Metavolcanic rocks and orthogneisses from Porongos and Várzea do Capivarita complexes: A case for identification of tectonic interleaving at different crustal levels from structural and geochemical data in southernmost Brazil

Abstract

The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Várzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D2 and D3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D1 structure is an alternation of non-mylonitic S1 and mylonitic-S1, developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L1) measurements along a great circle in stereoplots is interpreted to result from shearing along S1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma).