Partial Melting and Counterclockwise P T Path of Subducted Oceanic Crust (Sierra del Convento Melange, Cuba)

Partial melting of subducted oceanic crust has been identified in the Sierra del Convento mélange (Cuba). This serpentinite-matrix mélange contains blocks of mid-ocean ridge basalt (MORB)-derived plagioclase-lacking epidote ± garnet amphibolite intimately associated with peraluminous trondhjemitic–tonalitic rocks. Field relations, major element bulk-rock compositions, mineral assemblages, peak metamorphic conditions (c. 750°C, 14–16 kbar), experimental evidence, and theoretical phase relations support formation of the trondhjemitic–tonalitic rocks by wet melting of subducted amphibolites. Phase relations and mass-balance calculations indicate eutectic- and peritectic-like melting reactions characterized by large stoichiometric coefficients of reactant plagioclase and suggest that this phase was completely consumed upon melting. The magmatic assemblages of the trondhjemitic–tonalitic melts, consisting of plagioclase, quartz, epidote, ± paragonite, ± pargasite, and ± kyanite, crystallized at depth (14–15 kbar). The peraluminous composition of the melts is consistent with experimental evidence, explains the presence of magmatic paragonite and (relict) kyanite, and places important constraints on the interpretation of slab-derived magmatic rocks. Calculated P–T conditions indicate counterclockwise P–T paths during exhumation, when retrograde blueschist-facies overprints, composed of combinations of omphacite, glaucophane, actinolite, tremolite, paragonite, lawsonite, albite, (clino)zoisite, chlorite, pumpellyite and phengite, were formed in the amphibolites and trondhjemites. Partial melting of subducted oceanic crust in eastern Cuba is unique in the Caribbean realm and has important consequences for the plate-tectonic interpretation of the region, as it supports a scenario of onset of subduction of a young oceanic lithosphere during the early Cretaceous (c. 120 Ma). The counterclockwise P–T paths were caused by ensuing exhumation during continued subduction.