Geology and petrology of the mafic dike swarms of the Araguaia Belt: Evidence for distinct events of intracontinental magmatism in central-northern Brazil and correlations with large igneous provinces

Abstract

Extensive N–S and NNW-SEE-trending subparallel mafic dike swarms are intruded into metasedimentary rocks of the Tocantins Group, Araguaia Belt, Central-North of Brazil. They are under-examined, and there is little to no information about their origin and mantellic sources, as well as uncertainty about whether they are part of two different episodes of magmatism. Representative mafic dikes from the Santa Maria das Barreiras-Conceição do Araguaia region, at the boundary between the states of Pará and Tocantins, were studied in order to address these problems. It was possible to separate the dikes into two groups: one consisting of metadiabases affected by the Neoproterozoic regional metamorphism of the Araguaia Belt with varying degrees of transformations and mineral deformation; and the other consisting of unmetamorphosed and undeformed diabases and leucodiabases. The studied dikes are compositionally classified as subalkaline basalts with tholeiitic affinity and were interpreted to be originated in an intracontinental setting with the aid of Ti–V, Zr–Zr/Y, and Zr–Ti discrimination diagrams. However, metadiabases present an arc-like geochemical signature characterized by a pronounced Nb–Ta negative anomaly, whereas leucodiabases and diabases lack a negative Nb–Ta anomaly and show a LREE-enriched pattern, which resembles the signatures of plume-generated basaltic rocks. Also, there is evidence of an important contribution of enriched (EN) mantle components in the source of metadiabases and significant contribution of primitive mantle (PM) in the source of both leucodiabases and diabases. It was suggested, then, that the metadiabases represent the exposed plumbing system of arc-like intracontinental basalts which precede the regional Neoproterozoic metamorphism of the belt, and that the leucodiabases and diabases represent the exposed conduits of intracontinental basalts whose magmatism succeed the metamorphic event. Geochemical models were elaborated to demonstrate that fractional crystallization processes could account for the differentiation of the metadiabases and for the genesis of the leucodiabases via fractionation of the melt from which the diabases formed. The rocks from the earlier event share several similarities with Neoproterozoic mafic rocks from the eastern domain of the Araguaia Belt and nearby Tonian rocks of the 1100 Ma Rincón del Tigre-Huanchaca LIP event, while the rocks from the later event are remarkably similar to nearby CAMP basalts and diabase dikes.