Petrografia e química mineral da suíte shoshonítica gabro-norito-diorito ediacarana da região de São José do Campestre / RN, NE do Brasil

Abstract

The term shoshonitic series encompasses a series of volcanic rocks and their plutonic counterparts, relatively saturated in SiO2 with levels of alkalis (> 5%), high and variable concentration of Al2O3, low TiO2 levels, and enrichment in P2O5, Rb, Sr, Ba, Pb and LREE (light rare earth elements). In the Borborema Province, the shoshonite suite is represented by gabbroic, dioritic, monzodioritic, monzonitic, quartz-monzonitic and tonalitic rocks. In its easternmost portion, the so-called São José do Campestre Domain, there are lithotypes of this suite, still little understood concerning their petrography, mineralogy and mineral chemistry. This research aims to fill these gaps and better understand the chemical nature of shoshonitic magma, in an area located east of São José do Campestre. Gabbro-noritic, gabbroic, dioritic and monzonitic rocks compound a shoshonitic suite in the studied area. These lithotypes intrude the paleoproterozoic gneissic basement, associated or as intermediate enclaves to mafic in the porphyritic granitoids. The gabbroic rocks include gabbro-norite olivine and gabbro biotite, whose main mineralogy is enriched in MgO, CaO and FeO, expressed by the formation of forsterite, diopside, augite, hyperstene, biotite, hornblende (pargasite or Mg-hornblende) and plagioclase (labradorite to andesine). The diorites to monzonites rocks, and the mafic enclaves to intermediates are more chemically evolved than gabbroic rocks, carrying plagioclases of the andesian to oligoclase type. The chemical analysis of gabbro and intermediate to mafic enclaves shows similarity in terms of amphibole composition, with the presence in both of the Mg-hornblende type. The same feature is observed for the composition of biotites, which are rich in phlogopite, also present in dioritic to monzonites bodies. The integrated relationship of the mineralogical composition and the necessary chemical analysis that a shoshonite suite began to crystallize at high pressures and temperature promoting the formation of olivine bearing gabbro-noritic rocks. The residual magma of the process enriched in Na2O and FeO, made it possible to crystallize biotite-gabbro, dioritic rocks and intermediate to mafic enclaves.