Revista Brasileira de Geociências 12(1-3): 331-347, Mar.-Set., 1982 - Silo Paulo

PETROGRAPHIC AND CHEMICAL ASPECTS OF THE MAFIC-ULTRAMAFIC ROCKS OF THE CRIXAs, GUARINOS, PILAR DE GOlAS - HIDROLlNA AND GOlAs GREENSTONE BELTS, CENTRAL BRAZIL

RAIMUNDO MONTENEGRO GARCIA DE MONTALVÃO·, PETER DELF HILDRED·, PEDRO EDSON LEAL BEZERRA·, PÉRICLES PRADO· and SIMÃO DE JESUS SILVA·

AB8TRACT Mafic and uitramafic rocks occur in the lower parts of volcano-sedimentary sequences of Archean greenstone belt type, which form the Pilar de Goiás Group in the areas of Crixás, Guarinos, Pilar de Goiás-Hidrolina, and Goiás town, central Brazil. Some of the petro­ graphic and chemical parameters of these rocks are described and illustrated to provide data for comparison with rocks of similar cornposition in other greenstone belt arcas. Petrographic types include meta-peridorites and meta-pyroxenites showing skeletal and spinifex (piate and radiating) textures. and composed mainly of tremolite and serpentine, with chlorite, opaque minerais and devitnfied glass. Other varieties described comprise calc-serpen­ tinite, talc-serpentinite, talc-carbonate schist , chlorite schist, chloritite, tremolite schist, chlorite­ -tremolite schist, serpentine-chlorite-tremolite schist and meta-basalts. The principal rypes or alteration and greenschist facies metamorphic assemblages include those related to serpentiniza­ tion, talcification, carbonatization, amphibolitization, ehloritization, epidotization and local tourmalinization. The major-element chemistry of the dominantly ultramafic rocks, based on 56 analysis from the Crixás, Guarinos, Pilar de Goiás-Hidrolina bclts and 32 analyses from the Goiás belt; shows the following average. contenta, respectively: MgO 23.481'~, 20,31:;';" AI;OJ 8.66:;';;, 9.991'~; CaO 4,22~~" 3.01/;;; Ti02 OA2~", 0.17/~; K20 O,IO~;, 0.70/,,,; Na20 0.52:;';;, 1.23~", Despite theIow CaOjAI 203 ratios compared with other areas, the chemical parameters are generally consistent with a komatiitic composition. Plots of analytical data on various diagrams reveal mainly koma­ tiitic and tholciitic, and occasional calc-alkaline, trends for these rocks.

INTRODUCTION Volcano-sedimentary sequences oc­ granite-gneiss terrain with probable Archean nuclei and curring in the areas of Crixás. Guarinos, Pilar de Goiás, reworked during the Transamazonian, Uruaçuano, and Bra­ Hidrolina and Goiás town, in the State of Goiás, central siliano geotectonic cycles. This unit is composed of granites, Brazil (seeFigs. IA and IB for location) show compositional, granodiorites, tonalites, migmatites, granulites, chamo­ textural, stratigraphic, structural, and geotectonic features ckites and amphíbolites, with metamorphism in amphibolite characteristic of greenstone belt successions ofArchean age. to granulite facies, and localized retrogressive metamor­ The mafic and ultramafie rocks ofthese sequences are studied phism to greenschist facies. here in a prelirninary attempt at their charaeterization using The Barro Alto Complex, represented by its western petrographic and chemical parameters, including compa­ extremity in the central southeast portion of the area, is rison with rocks of similar composition in other areas. also assigned to the Archean, and comprises a wide variety Brief petrographic descriptions of some representa tive ofmostly mafic and u1tramaficlithologies, including dunites, lithological types sampled during geological mapping ope­ peridotites, pyroxenites, gabbros, norites, amphibolites, rations are followed by a consideration of the principal anorthosites granulites, and sillimanite gneisses. effects of metamorphism and alteration. The ehemical data The Pilar de Goiás Group, originally described by Danni available include 56 analysis of major-element oxides for and Ribeiro (1978), here includes the greenstone belt se­ lhe Crixás, Guarinos, and Pilar de Goiás-Hidrolina greens­ quences of Crixás, Guarinos, Pilar de Goiás-Hidrolina and tone belts considered as a group, and 32 analyses for the also that of Goiás (Goiás town), which is closely correlated Goiás (Goiás town) greenstone belt. The major-element with the type area. These sequences comprise a wide variety geochemistry of these roeks is reviewed and the analytical of lithologies, including the metamorphosed volcanic and results are plotted on a series ofvariation and discrimination hypabyssal mafic and ultramafic rocks which are the subjeet diagrams with the objective of identifying trends of petro­ -of this paper, subordinate acid meta-volcanic rocks, quart­ genetic interest. Sample locations are shown on Figs. IA zites, marbles, calc-silicate rocks, micaschists, graphitic (Crixás, Guarinos, Pilar de Goiás-Hidrolina) and IB (Goiásl. phyllites, meta-cherts, and banded iron formation. These lithologies are restrieted to partly interconnected belts of up to 50 km in length, striking between NW-SE and N-S, in the areas of Crixás. Guarinos, Pilar de Goiás and Hidro­ REGIONAL GEOLOGY A simplified geological map Iina, and surrounded by granite-gneiss "domes". ln the area of part of south-central Goiás, based on the mapping of northwest of Goiás town the greenstone belt forms a narrow Projeto RADAMBRASIL, is shown in Fig. 2. synclinal structure of NW-SE strike extending for over The oldest Iithostratigraphic unit in the area is considered 100 km and surrounded by granite-gneiss terrain or younger to be the Goiano Complex, forming a polyrnetamorphic metasediments.

*MME-SG-Projeto RADAMBRASIL, Avenida Universitária, 444, Setor Sul, CEP 74000, Goiânia, Goiás, Brazil 332 Revista Brasileira de Geocténcías, Volume 12 (1-), 1982

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IO~m • w5;~.~o:,,------.L-~'------L------.:L------.L---.-9-.Jo~55'

Figure JA - Map showlng locutíans oI analvzed maíic-ultranutjic rocks o] Crixás, Guarlnos, Piku: di' Goiás-Hidrolína greenstone betts

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Figure /8 - Mup: showtng tocations ol. analvzed mafic-ultramafic rocks oI Goiás (Goiás /0\\'11) groenstone hei' Revista Brasileira de Geociências, Volume 12 (1-3), 1982 333

~" . Hidrolino compl... cenceclc O.po.it. .~ pilar .. ooid. Oroup (undifferentioted 1 Mofle dikea Aroxd Gtoup pilar .. ooill. Group ( mo'ic~ullromo'ie rock. 1 Rood n Town , " earro Alto Complex Rev"" er TtlNlf Foult I B Foult " strotigropnic centeet Goiono Complu O Litholooicol contoct

Figure 2 - Simplified. geologícaí map of part of south-central Goiás (modified from Drago et al., 1981) 334 Revista Brasileira de Geocíências, Volume 12 (l·3), 1982

Migmatites are locally present in the greenstone belts, possible continuation to the north in the regions of Mara especially in the Pilar de Goiàs-Hidrolina belt, and io the Rosa and Porangatu. latter area ultramafic rocks probably in the basal portion Montalvão and Bezerra (1981) reported a petrological of the Pilar de Goiás Group are intruded by tonalitic to study of the basic and ultrabasic rocks of the Goiás greens­ granitic gneisses of the Hidrolina Complex. A whole-rock tone belt: including the principal chemical trends. Stra­ isochron age of2,929 ± lOS m.y. was obtained by Tassinari tigraphic descriptions ofthe Goiás area were made by Danni, and Montalvão (1980) on these intrusive rocks, providing Dardenne and Fuck (1981), dividing the "Serra de Santa a minimum age for the greenstone belt sequences. Rita .greenstone belt" into a lower ultramafic unit (meta­ The Araxá Group, of Middle Precambrian age, overlies peridotites and metapyroxenites, with metasediment in­ the Pilar de Goiás Group in this area, although the nature tercalations}, a basic and acid metavolcanic unit, and an of the contact is sometimes doubtful, The former unit is upper metasedimentary unit, and by Teixeira (1981), who widely distributed in the State of Goiás, and includes mica­ described these rocks under the heading ofthe Goiás Velho schists, garnet-staurolite-, kyanite- and sillimanite-bearing Group, similarly divided in three units: basal ultrabasic schists, quartzites, gneísses, amphibolites and amphibole vo1canics, metasediments intercalated with basic volcanics, schists. and upper metasediments. Other probable Precambrian rock units in the area include Drago el ai. (1981) provided general accounts of the a large number of small ultramafic bodies and mafic dikes. petrology, stratigraphy and structure of the Pilar de Goiás Cenozoic deposits are represented by Tertiary-Quaternary Group in Crixás, Guarinos, Pilar de Goiás-Hidrolina, and .cover sediments and recent alluvium. Goiás greenstone belts.

PREVIOUS WORK The earlier regional geological map­ PETROGRAPHY Within the mafic and ultramafic se­ ping projects which include the Crixás, Guarinos, Pilar de quences of Crixás, Guarinos, Pilar de Goiás-Hidrolina, and Goiás-Hidrolina and Goiás areas, incorporated the mafic Goiás town there is a great variety of lithological types, and uitramafic rocks and their enclosing sequences in the mainly of greenschist metamorphic fades, and often pre­ Araxá Group (Barbosa et ai., 1969; Lessa Sobrinho and serving primary textures and structures, such as porphyritic, Andrade, 1971; Pena et ai., 1975; compiled by Schobbe­ skeletal and spinifex textures, and vesicular, amygdatoidal, nhaus Filho et ai., 1975). More recent maps at various orbicular and pillow structures, and polyhedral jointing. scales, mainly by Metais de Goiás (Metago), Companhia Only the most importam petrographic varieties will be brie­ de Pesquisa de Recursos Minerais (CPRM), Projeto RA­ fly described in lhe following account, using representative DAMBRASIL, and the University of Brasilia, have con­ samples of each type, selected from lhe Crixás and Pilar firmed the existence, in these areas, of greenstone belt type de Goiás-Hidrolina belts. sequences of 'pre-Araxà age, Danni and Ribeiro (1978) discussed lhe stratigraphy of Meta-pyroxenite (skeletal texture} This rock from lhe the volcanic-sedimentary sequence of Pilar de Goiás and Crixás belt is composed principally of tremolite, chlorite, Guarinos, placing serpentinized peridotites and pyroxeni­ opaques and devitrified glass. Tremolite occurs as anhe­ tes, and mafic-ultramafic metavolcanics, at the base of the dral to subhedral prisms with variable orientation, and also Pilar de Goiás Group, which was compared with the sequen­ as acicular microcrystals or locally forming irregular aggre­ ce of Serra de Santa Rita, Goiás and a general greenstone gates. The crystals are immersed in a dark, fine-grained belt succession. Ribeiro Filho et ai. (1978) mapped the Pilar matrix of originally glassy material containing chlorite de Goiás and Mara Rosa areas and separated the Pilar de flakes of natural pale-green colour and very low birefrin­ Goiás Metamorphic Association, including mafic-ultra­ gence, derived from devitrification. Skeletal features are mafic.volcanic rocks and forming part of a proposed pre­ -Araxá mobile belt. However, the first specific references to greenstone belts in the areas of Crixás and Goiás were made by Sabóia (1979), with a division offhc Crixás sequence into three units, comprising mainly ultrabasic flows with spinifex texture and polyhedral jointing in the basal unit, basalts with pillow structures and metabasites in the middle unit, and metasediments in the upper unit. Additional details on the geology of the Crixás belt were summarized by Sabóia el ai. (1979), and more specific descriptions ofthe textures, structures, and internal divisions of the ultrabasic flows were made by Sabóia and Teixeira (1980) and Teixeira et ai. (1981); including comparison with similar features described in lhe area of Munro Township, Canada. ln the sarne area of the Crixás belt, Montalvão, Lopes and Pereira (1981) discussed lhe petrology of the komatiites, inclu­ ding descriptions of the textures and structures and the first detailed discussion about the chemistry of these rocks. Danni, Leonardos and Andrade (1980) described polyhe­ dral jointing, flow breccias and spinifex textures in the ultramafic rocks of the Hidrolina area, while Ribeiro Filho (1981) and Machado (1981) re-evaluated the geology ofthe Photo / - Mera-pvroxeníte wíth skeletal reXrUl'e. Opaque mínera!s greenstone terrains of the Pilar de Goiás arca and their borderíng tremolltc crvstuls. Crtxás hei! (natural lighr) RevistaBrasileira de Geociências, Volume 12 (1-3); 1982 335 very well represented in this type, with the tremolite prisms encrusted by opaque minerais showing very varied forms, including short, straight and curved, paralle! traces, rami­ fying forms and even shapes resembling a dorsal spine in appearance. Such features are extreme!y well exhibited in natural light (Photo I).

Meta-peridotite and meta-pyroxenite lspinifex texture) This petrographic group is characterized mainly by spinifex textures of radlatlng and plate type, Three sam pies with radiating spinifex texture can be des­ cribed. The first is located in lhe Hidrolina region, and is a meta-pyroxenite with small-scale radiating spinifex and composed mainly of tremolite, chlorite and opaques. Tre­ molite laminas of 0.5 to 1.0 cm in length are randomly orientated, mutually intercepting to form mostly triangular and other structures limited by the blades. Skeletal tremolite crystals forming elongate microprisms are arranged radially in fan patterns in "the inter-blade spaces, with abundant opaque minerais emphasizing the crystal morphology. Photo 3 - Meta-peridotite wíth radiating sptníjex texture. Serpentine Sparse xenomorphic tlakes of chlorite are also dispersed h/ades with tremolíte and opaque minerais. Crixás belt (po/arized between the blades (Photo 2). lightl

microblades of actinolite and rremolite form radial fan arrangements with varied orientation, and anhedral tremo­ lite occurs in various attitudes between the fans. Anhedral opaque minerais are randomly distributed as well as sur­ rounding crystals and outlining the radial structures. Chrysotile also occurs in a small continuous vein and di SR continuous films cutting across the rock. The rocks with pknetor tabulars spínifex textures exhibit a great variety of arrangements. One of the common exam­ pies of this texture is shown by a sample from the Hidrolina region, classified as a meta-peridotite and cornposed of serpentine, tremolite and opaques. Blades of up to 3 cm in length and 1 mm width are arranged in notably parallel orientation,with zones of tremolite consisting of aggregates of subhedral crystals with varied altitudes alternating with zones of serpentine and/or serpentine-trernolite. ln the essentially serpentine zones long, thin blades of olivine are Photo 2 - Meta-pvroxentte wíth radtating spinifex lexture. Tre­ moiíte h/ades tn matrix oftremolite, chlorite, and opaque minerais. Pilar de Goiás-Hídrolína belt (natural light)

A second sampie, from Córrego do Alagadinho in the Crixás belt, collected from a complete tlow unit where the A and B subunits can be recognized, is composed of serpen­ tine, tremolite, chlorite and opaques, and c1assified as a metaperidotite. Thin, elongated blades of serpentine with associated opaques are arranged at random, mutually in­ tersecting and forming most commonly triangular spaces containing anhedral microcrystals of tremolite in random arrangement and/or flakes of serpentine. Locally, serpen­ tine blades may occur parallel to one of the sides of the triangular structure and in distinct alternation with mag­ netite. Tremolite occurs as elongated microblades forming clusters in paralle! arrangement, and the discordant orien­ tation of tremolite crystals in adjacent strips gives a charac­ teristic appearance (Photo 3). Another sample from the area of Córrego do Alagadinho is classified as a tremolite-actinolitite derived through Photo 4 - Meta-peridotite with plate spínijex texture. Serpentine metamorphism of a clinopyroxenite, and cornposed of blades altematíng wíth tremollte. Pilar de Goiás-Hídroltna belt actinolite, tremolite, serpentine and opaques. Elongated ípolarízed fightJ 336 RevistaBrasileira de Oeocíênctas, Volume 12 (l~3). 1982

replaced transversally by serpentine and contain interstitial Talc-carbonate schist A schistose rock of lepidoblastic microlites of tremolite; microblades of serpentine in obli­ texture, with more than 95% oftale and carbonate and minor que orientation may also occur or tremolite microcrystals serpentine and opaques. Fine tale lamellae show a promi­ may be mixed in various proportions with flakes ofplumose nent orientation and anhedral carbonate porphyroblasts chlorite. Secondary, anhedral opaque minerais are always with irregular outlines are elongated on the schistosity. present, both associated with blades of serpentinized olivine Some carbonate crystals clearly exhibit rhombohedral clea­ and outlining tremolite crystals (Photo 4). vage. Sparse serpentine flakes and anhedral opaque minerais A second example of plate spinifex texture in a rock of are also present, while microfractures contain fibrous mi­ the same composition (tremolite, serpentine and opaques) nerals. is provided by a sampie from the area of Córrego do Ala­ gadinho, Large blades of serpentine up to 3 cm in length Chlorite schist A fine-grained rock composed almost en­ form triangular structures, while tremolite plates alterna­ tirely ofchlorite flakes generally arranged in minute fans or ting with thin serpentine blades form parallel groups aligned subparallel bunches with very varied orientation. The chlo­ with one of the sides of the triangle. Within the amphibole rite has a natural pale-green colour, low birefringence and plates, microprisms of tremolite arrange themselves inter­ fairly weak pleochroism. Minute anhedral crystals of opa­ posingly and obliquely to the serpentine blades, in parallel que mineral are sparsely distributed. arrangements which change in orientation from plate to plate, Opaque minerais occur mainly in discontinuous as­ Chloritite This Iithology is characterized by a curious fan sociation with the thin blades of serpentine, and also texture, with pale-green chlorite flakes forming radiating accompanying the inter-blade microprisms of tremolite. fan patterns of varied attitude in a fine-grained dense arran­ A tremolitic veio cuts across the rock. gement, with dispersed granules ofanhedral opaque mineral.

Calc-serpentinite This lithology from the Hidrolina region Tremolite schist A rock oflepidoblastictexture, cornposed is composed principally of serpentine, carbonate and opa­ of tremolite, olivine, chlorite, tale and opaques. Tremolite ques. Serpentine crystals form a fine-grained, dense mass forms subhedral prismatic microcrystals, Iocally with fibrous in which secondary carbonate occurs locally in aggregates. habit, following a preferential direction in a fine-grained, Using uncrossed polars, carbonates are frequently obser­ dense, foliated arrangement. Irregularly shaped, fractured, ved delimiting long, rectangular serpentinized crystals ongi­ porphyroclasts ofrelic olivine occur in the fine amphibolitic nally ofolivine, while other carbonates are probably derived matrix, together with chlorite flakes, fine tale lamellae and from the alteration of pyroxene. Cross-cutting veins of ser­ minute anhedral grains of opaque mineral. The effects of pentine are numerous. tectonism are evident both in the orientation of the matrix and in the stretching and fracturing of the porphyroclasts. Talc-serpentinite This rock, represented by a sample from

the Crixás belt, contains over 90:/0 serpentine and tale, with Chlorite-tremoiite schist This Iithology contains tremoli­ minor tremolite. Rounded, oval and prismatic serpentinized te, chlorite, olivine and opaques, with a fine-grained nemato­ forms from the alteration of olivine are enveloped by fine­ blastic texture. Tremolite occurs as long, thin prisms and -grained talcified material, which shows (specially with microfragments densely packed in a prominently oriented uncrossed polars) minute tremolitized and talcified laminas matrix containing irregular aggregates of chlorite. Intensely alternating with serpentinized laminas, locally forming semi­ fractured clasts and porphyroclasts of relic olivine with -radial structures. At first sight the rock seems to show the irregular form are common, and anhedral opaque mineral normal texture ofan extensively metamorphosed peridotite, (magnetite) is associated with the olivine and also commonly but with detailed observation, possible relics of spinifex occurs on the cleavage planes of chlorite, especially in the texture may be discerned (Photo 5). proximity of olivine crystals (Photo 6).

Photo 5 - Talc-serpentínite. Roundedserpentíne pseudomorphs with Photo 6 - Chloríte-tremolíte schíst, Fractured olívine crystals in tale, tremo/ire andserpentíne laminae;vestigesofspinifex texture (?). atteratton, with eh/orite, tremo/ite and opaqõe minerais. Crixás belt Críxás belt (natural Iight) (naturallight) RevistaBrasileira de Oeoaénctas; Volume 12 (1·3), 1982 337

Serpentlne-chlorlte-tremollte schist This rock of nema­ produced secondary minerais replacing the greenschist fa­ toblastic texture shows a well-marked division between tre­ cies assernblages, Matrix minerais and glass are recrystal­ rnolite-chlorite and serpentine-chlorite zones. The tremolite lized with theformation ofactinolite, chlorite, epidote, tale, forms subhedral to anhedral prisms generaIly without ori­ and other secondary minerais. entation, and with varied attitudes, often transverse to the The following types of alteration are considered to be the c1early defined preferential direction shown by the subhedral most important in the rocks under review. chlorite flakes. The serpentinitie zones contain fine lameIlae of serpentino forming a mass of blue interference colour Serpentinlzation A very common process leatíing to the from which oriented chlorite flakes stand out. formation of mostly antigorite and chrysotíle. Serpentine occurs most conspicuousiy as pseudomorphs of the blades Metebeselts The Iithology represented by a sample from and other crystals of olivine, and formation of serpentine the Crixás area is composed mainly of actinolite, quartz, ai the expense of pyroxene seems to be unlikely in these chlorite, and sphene, with a fine-grained metamorphic tex­ rocks. At least two generations of serpentine are present, ture. Actinolite, probably a metamorphic product of pre­ since veins and lenses of chrysotile cut serpentine (antigo­ -existing c1inopyroxene, is the dominant mineral,and occurs rite) crystals formed and oriented during the main defor- as idioblastic prisms generally with random orientation Dr mational phase, ' locally in radial arrangements. Chlorite is a minor cons­ tituent and sphene is distributed in smaIl, weIl-formed Talclflcatlon The formation of tale is directly related to crystals. Quartz occurs in mínutegrains in the aphanitic, the transformation of olivine, and often occurs as a sub­ devitrified matrix, together with possible feldspathic ma­ sequent alteration of serpentine, According to Turner and terial, and also in larger recrystaIlized areas. Although the Verhoogen (1960), carbon dioxide metasomatism of ser­ amount ofquartz in the matrix might suggest a rather more pentine accounts for the formation of tale and magnesite. acidic (andesitic?) composition, most of it is probably At high temperatures, hydrothermal metarnorphísrn of ui­ secondary, and the macroscopie features of the rock (vesi­ trabasic rocks containing original CaO, or exposed to cal­ cular, amygdaloidal and piIlow structures) support an ori­ cium-bearing solutions, tends to giveactinolite, or if A1 20J ginaIly basic composition, is present actinolite-chlorite, at the expense of serpentine. Another sarn pie from the sarne locality contains actino­ At low temperatures, within the greenschist faeies, the par­ lite, plagioclase, quartz, carbonate, epidote, chlorite, tour­ tial rcplacement of SiO, by CO, produces a talc-dolomite maline and opaques. The rock has a very fine grained ne­ association instead 01' actinolite. matoblastic texture produced by the orientation of actino­ lite prisms. Epidote and carbonate are weIl developed and Carbonatization A common process in the alteration of in considerable quantities. presumably resulting from me­ lhe mafic and principally ultramafic rocks. ln some sam­ tasomatism. Banding in the rock is'caused by lhe alterna­ pies the carbonate replaces serpentine, and in others it tion of bands richer and less rich in actinolite, epidote and results from lhe replacement of amphibole (tremolite). Si­ carbonate, possibly indicating a differentiation between ba­ multaneous replacement of serpentine and tremolite is saltic and andesitic composition in the original inter-pillow also possible, and occasionally pseudornorphs ofolivine and material. clinopyroxene are preserved after complete replacement by carbonate. Carbonatization of tale is also observed. Condie (1981 ) considers that the possible sources of carbonate in METAMORPHISM AND ALTERATION The mafic­ Archean greenstone belt voleanics are: I. late-magmatic -ultramafic rocks of lhe Crixás. Guarinos. Pilar de Goiás or deuteric fluids associated with voleanism; 2. volatiles -Hidrolina. and Goiás (Goiás town) grccnsrone belts have liberated by intrusive granites; 3. reaction with sea water undergone a series of mineralogical changes during late­ during or soon after eruption; and 4. mobilization and -magmatic, principal metamorphic and post-principal meta­ concentration 01' volatiles already present in volcanics. morphic (or later deformation) phases, The last phase is marked by lhe crystaIlization ofcross micas and amphiboles Amphlbolltlzatlon Tremolite is the most common mineral intersecting the maio schistosityi and replacement by secon­ in the ultramafic flows under consideration, especially in dary minerais, as well as the presence of chrysotile veins those with skeletal and spinifex textures. Most of the blades cutting serpentine oriented parallel to the schistosity. Late­ and other crystals of tremolite are believed to be derived -magrnatic transforrnations are the most difficult to recog­ from primary clinopyroxene by transformation in a late­ nize, although it is believed that the rocks preserving pri­ -rnagmatic phase and/or during low grade metamorphism. mary features such as porphyritic, cumulus, skeletal and More rarely tremolite rnay be derived also from serpentine, spinifex textures, and lackinginorientation , have been trans­ and tremolite and actinolíte can also result from the altera­ formed into this phase or into a low-grade metamorphism­ tion 01' orthopyroxene. However, the common occurrence without observable dynamic effects. of the cyclíc alternation of serpentino with tremolite and/or ln general themetamorphic mineral assemblages are com­ actinolite in lhe rocks with spinifex texturc, shows thal parable to those described in mafic and ultramafic rocks transforrnation 01' different minerais occurred during the by Turner andVerhoogen (1960), Williams (1971), Miyas­ sarne alteration phase. and that most amphibole follows cli­ hiro (1973) and Winkler (1977). According to Condie (1981 ), nopyroxene rather than olivinc. Trernolite and actinolite the most common types of alteration in Archean volcanic occur in at least two generations in the uitramafíc rocks, rocks are carbonatization, chloritization, silicification, epi­ while actinolite is also commonly formed by the low-grade dotization and serpentinization. The mineral associations metarnorphism of basalts and diabases. formed during alteration are similar to those of greenschist facies metamorphism, and may have resulted from the sarne Chloritization Chlorite appears to be an alteration product processo At other times post-metarnorphic alteration has of both primary olivine and pyroxene, or their derivatives, 338 Revista Brasileira de Oeocénctas, Volume 12(t-a), 1982

during various episodes of formation. At times, chlorite may be fairly variable even in komatiitic peridotites, while replaces tremolite, and tremolite porphyroblasts are obser­ komatiitic pyroxenites may show relatively high average ved to be formed previously to the metamorphic phase in AI,O, values, as in those of Munro Township, Abitibi belt which chlorite originated. ln other rocks there is a distinct (Arndt, Naldrelt, and Pyke, 1977). banding between tremolite and chlorite, or between tre­ CaO conterits show averages of 4.22~(j for the Crixás, molite-actinolite and serpentine-chlorite, with tremolite and Guarinos, Pilar de Goiás-Hidrolina greenstone belts and chlorite forrning in the sarne metamorphic phase. ln this 3.0l/ofor the Goiás greenstone belt, values which are com­ case chlorite was derived from a mineral other than tremo­ patible with those from similar sequences in other areas, lhe or clinopyroxene. ' although very low in relation to those of corresponding rock types from Munro Township and the Abitibi belt in Epidotization . Cornmonly present, especially in the mafie general. Ti02 contents are very low, with 'averages of0.42/,o rocks where the ca!cic plagioclases of the basalts are altered for Crixás, Guarinos, Pilar de Goiás-Hidrolina and only to epidote, carbonate and chlorite. 0.17/(; for Goiás. Considering the ratios of calei um, aluminium and tita­ Tourmalinization Localized and possibly related to boron nium, it is notable that the CaOjAl,O, ratios, 0.5 for metasomatism associated with residual granitic solutions Crixás, Guarinos, Pilar de Goiás-Hidrolina, and 0.3 for acting on the tremolite schists and tremolite- tale schists Goiás, are very low. due to the high AI,O, contents. This of the Crixás belt, and probably also present in other areas.. suggests that the CaOjAl,O, ratio aI' greater than 1 pro­ The formation of opaque mínerals. principally magnetite, posed as a criterion of komatiites by Viljoen and Viljoen is also connected to some of the alteration phases. During (1969) and Brooks and Hart 1974), and even the ratio 0.8 serpentinization magnetite accumulates around the pseudo­ proposed by Nisbet et ai. (1977) and accepted by Condie morphs of olivine, although primary magnetite may also (1981) as part of a modified definition, cannot be reliably be present in peridotite as an inter-cumulus phase. ln the used in the areas under discussion. The AI203/Ti02 ratio alteration of clinopyroxcne to trcmolitc-actinolite excess for the Crixás, Guarinos, Pilar de Goiás-Hidrolina green­ of iron is also Iiberated in the form of magnetite, which stone belts is 20.6 and for the Goiás greenstone belt 59.5, often outlines arnphibole blades ar concentrates on clea­ the latter value being very high in relation to those quoted vage planes. for komatiitic peridotites of Barberton, Abitibi and wes­ tern Australia (Condie, 1981). The CaOjTiO, ratio is 10.1 for the Crixás, Guarinos, Pilar de Goiás-Hidrolina belts and 17.9 for the Goiás belt. The average K20 contents are 0.1O/o for Crixás, Guarinos, Pilar de Goiás-Hidrolina and 0.70?" for Goiás, this being MAJOR-ELEMENT CHEMISTRY Geochemical considerably higher than comparable values in other areas studies aI' the mafic-ultramafic rocks of the Crixás, Gua­ or the proposed upper limit of 0,5:)'0 for komatiite of Brooks rinos, Pilar de Goiás-Hidrolina greenstone belts have been and Hart (1974). Average Na,O content is 0.52/" for Crixás, made by Montalvão, Lopes and Pereira (1981), and for Guarinos, Pilar do Goiás-Hidrolina and 1.23/0 for Goiás. the corresponding rocks of the Goiás greenstone belt by The chemical variations 01' the major elements 01' the Montalvão and Bezerra (1981). ln those studies diagrams mafic-ultrarnafic rocks ef the two greenstone belt sequences using selected minor and trace elements were presented in can be plotted in a scnes of diagrams for comparison with to major elements, but the present account only addition in trends established in similar sequences elsewhere. The rocks the major-elernent chemistry of the rocks will be considered. from various traverses in the Abitibi belt ofCanada sampled The analytical results for the oxides ofmajor elements are by Jollie (1975, in Condie, 1981) show a rapid variation of Iisted in Table I (Crixás. Guarinos, Pilar de Goiás-Hidro­ MgO for a small variation in total iron (FeO') in the korna­ Iina) and Table 2 (Goiás). The locations of the sam pies are tiitic series and a sharp increase in total iron for a slight in­ shown in Figs. IA and IB, and the corresponding Iithologies crease in MgO in the tholeiitic series, The mafic-ultramafic are Iisted in Table 3. rocks ofthe Crixás, Guarinos, Pilar de Goiús-Hidrolina belts The averages ofthe analytical results are shown in Table 4. prcscnt a similar trend to that of the komatiitic series of It should be emphasized that the overwhelming majority Abitibi (Fig.3A). For the rocks of the Goiás belt the curve of rocks considered here is of uitrarnafic composition, and resembles sornewhat the trend 01' the tholeiitic series of the few analyses aI' mafie rocks included in the calculations Abitibi. but is more open with an inílection in the upper do not significantly affect the results. part of the diagram. showing an enrichrnent in MgO High average MgO values are recorded in both the Crixás, (Fig. 3B). Howcver, the correlation between these rocks Guarinos, Pilar de Goiás-Hidrolina greenstone belts and those of Crixás, Guarinos, Pilar de Goiás-Hidrolina (23.48/,,), and in the Goiás greenstone belt (20.31:',,). indicates dominant kornatiitic tendencies for the sequence. The AI,O, contents are notably high in the two areas, On the AI,O, I'S. Alkalis (Na,O + K,O) diagram Kuno with values of 8.66~~,o for Crixás, Guarinos, Pilar de Goiás­ (1966) outlined the fields of alkali basalt (A), high-alumina -Hidrolina, and 9.99~() for Goiás. ln comparison, Condie basalt (B) and tholeiitic basalt (C). The rocks of both the (1981) shows the following average AI,O, contents: garnet Crixás, Guarinos, Pilar de Goiás-Hidrolina, and Goiás peridotite from a kimberlitic nodule 2.66:)-0' kornatiitic peri­ belts plot in the tholeiitic basalt field, with only two sarnples dotite from Komati Formation, South Africa, 3.44~o~ ko­ in the alkali basalt field (Figs, 4A and 4B). matiitic peridotite from western Australia 5.22~(p and ko­ ln the FeO' (total iron) I'S. MgO diagram, modified from matiitic peridorite from Abitibi belt, Canada, 7.46~,(). Thus, Glikson (1972), the rocks of the Cri xás, Guarinos, Pilar with the exception of the Abitibi belt, the other examples de Goiás-Hidrolina belt (Fig.5A) plot in various fields show much lower AI,O, values than are presently recorded from tholeiites to highly magnesian rocks, especially in in our analyses. However, it appears that AI203 content that of peridotites and dunites. The rocks of the Goiás belt Revista Brasileira de Geocíêncías, Volume 12 (1-3), 1982 339

Tabte 1 - Analysis of major-eíement oxides for mafíc-ultrumafíc rocks of Crixás, Guarinos, Pilar de Goiás-Htdroltna greenstone beits.

I 2 4 s 7 8 9 10 12 13 14 15 17 18 19 21 Oxides~ • " "

sto, 66,28 47.22 38.80 41.31 45.84 44.31 43.70 46.53 45.09 48.71 40.98 45.34 48.62 42.79 37.67 43.21 44.34 36.14 40.22

All0J 18.15 18.7] 10.77 6.59 7.48 7.62 6.81 4.00 8.93 15.40 9.55 6.72 15.17 7.81 3.92 8.12 6.55 8.37 5.42

FC20~ 1.00 14.65 6.24 5.52 4.42 4.88 4.52 3.34 7.10 5.65 5.15 4.53 3.14 5.02 5.33 7.36 6.46 8.57 6.29

FoO 1.32 1.35 4.07 2.75 3.10 2,~3 5.41 4.45 4.48 6.08 2.88 5.50 8.84 3.53 2.79 2.25 2.70 3.12 3.51

Moa 0.07 0.15 0.21 0.13 0.15 0.19 0.17 0.21 0.22 0.22 0.19 0.20 0.27 0.17 II ~I 0.18 0.21 0.26 0.17

Ti02 0.39 1.50 0.53 0.37 ·0.30 0,30 0.54 0.44 0.58 0.62 0.42 0.47 0.94 0.26 (L~(\ 0,39 0.39 0.26 0.32

CaO 1.88 5.18 2.45 4,01 6.38 5.01 4.27 1.95 6.16 9.77 4.04 7.17 10.48 5.36 1.81 5.14 3.34 0.33 4.67

MgO 3.31 3.01 27.29 29.60 26,44 28.38 27.47 30.59 22.55 9.06 27.24 24.29 9.22 26.85 34.76 26.53 27.45 31.14 29.41

Na20 4.33 0,29 0,24 0.24 0.58 0.24 0.24 0.24 0,62 2.49 0.37 0.66 1.83 0.41 0.25 0.16 0.20 0.20 0.12

K,O 2,29 0.12 0.12 0.04 0.08 0.04 0,04 0.04 0.04 0.16 0.08 0.20 0.08 0,04 0.04 0.04 0.12 0.04

PlOS 0.07 0.18 OJ>9 0.09 0.07 0.05 0.10 0.08 0.10 0.12 0.05 0,08 0,11 0.07 0.02 0.04 0.06 0.06 0.10

H,O 0,23 0.71 0.42 0.30 0.17 0.12 0.32 0.25 0.11 0,05 0.09 0.09 0.25 0.09 0.34 n.re 0.23 0.23 ---- L.O.I. 1.46 7.29 8.41 8.43 i 5.13 6.48 5,83 7.21 3.86 1.30 8.20 4.35 0.49 7.20 12.78 6.61 7,1(, 10.56 8.92

22 23 24 25 2. 27 28 29 30 31 32 33 34 35 36 37 38 39 4

s.o, 39,11 47.99 47.42 50.93 • 39.17 47.76 49.99 46.07 48.00 27.19 47.68 40.24 40.16 45.81 30.41 27.34 42.25 43.12 40.36

AI1O~ 8.53 4.47 4.98 7.60 15.14 12.63 5.05 11,88 11.34 17.72 1.85 3.99 3,84 4.51 20.7] 18.54 4.11 3.58 2.46

FclO.l 3.42 5,56 9,76 7.55 7.05 4.83 4.57 13.63 4.65 7.68 11m 6.40 4,66 6.45 14.45 6.43 18.93 17.10 3.65

FcO 4.24 4.66 4.14 1.68 4.94 6.58 3.96 .1.54 7.48 10.40 4.07 6.41 6.29 4.75 5.18 l.UI 3.53 4.11 5.64

MnO 0,17 0.16 0,19 0.24 0.19 0.20 0.18 0.25 0.225 Ü.172 0.255 0.192 0.153 0,152 0.14 0.155 0.252 0.204 0.157

no, 0.26 ü.28 0.43 0.13 0.65 0.74 0.15 1.07 0.54 0.94 0.41 ü.32 0.15 0.36 0.70 0,78 0,70 0.61 0.14

ClIO 1.55 1.53 0.35 4.62 7.79 9.98 7,61 9.48 0.29 9,31 3.55 0.22 7.70 0.08 0.09 2.92 6.57 0.05

MgO 31.11 28.03 25.19 25.18 18.26 12.47 22.06 8.19 11.57 21.09 20.70 28,31 32.41 24.03 10,63 19.15 19.78 19.25 34.36

N1I1O 0.12 0.12 O.OS 0.04 1.24 2.41 0.49 0.49 2.64 0.35 0,16 0.07 0.17 0.38 0.02 0.38 009 0.14 0.04

K,O 0.Q4 0.04 -- 0.04 0.08 0.24 0.19 0.02 0.02 0.01 0.01 0.04 0,03 0.01 0.01 0.01 0.005

PlOS O.OS 0.09 0.11 0.10 0.12 0.14 0.09 0.15 0.04 0.09 0.07 0.06 0.ü3 0.03 0.04 0.11 0.02 O.oJ 0.02

H,O 0.11 0.10 0.17 0.28 0.34 0.17 0.23 0.79 0.57 0.85 0,22 0.42 0.60 0.22 4.30 1.02 1.26 0.82 0.26

L.O.i. lO.39 6,39 4,99 6.34 7.55 3.67 3.75 5.23 2,75 11.32 3.55 8,79 11.75 4,93 11,63 11.43 6.95 5.40 11.12

41 42 43 44 4' 46 47 48 49 50 st 52 53 54 ss 56 57 58 Oxides~

sro, 40.90 40.24 42.10 33.85 43.31 42.88 49.92 45,18 47.19 49.59 53.24 43.18 51.33 24.46 40.43 46.34 43.26 47.36

A120~ 4.20 2.93 3,99 1.04 5.38 4.18 5.10 5.81 5.57 4.81 3.25 2.50 11.92 57.75 5.09 7.40 9.72 5.44

FclOJ 4.93 4.44 7.91 4,26 4,65 6.02 4.64 3.66 4.61 4.42 3.72 2.19 L70 0.51 10,05 7.80 4.62 3.23

FcO 6.45 6.51 4,53 3.91 4.16 3.84 3.70 4.06 5.3! 5.85 4.78 4.35 8.85 0.64 4.04 5.59 6.32 3.58

Moa 0.195 0.169 0.042 0.114 0.134 0.267 (l,183 0.177 0.205 0.283 0.057 0,127 0.255 0.054 0.191 0.l3 0.27 0.18

no, 0,26 0.18 0.27 0.01 0.16 0.16 0.11 0.20 0.33 0.25 0.25 0.09 0.80 0,06 0.69 0.10 0,69 0.32

ClIO 1.96 1.61 4.65 0.11 4.27 2.fJ7 3.4S 3.57 2.65 2.94 0.10 0.94 10.65 5.88 4.31 8.42 7.41 5.19

MgO 29.63 3US 26,72 32.15 29.21 32.95 29.66 2S.61 21.03 24.79 27.87 34.04 8.58 1.28 24.59 22,76 21.02 26.75

N1I2O 0.14 0.04 O.ll 0.08 0.23 0,07 0.08 0.18 0.28 1.30 0.03 0.16 0.16 1.42 0.11 1.42 0,70 0.20

K,O 0.02 0.005 0.02 0.01 0.02 0.02 0.01 0.02 0.05 0.24 0.002 0.01 0.03 0.03 0.02 0.12 0.04 0.04

P10S 0.03 0.Q1 0.003 0.003 O.oJ 0.07 0.03 0.05 0.05 0.03 0.01 0.24 0.06 0.01 0.03 0.10 0,13 0.06

H,O 0.33 0.18 I 0.30 1 0.13 I 0.16 0.20 0.20 0.23 0.20 0.02 I 0.57 0.14 0.28 0.06 0.69 ~ 0.20 0.21

L.O.I. 9.61 10.48 7.32 2l.77 6.46 S.5l 8.38 s.n 5.43 na 6.17 10.80 1.22 3.50 6.82 4.53 5.13 6.58 340 Revista Brasileira de Geocténaas, Volume 12 (1·3), 1982

Table 2 - Analysis of major-element oxides for mafíc-ultramafíc rocks of Goiás (Goiás town) greenstone beit.

I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Oxides~ sio, 53.72 46.72 44.92 49.02 25.22 50.40 47.50 46.74 39.04 39.36 58.60 41.68 50.70 39.60 52.62 45.30

Al z03 4.60 15.56 11.43 7.96 24.40 5.36 5.27 7.48 5.42 5.26 1.60 2.17 5.33 6.87 3.16 10.40

FczO,3 6.80 19.06 9.47 7.62 12.80 12.91 14.49 12.17 10.39 9.82 4.91 14.41 12.02 8.45 11.70 15.02

FeO 5.04 8.76 6.76 5.72 8.03 4.76 6.67 6.25 5.59 6.24 3.13 7.16 7.70 4.11 6.29 5.16

MnO 0.10 0.25 0.10 0.10 0.22 0.10 0.10 0.10 0.16 0.12 0.10 0.14 0.10 0.13 0.27 0.17

MgO 24.92 3.44 25.40 22.27 24.38 22.90 25.53 24.09 24.80 31.32 25.66 25.97 23.97 25.18 26.20 19.54

C,O 0.72 5.99 0.10 4.23 0.14 0.10 0.18 0.10 5.83 0.23 0.96 0.55 0.16 3.98 1.24 0.43

NazO 0.92 1.62 0.94 1.13 1.51 0.90 1.05 0.73 1.23 J.l7 0.62 0.72 J.l5 1.21 1.03 J.l2

K,O 0.10 0.44 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.16 0.40 0.43 0.36 0.25 0.14 0.40

no, 0.10 0.31 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.32 0.10 0.31 0.40 0.25 0.23 0.11

PzOs 0.13 0.36 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

L.O.1. 5.64 4.11 7.53 5.80 11.40 5.92 5.90 6.70 12.93 11.37 5.03 10.01 6.31 10.17 3.60 7.78

17 18 19 lO 21 22 23 24 25 26 27 28 29 30 31 32 Oxides~

so, 59.26 46.00 42.98 46.36 41.88 35.72 49.54 46.64 49.40 47.18 50.04 49.08 36.74 53.22 46.68 44.92

Al z03 14.16 9.12 37.00 6.60 25.28 28.52 7.17 7.65 5.90 7.94 13.81 4.40 12.91 4.98 3.85 8.30

FCZ03 8.62 10.60 2.23 12.00 16.64 8.00 11.10 8.14 9.80 8.90 12.74 12.00 9.50 7.79 15.29 10.30

FeO 6.11 7.50 0.24 8.90 1.94 1.21 7.17 4.88 6.20 4.62 8.03 8.99 5.65 5.39 8:18 4.74

MnO 0.12 0.10 0.10 0.20 0.10 0.10 0.17 0.10 0.15 0.13 0.21 0.27 0.15 0.10 0.23 0.22

MgO 9.70 24.81 0.81 19.76 2.46 9.34 16.80 25.99 22.90 23.91 8.07 18.68 25.62 25.29 15.00 25.60

C.O 0.25 0.25 0.22 10.11 0.20 0.24 7.35 0.15 6.13 4.00 12.07 11.23 4.26 0.18 14.72 0.10

NazO 1.00 0.87 1.43 1.28 1.02 2.08 1.24 1.53 J.l2 J.l7 .107 1.29 1.44 1.37 1.45 1.05

K,O 0.85 0.43 8.48 0.10 6.85 0.10 . 0.07 0.42 0.10 0.10 0.65 0.35 0.14 0.34 0.10 0.13

no, 0.11 0.35 0.10 0.10 0.10 0.15 0.10 0.21 0.17 0.10 0.10 0.44 0.22 0.10 0.10 0.10

r,o, 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

L.O.1. 5.48 7.38 4.19 3.97 6.20 3.30 4.72 7.63 4.80 6.71 0.71 3.20 7.88 5.95 2.31 7.10

(Fig. 5B) plot mainly in lhe field of pyroxenites with a few tional crystallization, while lhe calc-alkaline series exhibits in the tholeiite field. slight decreases in TiO, and FeO" The mafic-ultramafic Miyashiro (1975) discriminated between tholeiitic (TH) rocks of'the Crixás, Guarinos, Pilar de Goiás-Hidrolina belts and calc-alkaline (CA) series in diagrarns showing lhe plot on a verlical trend in lhe TiO, l's. FeO*/MgO diagram, variation of 8iO" TiO, and FeO* with increasing FeO*/ with abrupl enrichment in TiO" typical of fractional crys­ MgO. The CA series shows a more rapid increase of 8iO, tallization of the tholeiitic series (Fig. 7A). For lhe corres­ with advancing fractional crystallization, and in lhe 8iO, l'S. ponding rocks of the Goiás belt, rnost analyses plol on a FeO*/MgO diagram lhe mafic-ultramafic rocks of both vertical trend of similar type and interpretation, although lhe Crixás, Guarinos, Pilar de Goiás-Hidrolina and Goiás there is also an indication of a horizontal trend with cons­ belts show curves of very steep angle, approximating lo lhe tant vaIue of TiO, (Fig. 7B). ln lhe FeO* \'s. FeO*/MgO vertical, representing a calc-alkaline trend. The two sets of diagram lhe generally low values of FeO*/MgO prevent a rocks plot in both tholeiitic and calc-alkaline fields, although discrimination of lhe tholeiitic and calc-alkaline fields, but lhe generally low values of FeO*/MgO do not enable a good the overall lendency is tholeiitic as shown by the steep angle discrimination (Figs. 6A and 6B). ln lhe TiO, 01' FeO* vs. curves with rapid enrichmenl in FeO* for bOIh lhe Crixás, FeO*/MgOdiagrams lhe typical tholeiitic series shows initial Guarinos, Pilar de Goiás-Hidrolina, and Goiás areas (Figs. enrichment in TiO;! and FeO* with a maximum during frac- 8A and 8B). Revista Brasileira de Geociêncías, Volume 12 (l~3), 1982 341

Table 3 - Petrographíc classífícatíon ofanalyzed mafíc-ultramafic rock:sofCríxàs. Guarinos, Pilar de Goíàs-Hidrotína and Goiás (Goiás town) greenstone helts.

Crlxás, Guarinos, Pilar de Golás-Hldrelína Greenstone Belts Goiás Greenstone DeU

Sample Sample Petrographic c1assification Petrographic classificadon N." N."

I Tremolite-talc schist I 2 Amphibolite 2 Quartz-amphibole schist 3 -- 3 Chlorite-talc schist 4 Chlorite-tremolite serpentinite 4 Tremolite-chlorite-talc schist 5 Chlorite-tremolite serpentinite 5 Chlorite-talc schist 6 - 6 Tale schist 7 Serpentine-tremoli te-chlorite schist 7 8 Chlorite-tremolite schist 8 Tale schist 9 Serpentine-carbonate-tremolire-tale schist 9 Chlorite-talc schist 10 Chlorite-trernolite schist 10 Tale serpentinite II Amphibolite II Chlorite-talc schist 12 Meta-ultramafite 12 Serpentinite 13 13 Tale schist 14 Meta-dolerite (?) 14 Tremolite-chlonte schist 15 Serpentine-tremolite-talc-chlorite schist 15 Tremolite schist 16 Meta-ultramafite 16 Tale schist 17 Tremolite-chlorite schist 17 18 Meta-ultramafite 18 Tale schist 19 Chiorite serpentinite 19 20 20 Tremolite-actinolite schist 21 Tremolite serpentinite 21 Tale schist 22 Meta-ultramafite 22 Tourmalinite 23 Tremolite-chlorite-talc schist 23 Tremolite schist 24 Chlorite-talc schist 24 Tale schist 25 Talc-chlorite schist 25 Quurtz-amphibolc schist 26 Chloritite 26 Tale schist 27 Amphibolite 27 Amphibolitc 28 Meta-ultramafite 28 29 Epidote-amphibohte 29 Trcrnolite-chloritc schist 30 Chlorite-tremolite serpentinite 30 Tale schist 31 Chlorite schist 31 Meta-pyroxenitc ('?) 32 Meta-pyroxenite 32 Trcmolitc schist 33 Meta-peridotitc 34 Serpentinite 35 Meta-pyroxenite 36 37 Chlorite schistjChlorititc 38 Talc-serpentinite 39 Meta-peridotite 40 Serpentinite 41 Meta-peridotite 42 Serpentinite 43 Amphibolite 44 Talc-carbonate sebist 45 Chlorite-tremolite schist 46 Tremolite schist 47 Serpentine-chlorite-tremolite schist 48 Serpentine-chlorite-trernolite schist 49 Talc-chlorite-tremolite schist 50 Tremolite-chlorite-talc schist 51 Chlorite-talc schist 52 -- 53 Garnet amphibolite 54 _. 55 Meta-peridotite 56· Chlorite-tremolite schist 57 Tremolite-chlorite schist 58 Meta-ultramafitc 342 Revista Brasileira de Geociéncias, Volume 12 (1·3), 1982

Table 4 - Averages of major-element oxide contents for mafic-ultramajic rocks of the Crixás, Guarlnos, Pilar de Goiás-Hldroíina and Goiás (Goiás tmm) greenslone beíts

OXIDES sro, Alz03 FeZ0 3 FeO MnO MgO C-O NazO K,O 'no, r.o, H,O Crixás, Guarinosand Pilar de Goiás-Hidrolina 43.45 8.66 6.25 4.68 0.18 23.48 4.22 0.52 0.10 0.42 0.07 0.39 Greenstone Belts (56 samplesl

Goiús Grcenstone Bell 46.14 9.99 10.80 5.84 0.14 20.32 3.01 1.23 0.70 0.16 0.10 ---- (32 samples]

--0------0-- Mofic-ultromafic rocxe of tt'le crbúe, sucrinos. --a---o- Mofic-ultromofic rocks of the Goiás (Goiás PÍtar de Goiás - HícroIino greenstone belts. town I greenstone ben.

Kcmotiitic series. Komotiitic sedes. ------eotc-oIkoline series. ------coíc-olkoíine sedes ThOIeiihc series. ----- Tho6eiitic eeries

36. '0 '0

6

2 2

2 6 re 2 6 '0 18 feO' feO'

Figure 3A - MgOjFeO* diagram for mafíc-ultramafic rocks of Figure 38 _. MgOjFeO* diagram for mafic-ultramafíc rocks 0/ the Crixás, Guarinos, Pilar de Goiás-Hidrolina greenstone belts, com­ Goiás (Goiás town) greenstone belt, compared wíth komatiitic, tho­ pared wíth komatiitic, thoíeíitíc and calc-uíkalíne seríes oIAbitíbi íeíitíc and catc-atkattne seríes of lhe Abitibí belt (jrom Jolly, 1975, belt (jrom Jol/y, 1975, in Condie ; 1981) in Condie, /981)

Arndt, Naldrett and Pyke (197i) plotted the komatiitic The maflc-ultramaflc rocks of the Crixás, Guarinos, Pilar and tholciitic lavas of Munro Township on the MgO-CaO­ de Goiás-Hidrolina belts were plotted 00 a diagram of this -AI,O, (MCA) diagram, showinga trend with a CaO/AI,O, type (Fig. 9A) and shaw a similar trend of CaOjAI,O, ratio of approximaiely I, which bends towards lhe AI 20 , equal to I, with rather more dispersion, and an acccntuated comer (i.e. decrease in CaO/AI 20 , ). The removal of olivine inflection towards the AI,O, corner, and low CaO/AI 20 , is considered to be the dominant control in the trend of ratios. The rocks of lhe Goiás belt plot with greater dis­ decreasing MgO contents at constant CaOjAI203 ratio persion (Fig. 9B), with some sam pies following the MgO­ fram peridotitic through pyroxenitic lo basaltic komatiites, -AI,O, side of lhe diagrám and othcr showing a deviation at which stage clinopyroxene removal becomes responsible towards lhe MgO-CaO side, with CaO/AI20, ratios greater for the curvature towards lhe MgO-CaO side of'the diagram. than l. Revista Brasileira de Oeocíências, Volume 12 (l·3), 1982 343

AI2O. ."

2. / / :" / / / / / ". I / I / 22 / / / / / / B /A C I B / A \ \ / <, / 18 '" / '"...... / I / .o' " ." '\. " 2h "- -, " ~, ." -, ." \ \ 10 '" '~.• \ 00,. ","I' .24 \ 14 \ 07~23. \ e .-20 \ \ ~ 00.; -• • 1~30 \ orlO~·28 \ I~. 3! \ \ 4 \ ! ." \ \ 4 6 2 , 6 N02 0+K20 N020..K2 °

Figure 4A - Al 20J VS. alkalis díagram for mafic-ultramafíc rocks Figure 48 - A /20 J vs. alkalis diagram .fil,. majíc-ultramafic rocks oJ (Ir ,he Crixás, Guarinos, Pilar de Goiás-Hldrolina ~l'eenstol1(' belts, .Goíàs (Goiás /own) greenstone helt, with alkali (A), "nigh.atumtna with alkali (A), high alumina (8) and tholeíitic (C) basal! [ields tafter (B) and tholeiitic (C) basal! fields (after Kuno. 1966) Kuno, 1966)

FeO' FeO· ." . ~(, . ." 28. .1-0 20 • 20 To ". ." 08. 1fI 15 ." 06. •• 28• •0' .0 • • _29/32 15 15 ." •• 2\ fl. • • O.. .,.24 •• • o,· 10 e.. .:.. • ••• . . .. ro ." .... -II Tholeiites High-Mg PyroKenites •••• -. Tholeiites High-Mg Pyroxenires 8050lts • Peridotites • Bosalts Periõctites 5 cnd ounltes 5 ond Dunites • with CUmulo~ witn Cumulo- tes • • t.. •• 5 10 15 20 25 ligO 5 10 15 20 25 'i.go Figure 5A - FeO* vs. MgO diagram for mafic-ultramafic rocks of Figure 58 - FeO'" vs. MgO diagram for majic-uítrumajic rocks (~t Crixás, Guarínos: Pilar de Gaiás-Hidrolína greenstone belts, with Goiás (Goiás town) greenstone belt ; 11'1111 approxímate jields of approximate jie/ds of thoieiites, high-Mg basalts, pyroxenites and tholeíites, high-Mg basalts, pyraxenites, anel perídotites + dunites perídotítes + dunites (modified from Gtikson. /972) (modified from Gtikson, 1972)

ln lhe AFM diagram the mafic-ultrarnafic rocks of the CONClUSIONS Mafic and uitramafic rocks form an Crixás, Guarinos, Pilar de Goiás-Hidrolina belts plot mainly important component af the Archean greenstane belts af on the FeO*·MgO side, with an enrichment in iron typical Crixás, Guarinos, Pilar de Goiás-Hidrolina, and Goiás of lhe tholeiitic series, and a few samples deviate towards (Goiás town ), Primary textures and structures are typical lhe Na 20 + K20 comer, suggesting a possible calc-alkaline of volcanic and subordinately subvolcanic environrnents, trend (Fig. IOA). The corresponding rocks of the Goiás and spinifex and other skeletal textures provide evidence of belt also follow lhe FeO*·MgO side of the diagrarn with a probable komatiitic compasition. Greenschisl facies a possible inflection of lhe curve 'in lhe FeO* comer (Fig. metamorphism and alteration of lhe primary mineralogy af 10.8). Hawkesworth and O'Nions (1977, in Condie, 1981) mainly olivine, clinopyroxene, plagioclase (in mafic rocks) plotted the tholeiitic, komatiitic and calc-alkaline series of and opaques have produced assemblages of tremolite­ the Bulawayan volcanics of Zimbabwe (Rhodesia) and -actinolite, serpentino. chlorite, tale, epidote, carbonate observed that the komatiitic and tholeiitic series are indis­ and opaques. tinguishable on the AFM diagram, and that lhe rocks The major-element chemical composition of many of present a typically calc-alkaline trend. these rocks is compatible with that of komatiites from other 344 RevistaBrasileira de Geocíências, Volume 12 (1·3), 1982

sio, .8 •• " 510. 54 I •• ..,.1 (J~~ .0 'I 60 JI ." I

,. 5. / I 01. ~ I ea ". I " .J' I ~~ ." 4. 4. .~j ij%"..20 oe. 32\5"11I 44 44 ." ." ., 40 40 10'.09" ." ,. se ."

52 sa '" 26 26 ~,." .ao 24 ." 24

2 , 4 s 2 , 4 s 6 7 e feO"/MgO feO'/MgO

Figure 6A - SiO 2 vs. FeO*iMKD diagrumfor mufic-uhramajic rodes Figure 68 - 5i02 vs. reO· IMgO diagram for mafic-uítramafic rocks ojCrixás, Guarinos, Pilar de Goiás-Hldrolina green.wme bclts, wíth of Goiás (Goiás /own) greenstone belt ; wíth divislon between lho­ division between tholeíitíc (TH) and calc-alkaline (CA) series (afia tettüc (TH) mui calc-alkaline (C A) seríes tafter Miyashiro, /975) Miyashiro, 1975)

055 Ti02

050

04' »

040 ."

0>5 ." 10••12 ". 0'0

025 ." ...."." 020 ." O" ." ~"ro, .'" o to .27 .19 ". ~/46..' '" ~ ,,~ 010 ": ~6 00' /07 20/2! ...se .,. ? , s 6 7 4 e feO'/MgO

Figure lA - 110 2 vs. FeO""/MgO diagram for mafic-ultramafic Figure 78 - n02 vs. FeO*jMgO diagram for mafic-ultramafic rock:sofCrtxás, Guarínos, Pilar de Goiás-Hidroltna greenstone belts rocks of Goiás (Goiás town) greenstone belt Revista Brasileira de Geoci€ncias, Volume 12 0·3), 1982 345

22 ."

•O 20 .f 0"

18 0'9

16

14

12

10

8

6

4

2

5 FeO'/MgO Figure 8A - FeO* vs. FeO*/MgO díagram for mafic-ultramafic rocks of Crixás, Guortnos, Pilar de Goiás-Hídrotína greenstone belts, with dívision between thofeiitic (TH) and calc-alkaline (CA) series tafter Miyashiro, /975) ."

22 07.~•• .a;28 li' ." O 20 ." ." u, " 16.08 ." 18 151"6 ''0 R' 16 &"" '90." ." 14 3l\~4." 12 ". ·01

10 .22

8 .11

6

4

2

2 3 4 5 6 7 FeO'/M,O Figure 88 --FeO· vs. FeO*/MgO diagram for mafic-ultramafíc rocks of Golás (Goiás town) greenstone beít, wíth dívisíon between thoíeíític (71f) and catc-alkaltne (CAl seríes tafter Miyashiro, 1975) 346 Revisto Brasileira de Geociêncías, Volume 12 (1~3). 1982

M90 MlIO 100 100

40 .. • • • 60 40 .,. 60 ". ,~ ee• ",' • ." 60 40 60 40 • " • • '. i' 80 20 80 " 20 .'" " • • 00 .::.~ 40 60 80 100 100 20 40 60 eo 100 CoO AI.O, CoO AIoO,

Figure 9A - MgO~CaO~AI203 diagram for mafic-ultramafic rocks Figure 98 - MgO~CaO-AI203 ãiagram for mafic-ultramafic rocks of Crixás, Guarinos.• Pilar de Goíás-Hidrolina greenstone belts of Goiás (Goiás town) greenstone belt

FeO· 100

20 eo

40 60 40 •" • .'. 60 • 60

• 80 • o 80 ." 20 ,,' O' oa 100 20 40 60 eo 100 40 60 80 100 No.O+K.O MlIO MlIO

Figure IOA - AFM dtagram for mafic-ultrumajlc rocks o] Crixás, Figure lOB - AFM diagram for mafic-ultramafic rocks ofthe Goiás Guarinos, Pilar de 'Gotás-Hídroítna 'greenstone belts (Goiás town) greenstone belt

greenstone belts of the world, unless the high AI,O, con­ difficult to evaluate. Komatiitic and tholeiitic tendencies are tents and generally low CaO/AI20, ratios. Average CaO/ commonin both areas, together with occasional calc-alka­ AI,O, ratio is lower and average K,O content higher in the line trends, but considerably more systematic and detailed Goiás belt than in the Crixás, Guarinos, Pilar de Goiás­ work is necessary to characterize adequately the petrology -Hidrolina belts, although the possible effects of hydro­ and chemistry of these rocks. thermal, metamorphic or weathering modification are Revista Brasileira de Geocíênctas, Volume 12 (1·3), 1982 347

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