Provenance Studies of Natural Stones Used in Historical Buildings of the Peninsula de Barbanza, Galicia, Spain (North-Western Iberia)

Abstract

Traditionally, the building stones used in the architectural heritage of Galicia (Spain) during the past were mainly extracted from quarries located in the surrounding areas of the historical buildings. Thus, a great variety of monuments were built with the same type of granite but with different degrees of weathering depending on local conditions, geological context (facies), and period of construction. The main purpose of this work is to evaluate the probable origin of the construction materials of six historical buildings on the Barbanza Peninsula, Galicia (Spain), based on the degree of weathering and petrographic-mineralogical characteristics. The evaluation was performed on six different samples of granite according to the geological context in outcrops of places where there are old quarries. We used X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), and petrographic microscopy (PM) to attempt to address whether the origin is really local to the Barbanza Peninsula or whether the stone was brought from more distant places, based on our knowledge of the geological context of the study area. Based on the chemical, textural, and mineralogical analyses obtained, the material used for construction has a local origin and comes from small quarries spread over a wide area within the Peninsula itself. Barbanza-type granite is the most abundant within the geological context of the area and also the most used as a construction material. Other types of granites identified are the Confurco Granite and other granitoids of the Noya complex. The Chemical Alteration Index (CIA) shows low to moderate weathering in the granites, fitting petrographic observations. In monuments, samples show higher concentrations of Na and K due to salt crystallization. They show microcracks due to extraction, cutting, and finishing processes and common alteration processes of the main minerals, such as sericitization of plagioclase and chloritization of biotite. This work consists of a multidisciplinary study focused on the geological perspective for the identification and preservation of historic quarries. Knowledge of the original material also presents a unique opportunity for the restoration and/or reconstruction of monuments, which allows for the maintenance of their conceptual and constructive homogeneity.