gems, jade, pyroxene jade, jadeite, fei cui, Dominican Republic

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Río San Juan Complex (RSJC) of the northern Dominican Republic within serpentinite mélanges associated with a former intra-oceanic subduction zone. Allochthonous blocks occurring in lag deposits developed upon mélange outcrops, as well as boulders within riverbeds, are common. A particularly unusual feature of the RSJC is the presence of concordant layers and discordant veins, ranging from centimetres to decimetres in thickness, within blocks of jadeite-lawsonite- or omphacite-garnet-bearing blueschist in the mélange. Two suites of jadeite-rich rocks may be distinguished. The first consists of quartz-free jadeitite sensu stricto (>90 vol.% jadeite), so far known only from blocks and boulders. The second suite comprises quartz-bearing jadeitite sensu stricto grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). This second suite occurs not only as blocks and boulders but also as layers and veins within blueschist blocks. A single occurrence of a cross-cutting omphacitite vein within blueschist has likewise been documented. Additional minerals identified within both suites include omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have thus far been observed only within the quartz-free jadeitite sensu stricto, whereas almandine-rich garnet has been found exclusively within JLQ. The jadeite-bearing rocks occur in various shades of green, range from fine- to coarse-grained and are generally equigranular. Mineral distribution is commonly homogeneous, although JLQ may display a patchy distribution that imparts a distinctly mottled appearance. Cathodoluminescence imaging reveals oscillatory zoning in jadeite, zircon, apatite and calcite, indicating crystallization from aqueous fluids under open-system conditions. Zircons separated from quartz-free jadeitite sensu stricto contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, demonstrating coeval zircon growth. Zircon cores yield ages of 114.9 ± 2.9 Ma, indicating crystallization near the initiation of subduction within the Río San Juan Complex, at a time when relatively “warm” geothermal gradients of approximately 15°C/km prevailed. These ages contrast sharply with those of the blueschists hosting the younger quartz-bearing suite, which range from 80 to 62 Ma and correspond to the final stages of subduction-zone activity at around 55 Ma under cooler geothermal gradients of approximately 8–9°C/km. For the younger quartz-bearing suite, phengite compositions combined with available P–T–t paths for the host blueschists suggest crystallization temperatures of approximately 350–500°C at minimum pressures of 15–16 kbar. Conditions for the older quartz-free suite are more difficult to constrain, although phengite compositions together with the geothermal conditions prevailing in the early warm subduction zone imply minimum conditions of at least 500°C and 11 kbar. Temperatures and pressures as high as 600°C and 15 kbar, however, comparable to those documented for jadeitites of similar age exposed in eastern Cuba within the same subduction system, remain possible. The jadeitites and jadeite-rich rocks of the RSJC are therefore interpreted as having crystallized over a span of approximately 60 million years within a single thermally self-organizing and progressively cooling subduction zone, evolving from initial temperatures exceeding 500°C to later conditions near 350°C. The P–T conditions inferred for the younger quartz-bearing suite correspond closely with those of jadeitite formation south of the Motagua Fault Zone in Guatemala, the only other known locality worldwide where jadeitite containing both lawsonite and quartz appears to be common. Further evidence is nevertheless required to confirm whether the older quartz-free suite represents another example of the rare high-temperature jadeitite known from Cuba.