Autochthonous origin of the Encruzilhada Block, Dom Feliciano Belt, southern Brazil, based on aerogeophysics, image analysis and PT-paths

Abstract

The Encruzilhada Block lies between the eastern hinterland and western foreland of the Dom Feliciano Belt (southern Brazil) and its tectonic meaning in the orogenic evolution of the belt is still poorly understood. Its low magnetic signal is similar to that of the foreland supracrustal sequences, whereas the high gammaspectrometric signal appears due to the hinterland post-collisional granitoids. Satellite image lineament analysis points to tectonic similarities between the Encruzilhada Block and the foreland, as the structures in both domains show predominantly NNW and NNE trends, while the hinterland mostly presents NE-trending structures. The pre-orogenic connection between the Encruzilhada Block and the foreland is indicated by the correlation between the Várzea do Capivarita and Cerro da Árvore complexes. Both contain 800 – 780 Ma metaigneous rocks with arc-like geochemistry, and parametamorphic rocks with similar sedimentary provenance. Common syn-orogenic deformation history of both complexes is suggested by progressive to P-T o-th E-W transpressive deformation and similar peak metamorphic age (660 – 640 Ma). PT-paths suggest exhumation of the hinterland Várzea do Capivarita Complex from high-T/low-P conditions of 790–820 °C/4.4–4.8 kbar to 660–720 °C/2.5–3.4 kbar at ca. 630 Ma. On the other hand, the foreland Cerro da Árvore Complex was subject to progressive metamorphism from 555 to 565 °C/5.4–5.7 kbar to 560–580 °C/5.8–6.3 kbar, which is interpreted as a result of orogenic thickening. Evidence suggests that both complexes have originated in a similar, if not a single, basin on an attenuated lithosphere with high geothermal gradient, possibly a (back-arc?) rift. Oblique collision has caused basin inversion and thrusting of the lower crust over the rift margin, which may have been accompanied by lithosphere delamination. Progressive transpressional deformation and voluminous post-collisional magmatism (640 – 578 Ma) controlled by major lineaments have blurred the original tectonic contact and finally caused thermal metamorphism in both complexes at ca. 3 kbar, when the autochthonous Encruzilhada Block was stabilized in its present geological configuration. The intracontinental character of major shear zones limiting hinterland and foreland requires a review of the existing evolutionary model for the Dom Feliciano Belt in the context of Western Gondwana amalgamation.

Introduction

Distinguishing the native or exotic character of crustal blocks is essential to understand the geological evolution of orogenic belts, and this should be done based on multiple and independent criteria (e.g. Sengör and Dewey, 1990; Burton-Johnson and Riley, 2015). The pre-orogenic position of various crustal blocks is a central question in recent models for the Western Gondwana amalgamation (Fig. 1a), in which the major tectonic boundaries of the Mantiqueira Province (Almeida et al., 2000) mobile belts are interpreted either as sutures (e.g. Chemale Jr., 2000; Basei et al., 2005, 2008; Passarelli et al., 2011; Heilbron et al., 2020) or intracontinental shear zones (e.g. Florisbal et al., 2012; Oriolo et al., 2016a, 2017; Konopásek et al., 2018; Meira et al., 2015, 2019; Fossen et al., 2020; De Toni et al., 2020a, b; Percival et al., 2021).

The presence of high-T/low-P granulites in the Mantiqueira Province is conspicuous (e.g. Gross et al., 2006, 2009; Meira et al., 2019; Costa et al., 2020), and their tectonic setting is still a matter of debate. As pointed out by Kelsey and Hand (2011), high-T/low-P conditions are not expected for an overthickened lithosphere. Collision and tickening of a previously extended crust is assumed to account for the combination of high geothermal gradients and gently dipping contractional structures (Harris and Holland, 1984; Thompson et al., 2001; Hyndman et al., 2005; Clark et al., 2011; Fowler et al., 2015). A sequence of tectonic pull and push is expected when collision follows the opening of a rift or back-arc rift (Lister and Foster, 2009; Fowler et al., 2015). Recent models for the Musgrave Orogeny (central Australia) by Gorczyk and Vogt (2015) suggest that lithospheric delamination occured during contraction of previously weakened domains limited by cratonic blocks in an intracontinental setting, causing anomalously high geothermal heat flux distant from active plate boundaries.

In this study we investigate a “suspect terrane” of the Neoproterozoic Dom Feliciano Belt, southernmost Brazil. The Encruzilhada Block (as defined by Jost and Hartmann, 1984) is distinguished from the rest of the belt by its rock association, including the high-T/low-P Várzea do Capivarita Complex (VCC – e.g. Gross et al., 2006; Costa et al., 2020), abundant syenites and an anorthosite body. The block is separated from the hinterland Pelotas Batholith by the large-scale Dorsal de Canguçu Shear Zone, and from the foreland Tijucas Fold Belt by the Passo do Marinheiro Fault (Fig. 1b). The tectonic significance of the Encruzilhada Block remains poorly understood. Apart from its position between the orogenic hinterland and foreland, the block presents geophysical characteristics compatible with both domains (Hartmann et al., 2016). Large-scale aeromagnetometry and aerogammaspectrometry data together with structural data of the Encruzilhada Block are integrated with PT data from the high-T/low-P (cordierite + spinel)-bearing metapelite of Várzea do Capivarita Complex and a medium-grade schist of the easternmost foreland. Finally, we discuss the origin of the block and its role in the Dom Feliciano Belt tectonic evolution.