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Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton, SE Brazil

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Abstract

Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.

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Acknowledgments

We thank FAPESP (95/8399-0 and 07/0935-5 grants) Brazilian agency for its financial support. Pressi L.F. thanks CNPq (Brazilian agency) for MSc scholarship (133629/2009-0 grant). We thank Alex Fortunato Ribeiro for his help in both field work and AMS measurements. We also thank Fátima Martín-Hernández, an anonymous referee, and the Editor Manish A. Mamtani for their comments and suggestions.

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  1. Instituto de Geociências da Universidade de São Paulo, Rua do Lago, 562, São Paulo, SP, 05508-080, Brazil

    M. Irene B. Raposo, Leonardo Frederico Pressi & Valdecir de Assis Janasi

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  1. M. Irene B. Raposo
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  2. Leonardo Frederico Pressi
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  3. Valdecir de Assis Janasi
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Correspondence to M. Irene B. Raposo.

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Raposo, M.I.B., Pressi, L.F. & de Assis Janasi, V. Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton, SE Brazil. Int J Earth Sci (Geol Rundsch) 101, 773–786 (2012). https://doi.org/10.1007/s00531-011-0696-5

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