Ocean floor and Barrovian metamorphic events in the Mesozoic Peltetec ophiolitic belt (Ecuador)

Abstract

The Peltetec ophiolitic belt, exposed along the western margin of the Cordillera Real (Ecuador), comprises tectonic slivers of metamorphosed (greenschist facies) and variably deformed metaperidotites (serpentinites), gabbros (locally cumulate), greenschist and sub-greenschist rocks. New field data, deformation and petrographic observations, whole-rock major element, mineral composition and P-T determinations of the Peltetec metaophiolite allow determining the metamorphic-geodynamic events of a section of the active western margin of Gondwana during the Mesozoic. Two metamorphic events are proposed: (i) ocean-floor metamorphism during the formation of oceanic lithosphere under low-pressure amphibolite facies conditions (M), followed by (ii) prograde metamorphism up to the greenschist facies along an intermediate (Barrovian) P/T gradient (M; ~315°C and ~ 3.9 kbar, ca. 23°C/km). The first metamorphic event occurred under near-static stress conditions and rocks did not suffer significant deformation. During this event, pyroxenes were replaced by hornblende while plagioclase recrystallized. The second event produced strongly recrystallized and foliated greenschist facies rocks with a peak assemblage of actinolite + chlorite + albite + epidote + titanite ± phengite. We interpreted that this assemblage developed during the closure of a back-arc basin and the emplacement of the ophiolite onto the continental margin in the late Triassic or possibly by compressive events in the Early Cretaceous and/or Late Cretaceous. Later processes are related to the regional mostly strike-slip Peltetec fault/shear zone that produced displacement and mixing of blocks, as indicated by cataclastic structure present in most of the slivers, but without differentiable metamorphic recrystallization overprint.