Petrography and geochemnistary of the Elika Formation dolomites in Zal section, Julfa, NW Iran

Early to Middle Triassic carbonate sequences of Elika formation in Zal Section (South of Julfa, NW Iran), consist of two different lithology: limestone and dolomite. The thickness of this formation is about 377 m and contains nine different units. The two upper units (8 and 9) are mainly dolomites with thickness of 78 m. The transition between limestone and thick-bedded dolomite in this formation is gradual with the presence of medium-bedded dolomitic limestone lithology. Petrographic results and geochemical analysis clearly show that these dolomites are mainly type I (dolomicrite). Dolomite crystals formed in shallow burial environment (reduction conditions), which affected by marine fluids with little chemical change during dolomitization. The result of XRD analysis in different types of dolomites shows that most of the dolomites of the Elika formation are so close to stoichiometric state and the amount of Mol/Percent of MgCO3 in these dolomites are between 46.7 to 50.4. The Elika dolomites deposited in shallow marine environments (supratidal to intertidal) with high rate of evaporation and salinity, which led to form very fine dolomite along with some silt-sized quartz. The presence of ooid, primary sedimentary structures such as lamination, along with low amount of Fe and Mn and high amount of Na and Sr prove mentioned sedimentation situation. High amount of Na in these dolomites possibly indicate the presence of clay minerals, which identified by SEM and XRD studies.