QUÍMICA MINERAL E EVOLUÇÃO PETROLÓGICA DO MAGMATISMO ALCALINO NEOPROTEROZÓICO DO MACIÇO SÃO JOSÉ DE CAMPESTRE (RN-PB), EXTREMO NE DA PROVÍNCIA BORBOREMA

Abstract

The southern portion of the Sao Jose 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 Na 2 O+K 2 O higher than 8%, higher values of Fe number (Fe#=64-100) and on agpaitic index less than 1,0. These geochemical features are typical of the classical alkaline granitic suites. On the other hand, the low amounts of Nb, Ga, Y, Rb and Zr, as well as the very high Sr, low total rare earth elements and positive Eu anomalies (Eu/Eu*=1,5-2,9) are not compatible with the traditional alkaline magmas. The integration of petrographic and geochemical data suggests the fractional crystallization as the main petrogenetic process. Some mass balance calculations and trace element modeling point to less than 10% fractionation, of a cumulate composed of clinopyroxene, plagioclase, magnetite and zircon.