238 MORRO DO LOPES GRANITES: PALAEOPROTEROZOIC ALKALINE MAGMAS AND THEIR SIGNIFICANCE FOR SERRINHA NUCLEUS EVOLUTION, NORTHEASTERN BAHIA, BRAZIL Rios, D.C. 1 , Conceição, H. 1 , Davis, D.W. 2 , Rosa, M.L.S. 1 , Macambira, M.J.B. 3 , Peixoto, A.A. 1 , Marinho, M.M. 1,4 1. Laboratório de Petrologia Aplicada à Pesquisa Mineral – IGEO – UFBA, Rua Caetano Moura, 123, Federação, CEP: 40201-340, Salvador-Bahia, Brazil (debora@cpgg.ufba.br, herbet@ufba.br, lourdes@cpgg.ufba.br, aap@cpgg.ufba.br). 2. Jack Satterlly Geochronological Laboratory, University of Toronto, Ontario, Canada 3. Laboratório de Geologia Isotópica (Pará-Iso) – Universidade Federal do Pará, Caixa Postal 1611, CEP: 66075-900, Belém-Pará, Brazil (moamac@ufpa.br) 4. Companhia Baiana de Pesquisa Mineral – CBPM, 4 a Avenida, 460, Centro Administrativo da Bahia, CEP: 41750-300, Salvador- Bahia, Brazil (cbpmdt@cbpm.com.br) Keywords: Granites, Serrinha Nucleus, Alkaline, Bahia, Brazil INTRODUCTION Morro do Lopes Granites (MLG) is a terminology which group several (>30) small granitic stocks and innumerous dikes, representing the last Palaeoproterozoic magmatic expression at Serrinha Nucleus (SerN). The SerN is interpreted as one of three ancient (Archaean) nuclei which constitute the basement of the São Francisco Craton at Bahia State. SerN terrain is constituted by Archaean gneissic-migmatites basement and volcano sedimentary sequences that record evidence of green- schist to amphibole grade metamorphism. Plutonic activity is represent by Archaean calc-alkaline granites (2.8 to 3.1 Ga) and Palaeoproterozoic calc-alkaline and TTGs (2.15 Ga) followed by alkaline rocks (2.07 to 2.10 Ga). The volcano sedimentary sequences, with Palaeoproterozoic ages, are interpreted as greenstone- belts terrains which result from Transamazonic Orogeny activities. This study presents an overview of these late granitic bodies emplacement, which precisely constrain the end of Transamazonian Orogeny at the Northeastern Bahia State. MORRO DO LOPES GRANITES AT SERRINHA NUCLEUS Intrusive activity in the end of Transamazonian produced diverse alkaline plutons. The Morro do Lopes type was first recognized at the Southwestern area of SerN as the last products of regional extension following the back-arc basin closure. Later, compositionally similar granitoids were also recognized further North and East, presenting the same time range. A detailed geochronological study (Rios, 2002) of these stocks indicates that constituent rocks were emplaced during the interval 2070 to 2080 Ma. This late magmatic pulse produced K-granites similar to the early shoshonitic metaluminous monzonites (~2.1 Ga), although more differentiated and peraluminous. This magmatism was widespread and hot enough to generate zircon crystals and overgrowths at the early rocks of Serrinha Nucleus. MLG dominantly occur as small (< 10 km 2 ) circular stocks, been constituted by isotropic fine to medium faneritic rocks, which are not affected by the Transamazonian deformation although some of them still show a very slightly elongated north-south shape. In field, just a few of these granites display positive topography, which makes difficult to locate them. They also occur as tabular shaped dikes, showing vertical to sub-vertical dip and varied dimensions up to 15 m larger. They cut all lithologies present at SerN. The isotropic and homogeneous MLG´s rocks just present flow foliation structures, sometimes only perceptible by little variations on the gray tone of rocks. Younger aplogranite dikes, white colored and up to 5 cm large are quite common. Enclaves are extremely rare to absent, been represented by angular fragments of the gneissic-migmatitic and amphibolites from SerN basement. PETROLOGICAL, GEOCHEMICAL AND ISOTOPIC DATA Most petrological and geochemical data were obtained through detailed study of six distinct bodies: Marmota, Alto Bonito, Fazenda das Bananas, Maravilha, Barroquinha and Pedra Vermelha. The MLG are hololeucocratic and equigranular biotite-bearing, alkali- feldspar, grey monzogranites which grades to granodiorites and granites (s.l.), with rare plagioclase phenocrysts. Porphyritic texture is occasionally observable, although the phenocrysts of alkali feldspar and plagioclase constitute less than 20% of volume. The principal mineralogy is constituted by alkali feldspar, albite-oligoclase and quartz. Biotite is the principal mafic phase, and hornblende is absent on most outcrops. Allanite, apatite, zircon, titanite and occasionally opaque minerals (ilmenite, magnetite) and molibdenite represent