Hydrothermal origin and age of jadeitites from Sierra del Convento Melange (Eastern Cuba)

摘要

Jadeitite (jadeite jade) from Sierra del Convento (eastern Cuba) occurs in a subduction-related serpentinite-matrix mélange associated with a variety of high-pressure tectonic blocks including garnet-amphibolites and related anatectic trondhjemites. The eastern Cuban jadeitite is massive and characterized by rare quartz inclusions and omphacite exsolution in jadeite crystals, as well as replacement or infilling by omphacite. Minor minerals include epidote, biotite, albite, phengite, titanite, rutile, zircon, and apatite. Oscillatory zoning in jadeite crystals and zircon ages suggest hydrothermal crystallization in veins formed in serpentinized peridotite, probably of the mantle wedge. Al-Na-Mg-Ca-bearing fluids of variable composition but high pH (capable of mobilizing Zr and Hf at SiO_2-subsaturated conditions) deposited jadeitite in veins during episodic opening of the fractures at depth in the subduction environment. Late-stage crystallizations include omphacite, albite and epidote from fluids nearly saturated in SiO_2. The compositional gap of two coexisting pyroxenes indicates a temperature of jadeite formation higher than 500 °C. Zircon~(206)Pb/~(238)U ages of 107.4 ± 0.5 Ma and 107.8 ± 1.1 Ma attest formation during the earliest stages of subduction in the region. These ages and the high temperature of formation of jadeitite suggest a genetic link between the jadeite-forming fluids and fluids derived from associated anatectic trondhjemites crystallized at depth (15 kbar).