Pyrometamorphic Rocks in the Molinicos Basin (Betic Cordillera, SE Spain): Insights Into the Generation of Cordierite Paralavas
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minerals Article Pyrometamorphic Rocks in the Molinicos Basin (Betic Cordillera, SE Spain): Insights into the Generation of Cordierite Paralavas Isabel Abad 1,*, Mario Sánchez-Gómez 1, Matías Reolid 1 and Vicente López Sánchez-Vizcaíno 2 1 Departamento de Geología and CEACTierra, Universidad de Jaén, Campus Las Lagunillas s/n, 23071 Jaén, Spain; [email protected] (M.S.-G.), [email protected] (M.R.) 2 Departamento de Geología and CEACTierra, Campus Científico Tecnológico de Linares, Universidad de Jaén, 23700 Linares, Jaén, Spain; [email protected] * Correspondence: [email protected] Received: 27 September 2019; Accepted: 26 November 2019; Published: 30 November 2019 Abstract: A singular thermal anomaly occurred in the Molinicos Miocene, lacustrine, intramontane basin (Betic Cordillera). This gave place to vitreous vesicular materials (paralavas) and baked rocks (clinker) inside of a sequence of marly diatomites and limestones. The chemical composition of the paralavas (SiO = 52–57, Al O 20, Fe O = 10–20, K O + Na O < 2.5, CaO < 4.5, and MgO < 2 2 3 ≈ 2 3 2 2 1.5, % in weight), which is very different from typical igneous rocks, and their high-T mineralogy (cordierite, sillimanite, anorthite, mullite, and high-T silica polymorphs) suggest that they formed during a pyrometamorphic event. The occurrence of dry intervals in the lacustrine depositional system, the high Total Organic Carbon contents (>4% in weight) of dark clay layers and the existence of tectonic fractures give the right context for a combustion process. Short-term heating favoured the generation of paralavas, clinker and marbles. Thermodynamic modelling constrains the onset of melting at 870–920 ◦C for <10 MPa at equilibrium conditions. However, the presence of tridymite and/or cristobalite in clinker and paralavas and the compositional variation in both rock types suggests that the temperature at which first melting occurred ranged between 870 ◦C and 1260 ◦C due to melt fractionation processes. Keywords: cordierite; clinker; combustion metamorphism; Molinicos Basin; paralava; melt fractionation; P-T pseudosection; SEM 1. Introduction 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 [3–5], coal deposits [6–12], dumps from coal mines [13–15], mud volcanoes [16–18] and bituminous sediments [2,19–30]. 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 Minerals 2019, 9, 748; doi:10.3390/min9120748 www.mdpi.com/journal/minerals Minerals 2019, 9, 748 2 of 24 evacuation of combustion products [22]. At the same time, thermal isolation is necessary to accumulate Minerals 2019, 9, x FOR PEER REVIEW 2 of 24 locally enough heat until an activation threshold is reached [29]. Thus, depth and extension of the fires areenough commonly heat restricteduntil an activation to some tensthreshold of square is reached metres, [29]. limited Thus, in depth addition and by extension the position of the of fires the waterare table.commonly However, restricted much to larger some areas tens haveof square been metres, described limited reaching in addition hundreds by the of squareposition kilometres of the water [22 ]. table.During However, combustion much larger metamorphism, areas have been the sedimentary described reaching rocks around hundreds the of ignition square focikilometres are subjected [22]. to certainDuring changes combustion in addition metamorphism, to common the dehydration, sedimentary and rocks decarbonation around the ignition in case foci CaCO are 3subjectedis present. Firstly,to certain therocks changes are in hardened addition and, to common later, they dehydration, become sintered and decarbonation and recrystallised in case producing CaCO3 is present. a ceramic texture.Firstly, Thethe colourrocks are of hardened the rocks and, is also later, progressively they become changed sintered from and yellowrecrystallised to more producing intense orangea ceramic and red,texture. due to The iron colour oxidation, of the and rocks finally is also even progress grey andively black changed when reducing from yellow conditions to more are intense reached. orange In any case,and this red, intense due to iron thermal oxidation, metamorphism and finally triggerseven grey only and limited black when fusion reducing of the sediments.conditions are The reached. released meltsIn any may case, flow this along intense fractures thermal but metamorphism usually undergo triggers fast coolingonly limited producing fusion smallof the amountssediments. of The glass withreleased particular melts mineralmay flow assemblages along fractures and but textures usually similarundergo to fast those cooling of fine-grained producing small volcanic amounts rocks. Theseof glass rocks, with which particular show mineral features assemblages and parageneses and textures merging similar with to those those of of igneous fine-grained rocks, volcanic are called paralavasrocks. These and rocks, the associated which show baked features or partially and parageneses melted sedimentary merging with rocks those are of denominated igneous rocks, clinker are e.g.,called [24, 31paralavas–34]. According and the associated to Cosca and baked Essene or part [35ially] the melted mineral sedimentary assemblages rocks produced are denominated in paralavas areclinker partly e.g., a function [24,31–34]. of the According temperature, to Cosca degree and of Essene partial [35] melting, the mineral and oxidation assemblages state relatedproduced to localin gasparalavas buffers, are in addition partly a tofunction the original of the bulktemperature, composition degree of theof partial sedimentary melting, protolith. and oxidation Fast heating state andrelated subsequent to local gas fast buffers, cooling in cause addition only to incompletethe original bulk reaction composition and associated of the sedimentary chemical disequilibrium,protolith. Fast whichheating contributes and subsequent to the peculiar fast cooling mineral cause composition only incomplete of these rocks, reaction different and to associated other facies chemical of contact disequilibrium, which contributes to the peculiar mineral composition of these rocks, different to metamorphism [1]. other facies of contact metamorphism [1]. In this study, we document the occurrence of a subsurface combustion area in a sedimentary 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 basin located in South Spain, the Molinicos Basin, which is an upper Miocene lacustrine basin of the the Betic Cordillera (Figure1). This is the first case of pyrometamorphism described in the Betics. Betic Cordillera (Figure 1). This is the first case of pyrometamorphism described in the Betics. With With the aim of finding out the particular factors that triggered this process we have carried out field 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 observations, and mineralogical and geochemical analysis in the materials affected by the combustion metamorphism.metamorphism. In In addition, addition, P-TP-T pseudosectionpseudosection calculationscalculations from from the the plausible plausible protolith protolith are are compared compared withwith the the observations observations in in order order to to understand understand and and explain explain the the origin origin and and the the temperature temperature evolutionevolution of theof observed the observed variety variety of lithologies of lithologies and and its relationships.its relationships. FigureFigure 1. 1.Late Late Miocene Miocene lacustrine lacustrine basins basins from from the the eastern eastern Betics Betics and and main main related related structures. structures. NamesNames of theof basins the basins from from previous previous studies studies e.g., e.g., [36 –[36–38]38] and and this this study: study: 1, H1, íHíjajar; 2,r; 2, Isso; Isso; 3, 3, Molinicos; Molinicos; 4, 4, Elche Elche de lade Sierra; la Sierra; 5, Cobatillas; 5, Cobatillas; 6, Gallego; 6, Gallego; 7, Cenajo; 7, Cenajo; 8 Las 8 Minas-Camarillas;Las Minas-Camarillas; 9, El 9, Cañar; El Cañar; 10, F10,érez; Férez; 11, Letur.11, Inset:Letur. Location Inset: Location map of themap studied of the studied area in area the SEin the of the SE Iberianof the Iberian Peninsula. Peninsula. Minerals 2019, 9, 748 3 of 24 Minerals 2019, 9, x FOR PEER REVIEW 3 of 24 2.2. Geological Geological Setting Setting TheThe Molinicos Molinicos Basin Basin formsforms partpart ofof aa setset ofof severalseveral late