Granites really are magmatic: Using microstructural evidence to refute some obstinate hypotheses

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Abstract

Microstructural evidence can be used to distinguish between magmatic and solid-state features in granitoids, as well as to interpret most of the crystallization and post-crystallization history, though generally it can give little or no information about the earliest stages of crystallization or the order of nucleation of minerals. Structural evidence is critical for the evaluation of the following three, persistently promoted hypotheses, namely: (1) granitoids typically contain solid restite, (2) K-feldspar megacrysts form after most or all crystallization of granitic magma, and (3) granitoids change their grain shapes appreciably during slow cooling and so are effectively metamorphic rocks. Microstructural evidence questioning hypothesis (1) includes: a lack of convincing evidence for a restitic origin of xenocrysts and xenoliths; the igneous nature of microgranitoid enclaves; restite-poor feeder dykes; the tendency for solids to separate from liquid during flow; the tendency of restite to melt during ascent; and evidence for magmatic crystallization in many minerals in granitoids, especially oscillatory zoning, which reflects growth in liquid from the earliest stages. Microstructural evidence refuting hypothesis (2) includes: quenched felsic rocks with euhedral Kfeldspar megacrysts surrounded by an abundant fine-grained groundmass; widespread structural evidence of crystal movement, flow alignment and physical accumulation in granitic magmas; and evidence of mixing of megacrysts in hybrid mafic magma, implying the presence of megacrysts in liquid. Microstructural evidence refuting hypothesis (3) includes: preservation of euhedral crystal shapes, especially for quartz and feldspar; common preservation of oscillatory zoning without major truncations by grain boundaries; preservation of graphic intergrowths; euhedral, rather than rounded inclusion shapes; and arrangement of inclusions in crystallographic zones in the host crystals, rather than at random or in inclusion trails. Thus, microstructural evidence confirms that most granitoid minerals crystallize in liquid, even crystals ('antecrysts') that grow elsewhere and become entrained in the host granitoid, and that: (1) solid restite is generally absent or difficult to recognize, (2) K-feldspar megacrysts crystallize as normal phenocrysts, and (3) granitoids generally do not undergo major changes in grain shapes during slow cooling.

Original languageEnglish
Pages (from-to)1-36
Number of pages36
JournalThe Journal of the Virtual Explorer
Volume35
DOIs
Publication statusPublished - 2010

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