Pyrometamorphic Rocks in the Molinicos Basin (Betic Cordillera, SE Spain): Insights into the Generation of Cordierite Paralavas

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 2 = 52-57, Al 2 O 3 ≈ 20, Fe 2 O 3 = 10-20, K 2 O + Na 2 O < 2.5, CaO < 4.5, and MgO < 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.Minerals 2019, 9, 748 2 of 24 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 CaCO 3 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,[31][32][33][34]. 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 subs...