Abstract
Within the Upper Triassic successions in the Iberian Basin (Spain), the occurrence of both idiomorphic bipyramidal quartz crystals as well as pseudohexagonal aragonite crystals are related to mudstone and evaporite bearing sequences. Bipyramidal-euhedral quartz crystals occur commonly at widespread locations and similar idiomorphic crystals have been described in other formations and ages from Europe, America, Pakistan, and Africa. Similarly, pseudohexagonal aragonite crystals are located at three main sites in the Iberian Range and are common constituents of deposits of this age in France, Italy, and Morocco. This study presents a detailed description of the geochemical and mineralogical characteristics of the bipyramidal quartz crystals to decipher their time of formation in relation to the diagenetic evolution of the sedimentary succession in which they formed. Petrographic and scanning electron microscopy (SEM) analyses permit the separation of an inner part of quartz crystals with abundant anhydrite and organic-rich inclusions. This inner part resulted from near-surface recrystallization (silicification) of an anhydrite nodule, at temperatures that were <40 °C. Raman spectra reveal the existence of moganite and polyhalite, which reinforces the evaporitic character of the original depositional environment. The external zone of the quartz contains no anhydrite or organic inclusions and no signs of evaporites in the Raman spectra, being interpreted as quartz overgrowths formed during burial, at temperatures between 80 to 90 °C. Meanwhile, the aragonite that appears in the same Keuper deposits was precipitated during the Callovian, resulting from the mixing of hydrothermal fluids with infiltrated waters of marine origin, at temperatures ranging between 160 and 260 °C based on fluids inclusion analyses. Although both pseudohexagonal aragonite crystals and bipyramidal quartz appear within the same succession, they formed at different phases of the diagenetic and tectonic evolution of the basin: bipyramidal quartz crystals formed in eo-to mesodiagenetic environments during a rifting period at Upper Triassic times, while aragonite formed 40 Ma later as a result of hydrothermal fluids circulating through normal faults.
Highlights
Within the Upper Triassic sequences in the Iberian Basin, Spain (Figure 1a), and in other European countries,Within the occurrence both sequences bipyramidal quartz crystals1b) and the Upper of Triassic in the Iberian Basin, (Figure Spain (Figure1a),pseudohexagonal and in other aragonite crystals (Figure is common within mudstone, and1b)marls that represent salineEuropean countries, the 1c) occurrence of both bipyramidal quartzevaporites, crystals (Figure and pseudohexagonal mudflats and salt pond facies.1c)The aragonite crystals occur evaporites, associatedand with outcrops of pre-Hettangian aragonite crystals
Single crystals ranged from 0.0625 mm to 2.6 cm lengths (Figure 1b), while cluster appeared as groupings of quartz crystals that converged in a nucleus (Figure 1b)
Petrographic analyses revealed that the center of the crystal had numerous evaporate mineral inclusions (Figure 3a) resembling anhydrite laths (Figure 3b,c) and radial-pattern inclusion filled by organic matter (Figure 3d,e)
Summary
Within the Upper Triassic sequences in the Iberian Basin, Spain (Figure 1a), and in other European countries,Within the occurrence both sequences bipyramidal quartz crystals1b) and the Upper of Triassic in the Iberian Basin, (Figure Spain (Figure1a),pseudohexagonal and in other aragonite crystals (Figure is common within mudstone, and1b)marls that represent salineEuropean countries, the 1c) occurrence of both bipyramidal quartzevaporites, crystals (Figure and pseudohexagonal mudflats and salt pond facies.1c)The aragonite crystals occur evaporites, associatedand with outcrops of pre-Hettangian aragonite crystals Within the Upper Triassic sequences in the Iberian Basin, Spain (Figure 1a), and in other European countries,Within the occurrence both sequences bipyramidal quartz crystals. The 1c) occurrence of both bipyramidal quartzevaporites, crystals (Figure and pseudohexagonal mudflats and salt pond facies.1c)The aragonite crystals occur evaporites, associatedand with outcrops of pre-Hettangian aragonite crystals Hettangian alkaline magmatism and related to normal faults dated as Middle-to-Upper resulted from the mixing of infiltrated marine waters with hydrothermal fluids that circulated synrift formation from of infiltrated waters hydrothermal fluids faultsTheir [1]. The quartz crystals, on the contrary, appear in a wider range of locations close relation to major faults or volcanic deposits. They appear to be organic matter and have numerous saline mineral inclusions, indicating that the original sediments related to facies rich in organic matter and have numerous saline mineral inclusions, indicating that formed in environments under strong evaporitic conditions
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