The Higher Himalayan Crystallines (HHC) in the Siyom river section of eastern Arunachal Himalaya, display top-to- ESE compressional ductile shear from Pene to Menchuka. In this part of the Himalayan Metamorphic Belt (HMB), the base of the Main Central Thrust (MCT) is the southern boundary of the ductile shear fabrics in HHC. Across the MCT, a significant break in the grade of metamorphism is observed between the HHC and the Lesser Himalayan Sequences (LHS). The ductile shear fabrics documented in the HHC are primary S-C and S-C′ fabrics, asymmetric folds, porphyroclasts and porphyrobalsts, brittle- ductile structures and asymmetric boudins. Three phases of folding representing three deformation episodes (D1-D3) in HHC are recorded in the area. The rocks of the HHC in Siyom valley are completely transposed by the D2 deformation into NNW dipping C- plane. Ductile shear represented by grain scale structures include sigmoidal foliation, rotation of the inclusion trails in garnet porphyroblasts, asymmetric folds with consistent top-to-ESE sense of shear. The ductile to brittle-ductile shear fabrics in the area indicate that the thrust sense of shear in the HHC is consistent without any shear sense reversal from Pene to Menchuka. These compressional shear fabrics provide invaluable field evidences for constraining the evolution of this part of the Higher Himalaya.
A thin linear belt of Permian Lower Gondwana rocks occur in the Eastern Himalayas from Arunachal Pradesh to Sikkim. The Lower Gondwana Group of rocks consists of shale, siltstone, sandstone, carbonaceous shale and coaly matter and is sandwiched between Miri Group and Siwalik Group of rocks. Heavy mineral composition of sandstones is extensively used in the provenance studies as they are the surviving remnants of the rather abundant but unstable mafic components of the source rock. The sandstones of Lower Gondwana Group bear the heavy mineral assemblage of chlorite, biotite, zircon, ilmenite, epidote, garnet, amphibole, chloritoid, brown tourmaline, magnetite, staurolite, rutile, and opaques which is indicative of a provenance of metamorphic rocks with input of igneous rocks. Provenance sensitive mineral index ratios were also calculated to see the variation in the ratio of two or more stable minerals with the same characteristics.
The Permian Gondwana sequence of Arunachal Lesser Himalayas provides an excellent setup to study the provenance and associated tectonics during deposition due to its typical positioning at the margin of the Gondwanic Indian continent. Petrographic character of these Gondwana sandstones exposed in parts of East Siang and West Siang districts of Arunachal Pradesh and inferences on source rocks and tectonic setting are presented in this paper.
The modal analysis of the sandstones reveals that quartz is the dominant detrital framework grain with 58.1-78.30%, feldspar percentage varies between 0.5 to 4.7%, the percentage of rock fragments ranges from 1.0 to 12.6%, mica content varies between 1.3-14.0%, matrix percentage varies between 6.8-15% and cement constitutes 3.1-23.5%. The sandstones are classified as sublithic arenite and quartz arenite. The study suggests quartzose recycled orogen as well as transitional recycled source and indicates towards a low grade metamorphic source for the sediments. It is inferred that these recycled sandstones were originally having its source in the cratonic blocks and sedimentation in this Gondwana basin evolved through multiferous interplay of deformation due to thrust loading, changes in sea level resulting in progressive compaction and a humid climate.
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