Литосфера (May 2020)
Xenocrysts and megacrysts of alkaline olivine-basalt-basanite-nephelinite association Makhtesh Ramon (Israel): interaction with carrier magmas and crystallographic transformations
Abstract
The article considers xenocrysts and megacrysts hosted in the rocks of Early Cretaceous olivine-basalt-basanite-nephelinite association that outcropped in the erosion crater of Makhtesh Ramon (Natural Reserve of Mishmar ha-Nagev, Israel). This magmatic rock association contains a wide spectrum of xenoliths trapped at different crustal levels. These are upper mantle, lower and upper crustal xenoliths. Mantle xenoliths are represented by peridotites, olivine clinopyroxenites, clinopyroxenites, olivine websterites, websterites and their amphibole-bearing analogues. Lower crustal xenoliths are mafic granulites, such as metagabbros and plagioclasites, whereas upper crustal xenoliths are the fragments of Neoproterozoic tuffs. Xenocrysts and megacrysts are the fragments of xenoliths that chipped from them on their way to the surface. Alterations in xenoliths, xenocrysts and megacrysts caused by the host melt constitute a common petrographic feature. Xenocrysts and megacrysts are mainly represented by minerals that are compatible with the magmatic rock association. These are olivine, clinopyroxene, amphibole, nepheline, plagioclase, anorthoclase, apatite, magnetite and spinel. The xenocrysts of quartz and orthopyroxene are incompatible with the SiO2-undersaturated host rock of this magmatic association. Main reasons determining the interaction between magma and xenoliths include rapid decompression, metamorphism and metasomatism. Xenocrysts are subjected to metamorphism that corresponds to high-temperature facies of contact metamorphism, up to the partial melting of xenocrysts. Metasomatism is directed at equalising the compositions of xenocrysts and eponymous minerals that crystallised from the host melt. There are several important criteria adopted to identify xenocrysts and distinguish them from phenocrysts. These are partial melting, solid-phase decomposition, decrystallisation of primary (before-trapping) textures, recrystallisation and self-faceting of initially xenomorphic grains into the crystals with perfect habits. The chemical composition of xenocrysts has mineral and geochemical indications of xenogenic origin, as well as the signs of a newly-formed substance.
