Dolomitization models in the Sibzar Formation (Middle Devonian), Binalood Mountains (NE Iran): Based on the petrographic and geochemical evidence

Abstract

Lithostratigraphically, Sibzar Formation (Middle Devonian) in the Kharv section (Binalood Mountains, NE Iran) with a thickness of 334 m consists of three units (in ascending order): dolostone with intercalations sandstone, middle basaltic sill, and upper dolostone. Three types of dolomite have been identified based on crystal size and shape. Fine crystalline dolomite (D1; finer than 30 μm) consist of Nonplanar-a dolomites. Fine to medium crystalline dolomite (D2; 20 to 150 μm) consist of mostly Nonplanar-a dolomites and some dolomites are Planar-s and Planar-e. Vein dolomites (Vd; 40 to 700 μm) have a very diverse crystalline texture and are mostly Planar-s, Planar-e and some Nonplanar-a. Cathodoluminescence technique studies also showed that most of these dolomites have dull red luminescence, in some veins with bright red luminescence and zoning. Based on the X-ray diffraction (XRD) and ICP-OES, all dolomites are nearly stoichiometric dolomite and relatively well ordered. Elemental analysis and stable isotope studies show that the concentration of minor elements such as sodium (D1: 822–1365 ppm; D2: 1016–1241 ppm; Vd: 971–1072 ppm), strontium (D1: 257–963 ppm; D2: 134–1334 ppm; Vd: 144–379 ppm), manganese (D1: 560–3675 ppm; D2: 1039–2168 ppm; Vd: 1818–3454 ppm), and iron (D1: 15997-30080 ppm; D2: 8288–18456 ppm; Vd: 19304-25618 ppm) in these dolomites were high and the values of the stable isotope of oxygen (D1: -9.1 to −5.9‰; D2: -7.6 to −6.1‰; Vd: -11.1 to −7.2‰) and carbon (D1: -3.5 to −2.5‰; D2: -5.2 to −1.3‰; Vd: -2 to −1.6‰) were low. The high values of iron and manganese and depletion of the carbon and oxygen isotopes can be related to the effects of high-temperature diagenetic fluids caused by basaltic sill during the burial. These dolomites formed in tidal environment and burial diagenesis in this Formation. Seawater, connate seawaters, saline basinal fluid, and magnesium-rich hypersaline fluids are the main source of magnesium ion (Mg2+) for dolomites of this Formation.