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Magma Flow in Plumbing System-Type LIPs: Parameters from Anisotropy of Magnetic Susceptibility in Giant Dyke Swarms of the Equatorial Atlantic Magmatic Province

Antomat A. Macêdo Filho , Carlos F. Ávila, Carlos J. Archanjo, Maria Helena Hollanda, Alisson L. Oliveira

Abstract

Anisotropy of magnetic susceptibility (AMS) was applied to the Rio Ceará-Mirim Dyke Swarm, which intrudes Precambrian rocks in the northeastern corner of South America. This swarm consists of main subsets trending from NE-SW to E-W spanning over 1,000 km in length, along with minor NW-SE sub-sets. These dyke sets converge near a Cretaceous rift system formed during the fragmentation of West Gondwana. The dykes are predominantly high-TiO2 tholeiites. Zircon U-Pb (CA-ID-TIMS) dating indicates ages ranging from 133.38 to 133.28 Ma. AMS was investigated in 110 dykes distributed along the NE-SW sub-set and integrated with previous AMS studies from the E-W sub-set. Normal magnetic fabrics (foliation subparallel to the dyke plane) show steeply plunging magnetic lineations in the dike convergence, which is interpreted as the main magmatic feeder zone. Minor feeder zones were identified locally, such as within the E-W dykes near Tertiary volcanic necks. Away from the feeder zones, the magnetic lineations typically display shallow plunges. These results were summarized into a lithospheric swelling model with a main magmatic feeder focused on the dyke swarms convergence sector. The mafic magma would have flowed laterally exploring discontinuous crustal shear-band structures that propagate outward driven by far-field pressure increases originating from the central focus.