Megacrysts in basanites from Monti Lessini, Veneto Region, northern Italy

Abstract

A suite of megacrysts is present at many localities in undersaturated, alkali-olivine basalts and basanites from the Venetian Volcanic Province of Paleocene to Upper Oligocene age. The megacrysts (clinopyroxene, amphibole, spinels and feldspars) belong either to the group A or to the group B , according to the classification by Irving (1984). The clinopyroxene megacrysts are Al- and Ti-rich augites which plot within the compositional range of megacrysts in basalts defined on a world-wide basis. A composite clinopyroxene megacryst includes some orthopyroxene grains of Al- and Ti-rich bronzite. All the amphiboles megacrysts, either single or composite, can be classified as kaersutites. The very similar composition of: both single and composite amphibole megacrysts, suggests that they crystallized from a broadly similar liquid in a narrow range of physical conditions. The spinel megacrysts belong to either the group A and B of megacryst suites. Chemically they are hercynites (group A) and titanomagnetites (group B) with high ulvospinel contents. A high pressure origin, not necessarily from the present host basanites, is suggested for the hercynite-ferripleonaste megacrysts. The feldspar megacrysts vary in composition from K-rich oligoclase to calcic-anorthoclase and belong to the group B of megacryst suite.Because the large range in compositione.g., mg ratios, TiO2 etc. of the megacryst suite from the Veneto basaltic rocks, it is concluded that the present host rocks cannot represent the liquids from which the whole suite of megacrysts crystallized. Consequently, they may be considered xenocrysts, likely crystallized from parental and/or more evolved and fractionated melts of the present host rocks and later picked up by ascending magmas and carried to the surface.