Original paper
The incipient flash melting of scapolite and plagioclase megacrysts in alkali basalts of the Olot suite, Catalunya, Spain, and at Chuquet Genestoux, Puy-de-Dôme, France
Torró, Lisard; Martin, Robert F.; Schumann, Dirk; Cox, Julia; Galí Medina, Salvador; Melgarejo Draper, Joan Carles
European Journal of Mineralogy Volume 30 Number 1 (2018), p. 45 - 59
published: Feb 1, 2018
Abstract
Megacryst-bearing alkaline basic rocks of Cenozoic age occur in scattered centers in western Europe, from the Rhine graben to southern Spain. We focus on two volcanic fields, La Garrotxa in northeastern Spain, and in particular the Pomareda and Roca Negra monogenetic cones in the Olot suite, and the Chuquet Genestoux occurrence, Puy-de-Dôme, France. The explosive emplacement of the basanitic magma favored the entrainment of xenoliths of clearly crustal and mantle origin in both suites. We seek to characterize the megacrysts of scapolite and andesine. The sulfate-bearing scapolite crystallized in space group I 4/ m rather than the expected P 42/ n, likely because of disorder in the sulfate groups. The megacrysts in both suites are part of an igneous mantle-derived assemblage, to judge from δ18 O values, supported by δ13 C and δ34 S values of the sulfate-bearing scapolite. We document signs of flash melting in both scapolite and plagioclase megacrysts from Pomareda. A white rim of up to 2 mm on the scapolite megacrysts consists of skeletal individuals of calcic plagioclase; these crystals contain wollastonite, silvialite, diopside, and myriads of vacuoles. The andesine megacryst also underwent melting at its margin, to a sieve-textured assemblage of neoformed labradorite, glass, and free silica. The flash melting of individual megacrysts encapsulated in basanitic melt, documented here for the first time, has produced metastable products. Progress of the melting reactions was arrested by a thermal quench upon eruption.
Keywords
chuquet • spain • andesine • olot volcanic suite • flash melting • scapolite • genestoux • france • explosive emplacement • basanite • megacryst