The signatures of Permo-Carboniferous glacial and interglacial events are recorded in the form of sedimentary facies in two different basins, Rangit Pebble Slate Formation in the Lesser and Lachi Formation in the Tethyan Sikkim Himalaya. Integrated lithological, petrological, and geochemical approaches were used for evaluatio n of the Rangit Pebble Slate and Lachi Formation of Sikkim Himalaya. The geochemistry and petrography have been done with an aim to establish provenance, tectonic setting and weathering intensity for the sediment of the Rangit Pebble Slate (RPS) and Lachi Formation. The lithological association of these peri-Gondwana sequences from Sikkim Himalaya includes the diamictite facies at the base and the alternating sandstone-shale facies at the top of the sequence. Lowermost sequences of both the formations are characteristically recognized by boulder beds and diamictite facies which suggests a glacial environment of deposition, while the upper sequences of alternate sandstone and shale are considered marine transgression. The lenticular bedding with continuous thick lenses in the uppermost sequence of Rangit Pebble Slate Formation shows the evidence of ii tidal wave. The whole lithofacies sequence shows a series of fining-upwards cycles which might be due to transgression of Tethys Sea after the Permian glaciation event. The stratigraphic sequences of Rangit Pebble Slate and Lachi Formation indicate a transgressive tract system. The presence of marine fossils like Eurydesma also confirm the marine environment of deposition. The ternary and diamond diagram for the composition of sandstone indicate that sediments of Rangit Pebble Slate and Lachi Formation are dominant with monocrystalline quartz, feldspar, rock fragments and are classified as sub-arkose and arkosic arenites. Both coarse and fine-grained sandstones from the study area were evaluated and it was observed that these sediments were derived from plutonic and high-grade metamorphic terrain. The ternary plot of provenance and tectonic setting clearly suggests that the sediments of the Rangit Pebble Slate and Lachi Formation were derived from stable cratonic and interior continental areas which were originally deposited in passive continental margins. 2 against various other oxides in all studied samples. The plot of major elements K2O vs. Na2O quantifies the richness of quartz in all studied samples indicating that these sediments were derived from quartz-rich provenance. The tectonic discrimination diagram between K2O /Na2O Vs SiO2 infers the area of a passive margin. The enrichment of SiO2, Al2O3, TiO2, MnO, MgO, and K2O indicates that these sediments were mostly derived from felsic rock source areas. The chondrite normalized REE pattern with high value of LREE/HREE ratio with Eu negative anomalies also indicate that sediments of Rangit Pebble Slate and Lachi Formation were derived from felsic sources. The values of Chemical Index of Alteration (CIA) suggest the cold environment of deposition for the sediments of lower sequences whereas the deposition of sediments of upper sequences was warm and humid. The A-CN-K ternary plot also indicate that the source areas were subjected to prolonged intense chemical iii weathering from low to high grade due to shifting of cold to warm humid paleo-clima tic condition. The petrological and geochemical analysis of all studied samples of Rangit Pebble Slate and Lachi Formation shows close affinities with other peninsular Gondwana basins in terms of tectonic setting and provenance, i.e., passive tectonic setting and granitic source. Most of the studied samples of both the formations were plotted in the plutonic fields which indicate the granitic source of rock which might have been derived from the stable plutonic craton of Peninsular India i.e., Chtonagpur Granite Gneiss Complex (CGGC), Shillong Plateau Gneissic Complex, Proterozoic sedimentary rocks, etc. The Chotanagpur Granite Gneiss Complex (CGGC) and Shillong Plateau Gneissic Complex of the Indian shield lying to the southwestern of Sikkim-Darjeeling Gondwana basin, which is composed of high-grade migmatitic gneiss, khondalite, quartzite, and basic igneous rock. The other Permo-Carboniferous sedimentary sequences of Himalaya i.e., Arunachal Pradesh and Spiti region of Himachal Pradesh have shown close similarity in terms of provenance tectonic setting and climatic condition.