Abstract

The Tinaquillo peridotite is the least serpentinized of the ultramafic masses included in the N. 75 E.-trending belt of Cretaceous metamorphic rocks in Northern Venezuela. It is a south-dipping tabular mass of intensely crushed and foliated dunite bounded on the north by a south-dipping regional thrust fault and on the south by regionally metamorphosed hornblende-quartz-plagioclase gneisses. Near the intrusion, these gneisses have been contact-metamorphosed by the dunite to garnet-augite-hornblende-andesine gneisses. Large tabular masses of gabbroic composition within the peridotite are interpreted as inclusions of contact-metamorphosed country rock. The peridotite contains thin hornblende and pyroxene layers parallel to the foliation and, in addition, remarkable lath-shaped orthopyroxene phenocrysts flattened in the planes of foliation and oriented dimensionally and crystallographically parallel to one another. These laths were formed by the stretching-out of primary equant-shaped phenocrysts, probably during final-stage solid intrusion of the peridotite.

The structure and composition of the pyroxene and garnet minerals in the contact rocks indicate that the temperature of intrusion was between 800°C. and 1000°C. During cooling, some of the thin pyroxene layers were altered hydrothermally to hornblende. Later and at still lower temperature, a belt of dunite along the Manrique thrust fault was altered to serpentinite and asbestos.

The other peridotites in the coastal range of northern Venezuela, which are moderately to intensely serpentinized, have no contact metamorphic aureoles, and some show conflicting age relationships toward the country rocks. Repeated upward movement during successive orogenies may explain these phenomena associated with serpentinites both in Venezuela and elsewhere.

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