New petrological and structural constraints on the origin of the Mayarí-Baracoa ophiolitic belt (eastern Cuba) (original) (raw)

2003

Abstract

The Jurassic-Cretaceous Mayarí-Baracoa ophiolitic belt (MBOB) is situated in the easternmost part of the east-west trending Cuban ophiolitic belt. The MBOB comprises the Mayarí-Cristal and Moa-Baracoa massifs located, respectively, to the east and the west of the Riito de Miraflores fault. The Mayarí-Cristal massif is essentially made up of mantle tectonites (> 5 km thick) and subordinate gabbro-diabase dikes intruding the mantle section. These mafic dikes cut the peridotites but no diabase sheeted complex has been observed. We differentiate two main domains: 1) the lower domain, which outcrops in the southern and central part of the massif, is composed of porphyroclastic harzburgite tectonites. Dunites are widespread and occur as irregular patches and sub-concordant layers ( 2.2 km thick) topped by a thin crustal section made up of lower layered gabbros (ca. 300 meters thick) and discordant volcanics. Isotropic gabbros and diabase sheeted-dyke complex are lacking. This massif exhibits an unusual crust-mantle transition zone made up of residual harzburgite tectonites and minor dunites. Some peridotites are enriched in plagioclase and clinopyroxene and may form layers of ``impregnated peridotites'' and troctolites. In the transition zone concordant and rare sub-concordant bodies of dunite and chromitite and sills of gabbro also occur. Peridotites are cut by dikes of gabbro, pegmatitic gabbro, minor diabase, olivine norite and orthopyroxenite. The layered gabbros are composed of olivine gabbro and gabbronorites. The composition of olivine (Mg# = 85-86), clinopyroxene (Mg# = 78-90), orthopyroxene (Mg# = 73-87), and plagioclase (An = 60-77 wt %) are similar to those from ophiolitic cumulates associated to arc or back-arc settings. Many geochemical features suggest that peridotites from the MBOB were formed in a supra-subduction setting. The Cr# of spinel in peridotites (0.44-0.80) is in equilibrium with melts ranging in composition from MORB to boninites. Harzburgites and dunites are enriched in highly incompatible elements (Cs, Rb, Ba, Th, U and Nb) relative to REE (Th/La = 0.6-0.8) and display trace element normalized patterns with U spikes (U/Th > 1.3) suggesting the involvement of a volatile-rich fluid/melt. Moreover MBOB peridotites show high fO2 values typical of peridotites from subduction settings [delta logfO2 (FMQ) > 1]. We propose that the MBOB was initially part of an ocean basin formed by the rifting and spreading of the ``Protocaribbean'' ocean. This proto-ocean was subsequently affected by the development of a juvenile island-arc in a Cretaceous supra-subduction setting and by the late opening of a back-arc basin where most of the intrusive plutonic and volcanic crust of the MBOB was accreted.

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