Correia, D.R.

Corrêa-Gomes, LC.

Souza, M.N.

English

Cadernos Espinosanos (E-Journal)

BoLlG-USP, Publ.Esp., 10:9-15, 1991 CHILLEO ANO FROZEN MARGINS - EVIOENCE OF THERMAL CONTRAST IN MAFIC OYKES ANO THEIR SIGNIFICANCE FOR MAGMATIC CONSOLlOATION INTRODucnON L.C.Corrêa-Gomes 1 M.N.Souza2 D.R. Correia2 When a magma rislng through the Ilthosphere contacts "coId" wallrocks a thermal contrast occurs at the interface, which deminishes slowly inwards trom the contact. Ideally, a sequence of zonas parallel to the contact would be createcl having the following fabrics trom the margin to the centre of the condult: a) HoIohyaline (>90% glass) , b) Hyalocrystalline (90% to 10% glass), and c) HoIocrystalline « 1 0% glass). Grain size in the central zones depends obviously on the time necessary for total magma consolidation. AJthough rheologlcal expectation would saem to indlcate that the larger the thermal contrast and magma volume In the system, the thlcker the chillecl mar gins should be (JAEGER, 1967), observatlons on maflc dykas trom southem France (FABRE et ai., 1989) have provecl the contrary. Contlnuous magma movement wlthin the · condult, until total immobilization, is responslble for margin reheatlng and recluction by devltrification, "softening" and erosion due to magmatlc flux. Thus, the observation of maflc dyke contacts can provide important information on the thermal evolutlon of magmatic cooling, aspecially when dykes of different thicknesses trom the same area are compareci. A10ng the coast of the clty of Salvador, Bahia, several well-preserved dykes lUniversidade Federal da Bahia, Rua Caetano Moura, 123,40210 Salvador, BA, Brazil- Universidade Cat6lica, 40210 Salvador, BA, Brazil. 2Universidade Federal da Bahia, Rua Caetano Moura, 123,40210 Salvador, BA, Brazil. unaffected by metamorphism crop out wlth the above characterlstics and are, therefore, sultable for such analysis. LOCATlON ANO METHOOOLOGY The mafic dyke system under study out in the Rio Vermelho district of Salvador (Fig. 1) Is represented by a dyke having a complex blfurcate geometry (Flg. 2) In which the principal branch Is 26 m thlck, and the lateral ramification reaches 1.65 m in thlckness. Small maflc veins (8 cm thick) complete the local scenario. ---I...L.. ______ ....J '--"--______ ....J Agure 1 - Location map. AV = Alo Vermelho district. /L \~ .. .. , __ . •. . ~ ~ • ,.. 1 ." . . . • ~ //~ ' . " . '. ." 0 ' ° • . . = maficdyke ..... ,.. • /' "lo'" ..... .,,; / .. /' ./"" ' / / // / ::::- / " " . ... .. oountry rocks ,.. A It. ./ ~ -0-: fLDW Agure 2 - Schematic aspect of mafic dyke from beach at Hotel Meridien. 1)Principal body, 2) lateral branch, 3) veinleta. 10 The basement consists of Transamazonlan (2.0 Ga; CORDANI, 1973) mlgmatized granulltes that exhlblt banded to locaJly massive structures. These rocks are thought to belong to the Atlantlc granullte beIt (INDA & BARBOSA, 1978). The dykes are composed of labradorlte, pigeonlte andjor auglte, opaques, and rare oIlv1ne (TANNER de OUVEIRA & CONCEiÇÃO, 1982). In outcrop dykes exhiblt flne-grained margins of varlable thlckness, sometimes Irregular. These quenched margins paraJleI the country rock contact and are dlstingulshed by thelr fabrlc varlation, coIour, luster, roughness, and fracture (conchoidal, planar). OEVELOPMENT OF CHILLEO ANO FROZEN OYKE CONTACTS experimentai work on soIidiflcatlon of basaltlc magmas has indicated oooIlng rates on the order of up to 1000°Cjh for glass formatlon (LOEFGREN et ai., 1974; SCHIFFMANN & LOEFGREN, 1982). Spherulltes and microlltes form at cooIlng rates of about aoo oCjh (FABRE et aI., 1989). These are the most adequate therrnal condltlons for formation of chllled margins. However, If reaction phenomena related to metamorphism or internai exothermic reactlons durlng magmatic solidiflcation take placa, the temperature may rlse and, eventually lead to partlal recrystallization (BEAL & RITTLER, 1976), andjor "sottening" and retusion (LEBEAU & GIROD, 1987). Thus, when dykes are not affected by "metamorphism", the longer the duration of internai magmatlc flux, the thlnner will be the chilled margins. In Figure 3 the percentage of glass is plotted for dykes of dlffering thicknesses IOOF:::::~"<:::::---80 -..80 -.. -c c;4Q ~ 20 5 Figure 3 - Graph of glaaa percentage in dykes of dlfferent thickn .... s (100, 70, 50, 40 and 2 em) versus distance to borders (mm) (FABRE et ai., 1989). 11 agalnst dlstance from the country rock contact (FABRE 8t ai., 1989). Clearly defined are plateaus near the values of 90%, 30% and, 10% glass. These percentages may be related to those utilized to characterlze holohyallne, hyalocrystalline and holocrystalllne fabrics. The visual dlstinctlon of a glassy border Is falrly easy, but recognition of the other two glass leveis (I.e. 30% and 10%), is not, and therefore these have been consldered here as a single zone. Nevertheless, these leveis have received speclflc denominatlons, because they do reflect different coollng rates here Indicated as trozen margins (FM) and, more Internally, chUled margins (CM) dlstlnct trom the outer glassy border. The measurements summarized In Table 1 show some remarkable differences: - In the principal dyke (26 m thlck), the absence of FMs is evident, but CMs are thlck, reachlng 150mm; the latter contaln rare phenocrystals (xenocrystals) of plagioclase, 2.0 mm long, whlch are also found up to about 1 m trom the border. These crystals have a small tabular shape, common to feldspars formed by reaction between crystals and IIquid induced by intramagmatic exothermlc reactlons that ultirnately could result in the destructlon of FMs. As the temperature decreases, CMs could grow wlthout impediment. Inwardly, crystal size evolves to medium to coarse phaneritlc texture. - In the lateral branch (1.50 m to 1.65 m thick), the narrow parts show thicker CMs and FMs. Here also the typical irregular FM shapes are important as they evldence partial destruction by reheatlng and erosion in a continual-flow regime. -In the 8-cm-thick veins, the CM and FM thicknesses are proportlonally thlcker than In the dyke and lateral branch (see Table 1). In addltion to these features, other evidence regarding the differentiated latent caJorlfic potentlal of the dyke can be derived from the shapes of contacts: linear and well defined in the thlnner bodies, to Irregular and confused in the main body, the latter representing the largest therrnal echange with the country rocks~ A schematic summary of the principal aspects referred to above is shown in Figure 4. CONCLUSIONS Some important conclusions can be drawn from the FMs and CMs in the studied examples: 1) The choice and identification of margins as being of the FM and CM types are 12 Table 1 - Thickness of chilled (eM) and frozen (FM) margins in the dykes. THICKNESS DYKE CM FM (em) (mm) (mm) PRINCIPAL BODY* 2600 150 165 20 5 260 42 6 LATERAL 150 60 11 BRANCH 145 45 10 145 33 7 VEINLETS** 4 5 15 "average of 8 measurements; **average of 6 measurements. ~f~' 1 ' Ftt v v r v vi l~~ eM v v ICvM.., j I 1 " v v v v v v / " ... \I v Y li / /) ~ I 1 (/ /, I- v " v v Pt.~ IgC~ / / / / 1 \ v v v / I 1 " V' v V v v • Iv 1 ~ 8(."" ,.A" I v I \ ',.rI/V v V v . I I v v v v v ( / / 11 I 1/ /\ \ v v v '" V v v , I , 1/ /} v/ v- I ~ /(, \" v v v v I \ / v / -/ V' Y I". / /. --;, / 1 I v v I I I , v v 1\ / I v \ / /\ y I v v V I \ v " 1,/ I C- / /1 1 v I v I I v '/ \ \ ., v v I ,. / 1 \ v v v " v I , / 1 ... \ / / 1 I V v v 1 \ \ /. / I ~ , 150~ , \ ~ .., Agure 4 - Summary of principal aspects associated with mafic dykes. eM = chilled margins, FM = trozen margins. Plagioclase phenocrysts are indicated in the circle. 13 necessary for a better analysis of the thermal behaviour of dykes. The option of chilled margins alone is not enough to show complete thermal evolution. 2) The absence of a FM in the principal dyke (26 m), combined with the thickness of its CMs, indicates greater resistence of CMs to thermal transformation than FMs. 3) Phenocrysts with rounded shape, irregular contacts, the absence of FM, and the lack of evidence of metamorphism in the dykes allow the conclusion that the source of heat was InternaI. 4) The longer the cooling time of a magma the greater the destruction of the FM and the greater the possibility of CM formation. 5) The formationjdestruction stages of FMs can be observed by comparing their regular form in the veins with their irregular shape in the lateral branch and their absence in the principal body. REFERENCES BEAL, G.H. & RITTLER, H.L (1976) Basalt glass ceramics. American Ceramic Society Bulletin, 55:579-583. CORDANI, U.G. (1973) Evoluç o geológica precambriana da faixa costeira do Brasil entre Salvador e Vitória. S o Paulo, 98p. (Tese de Livre Docência-Instituto de GeociênciasjUSP). FABRE, P.; KAST, Y.; GIROD, M. (1989) Estimation of tlow duration of basaltic magma in fissures. Journal Volcanology and Geothermal Research, 37:167-186. INDA, H.A. & BARBOSA, J.F. (1978) Texto explicativo para o mapa geológico do Estado da Bahia. Salvador, SMEjCPM. Escala 1 :1.000.000. JAEGER, J.C. (1967) Cooling and solidification of igneous rocks. In: HESS, H.H. & POLDERVAART, A. (1967) Sasahs: the poldervant treatise on rocks of basaltic composition. New York, Interscience Publlshers. V.2, p.503-536. LEBEAU, M.J. & GIROD, M. (1987) Incorporation of simulated nuclear ashes in basalts: an 14 experimentai investigation. American Cera mie Society Bulletin, 66(11):1640-1646. LOFGREN, G.E.; DONALDSON, C.H.; WIWAMS, R.J.; MUWNS,O.J.; USSELMAN, T.M. (1974) An experlmentaJly reproduced textures and mineral chemistry of ApoIlo 15 quartz normative basalts. Proc. 5th. Lunar Conf.Geochem.Cosmochem. Acta, 1 :549-567. SCHIFFMAN, P. & LOFGREN, G.E. (1982) Dynamic crystallization studies on the Grande Ronde piliow basaJts, Central Washington. Journal of Geology, 80:49-78. TANNER DE OUVEIRA, MAF. & CONCEiÇÃO, H. (1982) Metamorfismo de alto grau da faixa costeira de Salvador, BA In: CONGRESSO BRASILEIRO DE GEOLOGIA, 32. , Salvador, 1982. Anais. Salvador. SBG. V.2, p.603-613. 15 

Chilled and frozen margins: evidence of thermal contrast in mafic dykes and their significance for magmatic consolidation

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Chilled and frozen margins: evidence of thermal contrast in mafic dykes and their significance for magmatic consolidation

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