⁴⁰Ar/³⁹Ar and Rb/Sr Geochronology of the Goiás-Crixás Dike Swarm, Central Brazil: Constraints on the Neoarchean-Paleoproterozoic Tectonic Boundary in South America, and Nd-Sr Signature of the Subcontinental Mantle

Abstract

The Goiás-Crixás tholeiitic swarm comprises high TiO 2 (> 1.5 wt %, HTI) and low TiO 2 (< 1.5 wt%, LTI) types that crosscut Archean metamorphic rocks of the Crixás-Goiás Block in central Brazil. Petrographically, the dikes were divided into three groups: diabases, metabasites, and amphibolites. Rb-Sr and 40 Ar/ 39 Ar analyses performed on the diabases and metabasites define the emplacement age at 2490 ± 40 Ma; field relations indicate that the dikes were emplaced under an extensional regime after regional uplift and cooling of the Neoarchean crust. The εSrT (-21.89 to +46.52) and εNdT (-4.84 to +0.53) values of the swarm are consistent with a DMM mantle source enriched by an EMI component. This signature is comparable to those exhibited by the Paleoproterozoic Uauá (ca. 2.2 Ga) and Lavras (1.9 Ga) dike swarms of the São Francisco craton. In comparison, the isotopic patterns of the Florida (1.70-1.75 Ga) and Salvador (1.2-1.0 Ga) dikes, from the Rio de La Plata and São Francisco cratons, respectively, display larger εNd and smaller εSr variations, suggesting a greater influence of the EMI component on these younger swarms. From the perspective of continental-scale evolution, the Goiás-Crixás dike swarm signals an extensional episode in the Brazilian Precambrian shield associated with Neoarchean-Paleoproterozoic tectonics.

Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil

“…The Faina greenstone belt is characterized by two sequences consisting of lower conglomerate, quartzite and pelite overlain by chemically precipitated rocks [93]. The maximum deposition age of sedimentary rocks for all greenstone belts is constrained by a mafic dike swarm emplaced between 2500-2300 Ma [94] that crosscuts Archean TTG complexes and volcanic rocks from the Goiás, Faina and Goiás belts (but not their upper sedimentary rocks). This field relationship implies that the sedimentary rocks must have been deposited following or during regional crustal extension after 2300 Ma.…”

“…2840 Ma [25]. The minimum age of the Dn event is provided by NE-and NW-trending mafic dikes dated by Rb-Sr (whole rock) and 40 Ar/ 39 Ar (hornblende) to 2490 ± 40 Ma, which crosscut the TTG complexes and lower volcanic rocks in the Faina, Goiás and Crixás greenstone belts [94].…”

“…Two main tectonic events are recorded in the GAB during the Paleoproterozoic [22]. An early crustal extension manifested by~2300 Ma epicratonic mafic dike swarm in TTG terranes [85,94] and a compressional deformation revealed by~2150 Ma metamorphic titanite [72]. Contrary to the age range reported for lower stratigraphic sequences of the Crixás, Faina, and Goiás greenstone belts at ca.3000-2800 Ma, volcanism at Guarinos and Pilar de Goiás greenstone belts dates approximately 2200 Ma [11,75,76].…”