First occurrence of diopside sanidine phlogopite lamproite in the Andean Cordillera: the Huacancha and Morojarja dikes, southern Peru

Abstract

Two dikes of diopside sanidine phlogopite lamproite composition, emplaced during the Plio-Quaternary, have been discovered along the main active fault system in the Altiplano region of southern Peru. These rocks have mineralogical characteristics and bulk-rock compositions typical of phlogopite lamproite (i.e., Ti-rich and Al-poor phlogopite, K-richterite, 4 < K2O/Na2O < 5, and very high contents of incompatible elements (Ba, Sr, light rare earth elements)). Moreover, these dikes show textural, mineralogical, and bulk-rock features that have been described only in phlogopite lamproite from the Leucite Hills Volcanic Province, United States, namely a microscale layering enriched in leucite or sanidine; phlogopite and clinopyroxene xenocrysts displaying higher Al contents than the phenocryst cores; late-stage aegirine; and primitive mantle normalized Ba/Th and La/Yb ratios of 2–4 and 64–109, respectively. The occurrence of diopside sanidine phlogopite lamproite argues for the presence of a thick lithospheric mantle containing apatite–phlogopite-rich clinopyroxenite beneath the Altiplano.