J. López Ruiz

Chemical variations in biotites during prograde metamorphism, Sierra de Guadarrama, Sistema Central, Spain

Abstract With increasing metamorphic grade, the biotites in the metapelites of the Sierra de Guadarrama show an increase of Al IV , Ti, Mn and K, and a decrease of Si and Al VI contents. On the contrary, the biotites in meta-arkoses show increasing Si, Mn and K and decreasing Al IV and Ti contents. The increase of Al IV (and consequently the decrease of Si) of the biotites in the metapelites could have been conditioned by increase of temperature, whereas the decrease of Al IV of the biotites in the meta-arkoses could be related with modal percentage of plagioclase. The variation trends of Mn and K can be related with resorption processes of garnets and progressive breakdown of muscovite, respectively.

The garnets of the eastern area of the Sierra de Guadarrama, Sistema Central, Spain

Abstract Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals. The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO 2 , Al 2 O 3 and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al 2 O 3 and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.