Awais, Muhammad (2024) Testing models of dolomitization on large scale Mesozoic outcrops from the Sorrento Peninsula (Italy). [Tesi di dottorato]

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Abstract

The present research work is focused on three research problems: (1) an experimental procedure has been tested for differential dissolution of calcite from mix lithologies (e.g., calcareous dolomites, partially dolomitized limestone) to acquire dolomite only for subsequent stable isotopes analysis, (2) testing reflux dolomitization in the Early Cretaceous carbonates of the Lattari mountains (southern Apennines, Italy), and (3) some late saddle dolomites were found, and their presence provided the possibility to test using U-Pb dating the previous model of single episode late stage, fracture-related dolomitization. An integrated approach has been deployed to assess the aforementioned research problems, including field work, petrography, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), Electron Microprobe analysis (EPMA), XRD, pXRF, stable (δ13C and δ18O) and Sr-isotopes (87Sr/86Sr) analyses, and U-Pb dating. (1) An experimental study has been conducted to test dissolution of calcite in carbonates (mix of calcite and dolomite i.e., partially dolomitized limestones, calcareous dolomites, etc) to acquire dolomite only. The study is an integration of experimental work with X-Ray Diffraction (XRD) and stable isotopes (δ13C and δ18O) analyses. In the present study, calcite (Carrara Marble), dolomite and synthetic mixtures (i.e., Calcite10%-Dolomite90%, Calcite30%-Dolomite70%, Calcite50%-Dolomite50% and Calcite70%-Dolomite30%) are used. These samples are experimentally tested with EDTA-acid (0.3M and 0.5M) and acetic acid buffer (1.75M and 3.5M) solutions. The experimental work and XRD analysis revealed less calcite is dissolved by 0.3M EDTA solution and more calcite is dissolved by 1.75M acetic acid buffer. The former solution is least reactive, and the latter is more aggressive and dissolved almost all the calcite of synthetic mixed samples. Similarly, the results show a gradual shift in dolomite percentage and isotopic values from calcite via mixtures towards dolomite in experiments using 0.3M EDTA to 1.75M acetic acid buffer solutions. The stable isotopes (δ13C and δ18O) cross-plots are demonstrating no obvious effect of different chemical treatments on isotopic signatures although some exceptions exist. Among the tested different solutions, 1.75M acetic acid buffer is more effective and suitable for dissolving calcite and acquiring dolomite only for subsequent geochemical analyses. (2) In the Cretaceous carbonates of the Lattari mountains, the intensity of dolomitization varies across the stratigraphy and is preserved as partially dolomitized limestones and completely dolomitized unit(s). The studied stratigraphic intervals include C (upper Barremian-lower Aptian) and B (lower Barremian-upper Hauterivian) intervals. The studied carbonates successions are made up of multiple stacked cycles. The C-interval consist of alternations of limestones and dolomites, and the B-interval is almost completely dolomitized unit. All sections of the C-interval consist of medium (Mdol) and coarse (Cdol) crystalline dolomites except Castello-C which contain Mdol only. Mdol and Cdol’s of the C-interval are usually Ca-rich non-stoichiometric having low degree of order (except for a couple of samples having Cdol’s with moderate degree of order) and few nearly stoichiometric dolomites. Mdol’s of the B-interval consist of Ca-rich stoichiometric, nearly stoichiometric and stoichiometric dolomites with low-moderate degree of order. The Cdol’s of the B-interval are nearly stoichiometric to stoichiometric with low-moderate degree of order. Zoned (dolomite crystals with cloudy cores and limpid rims) and corroded dolomites (worn-out crystals) are noticed in both C- and B-intervals. In the C2-interval the dolomite stoichiometry and degree of order varies from Faito-C through Moiano-C to Castello-C. These observations imply that the source of dolomitizing fluids was closer to the Faito-C Section with respect to the Castello-C Section. The geometry of beds/cycles, lithological variations, distribution of dolomites laterally and vertically, dolomites petrography and geochemistry indicate reflux dolomitization by downward and basinward flowing mesohaline fluids. The C-interval has experienced few episodes of refluxing brines due to limited occurrence of zoned dolomites. In the C-interval, the geological and geochemical characteristics of cycle 3 (selective partial dolomitization) indicate active reflux, and the dolomite mosaics (i.e., pervasive dolomitization) in cycles 1 and 2 might indicate latent reflux described by Jones et al. (2002). The B-interval is not completely dolomitized unit but pervasively dolomitized unit due to presence of calcite inclusions and micritic matrix. Same is also applied for the dolomite mosaics having zoned dolomites (cycles 1 and 2) of C-interval. The overdolomitization in Cdol’s (limpid rims on cloudy cores) is common in the B-interval indicating multiple episodes of refluxing brines. Consequently, there is superimposition of geochemical trends in single cycles (which can be seen in the C interval). This indicates that repeated lateral flow of dolomitizing fluids in the B interval have obscured vertical trends and the only variations are associated to type of dolomites. (3) The U-Pb dating of sparry saddle dolomites found in veins and vugs of the Mesozoic (Upper Triassic, Lower Jurassic and Early Cretaceous) carbonates of the Lattari mountains (southern Apennines, Italy) has been conducted. Petrographically, the dolomite crystals are very coarse (sometimes typical saddle dolomites), euhedral and granular anhedral, geodic, having sweeping extinction and occupied irregular vugs and fractures and is usually followed by poikilotopic calcite precipitation. The U-Pb data provided four main clusters of dates for saddle dolomites: Late Jurassic (160 Ma), Albian-Cenomanian (100-90 Ma), Paleogene (>70, ~65, 50 and 35 Ma), and two clusters for calcite: Miocene (11.5 Ma and 10 Ma) and Pliocene (4 Ma). Each age corresponds to well established extensional phases affecting the Apennine Carbonate Platform on a regional scale: the formation of intraplatform restricted basins during the Mesozoic and the pre-orogenic extension of the platform. The U-Pb data suggest that each of these tectonic events promoted fluid flow, and related hydrothermal dolomitization. Consequently, a proposed model implies the circulation of warm (hydrothermal) dolomitising fluids through Mesozoic and Paleogene extensional faults in the Apenninic platform.

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