PETROGRAPHY AND GEOCHEMISTRY OF DOLOMITES OF SIRBAN MEMBER, MIRPUR SECTION HAZARA REGION, NW HIMALAYAS PAKISTAN

Abstract

The Sirban Member of the Abbottabad Group at Mirpur Section, Abbottabad District is selected for research study. Research is based on petrographic analysis comprising of microfacies and geochemical study. The Sirban Member exposed in the study area having a thickness of 240 to 485 m, with lower part sandy, followed by thick sequence of dolomite imbricated with sandstone, shale, limestone, quartzitic sandstone and chert. During the field the features observed in the study area including chert bands, calcite veins, fractures, stylolites, dissolution, butcher-chop weathering. The petrographic study of the Sirban member reveals different phases of matrix and cement dolomite and diagenetic events have also been identified, including chertification, mechanical compaction, brecciation, stylolization, quartz influx, several stages of fracturing and dolomitization. The paragenetic sequence is developed for Sirban Member revealing the history. In the dolomite D1 the oxygen isotopic signatures of the Sirban member ranges from -2.97 to -3.22‰ VPDB while carbon signatures varies +2.42 to +1.83‰ V-PDB. Whereas, dolomite D2 in Sirban member shows oxygen and carbon values ranges from -4.12% to -3.57% V-PDB and +1.11% to +2.05% V-PDB respectivelly. Similarly the dolomite D3 in Sirban member shows oxygen values that varies from −6.49 to −7.07‰ V-PDB and carbon signatures +2.14 to +1.66‰ V-PDB . This means that these values lies in cambrian marine signature and show dolomatization has occured through the same fluids.Atomic Absorption shows values of different element like Iron (Fe), Manganese (Mn), Strontium (Sr) and Sodium (Na) in three types of dolomite.Based on field study petrography elemental geochemistry and stable isotopic signature it may be possible that the dolomite is formed by evaporation and subsequent reflux dolomitization.