megacryst

megacryst

[′meg·ə‚krist]
(petrology)
Any crystal or grain in an igneous or metamorphic rock that is significantly larger than the surrounding matrix.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The most common megacryst phase is plagioclase, followed by amphibole, all set in a fine-grained matrix (Fig.
Finally, a coarse muscovite grain from a quartz-feldspar-muscovite intergrowth surrounding a blocky K-feldspar megacryst was dated with the [sup.40]Ar/[sup.39] Ar spot laser technique and yielded a maximum age of 347.6 Ma for a core analysis and minimum age of 318.4 Ma for a rim analysis.
This dominantly monzogranitic pluton, generated by hybridization between mantle and crustal magmas, consists of mappable facies; end members range from less hybridized with high concentrations of very coarse K-feldspar megacrysts and abundant mafic microgranular enclaves, to more hybridized with low megacryst and enclave abundances.
A range of stages of development of massive crystalline aggregates is observed in the samples: individual microcrysts, framboids, framboid clusters, recrystallized megacryst overgrowths, and banded concentric zones.
Conticelli, "The origin of K-feldspar megacrysts hosted in alkaline potassic rocks from central Italy: a track for low-pressure processes in mafic magmas," Lithos, vol.
A close-up photograph of the outcrop along McDougall Inlet clearly shows pink megacrysts of potassium feldspar with rapakivi mantling characteristic of the Jimmy Hill Granite in contact with the equigranular John Lee Brook Granite (Fig.
Age and origin of gem corundum and zircon megacrysts from the Mercaderes-Rio Mayo area, south-west Colombia, South America.
Underplating of Mesozoic mantle-derived magmas in Tongling, Anhui Province: evidence from megacrysts and xenoliths.
It is full of feldspar megacrysts (some over 7 cm long) in a granitic groundmass.
Where K-feldspar megacrysts appear, flow textures are visible with a N130 main trend, following the contact of the granitic pluton in the most external areas.
The alkali feldspar, including microcline, is mostly perthitic and occurs as megacrysts that occasionally display simple twinning (Figures 3(a) and 3(b)).