The peralkalic magmatism in the Precambrian Cachoeirinha- Salgueiro foldbelt, northeast Brazil: geochemical aspects.

Abstract

Peralkaline rocks of probable Brasiliano age intruded metasediments of the Precambrian Cachoeirinha and Salgueiro groups and their basement, between 37oW and 40oW and 7oS and 8o15'S, Pernambuco and Paraiba states. They include two groups of aegirine and/or riebeckite-arfvedsonite-bearing rocks: 1) 'silica- saturated' quartz-alkali-feldspar greyish-green trachytes and alkali-feldspar syenite, and 2) 'silica-oversaturated' quartz-alkali-feldspar syenite to alkali-feldspar granite (fluorite and cassiterite-bearing). The first group forms a dyke swarm at Manaira, Paraiba and Terra Nova, Pernambuco, batholiths and dykes adjacent to the S border of the Cachoeirinha-Salgueiro foldbelt, constituting a syenitic line, besides small stocks piercing Cachoeirinha metasediments and the second one, dyke swarms, stocks and ring-dykes. The first group is characterized by normative nepheline and K2O (9.7-12.8%) > Na2O. Agpaitic index varies from 1.156 to 1.280 and in the second group ranges from 0.840 to 1.339. The SIGMA REE are usually low and patterns for the two groups are very fractionated, LREE-enriched, HREE-depleted and Eu anomaly is absent. A very discrete negative Eu anomaly is seen in some of the saturated rocks, while the oversaturated rocks exhibit, sometimes, a discrete positive Eu anomaly. Whole-rock delta 18O values are usually below 10per mille, except for the oversaturated subvolcanic dykes which intruded Cachoeirinha metasediments. Intense, early pyroxene fractionation from an alkali basic parental magma has been perhaps responsible for the generation of the Triunfo syenite batholith, Pernambuco, and other syenites in the peralkaline syenitic line. The peralkalinity of the Macacos Hill oversaturated ring-dyke next to Serrita, Pernambuco, may have resulted from Bowen's plagioclase effect in the crystallization of a magma with trondhjemitic affinity.-J.M.C.N.