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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.

Xenocrysts and megacrysts hosted in the rocks of Early Cretaceous olivine-basalt-basanite-nephelinite association that outcropped in erosion crater of Makhtesh Ramon (Natural Reserve of Mishmar ha-Nagev, Israel) are the topic of the current research. Magmatic rock association contains the 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 analogs. Lower crustal xenoliths are mafic granulites, such as metagabbros and plagioclasites, upper crustal xenoliths are the fragments of Neoproterozoic tuffs. Xenocrysts and megacrysts are fragments of xenoliths that chipped from them during their transportation to the surface. Different rate of xenoliths, xenocrysts, and megacrysts alteration by host magma and late fluids is a common petrographic particularity. The fluid alteration occurred at phreatomagmatic stage of magma crystallization. Alteration is observed by the appearance of new textures and products of reactional interaction. Xenocrysts and megacrysts are mainly represented by minerals that compatible with rock magmatic association. These are olivine, clinopyroxene, amphibole, nepheline, plagioclase, anorthoclase, apatite, magnetite, and spinel. Xenocrysts of quartz and orthopyroxene are incompatible to host rock magmatic association under-saturated in SiO2. Main reasons determining interaction between magma and xenolith are 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 smoothing out the composition of xenocrysts to the composition of the same minerals that crystallized from host melt. There are several important criterions, which permit to identify xenocrysts and divide them from phenocrysts. These are partial melting, solid-state decomposition, recrystallization of primary (before-trapping) textures, recrystallization and self-faceting of initially anhedral grains into the crystals with perfect habit. Chemical composition of xenocrysts has both mineral - geochemical indications of xenogenic origin and new-formed sings of alteration.