Geothermal evolution of the evaporite-bearing sequences of the Lesser Himalaya, India |
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Authors: | S P Singh B P Singh |
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Institution: | (1) Postgraduate Department of Geology, University of Jammu, Jammu, 180006, India;(2) Department of Geology, Banaras Hindu University, Varanasi, 221005, India; |
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Abstract: | Neoproterozoic evaporites occurring in the western part of the Lesser Himalaya in India, coeval to Pakistan, Iran and Oman
evaporites, were investigated in order to understand the degree of metamorphism in them and in associated carbonates. The
evaporite-bearing succession occurs in association of phyllite, quartzite and carbonate near the Main boundary Thrust. In
order to learn the details about the burial history of these evaporite rocks, the Kübler illite crystallinity index (KI) was
measured from the illite peaks of the clay minerals separated from the evaporite rocks and it indicated that this section
has reached a maximum temperature up to ~300°C. Microthermometric measurements on fluid inclusions present in the associated
dolomite show range of homogenization temperatures (Th), from 220 to 280°C, well within the temperature range of anchizone
metamorphism. Additionally, dolomite shows a highly negative δ18O signature (mean, −15.5‰PDB), which is more likely related to diagenetic overprint from deep burial conditions rather than original precipitation from
18O-depleted seawater. The evaporites (sulfates and chloride) probably were transformed many times after their precipitation,
but they have retained only the features developed during last one or two phases of alteration and deformation as they are
continuously susceptible to minor changes in temperatures and stresses. The final temperature range of 42–78°C in sulfates
and chloride gives thermal approximation estimate that is not in concordance with the thermal history of the basin and are
likely related to conversion of anhydrite into gypsum and recrystallization of halite during exhumation. Highly negative oxygen
isotopic composition, homogenization temperatures and KI values equivalent to a high anchizone metamorphism suggest a burial
depth of ~10 km for these terminal Neoproterozoic evaporite-bearing sequences of the Lesser Himalaya. |
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