Dolomitization of the Silurian Niur Formation, Tabas block, east central Iran: Fluid flow and dolomite evolution

Abstract

The Silurian Niur carbonates at Dahaneh-Kalut and Ozbak-Kuh sections (Tabas block, eastern Central Iran) have been pervasively dolomitized. Four types of dolomite, consisting of replacive dolomite (Rd1–Rd3) and saddle dolomite cements (Cd), were discriminated by texture, cathodolouminscence petrography and geochemical analyses. Rd1 dolomite occurred as a very fine (10–25 μm), planar-e to planar-s dolomite that replaced marine limestones. Rd2 dolomite is a planar-s to planar-e, medium crystalline dolomite (100–400 μm). Rd3 dolomite is a very coarse (400 μm–2 mm), non-planar crystalline dolomite with sweeping extinction, and Cd dolomite occurred as voids- and fractures-filling saddle dolomite (5–10 mm) with strong sweeping extinction. Field and petrographic observations demonstrated that Rd3 and Cd occur only at the Dahaneh-Kalut section and are related to mafic volcanic rocks and fractures at the base of the Niur Formation. The δ18O (−8.45 to −6.06‰ VPDB) and δ13C (−0.01 to 0.99‰ VPDB) values suggest that Rd1 dolomite was precipitated at the early stage of dolomitization from Silurian seawater, whereas Rd2 dolomite formed from similar fluids at higher temperatures upon burial. The δ18O (−13.80 to −9.47‰) and δ13C (−1.66 to 0.76‰) values and fluid inclusion data (Th: 122 to 175 °C, salinity: 18.68 to 24 wt% NaCl eq.) revealed that Rd3 and Cd were precipitated from hot, saline fluids with a magmatic origin. The Mn, Fe, Na and Sr concentrations of Rd3 and Cd, suggest that the responsible dolomitizing fluids were more enriched with respect to these elements and had a different origin relative to the Rd1 and Rd2 dolomitizing fluids. These results revealed that the late Ordovician to early Silurian passive rifting phase generated a local thermal gradient that increased heat flow and thermal convection of diagenetic fluids that led to the precipitation of Rd3 and Cd as pervasive hydrothermal dolomite. The network of fractures and faults that formed during the rifting phase provided open conduits for hydrothermal fluids. These results emphasize the importance of both tectonic setting and thermal conditions in the generation and circulation of dolomitizing fluids as well as in massive dolomitization.