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 th at outcrops as a 10 Km long vertical sheet near Los Alisos (Sierras Su bandinas, northern Argentina), are consistent with the classification of this body as an alnoite. The mineralogy of the outcrop indicates th at it is an ultramafic lamprophyre, being composed of euhedral serpent inized olivine phenocrysts (20% mode) enclosed in large plates of poik ilitic phologopite (36%). The body also contains Ti-rich garnet (4%), Ca-rich pyroxene (8%), perovskite (10%), calcite (5%), apatite (1%) an d titano-magnetite (4%). Chlorite, ilmenite and sulfides also occur sp oradically. Geochemical data indicate that the rocks are K-rich, under saturated, and strongly enriched in REE (mainly LREE), other incompati ble elements, Ni and Cr. The geochemical features and isotope data ([S r-87/Sr-86](i) = 0.70445-0.70458) suggest that the Los Alisos rocks ca n be produced by a slight partial melting (3-5%) of a metasomatized an d veined phlogopite-bearing garnet peridotite. The latter is geochemic ally comparable with some metasomatized xenoliths forming in subcontin ental deep lithospheric mantle. (C) 1997 Published by Elsevier Science Ltd.