Compositions of natural sillimanites from volcanic inclusions and metamorphic rocks: A reply
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Besides many granitic xenoliths of volcanic rocks， their original granitic rocks and the host volcanic rocks ranging from basalt to andesite， all found in Southwest Japan， were petrographically examined before the detailed studies of zircons in them were made． The xenoliths can be divided into 6ve groups according to the degree of rock alteration．In the granitic xenoliths ma丘c minerals are most easily decomposed to produce mainly iron-ores and brown glass． The microperthitic texture of the original potash feldspar in the greater parts of the xenoliths disapPeared and this mineral is readily resorbed after the mafic rninerals． Plagioclase and quartz are more resistant to fusion：the former is commonly clouded to various extent with minute inclusions， and the latter is usually crackled， while vitrified plagioclase is apt to show 6ne mesh works of glass along its cleavages． Feldspar optics shows that potash feldspar and plagioclase in all the xenoliths are completely or incompletely converted to the high．temperature form．The observations described above may support the conclusion that the temperature attained by the granitic xenoliths in question should be between 500°and 573°in one example and above 500°to 700°and probably below about 900° in the others． ContentgYoshifumi KムRAKIDム ToMITA． The results of my investigation show that some of character changes in the zircons under consideration may be attributed to volcanic heating， an answer to these problems having been already discussed in paper I(ToMITA and KARAKIDA， 1958)． It is the purpose of this investigation to describe the modes of occurrence of granitic xenoliths collected from some volcanic丘elds in Southwest Japan；to examine the alteration of their main constituent minerals；to estimate the temperatures attained by the xenoliths on the basis of those observations． These description and discussion would serve as a fundamental knowledge for understanding the petrologic signi丘cance of the natural change in zircon characters which will be dealt with in a series of papers now in preparation．
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In this paper, tectonic and geothermobarometric environments have been studied with respect to the combination of pyroxene, olivine and plagioclase in volcanic rocks in the southeast of Bam. The combination of volcanic rocks in the region consists of olivine basalt, basalt, alkaline basalt, andesite, trachyandesite and pyroxene andesite. This combination is the result of the processes of crystallization and sometimes contamination. Plagioclase, clinopyroxene, olivine, and amphibole constitute the major minerals (rock forming minerals) in these rocks. Porphyritic to mega-porphyritic textures with microlithic, glumero-porphyritic and amigdaluidal matrix are observed. Based on the thermometric calculations, plagioclase, pyroxene, and olivine minerals and the rocks of this region are crystallized at a pressure of 1.5 to 7 kb and temperatures ranging from about 700°C to 1250°C.
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Ultramafic xenoliths at The Thumb, a small minette neck near Shiprock, New Mexico, are divisible into three overlapping textural/compositional groups: coarse garnet peridotites with minerals relatively low in Fe and Ti and high in Cr, sheared garnet peridotites with minerals relatively high in Fe, Ti, and Cr, and megacrystalline rocks (medium- to ultracoarse- grained intergrowths of clinopyroxene, orthopyroxene, olivine, garnet, phlogopite, and ilmenite) with minerals very rich in Fe and Ti and low in Cr. The megacrystalline rocks are compositionally similar to the "discrete nodule" suite of Lesotho kimberlites, but are fragments of polycrystalline aggregates rather than phenocryst-like single crystals. Pyroxene compositions indicate that the inclusions equilibrated at temperatures ranging from 930–1230°C within a narrow vertical interval at around 130 km depth. The extreme textural and compositional heterogeneity of this restricted source terrane and the correlation between xenolith textures and mineral compositions are explained by the following model: The megacrystalline rocks are interpreted as precipitates from liquids introduced into an older, previously depleted terrane consisting of the coarse garnet peridotites. The Ti-Fe-rich sheared peridotites are thought to be the products of deformation and Ti-Fe-metasomatism of the coarse peridotite country rocks in tectonically disrupted aureoles surrounding the intrusive megacrystalline regions. The suggested metasomatizing medium is a residual liquid intermediate in composition between clinopyroxene and ilmenite. The model for The Thumb's inclusion suite may also be extended to explain the texturally and compositionally analogous suite of xenoliths described by Boyd and Nixon (1974) from Lesotho kimberlites. The Lesotho sheared garnet peridotites are viewed as the products of extensive metasomatism of previously less enriched mantle in response to intrusion of magma from which the discrete nodule association crystallized.
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This chapter contains sections titled: Introduction Classification and Mineralogy Chemistry Depth Relations of the Series and thier Origins
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Volcanic plateau
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