Comment on “A preserved early Ediacaran magmatic arc at the northernmost part of the transversal zone - central domain of the Borborema Province, Northeast of South America”, by B. B. de Brito Neves et al. (2016)

Abstract

The recent proposition for a long-lived (635-580 Ma) magmatic arc in the central portion of the Borborema Province is critically evaluated. Evidence favoring subduction include: low-T, high-P metamorphism; local occurrence of retroeclogites; and the probable participation of juvenile material in the genesis of 630-620 Ma-old plutons. However, several factors argue against the existence of a large ocean separating the Northern and Central subprovinces. First, early Tonian detrital zircons (1.0-0.9 Ga) in the Northern Subprovince indicate derivation from the Central Subprovince, and thus a connection between them until the time of deposition (ca. 660 Ma). Second, the age of peak metamorphism in the Northern Subprovince implies concomitant deformation in the upper and lower plates at 630-610 Ma. Third, 630-620 Ma-old granites have characteristics (e.g., high δO 18 values, low magnetic susceptibility) unlike plutons emplaced in continental active margins. Fourth, 590-580 Ma-old granites are clearly syn-transcurrent, i.e., post-collisional. Finally, the Patos shear zone, proposed to be a boundary transform between the two plates, is younger than 570 Ma, and thus cannot represent a suture zone. Therefore, if a magmatic arc existed, it had short duration and was developed at the end of the Cryogenian.

The role of airborne geophysics in the investigation of gold occurrences in the Itapetim Region, Borborema Province, Northeast Brazil

“…Borborema Province is in the Northeastern portion of Brazil and represents one of the major Neoproterozoic structural provinces of South America (Almeida et al 1981). It comprises basement domains formed by Paleoproterozoic gneissic and migmatitic rocks and some Archean nuclei (Dantas et al 2013, Costa et al 2015, Santos et al 2015, 2017a, Lima et al 2019) that are interleaved or partially covered by Meso-to Neoproterozoic metasedimentary and metavolcanic sequences and terranes, which are later intruded by several Ediacaran granitic plutons associated with the Brasiliano Orogeny (Van Schmus et al 1995, 2016, Neves et al 2006, Sial & Ferreira 2016, Basto et al 2019, Santos et al 2018. Based on the general displacement of the major shear zones (i.e., Patos and Pernambuco lineaments), the province is divided into three subprovinces: Northern, Transversal, and Southern (Santos et al 2000, as seen in Fig.…”

“…The occurrence and concentration of gold in the continental crust requires a combination of several geological processes, including specific tectonic settings, unique structural framework, and successive episodes of rock-fluid interaction (Groves et al 2003 and references therein). Orogenic-related (lode-type) deposits are established in accretionary and collisional orogenic systems, mainly in the late subduction stages, which are triggered by crustal shortening via compressional and transpressional/transcurrent regimes (Groves et al 2003, 2018, Goldfarb & Groves 2015. General geological aspects of these deposits comprehend association with trans-lithospheric faults, occurrence in terranes that experienced greenschist facies metamorphism, hydrothermal alteration, and a predominant quartz, carbonate, mica, chlorite, pyrite, scheelite, and tourmaline paragenesis (McCuaig & Kerrich 1998).…”

Accretionary models for the Neoproterozoic evolution of the Borborema Province: advances and open questions

“…In the past years, petrological, geochemical, and isotopic evidence is piling up, suggesting that a large number of granitic batholiths and stocks intruded in the BP mainly in the 635-600 Ma interval could also represent continental arc settings (Sial and Ferreira 2016). The most emblematic example is the ENE-WSW trend, 750 km long and up to 140 km wide Conceição magmatic arc of the Piancó-Alto Brigída Fold Belt in the Central BP (Brito Neves et al 2014Neves et al , 2016. About 90 different I-type plutonic bodies cover a large compositional range including "normal" calc-alkaline epidote-bearing, high-K calc alkaline and shoshonitic series, covering the spectrum of continental arc magmatism (Eyuboglu et al 2011).…”

“…It is speculated that such arc existed from 635 to 580 Ma, partially fitting with other Ediacaran subduction-related associations such as those from the Santa Quitéria Complex to the north and the Betânia arc to the south (see Caxito et al 2020a for detailed comparisons). The close association of the roots of this ancient magmatic arc with the Patos Shear Zone led Brito Neves et al (2016) to consider this structure as a former boundary transform, thus, marking another paleo-suture zone.…”

“…According to Neves (2018), a large ocean could not have existed along the Patos Shear Zone, because of the occurrence of early Tonian detrital zircon grains in the Northern BP, suggesting a connection with the Central BP and sourcing from the Cariris Velhos belt, as well as the lack of clear isotopic parameters of arc-related derivation of the early Ediacaran granites. In a general sense, compilations of detrital zircon data throughout the province reveal similar age distributions with main peaks at 2.15 to 2.05 Ga, an age range that characterizes most of the BP basement as well as the basement of the São Francisco Craton further south.…”