Journal of Petrology | Volume 38 | Number 1 | Pages 65-83 | 1997
© Oxford University Press 1997
High-Pressure (
2000 MPa) Kyanite- and Glaucophane-bearing Pelitic Schist and Eclogite from Cordillera de la Costa Belt, Venezuela
Department of Geology and Geophysics, Rice University MS-126, Houston, TX 77005-1892, USA
Received April 22, 1996; Revised typescript accepted August 5, 1996
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ABSTRACT |
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 TOP ABSTRACT IntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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Cretaceous melange of the Cordillera de la Costa belt, north–central Venezuela, there are knockers of eclogite, barroisite-bearing eclogite, and pelitic glaucophane schist. These occur in a metamorphic melange matrix that locally consists of marble, serpentinite, amphibolite, actinolite schist, feldspathic schist and gneiss, graphitic schist, chloritoid schist, and garnet-bearing mica schist. The protoliths for these various rock types exhibit a wide age range (Cambrian to Early Cretaceous?). Recently discovered knockers of pelitic glaucophane schist contain Mg-glaucophane + paragonite + kyanite + garnet + talc + graphite + rutile + quartz. The coexistence of kyanite and Mg-glaucophane suggests minimum P
2000 MPa at T > 600°C. Eclogite knockers from the same outcrop contain garnet and clinopyroxene which yield 500°C for cores, 700°C for rims, and P 
1200 MPa. The assemblage garnet–biotite–phengite–albite within schists of the melange matrix of this locality indicates metamorphic conditions of T = 450–520°C at P = 1800 MPa. Because all lithologies in this outcrop record high-P conditions, this metamorphic melange formed before or during peak metamorphism in a mid-Cretaceous subduction zone. KEY WORDS: geothermobarometry; high-P pelitic schist; eclogite; Puerto Cabello; Venezuela
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Introduction |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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Two belts of high-pressure (HP)–low-temperature (LT) metamorphic rocks are exposed in the Caribbean Mountain system of northern Venezuela, which is part of the complex east–west-trending boundary zone between the Caribbean and South American plates (e.g. Menéndez, 1966
, 1967). These two belts are the Cordillera de la Costa and Villa de Cura belts, both of which were metamorphosed during the mid-Cretaceous, presumably in a subduction zone related to the mid-Cretaceous Leeward Antilles volcanic arc (e.g. Pindell, 1993). From Jurassic to Eocene time North and South America were diverging and new proto-Caribbean sea-floor was created between them (e.g. Pindell, 1993). Therefore, the Leeward Antilles arc, as well as the entire Caribbean Mountain system, are allochthonous and formed far to the west as part of the Farallon–North/South American plate boundary zone (e.g. Pindell, 1993).
Although the two HP–LT belts have similar metamorphic ages, they differ greatly in lithology and metamorphic history. The Cordillera de la Costa belt consists of oceanic and passive continental-margin rocks intermixed with Paleozoic granites and granitic gneisses. In this belt, the eclogites and blueschists were retrograded to epidote–amphibolite- and greenschist-facies assemblages following an apparent P–T path typical for ‘Alpine-type’ subduction zones involving continental collision (Ernst, 1988). In contrast, the rocks of the Villa de Cura belt have only oceanic protoliths and were transformed to a coherent blueschist belt (Shagam, 1960; Navarro, 1983; Smith, 1996). They show little effect of retrogression. The P–T path is typical of ‘Franciscan-type’ inter-oceanic subduction zones (Ernst, 1988). The apparent contrasts in P–T history suggest that the two belts were metamorphosed in association with different parts of the convergent (Caribbean–South American) margin.
This study presents new petrologic and geothermobarometric data for rocks of the Cordillera de la Costa belt. We focus on eclogite and kyanite- and glaucophane-bearing pelitic schist knockers near Puerto Cabello to estimate the maximum P–T conditions for metamorphism during Cretaceous subduction.
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Cordillera de la Costa Belt |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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Prior studies of the metamorphic conditions for the high-pressure rocks in the Cordillera de la Costa belt have been mainly on Isla de Margarita (Blackburn & Navarro, 1977
; Maresch & Abraham, 1977, 1981; Navarro, 1981; Mottana et al., 1985; Chevallier, 1987; Guth & Avé Lallemant, 1990; Krückhans-Lueder & Maresch, 1992; Bocchio et al., 1996). However, the field relations and petrography of many eclogite occurrences near Puerto Cabello were described by Morgan (1967, 1970). Most of the eclogite localities studied here were previously identified by Morgan (1967), except for a few new localities to the east and north of Puerto Cabello (Fig. 1). Some of Morgan's locations could not be relocated, as a result of extreme tropical weathering and encroachment of civilization in the intervening years.
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The Cordillera de la Costa belt consists of three protolith associations: (1) one of oceanic affinities; (2) metasedimentary rocks derived from passive continental-margin sediments; (3) coarse-grained augen gneisses and granites. Although all these rocks are intimately intermixed, eclogite-bearing units are found only in a belt along the coast, whereas eclogite-free schists, gneisses and marbles occur in the south (Fig. 1).
The oceanic association consists of serpentinite, leucotonalitic gneiss, amphibolite, and actinolite-, garnet-, mica-, and chlorite-bearing schists, along with eclogite, barroisite-bearing eclogite, and blueschist knockers (Morgan, 1970; Avé Lallemant & Sisson, 1993; Ertan et al., 1995). These rocks were retrograded at epidote–amphibolite- and greenschist-facies conditions. Eclogite and barroisite-bearing eclogite of Puerto Cabello, Venezuela, are basaltic rocks metamorphosed at T = 525 ± 50°C, and P = 700–1200 MPa (Morgan, 1970; Sisson & Avé Lallemant, 1992). These conditions were probably attained during subduction of the proto-Caribbean oceanic lithosphere under the Caribbean (Farallon) plate (Pindell et al., 1988; Avé Lallemant & Guth, 1990). Thus, this association is allochthonous and has probably traveled very far. There are no available geochronologic data for minerals with a high closure temperature to constrain the timing of metamorphic events in this coastal belt.
The metasedimentary association consists of graphite–garnet–mica schist, quartzite, marble, and quartzo-feldspathic gneiss. These rocks may have been deposited on the northern passive continental margin of South America (e.g. Bellizzia, 1986; Burke, 1988). Although they are also allochthonous, they may not have traveled as far as the oceanic rocks.
Several large bodies of granite and augen gneiss (from tens of meters to kilometers in length) make up the third protolith association. Thin trondjhemitic dikes intrude the melange matrix near several eclogite localities. All these rocks have been dated by U/Pb zircon geochronology (Avé Lallemant & Sisson, 1993, and unpublished results, 1995) and yield Early Paleozoic ages. These rocks may be fragments of a Late Precambrian–Early Paleozoic orogenic belt which wraps around the northwestern and northern margins of the South American craton (e.g. Bartok, 1993), upon which the sediments of the passive margin association were deposited (e.g. González de Juana et al., 1980).
The rocks in the Cordillera de la Costa belt were strongly affected by five synmetamorphic (D1a to D1e) and two postmetamorphic (D2a and D2b) phases of deformation. The synmetamorphic phases occurred at sequentially shallower and cooler conditions following a clockwise exhumation path typical of ‘Alpine-type’ subduction zones (Ernst, 1988; Avé Lallemant & Sisson, 1993). The first occurred at eclogite-, and the second at blueschist-facies conditions. Because eclogite and blueschist occur only as knockers, no regional kinematic interpretation of these structures can be given. The others (D1c to D1e) occurred at epidote–amphibolite and greenschist-facies conditions. Kinematic analyses of the synmetamorphic structures indicate that these rocks were deformed by shortening normal to the plate boundary (D1c), dextral simple shear parallel to the plate boundary (D1d) and by extension parallel to the plate boundary (D1e). Generally, D1d structures deform D1c structures and are deformed by D1e; however, locally they can form all at the same time. This kinematic history of the HP–LT rocks is compatible with an origin in an arcuate, right-oblique convergent plate boundary (Avé Lallemant & Guth, 1990; Avé Lallemant & Sisson, 1993). These events took place when the subduction complex was situated far to the west, probably near the northwest corner of Colombia (e.g. Pindell, 1993). The extension parallel to the plate margin (D1e) may have been responsible for part of the uplift and decompression of these rocks (Avé Lallemant & Guth, 1990).
After the D1e phase of deformation, the Cordillera de la Costa belt was passively transported eastward along the south Caribbean plate boundary zone. In Eocene to Miocene time, the entire Caribbean Mountain system was thrust (D2a) southward onto the South American continental margin. Simultaneously, large displacements occurred along dextral east–west-trending strike slip faults (D2b) resulting from the west-northwest to east-southeast convergence of the North and South American plates (Pindell et al., 1988; Pindell, 1993). The D2 event and erosion were responsible for the second stage of uplift and exhumation of the HP–LT metamorphic rocks.
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Petrography and Mineral Chemistry |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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To constrain the metamorphic conditions, we concentrated our efforts on eclogite and glaucophane- and kyanite-bearing pelitic schists (Table 1). Most of the melange matrix schists do not contain suitable mineral assemblages for detailed geothermobarometric studies. This study does not thoroughly examine the subsequent retrogression, which locally obliterates the original assemblages.
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Eclogite
Eclogite and barroisite-bearing eclogite occur as centimeter- to meter-size knockers within pelitic and calcareous schists. We are using the term barroisite-bearing eclogite for eclogites with <10 modal % amphibole. Morgan (1970)
defined these as eclogite–amphibolites. As noted by Morgan (1967, 1970), there are only a few true eclogite knockers and the majority of the mafic knockers are the barroisite-bearing ones. The composition of the amphibole in these varies with both the protolith composition and the degree of retrogression. Typically, the long axes of the knockers are parallel to schistosity of the enclosing rocks (Fig. 2). Most have tholeiitic protoliths (Morgan, 1967). Now, they consist primarily of omphacite and garnet with minor phengite, glaucophane, quartz, rutile, titanite, and opaque minerals, as well as secondary calciferous amphibole (barroisite, hornblende or winchite). Primary plagioclase is absent. The eclogite contains garnet porphyroblasts (some as flattened idioblasts; Fig. 3a) which show little or no alteration, range up to 1.5 mm in diameter and contain abundant quartz, rutile (not oriented), sodic amphibole and/or zoisite inclusions. Some of the quartz inclusions in garnet and clinopyroxene are surrounded by radial cracks (Fig. 3b). The quartz inclusions are generally a single grain and lack features such as fine polycrystalline grains with subparallel subgrain boundaries and truncation of sets of subgrains which are typical of quartz pseudomorphs after coesite (e.g. Chopin, 1984; Chopin & Sobolev, 1995; Hacker & Peacock, 1995). Garnet in the barroisite-bearing eclogite knockers is up to 3 mm in diameter and contains only epidote and titanite as inclusions. One sample has a hornblende–plagioclase symplectite around garnet (91-45). The amount of secondary amphibole and/or chlorite varies from sample to sample. In eclogites which show high degrees of retrogression, their metamorphic foliation is well developed. In one sample (91-44a), eclogite minerals are surrounded by mats of secondary minerals consisting of acicular actinolite, plagioclase (An7), clinozoisite, garnet relics, quartz, rutile, and titanite. Often, green omphacite is replaced by glaucophane, barroisite, or actinolite. Small clusters of rutile (oriented parallel to the schistosity, where present) and opaque minerals (pyrite, magnetite, hematite) are common. In some samples (92-39b and 91-25), fine-grained talc occurs in the matrix and as larger flakes within or around garnet and amphibole.
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Kyanite–paragonite–garnet–glaucophane schist
Glaucophane-bearing pelitic schist occurs as a string of elongated knockers adjacent to eclogite knockers exposed in a road cut along the highway between Puerto Cabello and Valencia (Fig. 2). Both rock types are hosted by calcareous pelitic schists. The glaucophane-bearing pelitic schist has the assemblage kyanite, paragonite, colorless glaucophane, quartz, and garnet porphyroblasts that range up to 2 cm. Common accessory minerals are rutile, zircon, and allanite. The garnet contains inclusions of graphite, rutile, and mica (Fig. 3c and d). Some garnets have pressure shadows filled with quartz. Elongated, hypidioblastic or xenoblastic kyanite crystals have inclusions of graphite, quartz, glaucophane, rutile, and zircon. Jadeite, omphacite, and coesite are lacking, as well as relics of these phases. Talc is present as small inclusions within kyanite, and is associated with paragonite and phengitic white mica in the matrix.
Host schists
As mentioned above, the host melange matrix for the eclogite and kyanite-bearing pelitic knockers has a wide variety of protoliths. However, most of the eclogite knockers occur in the pelitic and feldspathic schists. The host pelitic and feldspathic schists have variable proportions of muscovite, dolomite, calcite, garnet, plagioclase, and quartz; some layers are biotite bearing (Table 1). Most of the feldspathic schists lack biotite and have phengite as their sole micaceous phase. They also lack other critical alumino-silicates, presumably because their protolith is not sufficiently aluminous. Many of the host schists have abundant graphite, rutile as well as other accessory minerals which include titanite, ilmenite, pyrite and chalcopyrite. In some pelitic layers, graphite is concentrated in 0.5–3 cm long ‘fish-like’ structures with up to 50 modal % C. Adjacent to the kyanite–glaucophane-bearing pelitic and eclogite knockers, the host calcareous pelitic schist has small garnets, only reaching 0.5 cm in diameter. Elsewhere, garnet in the host schist ranges between 0.1 and 1 cm in diameter. One pelitic schist outcrop near eclogite knockers in Choroní has the assemblage chloritoid–paragonite–garnet–quartz–muscovite–chlorite. The chloritoid is found as elongated porphyroblasts, up to 0.5 mm across, oriented parallel to schistosity. Garnet porphyroblasts contain chloritoid inclusions with the same compositions as the matrix chloritoid. Titanite is present as large crystals (2 mm) in the matrix.
Analytical procedures
Minerals were analyzed at Rice University on a Cameca SX-50 electron microprobe. Operating conditions were an accelerating potential of 15 kV and sample current of 15 nA, and counting times varied from 10 to 20 s. The beam diameter was set at 1 or 5 µm. Natural or synthetic mineral and oxide standards were employed. Most analyses represent averages of three or more individual spot analyses from homogeneous minerals. After data collection, analyses with poor totals or unacceptable stoichiometries were discarded. Compositions are generally reproducible to better than ±2% for major elements and ±10% for minor elements. Mineral compositions used for geothermobarometry were determined at rim-to-rim contacts for coexisting minerals. Spot-analysis traverses were made to check for zoning in garnet, amphibole, and pyroxene. All corrections were carried out by modified PAP procedures (Pouchou & Pichoir, 1987).
Garnet
Most compositions of garnet (rim and core) in eclogite, barroisite-bearing eclogite, and the host schists are similar to those of group C eclogite, but some fall in the group B field (Table 2; Fig. 4; Coleman et al., 1965). Many garnet grains are zoned, especially those in eclogite knockers (Fig. 4). Pyrope content ranges between 15 and 32% in eclogite and between 15 and 41% in kyanite-bearing schist. The amount of zoning decreases drastically with increasing degree of retrograde overprinting of the eclogite knockers. In samples with abundant greenschist-facies overprinting, composition is often fairly constant across the center of grains, with depletions and enrichments occurring only near the rims. The garnet cores are generally enriched in FeO, CaO, and MnO and depleted in MgO relative to the rims. This pattern is interpreted as evidence for growth as temperature rose.
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Pyroxene
Sodic clinopyroxene commonly occurs as elongated grains aligned along the D1a foliation. Clinopyroxene grains display very low acmite contents, and are therefore omphacite (Table 3). Some grains show very distinct zoning: in these, jadeite contents increase from core (XJd = 0.35) to rim (XJd = 0.50). This may indicate continuous growth during increasing pressure and rising temperature. In retrograded eclogite, omphacite relics are intimately intergrown with sodic amphibole and actinolite; this makes it difficult to obtain good analyses (e.g. 92-30a).
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Amphibole
The structural formulae of amphibole (Table 4) were calculated based on 23 oxygen atoms; Fe2+ and Fe3+ contents were estimated by normalizing total cations – K to 15 for sodic amphiboles, and total cations – (K + Na+Ca) to 13 for calciferous amphiboles (e.g. Tindle & Webb, 1994
). Each rock type has a distinct amphibole composition (Fig. 5). Mg-rich glaucophane with Mg/(Mg + Fe2+) of 0.92 is found only in kyanite schists (91-13e and 94-18a), whereas the sodic amphibole in the eclogite knockers is crossite. Sodic amphibole occurs in retrograded eclogite and in metasomatic rinds around some eclogite knockers. Both actinolite and/or hornblende are found in retrogressed samples. In other retrograde assemblages, chlorite, actinolite, and barroisite are more common than glaucophane.
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Other minerals
Phengite is found in eclogite knockers, whereas muscovite is limited to biotite-bearing schists. Secondary phengite is also found around paragonite. Paragonite occurs only in kyanite-bearing and chloritoid-bearing schist (Fig. 6). Chloritoid was found in a 3 m thick layer of the host schist. There is no compositional difference among chloritoid inclusions in garnet porphyroblasts, chloritoid porphyroblasts or matrix chloritoid; all have Mg/(Mg + Fe) = 0.12 (e.g. 92-47c in Table 5). Talc commonly occurs as large grains in barroisite-bearing eclogites. Their Mg/(Fe + Mg) ratio is
0.93 (Table 5). Finer-grained talc appears to be an alteration product of kyanite. Kyanite occurs only in glaucophane- and paragonite-bearing pelitic schists. It contains 0.20 wt % FeO. Plagioclase is found in a few eclogite knockers (Table 1). In one sample (91-44a), plagioclase (An7) occurs with amphibole in a reaction rim around garnet and pyroxene. Albite with <1% An content occurs in quartzo-feldspathic schist.
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Metamorphic Conditions |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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Several experimentally determined or thermodynamically calculated phase equilibria and geothermobarometers constrain metamorphic P–T conditions. The thermodynamic calculations are based on the data sets of Berman (1988)
and the GEØ-CALC program (Brown et al., 1988). Calculations for the kyanite- and glaucophane-bearing pelitic schists were restricted to Mg-rich systems such as NMASH, NCMASH, and KNMASH. The host schists are intercalated with marbles. They also commonly contain calcite and graphite; thus, CO2 may be a major fluid component.
Phase equilibria for the kyanite- and glaucophane-bearing pelitic schists constrain its peak metamorphic P–T conditions (Fig. 7). The talc + kyanite assemblage has a very large stability field at pressures between 600 and 4000 MPa and temperatures higher than 300°C (Chopin & Schreyer, 1983; Schreyer, 1988). The retrograde talc + phengite assemblage is stable at pressures <1100 MPa above the medium-pressure assemblage of biotite + chlorite (Chopin, 1981; Massonne & Schreyer, 1989). In the NMASH system, glaucophane + kyanite is stable between 2200 and 3100 MPa, as with increasing PH2O it forms from paragonite + talc. The glaucophane-bearing pelitic schists can be described by an NFMASH grid projected from garnet, quartz, and H2O (e.g. Guiraud et al., 1990; Massonne, 1995). Guiraud et al. (1990) subdivided their petrogenetic grid into low- and high-temperature stability fields between 630 and 650°C; the glaucophane + kyanite and paragonite + talc assemblages in the Venezuelan samples fall in their high-temperature stability field. The presence of garnet with glaucophane + kyanite also implies temperatures above 630°C (Massonne, 1995). The assemblage paragonite + quartz defines the upper thermal stablity of 750°C at 2000 MPa for this assemblage (Holland, 1979a). Thus, the glaucophane–kyanite–garnet assemblage probably formed at P <2000 MPa and T <630°C.
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Garnet–pyroxene thermometry
Fe2+–Mg partitioning between coexisting garnet-clinopyroxene rims yields average metamorphic temperature of
600–700°C (Table 6; Ellis & Green, 1979) at reference pressures of 1000–2000 MPa. The largest uncertainties in these values are the result of the estimation of ferric iron in clinopyroxene (e.g. Selverstone et al., 1992). Other calibrations of this geothermometer result in temperatures which are 50°C lower (e.g. Krogh, 1988; Pattison & Newton, 1989). Both garnet and clinopyroxene are zoned, which results in lower temperatures between 450 and 650°C for the core. It is difficult to constrain the temperatures of the cores, as these may not have an equilibrium relationship. There are no clinopyroxene inclusions in the garnets to verify these estimates. We tentatively interpret this as prograde zoning from core to rim, as these values reflect conditions with rising temperature during metamorphism. Each eclogite knocker records a different core and rim temperature (Table 6).
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Jadeite geobarometry
None of the samples investigated contain the requisite minerals for the geobarometer calibrated for the reaction albite = jadeite + quartz (Holland, 1980
). However, calculated minimum pressures are 1500 MPa at 700°C for omphacite with XJd = 0.49. As most of the omphacite in the eclogite knockers displays similar jadeite contents (Table 3), we assume they all were subjected to a similar minimum pressure.
Phengite geothermobarometry
Si content of phengite increases with pressure in phengite + talc + kyanite + quartz–coesite assemblages (Massonne & Schreyer, 1987, 1989). The eclogite knockers (e.g. 91-13a) that occur in the same outcrop as the kyanite- and glaucophane-bearing pelitic schist (e.g. 91-13e and 94-18a) yield phengite contents that correspond to minimum pressures of 1800 MPa at 650°C, which is consistent with the high P–T conditions for the kyanite- and glaucophane-bearing pelitic schist. These samples do not contain coexisting paragonite–muscovite except for samples which have formed a second mica during retrogression. Thus, temperature estimates using the muscovite–paragonite geothermometer of Blencoe et al. (1994) are 300°C for eclogites and the kyanite-bearing pelitic schist, which is only indicative of retrograde P–T conditions. The muscovite component of paragonite may be used to estimate a minimum temperature; this yields a minimum temperature of 410°C for the chloritoid schist (92-47c). This is compatible with the mineral assemblages in the melange matrix.
Hornblende–plagioclase thermometry
One barroisite-bearing eclogite (91-44a) has a retrograde amphibole–plagioclase symplectite around garnet and clinopyroxene. Holland & Blundy (1994) have proposed two calibrations for this assemblage as a geothermometer. Calculated temperatures for the calcic end-member are either 425°C at 1000 MPa or 485°C at 1500 MPa, consistent with their texture, which we associate with retrograde metamorphism during exhumation.
Garnet–biotite thermometry
The garnet–graphite–mica schist that hosts the eclogite knockers locally contains coexisting biotite and garnet. These were used to calculate equilibration temperatures using the calibration of Hodges & Spear (1982). Only two samples contain biotite which was not severely retrograded to chlorite. The garnet–graphite–mica schist (94-13b) within 0.5 m of the glaucophane–kyanite pelitic knockers records peak temperatures of 435°C at pressures of 1500–2000 MPa. This temperature is much lower than the maximum calculated temperatures for the glaucophane–kyanite pelitic schist and eclogite knockers. Another sample (92-32a) of the melange matrix records a temperature of 450°C. These temperatures are consistent with both the minimum temperature for the chloritoid schist and the low-temperature mineral assemblages reported throughout the melange matrix (e.g. Morgan, 1967).
Garnet–muscovite–biotite–plagioclase geothermobarometry
One sample of garnet–graphite–mica schist (92-32a) that contains biotite and phengitic white mica was found 200 m from eclogite knockers (92-34c). Metamorphic P–T conditions calculated from intersections for garnet–biotite and garnet–plagioclase–muscovite–biotite reactions (e.g. Applegate & Hodges, 1994) are T = 450–520°C at 1800 MPa (Fig. 7). However, Applegate & Hodges (1994) used thermodynamic parameters for muscovite from Chatterjee & Flux (1986), which only account for Na–K mixing. Thermodynamic mixing properties for phengite and other components in white mica are poorly known. As discussed by Guidotti et al. (1994), the Fe2+, Fe3+, and Mg contents increase with pressure. As this shifts the Na–K solvus towards a higher K content in muscovite, this may have the effect of underestimating the pressure calculated from the intersection of the two reactions.
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Discussion |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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The glaucophane–kyanite pelitic knockers near Puerto Cabello in the Cordillera de la Costa belt, northern Venezuela, formed at hotter metamorphic conditions (630–700°C,
2000 MPa) than the adjacent quartzo-feldspathic schists (450–520°C, 1800 MPa). Most assemblages of the eclogite knockers contain evidence for T = 650–700°C at P < 1500 MPa. It is unclear if the differences in these P–T estimates represent variable preservation of the peak metamorphic conditions during later retrogression, or mixing of rocks with different P–T histories during decompression. As both the garnet and omphacite zoning patterns indicate growth during rising temperature, we believe that this metamorphic event followed a clockwise path.
The P–T conditions estimated for the glaucophane–kyanite pelitic schist are comparable with those estimated for other localities around the world. For example, kyanite-bearing eclogite and related kyanite gneisses from the Tauern Window in the eastern Alps of Austria (Richter, 1973; Miller, 1977; Holland, 1979a) underwent peak metamorphism at 560–620°C and 1800–2500 MPa (Holland, 1979b, 1983). Both the Tauern Window and Venezuelan rocks contain Mg-rich glaucophane. There are no other reported localities with compositions close to end-member Mg-glaucophane. Kyanite and paragonite also occur in mafic eclogites such as the Tauern Window (Guiraud et al., 1990), Bitlis Massif, Turkey (Okay et al., 1985), central Rhodope, Greece (Liati & Siedel, 1996), and Dabie Shan, China (Okay, 1993, 1995).
The eclogite and barroisite-bearing eclogites of Puerto Cabello, Venezuela, are metamorphosed basaltic rocks (Morgan, 1970; Avé Lallemant & Sisson, 1993) that occur as discontinuous knockers. It is unclear from field relations whether they are dismembered pillow lavas (e.g. Puga et al., 1995) or highly deformed, once-continuous basaltic layers. The occurrences of eclogite relicts (92-39b) with glaucophane–paragonite schist rinds (92-39a) resemble those from the Central Alps near Zermatt (Oberhaensli, 1980). The latter display glaucophane schist rinds that surround garnet and pyroxene cores. Oberhaensli (1980) hypothesized that the glaucophane schist represents metamorphosed hydrated pillow rims, and that metamorphic conditions reached 600°C at 1400 MPa. However, glaucophane-rich rinds are very sparse in the Venezuelan localities. Furthermore, most of the shapes of the knockers that we have observed are sigma and delta structures that reflect intense deformation. Thus, we interpret the glaucophane–paragonite-rich rinds to reflect metasomatic processes of interaction between basaltic and pelitic bulk compositions via mechanisms broadly similar to those that created rinds around high-grade tectonic blocks in serpentinite-matrix melanges of California and Washington state (Sorensen & Grossman, 1989, 1993).
Glaucophane is also found as a retrograde mineral; in one eclogite sample (92-30a), omphacite is partially replaced by glaucophane and actinolite. This retrograde overprint occurred at much lower P–T conditions. As discussed above, some samples exhibit fresh cores of eclogite knockers with outer zones containing plagioclase-hornblende symplectite; this clearly shows the hydration of eclogite in the outer zone, while preserving the inner core during uplift. A similar occurrence has been reported from the Adula Nappe in the Central Alps (Heinrich, 1982, 1986).
Metamorphic P–T conditions for the basaltic eclogites from Isla de Margarita, northeastern Venezuela, are estimated at 500–650°C at 1200–1500 MPa (Maresch & Abraham, 1981; Bocchio et al., 1990, 1996; Stöckhert et al., 1995). Each eclogite lens on Isla de Margarita yields a different temperature, similar to the results for the eclogite knockers of Puerto Cabella. Kyanite-, staurolite-, phengite- and chloritoid-bearing assemblages in metapelites which host the eclogite lenses imply P = 1000–1400 MPa at T = 500–600°C (Krückhans-Lueder & Maresch, 1992). Slightly zoned omphacitic pyroxenes with jadeite-rich rims were also reported from Isla de Margarita (Maresch & Abraham, 1981). These yield comparable temperatures to those for the eclogite and barroisite-bearing eclogite from the Puerto Cabello region; however, the pressures in the latter are much higher.
Garnet zoning profiles from correlative eclogites from Isla de Margarita range from flat to discontinuous (Blackburn & Navarro, 1977; Maresch & Abraham, 1981; Bocchio et al., 1996); MgO shows both increases and decreases in different garnets in the same sample (Maresch & Abraham, 1981). Maresch & Abraham (1981) concluded that garnet growth occurred over a 100°C temperature interval during a single prograde metamorphic episode. In contrast, Blackburn & Navarro (1977) reported discontinuities in MnO zoning, which they interpreted as a growth hiatus during polymetamorphism. This interpretation is supported by textural observations that garnets display inclusion-rich cores with thin resorption zones overgrown by inclusion-free garnet rims (Bocchio et al., 1996). No discontinuities in MnO zoning are seen in garnet from the Puerto Cabello eclogites.
Decompression of the Puerto Cabello region is documented by several observations, including the alteration of kyanite to talc in the glaucophane-rich metapelitic knockers. In addition, the eclogite knockers must pass into the stability field of glaucophane and barroisite (Fig. 7) before passing into greenschist-facies. Finally, the radial cracks around quartz in garnets found in eclogite (Fig. 3b) formed in response to the different elastic behavior of quartz and garnet during relatively isothermal decompression (e.g. Van der Moolen & Van Roermund, 1986; Wendt et al., 1993).
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Conclusions and Tectonic Model |
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TOPABSTRACTIntroductionCordillera de la Costa...Petrography and Mineral...Metamorphic ConditionsDiscussionConclusions and Tectonic ModelREFERENCES |
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Eclogites from different localities within the Cordillera de la Costa belt record different metamorphic conditions. In both Puerto Cabello and Isla de Margarita, coexisting garnet–clinopyroxene core and rim analyses from eclogite yield similar temperatures between 500 and 700°C. However, our results suggest that the maximum pressure for peak metamorphism was 1800–2200 MPa for Puerto Cabello, whereas on Isla de Margarita the pressure was only 1000–1500 MPa. It should be noted that this is a minimum pressure estimate from jadeite geobarometry and is similar to pressure estimates calculated with this geobarometer for the Puerto Cabello eclogite knockers. However, a higher pressure for Puerto Cabello is estimated from the unusual assemblage of Mg-glaucophane–kyanite–paragonite and talc, which records maximum pressures of
2200 MPa at 650°C. Another indication for higher pressures for Puerto Cabello are the estimated minimum pressures, based on the Si contents of phengitic micas, of 1700–1800 MPa in both pelitic schist and eclogite knockers. A third high-pressure estimate, based on coexisting garnet–biotite–phengite–albite within host schist, indicates conditions of 450–520°C at 1800 MPa. Thus, we hypothesize that the Puerto Cabello region reached a deeper level than Isla de Margarita during metamorphism in the Cretaceous subduction zone.
The decompression path of eclogite and blueschist of the Cordillera de la Costa belt indicates that exhumation followed a clockwise P–T trajectory. Ernst (1988) suggested that such a path (which he termed ‘Alpine-type’) can best be explained by cessation of plate convergence and subsequent isostatic rebound and erosion or tectonic denudation, and that the cessation of subduction may be the result of continental collision. The Cordillera de la Costa belt is a mixture of oceanic rocks (serpentinite and metabasalt), continental-margin deposits (graphitic schist, marble, and quartzite), and early Paleozoic granitic rocks, a mixture of protoliths consistent with a collisional setting. It was, therefore, formed during the mid-Cretaceous, when the ‘Great Arc of the Caribbean’ (Burke, 1988) was migrating northeastward between North and South America. If the northwest corner of the South American continent (Colombia) collided very obliquely with a subduction zone (Fig. 8), it could have ended subduction and produced the Alpine-type characteristics of this region. In contrast, the Villa de Cura HP–LT metamorphic belt, showing a ‘Franciscan-type’ decompression history, may have formed in an inter-oceanic setting in the same mid-Cretaceous subduction zone (Smith, 1996; Smith et al., 1996).
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The uplift and exhumation of the HP–LT metamorphic rocks occurred in two stages. The first stage was the result of plate boundary parallel extension of the subduction complex in mid-Cretaceous time; the second resulted from the Eocene to Miocene obduction of the complex onto the South American continental margin.
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ACKNOWLEDGEMENTS |
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We thank Marino Ostos Rosales for his help in arranging logistics and accompanying us during our initial field work, and Milton Pierson for his assistance with the microprobe analyses. We thank Sorena Sorensen, George Harlow, Darrel Henry, and an anonymous reviewer for their very helpful and rigorous reviews. This work was supported by NSF Grants EAR9019243 and EAR9304377.
* Corresponding author. Telephone: (713) 285-5234 (O). Fax: (713) 285-5214. e-mail: jinnys{at}rice.edu
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