Abstract
Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) were used to evaluate the emplacement history of the Parashi stock and related dyke swarm situated in NW Colombia. The average magnetic susceptibility of 4.5×10-2 SI, in conjunction with low-coercivity components provided by the isothermal remanence and thermomagnetic curves with net Verwey and Curie transitions, indicates that multidomain magnetite records the anisotropy directions. The similar orientation and shape of the AMS and AAR ellipsoids indicate the absence of very fine magnetite with an inverse fabric. The magnetic foliation is the best-defined fabric element in these rocks and outlines a concentric structure, elongated parallel to the NE-SW direction of the pluton. Crystallisation age of the stock and dykes (51-47 Ma), along with pressure of emplacement determination indicate that the stock and the dyke swarm probably formed simultaneously, and they were emplaced in the shallow crust (
Highlights
Low-field Anisotropy of Magnetic Susceptibility (AMS) is a useful technique for recording fabrics in rocks that appear isotropic by visual inspection
Like AMS, Anisotropy of Anhysteretic Remanence (AAR) can be represented as a second-rank tensor, with the principal directions represented by an ellipsoid
This study examines the magnetic fabric of a quartz-dioritic stock and related dyke swarm from a time when the oblique convergence of the Caribbean plate triggered the exhumation and disruption of the former Late Cretaceous collisional orogen in the Guajira region
Summary
Low-field Anisotropy of Magnetic Susceptibility (AMS) is a useful technique for recording fabrics in rocks that appear isotropic by visual inspection. This anisotropy is based on the difference between the magnetic susceptibility measured in different directions of the rock, which itself depends on the preferred orientation of all the mineral grains weighted according to their concentrations and grain anisotropies (Hrouda, 1982). AMS records the bulk contribution of the diamagnetic (quartz, feldspars), paramagnetic (micas, amphibole) and ferromagnetic (magnetite, hematite) mineral assemblages. The Anisotropy of Anhysteretic Remanence (AAR) is a complementary magnetic method that records only the contribution to the magnetic fabric of minerals carrying a remanence, which is mostly magnetite in granitic rocks (Jackson, 1991). The AMS and AAR data, can provide valuable information about emplacement tectonics at different stages of pluton construction (Bouchez, 1997), especially in volcano-plutonic complexes in which earlier intrusions are often later invaded, usually in the shallower crust, by subvolcanic rocks and dykes (e.g., Magee et al, 2012; Tomek et al, 2017; Burton-Johnson et al, 2019)
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