Revista Brasileira de Geociências

Revista Brasileira de Geociências

Marcos Antonio Leite do Nascimento et al.

33(2):225-236, junho de 2003

QUÍMICA MINERAL E EVOLUÇÃO PETROLÓGICA DO MAGMATISMO
ALCALINO NEOPROTEROZÓICO DO MACIÇO SÃO JOSÉ DE CAMPESTRE
(RN-PB), EXTREMO NE DA PROVÍNCIA BORBOREMA*
MARCOS ANTONIO LEITE DO NASCIMENTO1, ZORANO SÉRGIO DE SOUZA1,3,
RIELVA SOLIMAIRY CAMPELO DO NASCIMENTO2 & ANTONIO CARLOS GALINDO1
Abstract MINERAL CHEMISTRY AND PETROLOGIC EVOLUTION OF THE NEOPROTEROZOIC ALKALINE MAGMATISM
IN THE SÃO JOSÉ DE CAMPESTRE MASSIF (RN-PB), NORTHEAST BORBOREMA PROVINCE The southern portion of the
São José de Campestre massif (NE Borborema Province) comprises a number of granitic rocks of alkaline affinity, associated with ductile shear zones of Neoproterozoic age. These bodies are constituted by rocks with very similar petrographic and textural features.
They consist essentially of hololeucocratic, white to pink alkali-feldspar granites with equigranular fine- to medium-grained texture.
Feldspar (microcline and albite) and quartz, moke up about 90 weight %. The main mafic phase is clinopyroxene of the types aegirineaugite or hedenbergite, in addition to accessory amounts of garnet, sphene, allanite, zircon, apatite and magnetite.
Microprobe analyses corroborate the petrographic distinction of clinopyroxenes into two types. In garnet bearing rocks, the pyroxene is hedenbergite, whereas in rocks with no garnet the pyroxene is aegirine-augite. Garnet is andradite rich (88-82 mol%), with subordinate amounts of the grossular, spessartine and almandine components. Plagioclase shows a remarkable uniformity in composition ranging from 0 to 9 mol% anortite, with the more calcium rich individuals found in rocks with garnet. The oxides are always Tipoor magnetite.
Chemically, the studied rocks show similarities with high silica (>70%) and high alkali granites. They are metaluminous to slighly peraluminous, with Na2O+K2O higher than 8%, higher values of Fe number