The composition of phyllosilicates in Precambrian, low-grademetamorphic, clastic rocks from the Southern Hesperian Massif (Spain) used as an indicator to metamorphic conditions

13 páginas
In order to obtain an indication of metamorphic conditions we have studied the mineralogy of the clastic rocks, in particular the phyllosilicates, from three Precambrian series in the Central-Iberian and Ossa-Morena zones of the Southern Hesperian Massif: the Esquisto Grauváquico Complex; the so-called Serie Negra; and the Azuaga Formation. For this purpose we have used the crystal-chemical parameters obtained via X-ray diffraction (d001, b0, ratios of the basal intensity of the white micas and chlorites, and the crystallinity index of the mica) together with electron microprobe data from selected areas, obtained via backscattered electron images. The mineral assemblages, commonly quartz+muscovite +albite +chlorite ±biotite, and the values obtained for the illite crystallinity index (between 0.18° and 0.20° 2Θ) in rocks with non-diagnostic mineral assemblages point to at least greenschist-facies metamorphic conditons. The phengite content of the mica indicates an overall pressure regime near the limits between low and intermediate pressure, but some significant differences exist between the series studied. The Azuaga Formation shows the lowest pressure (about 4 kbar), while the Serie Negra indicates a pressure of at least 1 kbar more. In the Esquisto Grauváquico Complex, when the samples are considered overall, the results are similar to those of the Serie Negra, but there is inhomogeneity in the rocks from the two sections studied. The various types of chlorite, both textural and optical, are homogeneous in composition in each rock specimen and their chemical composition depends primarily on the composition of the host-rock.