Origin of dolomites in oolitic carbonates of the Middle Jurassic Dorgali Formation, eastern Sardinia, Italy: Petrographic and geochemical constraints

Abstract

The Bathonian-Callovian (Middle Jurassic) Dorgali Formation in eastern Sardinia (Italy) consists of dolomitized oolitic grainstone and records a complex diagenetic history. Petrographic examinations revealed three main phases of replacive (D1, D2) and fracture-filling (D3) dolomites with crystal-size ranges of <4–50 μm (dolomicrite to dolomicrosparite D1), 50–200 μm (eu-to subhedral dolomite D2), and 50–400 μm (sub-to anhedral crystals of dolomite D3). The early D1 exhibits dull luminescence (CL) under cathodoluminoscope, whereas the later D2 shows dull to zoned CL, and D3 consistently zoned CL. The near-micritic grainsize, low Sr contents (136 ± 43 ppm) and estimated δ18O values of the parent dolomitizing fluids (−1.3‰ to −0.9‰ VSMOW) suggest an early-diagenesis origin for D1 at low temperatures of near-surface conditions. In contrast, the larger crystal size and microthermometric measurements of primary two-phase fluid inclusions (Th = 101.7 ± 8.0 °C) reveal that D2 precipitated during later stages at higher temperatures of mid-to deeper burial settings, which is supported by the high calculated δ18O values of D2 parent dolomitizing fluids (10.0‰–10.7‰ VSMOW). Petrographic examinations suggest that the vug- and fracture-filling D3 and coarse calcite cement (C3, up to 2 mm) precipitated at deeper burial conditions, which is consistent with the low δ18O (−5.2 ± 1.0‰ VPDB) and δ13C (−5.1 ± 2.6‰ VPDB) compositions of C3. The similarity in shale-normalized REESN patterns, strong negative Ce anomalies and lack of Eu anomalies, suggest a seawater origin for the parent dolomitizing fluids.