A Melt origin for spinifex-textured metaperidotite in the Cerro del Almirez massif, southern Spain

Abstract

Outcrops of spinifex-textured metaperidotite in the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogite-facies (1.6–1.9 GPa, ≈680 °C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown in thin section, and microprobe analysis (carefully avoiding accidental inclusions) shows contents of Cr2O3 and TiO2 in the range 0 to 0.4 and 0 to 0.1 weight percent respectively. These amounts of Cr2O3 and TiO2 are matched by olivine in peridotite xenoliths from the mantle, komatiites, and pseudotachylytes. According to laboratory experiments, at 2 GPa and a likely redox state near QFM, these levels of Cr2O3 signify equilibration at temperatures in excess of 1600 °C. Curvature of the elongate olivines, and outcrop-scale alignment of olivine splays suggest a connection with komatiites. Two early studies at Cerro del Almirez emphasized this connection, but all papers from the late-1990s to the present time have advocated a metamorphic origin, analogous to jackstraw-olivine rocks. Some 5 to 6 weight percent or 13 to 15 volume percent of H2O was released at the isograd. We hypothesize that this dehydration water induced embrittlement of chlorite-metaharzburgite, and frictional slip in shear zones raised the temperature by several hundred degrees C above the ambient, more than enough to melt wet or dry ultramafic rock and form spinifex texture on rapid cooling. The amounts of crystallized melt so formed are orders-of-magnitude larger than found in peridotite pseudotachylyte, but are perhaps not totally inconsistent with dehydration-induced seismic events repeated over a protracted period of time during Miocene subduction of an oceanic plate. The transient ultramafic melts yielded on rapid cooling up to 12 cm long olivines and interstitial aggregates of parallel to radiating orthopyroxene prisms, followed by crystallization from glass of assorted eclogite-facies minerals (chlorite, tremolite, talc).