Eclogite and Related High-Pressure Regional Metamorphic Rocks from the Andes of Ecuador

Resumen

High-pressure, regional metamorphic rocks (the Raspas Formation) constitute an inclusion more than 10 km long and 3 km wide in a protrusion of extensively serpentinized harzburgite in El Oro Province, southwestern Ecuador. The high-pressure rocks, all feldspar-free, consist of a dominant pelitic schist with quartzite layers, eclogite, eclogite amphibolite, garnetite, and retrograde mafic rocks. The pelitic schist is coarse grained and is composed of quartz + phengite + paragonite + garnet + chloritoid + rutile + graphite ± kyanite ± pyrite. Eclogite is fine to medium grained, massive to strongly foliated (average mode: omphacite (Jd42Ac6(Di + He)32), 38.2 per cent; garnet (Alm53 Spess2 Pyr19 Gross26), 26.6; barroisite (4.1 per cent Na2O), 22.6; clinozoisite, 4.1; rutile, 2.1; quartz, 5.7; other minerals, 0.7).Eclogite amphibolite is a medium-grained and massive to foliated rock composed of amphibole (3.1 per cent Na2O) + garnet + zoisite + kyanite + rutile + pyrite ±omphacite ± paragonite ± quartz ± apatite. Retrograde mafic rocks include glaucophane schist, greenschist, and coarse-grained, amphibole-rich rocks. Prograde metamorphism took place in an active Benioff zone. Based on phase relations in the pelitic schist, partitioning of Mg/(Mg + Fetotal + Mn) in garnet-amphibole and in omphacite-amphibole pairs (Dòbretsov et al., 1975), the absence of lawsonite, and other evidence, the conditions of metamorphism are estimated to have been T = 580 ° ± 20 °C and P total = 13 ± 3 kb (43 ± 10 km depth). P H2O ranged from ⋍ P total in pelitic schist, through ≤P total in eclogite amphibolitc, to «P total in eclogite. Retrograde metamorphism accompanied rapid uplift of the Raspas Formation during which the rocks passed through the stability field of glaucophane-epidote schist, but not the stability field of lawsonite. The inclusion of high-pressure rocks was carried upward intact in a protrusion of extensively serpentinized harzburgite which rose diapirically through the relatively denser amphibolite and greenschist which constitute the regional basement of El Oro Province. Serpentinization of harzburgite began at depth, and continued coevally with eclogitization. Protrusive rise occurred upon abandonment of the Benioff zone. Radiometric K-Ar ages on the uplift of the Raspas Formation and the youngest lavas of a volcanic are of Jurassic-Early Cretaceous age in eastern Ecuador are synchronous (132 m.y.).