Eclogitic rocks exposed in the Island of Margarita (Eastern Venezuela) can be divided into two main types: The first type, includes those found as isolated blocks and boudins within the schiats and quartzites of the Juan Griego Group. These are interpreted as being allocthonous. The second type (Pedro Gonzalez type) includes those rocks that occur as tabular masses conformable to achists and gneisses of the Greenstone Division (Juan Griego Group). They are restricted to the northeastern part of the Island of Margarita.

Several lithological types have been obaerved in these eclogites, from true eclogites to amphibolites. They have been derived from early orogenic basaltic lava flows of apilitic affinity which auffered metamorphism under a high P/T regime yielding ophiolitic eclogites. A later increase in temperature affected the same rocks producing common eclogites.

Determinatione of P-wave velocities gave values of 7.0 km/sec for true eclogites and 3.6 km/sec for amphibolites, indicating in all cases the crustal origin of these rocks, since higher values would be expected for mantle derived eclogites.

The mineralogy of the eclogites can be expreased in terms of their four principal mineral constituents: garnet, clinopyroxene, amphibole and white mica. Garnet is essentially almandine, with moterate grossularite content sut low spessartite and pyrope contents. Clinopyroxene is omphacite, with very high acmite component. Amphibole is of the sub-calcic type between barroisite and Ca-glaucophane. The white mica is essentially paragonite with minor potessium in solid solution.

The metamorphic history of these rocks can be summarized in two mayor events. A first event, typical of the blueschist facies (8-9 kb, 200-300°C), changes the original basaltic assemblage of the volcanic rocks lato an ephiolitic eclogite assemblage (lawsonite + jadeitic pyroxene + Fe-rich garnet). The second event, characterized by higher temperatures of metamorphism (amphibollte facies, 8-9 kb and 600°C) changed the prevlous assemblage into omphacite + Fe-Mg garnet + paragonite, typical of common eclogites. Later tranaformations of the eclogites under high P_H20 conditions gave a suite of amphibole eclogites, garnet amphibolites and amphibolites.

Investigations of the extent of the chemical equilibration domains for Mg, Fe, Ca and Mn in these rocks indicate that for different elements the domain sizes ant shapes are different. Extensive equilibration domains were observed for the above mentioned elements, probably due to original chemical homogeniety of the proto-rock rather than equilibration produced during metamorphism.

Very complex zonation patterns were observed studying the distribution of Mg, Fe, Ca and Mn in the garnets of several types of eclogitic rocks (true eclogites, amphibole eclogites and garnet amphibolites). In general, norma1 zonation profiles were observed for Mg and Mn, while Fe and Ca profiles proved to be oscillatory in many of the cases. At least two stages of garnet growth were identified froo the zonation profiles. Theee growth stages can be correlated with the two metamorphic events that affected the eclogitic rocks. Petrographic evidence of the later transformations in the garnets has also been observed.

An island arc-contlnent collision, mountain belt model seems to be the best explanation of the present geological situation observed in the venezuelan northern coast range of which the Island of Margarita is a part. Thie model also gives a satisfactory explanation of the two postulated metamorphic events that affected the eclogitic rocks.

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¹ University of Kentucky, Trabajo que luego fue presentado en la Universidad Central de Venezuela, Facultad de Ingeniería, Escuela de Geología y Minas, 1974.