Pyrometamorphic rocks are the highest temperature products owing to contact metamorphism induced by either igneous, combustion or lightning processes. Pyrometamorphism is an unusual metamorphic process typically characterized by low pressures and very high temperatures [1
]. Whereas maximum temperature conditions resulting from igneous activity are around 1200 °C at 1 kbar, under atmospheric conditions, the subsurface combustion of organic-rich rocks (i.e., carbonaceous sediments and coal) can lead to temperatures up to 1500 °C triggering peculiar pyrometamorphic processes called combustion metamorphism [2
The most common subsurface fires occur in peat beds [3
], coal deposits [6
], dumps from coal mines [13
], mud volcanoes [16
] and bituminous sediments [2
]. A particular case of subsurface fires is described in lakes with predominantly diatomitic sedimentation that are desiccated during drought periods [29
Except in the mud volcanoes, where gas discharge can facilitate ignition [17
], occurrence of self-ignition in natural subsurface fires needs tectonic joints that provide both oxygen access and evacuation of combustion products [22
]. At the same time, thermal isolation is necessary to accumulate locally enough heat until an activation threshold is reached [29
]. Thus, depth and extension of the fires are commonly restricted to some tens of square metres, limited in addition by the position of the water table. However, much larger areas have been described reaching hundreds of square kilometres [22
During combustion metamorphism, the sedimentary rocks around the ignition foci are subjected to certain changes in addition to common dehydration, and decarbonation in case CaCO3
is present. Firstly, the rocks are hardened and, later, they become sintered and recrystallised producing a ceramic texture. The colour of the rocks is also progressively changed from yellow to more intense orange and red, due to iron oxidation, and finally even grey and black when reducing conditions are reached. In any case, this intense thermal metamorphism triggers only limited fusion of the sediments. The released melts may flow along fractures but usually undergo fast cooling producing small amounts of glass with particular mineral assemblages and textures similar to those of fine-grained volcanic rocks. These rocks, which show features and parageneses merging with those of igneous rocks, are called paralavas and the associated baked or partially melted sedimentary rocks are denominated clinker e.g., [24
]. According to Cosca and Essene [35
] the mineral assemblages produced in paralavas are partly a function of the temperature, degree of partial melting, and oxidation state related to local gas buffers, in addition to the original bulk composition of the sedimentary protolith. Fast heating and subsequent fast cooling cause only incomplete reaction and associated chemical disequilibrium, which contributes to the peculiar mineral composition of these rocks, different to other facies of contact metamorphism [1
In this study, we document the occurrence of a subsurface combustion area in a sedimentary basin located in South Spain, the Molinicos Basin, which is an upper Miocene lacustrine basin of the Betic Cordillera (Figure 1
). This is the first case of pyrometamorphism described in the Betics. With the aim of finding out the particular factors that triggered this process we have carried out field observations, and mineralogical and geochemical analysis in the materials affected by the combustion metamorphism. In addition, P-T pseudosection calculations from the plausible protolith are compared with the observations in order to understand and explain the origin and the temperature evolution of the observed variety of lithologies and its relationships.
2. Geological Setting
The Molinicos Basin forms part of a set of several late Miocene lacustrine basins (Figure 1
) that mark out the northeastern limit of the Betic Cordillera [36
]. The paleolakes were placed on partially interconnected intramountain basins, which were formed during the tectonic uplift and closure of the so-called North-Betic Strait during the late Tortonian. These basins, ranging from a few km2
to 250 km2
, formed as rapidly subsiding troughs during the late Vallesian to late Turolian (Tortonian–Messinian of the marine chronostratigraphic scale) [37
], reaching a thickness of sediments up to 500 m.
The dominant elongation of the basins is ESE–WNW, roughly sub-parallel to the main fault in the vicinity (Figure 1
): the Socovos Fault [40
], a dextral strike-slip fault with lithospheric significance [42
]. Basins are located mostly at the north block of the Socovos Fault, and are bounded by anticline ridges (Figure 1
). Most of the deformation associated with the fault displacement occurs from early and middle Miocene until Tortonian [41
], but continues until nowadays at lower rates [43
]. The Molinicos Basin occurs at the western end of the set of lacustrine basins, crossed by the main trace of Socovos Fault (Figure 1
). A body of delta conglomerates and sands (≥100 m thick) limits the basin to the west. The observed maximum thickness of lacustrine sediments in this area ranges from 80 to 150 m [37
]. According to Foucault et al. [44
] and Elizaga [37
], the upper Miocene sequence is composed of: (a) conglomerates and sands (turbidites and pelagites), (b) claystones and marly limestones with gypsum, (c) marly limestones and diatomites with organic-rich levels (mainly plant remains), (d) slumped interval, (e) diatomites, limestones and sandy levels (Figure 2
In the Molinicos Basin, the sedimentary succession is specifically characterized by fine laminated diatomites and limestones (mudstones-wackestones) that can be correlated with the upper Tortonian of the regional sequence. Some limestone beds occur as gastropods- and ostracods-rich wackestones-packstones (Figure 3
a) and locally they present fossil mudcracks (Figure 3
b). Dark brown to black pelitic layers are found near the pyrometamorphic outcrop separated by a local discordance from the more calcareous sequence on top of them. Organic-rich levels similar to these are described in most of the neighbouring basins [38
]. Actually, some of these basins have well-known sulphur deposits formed after reduction of sulphates in contact with bituminous lutites or shales [46
]. The dark levels (10 cm to 30 cm thick, Figure 3
c,d) change their thickness in few metres and locally pinch-out. Two main types can be differentiated: (a) dark clay layers resembling black shales and (b) grey clay-rich layers with common vegetal remains, including well-preserved leaves. A metric-sized body of clays crosses through the overlaying beds forming an elongated structure that resembles a mud diapir. Here, the dark clays are oxidized, showing a light ochre colour, and include fragments of surrounding rocks such as green marls. This structure is sub-parallel to the strike of the Socovos Fault (Figure 4
The pyrometamorphic rocks in the sedimentary succession crop out in a reduced area of 0.4 km2
crossed by or very close to the estimated Socovos Fault trace (Figure 4
3. Materials and Methods
Fieldwork and mapping allowed us to characterize the distribution and appearance of the studied paralavas and baked rocks. Twenty-seven samples were collected corresponding to: (a) paralavas referring to the massive material relatively mobilized identified in the cross-section of Figure 5
a; (b) brick-red rocks forming the clinker which includes fused dark seams; (c) the whitened rocks above the clinker; (d) marbles and (e) rocks obtained from the nearest unaltered sedimentary sequence, which includes dark clay layers and marly diatomites.
Whole-rock analyses of the major elements of selected samples from the paralavas, clinker and unaltered rocks were carried out using X-ray fluorescence (XRF) in a Philips Magix Pro (PW-2440) spectrometer and trace elements were analysed using a NexION 300D inductively coupled plasma-mass spectrometer (ICP-MS) (both techniques in the Centro de Instrumentación Científica, CIC, Universidad de Granada, Granada, Spain).
The total organic carbon (TOC) of eight samples corresponding to dark clay layers close to the paralava outcrops was analyzed using a Shimazdu Total Organic Carbon Analyzer (TOC-V sch) from the Instituto de Recursos Naturales y Agrobiología (IRNAS) from CSIC-Sevilla.
X-ray diffraction (XRD) patterns of samples were obtained from powders and oriented aggregates with a PANalytical Empyrean diffractometer equipped with a θ
goniometer (Centro de Instrumentación Científico-Técnica, CICT, Universidad de Jaén). The CuK α radiation with a voltage of 45 kV and a current of 40 mA was used with a step size of 0.01° 2θ
and a count time of 40 s per step. Samples were scanned from 4° to 64° 2θ
. Following the XRD and optical study, carbon-coated polished thin sections were examined by Scanning Electron Microscopy (SEM), using back-scattered electron (BSE) imaging and energy-dispersive X-ray (EDX) analysis to obtain textural and chemical data. These observations were carried out with a Merlin Carl Zeiss SEM (CICT, Universidad de Jaén) and a Zeiss SUPRA40VP, at the CIC of the Universidad de Granada. An accelerating voltage of 20 kV, with a beam current of 1–2 nA and counting time of 30 s were used to analyze the minerals by SEM, using the following standards: albite (Na), periclase (Mg), wollastonite (Si and Ca), and orthoclase (K), and synthetic Al2
(Fe) and MnTiO3
(Ti and Mn). The very small size of some minerals in the studied samples was the reason for the selection of the SEM for the chemical analyses instead of the traditional electron microprobe (EMPA), since the SEM has a higher spatial resolution than the EMPA and the BSE images allow easy selection of very small, contamination-free areas for analyses. WDS analytical data are much more precise (may detect up to 0.02 wt. % of the elements analysed) and if compared to EDX, works under diffraction principles using selected crystals which avoid overlapping elements. In this particular case, EDX analysis of “not overlapping elements” using Na(Kα), Mg(Kα), Al(Kα), Si(Kα), K(Kα), Ca(Kα), Mn(Kα), and Ti(Kα) lines may provide very similar results to the EMPA analysis. In fact, Abad et al. [48
] demonstrated that SEM-EDX analyses obtained under the same conditions as EMPA, in particular a careful calibration with real standards and preparation of polished samples, produced equivalent results.
P-T phase-diagram sections (pseudosections) were calculated for bulk-rock compositions of possible suitable protoliths with the aim of estimating their melting conditions and the probable stability conditions of the mineral assemblages observed in the studied clinker and paralava samples. For the same bulk rock compositions, additional, isobaric melt fractionation calculations were made for a heating path, along which a set of incremental temperature steps was determined. After computing the initial stable assemblage for the specified bulk composition, at each temperature increment, the generated, fractionated melt is removed from the system, the system’s composition is adjusted and the stable phase assemblage is recalculated.
Calculations were made with Perple_X 6.8.7 [49
] in the system CaO–K2
O. We used the internally consistent thermodynamic database from Holland and Powell [50
] and the following solid solution models: clinopyroxene [52
]; melt [51
]; chlorite [54
]; olivine, spinel, staurolite, chloritoid, saphirine, garnet [50
]; orthopyroxene, biotite [55
]; white mica, cordierite [56
]; and ternary feldspar [57
Quartz, tridymite, andalusite and sillimanite were considered as pure phases. Mullite could not be considered in the calculations as it is not included in the employed thermodynamic database of minerals. We used the CORK equation of state for H2
], although fluids have been considered pure H2
O. This assumption is consistent with the very low CaO and MgO contents in these rock types (Table 1
) and the complete lack of carbonate phases in both clinker and paralava samples (Table 2
In the Molinicos lacustrine basin, subsurface combustion of organic-rich clay layers has triggered partial melting of the sedimentary rocks promoting the genesis of baked rocks (clinker) and paralavas. This is the first pyrometamorphic process identified in the Betics (SE Spain). This event was possible because of the particular characteristics of this sector: a shallow lacustrine basin that dried-up very often and a fractured context due to the proximity of a major active fault that permitted oxygen entrance at depth. Dark clay layers have been identified as the most probable protolith of the pyrometamorphic rocks according to their high TOC contents and bulk-rock composition.
Thermodynamic modelling comprising P-T pseudosections and melt fractionation calculations led to the following comprehensive evolution model of the reported thermal event that explains the observed field and textural relationships in the studied lithotypes:
(1) Main melting started at very low pressures (<10 MPa) and temperatures around 870 °C, at which melt coexisted with the mineral assemblage cordierite-sanidine-anorthite-sillimanite-tridymite that characterizes the paralavas. Nevertheless, due to short-term heating and extreme temperature gradients, the onset of first melting might have occurred in the range comprised between 870 °C and 1260 °C. Depending on temperature, the composition of fractionated melt would be different. This explains the compositional differences observed in both clinker and paralava samples.
(2) The common occurrence of tridymite in most of the studied rocks is compatible with simultaneous melting, at temperatures up to 1140 °C, if a melt fractionation model is considered. However, in spite of thermodynamic predictions, cristobalite was stable in these rocks at temperatures below 1260 °C.
(3) During the combustion, the increase of the oxygen fugacity produced an “out of sequence” mineralogy (corundum, hematite and hercynite) that was not predicted by thermodynamic modelling. Iron from the breakdown of the sedimentary clays produced ubiquitous hematite, which lent the typical reddish colour to the clinker. Thermometric estimations for coexisting corundum-hematite pairs result in a temperature stability range of 1150–1200 °C, in good agreement with thermodynamic models.
(4) Only a small volume of the overlapping carbonates was affected by the heat increase. Undisturbed limestone beds are turned into very fine-grained marbles, while along fractures networks it is induced the formation of a powdery matrix.
(5) A subsequent cooling hydrothermal stage produced textural alteration and new minerals that filled amygdules in the paralavas and clinker.