jimenez mejia 2006 macizo garzon

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Journal of South American Earth Sciences 21 (2006) 322–336

P–T–t conditions of high-grade metamorphic rocks of theGarzon Massif, Andean basement, SE Colombia

Diana Marıa Jimenez Mejıa a,*, Caetano Juliani b, Umberto G. Cordani b

a INGEOMINAS Bogota, Diag. 53 No. 34-53, A.A. 4865, Bogota, Colombiab Instituto de Geociencias, Universidade de Sao Paulo, R. Lago, 562, CEP 05508-080, Sao Paulo, SP, Brazil

Received 1 October 2004; accepted 1 February 2006

Abstract

The metamorphic evolution of the Garzon Massif, Colombia, is established on the basis of the textural, goethermobarometric, andgeochronological relationships of the metamorphic minerals. The geothermobarometric data define a clockwise, nearly isothermaldecompression path (ITD) for rocks from Las Margaritas migmatites, constrained by four P–T areas: 780–826 �C and 6.3–8.0 kbar,760–820 �C and 8.0–8.8 kbar, 680–755 �C and 6.6–9.0 kbar, and 630 �C and 4 kbar. For the a garnet-bearing charnockitic gneiss fromthe Vergel granulites, the path is counterclockwise, constrained by geothermobarometric data of 5.3–6.2 kbar and 700–780 �C and 6.2–7.2 kbar and 685–740 �C. The clockwise ITD path represents a loop followed by the orogen during the transitional granulite–amphibolitemetamorphic conditions, probably associated with a subduction process followed by a collisional tectonic event. This subduction frame-work produced continental crust thickening between 1148 and 1034 Ma and later collision with another continental block approximately1000 Ma ago. The orogenic exhumation occurred with moderate uplift rate. The counterclockwise trajectory and two metamorphicevents suggest a vertical displacement between the Vergel granulites and Las Margaritas migmatites units, because there is no isotopicdifference that indicates the existence of different terranes. The data confirm that the metamorphic evolution for this domain was moredynamic than previously believed and includes: (1) metamorphic processes with the generation of new crust with a possible mixture of oldmaterial and (2) metamorphic recycling of continental crust. These geological processes characterize a complex Mesoproterozoic orogen-ic event that shares certain features with the Grenvillian basement rocks participating in the formation of Rodinia.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Colombia; Geothermobarometry; Granulites; Grenvillian rocks; Andes

Resumen

La historia presion–temperatura–tiempo (P–T–t) de las metamorfitas del Macizo de Garzon – Colombia, fue establecida con base enlas relaciones texturales, termobarometricas y geocronologicas de los minerales metamorficos. Los datos P–T–t definen para las rocas delas Migmatitas de Las Margaritas una trayectoria horaria de descompresion cercanamente isotermal (ITD), definida por cuatro zonas enel grafico P–T: 780–826 �C y 6.3–8.0 kbar, 751–778 �C y 7.7–8.1 kbar, y 688–752 �C y 6.8–7.9 kbar, y 630 �C y 4 kbar . En contraste, paralas rocas de las Granulitas del Vergel el camino es anti-horario, definido por datos geotermobarometricos de 5.3–6.2 kbar y 700–780 �C,y 6.2–7.2 kbar y 685–740 �C. La trayectoria horaria (ITD) representa un camino seguido por el orogeno durante las condiciones meta-morficas en transicion anfibolita–granulita, probablemente asociado a un proceso de subduccion. Este ambiente de subduccion produceinicialmente, al interior del orogeno, una corteza continental engrosada entre 1148–1034 Ma, y posteriormente una colision con otramasa continental a 1000 Ma aproximadamente. La exhumacion del orogeno estuvo acompanada por tectonismo con moderadas tasasde levantamiento. La trayectoria anti-horaria y los dos eventos metamorficos permiten sugerir movimiento vertical entre las unidadesGranulitas del Vergel y Migmatitas de Las Margaritas, en vez de la existencia de terrenos diferentes, ya que no hay diferencias isotopicas.Los datos obtenidos confirman que la evolucion metamorfica de este dominio cortical fue un proceso dinamico que incluye los eventos de

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doi:10.1016/j.jsames.2006.07.001

* Corresponding author. Tel.: +57 1 2200273; fax: +57 1 2223764.E-mail address: [email protected] (D.M. Jimenez Mejıa).

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D.M. Jimenez Mejıa et al. / Journal of South American Earth Sciences 21 (2006) 322–336 323

(1) metamorfismo que involucra generacion de corteza nueva con posible mezcla de material mas antiguo y (2) metamorfismo caracter-izado por reciclaje de corteza continental. Estos procesos geologicos caracterizan complejos eventos orogenicos mesoproterozoicos quecomparten ciertos rasgos con rocas de basamento Grenvilliano participantes en la formacion de Rodinia.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

The concept of metamorphism as a dynamic process, asadopted by modern petrology, is based on the observationthat metamorphic minerals could preserve the chemicalzoning caused by changes in temperature and pressure dur-ing their crystallization (Spear and Selverstone, 1983;Thompson and England, 1984; Spear, 1995). Accordingly,geothermobarometry based on mineral chemical composi-tion provides quantitative data for the metamorphic eventin the P–T space.

High-grade metamorphic terrains provide importantinformation about the tectonic processes that result incrustal accretion and continental growth (Harley, 1989).The pressure–temperature–time evolution (P–T–t) of gran-ulites is usually well constrained, because many rocksinclude mineral assemblages suitable for geothermobaro-metric studies (Spear and Selverstone, 1983; Harley,1989). The P–T–t path is the loop recorded by the individ-ual rock unit and/or rocks from different crustal levelsaffected by an orogenic event during a certain time interval(Brown, 1993) and is considered a record of the tectonicevolution of the metamorphic terrain (England andThompson, 1984; Spear, 1995).

In granulite facies terrains, two types of P–T–t trajecto-ries are recognized (Bohlen, 1987; Harley, 1989): nearly iso-thermal decompression (ITD) and nearly isobaric cooling(IBC). In addition, the inferred P–T–t path of granuliteshas been considered a useful tool for determining the gen-esis and evolution of the lower crust, relative movement ofblocks inside the orogen, crustal unroofing, heating, andoverthickening and therefore to infer thrusting where thegeologic field evidence is ambiguous (Harley, 1989; Spear,1995).

The main aim of this article is to constrain the metamor-phic evolution of the Garzon Massif, the largest exposureof the Mesoproterozoic high-grade metamorphic basementof the Colombian Andes. These data enable the reconstruc-tion of the complex tectonic history of the Garzon Massifto the formation of the Andean belt in the Mesozoic–Cenozoic.

2. Geological setting

The Garzon Massif is an elongated inlier trending NE-SW, bounded by reverse faults related to Andean tectonics,and overlain by Mesozoic and Cenozoic sediments.

The massif comprises two main lithological units (Fig. 1):the Guapoton–Mancagua gneiss on the west and the Gar-zon Complex on the east. The Guapoton–Mancagua gneiss

is composed mainly of biotite–hornblende orthogneisseswith K-feldspar augens (Fig. 2a), whereas the GarzonComplex consists of banded quartz-feldspathic rocks withinterbedded metapelitic, metamafic, metaultramafic, andcalc-silicate rocks, metamorphosed from amphibolite togranulite facies (Kroonemberg, 1982; Ingeominas andGeoestudios, 2001; Rodriguez et al., 2002). These rocksare crosscut by pegmatite and lamprophyre dykes.

The Garzon Complex has been informally subdividedinto the Vergel granulites and Las Margaritas migmatites(Ingeominas and Geoestudios, 2001). The Vergel granulitesare essentially composed of charnockite, charnoenderbite,enderbite, mafic granulites, and quartz-feldspathic gneisses,as well as minor granitic to quartz syenitic gneisses andamphibolite bodies. The granulites are usually foliatedand have bands of hololeucocratic gneissified granitic rocks(Fig. 2b), which indicate a previous migmatization event(L). Ortho- and clinopyroxene also occur locally in theseleucocratic bands, suggesting possible migmatization inthe granulitic facies or metamorphic crystallization ofpyroxenes in lecosomes during the high-grademetamorphism.

The Las Margaritas migmatites unit is composed pre-dominantly of metasedimentary rocks and mafic dikesaffected by two migmatization events. The rocks of theolder migmatization event were intensely gneissified(Fig. 2c). The metamorphosed leucosomes (L) resultedin garnet-bearing biotite and quartz-feldspathic gneisses(Jimenez-Mejıa, 2003), and the mesosome led tobiotite–sillimanite–garnet gneisses, with prismatic sillima-nite oriented parallel to the metamorphic foliation.Between the mesosome and the leucosome, melanosomerelicts form millimeter-thick, biotite-rich layers withassociated sillimanite. The mafic rocks present in thisunit were also gneissified, resulting in boudinated struc-tures (Fig. 2d).

A second event of migmatization is observed as unde-formed granitic veins (L2) in gneissified migmatite andamphibolite boudins (Figs. 2c and d). Late granitic pegma-tites (L3) crosscut the whole sequence (Fig. 2d) and werelocally affected by brittle faults.

Lithological and geochemical data suggest that the mag-matic protoliths of the Garzon Massif were formed in anAndean-type tectonic environment (Kroonemberg, 1982,2001).

3. Analytical methods

Mineral analyses were performed at the ElectronicMicroprobe Laboratory of the Instituto de Geociencias,

Fig. 1. Geological map of the study area. (1) Cenozoic sediments and Cretaceous rocks; (2) Jurassic intrusives; (3) Jurassic–Triassic volcaniclasticsequences and Paleozoic sedimentary rocks; (4) Vergel granulites; (5a) Guapoton–Mancagua gneiss; (5b) Toro gneiss; (6) Recreo gneiss; (7) LasMargaritas migmatites; (8) Minas migmatites; and (9) Plata Orthogranite (modified from Ingeominas and Geoestudios, 2001).

324 D.M. Jimenez Mejıa et al. / Journal of South American Earth Sciences 21 (2006) 322–336

Universidade de Sao Paulo, equipped with a JEOL JXA-8600 electron microprobe. Operating conditions were20 nA as sample current, 15 kV as accelerating voltage,and beam diameter of 5 lm, except for the feldspars, whichwere analyzed with 10 lm beam diameter. Representativeanalyses of garnet, biotite, pyroxenes, plagioclase, andamphibole appear in Tables 1–4. The crystals were ana-lyzed from rim to rim, including the cores, to determinecompositional variations along the grains. Mineral abbre-viations in the text and figures are according to Kretz(1983).

The quantitative P–T conditions were evaluated usingthe TWQ (v. 2.02) computer program (Berman, 1991),which calculates the thermodynamic equilibrium of themineral assemblages on the basis of an internally consistentset of geothermometers and geobarometers that providethe location of a chemical reaction in a P–T space. Theintersections of the univariant chemical reactions curvesplot close to a point in an ideal case or define areas ofthe most probable metamorphic conditions of the paragen-esis when the thermodynamic equilibria between the ana-lyzed minerals are not perfect. The P–T conditions also

were estimated using a single equilibrium based onexchange and net transfer reactions. These calculationsused the GPT worksheets (Reche and Martinez, 1996),which contain geothermometer and geobarometer calibra-tions defined by several authors. These calculations appearin Table 5.

The chemical formulae of garnet, orthopyroxene, bio-tite, and plagioclase were calculated with Minpet 2.02 soft-ware (Richard, 1997). Biotite classification followed therecommendation of Rieder et al. (1998) (Tables 1–4), andthe amphibole classification coincides with the criteriaestablished by the International Mineralogical Association(Leake et al., 1997). The amphibole composition was calcu-lated on the basis of 23 (O, F, and Cl) atoms per formulaunit (a.p.f.u.), using the method proposed, in which theFe2+ and Fe3+ contents of calcic amphiboles are calculatedby averaging the results obtained, assuming a total of 15cations excluding Na and K and a total of 13 cationsexcluding Ca, Na, and K.

The P–T data were interpreted together with U–PbSHRIMP zircon ages, Sm–Nd garnet whole-rockisochrones, and Ar–Ar step-heating biotite analyses. The

Fig. 2. (a) Macroscopic aspect of the Guapoton–Mancagua gneiss; (b) banded granulites from Vergel granulites, with mafic granulite in dark colors andfelsic granulite (L) in light colors; (c) typical aspect of the foliated (L) and nonfoliated (L2) leucosomes of Las Margarites migmatites; (d) boudins ofamphibolites (dark colors) in Las Margarites migmatites. The leucosomes of the second event of migmatization (L2) crosscut the gnessified migmatite andamphibolite, and a dike of granitic pegmatite (L3) crosscuts the outcrop.


geochronological data from the Garzon Complex aredetailed by Cordani et al. (in press).

4. Petrography

4.1. Guapoton–Mancagua gneiss

This unit is composed of homogeneous leucocraticaugen orthogneisses, with elongated K-feldspars reaching3 cm in length. No migmatization or compositional band-ing is observed (Ingeominas and Geoestudios, 2001).Amphibolite, diorite, and granitic pegmatites dikes arelocally observed.

The gneisses have granitic composition. Brown biotiteand subordinate hornblende are the main mafic minerals.Microcline predominates over deformed K-feldspar igne-ous relicts, indicating that metamorphism occurred in tem-peratures compatible with lower-amphibolite to greeschistfacies (Heier, 1957; Martin, 1972). A greenschist retrometa-morphism is frequently observed in these rocks, represent-ed by sericitization and saussuritization of feldspars andchloritization of mafic minerals.

4.2. Vergel granulites

The composition of the Vergel felsic granulites rangesfrom enderbite to charnockite, with mafic minerals reach-ing 30% in volume locally. The association orthopyroxene+ clinopyroxene + garnet ± hornblende in granoblastic

texture is common in the least deformed rocks. Kroonem-berg (1982) also describes granulites with cordierite + orth-opyroxene + magnetite ± spinel around garnet crystals,interpreted as a product of contact metamorphism. How-ever, this texture also could be interpreted as generatedby decompression in high temperatures in granulite facieswith a nearly ITD metamorphic path (Harley, 1989). Inaddition to hornblende produced by the hydration ofpyroxenes, antiperthitic exsolutions in plagioclase, partialrecrystallization of mymerkite, and the formation of horn-blende–quartz and biotite–quartz symplectites are relatedto retrograde metamorphism.

Mafic granulites occur as decimeter-thick beds in felsicgranulites. Two types of mafic granulites are identified:andesine-rich (>50% modal) and hornblende-rich (�60%modal). The hornblende-rich rock has clinopyroxene, orth-opyroxene, hornblende, quartz, and biotite as essentialcomponents and magnetite, zircon, apatite, and K-feldsparas accessories. The andesine-rich rock has clinopyroxene,orthopyroxene, and andesine as essential minerals andquartz, biotite, apatite, opaque minerals, and zircon asaccessories.

According to Ringwood (1975), the assemblageorthopyroxene + clinopyroxene + plagioclase ± quartzand the absence of garnet are typical of intermediate-pres-sure granulite facies, limiting the metamorphic conditionsto up to 10 kbar at 850 �C. The assemblage plagio-clase + hornblende + pyroxenes ± biotite also occurs inthe mafic granulites; hornblende was formed by the

Table 1Representative analyses of minerals of charnockitic gneiss (sample V-332) of the Vergel granulites

Garnet Biotite Ortophyroxene Plagioclase

Core Rim Rim in contact with biotite Core Included in garnet Core Rim near garnet Core Rim near garnet

SiO2 37.64 37.54 37.80 36.09 35.78 49.06 49.34 59.63 59.69TiO2 0.05 n.d. 0.08 5.26 6.12 0.13 0.12 n.d. n.d.Al2O3 21.09 20.95 20.98 13.70 14.02 1.24 1.35 25.14 24.95FeOT 32.62 32.52 31.96 18.91 18.16 33.60 34.09 n.a. n.a.Fe2O3T n.a. n.a n.a. n.a. n.a. n.a. n.a. 0.16 0.11MnO 0.88 1.02 0.91 0.01 n.d. 0.26 0.35 0.02 0.01MgO 4.44 4.30 3.61 11.29 11.03 13.87 13.13 n.a. n.a.CaO 2.31 2.68 4.06 n.d. 0.02 0.37 0.34 6.81 6.59Na2O 0.02 0.01 0.01 0.04 0.37 n.d. 0.03 7.65 7.42Cr2O3 0.02 0.10 0.03 n.a. n.a. 0.10 0.05 n.a. n.a.BaO 0.01 0.03 0.08 0.33 0.28 n.a. n.a. n.d. n.d.K2O n.a. n.a. n.a. 9.39 8.75 0.02 n.d. 0.10 0.38Cl n.a. n.a. n.a. 0.40 0.82 n.a. n.a. 0.01 0.01F n.a. n.a n.a. 1.07 1.05 n.a. n.a. n.a. n.a.SrO n.d. n.d. n.d. n.a. n.a. n.a. n.a. 0.03 0.10

T 99.08 99.18 99.52 96.48 96.39 98.65 98.80 99.53 99.25

Number of ions on base of

24 oxygens 22 oxygens, OH = 0 6 oxygens 32 oxygens

Si 6.041 6.028 6.057 5.547 5.496 1.960 1.976 10.678 10.720Ti 0.007 n.d. 0.010 0.608 0.707 n.d n.d. n.d. n.dAlT n.d. n.d. n.d. n.d. n.d. 0.040 0.024 5.301 5.277AlIV n.d. n.d. n.d. 2.453 2.504 0.018 0.040 n.d. n.d.AlVI 3.986 3.962 3.959 0.026 0.031 n.d. n.d. n.d. n.d.Fe2+ 4.378 4.367 4.283 2.431 2.332 1.123 1.142 n.d. n.d.Fe3+ n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.021 0.015Mn 0.120 0.138 0.123 0.002 n.d. 0.009 0.012 n.d. n.d.Mg 1.063 1.029 0.863 2.588 2.525 0.826 0.784 n.d. n.a.Ca 0.398 0.462 0.697 n.d. 0.003 0.016 0.015 1.307 1.268Na 0.006 n.d. 0.003 0.013 0.110 n.d. 0.002 2.656 2.583Cr 0.002 0.013 0.004 n.a. n.a. 0.003 0.002 n.a. n.a.Ba n.d. n.d. n.d. 0.020 0.017 n.a. n.a. n.d. n.d.K n.a. n.a. n.a. 1.841 1.714 n.a. n.a. 0.023 0.087Cl n.a. n.a. n.a. 0.206 0.428 n.a. n.a. n.a. n.a.CF n.a. n.a. n.a. 1.035 1.021 n.a. n.a. n.a. n.a

16.001 15.999 15.999 16.770 16.880 3.995 3.997 19.986 19.950

Ab 66.60 65.60An 32.80 32.20Or 0.60 2.20

Analyses in wt%, T = total, n.d., not detected, and n.a., not analyzed.


hydration of pyroxenes during retrograde metamorphism.The final metamorphic equilibrium in the greenschist faciescaused the replacement of mafic minerals by chlorite and epi-dote and saussuritization and sericitization of plagioclase.

Leucocratic gneisses with compositions ranging fromtonalite to granite are associated with the granulites. Theserocks could represent a hydrous magmatic series associatedwith the charnockitic series or, alternatively, crustal por-tions relatively preserved by the granulitic metamorphism.The absence of orthopyroxene in these rocks also could bedue to a chemical composition unfavorable to their crystal-lization. A migmatization event (L) is recorded as foliatedquartz syenite and granite beds, where relicts of phaneritictexture and igneous perthitic K-feldspar are preserved.

K-feldspar exhibits perthitic to mesoperthitic textures,especially in more deformed portions, with string and flame

shapes. Metamorphic recrystallization of perthites also isobserved. Strong deformation and recrystallization accom-panied the retrograde metamorphism, which resulted infabric rearrangements, as observed by trails of quartzinclusions in feldspar and of quartz and biotite in poikilob-lastic garnet.

4.3. Las Margaritas migmatites

The Las Margarita migmatites unit is essentially com-posed of migmatized metasediments and granulites. Therelationship between the migmatites and granulites is notclear, but they could represent tectonic slabs of the Vergelgranulites inserted in the migmatitic terrains or zones ofdehydrated rocks affected by the high-temperature meta-morphism that caused partial melting of the country rocks.

Table 2Representative analyses of minerals of mafic granulite (sample C-32) of the Las Margaritas migmatites

Garnet Biotite Ortophyroxene Plagioclase

Core Rim Included in garnet Core Core Rim Core Rim

SiO2 37.67 37.79 35.28 35.92 50.55 50.14 58.55 58.44TiO2 n.d. n.d. 5.81 4.90 0.05 0.17 n.d. n.d.Al2O3 20.97 20.62 14.01 14.16 0.81 1.06 25.80 25.78FeOT 28.03 28.88 19.70 21.33 33.09 33.14 n.d. n.d.Fe2O3T n.a. n.a. n.a n.a n.a. n.a. 0.09 0.12MnO 2.24 2.52 0.07 0.03 0.82 0.85 n.d. n.d.MgO 3.46 2.76 10.50 10.12 14.30 14.12 n.d. n.d.CaO 6.52 7.03 0.12 0.05 0.76 0.92 7.70 7.81Na2O 0.02 0.03 0.08 0.02 0.02 0.04 7.11 6.98Cr2O3 0.01 0.01 n.a. n.a. 0.04 0.01 n.a. n.a.BaO n.d. n.d. 0.27 0.23 n.a. n.a. 0.01 0.01K2O n.a. n.a. 8.97 9.49 n.d. 0.01 0.31 0.28Cl n.a. n.a. 0.14 0.18 n.a. n.a. 0.01 0.01F n.a. n.a. 0.42 0.58 n.a. n.a. n.a. n.a.SrO n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.Y2O3 0.36 0.41 n.a. n.a. n.a. n.a. n.a n.a

T 99.28 100.02 95.37 97.00 100.43 100.44 99.57 99.43


24 oxygens 22 oxygens, OH = 0 6 oxygens 32 oxygens

Si 6.026 6.043 5.455 5.510 1.980 1.965 10.517 10.510Ti 0.002 n.d. 0.676 0.565 0.001 0.005 n.d. n.d.AlT n.d. n.d. n.d. n.d. 0.020 0.035 5.457 5.460AlIV n.d. n.d. 2.545 2.490 0.018 0.014 n.d. n.d.AlVI 3.962 3.883 0.005 0.068 n.d. n.d. n.d. n.d.Fe2+ 3.745 3.862 2.547 2.737 1.084 1.086 n.d. n.d.Fe3+ n.a. n.a. n.a. n.a. n.a. n.a. 0.013 0.016Mn 0.278 0.341 0.009 0.003 0.027 0.028 n.d. n.d.Mg 0.899 0.657 2.420 2.314 0.835 0.825 n.d. n.d.Ca 1.083 1.205 0.020 0.008 0.032 0.039 1.481 1.506Na 0.004 0.008 0.024 0.007 0.001 0.003 2.475 2.435Cr n.d. n.d. n.a. n.a. n.d. n.d. n.a. n.a.Ba n.d. n.d. 0.016 0.014 n.a. n.a. n.d. n.d.K n.a. n.a. 1.770 1.858 n.d. n.d. 0.070 0.065Cl n.a. n.a. 0.073 0.092 n.a. n.a. n.d. n.d.CF n.a. n.a. 0.413 0.563 n.a n.a. n.a. n.a.

15.999 15.999 15.973 16.229 3.999 4.000 20.013 19.992

Ab 61.50 60.80An 36.80 37.60Or 1.70 1.60

Analyses in wt%, T = total, n.d., not detected, and n.a., not analysed.


The mesosome of the migmatites usually contains garnetand sillimanite and is biotite rich or quartz rich, which sug-gests a predominantly sedimentary protolith, including pel-ites and greywackes (Jimenez-Mejıa, 2003). Sillimaniteinclusions in garnet and K-feldspar suggest that the reac-tion biotite + sillimanite + quartz + plagioclase = garnet+ K-feldspar + melt occurred at temperatures higher thanapproximately 760 �C and pressures of approximately8 kbar (Spear, 1995), probably in the beginning of thedecompression of the orogen (Fig. 3a). The partial meltingalso resulted in the breakdown of biotite by dehydrationand crystallization of garnet (Le Breton and Thompson,1988) in mesosome (Fig. 3b) and leucosome. With incre-ments of the melting rate, garnet was isolated in the melt,resulting, after cooling, in a garnet-bearing granitic

leucosome (L), which was later metamorphosed and gneis-sified (Sn).

The development of N-NNE–trending thrust faults,with low to medium W–NW dips, and NE-SW–trendingductile shear zones resulted in a superimposed foliation(Sn+1). The paragenesis biotite + sillimanite + muscovitefound in this foliation (Fig. 3c) indicates that the metamor-phism reached amphibolite facies in the beginning of theexhumation of the orogen. This retrograde metamorphismalso produced lamellar symplectites of green biotite andplagioclase (Fig. 3d), crystallized according to the reactionK-feldspar + garnet + H2O = biotite + plagioclase + quartz.This mineral reaction is indicative of a significant decom-pression event, which could be attributed to an ITD retro-grade metamorphic event (Escuder Virute et al., 2000).

Table 3Representative analyses of minerals of quartz-plagioclase rthogneiss(sample Gr-15) of the Las Margaritas migmatites

Garnet Biotite

Core Rim Included in garnet Core

SiO2 37.43 36.48 34.41 34.28TiO2 0.02 0.04 2.29 2.22Al2O3 20.78 20.59 16.82 16.80FeOT 32.49 33.89 23.99 23.91Fe2O3T n.a. n.a. n.a. n.a.MnO 1.94 2.66 0.05 0.05MgO 3.18 1.67 7.40 7.54CaO 3.81 3.48 n.d. 0.01Na2O 0.02 0.03 0.08 0.06Cr2O3 n.d. 0.02 n.a. n.a.BaO 0.02 0.08 0.13 0.01K2O n.a. n.a. 9.43 9.52Cl n.a. n.a. 0.23 0.22F n.a. n.a. 0.68 0.71SrO n.a. n.a n.a. n.a.Y2O3 0.24 0.28 n.a. n.a.

T 99.93 99.22 95.49 95.32


24 oxygens 22 oxygens, OH = 0

Si 6.013 5.977 5.436 5.424Ti 0.003 0.004 0.272 0.264AlT n.d. 0.023 n.d. n.d.AlIV n.d. n.d. 2.564 2.576AlVI 3.932 3.951 0.564 0.555Fe2+ 4.365 4.643 3.170 3.164Fe3+ n.a. n.a. n.a. n.a.Mn 0.264 0.369 0.006 0.007Mg 0.762 0.409 1.743 1.779Ca 0.655 0.611 n.d. 0.002Na 0.006 0.009 0.024 0.018Cr n.d. 0.002 n.a. n.a.Ba n.d. n.d. 0.008 0.001K n.a. n.a. 1.901 1.921Cl n.a. n.a. 0.122 0.118CF n.a. n.a. 0.676 0.713

16.000 15.998 16.486 16.542


Table 4Representative analyses of minerals of migmatite mesosome (sample Z-365) of the Las Margaritas migmatites

Garnet Biotite

Core Rim Included in garnet Core

SiO2 37.99 37.87 35.29 36.11TiO2 n.d. n.d. 4.40 2.75Al2O3 21.40 21.36 17.46 18.13FeOT 31.64 33.95 16.67 16.06Fe2O3T n.a. n.a. n.a. n.a.MnO 0.82 1.04 n.d. 0.02MgO 6.30 4.41 10.28 11.24CaO 0.92 1.00 n.d. n.d.Na2O 0.03 0.01 0.15 0.11Cr2O3 0.02 0.02 n.a. n.a.BaO 0.03 0.04 0.15 n.d.K2O n.a. n.a. 9.79 9.96Cl n.a. n.a. 0.14 0.13F n.a. n.a. 0.56 0.79SrO n.a. n.a. n.a. n.a.Y2O3 0.11 0.11 n.a. n.a.

T 99.26 99.81 94.88 95.28


24 oxygens 22 oxygens, OH = 0

Si 6.030 6.060 5.400 5.476Ti n.d. n.d. 0.506 0.313AlT n.d. n.d. n.d. n.d.AlIV n.d. n.d. 2.600 2.524AlVI 4.001 4.025 0.546 0.714Fe2+ 4.201 4.544 2.133 2.036Fe3+ n.a. n.a. n.a. n.a.Mn 0.111 0.141 n.d. 0.002Mg 1.490 1.053 2.345 2.541Ca 0.156 0.171 n.d. n.d.Na 0.008 0.004 0.045 0.031Cr 0.003 0.002 n.d. n.d.Ba n.a. n.a. 0.009 n.d.K n.a. n.a. 1.911 1.927Cl n.a. n.a. 0.072 0.067CF n.a. n.a. 0.540 0.754

16.000 16.000 16.107 16.385



Other textural evidence of the ITD path is the crystalliza-tion of thin plagioclase rims in garnet crystals and myrmek-ite when the plagioclase is in paragenesis with K-feldspar(Harley, 1989).

The granulites of the Las Margaritas migmatites unit aresimilar to those in the Vergel granulites. Their compositionranges from enderbite to charnoenderbite with ortho- andclinopyroxene. Tonalitic gneiss bodies and gneissifiedmigmatites are commonly associated with these rocks.

5. Mineral chemistry

Representative mineral compositions of a charnockitegneiss from the Vergel granulites and an enderbite andmigmatites from the Margarita migmatites appear inTables 1–4. Gneisses from Guapoton–Mancagua werenot analyzed, because the mineral compositions of the sam-

pled rocks were inadequate for geothermobarometriccalculations.

5.1. Vergel granulites

Sample V-332, garnet-bearing charnockitic gneiss, con-tains quartz + perthite + orthopyroxene (XFe = 1.08–1.15) + plagioclase (An35) + biotite + garnet as essentialminerals. The chemical characteristics of the minerals areas follows:

5.1.1. Garnet

The chemical composition of the garnet crystals ishomogeneous and close to Alm73.0–Prp17.5–Grs7.5–Sps2.0.Such homogeneity is typical of ion diffusion processes athigh temperatures, which erases the earlier compositionalrecord, as well the zoning formed in the progressive

Table 5Mean pressure and temperature according to geothermometer andgeobarometer calibrations defined by several authors

Associations P (kbar) T (�C) Temperaturecalibrations

Barometercalibrations

V-332-2 c 4.1–5.45 690–790 1, 2, 3, 4, 5,6 and 7

16 and 17V-332-3 c 4.55–5.55 665–740V-332-5 c 4.20–5.35 685–762.5V-332-6 c 4.47–5.60 705–774V-332-7 c 4.30–5.60 690–774V-332-9 c 4.85–6.00 735–798V-332-4 c 6.67–8.75 640–770V-332-13 r 7.12–8.70 665–770V-332-15 r 6.82–8.67 635–750

C-32-3 c 6.37–7.25 717–757 1, 4, 5, 6, 7,14 and 15

16 and 17C-32-7 c 6.37–7.27 717–761C-32-8 c 6.67–8.02 745–798C-32-11 c 6.65–8.30 742–857C-32-4 r 5.15–6.57 610–718C-32-16 r 5.34–6.87 618–734C-32-17 r 5.34–6.85 620–734

Gr-15-11 c 4 815 1, 8, 9, 10, and 11Gr-15-11 c 13 870Gr-15-12 c 4 834Gr-15-12 c 13 890Gr-15-1 c 4 805Gr-15-1 c 13 860Gr-15-3 c 4 794Gr-15-3 c 13 850Gr-15-6 c 4 750Gr-15-6 c 13 800

Z-365-1 c 4 795 1, 8, 10, 12 and 13Z-365-1 c 10 818Z-365-3 c 4 798Z-365-3 c 10 820Z-365-4 c 4 802Z-365-4 c 10 825Z-365-6 c 4 793Z-365-6 c 10 815Z-365-11 c 4 800Z-365-11 c 10 823Z-365-12 c 4 797Z-365-12 c 10 820

Calibrations: (1) Hodges and Spear (1982), (2) Pigage and Greenwood(1982), (3) Williams and Grambling (1990), (4) Sen and Bhattacharya(1984), (5) Bhattacharya et al. (1991), (6) Lavrent’eva and Perchuk (1990),(7) Lal (1993), (8) Thompson (1976), (9) Ganguly and Saxena (1984), (10)Perchuk and Lavrent’eva (1983), (11) Indares and Martignole (1985), (12)Lavrent’eva and Perchuk (1981), (13) Holdaway and Lee (1977), (14)Perchuk et al. (1985), (15) Dasgupta et al. (1991), (16) Bhattacharya et al.(1991), and (17) Newton and Perkins (1982). n, cores, r, rim.


metamorphic events. In contact with biotite, the garnetcomposition changes to Alm72.0–Prp14.0–Grs12.0–Sps2.0.The slight increase in the Fe/(Fe + Mg) ratio and grossularenrichment indicate chemical reequilibrium under restrict-ed cooling and at constant or increasing pressure (Figs. 4and 6).

5.1.2. OrthopyroxeneThe average composition, Fs57.0–En42.0–Wo1.0, indicates

that the orthopyroxene is a ferrossilite. No indication of

clear chemical zoning is observed, but close to garnet crys-tals, it exhibits a slight Fe enrichment.

5.1.3. Plagioclase

An andesine (Ab66.6An32.8Or0.6), it also occurs asmyrmekite.

5.1.4. Biotite

The composition ranges from K1.7(Mg2.5Fe2.3)Si5.5Al2.5

O22(OH,F,Cl)0.6 to K1.8(Mg2.6Fe2.5)Si5.6Al2.5O22(OH,F,Cl)0.4, and Fe numbers range from 0.48 to 0.50. Biotiteincluded in garnet shows the highest Ti contents (0.7a.p.f.u.), and crystals in contact or near garnet exhibitlower Ti contents (0.52 a.p.f.u.). This chemical variationsuggests that the inclusions crystallized at higher tempera-tures than those of the biotite of the matrix. The retrogrademetamorphism did not affect the composition of the inclu-sions, because they were ‘‘isolated’’ from the reactants bythe involving garnet (Guidotti, 1984; Spear et al., 1990).In contrast, metamorphic fluids favored partial reequilibra-tion between garnet rims and biotite crystals of the matrixduring the retrograde metamorphism.

5.2. Las Margaritas migmatites

5.2.1. Enderbite (Sample C-32)

This sample is mainly composed of plagioclase(An45) + quartz + biotite ± orthopyroxene (XFe = 1.06–1.10) ± garnet ± amphibole ± K-feldspar. The granuliticfacies paragenesis is partially replaced by biotite and pla-gioclase in the overprinted anastomosed Sn+1 foliation.The chemical compositions and mineral characteristicsare as follows:

Garnet. The average composition is Alm61.1Prp19.0

Grs15.5Sps5.2. The core and intermediate parts of the crys-tals exhibit a similar composition, close to Alm61.5Prp18.8

Grs14. 2Sps5.3. The rim, especially when in contact with bio-tite, shows a slight chemical zoning with a relative increaseof the Fe/(Fe + Mg) ratio and grossular contents anddecrease of pyrope contents, with compositions close toAlm62.0Prp19.2Grs13.8Sps5.1 (Figs. 5 and 7).

Orthopyroxene. The composition is of a ferrosilite withFs56.1En42.1Wo1.8. The association orthopyroxene–clino-pyroxene–garnet–plagioclase–quartz suggests that ortho-pyroxene crystallized as a result of hornblendedehydration, according to reaction (1):

hornblendeþ quartz ¼ garnetþ orthopyroxene

þ clinopyroxene

þ plagioclaseþH2O ð1Þ

(Spear, 1995). Crystals of orthopyroxene are usuallysheared, fractured, and partially replaced by amphibole,chlorite, and carbonate.

Amphibole. The composition is Na0.4Ca1.8(Fe2.2Al0.5)-Si6.2Al1.8O22.0, typical of the ferropargasite. The Fe/(Mg + Fe) ratio ranges from 0.51 to 0.57 and the Si content

Fig. 3. Photomicrographs of Las Margarites migmatites. (a) Leucosome of migmatite Z-365 showing K-feldspar (Kfs) inverted to microcline withsillimanite inclusions (Sil); (b) mesosome of migmatite Gr-15 with garnet (Grt), microcline (Mc), and quartz (Qtz); (c) Sn+1 foliation cutting the mesosomeof the migmatite Gr-15, with associated sillimanite (Sil) and biotite (Bt); and (d) detail of symplectite of biotite (Bt) and plagioclase (Pl) formed bybreakdown of garnet (Grt) and K-feldspar (Mc) of leucosome of migmatite Gr-15. Transmitted light and crossed polarizers.

1 2 3 4 5

Analyses

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fe/10

MgMn

Ca

Cat

ions

per

form

ula

unit

3

Bt

0.5 mm

1

2

4

5

Grt

Bt

Opx

a

3

bFig. 4. (a) Texture of garnet (Grt) in paragenesis with biotite (Bt) and orthopyroxene (Opx) of garnet-bearing charnockitic gneiss V-332 of Vergelgranulites (transmitted light, crossed polarizers). (b) Representative compositional profile of the garnet crystal in (a).


from 6.15 to 6.28 a.p.f.u. The amphibole crystallized dur-ing the retrograde metamorphism, according to reaction(2):

garnetþ orthopyroxeneþ plagioclaseþH2O

¼ hornblendeþ quartz ð2Þ(Spear, 1995).

Plagioclase. The average composition Ab60.6An37.7Or1.7

is close to that of andesine. Two plagioclase crystallizationevents could be associated with distinct microstructuraldomains. The older crystals occur in nonfoliated zones,show granoblastic texture, and are associated with pyrox-enes of granulite facies. The younger crystals are present

in Sn+1 metamorphic foliation domains generated by theretrograde metamorphic event of amphibolite facies.

Biotite. The composition ranges from K1.8(Mg2.4Fe2.5)Si5.4Al2.6O22(OH,F, Cl)0.4 to K1.8(Mg2.2Fe2.8)Si5.5Al2.5

O22(OH,F,Cl)0.3, and the Fe number varies from 0.51 to0.54. The coarser biotite inclusions in garnet have Ti con-tents of approximately 0.67 a.p.f.u., whereas biotite ofthe matrix and the fine-grained inclusions close to the gar-net rims have approximately 0.53 a.p.f.u.. The higher Ticontents reflect higher-temperature chemical compositionspreserved in coarser biotite inclusions. Reequilibration atlower temperatures occurred during the development ofthe foliation Sn+1.


5.2.2. Leucosome (Sample Gr-15)

It contains as essential assemblage quartz + plagioclase(An25) + microcline + biotite + garnet. The chemical com-positions and mineral characteristics are as follows:

Garnet. The average composition of the crystal cores isAlm72.1Prp13.2Grs10.5Sps4.1Uv0.1. The composition of therims in contact with biotite is Alm76.9Prp6.4Grs6.5

Sps10.2Uv0.1. Quartz inclusions are abundant in thepoikiloblastic garnets. Commonly, garnet rims are replacedby biotite–plagioclase symplectites formed under retro-grade metamorphic conditions.

Plagioclase. The chemical composition ranges fromAb60.9An23.4Or15.7 to Ab72.1An27.2Or0.7. The crystals ofthe matrix and those located close to the garnet crystalsexhibit the highest anorthite contents. Lower anorthitecontents are observed in the plagioclase–biotite inter-growths and moats around the garnets. These textures indi-cate that more albitic plagioclase crystallized during theretrograde metamorphism.

Biotite. It occurs as symplectitic intergrowths withplagioclase and disseminated in the matrix. The composi-tions are K1.8(Mg1.9Fe3.1)Si5.4Al3.3O22(F,Cl)0.8 andK1.9(Mg1.8Fe3.1)Si5.4Al3.3O22(F,Cl)0.6, respectively. The Ticontent of the biotite from the matrix is 0.21 a.p.f.u. andranges from 0.15 to 0.27 a.p.f.u. in the symplectites. Thelower values suggest that biotite was intensively recrystal-lized during the retrograde metamorphism.

5.2.3. Migmatite (Sample Z-365)

The mesosome contains quartz + plagioclase + K-feld-spar + biotite + sillimanite ± garnet ± sillimanite; thedeformed leucosome (L) quartz + K-feldspar + plagio-clase + biotite ± garnet ± sillimanite, and the melanosomeis biotite rich. The chemical compositions and mineralcharacteristics are as follows:

Garnet. In the leucosome (L), the core and intermediateparts of the crystals have a similar chemical composition,close to Alm70.7Prp24.3Grs3.0Sps2.0. These garnet crystalsshow high Mg contents and formed as a result of the dehy-dration melting reaction that consumed biotite and took

a

Fig. 5. (a) Texture of garnet (Grt) in paragenesis with orthopyroxene (Opx) anlight, crossed polarizers). (b) Representative compositional profile of the garn

place within the leucocratic portion. The garnet crystalsin contact with biotite show an increase in almandine anddecrease in pyrope contents from core to rim, without sig-nificant changes in spessartite and grossular contents. Thischemical pattern is due to Fe-Mg exchange between biotiteand garnet during the retrograde metamorphic cooling.

Plagioclase. The chemical composition ranges fromAb74.1An24.6Or1.5 to Ab74.9An24.6Or0.5. A slightly higheralbite content is obtained close to K-feldspar.

Biotite. Disseminated biotite associated with sillimanite,garnet, and K-feldspar of the mesosome has Ti contentsranging from 0.46 to 0.59 a.p.f.u. Biotite in contact withgarnet exhibits Ti contents of 0.28 to 0.31 a.p.f.u., reflectingthe retrograde metamorphic equilibrium proposed by Hen-ry and Guidotti (2002) and Henry et al. (2005).

6. Geothermobarometry

At metamorphic peak conditions, the chemical zonationof the garnet crystals formed during the progressive meta-morphic event was homogenized. However, retrogrademetamorphic reequilibrium is observed at the rims of thesegarnet crystals. Consequently, the composition of theirnuclei, as well as the core of other minerals present in thesame microstructural domains and the mineral inclusions,could record the P–T conditions of, or close to, the meta-morphic peak. These P–T conditions were estimated onthe basis of Fe-Mg exchanges and net transfer reactionsbetween garnet–orthopyroxene–plagioclase–biotite–quartzin the granulites. The temperatures were obtained usingthe orthopyroxene–biotite, orthopyroxene–garnet, andgarnet–biotite pairs. The pressures were estimated fromthe reactions between garnet–quartz–orthopyroxene–pla-gioclase and garnet–biotite–quartz–orthopyroxene–plagio-clase minerals (Table 5, Figs. 6 and 7).

The geothermobarometric calculations with the core ofmineral of garnet-bearing charnockitic gneiss V-332 inTWQ indicate that these rocks were formed between5.3–6.2 kbar and 700–780 �C and 6.2–7.2 kbar and685–740 �C at the rims (Fig. 6). The calibrations for

b

d biotite (Bt) of enderbite C-32 of Las Margarites migmatites (transmittedet crystal in (a).

0

1

2

3

4

5

6

7

8

9

10

350 400 450 500 550 600 650 700 750 800

Temperature (ºC)

Pre

ssur

e (k

bar)

SiI

And

Ky

Rims of mineralsCores of minerals

Fig. 6. Counterclockwise path for the charnockite granulite V-332 (Vergel granulites). The polygons include the intercepts of reactions betweenplagioclase–orthopyroxene–garnet–biotite–quartz. The black arrows indicate the metamorphic path defined by the mineral compositions from the core torim. Andalusite (And)–kyanite (Ky)–sillimanite (Sil) invariant point according Holdaway (1971). The chemical reactions used in the TWQ are asfollows: 2Alm + Grs + 3aQtz = 6Fs + 3An, 3En + Ann = 3Fs + Phl, 3En + Alm = 3Fs + Prp, 2Alm + Grs + 2Phl + 3aQtz = 6En + 3An + 2Ann,Grs + 2Prp + 3aQtz = 6En + 3An, and Alm + Phl = Prp + Ann. The thermodynamic data used for minerals come from Berman (1988) and activitiesfor the following minerals (from): plagioclase (Fuhrman and Lindsley, 1988), garnet (Berman et al., 1995), biotite, and orthopyroxene (Berman andAranovich, 1996).


garnet–biotite and garnet–orthopyroxene of Hodges andSpear (1982), Pigage and Greenwood (1982), Williamsand Grambling (1990), Sen and Bhattacharya (1984), Bhat-tacharya et al. (1991), Lavrent’eva and Perchuk (1990), andLal (1993) and of garnet–plagioclase–orthopyroxene–quartz and garnet–plagioclase–clinopyroxene–quartz ofNewton and Perkins (1982) and Bhattacharya et al.(1991) result in slightly higher pressure and temperaturefor this rock (Table 5).

Similarly, the geothermobarometric calculations withthe cores of minerals of enderbite C-32 in TWQ indicatethat the rocks were formed between 8.0–8.8 kbar and760–820 �C and 6.6–9.0 kbar and 680–755 �C at the rims(Fig. 7). The calibrations for garnet–biotite and garnet–orthopyroxene of Hodges and Spear (1982), Perchuket al. (1985), Dasgupta et al. (1991), Sen and Bhattacharya(1984), Bhattacharya et al. (1991), Lavrent’eva and Per-chuk (1990), and Lal (1993), as well as the calibrationsfor garnet–plagioclase–orthopyroxene–quartz and garnet–plagioclase–clinopyroxene–quartz of Newton and Perkins(1982) and Bhattacharya et al. (1991), show slightly higherpressure and temperature for this rock (Table 5).

The geothermobarometric calculations from the meso-some of migmatite Z-365 in TWQ were determined usingthe garnet–biotite geothermometer and the melting reac-tion phlogopite + quartz + sillimanite = pyrope + K-feld-spar + H2O. The pressures were calculated by chemicalreactions involving plagioclase–garnet–sillimanite–quartz.These geothermobarometers indicate that the rocks

formed between 780–826 �C and 6.3–8.0 kbar. A pressurehigher than 4 kbar for this mesosome is confirmed by thecrystallization of sillimanite + biotite + muscovite underretrograde metamorphic conditions (Fig. 7). The calibra-tions for garnet–biotite of Thompson (1976), Hodgesand Spear (1982), Perchuk and Lavrent’eva (1983), Lav-rent’eva and Perchuk (1981), and Holdaway and Lee(1977) confirm the values obtained from the TWQ for thisrock (Table 5).

The geothermobarometric calculations from the symp-lectitic biotite and reabsorbed garnet rims of the leucosomeGr-15 in TWQ indicate that they reached reequilibriumtemperatures between 625 and 640 �C (Fig. 7). The calibra-tions for garnet–biotite of Thompson (1976), Hodges andSpear (1982), Ganguly and Saxena (1984), Perchuk andLavrent’eva (1983), and Indares and Martignole (1985)resulted in higher temperatures for this rock (Table 5).

7. P–T trajectories

In rocks from the amphibolite–granulite transition, theearly progressive metamorphic history is usually reset dueto high ionic diffusion rates in and between minerals at hightemperatures, which precludes the preservation of a com-positional zoning that could reveal the complete metamor-phic history. As a consequence, the P–T trajectories ofhigh-grade metamorphic rocks commonly define retro-grade metamorphic events only (Harley, 1989; Spear,1995).

0

1

2

3

4

5

6

7

8

9

450 500 550 600 650 700 750 800 850 900 950

1

3

Ky

And

SiI

A

BC

P-T conditions of cores of minerals

P-T conditions of re-equilibrated rims of mineral AB

Enderbite C -32 Mesosome of migmatite Z-365

Main equilibrium between Grt-Sil-Bt-Kfs-Qtz

Grt-Bt geothermometerreactions curves

Leocosome Gr-15

CGrt-Bt reaction curves of symplectites

Temperature (ºC)

10

Pre

ssur

e (k

bar)

2

Fig. 7. ITD path for Las Margaritas migmatites. Reactions in the P–T grid: (1) melting–dehydration of biotite + plagioclase + sillimanite + quart = gar-net + K- feldspar + melt according to Le Breton and Thompson (1988); (2) muscovite + albite + quartz = K-feldspar + sillimanite + melt, according Peto(1976) in Le Breton and Thompson (1988); and (3) granite crystallization curve. Andalusite (And)–kyanite (Ky)–sillimanite (Sil) invariant point accordingHoldaway (1971). The chemical reactions used in the TWQ are as follows: 2Alm + Grs + 3aQtz = 6Fs + 3An, 3En + Ann = 3Fs + Phl,3En + Alm = 3Fs + Prp, 2Alm + Grs + 2Phl + 3aQtz = 6En + 3An + 2Ann, Grs + 2Prp + 3aQtz = 6En + 3An, and Alm + Phl = Prp + Ann. Thethermodynamic data used for minerals are from Berman (1988) and activities for the following minerals (from): plagioclase (Fuhrman and Lindsley,1988), garnet (Berman et al., 1995), biotite, and orthopyroxene (Berman and Aranovich, 1996).


The geothermobarometric calculations obtained for thecharnockitic gneiss from Vergel granulites define a retro-grade segment of a counterclockwise path in the P–T space(Fig. 6). This trajectory, characterized by the rise in pres-sure with almost constant temperature, is well indicatedby the compositional changes in biotite and garnet in con-tact, reflecting the reequilibrium under low water contentmetamorphic fluid. This P–T evolution could be responsi-ble for the textures of cordierite + orthopyroxene + mag-netite ± spinel around garnet crystals described byKroonemberg (1982).

Even though the prograde history of the metamorphismwas not revealed by the geothermobarometry of the LasMargaritas migmatites, a nearly ITD path was obtained,indicating a retrograde segment of a metamorphic trajecto-ry from granulite to amphibolite facies (Fig. 7). Such a tra-jectory can be envisaged with an earlier clockwise loop, in

which the baric peak preceded the thermal peak (Jimenez-Mejıa, 2003).

The trajectory in the upper part is defined by theequilibrium and reequilibrium conditions in the P–T

space of sample C-32. The lowest temperature obtainedfor samples Gr-15 and Z-365 defines the lower segmentof the metamorphic trajectory. The assemblage biotite-sillimanite-muscovite developed in association with theSn+1 foliation. The ITD path intercepts the reactionmuscovite + albite + quartz = K-feldspar + sillimanite +melt at a pressure higher than 4 kbar (Le Breton andThompson, 1988). This path is confirmed by texturalevidence, such as the generation of biotite–plagioclasesymplectites crystallized from garnet in gneisses andplagioclase moats in felsic granulites. Both texturesreflect cooling accompanied by a possible pressuredrop.


8. Discussion

The P–T–t path of Vergel granulites is counterclockwiseand, despite relatively small differences between the peakand cooling temperatures, is similar to an IBC path(Fig. 6). For the Las Margarita migmatites, the P–T–t pathis a clockwise and nearly ITD (Fig. 7). These two types oftrajectories have been defined for granulitic terranes allaround the world (Bohlen, 1987, 1991; Harley, 1989).

The IBC path of the Vergel granulites may be the recordof a thermally disturbed crust by magma intrusions and amagmatic arc environment, which could be followed by acollisional event that allowed thrusting of a lithosphericlower block (Brown, 1993; Spear et al., 1990; Spear, 1995).

The ITD path of Las Margaritas migmatites could beinterpreted as the result of continental collision, with tec-tonic thickening of the crust followed by the recompositionof the thermal perturbations toward a steady state, thenexhumation (Bohlen, 1991; Brown, 1993; Spear, 1995).

The link of geological and metamorphic P–T evidenceand the dating of the main events of the Garzon Massif(Jimenez-Mejıa, 2003; Cordani et al., in press) leads tothe reconstruction of the tectonic evolution of this impor-tant Proterozoic crustal portion of the Colombian Andes(Table 6).

The dynamic evolution of the Garzon Massif includesthe deposition of metavolcano-sedimentary sequences inan evolved continental domain with a passive margindeveloped along the Amazonian Craton (Cordani et al.,in press). This framework includes the onset of a subduc-tion process, the installation of a continental magmaticarc, and the development of an orogenic belt.

The metamorphic path observed in the Garzon Massifcould be explained by progressive metamorphism reachingthe amphibolite–granulite transition facies, probablyassociated with subduction processes along a continentalmargin with the development of a magmatic arc(Guapoton–Mancagua gneiss protoliths) at about

Table 6Tectono-magmatic events of Garzon Massif

Age (Ma) Garzon Complex

Guapoton–Mancagua gneiss Vergel granulites

800 Post-tectonic pegmCooling (Ar–Ar BCooling of collisioWR–Grt)

1000 Tectonic deformation (U–Pb SHRIMPZrn) (counter clockwise P–T path, LP)

Collisional metamZrn)

1200 Magmatic arc (U–Pb SHRIMP Zrn) Heritage (U–Pb S

1400 Heritage (U–Pb S

LP, lower plate; UP, upper plate; WR, whole rock.

1130 Ma (U–Pb zircon age). This regional metamorphism,exhibited by the Las Margarita migmatites (eastern oro-gen), occurred under high temperatures (up to 780 �C)and medium pressures (8 kbar) at a minimum age ofapproximately 1034 Ma (Sm–Nd age).

The metamorphic evolution of the orogen followed anearly ITD trajectory. During the early exhumation stages,the paragneisses were affected by partial melting, with abreakdown of biotite and subsequent crystallization of gar-net in the leucosome (L). The anatexis event, dated between1034 and 1015 Ma, culminated with the injection of ayounger granitic leucosome (L2) in gneisses and migma-tites, possibly generated by a higher rate of melting in deep-er rocks, and the recurrence of compressive stress thatallowed the migration of the melt.

The subduction process led to the closing of the precur-sor ocean and was followed by a continental collisionaround 1000 Ma. The exhumation and cooling of the oro-gen occurred at approximately 967 Ma, as defined by bio-tite Ar–Ar geochronology.

Fingerprints of this orogenic convergence are preservedinside the orogen by the development of an importantdeformational phase affecting the Guapoton–Mancaguagneiss (U–Pb metamorphic zircon age of �1000 Ma). Inaddition, as a response to the compressive stress phase,crustal thrusting developed inside the orogen, enablingthe formation of ductile shear zones in high-grade meta-morphic rocks. The thrusting produced the uprising ofthe Margarita migmatites over the Vergel granulites andfavored the decompression of the former. This decompres-sion caused melting in the roots of the orogen.

The vergence of the main thrusting was to the west.However, the tectonic zones formed at that time were reac-tivated during the Andean tectonics, in the Neogen, withthe opposite vergence.

The isotopic history of the Vergel granulites includeshigh-grade metamorphism at 1000 Ma (U–Pb zircon) andsubsequent cooling at 920 Ma (biotite Ar–Ar age). The

Las Margaritas migmatites

atitic phase (K–Ar Hbl) Cooling and thrusting (Ar–Ar Bt)t)nal metamorphism (Sm–Nd

orphism (U–Pb SHRIMP Cooling (Ar–Ar Bt and Hbl)Granitic magmatism (U–Pb SHRIMPZrn)Anatexis and deformation (clockwiseITD path, UP)

HRIMP Zrn) High-grade metamorphism (Sm–NdWR–Grt)

HRIMP Zrn)


counterclockwise path of these rocks is probably the resultof lower-block evolution, characterized by an increase ofpressure generated by magma intrusion within the crust,which resulted in crustal growth and thickening.

The diachronous thermal history of the Vergel granu-lites and Margarita migmatites points to different coolingrates for the tectonic blocks, with cooling ages of 920and 967 Ma, respectively. These data support an evolutioncharacterized by movement of blocks during theNeoproterozoic.

Finally, the study of the metamorphic conditions, thetrajectories in the P–T field, and the isotopic record inthe rocks of the Garzon Massif indicate that the geologicalevolution of this area is much more complex than previous-ly believed. As suggested by Cordani et al. (in press), theGarzon Massif, together with a few other Andean base-ment inliers, took part in the agglutination of the Rodiniasupercontinent (Hoffman, 1991), as revealed by their com-mon Grenvillian collisional history.

Acknowledgements

The authors acknowledge the support received by theBrazilian Ministry of Science and Technology (PRONEX41.96.0899.00). They are grateful to Juan E. Otamendiand Roberto D. Martino, who improved the manuscriptwith their constructive reviews.

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