High-Pressure (∼2000 MPa) Kyanite- and Glaucophane-bearing Pelitic Schist and Eclogite from Cordillera de la Costa Belt, Venezuela

JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 PAGES 65–83 1997 High-Pressure (~2000 MPa) Kyanite- and Glaucophane-bearing Pelitic Schist and Eclogite from Cordillera de la Costa Belt, Venezuela VIRGINIA B. SISSON∗, INCI EVREN ERTAN AND HANS G. AVÉ LALLEMANT DEPARTMENT OF GEOLOGY AND GEOPHYSICS, RICE UNIVERSITY, MS-126, HOUSTON, TX 77005-1892, USA RECEIVED APRIL 22, 1996 REVISED TYPESCRIPT ACCEPTED AUGUST 5, 1996 In the Cretaceous melange of the Cordillera de la Costa belt, north–central Venezuela, there are knockers of eclogite, barroisitebearing 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. INTRODUCTION Cabello, Venezuela 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 ∗Corresponding author. Telephone: (713) 285-5234 (O). Fax: (713) 285-5214. e-mail:  Oxford University Press 1997 KEY WORDS: geothermobarometry; high-P pelitic schist; eclogite; Puerto JOURNAL OF PETROLOGY VOLUME 38 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 ‘Franciscantype’ 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. NUMBER 1 JANUARY 1997 (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 (D 1a to D 1e) and two postmetamorphic (D 2a and D 2b) 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 (D 1c to D 1e) 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 (D 1c), dextral simple shear parallel to the plate boundary (D 1d) and by extension parallel to the plate boundary (D 1e). Generally, D 1d structures deform D 1c structures and are deformed by D 1e; 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, rightoblique 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 (D 1e) may have been responsible for part of the uplift and decompression of these rocks (Avé Lallemant & Guth, 1990). CO (...truncated)