Controls on dolomitization in extensional basins: An example from the Derbyshire Platform, U.K.

Abstract

Fault-controlled dolomitization has been documented in Lower Carboniferous (Viséan) platform carbonates at various localities in the Pennine Basin and North Wales. The largest of these dolomite bodies (approx. 60 km2) occurs on the Derbyshire Platform, on the southern margin of the Pennine Basin. This study tests the hypothesis that dolomitization occurred at this locality during deposition, platform drowning, and the earliest stages of burial, coincident with the transition from a late syn-rift to post-rift regime. It also assesses the importance of syn-rift volcanism on dolomitization. Planar, fabric-retentive dolomite with single-phase (i.e., low temperature) fluid inclusions occurs along NW–SE and E–W oriented faults, and in platform margin facies and in proximity to the Masson Hill Volcanic Complex. Oxygen isotope data are consistent with dolomitization from seawater, but slightly depleted δ13C values reflect mixing with magmatic fluids. Volcanic activity is likely to have produced a thermal drive for fluid circulation on the platform margin, and post-depositional alteration of basalts by CO2-rich fluids could have led to alteration of olivine and release of magnesium to convecting seawater. Consequently, the large volume of dolostone on the southern margin of the Derbyshire Platform is attributed to the increased geothermal gradient and a localized increase in the Mg/Ca ratio of dolomitizing fluids at this locality, compared to elsewhere in the Pennine Basin. The results suggest that syn-rift carbonate platforms in volcanically active areas of rift basins have a greater potential for dolomitization from seawater than non-volcanic platforms in the same basin.

Volcanic events‐related hydrothermal dolomitisation and silicification controlled by intra‐cratonic strike‐slip fault systems: Insights from the northern slope of the Tazhong Uplift, Tarim Basin, China

“…Fault‐related hydrothermal dolomitisation model dependent on seawater geothermal convection along fault planes has been well reported (e.g., Breislin et al., 2020; Dong et al., 2020; Hirani et al., 2018; Hollis et al., 2017; Martín‐Martín et al., 2015). As suggested by several studies, some of the main faults formed during rift tectonism can breach the surface, along which seawater could flow downward into deep aquifers and convect within permeable layers (Hirani et al., 2018; Hollis et al., 2017; Rustichelli et al., 2017).…”

“…Massive mixing of magmatic and igneous derived fluids into dolomitising fluids may lead to depleted δ 13 C, even close to the normal mantle value (−9‰ to −4‰ PDB; Breislin et al., 2020). However, in contrast to δ 13 C values of primary dolomites precipitated in equilibrium with Early Palaeozoic seawater (−2.5‰ to 1.5‰; Veizer et al., 1999), Cd, Sd and Qd dolomites don't have the obviously negative values as expected, indicating high water‐rock interactions with adjacent host rocks or low mixing ratios of magmatic material.…”

“…As suggested by several studies, some of the main faults formed during rift tectonism can breach the surface, along which seawater could flow downward into deep aquifers and convect within permeable layers (Hirani et al., 2018; Hollis et al., 2017; Rustichelli et al., 2017). Permeable platform margin facies can also provide sufficient seawater for convection (Breislin et al., 2020). Under conditions of sustained supply of (modified) seawater, high thermal gradients and appropriate paleohydrology, fluid circulation can be achieved by a combination of vertical transport along faults and lateral transport through permeable layers (Hirani et al., 2018; Hollis et al., 2017; Martín‐Martín et al., 2015).…”

“…Thus, it is very likely that ultramafic carbonation could have served as a guaranteed magnesium and silica source for dolomitisation and silicification during the Permian magmatism. In addition to deep-seated hydrothermal fluids interacting with intrusive rock, dolomitisation and silicification fluids may have been derived from a mixture of magmatic fluid and hydrothermal fluids in certain ratios, as suggested by some recent publications (e.g., Breislin et al, 2020;Dong et al, 2020).…”

Understanding controls on hydrothermal dolomitisation: insights from 3D reactive transport modelling of geothermal convection

“…The geochemical and isotopic signatures of the resulting dolomites would reflect the component of basement-derived fluids. We might expect, for instance, to see mixing trends between a 87 Sr / 86 Sr signature of downwelling fluids reflective of the seawater values, ∼ 0.7068-0.70907 (Burke et al, 1982), and more radiogenic basinderived fluids. Fluid inclusion and isotope analysis from several studies suggest that the dolomitising fluid might be formed by an evolved seawater or its interaction with rocks and/or other fluids (Gomez-Rivas et al, 2014;Jacquemyn et al, 2014;López-Horgue et al, 2010).…”