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Anorogenic Alkaline Granites from Northeastern Brazil: Major, Trace, and Rare Earth Elements in Magmatic and Metamorphic Biotite and Na-Ma®C Mineralsq

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Anorogenic Alkaline Granites from Northeastern Brazil: Major, Trace, and Rare Earth Elements in Magmatic and Metamorphic Biotite and Na-Ma®C Mineralsq Journal of Asian Earth Sciences 19 (2001) 375±397 www.elsevier.nl/locate/jseaes Anorogenic alkaline granites from northeastern Brazil: major, trace, and rare earth elements in magmatic and metamorphic biotite and Na-ma®c mineralsq J. Pla Cida,*, L.V.S. Nardia, H. ConceicËaÄob, B. Boninc aCurso de PoÂs-GraduacËaÄo em in GeocieÃncias UFRGS. Campus da Agronomia-Inst. de Geoc., Av. Bento GoncËalves, 9500, 91509-900 CEP RS Brazil bCPGG-PPPG/UFBA. Rua Caetano Moura, 123, Instituto de GeocieÃncias-UFBA, CEP- 40210-350, Salvador-BA Brazil cDepartement des Sciences de la Terre, Laboratoire de PeÂtrographie et Volcanologie-Universite Paris-Sud. Centre d'Orsay, Bat. 504, F-91504, Paris, France Accepted 29 August 2000 Abstract The anorogenic, alkaline silica-oversaturated Serra do Meio suite is located within the Riacho do Pontal fold belt, northeast Brazil. This suite, assumed to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional event. Preserved late-magmatic to subsolidus amphiboles belong to the riebeckite±arfvedsonite and riebeckite± winchite solid solutions. Riebeckite±winchite is frequently rimmed by Ti±aegirine. Ti-aegirine cores are strongly enriched in Nb, Y, Hf, and REE, which signi®cantly decrease in concentrations towards the rims. REE patterns of Ti-aegirine are strikingly similar to Ti-pyroxenes from the IlõÂmaussaq peralkaline intrusion. Recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe 1 Mg) ratios. Recrystallised amphibole is pure riebeckite. Magmatic Ti±Na-bearing pyroxene was converted to low-Ti aegirine 1 titanite ^ astrophyllite/aenigmatite. The reaction riebeckite 1 quartz ! aegirine 1 magnetite 1 quartz 1 ¯uid is also observed. Biotite and Na-ma®c minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 6008C between the NiNiO and HM buffers. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Anorogenic alkaline granites; Earth elements; Northeastern Brazil 1. Introduction genic alkaline suites, Rogers and Greenberg (1990) showed that post-orogenic suites are slightly richer in Granites related to the alkaline series are common in CaO and MgO with lower amounts of alkalis. Peralkaline within-plate, anorogenic (Murthy and Venkatenaman, types are more abundant in anorogenic suites while meta- 1964; Martin and Piwinskii, 1972), or post orogenic luminous types are largely dominant in post-orogenic asso- settings (Nardi and Bonin, 1991). Classical alkaline anoro- ciations (Nardi and Bonin, 1991). genic suites are exempli®ed by the Younger Granite In this paper, major, trace and rare earth elements data on province of Niger Ð Nigeria (Jacobson et al., 1958), the ma®c minerals from a Paleoproterozoic anorogenic alkaline Proterozoic Gardar province, South Greenland (Upton, suite are presented and discussed. Trace and rare earth 1974), and the Finnish rapakivi magmatism (Vorma, element data in sodic amphibole and pyroxene were 1976). Examples of post-orogenic alkaline suites are the obtained by ion microprobe, through the SIMS technical Permian±Triassic Western Mediterranean Province Bonin approach, showing the variation of these elements between (1980), Neoproterozoic Saibro Intrusive Suite, south magmatic and metamorphic grains. Brazil (Nardi and Bonin, 1991), and the Pan-African Arabian Shield. Comparing both post-orogenic and anoro- 2. Geological setting q This paper is part of the Special Issue: Alkaline and Carbonatitic Paleoproterozoic alkaline magmatism with potassic af®- Magmatism and Associated Mineralization±Part II. Guest Editors: L.G. nities have been described in the SaÄo Francisco Craton by Gwalani, J.L. Lytwyn. * Corresponding author. ConceicËaÄo (1990), ConceicËaÄo (1994), Rosa (1994), Rios E-mail address: [email protected] (J. Pla Cid). (1997) and Paim (1998). This potassic alkaline character 1367-9120/00/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00051-1 376 J. Pla Cid et al. / Journal of Asian Earth Sciences 19 (2001) 375±397 is ascribed to the peculiar composition of the Paleoproter- preserved structures (Couto, 1989) and associated alkaline ozoic mantle in northeastern Brazil, which has produced granites. alkaline magmas with compositions suggesting metasoma- The Neoproterozoic event at Campo Alegre de Lourdes tized mantle sources. region was characterised by frontal collision of continental The Serra do Meio Suite (SMS, Leite, 1997) is located blocks, with extensive thrust tectonics along ENE±WSW in the Riacho do Pontal Fold Belt (RPFB, Brito Neves, striking surfaces (Leite et al., 1993; Pla Cid, 1994 1975), in the Borborema Province of northeastern region Fig. 1C). NS-striking sub-vertical transcurrent zones were of Brazil (Fig. 1). The RPFB, located in the northwestern identi®ed in the northeastern part of this region (Fig. 1C), border of the SaÄo Francisco Craton (SFC), is one of the and interpreted as lateral ramps active during the frontal several Brasiliano (1.0±0.45 Ga, Wernick, 1981; Barbosa event (Leite, 1997). All geological units were affected by and Dominguez, 1996) fold belts surrounding this craton. this tectonic event which produced ductile shear structures AccordingtoJardimdeSa (1994), the Neoproterozoic along thrust planes and lateral ramps. Such a framework fold belts in northeastern Brazil were generated during follows Pla Cid (1994) in that lithological contacts in the intracontinental collisional events, with no evidence of Serra do Meio Suite result from strong superimposition of coeval subduction. tectonic regimes during the Neoproterozoic event and do not The Serra do Meio Suite is an alkaline granitic magma- represent the igneous geometry. tism, which is part of the Campo Alegre de Lourdes Alka- line Province (ConceicËaÄo, 1990). These alkaline granites have been studied by Leite (1987, 1997), ConceicËaÄo 3. Geology and petrography (1990), Pla Cid (1994) and Pla Cid et al., (2000). Isotopic determinations at Campo Alegre de Lourdes Alkaline The oldest rocks are represented by the late Archean Province point to magmatic ages related with the Transa- Gneissic±Migmatitic Complex which yields a whole-rock mazoÃnico Event (2.0 ^ 0.2 Ga, Wernick, 1981), as Rb±Sr age of 2.6 Ga as determined by whole-rock Rb±Sr evidenced by U±Pb data in zircon/baddeleyite grains of isotopic dating (Dalton de Souza et al., 1979). These the Angico dos Dias Carbonatite (2.01 Ga, Silva et al., gneisses have granite and trondhjemite±tonalite composi- 1988). In this area, the end of TransamazoÃnico Event tions with development of migmatitic structures and were was marked by crustal extension in a continental rift metamorphosed within the amphibolite facies. Metasedi- setting (Leite et al., 1993; Pla Cid, 1994), emplacement mentary rocks are represented by quartz±mica schist and of the Angico do Dias carbonatite complex, outpouring calcareous schist belonging to Paleoproterozoic Serra da of tholeitic to transitional basalts with cumulative Boa EsperancËa Unit (Silva et al., 1988) and by metapelitic A Brazil Bahia Pernambuco B Piaui 1 RPFB BAHIA a c Salvador South America b a - Phanerozoic Covers b - Brasiliano Covers c - São Francisco Craton Fig. 1. (A) Location of studied area in South America. (B) The SaÄo Francisco Craton is within the state of Bahia. The Serra do Meio suite is located between Bahia and Piauõ states, inside the RPFB terrain. (C) Geological map of the Serra do Meio suite, modi®ed after Leite (1997). RPFB (c) SMS Bahia Salvador 433 0' w 430 0' w N J. Pla  PIAUI Cid et al. / Journal of Asian Earth Sciences 19 (2001) 375±397 Sao Francisco Craton BAHIA Peixe Pedra Comprida 09˚ 30's Campo Alegre de Lourdes 04 8km Tra nscurrent Serra do Meio Suite State Limit Cenozoic Covers Zone Meta-Carbonate/Qtz-mica Schist Biotite Granite Roads and Thrust Fault Parnaiba Basin - Paleozoic Serra da Boa Esperanca Unity Biotite Magnetite Granite Tra ils Faults Toleithic Basic-Ultrabasic Complex/ Aegirine Biotite Magnetite Granite Inferred Contact Foliation Ultrabasic rocks with Fe-Ti-V Miner. Gneissic-Migmatitic Complex Riebeckite Aegirine Granite Geological Fold Axis Carbonatite Complex (2.01 Ga) Contact Undefined Alkaline Granite Fig. 1. (continued) 377 378 J. Pla Cid et al. / Journal of Asian Earth Sciences 19 (2001) 375±397 21(wt.%) 1.2 Biotite granites (NE) NaO+KO2 2 Agp. Index Riebeckite aegirine gr. Aegirine granites Biotite magnetite granites Alkaline Foid Syenite Granites 14 1.0 Foid Syenite Monzosyenite Granite Quartz Monzonite Monzonite Monzo- FeO/(FeO+MgO) 7 diorite 0.8 50 60SiO 70 80(wt.%) 0.7 0.8 0.9 1.0 2 (A) (B) 2 9 15 Al O CaO FeOt 23 Peralkaline trend Metaluminous trend 1 5 12 SiO SiO SiO 0 2 1 2 9 2 60 65 70 75 80 60 65 70 75 80 60 65 70 75 80 6 1.0 Na O 2 TiO2 Riebeckite-aegirine gr. Peralkaline Aegirine granites Biotite-magnetite granites Metaluminous Biotite granites (NE) 3.5 0.5 SiO SiO 1 2 0 2 60 65 70 75 80 60 65 70 75 80 (C) Fig. 2. (A) Total alkalis vs. silica (TAS) diagram, in wt.%, after Le Maitre et al. (1989), with chemical classi®cation and nomenclature of plutonic rocks, according to Middlemost (1994). (B) Agpaitic index vs. FeOt/(FeOt 1 MgO) diagram (Nardi, 1991), showing the usual ®eld for alkaline granites. (C) Harker diagrams of the Serra do Meio Suite. and metapsamitic rocks included in the Mesoproterozoic xenoliths are abundant within the alkaline granites. The Santo Onofre Group (Leite, 1997), both metamorphosed granites were deformed by the Brasiliano collision that to greenschist facies. produced gneissic structures with a marked foliation The Serra do Meio Suite includes several intrusions dipping to the SE or NW (Pla Cid et al., 2000). The pre- oriented along ENE±WSW trends (Fig. 1C) that intrude Brasiliano age of alkaline granites is suggested by whole- the Serra da Boa EsperancËa Unit. Quartz±mica schist rock Rb±Sr errorchrons yielding an age of ca.
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