THE NATURE OF SYNTECTONIC FLUIDS IN CHAKRATA FORMATION: EVIDENCE FROM rj FLUID INCLUSIONS

Abstract

The present study aims at understanding the nature of syntectonic fluid of the Chakrata Formation in the Lesser Himalayas. Samples were collected from the road section between Saknidhar and Devprayag, a stretch of 16 km. Quartz veins present in sandstone-shale facies were considered for the study. The area had en-echelon veins and samples from Bedding Parallel veins and Oblique veins were collected separately. At least two doubly polished wafers were prepared from each of the 10 samples collected and used for the fluid inclusion petrography and microthermometric heating-freezing experiments. Fluid inclusion petrography and Raman spectroscopy revealed the presence of at least three types of fluid inclusions: monophase aqueous inclusions; biphase aqueous inclusions and biphase aqueous-carbonic inclusions in all the samples. The secondary fluid inclusions occur in trails that were developed during the microfracturing in the vicinity of shear zones. Microthermometric data of 95 fluid inclusions are presented here. The studies revealed that the trapping conditions of the fluid inclusions in the two veins sets were the same. The eutectic temperature had a modal value of -34.5°C for both bedding parallel and oblique veins which corresponded to NaCI-MgCl2-H20 composition of the fluid (Goldstein & Reynolds, 1994). The final melting temperatures of these fluid inclusions lie between -3.6 to - 13.8°C corresponding to the salinity range between 8.14 to 17.61 wt% NaCI equivalent (Bodnar, 1993). The homogenization temperature (Th) vs salinity plot indicates no fluid mixing or boiling. However necking and decrepitation were observed in few inclusions. The intersection of the isochores with the thermobaric gradient defines the entrapment conditions of fluid inclusions as 200-230 MPa and 264-303°C. The high pressure and temperature of these fluids can be attributed to tectonically driven high pore fluid pressure and shear heating during the brittle-ductile shearing.