Lamproites are geographically widespread yet are volumetrically insignificant. Unlike kimberlites, which are found exclusively in Archaeancratons, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They also vary widely in age, from Proterozoic to Pleistocene, the youngest known example being 56,000 ± 5,000 years old.
Lamproites form from partially melted mantle at depths exceeding 150 km. The molten material is forced to the surface in volcanic pipes, bringing with it xenoliths and diamonds from the harzburgiticperidotite or eclogite mantle regions where diamond formation is stabilized.
Recent research, for example on the lamproites at Gaussberg in Antarctica, and lead-lead isotope geochemistry have revealed that the source of lamproites may be transition zone melts of subducted lithosphere which has become trapped at the base of the lithospheric mantle. This observation also reconciles the depth of melting with the peculiar geochemistry, which is most easily explained by melting of already felsic material under deep mantle conditions.
The mineralogy of lamproites is controlled by their peculiar geochemistry, with a predominance of rare silica-deficient mineral species and rare, mantle-derived minerals predominating.
Minerals typical of lamproites include: forsteritic olivine; high ironleucite; titanium-rich aluminium-poor phlogopite; potassium– and titanium-rich richterite; low aluminium diopside; and iron-rich sanidine. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8% potassium oxide. High chromium and nickel content is typical. The rocks commonly are altered to talc with carbonate or serpentine, chlorite, and magnetite. Zeolites and quartz may also occur.
Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases (Mitchell & Bergman, 1991):
- titanian (2-10 wt% TiO2), aluminium-poor (5-12 wt% Al2O3) phenocrystic phlogopite
- titanian (5-10 wt% TiO2) groundmass poikilitic “tetraferriphlogopite”
- titanian (3-5 wt% TiO2) potassium (4-6 wt% K2O) richterite
- forsteritic olivine
- aluminium-poor (2O3), sodium-poor (2O) diopside
- nonstoichiometric iron-rich (1-4 wt% Fe2O3) leucite, and
- iron-rich sanidine (typically 1-5 wt% Fe2O3)).
The presence of all the above phases is not required in order to classify a rock as a lamproite. Any one mineral may be dominant, and this, together with the two or three other major minerals present, suffices to determine the petrographic name.
The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.