Multiphase dolomitization in Devonian Shogram Formation, Chitral, Karakorum ranges, Pakistan: Evidence from outcrop analogue, petrography, and geochemistry

Abstract

This paper reports the first detailed microscopic and geochemical investigations of multiphase dolomitization in Devonian Shogram Formation, North-Western Karakorum constraining nature and origin of dolomitizing fluids. Field and petrographic studies revealed 4 different types of replacive dolomites, which are (i) fine grained anhedral dolomite (D1), (ii) medium grained subhedral to anhedral dolomite (D2), (iii) medium grained euhedral dolomite (D3), and (iv) coarse grained anhedral dolomite (D4) along with cement phase saddle dolomite (SD). These dolomites had high Fe and Mn concentraions and low Sr content. Stable isotope studies indicated high light isotope δ 18 O values for D1 and D2, higher lighter isotope δ 18 O value for D3, and highest lightest isotope δ 18 O values for D4 and SD, respectively. Petrographical and geochemical data suggested that D1 and D2 are likely formed in compactional flow regime in early stages prior to chemical compaction at shallow burial depths, whereas D3 is formed in late diagenetic deep burial settings. Lastly, D4 and SD are formed from high temperature hydrothermal fluids. These fluids could be related to widespread tectonic activity in the Karakorum region during collision or post-collisional time. Reshun fault acted as an important pathway that channeled Mg-rich hydrothermal fluids from underlying strata into carbonates, which caused intense dolomitization.