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.

INTRODUCTION

Geothermal convection of seawater can lead to dolomitization on carbonate-platform margins (e.g., Wilson et al. 2001; Whitaker and Xiao 2010). Recently, interest has grown in the relationship between dolomitization and volcanism. In particular, fault-controlled dolomitization has been related to syn-rift igneous activity on the Latemar platform (e.g., Zheng 1990; Carmichael and Ferry 2008; Blomme et al. 2017; Jacquemyn et al. 2017) the Levant region (Nader et al. 2004), and West Point Formation, Quebec (Lavoie et al. 2010). However, the contribution of syn-rift volcanic activity on dolomitization is still uncertain. For example, although heat flow is increased during volcanism, facilitating geothermal convection, submarine extrusion of lava can result in a net removal of magnesium from seawater through the formation of chlorite (Reed and Palandri 2006). Recent geochemical modeling by Robertson et al. (2019), however, has shown that ultramafic carbonation can substantially increase the Mg/Ca ratio of fluids, but the extent to which this increases dolomitization potential in nature has not been evaluated.

In the Pennine Basin and North Wales, UK, fault-controlled dolostone bodies are developed on the margins of Mississippian carbonate platforms that grew on the rotated footwalls of normal faults and a basement of lower Paleozoic metasediments (Fig. 1) that were reactivated during Carboniferous extension and basin inversion during the Variscan Orogeny (Fraser and Gawthorpe 2003). Dolostone is also observed in basinal and platform carbonate sediments (Gawthorpe 1987; Hollis and Walkden 2012). Although the size and shape of these geobodies have been consistently described in the literature (Fowles 1987; Gawthorpe 1987; Ford 2002; Bouch et al. 2004; Hollis and Walkden 2012; Frazer 2014; Frazer et al. 2014; Juerges et al. 2016), numerous dolomitization models have been proposed to explain their formation. Most of these dolomite bodies have been interpreted to have formed from basinal brines, expelled during post-rift thermal subsidence and basin inversion (e.g., Schofield and Adams 1986; Gawthorpe 1987; Bouch et al. 2004; Hollis and Walkden 2012; Frazer et al. 2014; Juerges et al. 2016).

The Derbyshire Platform, located at the southern margin of the Pennine Basin (Figs. 1, 2), comprises Lower Carboniferous (Viséan) platform carbonates deposited in a back-arc extensional regime (Fraser and Gawthorpe 2003). The Derbyshire Platform hosts the largest volume of dolostone of all the platforms in the Pennine Basin, estimated by Ford (2002) to be 50 km2 of dolostone, compared to approx. 8 km2 on the North Wales Platform (Juerges et al. 2016) and approx. 20 km2 on the Askrigg Platform and Craven Basin (Hollis and Walkden 2012). Despite its volume, however, there is still a lack of consensus as to the mechanism that formed this large dolostone geobody. Fowles (1987) proposed dolomitization occurred by downward-seeping brines in the Permo-Triassic whereas Schofield and Adams (1986), Ford (2002), and Hollis and Walkden (2012) proposed dolomitization from basinal brines expelled from juxtaposed hanging-wall basins.

Based on models of fluid flow and reactive transport models (RTM), Frazer (2014) and Frazer et al. (2014) recognized that while sufficient fluid volumes for dolomitization could have been supplied from basinal sediments in the hanging wall, such fluids probably contained insufficient magnesium to explain the observed volume of dolostone. Frazer (2014) showed that dolomitization could have occurred by geothermal convection of seawater along faults during platform growth, but to date there has been no petrographical or geochemical evidence to evaluate this hypothesis. As such, the current paper focuses on the first phase of dolomitization on the Derbyshire Platform and aims to a) determine the timing of the first phase of dolomitization, b) establish the source of fluid for dolomitization and c) evaluate the relationship between volcanism and dolomitization on the Derbyshire Platform.

GEOLOGICAL SETTING

Stratigraphy

During the Viséan, back-arc extension took place across the Pennine Basin, which was located north of the developing Variscan Orogen, with topography controlled by NE–SW, SW–NE trending faults (Fraser and Gawthorpe 2003). During this period, carbonate platforms developed on the footwalls of these reactivated Caledonian basement faults (Fraser and Gawthorpe 2003). The juxtaposed hanging-wall basins were filled progressively by pelagic and transported sediment during the Viséan–Serpukhovian. Post-rift thermal subsidence led to burial of the carbonate platforms to depths of 1–2 km beneath Bashkirian–Moscovian fluvio-deltaic systems that developed across a wide peneplain (Guion and Fielding 1988; Kelling and Collinson 1992). The entire basin was inverted in the Late Carboniferous to Permian, with the onset of the Variscan Orogeny (Fraser and Gawthorpe 2003).

The focus of this study is the southern margin of the Derbyshire Platform, which is composed of Viséan platform carbonates and syn-depositional, extrusive volcanics (Figs. 1A, 2A). The oldest exposed platform carbonates comprise the Holkerian Woo Dale Limestone Formation, a 105-m-thick succession of massive, fined grained, porcelaneous limestones (Fig. 1B). Overlying the Woo Dale Limestone Formation, the Asbian Bee Low Limestone Formation is composed of 75 m of skeletal packstone and grainstone. The Brigantian Monsal Dale Limestone Formation is at least 75 m thick, and is composed of gray to dark gray skeletal wackestone and packstone often containing chert (Aitkenhead and Chisholm 1982; Chisholm et al. 1983) (Fig. 1B). The limestone beds are separated by several volcanic tuff layers, one of which reaches 0.8 m in thickness at Masson Hill area (Ford 2001; Fig. 2B). The youngest carbonate sediments on the platform are limestones of the Eyam Limestone Formation, also of Brigantian age and ranging from 20 to 100 m in thickness. They are formed of thin, shelly limestones with shale partings with localized mud mounds (Ford 2002). The Viséan succession is overlain by the Serpukhovian-aged Millstone Grit Series, the total thickness of which is approximately 500 m. The total Upper Carboniferous cover is at least 1500 m and may have exceeded 2000 m (Smith et al. 1967; Aitkenhead et al. 1985).

Volcanism

Igneous activity during the Carboniferous was widespread across the UK from the early Viséan to the late Moscovian associated with lithospheric stretching (McKenzie 1978; Macdonald et al. 1984). On the Derbyshire Platform, extrusive volcanism was active in the Viséan (345–325 Ma) followed by intrusion of sills in the Serpukhovian (325–310Ma) (Macdonald et al. 1984). On the Derbyshire Platform, extrusive volcanism was concentrated around two main eruptive centers, Masson Hill and Miller's Dale (Macdonald et al. 1984). The southern volcanic center, concentrated at Masson Hill, has two major associated lavas; the Lower Matlock Lava (331–329 Ma), which forms the stratigraphic boundary between the Asbian and Brigantian, and the Upper Matlock Lava (330.9–329 Ma) (Macdonald et al. 1984; Fig. 2B). Away from the volcanic center, volcanic ash is present along karstic surfaces (Walkden 1972). The Lower Matlock Lava is a composite of several lava flows and tuffs, overlying the Bee Low Limestones (Chisholm et al. 1983) and is often taken as the boundary between the Asbian and Brigantian (Figs. 1B, 2B). The combined flows are about 45 m thick, but they reach 90 m thick at Masson Hill (Dunham 1952; Ixer 1975). Here, the lava appears to be in contact with the Bonsall dolerite intrusion with the thickness of the lava decreasing to the north and south from Masson Hill (Ford 2001). The Upper Matlock Lava lies in the center of the Monsal Dale Limestones, reaching 36 m in thickness (Smith et al. 1967; Ixer 1978) and averages 10–15 m elsewhere (Hurt 1968).

The geochemistry of the lavas indicates that volcanism was intra-plate and of tholeiitic affinity and they are composed of highly altered, fine-grained, vesicular, olivine-phyric and aphyric basalt (Walters and Ineson 1980; Macdonald et al. 1984). Both the lavas and the sills are enriched in light rare earth elements (LREE) in comparison to the heavy rare earth elements (HREE), with chondrite-normalized Ce (CeN) = 19–70 and YbN = 6–8. (Macdonald et al. 1984). Alteration of the primary igneous mineralogy probably resulted in the mobilization of Si, Ca, Mg, Na, K, Ba, Rb, and Sr, whilst Zr, Nb, Y, Ti , and P are interpreted to have been stable during alteration (e.g., Pearce and Cann 1973; Floyd and Winchester 1975; Pearce and Norry 1979; Macdonald et al. 1984).

Intrusive sills in the Masson Hill area, the Ible and Bonsall sills, are dated as Serpukhovian (325–310 Ma) and are medium to coarse-grained olivine dolerites (Macdonald et al. 1984). They commonly have serpentinized olivine phenocrysts, ophitic intergrowths of clinopyroxene (augite) and plagioclase (labradorite), and an absence or rarity of vesicles and amygdales (Smith et al. 1967; Macdonald et al. 1984). Accessory minerals include Fe-Ti oxide and apatite in a microcrystalline matrix, with plagioclase and secondary chlorite and accessory nepheline recorded from the Bonsall Sill (Smith et al. 1967).

Diagenetic Framework of the Derbyshire Platform

The Viséan succession of the Derbyshire Platform displays a complex but well-constrained diagenetic history (Gutteridge 1987, 1991; Walkden and Williams 1991; Hollis 1998; Hollis and Walkden 2002). Four principal phases, or zones, of calcite cementation (Zones 1–4, as defined by Walkden and Williams 1991) were described from carbonate matrix porosity (Zones 1–4) and fractures (Zone 3–4) (Walkden and Williams 1991; Hollis and Walkden 2002). Zones 1–2 precipitated from meteoric water in the freshwater lens during platform growth, followed by Zone 3, which is interpreted to have precipitated in the shallow-burial realm from groundwater that was driven down depositional dip by topographic flow from the East Midlands Platform during the Serpukhovian (Walkden and Williams 1991). Zone 3 is volumetrically the most important cement phase, occluding macroporosity, and is succeeded by Zone 4 calcite which is interpreted to have co-precipitated with hydrocarbon, fluorite, sulfates and sulfide mineralization (Hollis and Walkden 1996,2002; Hollis 1998).

Dolomite on the Derbyshire Platform occurs in the Holkerian Woo Dale Limestone near the northern to central part of the platform (Schofield and Adams 1986), and as two distinct geobodies in the Asbian Bee Low Limestone and Brigantian Monsal Dale Limestone on the southern Derbyshire Platform (Fig. 2A, C). One of these bodies is near to the southern platform margin, while the other is elongate, following the NW–SE Cronston–Bonsall fault in the west, and associated with the Masson Hill volcanic center in the east (Fig. 2). A zone of dolomite has also been identified within the basinal succession to the southwest of the Derbyshire Platform (Fig. 2A), as described by Breislin (2018).

The Carboniferous limestone of the Derbyshire Platform hosts economic quantities of galena, fluorite, and barite mineralization, classified as Mississippi-Valley-Type (MVT) ore deposits (Ixer and Vaughan 1993; Ford and Quirk 1995). Minerals are hosted in matrix porosity, faults, and fractures in dolostones and limestones. The presence of ore bodies in igneous rocks and localized beneath lavas (Walters and Ineson 1980) suggests that the igneous bodies acted as aquitards, as well as hosts to the mineralization (Walters and Ineson 1980; Ford 2002). Mineralization in the Lower Carboniferous of the Derbyshire Platform commonly occurs at dolostone–limestone boundaries, suggesting a spatial association of dolostone reaction fronts and mineralization (Ford 2002). A conceptual model of fluid flow as a result of overpressure release and seismic valving of fluids from the hanging-wall basins onto the Derbyshire Platform has been proposed based on the distribution of fracture-controlled calcite cementation and associated hydrocarbon, galena, fluorite, and barite mineralization (Coleman et al. 1989; Ixer and Vaughan 1993; Hollis and Walkden 2002).

METHODS

Rock samples were collected from fifteen outcrops, five mines, and three boreholes (Fig. 2A) in the Asbian to Brigantian succession of the Matlock area. The area was geologically mapped and logged, with detailed petrographic analysis of thin sections prepared from samples. Each locality and core was sampled systematically to cover the stratigraphic range and diversity of lithofacies and logged to record texture and composition, sedimentary features, skeletal content, and diagenetic features. A total of 53 polished thin sections were prepared from resin-impregnated subsamples (Table 1).

All thin sections were examined using optical transmitted-light and cathodoluminescence (CL) microscopy and scanned using an EPSON Perfection V600 Photo at 2400 dpi resolution. Optical microscopy was undertaken using a Nikon Eclipse LV100NPOL microscope fitted with a Nikon DS-Fi2 camera. Dolomite textures were described according the scheme of Sibley and Gregg (1987). CL observations were made using a CITL CCL 8200 Mk3 cold optical cathodoluminescence system coupled with a Progress C10 Laser Optik digital photographic system. Operating conditions for CL were set to 10 kV and 300 μA at a pressure of c. 0.2 Torr and maintained by microcomputer control.

Fluid-inclusion screening was conducted on selected dolomite samples using a Nikon Eclipse LV100NPOL microscope to determine the phases present in primary and pseudo-secondary inclusions, and their distribution in crystals. Based on the observation that the primary inclusions contain single-phase fluid inclusions, fluid-inclusion microthermometry was not conducted.

Standard X-ray diffractometry (XRD) was conducted on powders, acquired using a precision drill. These were then prepared following the methods of Charlier et al. (2006). The analysis was carried out using a Bruker D8Advance diffractometer at the University of Manchester. A tube voltage of 40 kV and a tube current of 40 mA, with a step size of 0.02° and time constant of 2.00 seconds was employed, using CuKα radiation. Semi-quantitative estimates of bulk mineralogy fractions were carried out using peak-area measurements (sensuSchultz 1964). All results contain an analytical error of 0.5%. Dolomite stoichiometry (%mol MgCO3) was calculated using the methodology of Jones et al. (2001) to determine the amount of calcium in excess of stoichiometric (Ca50Mg50) based on the position of the dolomite 104 reflection.

Inductively coupled plasma mass spectrometry (ICP-MS) analysis was conducted on powdered samples of host-rock limestone, bulk dolostone, and individual cement zones. Powders were digested in 2 ml of 20% HCl before dilution to 2% HCl. In addition, two blank samples were prepared in the same way to ensure reproducibility. Corresponding samples were drilled from hand specimens and their mineralogy checked using XRD to ensure no contamination in dolostone samples by calcite cements. REEs and Y concentrations were measured using ICP-MS with an Agilent 7500cx mass spectrometer. Samples were run against internal standards and reproducibility of 0.1‰ (1σ). All samples are reported in mg/l or ug/l (ppm or ppb). All REE results have been PAAS normalized following the methods of Nance and Taylor (1976).

Stable oxygen and carbon isotope analysis was conducted on powdered samples of dolostone, limestone, brachiopods, and discrete cement phases, acquired using a tungsten-tipped dentist drill. The mineral composition of the samples was assured by XRD analysis of a small amount of powder to ensure that there was no analysis of mixed calcite–dolomite samples. Samples of calcite were reacted (to completion) with phosphoric acid under vacuum, at temperatures of 25° C (sensuMcCrea 1950). Gases were measured by dual-inlet, stable-isotope-ratio mass spectrometry using a VG SIRA10 mass spectrometer. Isotope ratios were corrected for 17O effects following the procedures of Craig (1957). In order to calculate precipitation temperatures and fluid δ18O compositions, the equilibrium oxygen isotope fractionation of Friedman and O'Neil (1977) was used for calcite samples and the equilibrium fractionation factor of Matthews and Katz (1977) was used for dolomite samples. Samples were run against an internal standard, and all results were reproducible to ± 0.1‰ (2σ). All isotope data are reported relative to the VPDB calcite standard.

Strontium isotope analysis was conducted on powdered whole rock dolostone and discrete dolomite cements obtained using a tungsten-tipped micro-mill following the methodology of Charlier et al. (2006) and Pollington and Baxter (2010). Sample mineralogy was checked by XRD. Sr (Oak Ridge 84Sr, 1.1593 ppm) and Rb spikes were added to each sample to boost the signal. Strontium was extracted from the rock powders for isotopic analysis using Sr Spec extraction chromatographic resin. Column preparation and chemical separation were conducted following the methods of Deniel and Pin (2001) and Charlier et al. (2006). Before analysis, the Sr samples were mixed with tantalum (Ta) emitter solution, loaded on to single Rhenium (Re) filaments and subjected to a 2A current. Sr isotope ratios were measured on the British Geological Survey (NIGL) Thermo-Finnigan Triton TIMS instrument in static collection mode. An internal NBS 987 standard was used for calibration with a reproducibility of + 0.1%0 (2σ). During TIMS analysis, spiked samples are run in the same fashion as unspiked samples. However, the results of spiked samples were corrected, off-line, for spike contribution. All results are reported to the NBS 987 standard.

RESULTS

Distribution of Dolostone

Both fabric-selective, stratabound dolostone and fabric-destructive, non-stratabound dolostone were observed on the Derbyshire Platform. Five unique dolomite textures are identified (D1–D5; Table 1, Fig. 3). Phases D1–D3 are replacive phases; D1, the principal focus of the current study, is the oldest phase and is cross-cut and replaced by D2, which is itself cross-cut by D3. Phases D4 and D5 are dolomite cements that are identified petrographically. Based on the mapped distribution of D1 from outcrop and core, it is estimated to form 50% of the total volume of dolostone on the Derbyshire Platform (Fig. 2C). The thickest section of D1 is observed at the platform margin in Golconda Mine, where it is approximately 120 m thick. Away from the platform margin, on the platform top, the thickness of D1 decreases to an average of around 40 m (Fig. 2C).

D1 dolostone occurs in platform-margin grainstones and packstones and commonly occurs along or near to, the southern margin of the Derbyshire Platform (Fig. 2A). In outcrop, D1 is buff-gray, with a sucrosic texture. It is fabric preserving, with retention of bedding, cross-lamination, and bioturbation (Fig. 4). D1 dolostone is best exposed at Harborough Rocks, where an upward-shallowing succession of facies has been pervasively dolomitized; a basal layer of slumped and dewatered beds is overlain by pervasively bioturbated beds (by Thalassinoides) and capped by crinoidal and coral-bearing grainstones. Vuggy and moldic porosity is abundant, and D1 dolostone is cross-cut by stylolites. Elsewhere, the dolostone is occasionally overprinted by D2 and D3 (Figs. 4A, 5A). Reaction fronts are diffuse (Fig. 4A), with patchy dolomite bodies ranging from 5–30 cm in diameter, occurring up to 3 m away from the main dolostone body.

D1 dolostone also occurs in isolated reef mounds in the platform interior at Wyns Tor and Grey Tor and adjacent to extrusive Viséan lavas in the Matlock Volcanic Complex (Fig. 2B). In the Matlock area, D1 is strongly stratabound, preferentially forming both above and below the Upper and Lower Matlock Lavas, and above tuffs (Fig. 4C, D), as well as above or below exposure surfaces. Galena–fluorite–barite–calcite mineral assemblages are observed at the contact between dolostone and limestone, as well as along exposure surfaces. In Wapping Mine, at the center of the Matlock Volcanic Complex, dolostone replaces limestone, and white, opaque dolomite cement also occurs within a fault breccia along an E–W oriented fault (Fig. 5B, C). At Cromford (Fig. 5A), within the Masson Hill area, several E–W oriented faults formed as splays to the NW–SE Cronkston–Bonsall fault, with horizontal slickensides providing evidence of lateral movement. These E–W faults host D1 dolostone, including dolomite-cemented breccias. Dolomite cement is cross-cut by calcite, fluorite, barite, and minor galena, with slickensides that indicate strike-slip movement.

The structural lineaments on the Derbyshire Platform are predominantly NW–SE and NE–SW oriented crustal faults, with associated E–W and N–S trending faults (Figs. 2, 3). The Cronkston–Bonsall fault is a major NW–SE trending, deep-seated crustal fault, which extends approximately 20 km from the NW of the Derbyshire Platform (near the town of Buxton), to the Masson Hill area in the SE (Fig. 2). It is slightly sinuous and locally compound in nature, with several parallel faults and associated splays. Replacement of the host rock by fabric-destructive, non-stratabound (D2) dolostone with non-planar textures occurs along the length of the Cronkston–Bonsall Fault.

N–S faults cross-cut the E–W oriented faults, and are surrounded by a halo of non-fabric selective, non-planar dolostones up to 50 cm wide, and classified as D3 dolostone (Fig. 5A). At the intersection of E–W and N–S faults, large cavities up to a meter in diameter are filled with calcite, fluorite, and barite. Brecciation is commonly observed along these N–S faults, with the matrix commonly hosting galena and fluorite (Fig. 5C). The N–S faults also display slickensides, indicating strike-slip movement, and have greater quantities of sulfide and fluorite mineralization than the E–W oriented faults.

Petrographic and Crystallographic Characteristics of D1 Dolostone

In thin section, D1 dolostone is fabric destructive, with dolomite crystal size preserving a subtle fabric in packstone facies; in grainstone facies, dolomitization is fabric destructive in thin section. D1 is composed of clean, limpid, planar-e to planar-s dolomite (sensuSibley and Gregg 1987). The coarsest dolomite crystals (up to 1.45 mm diameter) are in allochem-rich grainstones towards the platform margin, whereas crystal size is smallest (approx. 0.1 mm) where replacement of micrite matrix occurred. Crystals become more euhedral as crystal size increases. Fluid inclusions in these crystals are single phase. Where dolomitization is incomplete, individual dolomite crystals occur in micritized allochems; where dolomitization is pervasive, dolomite crystals form a lining to the base of crinoid molds (Fig. 6B). The dolomite has a distinct yellow-orange cathodoluminescence, with small, red luminescent inclusions (Fig. 6B). Where recrystallization by D2 and D3 dolomite has occurred, non-planar textures are more common, alongside a change from orange to red luminescence in CL. Dull luminescent, sparry, pore-filling calcite cement (Zone 3 calcite cement of Walkden and Williams 1991) is not observed in D1 dolostone, although it is present in minor volumes within adjacent limestones (Fig. 6C, D).

Volcanics

Lava samples were taken from Temple Mine, Gratton Dale, and Gurdall Wensley borehole (Fig. 7). The lavas from Temple Mine in the Masson Hill Volcanic Complex are highly altered, with less intense and very minor alteration observed in the samples from Gurdall Wensley and Gratton Dale, respectively. The lavas are commonly vesicular, fine-grained olivine basalts containing amygdales of carbonate or chlorite (Fig. 7A–D). Where unaltered, the primary igneous mineralogical assemblage is composed of olivine, clinopyroxene, orthopyroxene, and plagioclase. The lavas are highly brecciated and cut by calcite-cemented fractures.

Petrographical and XRD analyses shows that primary olivine and pyroxene has been altered to clay minerals, (illite, chlorite–smectite, and kaolinite), calcite, Fe-oxide, chlorite, quartz, and minor pyrite (Fig. 7C) with pyroxenes commonly replaced by chlorite or calcite (Fig. 7B). This alteration is evidenced in hand specimen by a change from dark gray-green to a paler and greener color (Fig. 7A). Calcite cement occludes amygdales, and has replaced the matrix of the lava as anhedral, poikilotopic crystals up to 2 mm in diameter. There are monomineralic chlorite reaction rims around these calcite-filled amygdales, with the calcite displaying metasomatic textures (Fig. 7D). Under CL, the calcite has a yellow-orange color.

Silicification and Mineralization

Silicified limestone was observed on the Masson Hill anticline, on the southeast platform margin (Fig. 2), as well as parallel to an E–W oriented fault at Gratton Dale. These localities are proximal to the Viséan extrusive volcanic horizons at Gratton Dale and Masson Hill (Fig. 2). In the field, silicification is not facies selective, but occurs as discrete bodies, up to 5 m in diameter, located at the apex of the Masson Hill anticline, and close to faults. The silica is replacive, forming an anhedral mosaic composed of 0.5-mm-size crystals, where it is most pervasive. There is usually a diffuse boundary between silicified and unaltered limestone (Fig. 7E). Where silicification is incomplete, individual crystals are euhedral and finely crystalline (0.1–0.3 mm) (Fig. 7F). Although replacive silica and D1 dolostone are not observed together, intercrystalline porosity and fractures containing fluorite, barite, and calcite, cross-cutting silicified limestone and D1 in horizontal, sub-vertical and vertical fractures, are seen (e.g., Golconda Mine, Temple Mine, High Peak Quarry, Masson Cavern in Fig. 2), particularly at the boundary between dolostone and limestone. Therefore, both D1 dolomite and silica predate mineralization, but the paragenetic relationship between D1 dolostone and silica is equivocal.

Geochemistry

D1 dolostone is nearly stoichiometric with MgCO3 = 47.9 to 51.2%, with an average of 50.5%. Isotopic data is summarized in Table 3. The isotopic signature of unaltered brachiopod shells ranges from δ13C = 0.3 to 0.8‰ VPDB and δ18O = –4.1 to –4.2‰ VPDB (n = 2) and whole-rock limestone from δ13C = 1.4 to 0.8‰ VPDB and δ18O = –5.2 to –3.2‰ VPDB (n = 5). Dolostone ranges from δ13C = –2.7 to 0.8‰ VPDB and δ18O = –2.8 to –5.01‰ VPDB (n = 23). The mean δ13C values (δ13C = –0.6‰ VPDB, n = 23) of the dolostone are more depleted than those of whole-rock limestone and brachiopods (δ13C = 2.2‰ and 0.5‰ VPDB, respectively). There is a spatial variation in δ18Odolostone, with more depleted values (δ18O = –4.1‰ VPDB, n = 11) in the Matlock area, proximal to the volcanic center and extrusive volcanics, compared to the platform margin (δ18O = –3.4‰ VPDB, n = 12). No spatial trend is observed in the δ13C of the dolostone. The dolomite cement taken from the E–W fault breccia has one of the lowest mean δ13C and δ18O values of all the investigated dolostones (δ13C = –1.52‰ VPDB, δ18O = –5.01‰ VPDB, respectively) and dolomitized wall rock adjacent to this fault also has a light isotopic signature (δ13C = 0.28‰ VPDB, δ18O = –4.79‰ VPDB). Calcite cement precipitated in amygdales in the lavas are more depleted than Viséan seawater (δ13C = –1.6‰ VPDB and δ18O = –3.0‰ VPDB).

Unaltered brachiopod shells have 87Sr/ 86Sr_tr 0.7078 ((± 0.00004) that matches Viséan seawater (Burke et al. 1982). Average dolostone 87Sr/ 86Sr = 0.7084 (± 0.00004, n = 13) are more radiogenic than seawater, with a slight spatial trend whereby more radiogenic dolostone is recorded at the Matlock Volcanic Complex (87Sr/ 86Sr = 0.7083 (± 0.00004, n = 3) (Fig. 8) compared to the platform margin at Harborough Rocks and platform-interior isolated reefs (87Sr/ 86Sr = 0.7085 (± 0.00004, n = 10). The strontium signature of unaltered basalt taken from Gratton Dale is significantly less radiogenic than seawater at 87Sr/ 86Sr = 0.7046 (± 0.00004, n = 1; Fig. 8A). Altered lavas sampled from the Matlock Volcanic Complex have a strontium signature close to Viséan seawater at 87Sr/ 86Sr = 0.7080 (± 0.00004, n = 1).

Total REE concentrations (ΣREE + Y) in the dolostone range from 1.41 to 11.35 ppm, and are lower than that observed within the whole rock limestone samples, (4.93 to 77.5 ppm) (Fig. 9B, C). REE patterns are similar for all dolostone samples analyzed, both at the platform margin and from samples in the Matlock Volcanic Complex (Fig. 9). Most dolostone and limestone samples display a negative Ce anomaly, and a slight middle rare earth element (MREE) bulge (Fig. 9C). Y/Ho ratios for whole-rock limestone range from 30.4 to 38.5, and are slightly higher in dolostone (D1 platform margin = 37.3–49.4; D1 Matlock Volcanic Complex = 35.8–51.2).

Interpretation
Paragenesis

Several features are consistent with dolomitization in a near-surface or shallow-burial setting. Firstly, D1 is dominated by planar-e and -s dolomite crystals that contain mono-phase primary fluid inclusions. Secondly, D1 is fabric retentive and stratabound and occurs only in platform-margin grainstones and packstones, or along syn-depositional E–W oriented faults, suggesting a depositional and structural control on dolomitization. The presence of dolostone both above and below lavas in the Masson Hill area strongly suggests that lavas controlled dolostone distribution. D1 is cross-cut by stylolites, indicating that it formed before deep burial of the platform, and is observed along E–W trending normal faults which would have been open during N–S extension. Given that post-rift thermal subsidence is recorded across the Pennine Basin in the Serpukhovian (Fraser and Gawthorpe 2003), it is suggested that D1 dolomitization occurred either during or soon after deposition in the late Visean.

Zone 3 calcite cements are ubiquitous across most of the Derbyshire Platform, and also pre-date stylolitization, but are absent in D1 dolostone. This implies that dolomitization was either concurrent with, or post-dated, precipitation of this phase of calcite. Across most of the Derbyshire Platform, Zone 3 calcite cements occluded up to 70% of the interparticle pore space (Walkden and Williams 1991), thereby restricting the circulation of subsequant diagenetic fluids to faults and fractures (Hollis and Walkden 2002). Bingham (1991) noted a decrease in the volume of Zone 3 calcite towards the platform margins, however, and given the localization of D1 to the platform margins it is possible that dolomitization took place during the Serpukhovian, concurrent with Zone 3 cementation. Certainly, D1 dolomite was emplaced before reactivation of NW–SE and N–S trending faults, which host D2 and D3 dolostone, respectively, in the Bashkirian.

Three main fault trends are observed on the Derbyshire Platform, NW–SE, E–W, and N–S, with the E–W and N–S faults forming splays that originate from crustal-scale NW–SE faults (Fig. 1). Cross-cutting relationships show that E–W faults were created before the N–S trending faults, and the association of D1 dolostone with E–W faults strongly suggests that they were open during seawater circulation and D1 formation. These faults were later sealed by dolomite cement. In comparison, N–S faults host halos of a later phase of dolomite (D3) and breccias that are cemented by dolomite, galena, barite, and fluorite (Fig. 5). The cross-cutting relationship between N–S and E–W faults, along with the association of higher temperature dolostone phases and mineralization along N–S faults, is consistent with them post-dating D1 dolomitization and E–W faulting. Areas of fault interaction and intersection are often important sites for fluid flow and mineralization (Rotevatn and Bastesen 2014) and the presence of mineralized breccias and dissolution cavities at the intersection of E–W and N–S faults is consistent with this. Overall, the presence of fluorite–barite–galena–calcite assemblages in porosity in D1 dolostone, and in fractures and faults that dissect D1 and later phases of dolomitization (D2–3), is consistent with prior work, which proposed that mineralization took place as a result of basin dewatering during the Variscan Orogeny (Hollis and Walkden 2002 and references therein; Frazer et al. 2014).

Timing of Volcanic Alteration

Unaltered lavas sampled from Gratton Dale, located 4.5 km from the platform margin and away from D1 dolostone, have 87Sr/86Sr_tr = 0.70453, typical for basalts (Menzies and Seyfried 1979). Lavas proximal to dolomitization in the Masson Hill area, which show significant alteration, have a signature that resembles Viséan seawater (0.7078 87Sr/86Sr_tr; Fig. 8B) suggesting that alteration took place from seawater. Identification of altered lavas within boreholes, away from the effects of surface weathering (Fig. 7A) corroborates the notion that alteration is not likely to be the result of modern surface processes. The replacement of primary iron-magnesium rich mafic minerals (olivine and pyroxene) to produce an assemblage of clay minerals, (illite, chlorite–smectite, and kaolinite), calcite, quartz, and minor pyrite is consistent with the moderate temperature (< 100 °C) alteration of the lavas via interaction with seawater and/or meteoric fluids (Lovering 1949). The high porosity in the lavas before calcite cementation would have facilitated fluid flux.

The oxygen-isotope signature of the calcite cement formed in the lava groundmass and in amygdales (δ18O = –3‰, n = 1) is similar to calcite precipitated from seawater; however, the δ13C is isotopically lighter (δ13C = –1.6‰) than seawater (Fig. 9A). Assuming a mantle signature of around δ13C = –4‰ (Deines 2002), carbon sourced from the lavas during alteration would be isotopically depleted compared to seawater, leading to precipitation of isotopically depleted calcite cements at high fluid-rock ratios.

Dolostone Geochemistry

Calculation of the isotopic composition of the water responsible for dolomitization used Matthews and Katz (1977), assuming a fluid temperature of less than or equal to 50° C based on the presence of mono-phase fluid inclusions in D1. Viséan seawater is calculated to have δ18Owater = –0.6‰SMOW at 25° C in the study area, based on measured δ18Ocalcite = –3‰, from brachiopods. The average δ18Owater composition of the fluids responsible for dolomitization on the platform margin was calculated as δ18Owater = –2.7 ‰ SMOW (30° C), –0.6‰ SMOW (40° C), and 1.3 ‰ SMOW (50° C), using an average δ18Odolomite = –3.4%0, implying that the dolomitizing fluids were similar to seawater at temperatures above 40° C. Samples from the Matlock Volcanic Complex have a lighter average δ18Odolomite (–4.9‰ PDB) than the platform-margin, giving an estimated δ18Owater = –4.2‰ SMOW at 30° C, δ18Owater = –2.1‰ SMOW at 40° C, and –0.2‰ SMOW at 50° C (Fig. 9B). The isotopically light δ13C values measured in both dolostone and calcite, precipitated in the lavas near to the Matlock Igneous Complex suggest that the carbon pool was not entirely marine, and that fluid/rock ratios were high enough to preserve the δ13Cwater.

The strontium-isotope composition of D1 dolostone is slightly enriched in comparison to Viséan seawater. The distribution coefficient (DSrdolomite) for Sr in stoichiometric dolomite is 0.0118, increasing by approx. 0.0039 for each additional mol% CaCO3 in non-stoichiometric dolomite (Vahrenkamp and Swart 1990). D1 dolostone is nearly stoichiometric with MgCO3 = 47.88 to 51.06%, and a decrease in Sr concentration with increasing MgCO3, consistent with expected behavior (Vahrenkamp and Swart 1990) (Fig. 8B). Enrichment in the 87Sr/86Sr ratio implies fluid interaction with a source of radiogenic strontium, which is usually supplied by siliciclastic minerals such as clays.

Marine carbonates inherit the REE signatures of ambient seawater, except for a slight depletion of heavy REE (HREE) (Liu et al. 2017). These REE signatures are not radically changed during dolomitization, provided that the dolomitization occurs in seawater-like fluids (Banner et al. 1988; Qing and Mountjoy 1994; Miura and Kawabe 2000; Kamber and Webb 2001; Nothdurft et al. 2004). Seawater and marine limestones generally exhibit a rare earth element (REE) profile with a marked negative Ce anomaly, but decreasing levels of oxygenation can allow greater incorporation of Ce3+, removing this negative anomaly, and also leading to depletion of HREE (Hayley et al. 2004). The relatively flat REE profile, with a subtle MREE bulge, measured in D1, implies that dolomitization took place at high fluid/rock ratios either under sub-oxic conditions, at high temperature, or by interaction with another fluid. The field and paragenetic relationships, as well as petrographic and geochemical character, are inconsistent with dolomitization from a high-temperature fluid, but it is plausible that dolomitization took place beneath the sediment–water interface under sub-oxic or anoxic conditions. Comparison of normalized Pr* and Ce* values shows that although most D1 dolostones have a negative Ce and positive La anomaly, typical of seawater, some samples have a positive La anomaly (Fig. 9D), potentially reflecting sub-oxic or reducing conditions during dolomitization, post-dolomitization diagenetic overprinting, or potentially inadvertent incorporation of clay material inherent in the rock during leaching (Tostevin et al. 2016). The calculated Y/Ho ratios (35.8–51.1) are consistent with seawater (40–70; Bau and Dulski 1995), which could reflect some post-dolomitization hydrothermal modification (e.g., Ozyurt et al. 2019).

DISCUSSION

Composition of Dolomitizing Fluids

D1 dolostone reflects the first phase of dolomitization observed on the southern margin of the Derbyshire Platform. It is stratabound and variably fabric retentive with rhombic, sucrosic crystals, which are consistent with low-temperature dolomitization (Sibley and Gregg 1987). The presence of single-phase fluid inclusions suggests temperatures of up to approximately 50° C (Goldstein and Reynolds 1994). In this low-temperature setting, seawater would provide a volumetrically significant source of fluid with a sufficiently high Mg/Ca ratio for dolomitization to occur (e.g., Budd 1997; Kaczmarek and Sibley 2007; Whitaker and Xiao 2010). At temperatures of 50° C, the calculated δ18Owater, based on δ18Odolomite, varies between approx. 0.2 and 1.3‰, based on the average δ18Odolomite of D1 dolostone from the Matlock Igneous Complex and the platform margin, respectively. This isotopic range is consistent with Carboniferous seawater ( approx. 0 to –2%0; Veizer and Prokoph 2015). However, if cooler temperatures are invoked then the dolomitizing fluids must have been more isotopically depleted than seawater. This suggests either that fluid temperature was > 40° C, or that there was mixing with an isotopically lighter fluid.

Rare earth element data (Fig. 9C, D) has been interpreted to indicate dolomitization from oxic to suboxic seawater. Suboxic conditions would be consistent with dolomitization at slightly higher temperatures than ambient seawater in the shallow subsurface. The alteration of the REE profile of the whole-rock limestone during dolomitization reflects high fluid/rock ratios. Similarly, lighter δ13Cdolostone than would be expected for Viséan seawater suggests mixing with a fluid other than seawater at high fluid/rock ratios. In order to assess this, mixing of seawater in one of two potential fluids can be considered (Fig. 10):

  1. Magmatic or deep-seated crustal fluids, which have δ13C values similar to the normal mantle values (from –9 to –4‰ PDB) and isotopically enriched δ18Owater (> 0‰ SMOW)

  2. Subsurface meteoric water or groundwater, with isotopically light δ18O due to latitudinal and altitudinal fractionation, and isotopically light carbon due to an input of isotopically light CO2 derived from soil processes. Walkden and Williams (1991) interpreted δ18Owater of Serphukhovian groundwater to be as light as –6‰ from analysis of Zone 3 pore-filling calcite cements.

If dolomitization was concurrent with precipitation of Zone 3 calcite cements, then the δ18Owater of the dolomitizing fluids could have been lowered by mixing of seawater and groundwater. Mixing with groundwater could also have lowered the δ13C of the dolomitizing fluid. However, most groundwater is depleted in Mg, and fluid mixing would therefore lower the Mg/Ca ratio of the fluid, and hence reduce its dolomitizing potential. Zone 3 calcite is also only slightly isotopically depleted in carbon (δ13C = approx. –0.5 to +2.5‰) due to isotopic equilibration of pore waters with limestone over the long transport distances in the aquifer (Walkden and Williams 1991), and might not therefore have reduced δ13Cdolomite.

Alternatively, the δ13C of the dolomitizing fluid could have been lowered by mixing with magmatically derived CO2. CO2-rich waters are known to occur at the boundaries of volcanic geothermal systems and around active volcanoes (Reising and Sansone 1999) and the association between dolomitization and volcanic deposits on the southern Derbyshire Platform means that it is plausible to consider a magmatic source of CO2. On this basis, the composition of the dolomitizing fluid was modeled using end-member fluid compositions and the general hydrothermal carbon–oxygen isotopic mixing equation from Zheng and Hoefs (1993). It was assumed that Fluid A is Viséan seawater with a composition of δ13C = 2‰ PDB and δ18O = 0‰ SMOW whilst Fluid B is mantle water with a composition of δ13C = –4‰ PDB and δ18O = +6‰ SMOW. The concentration ratio (P) of total dissolved carbon in fluid B (magmatic water) to fluid A (seawater) required to explain the isotopic composition of D1, assuming a temperature of formation of 50° C, ranges from > 20 to < 2, with most values consistent with mixing < 10 times magmatic water with seawater (Fig. 10).

Timing of Dolomitization

On the basis of oxygen-isotope and fluid-inclusion data, as well as paragenetic relationships, it would appear that formation of D1 took place in seawater. Depleted carbon-isotope ratios for some samples, beneath expected values for Carboniferous seawater (δ13C +4%0; Prokoph et al. 2008), however, suggest an input of organic carbon or mixing with mantle-derived fluids. The mixing model described in the previous section suggests that seawater mixing with magmatic fluids can reasonably explain the measured δ13Cdolomite. It is likely that mantle fluids would have been present on the platform in the late Visean as a result of extrusive volcanism around the Matlock Igneous Complex. Mantle fluids may not have influenced dolomitization away from this area, and therefore such a fluid-mixing model cannot fully explain the isotopic composition of platform-margin dolostone. Furthermore, interaction of seawater and basalt leads to chlorite formation by reaction of Mg in seawater with Al and Si in the basalt during extrusive volcanism on the seafloor (Reed and Palandri 2006) and it is therefore unlikely that submarine volcanism at this time improved the dolomitizing potential of seawater. Nevertheless, reactive-transport models indicate that increased heat flow could have facilitated dolomitization across the study area (Frazer 2014).

Although carbonate sedimentation terminated on the Derbyshire Platform in the Viséan, the platform remained as a submarine ridge during the Pendleian (early Serphukhovian), becoming draped by organic-rich mudrocks as the basin underwent thermal sag subsidence (Kelling and Collinson 1992). The platform might not have been completely buried until the early Bashkerian (Collinson 1988), a period of more than 10 My after the carbonate platform drowned. As such, seawater convection could still have occurred on the platform margin and along faults. During this time, the platform underwent a series of intrusive volcanic events, leading to the emplacement of doleritic sills. The geothermal gradient on the Derbyshire Platform would have remained high, therefore, facilitating convection of seawater along the platform margin and along open faults (sensuHollis et al. 2017) leading to dolomitization beneath the platform top, under sub-oxic conditions.

Lowenstern (2001) demonstrated that CO2 solubility in water varies as a function of depth of volatile exsolution, and CO2 solubility in basalts displays a prograde nature. As such, deeply sourced CO2 would result in the generation of acidity towards the surface, and therefore mixing of seawater and even a small volume of mantle-derived fluids could have lowered the pH of the fluid, as well as its δ13C. The increased acidity would have increased the rate of dissolution of mafic minerals, such as olivine and pyroxene (Reising and Sansone 1999; Robinson et al. 2019) in the basalts that are interbedded with dolostone in the vicinity of the Matlock Igneous Complex. As a result, Mg would have been released through carbonation of interbedded basalts, increasing the Mg/Ca ratio of the fluid and also liberating silica (Robertson et al. 2019; Koeshidayatullah et al. 2020). The association of dolomitization, volcanism, and silicification of limestone in the vicinity of the Matlock Igneous Complex can consequently be explained by this process. The interpretation is supported by strontium-isotope data; unaltered lavas sampled from Gratton Dale, located 4.5 km from the platform margin and not associated with dolomitization, have a depleted strontium signature of 87Sr/86Sr_tr 0.70453, typical of basalts (Menzies and Seyfried 1979). Conversely, altered lavas in the vicinity of dolostone have signatures that resemble Viséan seawater (0.7078 87Sr/86Sr_tr). The cause of enrichment of radiogenic strontium in D1 dolostone is not entirely clear, but it is possible that it reflects an increase in clay content in the basin, with deposition of mudrocks in the deepest part of the basin from the earliest Serpukhovian. This might also have led to some minor clay contamination in samples that is also reflected in the REE results (see Tostevein et al. 2016).

The REE signature of D1 dolostone is interpreted to reflect varying degrees of oxygenation of the dolomitizing fluid. However, mixing with magmatic fluids under high water/rock ratios could also have altered the REE composition of the resultant dolostone (Murray et al. 1991; Webb and Kamber 2000; Shields and Stille 2001; Nothdurft et al. 2004; Webb et al. 2009; Liu et al. 2017). REEs can be mobilized during hydrothermal alteration (Taylor and Fryer 1980; Palacios et al. 1986; Fulignati et al. 1999), particularly during the interaction between seawater and mafic volcanics (Frey et al. 1977; Wood et al. 1976; Hellman and Henderson 1977; Ludden and Thompson 1979) and this could have led to the observed depletion in the HREEs, creating a slight MREE bulge. This alteration would also have depleted Y concentrations, lowering the Y/Ho ratio.

Conceptual Model of Diagenetic Fluid Flow and Dolomitization

Dolomitization on the southern margin of the Derbyshire Platform is broadly stratabound and occurs in platform-margin facies, along E–W faults, and close to lavas in the Matlock Igneous Complex. The association of the dolostone with E–W faults indicates that these faults were open during seawater circulation and formation of D1 dolostone. The fabric and texture of the dolostone, along with primary monophase fluid inclusions, indicate formation at temperatures up to 50° C. During the Viséan (Fig. 11A) and the Serphukhovian (Fig. 11B), until the platform was draped by onlapping clastic sediments, geothermal convection could have occurred along the platform margin and also along E–W faults, driven by the temperature differential between the cooler seawater in the Widmerpool Basin and the warmer, volcanically active platform top (Frazer et al. 2014; Frazer 2014). Within the vicinity of the Matlock Igneous Complex, interaction of seawater with submarine basalts that were extruded onto the seafloor could have removed Mg from seawater (Reed and Palandri 2006), however, reducing the dolomitizing potential of seawater in the vicinity of lava extruded during carbonate-platform growth. Consequently, dolomitization may only have occurred initially on the platform margin, and along E–W faults, perhaps facilitated by the increased heat flux associated with volcanic activity. Nevertheless, the preservation of olivine in some basalts suggests that serpentinization was not pervasive, and that the diagenetic potential of the lavas remained high.

With the onset of post-rift thermal subsidence in the Serpukhovian, the Derbyshire Platform was drowned and the surrounding hanging-wall basins accumulated fine-grained mudrocks (Fig. 11B). During this time, gravity-driven meteoric fluids flowed westwards from the East Midlands Platform, eventually leading to porosity occlusion by calcite cement phase Zone 3 on the Derbyshire Platform, but decreasing in volume towards the platform margin (Bingham 1991; Walkden and Williams 1991). At the same time, volcanism changed from extrusive to intrusive, leading to emplacement of doleritic sills (Macdonald et al. 1984). This would have facilitated dolomitization by maintaining a high heat flow and by increasing pCO2 (e.g., Sibley 1990). Furthermore, CO2 degassing would have lowered the pH of fluids, allowing magnesium to be leached from olivine and pyroxene in basalts, increasing the Mg/Ca ratio of the fluid (e.g., Robertson et al. 2019; Koeshidayatullah et al. 2020). Alteration of the basalts would also have released silica, implying a genetic link between the occurrence of dolostone the occurrence of silicified limestones close to the igneous intrusive sills and extrusive lavas, parallel to E–W oriented faults.

Once dolomitization was complete, dolomite cements precipitated along, and sealed E–W trending faults. There was also cementation of the extrusive lavas by calcite, such that later diagenetic fluids could not exploit the same fluid pathways as the dolomitizing fluids that formed phase D1. Support for this model comes from both the geochemical fingerprint of the dolostone and the localized silicification observed in the vicinity of the Matlock igneous center, since quartz is a by-product of the carbonation of ultramafic rocks (Lovering 1949). By the Bashkerian, the platform was buried underneath upper Carboniferous sediments (Collinson 1988), which became overpressured until the onset of the Variscan Orogeny led to rupture and fluid expulsion (Frazer et al. 2014). At this point, dolomitizing fluids were expelled along faults, leading to the formation of D2 dolostone along the Cronkston–Bonsall Fault, probably overprinting precursor D1 dolostone (Fig. 11C).

Implications to Dolomitization Within Extensional Basins

In the Pennine Basin, dolomitization commonly occurs on the margins of age-equivalent carbonate platforms, formed under the same tectono-stratigraphic regime (e.g., Bouch et al. 2004; Hollis and Walkden 2012; Juerges et al. 2016), often at zones of fault intersection or termination (Fig. 1). This is consistent with observations from the Suez rift, where fault-controlled dolomitization occurs in zones of structural complexity (e.g., Hollis et al. 2017; Hirani et al. 2018). However, the volume of dolostone at other localities in the Pennine Basin is significantly less than on the Derbyshire Platform, with body sizes typically less than 20 km2. They are also not associated with igneous activity; volcanism in the Pennine Basin in the Carboniferous was restricted to the Derbyshire Platform. From field and petrographical data, it is estimated that up to 50% of the dolostone mapped on the southern Derbyshire Platform was formed from modified seawater during the Visean and Serpukhovian, i.e., potentially > 25 km2, whereas the remainder formed from basinal brines expelled from juxtaposed hanging-wall basins (Frazer et al. 2014).

Where D1 dolostone has not been recrystallized, it has a well-connected intercrystalline pore network (Fig. 6A), indicating that the petrophysical properties of limestone can be positively affected by dolomitization through geothermal convection of seawater. Consequently, the potential for these dolostones to host mineralization, hydrocarbons, or groundwater is high. For the same reason, the ability to flux subsequent diagenetic fluids was also improved. D1 dolomite crystals could have nucleated subsequent phases of dolomitization. On the Derbyshire Platform, later dolomitization events occurred along NW–SE and N–S trending faults, thus producing phases D2–D5 which cross-cut D1. This suggests that the presence of a large area of dolomitization on the platform could have been a vital template for subsequent phases of dolomitization in the area, enlarging the volume of dolostone to > 60 km2 (Breislin 2018).

Globally, carbonate platforms commonly form on the footwalls of normal faults in extensional basins (Bosence 2012). The coincidence of active tectonism, high heat flow, seawater, and volcanic activity in these basins means that dolomitization from geothermal convection of seawater that has interacted with magmatic fluids should be common, as suggested by some well-documented case studies (e.g., Nader et al. 200; Blomme et al. 2017; Jacquemyn et al. 2017). On the Latemar Platform, for example, dolomitization is interpreted to be the product of seawater interaction with mafic dikes, forming dolostone halos (0.5 to 2.5 m wide) (e.g., Zheng 1990; Carmichael and Ferry 2008; Blomme et al. 2017; Jacquemyn et al. 2017). In the Levant region of Lebanon, hydrothermal dolomitization (TH = 50–80° C) has been related to volcanism in the Late Jurassic, which created a locally high geothermal gradient, optimizing the thermodynamic and kinetic potential for dolomitization (Nader et al. 2004). There are not many case studies, however, perhaps for several reasons. Firstly, dolomitization in extensional basins might have been attributed to hydrothermal processes, particularly where dolostone forms along faults, without a full explanation of the potential source of the fluids. This could reflect three scenarios. 1) The generic assumption that dolomitization was from upward-flowing, crustal fluids—often invoked for hydrothermal dolomitization (e.g., Davies and Smith 2006 and discussion in Hollis et al. 2017). 2) An indirect control of extensional tectonics on dolomitization, through an increase in the geothermal gradient. In this case, high heat flow might have facilitated dolomitization, but might not be recognized geochemically. This would be also be the case if there were low volumes of fluid mixing with seawater, such that the REE and/or isotopic signature of the dolomitizing brine was not altered by fluid mixing. 3) Recrystallization of the earliest phase of dolomite, with hotter, more saline brines. Alternatively, the coincidence of a CO2-rich brine, circulating through mafic or ultramafic rocks, adjacent to permeable carbonate strata, might be rare. In this case, dolomitization by the process invoked in this study might be uncommon. In order to test the model, evaluation of dolomitization processes in rift basins with widespread carbonate sedimentation, could be critical to assessing the wider occurrence of dolomitization by convection of mixed seawater and magmatic water. The observations made in this study could also go some way to explain the commonly observed association of replacive silica in fault-controlled dolostones in extensional basins.

CONCLUSIONS

Based on the data presented, the following conclusions have been drawn:

  • Dolomitization of the Derbyshire Platform was a multi-phase event, with the earliest dolostone phase, D1, accounting for approximately 50% of the total dolostone volume (approx. 25 km2 of a total of 60 km2 of dolostone on the platform).

  • D1 dolostone is stratabound and facies controlled and occurs in platform margin facies, close to interbedded basaltic lavas around the Matlock Volcanic Complex.

  • D1 dolostone is fabric retentive, composed of buff-gray, clean planar-e crystals. It is cross-cut by stylolites, and primary fluid inclusions are predominantly monophase, indicating formation at shallow burial depths and temperatures < 50° C.

  • Geochemical evidence points towards early dolomitization from seawater that circulated on the platform margin and syn-rift, E–W trending faults in the Viséan, which became progressively modified, through mixing with mantle-derived CO2, in the Serpukhovian, during platform drowning.

  • The addition of CO2 to circulating seawater lowered the pH of the brine, leading to carbonation of interbedded basalts. This resulted in calcite cementation of vesicles and an increase in the Mg/Ca ratio of the fluid, which may have further facilitated dolomitization. Concomitant release of silica during this process also led to localized areas of silicification close to the Matlock Volcanic Complex.

  • This model could account for variability in the occurrence of dolomite on carbonate platforms in rift basins, with larger and more pervasive zones of dolomite occurring where there is alteration of underlying or interbedded mafic rocks.

ACKNOWLEDGMENTS

Catherine Breislin gratefully acknowledges funding by Natural Environment Research Council Doctoral Training Programme grant number NE/L002469/1 with CASE support by Shell International Exploration and Production and the British Geological Survey. The authors thank Jon Fellows and John Waters at The University of Manchester and James Utley at The University of Liverpool for analytical services provided. The British Geological Survey is thanked for the loan of material. VJB, IM, and JBR publish with the approval of the Chief Executive Officer, British Geological Survey (NERC). Sarah Newport (University of Manchester) and Dr. Richard Shaw are thanked for their field assistance. Alanna Juerges, Conxita Taberner, and the Carbonate Research team at Shell International Exploration and Production and Hamish Robertson, University of Bristol, are thanked for their helpful discussions. The comments of two anonymous reviewers and Associate Editor Stephen Kaczmarek and Chief Editor Peter Burgess greatly improved a previous version of this paper.