Geostatistical approach to the study of the magnetic susceptibility variation: Lamas de Olo Pluton case study
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RESEARCH PAPER
Geostatistical approach to the study of the magnetic susceptibility variation: Lamas de Olo Pluton case study Cláudia Cruz1 · Joaquim Góis2 · Helena Sant’Ovaia1 · Fernando Noronha1 Received: 21 November 2019 / Accepted: 13 May 2020 © Universidad Complutense de Madrid 2020
Abstract The Lamas de Olo Pluton, in northern Portugal, is a post-tectonic pluton located in the northern part of the Central Iberian Zone of the Iberian Variscan belt. It is a composite massif comprising different granites: Lamas de Olo, Alto dos Cabeços and Barragem. Magnetic susceptibility was measured in 349 oriented cores from 48 sampling sites across the pluton. The measured magnetic susceptibility (Km) values range between 21 µSI (or 21 × 10− 6 SI) and 2769 µSI for all studied granites. The application of the geostatistical methodologies leads to a more detailed mapping of the magnetic susceptibility data and provided a quantification of its spatial dispersion. Kriging interpolation allows the visualization of two different areas in the Lamas de Olo Pluton: (1) areas with higher magnetic susceptibility values, typical of magnetite-bearing granites (Km ≥ 1200 µSI) and (2) areas with lower values, which correspond to ilmenite-type behavior granites (21 µSI Muscovite) and (3) Barragem, a leucocratic fine- to medium-grained and slightly porphyritic granite (Biotite = Muscovite) (Pereira 1989; Helal 1992; Fernandes et al. 2013). In the field, the contacts between Lamas de Olo and Alto dos Cabeços granites are generally
diffuse, but field observations show that Barragem granite crosscuts Lamas de Olo. The LOP intrudes the Armorican Quartzite Formation and the schists from Upper Silurian to Middle Ordovician (Lower Paleozoic Formations), the Desejosa Formation (belonging to the metasediments from Upper to Medium Cambrian of the Douro Group) and the two-mica syntectonic granites from the Vila Real Massif (Fig. 1; Pereira 1989). The two-mica granites yielded and weighted average 207Pb/235U age of ca. 311 ± 1 Ma for the Cabeceiras de Basto granite (Almeida et al. 1998) and of ca. 316 ± 7 Ma for the Carrazeda de Ansiões granite (Teixeira 2008) for monazite and zircon. 207Pb/235U dating yielded an age of 297.19 ± 0.73 Ma for the Lamas de Olo granite (Fernandes et al. 2013).
3 Materials and methods The relationship between the magnetization M induced in a material and the external field H is defined as M = K H, where K is the magnetic susceptibility (Hrouda 1982; Bouchez 1997). K reflects the mineral composition of the whole rock, comprising the diamagnetic (e.g. quartz, feldspar), paramagnetic (e.g. biotite, muscovite or hornblende) and ferromagnetic s.l. minerals (e.g. magnetite or hematite). K measurements were performed using a Agico KLY-4S Kappabridge susceptometer (Czech Republic) from the Instituto de Ciências da Terra-Pólo Porto, in a 300 A/m field. For each site, the ANISOFT 4.2 program package (Chadima and Jelinek 2009) enabled us to calculate the mean susceptibility,
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Km. The Km values are important in gran
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