The Ribeirao da Folha ophiolite-bearing accretionary wedge (Aracuai orogen, SE Brazil): New data for Cryogenian plagiogranite and metasedimentary rocks

Author(s)

Amaral, Leandro ^[1] ; Caxito, Fabricio de Andrade ^{[1, 2]} ; Pedrosa-Soares, Antonio Carlos ^{[1, 2]} ; Queiroga, Glaucia ^{[2, 3]} ; Babinski, Marly ^{[4, 2]} ; Trindade, Ricardo ^{[4, 2]} ; Lana, Cristiano ^{[2, 3]} ; Chemale, Farid ^{[2, 5]}

Affiliation

^[1] Univ Fed Minas Gerais, CPMTC IGC UFMG, Programa Posgrad Geol, Av Antonio Carlos 6627, BR-31270901 Belo Horizonte, MG - Brazil

^[2] Brazilian Sci Council CNPq, Brasilia, DF - Brazil

^[3] Univ Fed Ouro Preto, Dept Geol, Escola Minas, BR-35400000 Morro Do Cruzeiro, MG - Brazil

^[4] Univ Sao Paulo, Inst Geociencias, Rua Lago 562, BR-05580080 Sao Paulo, SP - Brazil

^[5] Univ Vale Rio dos Sinos, Escola Politecn, BR-93022750 Sao Leopoldo, RS - Brazil

• Document type: Journal article
• Source: Precambrian Research; v. 336, JAN 2020.
• Web of Science Citations: 3

Abstract

The Aracuai orogen and West Congo belt make up a singular confined orogenic system, embraced by the San Francisco - Congo craton: the AWCO. It includes a southern sector with ophiolite bodies and magmatic arc, and a northern sector free of them, suggesting the precursor basin was an embayment partially floored by oceanic crust. The northernmost ophiolitic rock-assemblage found in the Aracuai orogen comprises metamafic and meta-ultramafic rocks with signatures of ocean-floor magmas, and associated pelagic to oceanic metasedimentary rocks of the Ribeirao da Folha Formation. Although tectonically dismembered and metamorphosed, those rocks resemble the classical ophiolite pseudostratigraphy. The Ribeirao da Folha Formation comprises rocks expected to be found in the upper units of an ophiolite edifice, like Al-rich micaschist (pelagic pelite), graphite-rich schist (black shale), sulfide-bearing metachert, diopsidite with massive sulfide, and banded iron formations (chemical-exhalative sediments), and sulfide-bearing fine-grained ortho-amphibolite with thin metachert intercalations (mafic volcanic unit). That formation hosts tectonic slices of banded ortho-amphibolite (dolerite) with plagiogranite veins, and coarse-grained massive ortho-amphibolite (gabbro), representing dismembered slivers from deeper mafic units, and slices of metaultramafic rocks from the deepest ophiolite units. Zircon crystals from a plagiogranite vein yielded the U-Pb SHRIMP age of 645 +/- 10 Ma, providing a new time constraint for ocean-floor emplacement, Micaschist samples show chemical attributes typical of distal passive margin pelites. Among three progressive deformation phases, the main ductile phase (D-n) shows kinematic indicators related to top to SW mass transport, associated with intermediate P-T (St, Ky, Sil) metamorphic zoning. Although the few youngest grains of detrital zircon from three siliciclastic samples have distinct ages (around 599 Ma, 741 Ma, and 816 Ma), their wide-range age spectra and Hf signatures suggest similar sediment sources. The wide lithological variety and stratigraphic complexity along with the intricate tectonic framework of the Ribeirao da Folha region, comprising thrust slices of ophiolitic rocks tectonically interleaved with older rift-related rocks, characterize an accretionary wedge that was scrapped off the subducted slab and involved in collisional tectonics, marking the AWCO suture zone for some 250 km between the Guanhaes basement block (lower plate) and Rio Doce magmatic arc (upper plate). (AU)

• FAPESP's process: 16/06114-6 - The Neoproterozoic Earth System and the rise of biological complexity
• Grantee: Ricardo Ivan Ferreira da Trindade
• Support Opportunities: Research Projects - Thematic Grants