General Characteristics and Lithogeochemistry of the Itaguara Layered (Ultramafic-Mafic) Sequence, Southern São Francisco Craton
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Geochimica Brasiliensis, 22(1)045 - 072, 2008 GENERAL CHARACTERISTICS AND LITHOGEOCHEMISTRY OF THE ITAGUARA LAYERED (ULTRAMAFIC-MAFIC) SEQUENCE, SOUTHERN SÃO FRANCISCO CRATON L.E.A. Goulart, M.A. Carneiro1,* 1Departamento de Geologia; Escola de Minas, universidade Federal de Ouro Preto, Morro do Cruzeiro Campus Universitário, 35400-000, Ouro Preto, MG. E-mail: *[email protected] Recebido para publicação em setembro de 2007, aprovado para publicação em abril de 2008 ABSTRACT The Itaguara Layered Sequence (SAI) is a layered body of ultramafic-mafic (lherzolitic- harzburgitic, websteritic, orthopyroxenitic, gabbronoritic and gabbroic) composition, intrusive in the sialic crust of the southern portion of the São Francisco Craton (Minas Gerais, Brazil). The finer-grained varieties present mono- and polymineralic adcumulatic textures or, less frequently, mesocumulatic texture. The coarser-grained varieties show heteradcumulatic texture with or without intercumulus phases. The SAI lithogeochemistry characterizes it as of komatiitic affinity and corroborates to the hypothesis that the ultramafic-mafic magmatism, related to the Neoarchean Rio das Velhas Tectonothermal Event, was continuous in time and affected all the southern portion of the São Francisco Craton. RESUMO A Seqüência Acamadada Itaguara (SAI) é um corpo acamadado de composição ultramáfica- máfica (lherzolítica-harzburgítica, websterítica, ortopiroxenítica, gabronorítica e gabróica), intrusivo na crosta siálica da porção meridional do Cráton São Francisco (Minas Gerais). As variedades mais finas apresentam textura adcumulática mono e poliminerálica ou, menos freqüentemente, textura mesocumulática. As variedades de granularidade mais grossas apresentam textura heteradcumulática com ou sem fase intercumulus. A litogeoquímica da SAI demonstra a sua afinidade komatiítica e fortalece a hipótese de que o magmatismo ultramáfico-máfico, relacionado ao Evento Tectonotermal Rio das Velhas de idade neoarqueana, foi temporalmente contínuo e espalhado por toda porção meridional do Cráton São Francisco. INTRODUCTION portions. In the latter, Archean and Proterozoic supracrustal sequences, represented by the The São Francisco Craton is located in central-eastern Brazil and encompasses Rio das Velhas, Minas and São Francisco a crustal segment tectonically sTable since supergroups, overlie an Archean sialic crust the end of the Neoproterozoic, when the (Teixeira et al., 2000). The Quadrilátero Brasiliano-Panafrican Orogenic Cycle Ferrífero is inserted in this geologic context and terminated (Cordani et al., 2000). It is is constituted by greenstone-type sequences informally divided in northern and southern and metasedimentary units of the Rio das 45 L.E.A. Goulart, L.E.A., Carneiro, M.A. / Geochim. Brasil., 22(1)45 - 72, 2007 Velhas and Minas supergroups. The sialic Fernandes, 2001, Machado & Carneiro, 1992, crust of the southern São Francisco Craton Teixeira, 1985, Teixeira et al., 1996, 2000). (Figure 1) is composed of high-metamorphic grade, TTG-type (tonalite-trondhjemite- This crust was formed in the granodiorite) suites and comprises several Mesoarchean and is related to several crustal metamorphic complexes (Carneiro, 1992, accretion processes that occurred during its Carneiro et al., 1998a, b, Endo et al., 1996, geologic evolution (Fernandes, 2001), in Figure 1: Geology of part of the Southern São Francisco Craton, modified after Campos (2004). 46 L.E.A. Goulart, L.E.A., Carneiro, M.A. / Geochim. Brasil., 22(1)45 - 72, 2007 special the Rio das Velhas Tectonothermal in the direction of Morro da Onça, where it is Event (Carneiro, 1992). As products of this found in (undefined) contact with komatiitic event, besides reworking of the sialic crust flows, correlated with the Rio das Velhas previously formed, at least two important Supergroup (Pinheiro, 1998). In a broad magmatic events took place. One of them, sense, SAI is intrusive in the Neoarchean of ultramafic-mafic nature (Padilha, 1984, sialic crust of the southern São Francisco Schrank et al., 1990, Costa, 1995, Pinheiro, Craton, once gneiss xenoliths are found in 1998, Zucchetti, 1998), generated the Rio the rocks of this sequence. SAI also intrudes das Velhas Supergroup basal units (SGRV a meta-volcanosedimentary unit, whose - greenstone belt-type sequences), and also metasedimentary portion is constituted by several bodies distributed in the southern quartzites, sillimanite-quartzites, garnet- São Francisco Craton and neighboring sillimanite-quartzites, garnet-sillimanite- areas (Carneiro et al., 1996, Carvalho Jr. schists, and, to a lesser extent, by garnet- et al., 1998a,b, Carvalho Jr. & Carneiro, biotite-plagioclase-quartz-schists (Goulart, 1999, Carvalho Jr., 2001). Among them, the 2006). The metavolcanic portion is composed Piumhí and Fortaleza de Minas greenstone of metagabbros, amphibolites and tremolite- belts (Valença et al., 1998, Schorscher et schists. Relicts of a contact aureole are al., 1998, Szabó, 1996) and the Ribeirão observed in the field, generated by the dos Motas Layered Sequence stand out intrusion of SAI in metasediments. SAI is in (Carneiro et al. 1996, Carvalho Jr. et al. turn (Figure 2) intruded by two (a mafic and a 1998a,b, Carneiro et al. 1999, Carvalho Jr. granitic) dike systems (Goulart, 2006). These & Carneiro 1999, Carvalho Jr. 2001). Units dike systems are correlated with the mafic similar to the Ribeirão dos Motas Layered dikes of the Lençóis and Timboré systems Sequence (SARM) occur close to Cláudio (Carneiro et al., 2006). and Carmópolis de Minas cities (Couto, 2004, Carneiro et al., 2006, Couto & Carneiro, GENERAL CHARACTERISTICS AND ROCK TYPES 2007). The other magmatic event, related to the Rio das Velhas Tectonothermal Event, The Itaguara Layered Sequence (SAI) is of andesitic to tholeiitic, calc-alkaline encompasses a sequence of deformed and nature and is represented by granitoids and metamorphosed ultramafic-mafic rocks Neoarchean mafic dikes (Carneiro, 1992, (Goulart, 2006). Despite folding and faulting, Carneiro et al., 1998a,b, Teixeira et al., 2000, its igneous layering is still preserved. Campos et al., 2003). A new occurrence This layering is characterized by a cyclic of layered ultramafic-mafic magmatism compositional variation, with thickness has been recently identified between varying from centimeters to some meters. Not Itaguara and Crucilândia cities (Figure 1), rarely, grain-size gradation occurs internally apparently related to SARM (Goulart 2006). to the stratifications (Figure 3). In this work a preliminary petrographic and Under the microscope, this relict lithogeochemical characterization of the feature is composed of high metamorphic rocks of this occurrence is presented, which grade parageneses (amphibolite to granulite is from now on named Itaguara Layered facies), which are usually in equilibrium Sequence (Figure 2). with the cumulatic textures and primary parageneses. The primary parageneses were LOCATION AND GEOLOGIC SETTING little affected and enable the examination of The Itaguara Layered Sequence (SAI) the protolith petrographic characteristics (see crops out at the Quadrilátero Ferrífero discussion in the next item). Another striking western border, in a narrow NE – SW belt characteristic is the compositional variation (Figures 1 and 2), between Itaguara and of the SAI rock types, from peridotitic, Crucilândia. It stretches out northeastwards, pyroxenitic, hornblenditic and even gabbroic 47 L.E.A. Goulart, L.E.A., Carneiro, M.A. / Geochim. Brasil., 22(1)45 - 72, 2007 Geology of the study region, modified after Goulart (2006). : Figure 2 48 L.E.A. Goulart, L.E.A., Carneiro, M.A. / Geochim. Brasil., 22(1)45 - 72, 2007 Figure 3: A) Igneous bedding of the pegmatoidal meta-orthopyroxenite: The white strips indicate bedding intervals: B) meta-harzburgite showing grain-size gradation. Photograph taken from a fold flank, where foliation crosscuts the igneous bedding at a high angle. compositions. These varieties, as well as their generated by the growth of amphibole at the petrographic characteristics are presented expense of clino- and orthopyroxene. This below. texture is characterized by the orientation of tremolite, Mg-hornblende and, more rarely, META-PERIDOTITES anthophyllite, all included in ghost and/or relict orthopyroxene crystals. Although The SAI meta-peridotite varieties are less frequent, adcumulates of olivine and meta-harzburgites and meta-lherzolites. They spinels also occur, sometimes surrounded are characterized by alternating olivine- and by clino- and orthopyroxene. Phlogopite pyroxene-rich, adcumulatic micro-rhythms is rare and, when present, is altered to or by the presence of heteradcumulates clinochlore. The presence of tremolite and constituted by olivine, spinels and opaque Mg-hornblende (Figure 4 A), as well as the minerals, surrounded by xenomorphic overgrowth of green spinel (hercynite) on orthopyroxene or, less frequently, by brown spinel, indicates that the sequence clinopyroxene. Locally, adcumulate relicts re-equilibrated at high-grade metamorphic of olivine and orthopyroxene occur intact. conditions (amphibolite to granulite facies). These relicts are found where talcification/ The evidences for retrograde metamorphism serpentinization of olivine and orthopyroxene are given by the overgrowth of opaque spinel did not alter the textures of the primary micro- (Cr-magnetite?) on the rims of zoned spinel layering. These portions may or may not be crystals (brown nucleus/intermediate green associated with the nematoblastic texture zone/opaque rim),