Fault-controlled dolomitization in the Montagna dei Fiori Anticline (Central Apennines, Italy): record of a dominantly pre-orogenic fluid migration

Abstract

Abstract. The Lower Jurassic platform and basinal deposits exposed in the Montagna dei Fiori Anticline (Central Apennines, Italy) are pervasively affected by dolomitization. Based on the integration of field work, petrography, and geochemistry, two fault-related dolomitization events were recognized and interpreted as having occurred before and during the Apenninic orogeny. Fluid inclusion analysis indicates moderate to elevated salinity values of 3.5 to 20.5 and 12.8 to 18.6 eq. wt % NaCl in the first and the second event, respectively. The estimated salinities, in combination with δ18O values and 87Sr∕86Sr ratios, suggest significant involvement of evaporitic fluids in both events, most likely derived from the underlying Upper Triassic Burano Formation. In addition, the 87Sr∕86Sr ratios up to 0.70963 suggest the circulation of deep-sourced fluids that interacted with siliciclastic rocks and/or the crystalline basement during the dolomitization events. Two major dolomite types (D1 and D2) were recognized as pertaining to the first event, both postdated by high-amplitude bed-parallel stylolites, supporting a syn-burial pre-layer-parallel shortening dolomitization. A possible geodynamic framework for this dolomitization event is Early Jurassic to Late Jurassic rift-related extensional tectonism. The second dolomitization event (D3, D4, and D5) is characterized by a temperature upturn (up to 105 ∘C) and interpreted as associated with the inflow of hydrothermal fluids, possibly related to major changes in the permeability architecture of faults during early- to syn-thrusting and folding activity. Based on the timing of deformation in the Montagna dei Fiori Anticline, the second dolomitization event likely occurred in Late Miocene to Pliocene times. The findings regarding characteristics and timing of dolomitization here illustrates the long-term controlling role of the evaporitic detachments in the dolomitization process. This study shows that the Mg-rich fluids that were most likely derived from evaporites may prime the tectonically involved successions for repeated dolomitization, and hence the formation of potential reservoirs during sequential tectonic modifications (extensional vs. compressional).

Regional-scale paleofluid system across the Tuscan Nappe – Umbria Marche Arcuate Ridge (northern Apennines) as revealed by mesostructural and isotopic analyses of stylolite-vein networks

“…6c-d). Previously published isotopic and thermometric data for contractional fluids flow in the easternmost part of the UMAR reported infill of hydrothermal (100°C) dolomitizing fluid flow during contraction (Mozafari et al, 2019;Storti et al, 2018). These hydrothermal dolomitizing fluids have the same range of signatures of δ 18 Ofluids than the ones precipitating at the thermal equilibrium we document in the other folds of the UMAR (except monte Subasio).…”

“…During folding, we also document the hydrothermal fluids, but also fluids characterized by negative δ 18 Ofluids that precipitated at a temperature consistent with predicted depth, interpreted as an input of meteoric water through fractures in the reservoir. Note that the hydrothermal dolomitizing fluids documented at the Montagna dei Fiori (Mozafari et al, 2019;Storti et al, 2018); have an isotopic signature much lower (6‰ SMOW) than the ones from the fluids involved in the Monte Subasio and Monte Corona (15‰ SMOW), supporting that a different fluid system was prevailing in this part of the belt during LPS, folding and LSFT.…”

Regional-scale paleofluid system across the Tuscan Nappe–Umbria–Marche Apennine Ridge (northern Apennines) as revealed by mesostructural and isotopic analyses of stylolite–vein networks

“…First, mass spectrometer nonlinearity was corrected by applying a "pressure baseline correction" (Bernasconi et al, 2013). Next, the 47 results were projected in the absolute reference frame or carbon dioxide equilibrated scale (CDES; Dennis et al, 2011) based on routinely measured ETH1, ETH2, ETH3, ETH4 and Carrara marble (ICM) carbonate standards (Meckler et al, 2014;Muller et al, 2017). The last correction to the raw 47 was to add an acid correction factor of 0.082 ‰ to obtain a final 47 CO 2 value (Defliese et al, 2015).…”

“…10). A hydrothermal dolomitizing fluid migration event was documented in the southeastern part of the UMAR (Montagna dei Fiori) and interpreted as vertical fluid migration from deeper Jurassic reservoirs (Mozafari et al, 2019;Storti et al, 2018). But the reconstructed fluid temperatures (100 • C) and δ 18 O fluids (6 ‰ VSMOW) are still much lower than the ones reconstructed from the fluids involved in Monte Subasio (105-140 • C; 9 ‰ to 15 ‰ VSMOW), supporting that a different fluid system was prevailing in the westernmost part of the belt during layer-parallel shortening, folding, and late-stage fold tightening compared to central-eastern UMAR.…”

“…In other cases, mineralization records an infill of meteoric fluids flowing downward or of hydrothermal fluids (i.e. hotter than the host rock they precipitated in) flowing upward, either from the basin or the basement rocks through large-scale faults or décollement (Roure et al, 2005;Vandeginste et al, 2012;Cruset et al, 2018;Lacroix et al, 2011;Travé et al, 2000Travé et al, , 2007De Graaf et al, 2019;Callot et al, 2010;Beaudoin et al, 2011Beaudoin et al, , 2014Bertotti et al, 2017;Gonzalez et al, 2013;Lucca et al, 2019;Mozafari et al, 2019;Storti et al, 2018;Vannucchi et al, 2010).…”