Cathodoluminescence Petrography and Stable Isotope Geochemistry of Carbonate Rocks to Evaluate Diagenetic Evolution of the Middle Jurassic Samana Suk Formation (SSF), Kahi Section, Nizampur Basin, NW Himalayas, Pakistan

Diagenetically modified carbonate rocks are more common in the rock record. These modifications, have altered the carbonate rocks significantly. The Middle Jurassic carbonates of the SSF are extensively altered by the diagenetic evolution and several dolomitization process in the Kahi Section, Nizampur Basin. The primary objective of this study is to investigate the diagenetic evolution and multiphase dolomitization through cathodoluminescence petrography and stable isotopic studies. Field investigations show both host limestone (oolitic, fossiliferous and massive) and dolomites. Different types of dolomites were recognized on the basis of color contrast as dark grey color replacive dolomite, light grey dolomite, brownish dolomite and yellowish dolomite. Beside replacive phase voids and fracture filling cementing saddle dolomite, and cementing calcites are also recognized in the field. Petrographic studies show the complex diagenetic history of Samana Suk Formation from near surface diagenesis including micritization, neomorphism and several varities of dolomites. These verities are: RD1 is very fine to fine grained dolomite, RD2 is medium to coarse grained and anhedral to subhedral dolomite, RD3 is coarse to very coarse grained and planner euhedral zoned dolomite, and RD4 is coarse grained euhedral to subhedral ferroan dolomite. In addition, cementing saddle dolomite SD have large crystal, curved faces with sweeping extinction. cementing calcite phases are CC1 is granular mosaic, CC2 is twin, CC3 is fracture filling, and CC4 is ferroan calcite. The stable isotope values of limestone (δ18O: –7.13 to –0.73‰ V-PDB and δ13C: –0.05 to 1.32‰ V-PDB) showing depletion in from the Jurassic marine signature. The multiphase dolomites RD1–RD4, SD values (δ18O: –8.65 to –3.16‰ and δ13C: –3.56 to +2.09‰) indicate multiphase dolomitization. The C1–C3 values (δ18O: –11.07 to –8.97‰ and δ13C: –2.14 to +0.76‰) indicate highly depleted values of δ18O showing its source from the hydrothermal origin. From field, petrography and stable isotopic geochemistry data it is deduced that possible source of the Mg for hydrothermal dolomites is through activation of faults and fractures during active tectonic regime in the area and can be related to activation and reactivation of Kahi Thrust system.