Regularities in distribution of nickel and other trace elements in olivines from rocks of the ultramafic massif Roseta (Southern framing of theSan Francisco Craton, Brazil)
Pinheiro M. A. P., Lesnov F. P., Suita M. T. F., Karolyuk V. N.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 5. P. 57-70
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Language: Russian
Abstract
The massif Roseta, located in the south-eastern part of Brazil, occurs among Neoproterozoic metamorphic strata of the southern margin of San Francisco craton. It is composed of the complex of lherzolites, clinopyroxene and clinopyroxene-free harzburgites, as well as having a limited spread of olivine websterites. There was detected in the massif a kind of amphibole-olivine-orthopyroxene-clinopyroxene rocks with a dash of green spinel having a metasomatic nature. Olivines from lherzolites, harzburgites and olivine websterites are different, both in major and minor components’ composition. The average values of the Fo parameter in olivine decrease from clinopiroxene-free to clinopyroxene harzburgites, lherzolites and olivine websterites. Olivine from lherzolites is characterized by the very high content of NiO at moderate contents of MnO. This mineral from clinopyroxene and clinopyroxene-free harzburgites has relatively high contents of NiO with a lower content of MnO. Olivines from olivine websterites are comparable by the NiO content with this mineral from harzburgite, and they are differing from olivines from harzburgites and lherzolites the higher MnO contents. The lowest NiO contents were defined in «harzburgites» with green spinel. In the overall, analysis of olivine rocks has revealed an inverse relationship between values of the Fo parameter and the content of MnO. Lherzolites and harzburgites of the Roseta massif represent restites depleted in various degree, which were formed with different degrees of partial melting of the upper mantle protolith. Taking into consideration the fact that within a relatively small tectonic unit of restitogenic ultramafic rocks, as it is with the Roseta massif, wherein in one erosion level there are present ultramafic restites differing in the degree of depletion, it may be assumed that the partial melting of the upper mantle protolith in this region was uneven within a relatively small volume.
Key words: olivine, lherzolite, harzburgite, websterite, nickel, manganese, electron probe microanalysis, ophiolite, Brazil.
