Magnetic Fabric and Geochronology of a Cambrian "Isotropic" Pluton in the Neoproterozoic Aracuai Orogen

The Aracuai Orogen (AO) has been interpreted as a Neoproterozoic example of a large, "hot" orogen, based on a broad zone (250 km) of midcrustal metamorphic assemblages with a long, 70-Myr history of crustal melting and episodic magmatism throughout the late Neoproterozoic and earliest Paleozoic. Here, we present results of U-Pb sensitive high-resoution ion microprobe (SHRIMP) zircon dating and detailed anisotropy of magnetic susceptibility (AMS) study on a late Cambrian, bimodal pluton related to final period of collapse of the AO. New U-Pb zircon ages constrain the crystallization age of different suites within the Santa Angelica Pluton, 506 +/- 3 Ma for the early felsic phase and 498 +/- 5 Ma for the mafic core. New AMS data indicate that the emplacement of the Santa Angelica Pluton corresponds to two coupled plutons with concentric structures arrayed about a twin, bull's eye pattern. During the final stages of intrusion, upward relative movement of the northeastern lobe exposed the deeper levels of the intrusion, relative to the more shallowly eroded southwestern lobe. These observations suggest that magma emplacement was controlled by magma buoyancy forces, with little influence of regional tectonic stress. This behavior contrasts with the well-defined, tectonic-controlled fabrics of coeval plutons occurring to the north which was still hot, therefore highlighting the contrasting thermal evolution between different sectors of the orogen during its final stages.