Estudio petrológico y estructural del área antiformal del oeste de Pereruela (provincia de Zamora)

Abstract

The present work reports on a mapping, pétrographie and structural study of the area of the Pereruela antiform (central Zamora). Pétrographie and geochemical characterization of the gneisses of the core antiform is emphasized, as well as the formation of tourmaline. From a structural point of view, this area displays an outstanding variability, because the Pereruela-Villalcampo shear zone is overlapped on the phase III hercynian antiform. Conventionally, six sectors of different petrological and structural meaning are established; so Northwards a shistose and quarzitic sector, with likely extensional crenulation cleavage, follows Sayago anatectic granite, of which pelitic country rock appears phyllonitized. Concerning metamorphism it is worthy to note the subparallel geometry of Ml-metamorphism isograds, with chloritoid, garnet and relict staurolite, with regard to M2-metamorphism isograds, with andalusite, cordierite and prismatic sillimanite. The Sayago granite boundary is reworked by the shearing, so its contact becomes more parallel to these metamorphic isograds as well as in relation to the mega-structures of the country rock. On the other hand, the fluids generated by the anatectic process are mobilized by shearing towards a peripheral area of the core antiform; consequently, tourmalinites and a great quantity of Sn-W rich veins and aplopegmatitic segregations are developped. In particular, a preference for boron in plagioclase-biotite bearing gneisses and in sillimanite-biotite bearing schists can be deduced. Furthermore, the appearance of tourmaline in the leucocratic granites could be seen as a subsolidus evolution during shearing, in the same way as common myrmekites of the feldspars of the microglandular gneisses and porphyritic Sayago-type granite.