Early Cretaceous subduction initiation of the proto-Caribbean plate: geochronological and geochemical evidence from gabbros of the Moa-Baracoa ophiolitic massif, Eastern Cuba

Abstract

Ophiolites in Cuba are fragments of either the proto-Caribbean or Caribbean plate, however, neither the time of initial subduction nor the mechanism by which it was induced are well constrained. The Moa Baracoa ophiolitic massif, eastern Cuba, consists chiefly of harzburgite, layered gabbro, gabbro sills and dykes, and lavas. The gabbro sills and dykes yielded zircon UPb ages of 136.7 ± 1.8 Ma and 122.2 ± 0.5 Ma, respectively; in contrast, zircons from the layered gabbros yielded a wide range of ages (158 to 2556 Ma). The layered gabbros mostly show high MgO, Al2O3, and CaO, coupled with low TiO2, Na2O + K2O and incompatible elements, suggesting a cumulate origin. Their crystallization sequence (plagioclase before clinopyroxene), low plagioclase An values (69.8–73.5), and clinopyroxene Mg# values (80.4–81.6), are similar to those of mid-ocean ridge gabbros, however, modeled equilibrated melts based on clinopyroxene compositions have a forearc basalt geochemical affinity. In the gabbro sills, crystallization of clinopyroxene before plagioclase, high plagioclase An values (83.7–88.5), high clinopyroxene Mg# values (85.1–89.5), and the presence of magmatic amphibole require hydrous magmas from a supra-subduction zone. The gabbro dykes have moderate plagioclase An values (76.1–80.5) and clinopyroxene Mg# values (83.2–88.1), and modeled melts of the gabbro sills and dykes also show subduction-related geochemical features. High positive whole-rock εNd(t) (+8.48 to +10.81) and low (87Sr/86Sr)i ratios (0.702500–0.703645), coupled with high positive εHf(t)(zircon) (+12.0 to +17.7) suggest that all the Moa-Baracoa gabbros were derived from a depleted source. However, strongly heterogenous δ18O(zircon) values (0.41–6.32‰) in the gabbro sills and dykes indicate addition of a subduction-related fluid. Our data suggest that the Moa-Baracoa ophiolitic massif originated in a forearc environment in the early Cretaceous during subduction initiation. Combined with the regional geological background, we propose that subduction of the proto-Caribbean plate was initiated before 137 Ma. Thus, the zircon in the layered gabbros (δ18O = 6.28 to 8.39‰, εHf(t) = −1.94 to −9.10) must be detrital grains derived from subducted crustal material. The youngest zircon in these rocks implies that some of the crustal material may have been recycled in as little as 20 Myr.