Nota previa sobre la petrología y geoquímica de los granitoides de la Sierra de Guadarrama (España)

Abstract

This is a previous note of an overall geochemical and geochronological work now in progress about the late and post-kinematic hercynian granitoids of the western Spanish Central System. Sampling, up to the present, amounts to 230 rocks (thirty of them inclusions and late dykes) coming from the Sierra de Guadarrama, on which complete chemical analysis, i.e. major elements as well as the most meaningfull trace elements, have been carried out. Based on macroscopic and pétrographie data along with the chemical-mineralogical criteria devised by DEBON and LE FORT (1982), we have distinguished the following groups of rocks, excluding inclusions and dykes. 1. Intermediate rocks. Meta-aluminous amphibole-biotite quartz-diorites and granodiorites and related microgranular inclusions. These rocks are scarce, being more abundant to the west of the studied area. They often show planar and piano-linear fabrics. 2. Adamellites with granodioritic affinities. Most of them are per-aluminous rooks with biotite and sometimes Al-silicates, mainly cordierite and less often sillimanite, in varying proportions. Two sub-groups i.e. porphyritic and non-porphyritic adamellites can hp extinguished. On a first analysis the non-nornhvritic rocks seem to follow an aluminous-cafemic trend (rocks with amphibole occasionally exist), whilst the porphyritic types plot along a more aluminous trend. Nontneies% gradual transitions among bol h types seem to be common. Microgranular inclusions are abundant. Adamellites are by far the volumetrically most important rocks and usually display a weak to moderate planar and piano-linear fabrics. 3. Coarse-grained granites. They are aluminous quartz-rich rocks with biotite and little or none moscovite. Microgranular inclusions are almost lacking. Furthermore they do not show macroscopic fabrics. 4. Medium and fine-grained two-mica granites. They form an aluminous association normal in quartz. Along with the micas, Al-silicates like cordierite or garnet and sillimanite or andalusite are usually present. Microgranular inclusions are never seen. From a structural point of view these rocks are isotropic. Statistical taxonomic analysis of granitoids match to a large extent the classification put forward above. Factorial analysis of the geochemical data shows that the Sierra de Guadarrama granitoids form an homogeneous group of rocks ranging from an Al-Ca-Fe-Mg pole to a Si-K pole. In spite of the present lacking of enough radiometric data, macroscopic observations point to a relative chronology broadly in agreement with the classification order above. Meta-aluminous rocks are conserved either as relics near the metamorphic bodies or as large inclusions inside the adamellites. On the other side coarse grained biotite granites and two-mica granites form as a rule small massifs cutting across the adamellites. Emplacement of the first group of rocks and, may be also, the most deformed types of the second group, are thought to be late-kinematic with regard to a main compressive phase (F3). On the other side, coarse-grained biotite granites and two-mica granites are clearly post-kinematic. For the moment it is uncertain weather a single or more superposed magmatic trends exist in the Central System.