Contact metamorphism and dolomitization overprint on Cambrian carbonates from the Ossa-Morena Zone (SW Iberian Massif): implications to Sr-chronology of carbonate rocks

Abstract

The Cambrian Series 2 Carbonate Formation from the Alter do Chão Elvas-Cumbres Mayores unit (Ossa-Morena Zone, SW Iberian Massif) is composed of regionally metamorphosed marbles and marlstones that underwent chlorite zone metamorphism and preserve the primaeval limestone 87Sr/86Sr ratios (0.7083–0.7088). These are consistent with the established Lower Cambrian seawater curve, and therefore used for age constraints in formations lacking fossil contents. The regional mineralogical and Sr-isotopic features of the carbonate rocks are frequently overprinted by the effects of contact metamorphism induced by magmatic bodies emplaced during rift-related and synorogenic events of the Palaeozoic, as well as by post-metamorphic dolomitization processes. The development of calc-silicate minerals due to contact metamorphism is common in the rocks of the Carbonate Formation and apparently results from the interaction of the protolith with fluids of different origin: (i) internally produced fluids released by conductive heating (observed in external contact aureoles) and (ii) external intrusion-expelled fluids that, besides leading to the appearance of distinctive assemblages, also promote an influx of strontium content (observed in roof pendants). Calc-silicate mineralogy varies substantially throughout the region, likely due to the heterogeneous distribution of silicate minerals of the protolith, progression of intrusion-driven fluids, and the irregular effect of thermal gradients. Results suggest that high-grade contact metamorphism (hornblende facies or higher) and dolomitization processes imposed on the Carbonate Formation significantly influence the isotopic signatures of the carbonates, providing limitations in applying Sr-isotopic chronology.