Los anfíboles de las rocas básicas e intermedias de afinidad shoshonítica en el domo del Tormes

Zusammenfassung

Amphiboles in shoshonitic-affinity rocks from the Tormes dome show a compositional range between magnesiohornblende and actinolite. Most of them seem to be secondary, formed from clinopyroxene. Nevertheless, some of them, in textural equilibrium with K-feldspar and located in vugs, could be of primary origin. According to their chemical and isotope composition, they are not metamorphic but igneous, the amphiboles of the monzonitic association being magmatic and those of the vaugnerites being post-magmatic. Thermometric results indicate subsolidus conditions mainly for vaugnerites, suggesting an exsolved fluid phase from the magma that re-equilibrated the system. According to the high magnesium contents of the re-equilibrated amphiboles, the possibility of oxidizing conditions of the exsolved phase is suggested, as can be inferred from the high abundance of titanite. Maximum emplacement conditions of 2.5 kbar, based on the Al-content of the amphibole would be coherent with the presence of two feldspars in these rocks (subsolvus rocks) but, since Al-tschermak substitution is not the only that takes place in these amphiboles, underestimated pressure conditions should be inferred.