The mineralogy,geochemistry and origin of Iherzolite inclusions in Victorian basanites

The mineralogy of Iherzolite inclusions in Victorian basanites indicates an upper mantle origin, but a range of temperatures from igneous to metamorphic (subsolidus) is indicated by the mineral compositions. Pyroxene textural features exhibit a slow cooling history consistent with isotopic evidence that these inclusions are accidental xenoliths. Clinopyroxene-rich inclusions (10–20 vol. % cpx) have higher abundances of Ca, Na, AI, Sc, V, Cr and heavy REE, lower Mg/Mg + Fe²⁺, lower Ni abundances, and more fayalitic olivines than clinopyroxene-poor inclusions (<5 vol. % cpx). A surprising result is that the refractory Mg-rich, clinopyroxenepoor inclusions contain the highest abundances of incompatible elements such as P, K, Ti, light REE, Th and U. We believe these inclusions are composed of two components (A and B). Component A determines the major element abundances and primary mineralogy of the inclusions. Based on Ni abundances component A is interpreted as a melting residue rather than a crystallization accumulate. Component B forms a small and varying portion of the inclusions, and it contributes P, K, Ti, light REE, Th and U. This component has the geochemical characteristics of a liquid formed in equilibrium with garnet.The following model is presented for the origin of Iherzolite inclusions. Residual Iherzolite (Component A) is left in the lithosphere after partial fusion, and it is later modified by a melt which has migrated to the top of the low velocity zone. Because this liquid (Component B) results from a small degree ( <6 per cent) of melting (probably limited by water abundance), and has equilibrated with garnet, it will be very enriched in P, K, Ti, light REE, Th and U. Subsequent cooling and recrystallization forms the present mineralogy. Finally, explosive volcanism, characteristic of silica-undersaturated magmas, incorporates mantle fragments (Iherzolite inclusions), and the increasing temperature and decreasing pressure during ascent causes incongruent melting of minor hydrous phases such as phlogopite and amphibole.