The occurrence and timing of high-pressure metamorphism on Margarita Island, Venezuela: a constraint on Caribbean-South America interaction

Abstract

Abstract The metamorphic rock sequences exposed on the Island of Margarita, Venezuela, located in the southeastern corner of the Caribbean Plate margin, are composed of a high-pressure/low-temperature (HP/LT) nucleus subducted to at least 50 km depth, now structurally overlain by lower-grade greenschist-facies units lacking any sign of high-pressure subduction-zone metamorphism. The HP/LT nucleus involves protoliths of both oceanic (metabasalts and intimately associated carbonaceous schists of the La Rinconada unit; peridotite massifs) and continental affinity (metapelites, marbles and gneisses of the Juan Griego unit). All HP/LT units were joined together prior to the peak of high-pressure metamorphism, as shown by their matching metamorphic pressure–temperature evolution. The metamorphic grade attained produced barroisite as the regional amphibole. Glaucophane is not known from Margarita. Contrary to a widely propagated assumption, there are no major nappe structures post-dating HP/LT metamorphism anywhere within the high-pressure nucleus of Margarita Island. U–Pb zircon dating of key tonalitic to granitic intrusive rocks provides the following constraints: (1) the Juan Griego unit is heterogeneous and contains Palaeozoic as well as probable Mesozoic protolith; (2) the peak of HP/LT metamorphism, that is maximum subduction, is younger than 116–106 Ma and older than 85 Ma, most probably c. 100–90 Ma, a time span during which the southeastern Caribbean/South American border was clearly a passive margin. The assembly of Margaritan protoliths and their HP/LT overprint occurred far to the west in northwestern South America, a scenario completely in accord with the details of the Pacific-origin model outlined by Pindell & Kennan. Juxtaposition of the greenschist-facies units occurred after exhumation into mid-crustal levels after c. 80 Ma.