THE PHYLLOSILICATES IN DIAGENETIC-METAMORPHIC ROCKS OF THE SOUTH PORTUGUESE ZONE, SOUTHWESTERN PORTUGAL

Abstract

Devonian and Carboniferous metasedimentary sequences in the southernmost sector of the Iberian Massif, part of the Hercynian Orogen in southwestern Portugal, contain shales and slates of low but variable metamorphic grade. These rocks have been studied by optical microscopy and XRD to obtain basic data on their clay mineralogy and crystal-chemical parameters (IC, the index of “crystallinity”, b and d001). Electron-microscopy techniques (SEM, HRTEM and AEM) have been used to characterize the textural relationships and chemical evolution of the minerals, mainly the phyllosilicates. A good correlation has been foun d between the data determined by XRD methods and by electron microscopy. The main mineral association is quartz + dioctahedral K-rich mica + feldspars, but Na–K intermediate mica, paragonite, kaolinite and chlorite also are present, although discontinuou sly. The thickness of crystallites measured directly from lattice-fringe images is related to the IC, providing information on the evolution of the metamorphism through the sequences, with decreasing IC from southwest to northeast. The IC values for both the Brejeira and Mira formations correspond to diagenesis and anchizone, the Mertola and Phyllite–Quartzite formations, to high anchizone–epizone, and the Pulo do Lobo Formation presents values typical of epizonal conditions. Despite this pattern of evolution, however, the changes in metamorphic grade are not progressive, and the abrupt changes found between the characterist ic values of each stratigraphic formation are interpreted as being associated with thrust surfaces. Textural data provide a clear contrast between the samples of different grade, with the lowest-grade samples showing a bedding-parallel microfabric and stacks of phyllosilicates. The most strongly metamorphosed pelites, on the other hand, present a crenulation-cleavage microfabr ic and a recrystallized matrix. As grade of metamorphism increases, crystal thickness increases and defects decrease in the phyllosilicates, although chlorite shows a higher density of strain-related defects than mica. There is wide scatter in the che mical data for both dioctahedral micas and chlorites, but some chemical characteristics can still be discerned. XRD and AEM reveal a tendency for the micas to be more similar to muscovite and less to illite with increasing metamorphic grade, although both compositions are present in all samples. The extent of phengitic substitution is extremely low, indicating low-pressure conditi ons. The formation of chlorite and its composition are mainly controlled by the host-rock composition.