Idade (Pb-Pb) e aspectos petrográficos e litogeoquímicos do Complexo Alcalino Floresta Azul, Sul do Estado da Bahia
Maria de Lourdes da Silva RosaHerbet ConceiçãoMoacir José Buenano MacambiraMoacyr Moura MarinhoLeila Soares Marques
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Abstract:
Pb-Pb AGE AND PETROGRAPHIC AND LITHOGEOCHEMICAL ASPECTS OF THE FLORESTA AZUL ALKALINE COMPLEX, SOUTHERN STATE BAHIA The Floresta Azul Alkaline Complex (CAFA), is represented by a NE-SW elongated igneous body, with an exposed area of 200 km 2 , located in the central-northern region of the Southern Bahia State Alkaline Province. Two types of magmatic intrusion were recognized: (i) at the eastern part of complex, occur granitic rocks, presenting many dior itic inclusions, and (ii) the syenitic rocks are located in the western region. Pb-Pb ages obtained in single zircon crystals from g ranite (696 ± 11 Ma); diorite (688 ± 2 Ma) and syenite (688 ± 10 Ma) indicate that CAFA rocks were emplaced synchronously during a Neoproterozoic event. The main mafic mineral of the CAFA is biotite, whereas sodic amphibole and aegirine are sporadically foun d in the granitic and syenitic lithotypes. The textural features of these rocks, typified by the angular distribution of well-for med feldspar crystals, reflect the lack of significant deformation after crystallization. All the lithologic types from the complex have alk aline signature, characterized by the high contents of Nb (86-243 ppm), Y (22-43ppm), F (760-2600) and Cl (220-10.000). The distribut ion patterns of minor and trace elements suggest that the different rocks from CAFA are cogenetic.Keywords:
Diorite
Actinolite
Amphibole
Pb-Pb AGE AND PETROOLOGICAL ASPECTS OF THE BLUE SODALITE MINERALIZATION OF THE ITARANTIM NEPHELINE-SYENITE MASSIF, SOUTHERN STATE OF BAHIA The Itarantim nepheline-syenite massif (220 km 2 ) is a late proterozoic intrusion (727±30 Ma; Rb-Sr) of the southern region of the State of Bahia. It is mostly constituted by nepheline-be aring syenites, with biotite or aegirine, and by alkaline dykes. The dykes are more abundant in the biotite-nepheline-syenites and ar e the most evolved rocks of the massif. Recently, a blue sodalite-syenite dyke has been identified in the southwest part of the massi f, hosted by biotite-nepheline syenites. The blue syenites have been exploited as dimension stone. The dyke has magmatic sodalite that crystallized between 720 ± 9 Ma and 732 ± 24 Ma (Pb-Pb Zr ), and cuts the magmatic foliation of the host syenite, imprinting a metasomatism with sodalite formation. Petrographic data indicate that the magmatic sodalite formed during the late stages by a volatile rich magma (500-600 o C). The high contents (ppm) of Zr (>5,000), Hf (141), Ta (169), U (74) and F (1200) of the blue syenite suggest that it represents a highly evolved product from differentiation of the nepheline-syenitic magma. 2 ) constitui uma intrusao brasiliana (idade Rb-Sr de 727 ± 30 Ma) localizada no extremo sul do alinhamento de rochas alcalinas existente no sul do Estado da Bahia. Ele e constituido essencialmente por nef elina- sienitos com biotita ou egirina e por diques alcalinos, mais abundantes no biotita-nefelina-sienito. Estes diques constituem as rochas mais evoluidas deste macico. Recentemente foi identificado um dique de sodalita-sienito de cor azul encaixado no biotita-nefeli na- sienito, localizado na regiao sudoeste do macico e que esta sendo explorado como rocha ornamental. Este dique, portador de soda lita magmatica e cuja idade de cristalizacao esta compreendida entre 720 ± 9 Ma e 732 ± 24 Ma (Pb-Pb Zr ), trunca a foliacao magmatica do sienito encaixante e provoca formacao de sodalita metassomatica. Os dados petrograficos indicam que a sodalita magmatica do diq ue forma-se no final da cristalizacao (500-600 o C) de um magma particularmente rico em fluidos. Os elevados conteudos em ppm de Zr (>5.000), Hf (141), Ta (169), U (74) e F (1200) obtidos para os sienitos azuis estudados sugerem que eles representem produtos fortemente evoluidos da diferenciacao do magma nefelina-sienitico. Palavras-chave: Idade Pb-Pb, sodalita azul, Macico Nefelina-Sienitico Itarantim.
Nepheline
Nepheline syenite
Sodalite
Massif
Carbonatite
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Baddeleyite
Batholith
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