Metamorphic evolution from diagenesis to epizone in Cambrian formations from NW Zafra (Ossa-Morena Zone, SW Spain)

Abstract

The metamorphic evolution of clastic and volcanic rocks has been studied in the Alconera Unit as a representative series of the Ossa-Morena Zone Cambrian (Hesperian Massif, SW Spain). The phyllosilicate crystalchemical parameters were obtained by X-ray diffraction (d001, bo, basal intensity ratios and illite crystalinity index (IC), SEM and EDX analyses from selected areas as well as electron microprobe analyses from metamorphic minerals in volcanic rocks. In the detritic lithologies, the mineral assemblages quartz + albite + mica +- (chlorite and berthierine) are non-diagnostic in sub-greenschist facies metamorphic conditions. The white mica IC shows a temperature decrease from bottom (epizone conditions) to top (diagenetic conditions). The phengite content indicates intermediate pressure conditions in the Lower Cambrian. The b0 values from Middle and Upper Cambrian rocks correspond to low-pressure values, but are highly affected by detrital mica components due to a lack of reequilibration, as these rocks have not overcome diagenetic conditions. Detrital biotites, berthierine and the 1M dioctahedral-mica polytype are other indications of the absence of equilibrium at the top of the sequence. In the volcanic rocks, the very low-grade metamorphism has produced chlorite, silica varieties and calcite, but index clac-silicates are absent due to high aCO2. The proportion of chlorite versus smectite layers (Xc), determined from electron microprobe analyses of chlorite, increases from 0.81 at the top of the sequence to 0.91 in the Middle/Lower Cambrian boundary. The metamorphism may be of a sedimentary burial type; however, an anomalously high temperature increase from top to bottom seems to indicate an effect due to igneous rock intrusions or diastathermal metamorphism.