Dados da Submissão

Título

Metadunites and metaharzburgites from the Cerro Mantiqueira Ophiolite, São Gabriel Terrane, southern Brazil: petrographic features and major element geochemistry.

Texto do resumo

The mafic-ultramafic complex of Cerro Mantiqueira represents an ophiolitic remnant situated within the context of the Palma Accretionary Prism, a petrotectonic association of the São Gabriel Terrane, geotectonic unit of the Dom Feliciano Belt. This association is related to the São Gabriel orogenic event (0.77 – 0.68 Ga), which involved the closure of the Charrua Ocean, separating the continental areas of the Rio de La Plata craton and the Nico Perez microplate. With the aim of contributing to the petrographic and geochemical understanding of this complex, five samples were collected and analysed. The collected rocks were classified according to IUGS and CIPW-normative parameters as dunite (>90% olivine) and harzburgite (>40% olivine and >5% orthopyroxene). Due to the recrystallization of primary magmatic phases, metamorphic textures, and intense mineral alterations, the rocks were categorized as metadunite and metaharzburgite. The metadunite samples are characterized by relict olivine and pseudomorphic textures of olivine replaced by serpentine in the matrix and along intracrystalline fractures, forming a mesh texture. Chromite and magnetite crystals occur disseminated and predominantly exhibit concave and nodular shapes, with irregular fractures and cavities related to magmatic growth with silicate inclusions, commonly sheared. The metaharzburgite samples consist of enstatite, tremolite, and anthophyllite crystals, immersed in a matrix composed of serpentine and talc that formed during olivine alteration. Furthermore, the thin sections present evidence of recrystallization by hydration along orthopyroxene cleavage planes indicative of uralitization processes. Accessory minerals include hypidiomorphic crystals of magnetite, chromite, and chalcopyrite. Geochemical analyses by XRF revealed SiO2 (33–38 wt.%) and Mg# (80–92), compatible with mantle compositions dominated by forsterite-fayalite and hypersthene. The samples are characterized by low concentrations of Na, K, Ca and Ti, moderate Al contents and high Fe and Mg concentrations, which position them in the field of sub-alkaline ultramafic rocks in the De La Roche diagram. Geochemical modelling indicated that the separation of approximately 77.39% of olivine (65.47%), orthopyroxene (19.83%), amphibole (5.69%), and magnetite (9.01%) from metaharzburgite could produce a restite with compositions compatible with the metadunite, fitting reasonably well with the observed ranges for the most compatible trace elements (Ni, Cr, Co). Modelling the formation of these metaperidotites based on their geochemical and petrographic signatures is crucial for understanding the processes involved, thereby providing novel insights into the evolution of the Cerro Mantiqueira ophiolites.

Palavras Chave

Metaperidotite; Palma Accretionary Prism; Dom Feliciano Belt.