Glaucophane Schist

Canadian Mineralogist Vol. 20, pp. 4l-47 (1982)

GLAUCOPHANESCHIST IN THEANDES AT JAMBAL6,COLOMBIA

TOMAS FEININGER DApartementde Giologie, Universitd Laval, Qudbec,euibec GIK 7p4

ABSTRACT du Cr6tac6 inf6rieur, situ6esA 800 km de distance dans le Sud-Ouest de l'Equateur. Dans ce cas, Glaucophane schist at Jambal6 (2o46.9rN: cette indiquerait une importante p6riode 76"19.6,W) ceinture in the Colombian Andes includes the de subduction s'est termin6e soudainementau Cr6- assemblages albite-quartz-paragonite-glaucophane_ garnet-chlorite-calcite tac6 inf6rieur. D'autres affleurements de telles ro- and quartz-albite-paragon- ches constitueraientalors une ceinture imm6diate- ite-muscovite*glaucophane--epidote-chlorite, Meta- ment i I'est de la faille Romeral. Cette faille aurait morphism glaucophane of the schist at Jambal6 is form6 la frontidre active entre la plaque conti- interpreted to have taken place at 350--400oC nentale Sud-Am6ricaine et une plaque oc6anique under - P.61 5-7 kbar, with a mixed-volatile fluid vers le Nord-Ouest,du moins au d6but du Cl6tace. phase (Pr. = P.or) of locally variable H4O/CO3 ratios. This interpretation is based on phase petrol- (Iraduit par la R6daction) ogy supported by microprobe analyses and a glaucophane, comparison with analogous schists in the Mots-cMs.schistes bleus, Andes, Co- lowesr- lombie. grade part of the epidote zone from the Ou6goa district, New Caledonia. High-pressure rocks ar Jambal6 constitute a new occurrence on the dis- INrnopucuoN continuous circum-Pacific belt. They may have an origin similar to that proposed for eciogite and Metamorphic rocks of high-pressure baric associated rocks, also of Early Cretaceous ag€. type, including glaucophane schist, have been 800 km away in southwestern Ecuador. If so. it mapped by Colombian geologistsnear the town would suggest that (1) a major period of subduc- of Jambal6 (2"46.9N, 76"19.6\l) in the An- tion terminated abruptly in Early Cretaceous time; dean Central Cordillera (Orrego et al. l980a). (2) other outcrops of high-pressure rocks may be In the present report the phase petrology of just expected in a belt east of the Romeral fault: the glaucophane schist at Jambal6 is described and (3) the Romeral fault was the active boundarv briefly, and comment is made on the regional between the continental South plate American and significance of the occurrence. an oceanic plate to the northwest, at least duriug the beginning of the Cretaceous period. RrcloNar SnrrrNc Keywords: blueschist, glaucophane, Andes. Colom- bia. The bulk of the Andean Central Cordillera of Colombia is underlain by diverse low-pressure SoMlrerne metamorphic rocks of Paleozoic age, cut by granitic plutons chiefly of Mesozoic age (IN- ks schistes i glaucophane de Jambal6, dans les GEOMINAS 1976\. Continental volcanic de- Andes colombiennes (2o46.9rN, 76019.610) con- tiennent les assemblages albite--quartz-paragonite_ posits of Tertiary and Quaternary age blanket glaucophane-grenat-*hlorite-calcite et quartz_albi parts of the Cordillera in the south. A major te-para gonite-muscovite-glaucoph ano-epidote-chlo- inactive transform fault system, called the rite. A la lumidre des relations entre les phases, Romeral fault, follows the Cenral Cordillera des donn6es analytiques (microsonde) et d,une com- near its western base throughout Colombia. paraison avec des schistes analogues dans la por- The high-pressure metamorphic rocks at Jam- tion la plus faiblement m6tamorphos6e de la zone bal6 lie between the Romeral fault to the north- i 6pidote du district Ou6goa (Nouvelle-Cal6donie), v/est and basement gneissesand schists of the lc .mdtamorphisme des schistes glaucophane i de Central Cordillera to the southeast (Fig. 1). Jambal6 impliquerait les conditions suivantes: T The rocks at Jambal6 have been divided entre 350 et 400oC, P566 €Dtre 5 et into 7 kbar avec (l) pr6sence d'une phase fluide mixte (pn = p-.,") three units: Jambal6 Glaucophane Schist, ru" (2) de rapport H*O/CO2 variable localement. Ces ro- La Mina Greensshistand (3) San Antonio ches de haute pression constituent une nouvelle Amphibolite (Orrego et al. l980a). Although occurrence dans la ceinture discontinue circumpa- the Jambal6 Glaucophane Schist belongs to a cifique. kur origine pourrait 6tre analogue i cele blueschist facies, there is no evidence that the des 6clogites et des roches associ6es, ditant aussi two other units belong to or ever were part of 4l Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/20/1/41/4005663/41_20_1_cm.pdf by guest on 30 September 2021 42 THE CANADIAN MINERAI-OGIST

a high-pressurefacies series.Indeed, each of the three units constitutes a discrete block, isolated from the other units and from country rocks by faults tlat are in part filled by sheared ser- pentinite. Neither structural nor rnineralogical evidence (Orrego et al, l980a) supports a gen-

TABLEI. MODESOF TWO,GLAUCOPHANE SCHISTSFROM JAMBALO, COLOMBIA Sample J-l J-Z Quartz (r Al bi te )rs.z ) ):s.s tzl Paragonite 16.8 18.0 Muscovite 0.0 6.0 (3) - Chlorite 5.0 5.7 8oJ G1aucophane 43.3 28.2 %f /^-*-, Garnet 1.0 0.0 '\i:E--./' Epidote 0.0 2.1 Sphene 6.7 2.9 Apatite 0.2 0.3 Calcite 10.B 0.0 Pyrite 0.4 1.3 TOTAL 99.9 100.0 ,/b q Density(5) 2.97 2.92 A) r" 'l q' Notes: ) Al bi te > quartzt 2) quartz > albite; 3) Paragonite 0lo?o /muscoviteratio estimatedbY relative intensities of basa'l Scob (lnl reflections measuredon a djf- fractogram of a mica concen- Fro. l. Hieh-pressure metamorphic rocks at JaB- trate; 4) spacedon a^ 0.6 x bal6, Colombia, and the Raspas Formation, 0.6 rm grid; 5) gm.cm-r. Ecuador.

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The glaucophang: schist is a thoroughly re- Quaftz and. albite crystallized, fine- to medium-grained, grey to blue-grey rock. Dark slender prisms of glauco- Both minerals occlu as clear, nonstrained, polygonal (Anr.o phane, commonly less than 5 mm long, are, in anhedra. Albite to Anlg de- places, oriented and impart a strong lineation termined by microprobe analysis) is mostly to the schist. Nowhere have vestiges of pre- untwinned. metamorphic textures been preserved. In some samples, metamorphic segregation has imparted l{hite mica a pronounced lamination of felsic glauco- ver.rus (d.* phane-rich layers. Paragonite 3.216 A, roO o.7OVo) is the only mica present in J-1. paragonite Two samples of glaucophane schist were b I-2, (dw 3.21O A, IGO O.35Vo coexists with phen- selected for detailed study (Table I Specimen ) ). gitic muscovite (dw 3.325 A). The two micas J-l is a fine grained, dark blue-grey schist with the mineral assemblageglaucophane-paragonite- occur in subhedral, sligbtly bent flakes that are optically indistinguisha,ble from one another, albiteauartz-calcite-chlorite-garnet. Specimen J-2 is a fine- to medium-grained, laminated Glaucophane and lineated schist with the mineral assemblage glaucophane-paragonit*quartz-albite- Strongly pleochroic (X very pale yellow, Y muscovite-chlorite-epidote. Accessory sphene, medium lavender, Z medium blue; X 1 Y = Z) apatite and pyrite are common to both samples. sodic amphibole, chiefly pressnt in subhedral All phases are in mutual contact and have to euhedral prisms, is the most prominent smooth boundaries. These observations, soupled mineral. The amphibole is glaucophane (Table with the nearly equigranular texture of the rocks, 2), because charge-balance calculations indicate indicate that mineralogical equilibrium was at- a low content of ferric iron; the relatively high tained during metamorphism, at least on the Al"r content in each formula unit suggestsFe8+,/ scale of a thin section. (Fe'* + Al"r) ratios below those found in crossite. Corresponrlingly, Mgl(Mg * Fe*) ratios are artificially low, because all iron was calculated as Fes+. The glaucophanes are zoned, TABLE2. ELECTR0N-I4ICRoPR0BEAI{ALYSES 0F GLAUCOPHIINESwith rims consisteutly sharacterized by enrich- FROIiISCHIST AT .'AfiBALO,COLOI.IBIA ment in AtO" and highep Mgl(Mg + Fe*) il-I (core) ,t-l (rim) ,I-2 (core) it-2 (rtm) ratios relative to cores. This zoning is visible si02 55.94 55.88 55.45 56.56 optically because of the relatively stronger ab- Ti02 0.08 0.03 0.l0 0.00 sorption of amphibole cores. The cores of 41203 10.07 11.90 9.45 10.56 glaucophane in J-1 may be ferroglaucophane Fe0* I 7.88 15.35 16.71 '14.04 (Table 2). Mn0 o.z5 0.19 0.19 0.15 Mgo 6.88 6.83 8.54 8.85 Garnet Ca0 0.78 0.38 0.70 0.24 Subhedral to euhedral small porphyroblasts Naro 7.05 7,48 7.37 7,45 of almandine garnet, averaging O.75 mm in 0.01 0.01 90 0.04 0.02 diameter, constitute lVo of. sample J-1. The ToTAL98.94 98.05 98.s5 97.87 porphyroblasts are uniformly speckled with in- Nunberof lons on the baslsot clusions (chiefly qrnrtz, glaucophane, calcite and sphene) that in places defrne planes that lie 8..0000 Atlv 0.10 at low angles to the foliation of the schist. The Alvl l.sg 1.82 foliation is not deflected by the porphyroblasts. Tl 0.01 0'00 Relative to the cores, the rims of the garnets 5.00 't.43 c. 00 Mg 1.45 are strongly enriched in almandine, weakly 'I FeZ* 1.96 .75 enriched in pyrope and depleted in spessartine (Table 3). zoning general pat- Fe2* o.l5 0.061,, Tne follo{rs the tern observed by Dudley (1969) garnets Mn 0.03 o'02 in of 2.OO .0000 glaucophane schists from a variety of localities. Ca 0.12 0.06 The rims of the Jambal6 garuets contain about Na 1.70 1.85 twice as much spessartine as those reported by Na 0.23 1,, Dudley, feature that probably reflects o'23 I o.tl8 a a K 0.00 o.oo difference in the bulk composition of the schist from Jambal6.

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TABLE3. ELECTRON-MICROPROBEANALYSES the other optical properties are dissimilar. 0F A GARNETFR0M gLAUCoPHANE Chlorite that coexists with garnet in J-l is SCHISTAT JAMBALO,COLOMBIA length-slow, shows no birefringence, and has a deep blue anomalous interference color. Chlo- J-] (core) J-l (rim) rite in garnet-free J*2 is length-fast and has a si02 36.97 36.33 golden yellow interference color, with 2Vy = O'. Chlorite in J-1 is considerably more iron- - At 20.71 20.96 rich [Mgl(Mg * Feo) O.47) than that in 203 - Fe0* 17.03 22.73 Ia lMe/(Mg + Fef) 0.591. This differ- 't4.81 ence reflects the dissimilar bulk composition of Mn0 9.09 the two rocks. Mgo 0.46 0.63 GleucopneNs Scnrsrs er Jerranu,6 AND rN TrrE Ca0 8.94 9.18 Ou6coe Drsrrucr, Nrw CerrooNre TOTAL 98.92 98.92 The Jambal6 Glaucophane Schist is strik- Numberof ions on the basis of 12 (0): ingly similar to glaucophane schist from the si 3.0rI 2.e61 Ou6goa district of New Caledonia (Black 1977). |3.01 13.00 This offers a useful comparison, because the Aliv 0.00! 0.04) phase petrology and isotope geochemistry of the Alvi ,.rrlr.oo r.$lr.oo unusual rocks from the Ou6goa district have been described in great detail (Black 1973a, b, Fe3r 0.01) 0.021 1974, 1975). 'l Fe2+ r.r4l .53I Mineral assemblagesof the Jambal6 Glauco- I phane Schist are duplicated in metasedimentary Mn r.0, lr. oo o.u,l, . oo rocks from the lower epidote zone at Ou6goa Mg 0.06 0.081 (Black 1977, Fig, 2). Also, like the rocks at | Ou6goa, samples from Jambal6 lack the critical Ca 0.78t 0.80, minerals lawsonite, spessartine, jadeite, horn- blende, actinolite, aragonite and rutile. alm 38.0 50.3 The compositions of minerals are similar. spess 34.0 20.7 Jambal6 paragonite has a potassium conte-nt (0.7OVoK,O) and a dooovalue (3.210-3.216 A) pyr 2.0 2.6 comparable to that of analyzed paragonite from gross 25.7 25.7 Oudgoa (l.SVo K,O, d,y,"3.2O9 {; Black 1975' fabG t). The dooevalue (3.325 A) of phengitic and 0.3 0.7 muscovite from Jambal6 is slightly greater than the limit of. d.ooeG.302-3.323 A) ot phensites * Total iron as FeO;Fe3+/Fe2+ chosen to from the Ou6goa schist (Black 1975). Glauco- satisfy stoich'iornetry,with two trivalent phane rims from Jambal6 are comparable to 'ionsin sixfold coordinationper formula sodic amphiboles from metasedimentary rocks unit. in the epidote (rather than the lawsonite) zone at Ou6goa in terms of a plot of Na/(Na * Ca) versusMg/(Mg + FeE) (Black 1973b,Fig. 1). Epidote Garnet from schist at Jambal6 shares the com' position and pattern of zoning of metasedi' Subhedral, strongly birefringent, colorless to mentary garnets in the epidote and lawsonite- faintly chartreuse prisms of epidote, commonly epidote transitional metamorphic zones of the about 0.1 mm in length, constitute 2.lVo of. Ou6goa district (Black 1973a,Fig.3). Epidote sample J-2. The epidote is poor in iron. A from Jambal6 with Fe*,/(Fe* + Al) = 0.18 partial microprobe analysis gave Fes/(Fe* * is similar to Ou6goa metasedimentary epidotes, Al) = 0.18. which have (Feo * Mn * Mg)/(Fe* * Mn *Mg + Al) ratios that cluster around 0.16 Chlorite (Black 1977, Fig. 5).

The chlorite is pale and pleochroic (straw CoNpltloNs or MsreIuoRPHrsM yellow to light grey-green), and forms robust flakes in both samples J-l and J-2. However, The presence of glaucophane in the schist

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at Jambal6 points to metamorphism at relatively epidote) was favored Dy P(HrO) high pressure, as part o1 I high-P, low-T facies whereas the assemblage glaucophane a epidote series. As the samples from Jambal6 are taken was favored by P(HzO) = Ptot"r. This con- from outcrops in the western side of the unit, clusion is borne out qualitatively by the rocks the conditions of metamorphism here adduced at Jambal6. Sample J-1, which is epidote-free, apply only to that portion of the glaucophane contains glaucophane * paragonite + quanz schist. The possibility of a change in metamor- * calcite. The assemblage paragonite-quartz* phic conditions eastward (across strike and calcite is stable only under relatively high structurally downward) remains unstudied. P(CO), in a mixed-volatile fluid phase com- In J-1, a metamorphic temperature of 350- posed of HrO + COz (Chatterjee 1972).If. P, 400'C is given by the partitioning of Mg/ was broadly rtniform from place to place dur- (Mg + Fe * Mn) between rims of coexisting ing metamorphism, P(HzO) must have been garnet and glaucophane, using the Perchuk lower in sample J-l than in 1{,, which bears diagram reproduced in D6bretsov et al, (1975, no petrographic evidence of appreciable CO, Fig, 23), This corresponds well with an estimate having been present in the fluid phase. Of the of 385'C obtained from Fe'-Mg partitioning two samples, J-2, with the assemblageglauco- between rims of the same two minerals, using phane * epidote, was the "wetter" rock. the experimental-empirical geothermometer of The universal presence of accessory pyrite in Church (1978). In close agreement with these the schist at Jambal6 shows that f(O,) /t(9") temperatures, tle distribution of oxygen isotopes was relatively low during metamorphism. This between phases in metasedimentary rocks led is in keeping with the observation that ferro- Black (1974) to conclude that the metamorphic glaucophane rather than crossite constitutes the temperature of the lowest-grade part of the law- cores of sodic amphibole in J-l (Black 1977). sonite-free epidote zone in the Ou6goa district In summary, the conditions of metamorphism was 4O0"C. of the Jambal5 Glaucophane Schist are assumed A knowledge of the temperature of meta- to have been: T between 350 and 400'C and morphism allows limits to be placed on the P.*r in the range 5-7 kbar. A mixed-volatile pressure that prevailed. The absense of law- fluid phasewith locally variable H,O/COg ratios sonite shows that metamorphism took place at probably approximated Poar. Low l(O')/l(Sr) a lower pressure [P"r = P(HzO)] than that prevailed. of the stability field of lawsonite, which at 40O'C is about 6.5 kbar (Nitsch 1972, Fig. 5). RscroNAL SrcNrrrceNcB oF THE Jerrrner-6 At the same temperature, tle minimum pres- GreucopneNr Scnrsr sures at which aragonite and jadeite are stable are 9 and 72 kbar, respectively (Johannes & Until recently, the discontinuous belt of cir- Puhan 1971, Fig. 5; Newton & Smith 1967, cum-Pacific high-pressure metamorphic rocks Fig. 6), 'which accounts for the absence of had not been recognized on the west coast of these minerals at Jambal6. The minimum pres- South America north of latitude 41" S @5bret- sure possible during metamorphism would be sov et al.1975,Fig.26). This situationchanged the maximum pressure of the greenschist facies, with the discovery of the glaucophane schist in which case glaucophane would have been at Jambal6 (Orrego et al. l980a this paper) -r replaced by the assemblagechlorite * albite and eclogite and associatedhigharessure meta- actinolite. This is a divariant reaction that has morphic rocks from the Raspas Formation of not been determined experimentally. The min- southwestern Ecuador (Feininger 1980). The imum pressure for the stability of natural, Fe- two occurrences, although 800 km apart, are poor glaucophaneo however, has been studied sufficiently similar to warrant speculation about in the laboratory by Maresch (1977). His curve a common origtn. passes through the points 4 kbar and 35OoC, The Jambal6 Glaucophane Schist and the 6 kbar and 4@oC. 8 kbar and 450oC. Raspas Formation are both located on tle Metamorphism took place in the presence of ancient, continental South .American plate, a fluid phase probably composed nearly ex- within 50 km of its edge at the Romeral clusively of HrO and COe, with Pt"t"r = Pr. fault. Rocks northwest of the Romeral fault The composition of the fluid phase, buffered lie on relatively younger oceanic lithosphere, by local parageneses, must have varied from which may have been added by accretion and place to place. For example, chemographic an- a seaward jump of the continental-border sub- alyses using Schreinemi*ers bundles led Black duction zone during Cretaceow and Paleogene (1977) to conclude that at Oudgoa, the as- time (Feininger & Bristow 1980). The Colom- semblage glaucophane 1 paragonite (without bian and Ecuadorian high-pressure rocks occur

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in fault-bounded bodies (the Raspas Formation MINAS kindly provided a suite of samples in is partly eneased' iln. serpentinized harzburgite) advance of my visit to the outcrops, which that were emplaced tectonically. Finally, rocks was guided by Abigail Orrego. Mr. Jean-Pierre from the two widely separated localities both Tremblay carried out the mineral analyses in have Earliest Cretaceous K-Ar uplift ages: 125 the Laboratoire de la microsonde, Universit6 -+ 15 Ma at Jambal6 (Orrego et al. I98Ob) Laval. The manuscript was improved by the and 132 -+ 5 Ma for the Raspas (Feininger comments of Associate Editor A.C. Turnock, 1980). Professors Philippa Black and W.R. Church In my report on the Ecuadorian high-pres- and an anonymous reviewer. Permission to sure rocks, I suggested that the Raspas Forma- publish was given by Dr. Hermaun Duque C., tion was metamorphosed in a subduction zone Director Encargado, INGEOMINAS. This work beneath the South American continental border; was financed by the Natural Sciences and Engi- the formation was emplaced diapirically in rocks neering Research Council of Canada, grant of relatively high dsnsily, buoyed upward by 47348. serpentinized harzburgite, tlegrrnning when subduction ceased. A synchronous radiometric date RsrnnrNcrs on the youngest lava of the ceval volcanic arc in eastern Ecuador was cited in support of this Br.acr, P.M. (1973a): Mineralogy of New Cale- proposed mechanism. A similar history could donian metamorphic rocks. I. Garnets from tlie apply to the Jambal6 Glaucophane Schist, as Ou6goa District. Cont. Mineral. Petrology 88, most of the required elements are presenl The 22t-235. schist occurs in a fault-bounded block, in part (1973b): Mineralogy of New Caledonian emplaced against deuse metabasites. Serpentin- metamorphic rocks. II. Amphiboles from the ite, although not occurring in large masses, Ou6goa District. Contr. Mineral. Petrology 89, is found as sheared bodies along the fault be- 55-64. tween the glaucophane schist and the San Anto- (1974): Oxygen isotope study of meta- nio Amphibolite. possibly Roots of a coeval morphic rocks from the Ou6goa District, New arc constitute immense granitic batholiths of Caledonia. Contr, Mineral. Petrology 47, t97- Jurassic and Triassic age, between 70 and 140 206. km southeast of Jambal6 (INGEOMINAS 1976). (1975): Mineralogy of New Caledoniran If the Jambal6 Glaucophane Schist and the metamorphic rocks. IV. Sheet silicates from the Ou6goa District. Cont. Minerul. Petrology 49, Raspas Formation share an analogous and 269-284. synchronous origin, as I suggest they may, one can draw three conclusions with important (1977): Regiooal high-pressuremetamor- regional significance: (l) A major episode of phism in New Caledonia: phase equilibria in subduction in northwestern South America the Ou6goa District. Tectonophys.43, 89-107.' terminated abruptly in Early Cretaceous time. (2) Outcrops of high-pressure metamorphic Cnerren.rnr, N.D. (1972)t The upper stability of paragonito{uafiz and its rocks of Early Cretaceous age may occur else- limit the assemblage just natural occuren@s. Conff. Minerul. Petology 84, where east of the Romeral fault in a 50- 288-303. km-wide belt on the geologically complex northwestern corner of the South American con- C.trunqr, W.R. (1978): Eclogite-bearingamphi- tinent. These rocks may extend from &e ,Raspas bolites from the Appalachian mobile belt, norti- Filrmation in the south to a -kmpoint 170 km west Newfoundland: dry versus wet metamor- north of Medellfn (and 550 north of phism: a discussion.J. Geol. 86, 655-659. Jambal6), where the crystalline Central Cordil- Ddinrrsov, N.L., Sdsolrv, V.S., Sdnor.ev, lera plunges beneath N.V. & Tertiary and Quaternary Knr-6srov,V.V. (1975): The Faciesof (Fig. Regional sedimentary rocks in northern Colombia Metamorphism at Hieh Prcssures (translated l). (3) The Romeral fault marked the active from Russian by D.A. Brown). Australian Na- boundary between the northwestern corner of tional University, Canberra. the continental South American plate and an oceanic plate to the northwesf at least during Duor,ev,P.P. (1969): Electronmicroprobe anqlys€s the beginning of the Cretaceous Period. of garnet in glaucophaneschists and associated eclogites.Amer. Mineral. t4, 1139-1150. AcKNoWLEDGEMENTS FEnTNGER,T. (1980): Eclogite and related high- pressure regional metamorphic rocks from the Jairo Vesga and Ricardo Escovar of INGEO- Ardes of Ecuador. J, Petrology 21", tW-140,

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& Bnrsrow, C.R. (1980): Cretaceousand NIrscH, K.-H. (1972): Das P-T-Xcor-Stabilitiits- Paleogene geologic history of coastal Ecuador. feld von Lawsonit. Contr. Mineral. Petology 84, Geol. Rundschau69, 849-875. lt6-134.

INGEOMINAS (1976): Mapa geol6gicode Colom- ORRrco, A., Crruoe, H. & Ropnicurz. G.I. (1980a): bia [:1,500,0001. Min. de Minas y Energla, Esquistos glaucofdnicos en el 6rea de Jambal6, Bogot6. Cauca, Colombia (nota preliminar). Geol. Noran- IoneNxrg W. & Purrex, D. (1971): The calcite- dina (Bogotd) 1, 5-10. aragonite transition reinvestigated. Contr. Min- eral. Petrology 31, 28-38. Rns"rnnro, J.J., Toussenvr, J.-F. & Lwenrs, E. (1980b): Dataci6n de un esquisto sericltico MAREscn,W.V. (1977): Experimentalsturdies on de Jambal6, Cauca. Publ. Espec. Geol.25, Univ. glaucophane: an analysis of present kno.wledge. Nal., Medell'tn. Tectonophys.48, l@-125. NEwroN, R.C. & Srrdrr, J.V. (1967): Investiga- tions concerningthe breakdown of albite at depth Received lane. 1981, revised manuscript accepted in the earth. t. Geol. 75, 268-286.. November 1981.

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Photomicrographs of glaucophane schists from Jambal6. Plane light, field of view, 1.1 X 0.75 mm. A. Sample I-1. Euhedral garnet (eft), calcite (righ0, glaucophane (grey with dark cores), albite and quartz(colorle.ss),chlorite(boot-shapedgrain,centre), paragonite (clear flakes), sphene (very dark), and pyrite (lower right corner). B. Sample I-2. Subhedral prismatic glaucophane, quartz and albite (colorless), paragonite and muscovite Oottom), chlorite (centre),epidote (small grains with high relief), sphene (wedge- shape grains with very high relief), and pyrite (upper left).

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