Trace-element and isotope constraints on the origin of ultramafic lamprophyres from Los Alisos (Sierra Subandinas, northern Argentina)

Petrologic and geochemical analyses of an ultrabasic intrusive rock that outcrops as a 10 Km long vertical sheet near Los Alisos (Sierras Subandinas, northern Argentina), are consistent with the classification of this body as an alnoite. The mineralogy of the outcrop indicates that it is an ultramafic lamprophyre, being composed of euhedral serpentinized olivine phenocrysts (20% mode) enclosed in large plates of poikilitic phologopite (36%). The body also contains Ti-rich garnet (4%), Ca-rich pyroxene (8%), perovskite (10%), calcite (5%), apatite (1%) and titano-magnetite (4%). Chlorite, ilmenite and sulfides also occur sporadically. Geochemical data indicate that the rocks are K-rich, undersaturated, and strongly enriched in REE (mainly LREE), other incompatible elements, Ni and Cr. The geochemical features and isotope data ([87Sr86Sr]1 = 0.70445–0.70458) suggest that the Los Alisos rocks can be produced by a slight partial melting (3–5%) of a metasomatized and veined phlogopite-bearing garnet peridotite. The latter is geochemically comparable with some metasomatized xenoliths forming in subcontinental deep lithospheric mantle.