Miarolas del plutón de La Cabrera (Madrid). Clasificación y geocronología de los rellenos hidrotermales

Abstract

A large part of the hydrothermal processes that took place in the Sierra of Guadarrama in the last 300 Ma, are recorded in the infillings of miarolitic cavities that are common in the La Cabrera granitic pluton. Ca-rich minerals such as epidote, prehnite, laumontite, and hexagonal calcite, and to a smaller extent K minerals (muscovite, microcline and apophyllite) are abudant in these cavities and resulted from protracted hydrothermal activity. The miarolitic cavities have been classified in four types: a) cavities with calcic minerals (epidote, prehnite, laumontite and hexagonal calcite), b) cavities with K minerals (muscovite and microcline), c) mixed cavities and d) cavities with quartz, clorite (Mg) and pyrite. Model ages (87Sr/86Sr) of epidote, microcline and apophyllite are 278±18, 143±0.5 and 8.47 Ma, respectively. K/Ar ages of four microdines are 141 ±4, 142±4, 119±3 and 102±9 Ma, and one apophyllite has given 10±2.0 Ma. Epidote model ages are coincident with the age of episyenites and that of some barren quartz veins. Microcline ages match those of some F-Pb-Ba lodes and of younger barren quartz veins. Apophyllite formation is correlated with the recent uplift of the Sierra of Guadarrama.