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- </' etic link between the Jambal6 Glaucophane 'll Schist and the I-a Mina or San Antonio units. c\ fe JeuBer.6 GleucopneNn Scnrsr The block of Jambal6 Glaucophane Schist is elongated north-northeast, parallel with the regional Andean tectonic trend. The long axis of the block is at least 25 km, the transverse axis 7 km. The northern contact is incom- pletely mapped. Foliation in the schist dips uniformly to the west (Orrego et aI. 1980a). A single whole-rock K-Ar determination has given an Early Cretaceous age of 125 +- 15 Ma (Onego et al. 1,98Ob).Three small stocks of andesite porphyry of Tertiary age cut the body in the south. 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. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/20/1/41/4005663/41_20_1_cm.pdf by guest on 30 September 2021 GLAUCOPHANE SCHISiT IN THE ANDES 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 assem- blage 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.