Integrating Ultramafic Lamprophyres into the IUGS Classification of Igneous Rocks: Rationale and Implications

Abstract

RECEIVED JULY 16, 2004; ACCEPTED MARCH 16, 2005ADVANCE ACCESS PUBLICATION APRIL 29, 2005We introduce a modification to the current IUGS classificationsystem for igneous rocks to include ultramafic lamprophyres, whicharecurrentlyentirelyomitted.Thisisdonebyincludinganewstepinthe sequential system, after the assignment of pyroclastic rocks andcarbonatites, that considers ultramafic inequigranular textured rockswith olivine and phlogopite macrocrysts and/or phenocrysts. At thisstepultramaficlamprophyresareconsideredtogetherwithkimberlites,orangeites (former Group 2 kimberlites) and olivine lamproites.This proposal allows the correct identification and classificationof ultramafic lamprophyres within the IUGS scheme. Only threeend-members are required for describing the petrographic and com-positional continuum of ultramafic lamprophyres: alno¨ite (essentialgroundmass melilite), aillikite (essential primary carbonate) anddamtjernite (essentialgroundmass nepheline and/or alkali feldspar).It is argued that all ultramafic lamprophyre rock types canbe relatedto a common magma type which differs in important petrogeneticaspects from kimberlites, orangeites, olivine lamproites and theremainder of lamprophyres such as alkaline and calc-alkaline vari-eties. Ultramafic lamprophyres can be readily distinguished fromolivine lamproites by the occurrence of primary carbonates, and fromkimberlites by the presence of groundmass clinopyroxene. In othercases distinction between aillikites, kimberlites and orangeites mustrely on mineral compositions in order to recognize their petrogeneticaffinities.