Fluid–rock interaction across the South Tibetan Detachment, Garhwal Himalaya (India): Mineralogical and geochemical evidences

Abstract

The Malari Leucogranite in the Garhwal Himalaya is cut across by a continental-scale normal fault system called the South Tibetan Detachment (STD). A mineralogical, geochemical and fluid inclusion study of samples from the fault zone of the Malari Granite was performed to reveal the imprints of fluid-rock interaction. Fluid inclusion assemblages observed in the alteration zone indicate the presence of NaCl-dominated aqueous fluids with varied salinity of 6-16 wt.% of NaCl equivalent. Mineralogical changes include the alteration of feldspar to muscovite and muscovite to chlorite. This alteration took place at temperatures of 275 •-335 • C and pressures between 1.9 and 4.2 kbars as revealed by the application of chlorite thermometry, fluid isochores, and presence of K-feldspar+muscovite+chlorite+quartz mineral assemblage. Geochemical mass-balance estimates predict 32% volume loss during alteration. An estimated fluid/rock ratio of 82 is based on loss of silica during alteration, and reveals presence of a moderately low amount of fluid at the time of faulting. Results of fluid inclusion and alteration mineralogy indicate that the Malari Leucogranites were exhumed due to normal faulting along the STD and erosion from mid-crustal levels. Most of the leucogranites in the Himalayas occur along the STD and possibly a regional-scale fluid flow all along the STD might have caused similar alteration of leucogranites along this tectonic break. Regional fluid flow was probably concentrated along the STD and channelized through mesoscopic fractures, microcracks and grain boundaries.

Pressure‐Temperature‐Deformation‐Time Constraints on the South Tibetan Detachment System in the Garhwal Himalaya (NW India)

“…Brittle faulting was localized in the upper part of the study transect overprinting the medium‐temperature deformation within the Malari granite, probably below 0.50 GPa (Figure ). These P‐T conditions (300 ± 50°C; <0.50 GPa) are in agreement with fluid inclusion data and chlorite‐white mica thermobarometry (Saxena et al, ) on the fault‐related alteration of the Malari granite. Cooling below the closure temperatures of fission track dating of zircon (~240°C; Bernet, ) and apatite (~110°C; Gleadow & Duddy, ) occurred around 5.0 and 3.6 Ma, respectively (Patel & Carter, ).…”

Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

“…This is in good agreement with the lightest δ 18 O fluid value (7.71‰) that characterizes sample XRB-37 located closest to the STDS (Figure 10). (Gébelin et al, 2017), and the high regional fluid flow was probably concentrated along the STDS and channelled through mesoscopic fractures and microcracks (Saxena et al, 2012). For the veins in the hanging wall rocks, the δ 18 O fluid increases slightly from 12.89 to 15.79‰, and the R data range from 24 to 15% with increasing distance from 0.1 to 1.9 km relative to the STDS (Table 1; Figures 9 and 10).…”

“…The STDS is the largest syn-contractional detachment fault in the world (e.g., Kellett, Cottle, & Larson, 2018). However, limited studies have investigated the STDS as a fluid conduit and its fluid activity (Gébelin et al, 2013(Gébelin et al, , 2017Saxena, Sachan, Mukherjee, & Mukhopadhya, 2012). Gébelin et al (2017)…”

Infiltration of meteoric water in the South Tibetan Detachment (Mount Everest, Himalaya): When and why?

“…Assessing the role of fluids in detachment systems is important because fluids may influence the mechanisms and rates of detachment faulting from the grain scale to the scale of entire faults/shear zones [e.g., Mulch et al ., ; Whitney et al ., ]. Fluid infiltration along and within shear zones can induce metamorphic reactions [e.g., Mertanen and Karell , ; Saxena et al ., ] and influence deformation mechanisms (recrystallization and recovery) by formation of rheologically weak phyllosilicate layers that may localize deformation [ Wintsch et al ., ; Warr and Cox , ] or by enhancing crystal‐scale recrystallization [ Rutter and Mainprice , ; Paterson , ]. The presence of phyllosilicates may favor intergranular pressure solution processes [ Renard et al ., ] and thus reduce the porosity and permeability of deforming rocks.…”