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

Abstract

Calc-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.

Melting Conditions of Granitoid Xenoliths in Contact With Alkaline Mafic Magma (Gusinoozerskaya Dyke, Western Transbaikalia): To the Problem of the Origin of Ultrapotassic Acid Melts

“…Термин «эссекситы» в полной мере отражает щелочно-ба-зитовый состав (с нормативным нефелином) этих пород, образовавшихся в гипабиссальных условиях. Однако с учетом того, что породы дайки имеют порфировую структуру и вкрапленники представ-лены исключительно цветными минералами, а по-левошпатовые фенокристаллы и фельдшпатоиды отсутствуют, согласно [Rock, 1984], данные породы являются шошонитовыми (известково-щелочны-ми) или полевошпатовыми лампрофирами [Petrographic Code…, 2009].…”

“…Практически повсе-местно данные породы характеризуются нефелин-нормативным составом (ne ~ до 10 %), K 2 O/Na 2 O варьируется от 0.7 до 0.9, при содержании K 2 O ~3.5-5.0 мас. %, концентрация Sr составляет 2300-2500 ppm и Ba -до 3000 ppm, что типично для шошонитовых (известково-щелочных) лампрофи-ров [Rock, 1984].…”

Geochemistry of late Caledonian minettes from Northern Britain: implications for the Caledonian sub-continental lithospheric mantle

“…Felsic phases are restricted to the groundmass, where K-feldspar, often sanidine, dominates and is commonly spherulitic. Mica phenocrysts are fresh and display skeletal form; often described as castellated (Rock, 1984).…”

“…Probe analyses of unaltered pyroxenes show they have a narrow range covering diopside, endiopside, augite and salite compositions. Groundmass minerals are varying amounts of Kfeldspar, well-shaped apatites, some of which may be (Rock, 1984;Henney, 1991;Shand et al, 1994). Representative data for clinopyroxene and mica phenocrysts in minettes from the Northern Highlands and the Southern Uplands are presented in Tables 1 and 2, and Figs 2 and 3.…”

“…This paper presents new data for minettes from the Affric-Kintail and Creagan Rhambher areas of the Northern Highlands and the Onich area of the Grampian Highlands and compares them with others from across the Caledonides to investigate the relationship between chemistry and geographic or tectonic setting. Only minettes that satisfy the chemical and mineralogical screens of Rock (1984) have been used. The sample locations and their relationship to the Caledonian tectonic architecture are shown in Fig.…”