Neoproterozoic magmatic arc volcanism in the Borborema Province, NE Brazil: possible flare-ups and lulls and implications for western Gondwana assembly

Highlights

• A Cryogenian-Ediacaran continental magmatic arc in northeast Brazil

• U-Pb zircon data indicates five possible flare-ups from ca. 880 to ca. 600 Ma

• West Gondwana formed by both extroversion and introversion arc accretion mechanisms

Abstract

New zircon U-Pb (SHRIMP and LA-ICPMS), elemental and Nd-Sr geochemistry data on rhyolitic metavolcanic and metavolcaniclastic rocks of NE Brazil characterize widespread arc-related phenomena during the Neoproterozoic, related to the Conceição-type or Stage I plutonic rocks. U-Pb zircon dating pinpoint the main phase of magmatic activity at ca. 635-600 Ma in the 700-km long sigmoidal Piancó-Alto Brígida domain, but other important flare-ups might have taken place at ca. 670-690, 730-760, 810-820 and 860-880 Ma. A comprehensive compilation of detrital zircon data from metavolcanosedimentary successions of the entire Borborema Province (n=5532) confirms the occurrence of a quasi-continuum Neoproterozoic spectra punctuated by peaks at those same age intervals separated by minor lulls. Low Th/U rims of zircon crystals dated at ca. 577 Ma provide an estimate of the age of regional transpressional metamorphism. Samples of all age ranges are mostly calc-alkaline, magnesian and peraluminous, with moderately to highly fractionated LREE enrichment, negative Nb-Ta anomalies akin to convergent settings, and plot mainly within the volcanic arc field in tectonic discrimination diagrams. Nd-Sr isotope systematics indicate the involvement of juvenile Neoproterozoic melts from the mantle wedge, which upon mixing with Archean-Paleoproterozoic basement and contamination with the host metasedimentary rocks yield Mesoproterozoic T_DM mainly at 1.14-1.44 Ga, near-chondritic εNd(t) and ⁸⁷Sr/⁸⁶Sr_i 0.703-0.710. We put forward a model involving a major continental back-arc zone related to the development of the Conceição magmatic arc, akin to the modern-day Taupo volcanic zone of New Zealand, crosscutting NE Brazil and presumably continuing through the schist belts of Nigeria and Cameroon. The main magmatic flare ups might have been induced by extra-arc phenomena, such as collision of the West African paleocontinent with the northwestern Borborema edge due to closure of the exterior Goiás-Pharusian Ocean, force-speeding subduction in the interior V-shaped oceanic basins that constituted the Transnordestino-Central African Ocean and generating clockwise windshield-wiper-like rotation of the blocks back towards the São Francisco-Congo paleocontinent in a complete Wilson Cycle.

Introduction

Enduring magmatic arc systems evolve through protracted flare-up/lull cycles, with 100-1000 times more magma added to continental arcs during episodic flare-ups than during lulls (Paterson and Ducea, 2015). Although this pattern is now well recognized for Phanerozoic arc systems (e.g. Kirsch et al., 2016), it is still unclear if Proterozoic magmatic arcs evolved in the same way, and the role of protractred arc systems in the evolution of supercontinents such as Gondwanaland.

In this respect, the Borborema Province (Almeida et al., 1981; Brito Neves et al., 2000) of NE Brazil (Fig. 1) and its counterparts in NW Africa (Caxito et al., 2020a and references therein) are key elements for the understanding of Precambrian tectonic processes involved in western Gondwana amalgamation, situated right in the confluence between the major cratonic components of the São Francisco-Congo, West African and Saharan provinces. The Borborema Province was structured during the Brasiliano/Pan-African Orogeny, with widespread orogenic deformation, magmatism and metamorphism recorded in the ca. 630-510 Ma time range (Brito Neves et al., 2000; Caxito et al., 2020a).

Despite the relatively large amount of recent publications regarding the geology, geophysics and geodynamic evolution of this region (for a comprehensive review see Caxito et al., 2020a), models for the tectonic evolution of the province are contentious. An important recent advance is the interpretation of a plethora of calc-alkaline and high-K calc-alkaline plutons intruded in the central Borborema Province (Piancó-Alto Brígida and Alto Pajeú domains of the Transversal Zone), also known as Conceição-type plutons (or Stage I magmatism according to the nomenclature proposed by Van Schmus et al., 2011), as representing a Cryogenian-Ediacaran (ca. 650-610 Ma) continental magmatic arc, a proposition made independently by Sial and Ferreira (2015) and Brito Neves et al. (2016). This interpretation is however still under dispute (Neves, 2018).

Another batholith known as Tamboril-Santa Quitéria is also interpreted as a magmatic arc system developed at the province’s western border. The Tamboril-Santa Quitéria Complex is much more studied and interpreted as developed from subduction of the Goiás-Pharusian oceanic crust to the west at 880-860 Ma (early juvenile or transitional stage known as Lagoa Caiçara Arc) and ca. 650-610 Ma (late continental arc stage; Fetter et al., 2003; Santos et al., 2008; Ganade de Araujo et al., 2012a, Ganade de Araujo et al., 2012b, Ganade de Araujo et al., 2014b). Recently, other calc-alkaline granitic to granodioritic plutons in the same age range (630-600 Ma) in the southern zone of the Borborema Province have also been proposed as developed within a continental magmatic arc setting, such as the Betânia pluton of the Riacho do Pontal belt (Perpétuo, 2017), the Major Isidoro pluton of the PEAL domain (Silva et al., 2015) and both granodiorites (Oliveira et al., 2015a, Oliveira et al., 2015b) and mafic-ultramafic rocks within the Macururé domain (Pereira et al., 2020) of the Sergipano belt.

Thus, the infrastructure (i.e, the granitic and gabbroic plutons) of continental arc systems developed in the Borborema Province in the late Cryogenian-early Ediacaran is becoming progressively constrained. On the other hand, the suprastructure of those arc systems (i.e., their volcanosedimentary edifice) has not yet received proper attention. In this paper, we present a comprehensive study of metavolcanic and metavolcaniclastic rocks interleaved with turbiditic metagreywackes of the Piancó-Alto Brígida (PAB) domain, through field, geochemical, geochronological and isotopic data. The novel data sheds light on the supracrustal structure of the Transversal Zone (central Borborema Province) and suggests a direct link between the volcanic rocks and the plutonic counterpart.

Altogether, the Conceição-type plutons and the volcanic and volcaniclastic rocks forms a coherent infrastructure-suprastructure set that characterizes a continental magmatic arc system developed during the Neoproterozoic in the Borborema Province. This interpretation has far-reaching consequences for evolutionary models of western Gondwana amalgamation, specifically adding to the ongoing debate if whether the tectonic processes taking place in this portion of Gondwanaland in the Neoproterozoic were mostly intracontinental (e.g. Neves, 2003) or involved in Phanerozoic-style extroversion and introversion plate tectonics (e.g. Caxito et al., 2020a). The results and interpretations presented here also add to the understanding of Proterozoic arc systems and how they have evolved through time.