H2O-driven generation of picritic melts in the Middle to Late Triassic Stuhini arc of the Stikine terrane, British Columbia, Canada

- Ultramafic pyroclastics are part of Upper Triassic Stuhini Group of Stikine terrane.
- The pyroclastics are olivine cumulates formed from picritic parental magma.
- Picritic magmas of Stuhini Group were rich in magmatic volatiles (5-7 wt.% H2O).
- Eruptive temperature of Stuhini Group picrites was ∼1200 °C.
- Picrites formed by melting of hydrous mantle NOT anomalously hot mantle.

Basaltic to andesitic compositions predominate island arc magmatism; ultramafic magmas are rare. Ultramafic (MgO = 21-33 wt.%) tuff breccia, lapilli tuff, and ash tuff of the Middle to Upper Triassic Stuhini Group were erupted in the Stikine arc of the North American Cordillera shortly preceding an episode of prolific porphyry Cu-Mo(-Au) mineralization. The ultramafic tuff shows accumulation (20-65%) of olivine (Fo91) and minor chromite into a subalkaline picritic parental magma with MgO ∼16 wt.%. Despite the inferred high MgO content of the parental liquid, chromite phenocrysts record relatively low liquidus temperatures (<1200 °C) suggesting crystallization from relatively low temperature, hydrous melts, at oxygen fugacities one to three log units above the fayalite-magnetite-quartz (FMQ) buffer. The primary picritic magmas likely contained 5-7 wt.% H2O, inferred on the basis of olivine-liquid thermometry and thermal models for subduction zones, thus alleviating the need for catastrophic thermal perturbations in the mantle wedge. Instead, efficient release of water through slab dehydration at 2.5-3.0 GPa allows generation of picritic melts at ordinary mantle wedge temperatures through moderate degrees (F=0.10-0.15) of hydrous flux melting. The volatile-rich nature of the melt and the predominant extensional regime in the overlying lithosphere of Stikinia facilitated the near-adiabatic ascent of the Stuhini Group picrites. The high H2O content of the rapidly ascending picrite melt may have played a key role in transport of metals into the crust of the Stikinia and subsequent porphyry mineralization.