Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins

Abstract

Lamproite is a rare ultrapotassic alkaline rock of petrological importance as it is considered to be derived from metasomatized lithospheric mantle, and of economic significance, being the host of major diamond deposits. A review of the nomenclature of lamproite results in the recommendation that members of the lamproite petrological clan be named using mineralogical-genetic classifications to distinguish them from other genetically unrelated potassic alkaline rocks, kimberlite, and diverse lamprophyres. The names “Group 2 kimberlite” and “orangeite” must be abandoned as these rock types are varieties of bona fide lamproite restricted to the Kaapvaal Craton. Lamproites exhibit extreme diversity in their mineralogy which ranges from olivine phlogopite lamproite, through phlogopite leucite lamproite and potassic titanian richterite-diopside lamproite, to leucite sanidine lamproite. Diamondiferous olivine lamproites are hybrid rocks extensively contaminated by mantle-derived xenocrystic olivine. Currently, lamproites are divided into cratonic (e.g. Leucite Hills, USA; Baifen, China) and orogenic (Mediterranean) varieties (e.g. Murcia-Almeria, Spain; Afyon, Turkey; Xungba, Tibet). Each cratonic and orogenic lamproite province differs significantly in tectonic setting and Sr–Nd–Pb–Hf isotopic compositions. Isotopic compositions indicate derivation from enriched mantle sources, having long-term low Sm/Nd and high Rb/Sr ratios, relative to bulk earth and depleted asthenospheric mantle. All lamproites are considered, on the basis of their geochemistry, to be derived from ancient mineralogically complex K–Ti–Ba–REE-rich veins, or metasomes, in the lithospheric mantle with, or without, subsequent contributions from recent asthenospheric or subducted components at the time of genesis. Lamproite primary magmas are considered to be relatively silica-rich (~50–60 wt.% SiO2), MgO-poor (3–12 wt.%), and ultrapotassic (~8–12 wt.% K2O) as exemplified by hyalo-phlogopite lamproites from the Leucite Hills (Wyoming) or Smoky Butte (Montana). Brief descriptions are given of the most important phreatomagmatic diamondiferous lamproite vents. The tectonic processes which lead to partial melting of metasomes, and/or initiation of magmatism, are described for examples of cratonic and orogenic lamproites. As each lamproite province differs with respect to its mineralogy, geochemical evolution, and tectonic setting there is no simple or common petrogenetic model for their genesis. Each province must be considered as the unique expression of the times and vagaries of ancient mantle metasomatism, coupled with diverse and complex partial melting processes, together with mixing of younger asthenospheric and lithospheric material, and, in the case of many orogenic lamproites, with Paleogene to Recent subducted material.

Dynamic Metasomatism Experiments Investigating the Interaction between Migrating Potassic Melt and Garnet Peridotite

“…However, orangeites and lamproites may be related, although the exact relationship is currently uncertain. Some refer to them as carbonate-rich lamproites [13,58], a Kaapvaal equivalent of carbonate-poor lamproites elsewhere that have been shown to originate in reduced source regions where carbonate would not be stable [24,59]. This mode of origin also occurs in eastern Zambia [60].…”

Petrogenesis of Mediterranean lamproites and associated rocks: The role of overprinted metasomatic events in the post-collisional lithospheric upper mantle

“…Lamproites are Mg-rich alkaline ultrapotassic volcanic to hypo-abyssal rocks (Foley and Venturelli 1989;Mitchell 2020). They are characterized by relatively low Al 2 O 3 , FeO Tot , CaO and Na 2 O counterbalanced by extremely high MgO, and extremely variable silica contents, the latter ranging from basic to intermediate compositions.…”

“…, 2012Prelevicé t al. , 2015Davies et al 2006;Prelevicá nd Foley 2007;Tommasini et al 2011;Ammannati et al 2016;Mitchell 2020).…”

“…Occurrences of lamproites are described in both anorogenic (i.e. within-plate) and orogenic, postcollisional tectonic settings, rarely within ancient cratons, more often in areas of thickened crust at cratonal margins, which experienced one or several episodes of compression or post-collisional collapse (Mitchell and Bergman 1991;Mitchell 2020).…”

Simple steps for the detection and classification of different lamprophyric rocks: a case study from Greece

“…proschists, appinites (sensu Rock 1991), para-lamproites (sensu Mitchell 2020) or lamproitoids and many local and obsolete varie es. These petrological terms are not IUGS accepted but are included since they are used by several igneous and explora on petrologists.…”