Correlation of magnetic susceptibility with 18O data in magnetite- and ilmenite-type granites from Iberian massif

Abstract

The relationship between oxygen isotopic values and magnetic susceptibility composition on 11 Variscan Portuguese granites has been investigated. Whole-rock oxygen-isotope (18O ) values for Vieira do Minho (VM), Vila Pouca de Aguiar (VPA), Chaves, Castelo Branco (CB), Manteigas and Serra da Estrela (SE) granitoids, were compilated from bibliography [1,2,3,4], and 18O for Santa Eulalia Plutonic Complex (SEPC) were obtained by laser fluorination at the Stable Isotopic Laboratory of Salamanca. Magnetic susceptibility (Km) values were obtained with a Kappabridge equipment from Toulouse University and Geology Centre, Porto University [2,5,6,7,8]. In this study is shown that there is a significant inverse correlation between Km and 18O. Magnetite-type granites (Manteigas granodiorite and SEPC external facies) have Km>10-3 SI and low 18O values ranging from 8.9 to 10.3 0/00 instead those of ilmenite-type (all the other granites) have Km 10-4 SI and are 18O enriched (9.3 to 13.5 0/00). The I-type granites (VM, VPA, Chaves, Manteigas and SEPC external facies) show lower average 18O (10.2 0/00) and higher Km values (100x10-6 SI) than the S-type granites (SE and CB) with 18O = 12.6 0/00 and Km = 65x10-6 SI. Figure 1: Semi-log plot of Km (in Si units) versus 18O.

This work has been financially supported by PTDC/CTEGIX/ 099447/2008 (FCT-Portugal, COMPETE/FEDER).

[1] Martins et al. (in prep.) [2] Martins et al. (2009) Lithos 111, 142-155. [3] Antunes et al. (2008) Lithos 103, 445-465. [4] Neiva et al. (2009) Lithos 111, 186-202. [5] Sant’Ovaia et al. (2010) JSG 32, 1450-1465. [6] Sant’Ovaia et al. (2000) TRSE, ES 91, 123-127. [7] Sant’Ovaia et al. (2008) 33rd IGC CD. [8] Sant’Ovaia et al. (2011) Min. Mag. 75, 3, 1795.