Mineralogy, petrography and geochemistry of equigranular and porphyritic metadiabase dykes in Bom Sucesso region, Minas Gerais state, Brazil

Abstract

A swarm of metadiabase dykes crops out in the northwest region of the Bom Sucesso and Ibituruna ridges. These dykes have strike N20º-40ºE and cut metaultramafic rocks and the Archaean Aureliano Mourão, Bom Sucesso and Capela São Francisco metagranites. The metadiabase dykes have similar mineralogy, represented by plagioclase, amphibole, ilmenite, magnetite and apatite, and can be subdivided into porphyritic and equigranular types. The porphyritic metadiabase dykes are rare and present bytownite phenocrysts, which vary in size, shape and proportion. Tabular grains of labradorite predominate in the matrix. The equigranular dykes are abundant, exhibit fine to medium granulation and vary from isotropic to strongly foliated. Plagioclase corresponds to andesine and labradorite and was subdivided chemically into two groups. The first is more enriched in most elements, exhibiting slight REE fractionation and incipient negative Eu anomaly. The second group is more depleted in all elements and exhibits horizontalized ETR pattern. The porphyritic metadiabase and the second group of metadiabase dykes are chemically similar and may be cogenetic.