Geologia e petrologia dos enxames de diques máficos da região de Santa Maria das Barreiras-Conceição do Araguaia: evidências de eventos distintos de magmatismo intracontinental no Centro- Norte do Brasil.

Abstract

N-S and NNW-SEE-trending subparallel mafic dike swarms are intruded into metasedimentary rocks of the Tocantins Group, Araguaia Belt, central-north Brazil. They are under-examined and there is little to no information about their origin and mantellic sources and uncertainty about their ages. 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 diabases affected by the Neoproterozoic regional metamorphism of the Araguaia Belt with varied 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. 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. Both group of dikes were interpreted to be originated in an intracontinental setting with the aid of Ti–V, Zr–Zr/Y and Zr–Ti discrimination diagrams. There is evidence of important contribution of enriched (EN) mantle components in the source of metadiabases and significant contribution of primitive mantle (PM) to the source of both leucodiabases and diabases. We suggested that the metadiabases represent the exposed plumbing system of arc-like intracontinental basalts which precede the regional Neoproterozoic metamorphism of the area and the leucodiabases and diabases represent the exposed conduits of intracontinental basalts whose magmatism succeed the metamorphic event. The rocks from the older 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 newer event are remarkably similar to nearby CAMP basalts and diabase dikes.