Nature and origin of calc-alkaline lamprophyres: minettes, vogesites, kersantites and spessartites

Abstract

ABSTRACTCalc-alkaline lamprophyres are porphyritic dyke-rocks, richer in amphibole, biotite, ultramark elements (Mg, Cr, Ni) and incompatible elements (K, F, P, Rb, Sr, Zn, Nb, Ba, REE, Th, U) than other rocks of comparable colour index (35–67) or % SiO2(46–57). Field and petrological criteria are suggested for identifying them uniquely. New average compositions, based on some 600 screened analyses, confirm the chemical variability but possible heteromorphism of vogesites, kersantites, spessartites and some minettes. Calc-alkaline lamprophyres probably crystallise from volatile-rich, crystal-laden fluids. Microdioritic ‘porphyrites’, K-rich syenites and quartz-feldspar porphyries commonly form from these fluids by acidic hybridisation, and more rarely byin situdifferentiation. Calc-alkaline lamprophyres have close compositional equivalents among, for example, shonkinites and absarokites, but not among kimberlites or common plutonie or volcanic rocks. They are compositionally more ‘crustal’ than lamproites and leucitites, despite some overlap. They are far more widespread than other K-rich or alkaline rocks. Three genetic petrological associations are confirmed: with calc-alkaline granitoid plutons (A), with shoshonitic volcanic and subvolcanic suites (B), and with appinite—breccia-pipe complexes (C). Most calc-alkaline lamprophyres, from association A, perhaps form by crustal modification of primary lamproitic or leucititic magmas; a very few, carrying rare mantle-type xenoliths, may represent relatively unmodified, but otherwise similar, primary magmas. Those of association B may form merely by volatile enrichment of shoshonitic magmas during subvolcanic crystallisation. Different origins for minettes in these associations are suggested by compositional differences, revealed by discriminant analysis.