PETROLOGIA E GEOQUÍMICA DOS STOCKS GRANÍTICOS DE SERRITA, PERNAMBUCO

Abstract

Systematic geologic mapping of three granitic stocks intrusive in intermediate-grade metasediments of the Salgueiro Group, next to Serrita, Pernambuco, revealed ring-structures in two of them. Epidote-bearing and biotite-granodiorites predominare in the cores of the stocks but the rims, usually in topographic relief, are composed of aegirine-granites. Normative compositions of the biotite-granodiorites lie in or next to the Barker's trondhjemitic field in the Ab-An-Or diagram, whereas the aegirine-granites lie in the granite field. The biotite-granodiorites are meta lo peraluminous (agpaitic index between 0.67 to 0.93) and the aegirine granites show peralkaline tendency (agpaitic index from 0.93 to 1.02). The high Ba and Sr values in the rock groups are common to cummulale rocks and the trace elements are comparable (Nb, Zr, Y and Σ REE somewhat higher in the granites). REE patterns are characterized by high Ce/Yb values, discrete positive Eu anomaly and by an extreme improverishment of HREE in the biotite granodiorites and lack of Eu anomaly in lhe aegirine-granites. δ 18 0 values increase towards the core of lhe stocks, generating an elongate pattern in the SE-NE direction. A generation through partial fusion of garnet-amphibolite source and subsequent crystallization at pressures between 6 and 8 kbar for the granodiorites is compatible with the present data, whereas the granites may have resulted from the crystalllzation of a residual granodioritic liquid at lower pressures or from the remelting of the granodiorites caused by a rapid uplift.