GEOCHEMISTRY AND PETROGENESIS OF THE SERRA DA MACAMBIRA PLUTON, NEOPROTEROZOIC OF THE SERIDO BELT, BORBOREMA PROVINCE (NE BRAZIL)

Abstract

. The Serra da Macambira pluton intrudes Caicó Complex orthogneisses and metasediments of the Seridó Group of the Rio Grande do Norte Domain in the Borborema Province. The Serra da Macambira pluton comprises intermediate enclaves (quartzbearing monzonite and biotite-bearing tonalite), porphyritic monzogranite, equigranular syenogranite to monzogranite, and late granitic and pegmatitic dykes. The quartz-bearing monzonite has mainly Kfeldspar, plagioclase, biotite, hornblende, whereas in biotite-bearing tonalite hornblende is absent. Porphyritic and equigranular granites display mainly biotite and rare hornblende, myrmequite and pertitic textures, and zoned plagioclase indicating the relevance of fractional crystallization during magma evolution. Granitic dykes are hololeucocratic and granophyric demonstrating conditions of hypabissal crystallization. Equigranular and porphyritic granites have similar geochemical characteristics, which suggest they were formed from similar parental magmas. Such granitic rocks have Rare Earth Element (REE) patterns with negative Eu anomalies and light REE enrichment (normalized La/Yb from 8.9 to 24.6, and normalized Yb from 6.2 to 45.5). They are metaluminous to slightly peraluminous, following a classical high-K calc-alkaline path. Major and trace element contents suggest that the parental magma was generated by 20-35% partial melting of hydrated lower continental crust, leaving a restite of orthopyroxene, plagioclase, quartz, K-feldspar, hornblende, apatite, magnetite, ilmenite, and zircon. Such magma evolved through 20-25% of fractional crystallization, with a cumulate composed by plagioclase, K-feldspar, biotite, hornblende, titanite, magnetite, apatite, zircon and allanite. Discriminant diagrams and textural and structural characteristics may imply that the Serra da Macambira pluton is a late- to post-collisional intrusion formed during the collapse stage of the Neoproterozoic Brasiliano / Pan-African orogeny.