The matrix is sometimes micropegmatitic or granophyric. The glassy rhyolites include obsidian , pitchstone , perlite , and pumice. The chemical composition of rhyolite is very like that of granite.
Age and Tectonic Implications of Granitoid Rocks from the Indian Plate of Northern Pakistan
This equivalence implies that at least some and probably most granites are of magmatic origin. The phenocrysts of rhyolite may include quartz , alkali feldspar , oligoclase feldspar, biotite , amphibole, or pyroxene. If an alkali pyroxene or alkali amphibole is the principal dark mineral , oligoclase will be rare or absent, and the feldspar phenocrysts will consist largely or entirely of alkali feldspar; rocks of this sort are called pantellerite.
Certain differences between rhyolite and granite are noteworthy. Muscovite, a common mineral in granite, occurs very rarely and only as an alteration product in rhyolite. In most granites the alkali feldspar is a soda-poor microcline or microcline-perthite; in most rhyolites, however, it is sanidine , not infrequently rich in soda.
A great excess of potassium over sodium, uncommon in granite except as a consequence of hydrothermal alteration, is not uncommon in rhyolites. Rhyolites are known from all parts of the Earth and from all geologic ages. They are mostly confined, like granites, to the continents or their immediate margins, but they are not entirely lacking elsewhere. Small quantities of rhyolite or quartz trachyte have been described from oceanic islands remote from any continent.
Age and Tectonic Implications of Granitoid Rocks
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See Article History. Age data on these volcanites are quite scattered: 3.
https://manfsumbattmatvie.tk Fig 1: Schematic geologic map of S. Vincenzo area. After Pinarelli et al.
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Vincenzo volcanites have a porphiritic texture with glassy, cripto- or micro-crystalline groundmass. The phenocryst assemblage is dominated by quartz, alkali feldspar, plagioclase, and biotite with lesser amounts of cordierite; apatite, epidote, monazite, ilmenite, and zircon are present as accessory phases. Groundmass phases include plagioclase, biotite, apatite, zircon, and ilmenite.
Some lavas contain clinopyroxene and orthopyroxene xenocrysts and latite enclaves. Some the authors report a division of the S. It is to note that the two groups reflect different geographical settings. At San Vincenzo pure anatectic rocks and rocks derived by interaction among pure anatectic and mantle derived melts were observed.
The mixed rocks derived by a two end-member mixing process, with scarce, if any, fractional crystallization, between a felsic end-member similar to the NMG rocks and a mafic endmember presumably similar to Capraia magmas. Evolution of S.
Vincenzo volcanism can be envisaged as follow: Mafic magmas were injected into a magma chamber filled by anatectic felsic melts. The chamber became stratified with the upper part constituted by felsic magmas and the lower part constituted by mixed magmas. The repeated injection of the mafic magma triggered sequential extrusions of the felsic magmas and of the mixed magmas.
The first volcanic event filled the west part of the area, and, forming a structural high, prevented the second volcanie event to outpour on the same part. Consequently the MG magmas filled the east part of the area.
Cordierite phenocrysts. San Vincenzo rhyolite sample.