Variscan metamorphism and fluid-rock interaction in Lower Cambrian carbonates of the Central Iberian Zone (Toledo Mountains, Spain)
Abstract
In the southern Central Iberian Zone of the spanish Variscan massif, at the Urda village and nearby locations, there outcrop carbonate-siliciclastic rocks of the lower Cambrian Navalucillos Formation. These rocks are impure limestones ± dolostones affected by metamorphism and deformation during the Variscan orogeny (Carboniferous: middle to upper Missisipian). Some of the textures observed consist in quartz (± sericite) domains that include calcite nodules where archaeocyathan bioclasts are abundant, with silicate rims of Mg-biotite ± clinochlore ± iron-sulfides ± K-feldspar. Outside the rims, the silicate phases are Mg-biotite, ± muscovite, ± K-feldspar, ± chlorite-clinochlore, ± titanite, ± calcic-plagioclase, and iron sulfides. This mineral assemblage requires metamorphic/ metasomatic conditions to form phases such as Mg-biotite. Two hypothesis are considered: 1) A chert and clay mineral assemblage that partially encloses calcite/dolomite domains. These set of lithofacies were deformed and affected by the regional metamorphism with a moderate input of external dehydration-decarbonation fluids. 2) Initial impure limestones and dolostones were infiltrated by magmatic H2O ± CO2 external fluids carrying variable amounts of Si and other minor components during the variscan orogenic event. This fluid could have been related to granite intrusions located farther east. P-T-XCO2 thermodynamic modeling was performed with an effective bulk rock composition of the chert-calcite nodules and its rim of silicate phases. The calculated mineral assemblage matches fairly well with the observed one for low pressure conditions of metamorphism, low to moderate XCO2 fluid (≈ 0.2-0.5) and a T range of ≈ 300-375ºC. The absence of amphibole or clinopyroxene in the studied rocks indicates temperatures below ≈ 400-450ºC. The presence of chlorite/clinochlore suggests that the fluid CO2 content was low to moderate and hence this carbonate system could have been infiltrated by H2O external fluids. The CO2 was produced by progressive decarbonation during temperature increase and H2O interaction. The model predicts an iron-carbonate phase at lower temperature (ankerite).The presence of S species (not modeled) probably favored the development of Fe sulfides instead. Hypothesis 1 of regional metamorphism seems more plausible: the granite outcrops are, apparently, too far away from the studied rocks. Nevertheless, the external H2O might come from dehydration of metapelitic domains and/or exsolution by granite intrusions. External H2O input is also favored by the moderate to high variability (up to 6‰) observed in the isotopic δ18O values (Menéndez et al., 2010). References Menéndez, S., Rodríguez-Martínez, M., Moreno-Eiris, M., Perejón, A. & Reitner, J. (2010): Palaeo environmental and geochemical approach of archaeocyath-rich facies from Lower Cambrian of western Gondwana margin at Central Iberian Zone (Urda, Toledo Mountains, Spain). EGU General Assembly Conference Abstracts 12: 9359.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2018
- Bibcode:
- 2018EGUGA..20.2281M