Comprehensive petrophysics of rocks from the Monesterio Antiform (Ossa Morena Zone, SW Spain)

Abstract

The aim of this paper is to analyse the variation of petrophysical properties measured in field and laboratory from surface rocks of the Monesterio Antiform and their relationship with the available geophysical data. It is shown how this variation explains some conspicuous anomalies recorded in recent ground gravity, and airborne magnetic and radiometric surveys carried out over the Antiform. In this work 916 density and magnetic susceptibility samples, 225 remanent magnetization samples, and 263 “in situ” spectrometric assays (natural gamma radiation: % K, ppm Th, ppm U) have been measured. Main petrophysical groups have been characterized: high density rocks (spilites, amphibolites, gabbroic and ultramafic suites), low density rocks (metasedimentary rocks, felsic volcanics and granitoids), the magnetic markers of the area (spilites, keratophyres, serpentinites, albitic granites and some gabbros), and finally, the paramagnetic set (metasedimentary and granitic rocks). Induction dominates over remanence, which is only important in some volcanites and the gabbroic rocks. Multidomain magnetite is the main ferromagnetic mineral. This fact simplifies the interpretation of total field magnetic survey results. Ground natural gamma data have been collected only on igneous rocks, which depict a wide range of radiometric values. The anomalous values are mainly of potassium–thorium or potassium–uranium character (subalkaline, calcalkaline–peraluminous acidic igneous rocks) except those from sodic granites with moderate to high radioactive actinide content. Gabbros, being the less radioactive bodies, can be distinguished by radioactive level and dissimilar evolutionary K–Th trends. The maximum spread in the spectrometric data is recorded on several hybrid igneous complexes. These show a great variety of radioactive element contents and evolutionary histories, from the gabbro-dioritic group to the late, most evolved granitic rocks, through the intermediate rock groups of all complexes, which are the three main discernible radioactive suites in the data. The petrophysical measurements presented here illustrate some natural patterns exhibited by the analysed rocks. The data concerning igneous rocks constitute a very compact series. These results will contribute significantly to improve new geological mapping projects and structural interpretations with the aid of the high-resolution airborne and ground surveys available in the Monesterio Antiform. It should be emphasized that the measurement and analysis of a thorough set of physical properties, relevant for petrological and geological interpretation of geophysical surveys, especially in an extensive area, is an important methodological aspect of this study, not commonly carried out in petrophysical work.