| Description |
viii, 56 leaves : illustrations ; 28 cm |
| Summary |
"The origin of quartz glomerocrysts a distinctive petrographic feature of the rhyolite dike in Medicine Park, Oklahoma, was investigated using transmitted light and cathodoluminescence microscopy to determine if quartz glomerocrysts formed during quartz crystallization or during quartz dissolution. Quartz glomerocrysts are typically comprised of two to six individual phenocrysts of quartz and commonly exhibit subhedral partially embayed crystal forms with very rare euhedral phenocrysts in both glomerocrysts and individual quartz phenocrysts. The size range of the individual quartz phenocrysts are 0.08mm to 1.7mm while the size range is 0.08mm to 3.25mm for quartz glomerocrysts . Cathodoluminescence revealed that individual quartz phenocrysts which comprise glomerocrysts showed abrupt truncation of internal compositional growth zonation along the shared resorbed crystal surfaces, demonstrating the quartz glomerocrysts formed after initiation of quartz dissolution. The driving force behind quartz dissolution is consistent with decompression during magma ascent resulting in a decrease in the stability field for quartz due to the shift in the position of the coetectic in the system Q-Ab-Or-H₂O. Juxtaposition of dissolving quartz phenocrysts during magma ascent leads to the formation of glomerocrysts as a result of crystallization of the overlapping boundary layer melts that surround the dissolving quartz phenocrysts. The common occurrence of glomerophyric quartz phenocrysts in granites, akin to those observed in the rhyolite dike, may have also formed as a result of decompression dissolution, thus providing a textural record of magmatic ascent"--Abstract, leaf iii. |
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