Revista Brasileira de Geociências 17(4):512-518, dezembro de 1987

METASOMATIC EVOLUTION OFGRANITES, NORTHEAST , BRAZIL

ANTÔNIO CARLOS PEDROSA SOARES*, R UI Luís BAPTISTA PEREIRA MONTEIRO*, JOSÉ MARQUES CORREIA-NEVES*, OTHON HENRY LEONARDOS** and KAZUO FUZIKAWA***

ABSTRACT Geological mapp ing, petrographic, geochemical (major, trace elements and REE), and fluid inclusio n studies were carried out on some intrusive granites outcropping in lhe Coronel Murta-R ubelita regio n (Middle Valley ), norlheast Minas Gerais State , Braz il. The granites were forcefull emp laced in quartz-rnica schists and quartzites that belong to lhe Salinas Group (undifferentiated Pro terozoic). Salinas Group underwent a Barrowian type metamorphism at about 650 Ma in lhe domain of lhe Araçua í Foldbelt. The granitic plutons were characterized as post-tectonic intrusions (525 2: 30 Ma) that clearly disturbed the large open folds of lhe last regional deformation phase. The main granitic types recogn ized are: biotite granite, biotite (-muscovite) granite, muscovite (-biotite) granite, pegma toid granite and rnuscovite-tourmaline or albite-tourmaline granite, Gran ites s.s. are largely dominant over granodiorites. The granites are alkaline, K-rich, and metal uminous (biotite granite) to peralum inous (other types). Biotite granite appears to be more representative of lhe original anatetic Iiquid while the other types, excepted lhe pegmatoid granite, are metasomatic facies wilhin each intrusion. Sili cated residues of lhe granitic melt were crystaIlized along lhe cupolas of some plutons, yielding a thin cap of pegmatoid granite, An evolution model for the granites is outlined. The sequence of lhe granitic facies wilhin the plutons starting from their bottom and inner parts (cores) to their rims and cupolas is exempl ified in the sucession of biotite granite, biotite (-muscovite) gran ite, muscovite (-biotite) granite, muscovite granitoids more or less albitized and/or tourmalinized and pegrnatoid granite. This sequence also defines the palh of the metasomatic fluids.

RESUMO Sob re os granitos intrusivos da região de Coronel Murta- (Médio Vale do Jequitinhon ha, NE de Minas Gerais ) foram realizados mapeamento geológico e estudos petrogrâficos, geoquímicos (elementos maiores, traços e terras-raras) e de inclusões fluidas . Os granitos constituem intrusões forçadas em quartzo-mica xistos e quartzitos do Grupo Salinas (Proterozõico indiferenciado). O. Grupo Sali nas sofreu metamorfismo de tipo barrowiano há cerca de 650 Ma, n!?- domínio da Faixa Araçuaf. Os plútons graníticos foram caracterizados como intrusões pós tectônicas (525 _ 30 Ma) que deformaram as macrodobras abertas da última fase deformativa regional. Os principais tipos graníticos identificados são: biotita granito, biotita (muscovita) granito, muscov ita (-biotita) granito, granito pegmatóide e rnuscovita-turmalina ou albita-turrnalina granito. Gran itos s.s. são largamente dominantes sobre os granodio ritos. Os gran itos são alcalinos, ricos em K e metaluminosos (biotita granitos) a peraluminosos (demais tipos). O biotita gran ito parece ser mais representativo da fusão anatética original, ao passo que os demai s tipos, exceto o granito pegrnatéide, são fácies metassomáticas em cada intrusão. Os granitos pegmat õides resultaram da cristalização dos resíd uos da fusão granítica nas cúpulas dos plútons. Um modelo evolutivo para os gran itos é apresentado. Nele, a seqüê ncia das fácies graníticas, das porções mais interiores às zonas externas e cúpulas dos plútons, é exernp lific ada pela sucessão biotita gran ito, biotita (-muscovita) gran ito, muscovita (- biotita) granito, granitõides muscovíticos mais ou menos albitizados eJou turmalinizados e granito pegmatóide. Esta seqüência de fácies também indica o caminho seguido pelos fluidos metassomáticos.

IN T RODUCTlON Large expositions of granitic rocks named as "proterozoic unit generated in or reworked plutonites are found in the Middle Jequitinhonha Valley during Brazilian Cycle" by Schobbenhaus et al. '(1981, (Fontes et al. 1978, Schobbenhaus et al. 1981, 1984), 1984). northeast Minas Gerai s State. The Salinas Group may be locally (Fig.l) subdivided into a This pape r presents a model for the evolution of lhe schistose unit (Psx) and a quartzitic unit (Psq). intrusive granites of the Coronel Murta-Rubelita region (Fig. Quar tz-rnica schists are the dominant rocks in the Psx unit 1). and are intercalated with orthoconglomerate, calc-silicated Geological mapping was carried out in the scale 1:40,000 rocks and carbonatic schists . Calc-silicated rocks are often (Correia-Neves et al, 1983, Pedrosa Soares 1984 and mineralized in scheelite closer to granitic cupolas Mont eiro 1986). Petrographic and lithochemical (major and (Pedrosa-Soares et a!. 1983, Monteiro 1986). The Psq unit trace elements) data of the granites and their host rocks are comprises pure quartzite and rnicaceous, feldspathic or found in Correia- Neves et al. (198 2), Pedrosa-Soares (1984) car bona tic quartzite. and Monteiro (1986). Rare earth element (REE) con tents and From Rb/Sr whole rock isochro ns, Sá (1977) and Siga Jr. their distribution in the Coronel Murta granites were (1986) dated the regional metamorphism of the Salinas Group presented by Dutra et al. (1986) and preliminary fluid as 650 Ma. This age corresponds to the Brasiliano tectonic inclusion studies by Monteiro (1986). event (Almeida & Hasui 1984). At the end of the Brasiliano Cycle a great quantity of granitic and pegmatitic material was GEOLOG ICAL SETT I NG Precambrian metamorphic generated either in the Araçuaí Foldbelt or in the Atlantic rock s in the area belong to a supra -crustal schistose sequence Belt (Leonardos & Fyfe 1974, Litwinski 1985, Correia-Neves of the Araçuaí Foldbelt , the Salinas Group (Cobra 1970, et al. 1986, Siga Jr. 1986). Pedrosa-S oare s et al. 1984). The Araçuaí Foldbelt shows The regional distribution of almandine, sta urolite, kyanite, vergence and thrust transport from east to west , against the and sillimanite in schists of the Salinas Group points to a São Franci sco Craton (Almeida 1977). The stud ied grani tes Barrowian type metamorphic zoning (Pedrosa-Soares et al. occu r in the inner tectonic zone of that foldbelt where the 1984). Salinas Gr oup comprises the major part of a sequence of A strong regional schistosity (Sn) imprinted on the Salinas

Departamento de Geo logia . CPMTC . Universidade Federal de Minas Gera is. Caixa Postal 2608. CEP 30161. , MG, Brasil ** Departamento de Geoc iências, Universi dade de Brasília , Campus Univesitário. Asa Norte , Caixa Postal 15, CEP 70910, Brasflia , DF, Brasil *** Nuclebrás. Av .Uruguai, 53 1, CEP 30130, Belo Hor izonte, MG, Brasil Revista Brasileira de Geociências. Volume 17, 1987 513

TERTIARY I -\ / " I , I ' / ~

" NG SÃO DOMINGOS FORMATION ( SI LTIC - PSAMMI TIC- ARGI LLACEOUS SEOIMEN T S)

~" 42" t LATE PROTEROZOIC I ECRLEClZOIC 1---=-+7-,,-+-- -0· ~ ~ I"/'ip",I BIOTlTE GRANITE MUSCOVITEI-BIOTITEI PEGMATOID

GRANITE ANO BIOTITE

1-t.lJSCO'IfTE) GRANfTE

UNDIFFERENTlATED PROTEROZOIC ~L1NAS GROUP ~ I:-hq:-] QUARTZ ' MICASQiISTS. QUARTZITES

META' OÀTHOCOI'G..OMERATE.

CALe · SlLlCA TEO ROCKS

FAULT

~ ROAOS RIVER

LOCAlITY Pu

o 2.5 5 7.5 10 Km ! , • I I

Figure 1 - Geological map oi the Coronel Murta-Rubelita area, Middle Jequitinhonha Valley, northeast Minas Gerais State. (Simplified from Pedrosa-Soares 1984 and Monteiro 1986) rocks is parallel to a compositional banding that generally Cascalheira mount pluton, figure I); trends NE to NNE and to the axial planes of the tight to • a tendency of Sn and Dn fold axes to be verticalized closer isoclinal folds of the main deformation phase (D n). The last to the intrusive plutons; regional deformation phase (D n+ 1) is marked by large open • pegmatitic and granitic apophysis leaving the granites folds and an incipient c1eavage. crosscut the intrusive contacts; The post-tectonic intrusive nature of the Coronel • presence of xenoliths and roof-pendants of the Murta-Rubelita granites is characterized by the following country-rocks; field evidences (Correia-Neves et aI. 1983, Pedrosa-Soares • pegmatoid granite contacts are marked by thin chilled 1984, Monteiro 1986): margins. • absence of deformation structures; The granites are 'also responsible for a medium pressure • where measurable, their incipient foliation is parallel to the contact metamorphism on the enclosing rocks, as granite contacts and thus crosscutting the regional tectonic corroborated by post-kinematic mineral associations and trend in many places; textures , decrease of rock fissility, and the generation of • this fol iation is an igneous flow structure which tends to fleck-structure closer to lhe contact. Mineralogical disappear towards the center of the plutons; assemblages suggest pressures between 4 to 5 kb and • the plutons tend to have circular shapes; maximum contact temperatures in the range of 600 to 7000C • the large antiforms and sinfonns of the last regional for the Coronel Murta granite aureoles (Pedrosa-Soares 1984, deformation phase (D n+ I) are c1early disturbed to Pedrosa-Soares et ai. 1984). Metasomatic contact process accommodate the forcefull intrusives (see, for example, the related to scheelite mineralization is detailed by Monteiro relation between Lagoa Nova plateau antiform and the (1986). 514 Revista Brasileira de Geociências. Volume 17, 1987

Siga Jr. (1986) dated the Coronel Murta granites as 525 ± is mineralogically similar to the biotite granite ('Y B)' However, 30 Ma (Rb/Sr whole rock isochron). The post-tectonic alkalic biotitc (- lllllscovitd granitc is Iound in a close fickl biotite and/or muscovite granites near ltinga (50 km east of Coronel Murta) gave an isochronic Rb/Sr age of 538 Ma, while KI Ar ages around 500 Ma were obtained from muscovites of pegmatites related to these granites (Sá 1977). The high Sr87/Sr86 initial ratios (0.711, Coronel Murta; and 0.713, ) are compatible with an anatetic crustal origin for such granites.

PETROGRAPHY AND FIELD RELATIONS OF THE GRAN ITES The granitic types in the Coronel Murta-Rubelita region were grouped (Fig. 1) into biotite granite ('Y B), two mica granites ( 'Y ), and pegmatoid granite ( 'Y p), Numerous pegmatites are found in the area (Pedrosa-Soares 1984, Correia Neves et al. 1986). Sixty modal contents plotted in the Streckeisen's (1973) QAP diagram indicate that granites s.s. are largely dominant over granodiorites (Fig. 2).

o

• CORREIA NEVES et st. (1982b) 1· GRANITE • PEDROSA SOARES (1984) 90 2 • GRANODIORITE • MONTEIRO (1986) 3 · QUARTZ MONZONITE 4 • QUARTZ MONZODIORITE

3 •

Figure 2 - The granitoids outcropping in the Coronel Murta-Rubelita area plotted on the IUGS' modal diagram

Biotite granite massifs ('YB) occur west and south of Coronel Murta where no muscovite rich types are found. Textures varies from equigranular to sub-porphyritic with hypidiomorphic orthoclase (triclinicity = O) fenocrists. Quartz, K-feldspars, oligoclase and biotite are the essential minerais. Zircon, apatite, epidote, sphene, and rare rutile inclusions in biotite are the accessory minerais. Sericitization and chloritization are rather rare. Two mica granite massifs ('Y) have muscovite rich facies in their outer zones. Those facies comprise muscovite (-biotite) granite ('Y M-B) and at the contact, fine grained rnuscovite-tourrnaline or albite-tourmaline granites may be also found associated with pegmatoid granite cupolas. Coarse Plote I - Photos: I . Typical aspect in thin scctio n IX 1'01.) o/ to fine grained muscovite-enriched granodiorites occur in the biotite (-muscovite) granite- 'YB-Mfo und ill the 'Y massifs. veins and rarely along 'Y granite borders. Mineral reactions Metasomatic alteration is incipient. 2. Typical aspect in thin such as biotite to muscovite and/or chlorite, microcline to section (X pol.) of the muscovite (-biotite) granite- 'YM_Bfoulld muscovite, garnet to biotite and/or chlorite and fibrolite to in 'Y- massifs. Sericitization of the feldspars is conspicuous. muscovite are common. Albitic metasomatic rims in Monteiro (1986) showed that the muscovite (M) are W plagioclase crystals are conditioned to microcline contacts. enriched. 3. Detail of muscovitization and chloritization of Biotite (-muscovite) granite (.'Y B-M) tends to prevail inwards biotite (thin section. X pol.). Note remnants ofchlorite (C) into the two mica granite massifs ('Y ). This facies (PI. I Photo 1) muscovite (M) Revista Brasileira de Geociências. Volume 17. 1987 515

A ST~ ANTÔNIO ROlE LAGOA lOJA PLATEAU CASCALHEIRA x x MOUNT 't x/ / I APOPHYS IS x, I , x, + 'te x I I I + "\. "\. I P SX I I "\. "I.. + I I I I > +

c o sw NE

PSX

x 't x ..... -,...::. Yp:', " ' ... -c, <; x \_' r:::"' ,-""""'" -.,~~ ~ : , PSx -c, -' /" <,' ,' ""- ,..- r (?) --=~ \, PEGMATITE DYKES x v x ,»

Figure 3 - Sketch profiles showing differen: field relations between granitic massifs and their host rocks. A-R and C-D are indicated infigure 1. SW-NE profil is located on the small pegmatoid massifnear the southern slope ofthe Sede ridge, 25 km north ofOuro Fino. Scales omitted

association with muscovite (-biotite) granite and it is more south of Rubelita, 20-32; metasomatic altered than the biotite granite. Some features of • pegmatoid granite massifs (1 p), 2.5 km south of Ouro metasomatic alteration described in muscovite (-biotite) Fino, 16 and 19; 2.5 km north of , 17; and 3 km granite thin sections are shown in plate 1 (Photos 2 and 3). south of Rubelita, 33. Pegmatoid granite crystal size varies considerably but Biotite granites ("IB) are metaluminous but muscovite rich medium grained texture (2.5-12 mm) appears to be dominant. facies and biotite (-muscovite) granites are scattered into At the contacts fine grained chilled margins are common. peraluminous field (Fig. 4).Tourmaline granites and Perthitic K-feldspar and quartz are the essentiaI mineraIs. The pegmatoid granite, except the chilled margin sample (33), are K- feldspar triclinicity ranges from zero (orthoclase) in the also peraIuminous rocks. Corindon is a cornrnon normative fine grained portions to high vaIues (microcline) in the mineral in "I samples (both 1 B-M and 1 M -B) while it is medium or coarse grained textures, Plagioclase, muscovite, absent in biotite granites ( 1 B)' tourmaline, garnet, biotite, and beryl are the accessory An alkalic composition is evident from Wright's (1969) mineraIs. diagram in which lithochemical data were plotted (Fig. 5). In Xenoliths, oriented or not, and roof-pendants of the the AFM diagram , biotite granites and muscovite (-biotite) country rocks are very common on the pegmatoid granite granites sucessively falI toward s the A vertex ( 1 p) and two mica granite ( 1 .) massifs but no roof-pendant (pedrosa-Soares 1984). was found on biotite granite massifs ( 1 B)' A homogeneous composition for biotite granite massifs The different field relations between granitic massifs and (1 B) is defmed in the clusters of various lithochernical their host rocks are shown in figure 3. diagrams (Figs. 4, 5 and 6, see also Pedrosa-Soares 1984). In The following Sn dip patterns recorded in the country lhe other hand, "I samples show disperse plots in those rocks around the granitic plutons (Fig. 3) yields a good diagrams. This is coherent with greater metasomatic evidence measurement of the erosion leveI of the pluton: in lhe two mica granite ( 1 ) thin sections. • around "I B massifs the SJI shows medium to high angle Silica contents averages 69.4 % iD 1 B, 70.9% in 1 B- M, centripetal dips (lower or bottom pari of the pluton); and 73.6% in 1 M -B as expected from the greater quartz • the centrifugal dips are generally rneasured near "I massifs modal percentages in the latter. (middle to upper part); Normative hypersthene, magnetite , ilrnenite, apatite, and • centrifugal low angle or vertical dips are related to anortite values are higher in both biotitic granites ( 1 B and pegmatoid cupolas (upper to top part), r B -M) than in lhe muscovitic facies (r M-B)' GEOCHEMISTRY ANO FLUIO INCLU­ Fluid contents also differ, being about two times greater in SIONS Thirty-three samples were analysed for major lhe 1 M -B than in lhe 1 B-M (Table 2). This reflects a greater and trace elements (Table 1 and 2). overaIl fluid participation in lhe muscovitic granites (1 M -B )' The general distribution of the analysed samples in figure 1 Coincidentally, the presence of larger and more abundant is presented as follows: fluid inclusions was observed in "IM-B (Monteiro 1986). • biotite granite massifs (1 B) of the Cascalheira and Frade Fluid inclusion studies were made ón three samples (1 B-M mounts, 1-6; with W < 3 ppm, 1 M-B with W = 20 ppm, and quartz vein • two mica granite massifs (1) of the Santo Antônio and carrying scheelite and wolframite mineralization from Itinga Sede ridges, 7-15 and 18, northwest of Sede Ridge and 10 km area). Small and scarse : dominantly aqueous secondary 516 Revista Brasileira de Geociências. Volume 17. 1987

inclusions were detected in .'Y B-M. Ali the 'Y M -B inclusions are seconda ry and their fluid content is composed of water, \ carbon dioxide, and possibly methane or N2. The quartz vein shows secondary inclusions in which C02 is the dominant \ fluid phase but water, CH4 and/or N2 are also present. Trapping temperatures of 6500C and 450 to 3(){)OC, \ considering pressures of 3 and 2 kb (pedrosa-Soares et ai. . 1984, Costa et ai. 1984), were respectively obtained to lhe . \' granites and quartz vein fluid inclusions (Monteiro 1986). \ / x , \ / 1 / 2 .5 I CALC­ / + " 10 I, /1 ...) I ALKALlNE l+ "./ • I ~) x, y / ". + ,/ B / / I I ~ _ / ,. ALKALINE I /PERALKALl NE 2.0 / I METALUMI NOUS • /

.. +-----c---'--~---~---~--L....,

AI Z 0 3 .. C a O ... K 2 O .... N a 2 O

A'2 0 3 + C o O- ( K 2 O + N o 2 O )

1.5 cn , , o:) Figure 5 - The Coronel Murta-Rubelita granitoids plotted on 1l.1l "'... z the Wrights (1969) diagram: + - biotite granitesfrom Coronel ~ ...... " / ':; \1 . ~ ' • Murta ('Y B); X- Mo mica granites ( 'Y M-B and 'Y B-M); Â ­ 't B :) \. + 'I', ...J pegmatoid granites ('Yp); •- muscovite-enriched / + Ix x +~

I K 2 o + N02 o t C oO anomalous field. Pegmatoid granite, as expec ted, appears as I the more differentiated granitic type in figure 6. 0 .5 REE studies were carried out by Dutra et ai. (1986) 00 0.5 1.0 1.5 2.0 biotite granite ('Y B- 4 samples), muscovite (-biotite) granite

Figure 4 - The Coronel Murta-Rubelita granitoids shown in a graphical representation of the alumina saturation: MP ­ Rb molecular proportion data; + - biotite granites from Coronel Murta ('Y B) ; X - Mo mica granites ( 'Y M-B and 'Y B-M); Â ­ pegmatoid granites ('Y r ): • . muscovite-enriched granodiorites + BIOTITE GRANITE ( l"e) x MUSCO/IT E (-BIOTI TE I GRANITE • BIQTITE ( .MUSCOVlTE) GRAN1TE

Although the decrease in the FeO, MgO, CaO, Ba and Sr o ALBm:-TOURMALlNE GRAN ITE o MUSCOIIlTE GRANOOlORlTE values from 'Y B to 'Y M-B granites might be suggestive of a , slight magmatic differentiation between the inner and outer O PEGMATOlD GRANITE o facies, lhe strong decrease of lithium from 'Y B to 'Y M-B grani tes, the distribution of lhe Ba-Rb-Sr relations in figure 6, the fact that 'Y samples are irregularly enriched or depleted in various elements in relation to 'Y B, the scattered diagranunatic representation of lithochemical relations , as well as mineral replacements early reffered, brings .metasomatism as an important process to account for lhe .. chemical evolution of granitic types. ++ + In despite of their biotitic composition, 'Y B-M Bo Sr geochemically resemble more the 'Y M -B than the 'Y B, being more altered than 'Y B but less than 'Y M-B granites. Figure 6, for example, shows a clear separation between Figure 6 - Rb-Ba-Sr relations for the Coronel Murta-Rubelita 'Y B and 'Y samples. It must be noted that lhe great majorit y granitoids. Diagram fields from El Bouseily & El Sokkary of compositions that fali in the granodiorite and quartz diorite (1975) Revista Brasileira de Geociências. Volume 17, 1987 517

( "'I M-B- 3 samples), tourmaline granit é (2 samples) and • "'I p - cupolas mainly composed of pegmatoid granite, pegmatoid granite (one sample). A strong REE depletion from locally fine grained muscovite -tourmaline and/or albite­ biotite granite to muscovite (-biotite) granite is evident from tourmaline granites occur; figure 7. Depletion ratios in this system are as follows: • ':Y - rims consisting of muscovite (-biotite ) granite REE-lO, LREE-12, and HREE-5. Depletion ratios in the ('Y M-S) and minor biotite (-muscovite) granite ('Y B-M), at system muscovite (-biotite) granite ~ tourmaline granite the contacts tourmaline granites or muscovite-enriched are REE-3, LREE-3, and HREE-1.3 (Dutra et al. 1986). granodiorites may be found; petrographic and geochemical Besides the general REE depletion it is also clear that in the metasomatic alteration evidences were recorded in all facies; outer granitic facies of the plutons the heavy REE (Gd to Lu) • "'I B - biotite granite cores; no or incipient metasomatic were enriched relatively to light REE (La to Sm). Enrichment alteration was detected. of the heavy REE in hydrothermal fluids (as COz, Cl or F) and in metasomatized granitoids has been reported by several authors (Mineyev 1963, Kovalenko et ai. 1966, Kosterin c:>-.!.->.c.:...:~~:;.;:----- 't P • PEGMATOID CUPOL A 1959, Muecke & Clarke 1981, Taylor & Fryer 1983). In the studied granites, the role of COz in the REE mobilization and ->,-__ 't . MUSCOVITEENRICHED RIM fractionation under somewhat high temperatures seems to have been important, as pointed out by fluid inclusion data.

,------,1000 ,------,<>00 BIOTlTE GRANlTES ( 'tB) MUSCOVITE I· BIOTlTE) GRANITES ( 1M•B)

~--""------l 1OO - ----+- --'ts· SIOTITE GRANITE CORE

". SAMPLE 198

r---";j"r<:------1'O \ + "\ \ Eu , \ LACe Nd Sm Gd Oy Ho Er Tm Yb Lu La Ce Nq Sm

,------,100 ,------,'00 Figure 8 - An ideal pluton as inferred from the presented TOURMALlNE GRANITES PEGMATOIDGRANITE evidences for the Coronel Murta-Rubelita area. Heavy arrows point to the path of metasomatic (magmatic + metamorphic) Ê fluids ~------1 ' O

This rnodel takes metasomatism as the main process to explain the evolution of a single pluton leading to the pattern shown in figure 8. Biotite granite and minor associated biotite .... MORE ALTERED ( SAMPLE 19 131 granodiorite ("'I B) appear to be more representative of the original anatetic liquid while the other types ( "'I), except the Eu pegmatoid granite , are metasomatic facies within each La Co Nd Sm intrusion. Indeed, the silicated residues of the granitic melt were crystallized along the cupolas of some plutons, yielding a thin Figure 7 - Chondrite-normalized REE patterns for the cap of pegrnatoid granite that was emplaced on the space 'Coronel Muna granitoids, as shown by Dutra et alo (1986) generated by the cooling pluton. The sequence of the granitic facies within the plutons starting from their bottom and inner parts (cores) to their rims and cupolas is exemplified in the sucession of biotite granite , Biotite granite don't show Eu anomaly. It appears in the biotite (-muscovite) granite , muscovite (-biotite) granite, muscovite (-biotite) granites, markedly increasing in the more muscovite granitoids more or less albitized and/or altered albite-tourmaline granite (sample 1913) and in the tourmalinized and pegmatoid granite, which defines the path pegmatoid granite (Dutra et al. 1986). of the metasomatic fluids. Sample 198 (Fig. 7) and biotite granites have equivalent REE contents. However, sample 198 is a less metasomatic Acknowledgements We are indebted to the Conselho altered muscovite (~biotite) granite in which the modal ratio Nacional de Desenvolvimento Científico e Tecnológico muscovite/biotite is 1.5 while it is 4 in the other two samples (CNPq) and to the Conselho de Pesquisa da Universidade (compare also chemical analysis, Tables 1 and 2). Federal de Minas Gerais (CPq- UFMG) for the financial supports. Our gratitude to Hilnaide M.C. Ferreira for the THE METASOMATIC EVOLUTION MODEL Based typewriting and to Frederico Rosa e Silva for the drawings. on field relations, petrographic, lithochemical, fluid inclusion, and REE data an evolution model for the granites is outlined in figure 8, as follows: 518 . Revista Brasileira de Geociências, Volume 17, 1987

REFERENCES

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Talvez o maior de todos os estímulos da análise estruturaI é a oportunidade de viajar de uma visão de detalhe para outra à distância e retomar, para ganhar novas inspirações para um trabalho criativo.

George H. Davis, 1984. Structural Geology of'rocks and regions,