Petrographic and geochemical characteristics of dolomites in the Devonian Shogram Formation, Karakorum Ranges, North Pakistan

Excellent outcrops of Devonian carbonate rocks are present in the Karakorum Ranges of North Pakistan that are highly dolomitized. The Devonian Shogram Formation comprises both matrix and cement dolomite. Petrographic studies revealed the texture of matrix dolomite as fine-grained anhedral dolomite (D1), medium-grained subhedral to anhedral dolomite (D2), medium-grained euhedral dolomite (D3) and coarse-grained anhedral dolomite (D4). The saddle cement dolomite phase of hydrothermal origin precipitated in vugs and fractures. The origin of various dolomitization phases has been interpreted based on petrographic associations, elemental and stable isotopes signatures of each phase. The first two phases of dolomitization are affiliated with the compaction flow of overlying shales based on evidence of low Fe and Mn content and isotopically light signatures of d 18O. The increase in burial resulted in the third phase of dolomite showing signatures of redox conditions and high depleted d 18O compared to the original marine signatures. The final phases of matrix dolomite and saddle dolomite cement are associated with tectonic activity in the Karakorum region, which brought the deep-source Mg-rich fluids along the Reshun Fault.