Magnetic fabric of the early-Ediacaran Itapetim monzogranitic pluton: magma flow during oblique extension along strike-slip shear zones (Eastern Brazil)

Abstract

The Itapetim pluton is a sigmoid-shaped intrusion bounded by the NE-SW, sinistral Itapetim and Taperoá shear zones. The Anisotropy of Magnetic Susceptibility (AMS) and Anhysteretic Remanent Magnetization (AARM) techniques and microstructural analysis were employed in order to characterize the pluton's emplacement mechanism and its relationships with regional tectonics. Microstructures are mainly sub-magmatic/incipient solid-state. Magnetic susceptibilities are typically paramagnetic (k ≤ 0.5 mSI). Thermomagnetic curves and hysteresis loops further indicate that negligible ferrimagnetic susceptibilities may be the result of fine-grained, oxidized magnetite inclusions. AARM fabrics do not show direct correlations with AMS fabrics. A WNW to E-W trending, shallowly-plunging magnetic lineation is associated with an intermediate-to steep-dipping magnetic foliation. The magnetic lineation locally displays intermediate plunges that may represent possible feeder zones for magma ascent during the initial stages of emplacement. The proposed emplacement model for the Itapetim pluton combines bulk NNW-SSE shortening with NE-SW trending simple shear along the Itapetim and Taperoá shear zones, and a WNW-ESE/E-W dextral stretching in the splay between both shear zones. The crystallizing magma mush flowed laterally along the splay during bulk NNW-SSE transpression, which is consistent with continental collision and subsequent extrusion of the Central Domain of the Borborema Province during the Neoproterozoic.