Experimental petrology constraints on kamafugitic magmas

Abstract

Kamafugites are rare volcanic igneous rocks, characterized by the presence of kalsilite and variable amounts of leucite, nepheline, melilite, clinopyroxene, olivine and phlogopite, which may not necessarily be present all together. Kamafugites are silica-poor (moderately ultrabasic to basic), CaO- and alkali-rich (mostly ultrapotassic) lithologies, generated from strongly metasomatized and heterogeneous mantle sources, with abundant phlogopite and little or no orthopyroxene. Melting of phlogopite- and carbonate-bearing veins is often invoked as being responsible for the ultrapotassic and ultracalcic signatures observed in many kamafugites. Nevertheless, many questions still persist about their mantle sources, such as the paragenesis of the metasomatic veins within the lithospheric mantle and the degree of interaction between the initial melts and the peridotite matrix. We experimentally investigated four natural kamafugite samples to determine the mantle assemblages that were in equilibrium with these melts at the onset of partial melting and their genesis. The kamafugites were collected from the three known areas where they occur: Uganda, Italy and Brazil. Near-liquidus experiments were carried out at 1 to 2 GPa and temperatures from 1250 to 1380 °C. These experiments provide information on the mineralogy of the potential mantle sources in each of the volcanic provinces, also allowing a comparison among them. The experiments confirm the common presence of clinopyroxene and phlogopite as the main near-liquidus phases, with olivine joining the near-liquidus phase assemblage in one Italian sample (San Venanzo) and in the Brazilian kamafugite. Other minor phases (apatite and Fe-Ti oxides) also crystallized in near-liquidus conditions, highlighting their importance for at least the Ugandan and Brazilian kamafugites. Our results demonstrate that various amounts of clinopyroxene (g 1/440% in Italy and 50 %-60 % in Uganda and Brazil), phlogopite (g 1/420%-30 % in Brazil, g 1/440% in Uganda and g 1/460% in Italy) and accessory phases (up to 4 % titanite in Uganda, up to 3 % apatite in Uganda and up to 5 % oxides in Uganda and Brazil) are required for the formation of kamafugite melts. The contribution of olivine differs among the four samples, being negligible for the Ugandan kamafugites and in one of the Italian kamafugites but up to 5 % in the second Italian kamafugite and 10 % in Brazil. Show more