No abstract

Mizoram is part of Surma basin which later evolved into the present state of geological terrain due to Indo- Myanmar tectonic collision during the Oligocene period. The present work deals with geochemical characteristics of Barail sandstones exposed in Champhai area of eastern region in Mizoram. The major/minor oxides, trace elements and rare earth elements data are used to infer the geological history and evolution of the sandstone in the study area of Champhai. The petrographic study shows the presence of various detrital grains like quartz, lithic fragments, feldspar, chertz, mica, etc., which are cemented by siliceous and ferruginous materials. Geochemically, the sandstones indicate high wt% of SiO2, Al2O3 and MgO compared to Upper Continental Crust (UCC) while rest of the major oxides indicate low concentrations. The geochemical classification indicated the sandstones as litharenite and wacke. The chondrite normalised REE pattern shows the enrichment of HREE and depletion of LREE with negative Eu anomaly. The value of Eu/Eu*, La/Lu, La/Co, Th/Sc, Th/Co, Cr/Th and high ratio of LREE/HREE of Barail sandstone suggest felsic source rock. The analysis of paleoweathering history indicated moderate to intensive weathering in the provenance. Various tectonic discriminant function diagrams suggested Active Continental Margin settings.

The Precambrian terrain of the Sonapahar area is characterised by the presence of a thin, long, discontinuous band of quartz-sillimanite schist which hosts the massive sillimanite-corundum deposits. The quartz sillimanite schists are generally siliceous, SiO 2 (60.59-76.82 wt %) and are characterized by high Al 2 O 3 content with average value (27.22 wt %), low P 2 O 5 , Fe 2 O 3 and MgO, MnO Na 2 O, K 2 O and TiO 2. Average MgO, CaO, K 2 O and Na 2 O values are generally less than 0.15 wt %. TiO 2 values range from 0.13-0.35 wt %. Compared with the post-Archean metasediments, the Sonapahar schists are richer in SiO 2 and Al 2 O 3 but depleted in other oxides. In Harkers variation diagram, SiO 2 shows a linear inverse correlation with Al 2 O 3 and TiO 2, but there are no significant correlations observed with other oxides. In general, Al 2 O 3 and SiO 2 constitute more than 97 wt % in most of the samples. Among trace elements, Zr, Hf and Pb are characterized by high concentration and low La/Th and Th/U. The schistose rocks are generally low in Sr/Ba ratios (< 0.88 %) and relatively high content of Rb in contrast to Ba. Chondrite normalize REE patterns of the schists reveals a strongly fractionated patterns with enriched LREE and low HREE values [(La/Yb)N = 27.38-43.98] with prominent negative Eu-anomaly. The concentration of V (10.28-51.49) shows the apparent mobility in these quartz sillimanite schists and the oxidizing conditions prevailed during weathering. Geochemical characteristics suggested that quartz sillimanite schists from Sonapahar area are derived from post-Archean recycled upper crust sources and the precursor sediments are possibly derived from felsic rocks and probably not mafic rocks.

The area around Champhai district contains exposed Barail Group of rocks (Oligocene) which comprises of buff-coloured sandstones with alternating beds of shale and siltstone. This study focussed on the provenance and tectonic setting of the Barail sandstones with the help of detailed petrographic studies and quantitative and qualitative modal analysis by point counting technique. The general lithological units consist of sandstone, shale, crumpled shale, silty-shale and siltstone. A modal analysis of the 25 sandstone samples was carried out on selected detrital grains like quartz, feldspar and lithic fragments which indicated that quartz ranges between 69.75 to 82.25%, feldspar ranges between 1.80 to 6.91% and lithic fragments ranges between 13.82% to 29.43%. The sandstones belong to sublitharenite, litharenite and feldspathic litharenite indicating predominant presence of quartz in the sandstones with lesser components of feldspar and lithic fragments. Different variants of quartz such as undulatory monocrystalline quartz and polycrystalline quartz and other lithic fragments like schist and gneiss indicate metamorphic source rock. The common occurrence of micas in almost all the sandstone samples may also indicate the common occurrence of mica-bearing metamorphic schists or gneiss in the provenance. Various characteristics of feldspar like strong alteration in orthoclase, deformed twinning of plagioclase and presence of micas may be indicative of low to high grade metamorphic rocks and acidic to basic igneous rocks. On the basis of different tectonic discrimination diagram using QFL and QmFL, the tectono-provenance of the sandstones was found to be Quartzose Recycled Orogen. It is concluded that the source area might be the tectonically active continent-continent collision zone of eastern Himalaya and the Indo-Burmese belt and that the associated extrusive and intrusive igneous rocks are prone to weathering and erosion by various denudation processes.