Application of magnetic fabrics to the emplacement and tectonic history of Devonian granitoids in central Argentina
M. G. López De Luchi, A. E. Rapalini, S. Siegesmund, A. Steenken, 2004. "Application of magnetic fabrics to the emplacement and tectonic history of Devonian granitoids in central Argentina", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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Regional considerations on the tectonic regime during the emplacement of the Early Devonian magmatic units in the Sierra de San Luis are inferred from combined field, petrographic and AMS observations. Devonian granitoids of the Sierra de San Luis, in central Argentina, constitute elliptical composite batholiths and make up the most voluminous magmatism that appears in the Sierra. Detailed fabric studies have been carried out on the La Totora batholith (33°09′S, 65°42′W), which complement previous studies on two of the largest plutons in the Sierra de San Luis: The Renca and Las Chacras-Potrerillos batholiths. The studies comprised systematic field surveys, petrographic observations and anisotropy of magnetic susceptibility (AMS) measurements. Microstructural studies indicate that the batholith rocks are mainly characterized by magmatic microstructures with limited sub-magmatic to high-temperature solid-state deformation. All three batholiths possess concentric foliation patterns. The average magnetic foliation patterns in the studied plutons agree well with the macroscopic fabrics measured in the field indicating that the AMS-data can be used to study the orientation of fabric elements. Bulk susceptibility indicates a predominance of ferromagnetic contributions, although some paramagnetic sub-units are also present. Most foliations and lineations reflect magmatic flow and their attitude is linked to the interference between regional deformation and batholith inflation, i.e. fabrics may be due to regional strain in combination with the internal dynamics of the magma bodies. Rock fabrics are mainly described by oblate magmetic fabric ellipsoids. Magnetic lineations generally show a NNE-SSW trend that is interpreted to be controlled by the opening transtensional pull-apart structures during batholith inflation. It turns out that the Devonian batholiths intruded the basement syn-kinematically with respect to the Achalian deformational cycle.
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.