Interpretation of diagenetic sequence and geochemistry of Parvadeh Formation in East-Central of Iran (Tabas Block)

After the Mid-Cimmerian tectonic event in the Central-East Iranian Micro-continent (CEIM), the marine sediments of the Parvadeh Formation were deposited in the Middle Jurassic, with a significant expansion in the north of the Tabas block and mainly formed from dark gray fossiliferous limestone (Aghanabati, 2014; Seyed-Emami et al., 2004). In this study, two sections of Parvadeh Formation in Ravar-Mazino sub-block (92 km west of Tabas) and Kalshaneh syncline in Kalmard block (83 km north of Tabas) have been selected for diagenesis and geochemistry studies. Diagenetic events are an important key in sequence stratigraphy studies, identifying surface exposure phases and changes in environmental conditions (Moore and Wade, 2013). Analysis of trace and major elements also leads to a better understanding of environmental conditions characteristics and better identification of fluid systems associated with diagenetic processes (Nader, 2017; Oliveira and Truckenbrodt, 2019). A combination of field, petrographic and geochemical data was used to investigate the effect of diagenesis on the studied deposits.

After completion of field studies, 113 rock samples were selected and studied by Cathodoluminescence (CL) and polarizing microscope after the thin sections were prepared, while Dickson (1966) method was used to detect calcite from dolomite. Also, 11 limestone samples were selected for geochemical analysis and then tested by Zarazma Mineral Studies Company using the ICP-OES Varian model to determine trace and major elements.

Petrographic studies indicate that the Parvadeh Formation has been affected by micritization, cementation (Isopachus, Granular, Drusy, Blocky, and Poikilotopic), neomorphism, replacement (dolomitization, silicification, and pyritization), physical and chemical compaction, iron oxide, and fracturing processes. Calcite was pink to red in staining with red alizarin, while dolomite was not stained. In addition, elemental analysis of samples from the Parvadeh Formation showed that the amount of Sr varied from 269 to 678 ppm, and the amount of Na varied from 604 to 1189 ppm. The changes ranged between 134 to 443 ppm for Mn, 1978 to 13183 ppm for Fe, 33.15 to 38.24% for Ca, and 2785 to 4639 ppm for Mg. Positive trend changes in the Fe/Mn, Fe/Na, the low Sr/Mn ratio in the studied samples, and its inverse linear relationship with Mn values showed the highest effects of meteoric diagenesis and, to some extent, burial diagenesis. Based on the Sr/Ca plot versus Mn, it is possible to figure out the open and closed diagenetic system, the ratio of water-to-rock interactions, and the oxidation and reduction conditions of the environment (Brand and Veizer, 1980).The bivariate plot of Sr/Ca versus Mn shows that fluids in an open diagenetic system have stabilized the limestone of the Parvadeh Formation. Modern and ancient tropical carbonates differ from their non-tropical counterparts by their Sr/Na ratio and Mn contents (Rao, 1991; Winefield et al., 1996). In the Parvadeh limestone, Sr/Na concentrations range from 1.44 to 0.3 (mean: 0.6), similar to carbonate samples and the subtropical-temperate climate of the recent regions. The plotting of Sr/Mn with Mn provides a valuable measure of the degree of dissolution in limestone (Rao, 1991). The average Sr/Mn ratio in the Parvadeh Formation carbonates is 1.61. The low Sr/Mn ratio in the studied samples and its inverse linear relationship with Mn values show the highest effects of meteoric diagenesis and, to some extent, burial diagenesis.

Micritization, cementation, neomorphism, physical and chemical compaction, dissolution, Viens filling and fracturing, dolomitization, silicification, and pyritization are the most important diagenetic processes of the Parvadeh Formation. These processes have been carried out in four marine, meteoric, burial, and uplift diagenetic environments, with meteoric and burial diagenesis having the most significant impact on the Parvadeh Formation, respectively. Micriticization processes, formation of microcrystalline dolomites, and some cement (overgrowth) have occurred in the stage of marine diagenesis. In contrast, dissolution, neomorphism, and appearance of some cement (Blocky, Granular, Drusy) have taken place in the stage of meteoric diagenesis. Also, physical compaction, stylolite, formation of some cement (Blocky and Drusy), and medium crystalline dolomites have occurred in the burial diagenesis stage, while the development of fractures and their filling has taken place in the uplift stage. The positive trend of changes in Fe versus Mn and Fe versus Na graphs indicates the effect of meteoric diagenesis. The low Sr/Mn ratio in the studied samples and its inverse linear relationship with Mn values show an increase in burial depth and a decreasing trend of Sr/Na versus an increase in Mn, indicating a close combination of recent temperate carbonates, whose geochemistry results are consistent with subtropical to temperate climates obtained during the Middle Jurassic. Also, changes (1000 * Sr/Ca (wt)) versus Mn indicate the open diagenetic system in the Parvadeh Formation.