Timescales of metasomatism in a hot subduction channel: a microstructural and (radio-)isotopic study of Sierra del Convento jade

The Sierra del Convento mélange in southeastern Cuba is one of a limited number of jadeite jade occurrences worldwide. This exhumed, pervasively serpentinized fossil subduction interface hosts tectonic blocks up to several tens of meters in scale that were reworked during intense metasomatism. The flat-lying mélange spans a 300 m thick vertical exposure, progressing from a block-rich lower portion toward a block free, serpentinite-dominated upper region. We conducted structurally-controlled sampling of blocks and matrix to produce a spatially-resolved geochemical and microstructural profile through the thickness of the Convento mélange. Pale green, nearly pure jadeitite constitutes the dominant block population in the southern part of this mélange, and may contain up to 30% epidote and/or white mica by volume. Minor metasedimentary and metamafic block populations, recording variable degrees of HP metamorphism, coexist alongside the jade blocks. We report a newly identified metre-scale zoning within the jadeitite bodies, consistent with that documented in other jade localities. Green jadeitite locally occurs surrounding an older core of zoisitite containing relict jadeite crystals rimmed by omphacite. Green jade is cross-cut by fractures infilled by dark-colored brecciated jade, which is in turn rimmed by a late, pyroxene-free rind composed mainly of weakly foliated phengite + albite ± epidote. The latter facies occupies a similar position to chloritite blackwalls previously described from the Convento jade occurrence. All jade varieties except for these latest phengite-albite rinds and chloritite blackwalls are cross-cut by fractures infilled by jadeite and omphacite. Parts of the main jade bodies exhibit prismatic radial and comb jadeite microstructures, consistent with descriptions of P-type jade, which precipitate directly into open fractures from hydrothermal fluids. However, Convento jade contains paragonite with up to 2 wt.% K₂O and jadeite-omphacite exsolution domains brecciated into jigsaw-like fragments recemented by jadeite and/or omphacite. These observations are consistent with at least part of the jade in the Sierra del Convento mélange representing near-total high-temperature metasomatic replacement of high-pressure anatectic trondhjemite protoliths, which originated as partial melts of garnet amphibolite at ~15 kbar, resembling an R-type (replacement) jade paragenesis. To overcome the considerable ambiguity inherent to geochronology datasets from HP igneous and metasomatic rocks from this locality (105 to 115 Ma, U-Pb zircon), we are conducting detailed multi-mineral and multi-system geochronology, including further U-Pb on zircon, titanite, and apatite, Ar/Ar on white mica, and Rb-Sr on white mica. This multi-chronometric approach will establish relative and numerical chronology for the diverse jade facies of the Convento occurrence, resolving timescales of the multiple associated fluidization events within the subduction channel.