Topic
Phenocryst
About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.
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Abstract: The subsolvus biotite-hornblende-granite at Megantic contains mafic to leucocratic enclaves. The groundmass of leucocratic enclaves is microgranitic; that of mafic enclaves is meladioritic. Apatite is commonly acicular in the enclaves, suggesting quenching of mafic magmas commingled with the granitic host. Mafic enclaves contain mafic clots (hornblende cores, biotite rims), some with relic cores of viAl-Cr-rich diopsidic augite similar to phenocrysts in associated basaltic dykes. The clots are interpreted to be pseudomorphs after mafic phenocrysts. The enclaves also contain granitic microxenoliths, xenocrysts of quartz with hornblende reaction rims, partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts. The xenocrysts record hybridization between a mafic enclave magma and a granitic magma and/or rock. Enclaves define mixing lines on many variation diagrams. However, Rb, Sr, Ba, Zr, Nb, Y, REE, Zn, Pb, and F do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles. The data suggest that the granite is anatectic, heat being provided by crystallization of associated basalts. Mixing calculations suggest that as little as 5% hybridization with basalt or hawaiite can account for the Sr isotopic heterogeneity of the granites.
70 citations
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TL;DR: The Siletz River Volcanic Series of northwestern Oregon offers an ideal opportunity to determine the direction of the Earth's magnetic field in the western United States during the early Tertiary.
Abstract: THE Siletz River Volcanic Series of north-western Oregon offers an ideal opportunity to determine the direction of the Earth's magnetic field in the western United States during the early Tertiary. The formation consists of 3,000–5,000 ft. of mainly submarine basaltic flows, flow breccia and tuff. Interbedded clastic sedimentary strata have yielded more than thirty species of fossils including brachiopoda and Turritella andersoni cf. subsp. susanae Merriam, on the basis of which J. Durham correlated this formation with the Capay shale of California1. Additional faunal assemblages, including some foraminifera collected subsequently to those referred to above, confirm this correlation and clearly establish the age of the Siletz River Volcanic Series as early middle Eocene to lower Eocene. Petrographically, the flows are porphyritic in texture, with phenocrysts of plagioclase and augite in a ground mass which consists of volcanic glass, plagioclase laths, and granules of augito and magnetite, the latter constituting as much as 15 per cent of the rock1.
70 citations
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TL;DR: In this article, chemical and isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton.
Abstract: Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H 2 O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.
70 citations
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TL;DR: In this paper, the authors reported the occurrence of inclusions of low-Ca and high-Ca pyroxenes in the olivine phenocrysts in the Udachnaya-East hypabyssal kimberlite (Yakutia, Russia).
70 citations
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TL;DR: A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis as mentioned in this paper.
70 citations