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Magnetic susceptibility of Variscan granite-types of the Spanish Central System and the redox state of magma

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Villaseca González, Carlos and Ruiz Martínez, Vicente Carlos and Pérez-Soba Aguilar, Cecilia (2017) Magnetic susceptibility of Variscan granite-types of the Spanish Central System and the redox state of magma. Geologica acta, 15 (4). pp. 379-394. ISSN 1695-6133

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Official URL: http://dx.doi.org/10.1344/GeologicaActa2017.15.4.8


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Abstract

Magnetic susceptibility (MS) has been measured in Variscan granites from central Spain. They yield values in the order of 15 to 180 mu SI units for S-and I-type granites, indicating that both types belong to the ilmenite series. Only samples from magnetite-bearing leucogranites from the I-type La Pedriza massif show high MS values, in the order of 500-1400 mu SI, reflecting the presence of this ferromagnetic mineral. Mineral chemistry of magmatic Fe-rich minerals (mainly biotite) suggests similar oxidation values for both granite types. MS values change in highly fractionated granites accordingly either with the presence of rare new Fe-oxide phases (some I-type leucogranites) or with the marked modal amount decrease of Fe-rich minerals (I-and S-type leucogranites). The redox state in highly fractionated granite melts is mostly controlled by magmatic processes that modify redox conditions inherited from the source region. Thus, the occurrence of magnetite or ilmenite in granites is primarily controlled by the oxidation state of the source material but also by the differentiation degree of the granite melt. The presence of magnetite in some Variscan I-type leucogranites might be a consequence of crystal fractionation processes in a more limited mafic mineral assemblage than in S-type granite melts.


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© C. Villaseca, V.C. Ruíz-Martínez, C. Pérez-Soba, 2017
We thank Alfredo Fernandez Larios for his assistance with the electron microprobe analyses in the Centro Nacional de Microscopia Electronica Luis Bru (UCM). Comments and revision of an early version of the work by Dr. Encarnacion Roda-Robles were very fruitful. We also thank the comments and suggestions made by Montserrat Liesa (Editor) and two anonymous reviewers that greatly increased the quality of the manuscript. This work is included in the objectives and supported by the CGL2016-78796 project of the Spanish MICINN and the Santander-UCM PR26/16-20294 project.

Uncontrolled Keywords:Central iberian zone; Lower continental-crust; i-Type; Central Spain; Peraluminous granites; Hercynian granitoids; Zircon composition; Northern portugal; s-Type; Rocks; Magnetic susceptibility; Granite type; Biotite chemistry; Oxygen fugacity
Subjects:Sciences > Physics > Geophysics
Sciences > Physics > Meteorology
ID Code:46389
Deposited On:08 Feb 2018 18:10
Last Modified:09 Apr 2018 15:02

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