Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited

摘要

Kimberlite magmas from the Kimberley area of South Africa have sampled two main types of phlogopite-rich mafic xenoliths which represent deep mantle segregations from highly alkaline melts. The first group corresponds to the MARID rocks characterised by the mineral association mica (phlogopite)–amphibole (K-richterite)–rutile–ilmenite–clinopyroxene and the second group consists of the PIC rocks characterised by the mineral association mica (phlogopite)–ilmenite–clinopyroxene–minor rutile. The two groups are clearly distinguished from one another by their mineral paragenesis, by the major element composition of their phlogopite and ilmenite, by the trace element content of their clinopyroxene and by their clinopyroxene and whole rock Sr and Nd isotope ratios. The combined major and trace element variations are interpreted to indicate a genetic relationship between the PIC rocks and group I kimberlite magma, and between the MARID rocks and group II kimberlite magma. The two types of parental melts percolated through, and metasomatised, the upper mantle beneath the Kimberley area as indicated by the trace element characteristics of the clinopyroxenes of the studied phlogopite-bearing peridotites.