Fenómenos de alcalinización en granitoides hercínicos de la Sierra de Guadarrama (Sistema Central)

Zusammenfassung

This paper describes two panicular cases of granites which are dequanzified and alkalinified along their fractures planes. The outcrops are small and lenticulal, shaped (< 5 m width) with scarce longitudinal continuity (< 200 m length) and centimetrical transits with the not transformated granite. In the pluton of Villacastín this piesienite types appear in fractures N50E defining outocrops with a complex compositional banding whose more transformated varieties are composed by albitized plagioclase, ferroaugite and ferrohornblende, usually associated with interstitial microcline. The episienites of the plutonic complex of La Pedriza are aligned in N85E fractures, and they are an hydrothermal transformation of the porphiritic cordierite granites of the intermediate zone of this complex. These rocks reach a peralkaline mineralogy with albite, residual microcline, aegirine, riebeckite and limited quartz as main phases. The episienites of the Villacastín have a middle metamorphic grade paragenesis of low pressure (an estimated temperatures of 450º-500ºC and pressures less than 0.6 kb), whereas the ones of La Pedriza are of lower temperature, i. e. low metamorphic grade (temperatures of 250º-450°C and a pressure less than O.5 kb). These pyroxenic-amphibolitic episienites have not been described in others areas of the Iberian Hercynian Belt.