Search

Clastic rocks of foreland basin from Nandhaur Sanctuary area, Kumaun Sub-Himalaya were studied to interpret their lithological composition, provenance, source area weathering characteristics and tectonic setting. Detailed geochemical investigation on 33 mudstone and petrographic study of 25 sandstone samples have been carried out. Sandstones are classified as quartz-arenite to sublith-arenite; lithic to subarkosic arenite and lithic to arkosic greywacke using detrital mode analysis. Chemically, mudstone is identified as greywacke to lithic-arenite and also straddles between shale and wacke with a few in lithic-arenite field. CIA values vary from 55.8 to 78.7 for Lower Siwalik, 53–78.3 for Middle Siwalik and, 58.8–81.7 for Upper Siwalik sediments. These results suggest that differential weathering conditions acted upon the sediment provenance: under steady to non-steady-state, variable climatic conditions, and Cenozoic tectonic uplifting along Himalayan thrusts. Chemical data in Al2O3-CaO-Na2O-K2O (A-CN-K) plot depart from the granite-granodiorite field towards the illite-muscovite indicating weathering trend in provenance. Chemically, mudstone bears the signature of Passive Margin tectonic setting of deposition which is inherited from the source rock. High field strength element (HFSE) and Rare Earth Element (REE) content of mudstone support the felsic upper crustal material as source, whereas transition elements advocate minor contribution from mafic rocks. Chondrite normalized high Light REE (LREE)/ Heavy REE (HREE) ratio (11.15–15.11), sloping LREE and near-flat HREE, negative Eu anomaly (Eu/Eu* = 0.6–0.88) indicate the evolved upper crustal material as the prominent source. The trace element ratio in mudstone; La/Sc (0.71–5.86), Cr/Th (2.42–9.50), Th/Co (0.07–0.95), and La/Co (0.15–2.31), supports the sediment derivation from felsic sources however Th/Co values indicate the mixed source rocks. The geochemistry of mudstone shows affinity with the rocks of high-grade Higher Himalaya and low-grade Lesser Himalaya and is a useful proxy for understanding the various tectonic and paleo-climatic conditions that prevailed during the evolution of Himalayan Foreland Basin.

Abstract: Bhilangana granitoid is a batholithic size body exposed in Bhilangana Valley and later thrusted over rocks of Lesser Himalaya. It comprises mylonitized porphyritic granite gneiss, psammitic gneiss, and its variants, containing primary biotite and muscovite, and S-type in nature. Bhilangana granitoid is intruded by basic intrusive. The variation in chemical signatures and associated petrogenetic issues of Bhilangana granitoid are dealt with detailed petrographic, geochemical and mineral chemical study. In the vicinity of basic intrusive, the porphyritic granite gneiss shows enrichment of MgO, whereas its evolved part, psammitic gneiss shows more boron enrichment with the presence of tourmaline nodules surrounded by quartzo-feldspathic rim. The major mineral assemblage in Bhilangana granitoid is quartz–K-feldspar–plagioclase–biotite–muscovite with tourmaline, epidote, zircon, ilmenite, apatite and monazite. The granite is peraluminous with ASI values always >1. Biotite shows variation from Fe-rich siderophyllitic to Mg-rich eastonitic composition, indicating involvement of both Al-rich crustal and Mg-rich mafic sources, inferring source heterogeneity or mixing of felsic-mafic magma. Granitoid melt was buffered below QFM to NNO and partly above HM, indicating a strong reducing to the oxidizing condition of melt. The melt was saturated with the presence of 3.3–6.8 wt.% H2O content. The crystallization temperature of Bhilangana granitoid (TZr) ranges between 697° and 807°C with a solidification pressure of about 2.92–4.31 kbar, implying crystallization at a depth of 8.76–13 km. The presence of greisens assemblage of quartz–mica–tourmaline–epidote, along with the presence of fluorite, is a representative greisens assemblage for Sn–W mineralization. Thus, the Bhilangana granitoid represents the fertile nature and potentiality of Sn–W minerals. Research highlights:

Bhilangana granitoid is S-type, peraluminous granitoid, formed by anatexis of crustal material

The biotite chemistry reveals the variation in the redox condition and modification in the oxidation state, preserved in the different variant of granite representing the magma mixing or source heterogeneity in protolith.

The Bhilangana granitoid has attended the crystallization temperature (TZr) of 697–807 °C under pressure of about 2.92–4.31 kb implying the depth of crystallization of 8.76–13 km.

The presence of greisenization favours the potential of Sn–W minerals. The geochemical results shows abnormal enrichment of W and Sn in the Bhilangana granitoid.

The Bhilangana granitoid appears to have fertile nature for Sn–W mineral potential.

The present study reports and investigates ‘lazulite’ occurring in the vicinity of a highly tectonized zone of the Main Central Thrust (MCT) in the Himalaya. The azure blue lazulite, hosted in quartz veins, occurs in fractured Berinag quartzite, which forms the footwall of the MCT near Sobla village in NE Kumaun Himalaya, India. Lazulite was investigated using SEM-EDX, micro Raman spectroscopy, fluid inclusion microthermometry and electron probe microanalysis (EPMA). Lazulite contains inclusions of rutile and hematite and has Mg/(Mg+Fe) ratios of 0.86 to 0.90. The phosphorus in lazulite shows a negative trend with Mg+Al contents. This lazulite is an intermediate solid solution near the lazulite end-member with a cationic composition in the structural formula: Mg0.81–0.89Fe0.10–0.13Al1.88–1.98P2.00–2.07. Its composition in the lazulite–scorzalite stability field points to a higher temperature of its formation. Fluids trapped as inclusions in lazulite and the associated quartz are generally C–O–H fluid. The fluid inclusion isochors for lazulite, together with the temperature calculated for metamorphism of the equivalent structural level in the adjacent area suggest 500–600°C and 7.25 to 9.25 kbar, which match the peak metamorphic temperature–pressure derived elsewhere for the Higher Himalayan Crystallines. Moderately enriched δD‰ values and H2O–CO2–low NaCl fluid suggest that water from a deep reservoir, more likely a metamorphic fluid, participated in lazulite formation. Classic sigmoidal fluid inclusions in lazulite reveal their development during MCT shearing, whereas the overpressured fluid inclusions suggest a post-lazulite uplift. The MCT lazulite is interpreted to have formed during Himalayan shearing and concurrent metamorphism. The present study also implies that this refractory mineral can sustain fluid inclusions within it against intense deformation conditions, such as in the MCT.

Abstract: Ghuttu tectonic window is a structural window and is exposed around Ghuttu village in Bhilangana Valley, tectonically surrounded with overlying ductile deformed and sheared granite gneiss of Bhilangana Formation. The dominant lithology in the window comprises geochronologically undifferentiated quartzite, dolomitic limestone, carbonaceous slate and argillite/calc-argillite belonging to Lesser Himalayan sequence. The granite gneisses of Bhilangana Formation are metamorphosed up to lower amphibolite facies and tectonically lie above the low-grade greenschist facies rocks of Ghuttu tectonic window along Main Central Thrust (MCT-III). Both the hanging and footwall units are co-folded into doubly plunging antiformal structure of the window. The signatures of multiple ductile deformations as the folding of various generations, development/effect of ductile shear zone, and related modifications in the pre-existing and Himalayan shear-related folds are unequivocal. The various deformation events, viz., D1–D4(type of folds) are identified and their kinematic history is studied. We propose a tectonic model of the structural evolution of both mechanically anisotropic and layered litho-tectonic units in Ghuttu Window and overlying mylonitised Bhilangana Formation. Research highlights:

The structural and lithological mapping of Bhilangana Formation and Ghuttu window reveals the development of sequential deformation.

One Pre-Himalayan and three Himalayan deformations (two orogen-transverse and one orogen-parallel) are identified under various stages of southward movement of Bhilangana Formation with minimum transportation distance of ~20 km along Bhilangana Thrust during Tertiary Tectonics.