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Geochemistry of Massibe and Gneissic Varities of the Montpelier Anorthosite, Goochland Terrane, Virginia, and Comparisons with Other Alkalic Anorthosites
Iriarte, Kristen
Iriarte, Kristen
Abstract
The Montpelier anorthosite is a small pluton located 5.5 km SW of Montpelier, VA. Two dominant textural varieties of anorthosite occur at Montpelier: 1) a massive light gray variety with megacrysts of plagioclase in a recrystallized matrix; and 2) a gneissic variety that is strongly foliated with layers of feldspar and bands of rutile, ilmenite and biotite. Previous investigations have suggested that the gneissic variety is the highly deformed version of the massive variety. Whole-rock major and trace element compositions determined in this study show that both varieties are chemically similar, confirming previous suggestions, although gneissic samples tend to be slightly lower in normative An. Normative feldspar compositions in all samples range from An24~An28 and Or)s~Or2), compositions that are vastly different from other massif anorthosites worldwide (typically in the andesine to labradorite range). Sr and Ba are the most common trace elements in both textural varieties, with concentrations of 878-1180 ppm and 1057-1452 ppm respectively. The likely source for this extremely alkalic composition is from anomalously alkalic source material, for example, enriched lithospheric mantle. Montpelier is chemically most similar to the Roseland anorthosite in Virginia, but is likely the most potassic anorthosite in the world.
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2004-01-01
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Geology