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Timing of subduction and exhumation in a subduction channel: Evidence from slab melts from La Corea Mélange (eastern Cuba)

Abstract:

High pressure igneous rocks (tonalites), generated by partial melting of subducted basaltic rocks accreted to the mantle wedge, are present in the La Corea serpentinite-matrix mélange (eastern Cuba) as centimeter- to meter-sized blocks and as concordant to crosscutting veins within high-pressure parent amphibolite blocks. The slab melts have adakitic signatures, in agreement with formation after partial melting of metabasite. Thermobarometric calculations indicate 620-680°C and 13-15kbar during crystallization of tonalites and down to 250-300°C, 6kbar during retrogression, indicating counter-clockwise P-T paths (hot subduction-cool exhumation). Free water required for melting of amphibolite at moderate temperature (700-750°C) and moderate pressure (13-16kbar) close to the wet basaltic solidus is inferred to have been provided after dehydration of sediments, altered basaltic crust and serpentinite of the subducting Proto-Caribbean lithosphere. Single zircon (SHRIMP) and phengite 40Ar/39Ar age data constrain the P-T-t evolution of the mélange from the timing of crystallization of melts at ~110-105Ma to cooling at ~87-84Ma, ca 350°C, ca 9kbar. These figures are consistent with subduction of an oblique ridge, shortly before 115Ma. Furthermore, our data indicate very slow exhumation (ca. 1mm/yr) in the subduction channel during the oceanic convergence stage (120-70Ma) until final fast exhumation to the surface occurred at 70-65Ma during a regional arc-platform collision event. © 2011 Elsevier B.V.