Abstract:
The present study aims at understanding the nature of syntectonic fluids along the Main Boundary Thrust in the Lesser Himalayas. The field work was carried out in Rishikesh on the right bank of R. Ganga. Calcite veins present in Shaly limestone of Krol A formation were mapped and their relationship with the host rock and chronology was determined. The study revealed the presence of at least three sets of veins in the chronological order: Bedding Parallel Vein (BPV), Oblique Vein 1 (OVI), Oblique Vein 2(OV2).
Fluid inclusion petrography was difficult owing to the small size of the inclusions (<5 μm). Without any ambiguity we report here the presence of biphase aqueous liquid vapour inclusions in all the three sets. Presence of the inclusions in clusters suggests their secondary origin but the absence of grain transacting behavior makes it difficult to unequivocally state their genetic classification.
Microthermometric data of fluid inclusions from BPV and OVI set are presented here. No fluid inclusions could be worked out in the OV2 vein set. The bubble with Brownian motion in the biphase aqueous L-V inclusions helped to record the homogenization and final melting temperatures even when the size was so small. The inclusion size being too small militated against the estimation of first melting temperatures that makes it difficult for us to specify the cations present in the fluid. Laser Raman Spectroscopy revealed the presence of aqueous fluid inclusions. No carbonic phase was identified. The Sodium Chloride equivalent salinity (wt %) of each inclusion is determined by using the table prepared by Bodnar (1993). Freezing data reveals that the OVI is more saline (modal value in the range 11 - 13%) as compared to the BPV (modal value in the range 3 — 5 %) while the homogenization temperatures were comparable (modal value in the range 100-110°C). This is explained by suggesting that fluids in the BPV set were a mixture of freshwater derived from clay dehydration and the formational brines. Over time influx of freshwater ceased and only formational water brine was present.
