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A petrographic study of phlogopite phenocrysts from the Leucite Hills, Wyoming



A petrographic study of phlogopite phenocrysts from the Leucite Hills, Wyoming



Abstracts with Programs - Geological Society of America 32(4): 5



The Leucite Hills in southwestern Wyoming are composed of 21 lamproite rock bodies consisting of lava flows, cinder cones, dikes, necks, and plugs. These rocks are characterized as diopside leucite phlogopite lamproites (orendites), diopside leucite phlogopite lamproites (wyomingites), and diopside phlogopite lamproites (madupites). They consist mostly of phlogopite and diopside phenocrysts set in a varying groundmass of leucite, sanadine, richterite, apatite, priderite, perovskite, and wadeite. Preliminary inspection of thin sections indicated that a wide variety of phlogopite morphologies exist. Orendites and wyomingites carry euhedral phlogopite phenocrysts, whereas the madupites contain poikilitic phlogopites. A few rocks, characterized as transitional madupites, contain euhedral phenocrysts with poikilitic rims of phlogopite. Some phenocrysts display apparent compositional rims as well. In order to further quantify and assess phlogopite characteristics, phenocrysts from all 21 exposures were evaluated for several characteristics including, modal percentages, phenocryst length and width, aspect ratios, and any other distinguishing features using a petrographic microscope and computer image analysis. These results were then compared to rock composition, rock body type, geographical distribution, and exposure age. Results indicate several connections between phenocryst characteristics and rock body type and compositional variety. Lava flows exhibited both the largest aspect ratios and the longest crystals. This is consistent with experimental studies, which show that large aspect ratios are associated with high rates of undercooling. The highest phlogopite content was observed in the volcanic plugs and dikes, which presumably cooled at a slower rate. Data analysis is ongoing and additional parameters, such as crystal inclusions and alteration rims, are being evaluated.

Accession: 029742548

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