PETROLOGICAL CHARACTERIZATION OF THE SOURCE COMPONENTS OF POTASSIC MAGMAS - GEOCHEMICAL AND EXPERIMENTAL CONSTRAINTS

The source mineralogy and conditions of origin of the three main groups of ultrapotassic rocks are outlined by combining experimental constraints and an abstraction of evidence from whole-rock chemistry (including volatiles), tectonic setting and xenolith contents. Lamproites originate from a depleted source rock which was strongly re-enriched at a later stage, thus producing mica-harzburgite. Melting conditions are H2O-rich and in most cases strongly reducing. Kamafugites originate from a clinopyroxene-rich source, also with abundant mica, in more oxidizing, CO2-rich conditions. Members of the third group form in a relatively fertile spinel-peridotite also containing abundant clinopyroxene and mica. Contrasting effects of variation in (i) pressure of melting and (ii) oxygen fugacity, emphasize the importance of these parameters in the sources of ultrapotassic rocks. Currently popular models for the origin of ultrapotassic melts by partial melting of phlogopite-bearing Iherzolite are inconsistent with the now extensive array of liquidus experimental results on ultrapotassic rock compositions. The discrepancy between partial melting models and liquidus results is attributed to the implicit, invalid assumption in the partial melting models that incompatible elements are homogeneously distributed on a large scale. Non-peridotitic assemblages rich in mica and pyroxenes which may be completely free of olivine must have an important role in the genesis of potassic rocks as spatially restricted components of inhomogeneous source regions.