Portal de Periódicos da CAPES

Artigo Acesso aberto Produção Nacional

BIBTEX RIS

THE PERALKALlC MAGMATISM IN THE PRECAMBRIAN CACHOEIRINHA-SALGUEIRO FOLDBELT, NORTHEAST BRAZIL: GEOCHEMICAL ASPECTS

1986; Sociedade Brasileira de Geologia; Volume: 16; Issue: 1 Linguagem: Inglês

10.25249/0375-7536.19867385

2177-4382

Valderez P. Ferreira, Alcídes N. Sial,

Geological and Geochemical Analysis

Peralkalic rocks of probable Brasiliano age íntruded metasediments of the Precambrian Cachoeirinha and Salgueiro Groups and their basement, between the geographic coordinates 37ºW and 40ºW longitude and 7ºS and 8°1S'S latitude, Pernambuco and Paraíba states. They enclose two groups of aegírine and/or riebeckite-arfvedsonite-bearing rocks: (i) silica-saturated quartz-alkali-feldspar grayish-green trachytes and alkali-feldspar syenite, and (ii) silica-oversaturated quartz-alkali-feldspar syenite to alkali-feldspar granite (fluorite and cassiterite-bearing). The first group forms a díke swarm at Manaíra, Paraíba, and Terra Nova, Pernambuco, batholiths and dikes adjacent to the southern border of the Cachoeirinha-Salgueiro foldbelt, constituting a syenítoid line, besides small stocks piercíng Cachoeirinha metasediments and the second one, dike swarms stocks and ring-dikes. The first group is characterized by normative nepheline and K 2 0 (from 9.7 to 12.8%) greater than Na 2 O. Agpaitic index varies from 1.156 to 1.280 and in the second group ranges from 0.840 to 1.339. The Σ REE are usualy low and patterns for the two groups are very fractionated, LREE-enriched, HREE-deplete and Eu anomaly is rather missing. A very duscrete negative Eu anomaly is seen in some of the saturated rocks, while the oversatured rocks exhibit, sometimes, a discrete positive Eu anomaly. Whole δ 18 O values are usually below 10 permilSMOW, except for the oversaturated subvolcanic dikes which intruded Cachoeirinha metasediments. Intense, early pyroxene fractionation from an alkali basic parental magma has been perhaps responsible for the generation of the Triunfo-syeníte batholith, Pernambuco, and other syenites in the peralkalic syenitcid line. The peralkalinity of the Macacos hill oversaturated ring-dike next to Serrita, Pernambuco, may have resulted from Bowen's plagioclase effect in the crystalizaticn of a magma with trondhjemitic affinity.