Abstract

The Hyde School and Reservoir Hill alaskite bodies in the Grenville part of the northwest Adirondacks have a mineral, textural, and chemical zonation that strongly suggests relict stratigraphy. These and other bodies constitute a wide-ranging sequence of volcanic and sedimentary rocks that is a useful marker horizon in the Grenville stratigraphic sequence.

The thick feldspathic rock sequence of the two bodies includes granitic alaskite, granite, trondhjemite, quartz diorite, leucodiorite, and charnockitic alaskite that, in bodies throughout the region, are interlayered with garnet-sillimanite gneiss, biotite schist, carbonate, calc-silicate, and thin amphibolite masses. The Reservoir Hill body is petrographically and chemically less variable than the Hyde School body and consists chiefly of the granitic alaskite member. The Hyde School body is zoned with respect to alkali feldspar, texture, and anorthite content of the plagioclase. Rocks are chemically gradational between siliceous granitic alaskite and plagioclase-rich varieties. Trend surface analyses for each element oxide show contours that conform with gross structure of the alaskite bodies, suggesting that layers of similar chemistry have been folded.

Rocks are believed to be recrystallized liquid and (or) ash outpourings of dominantly rhyolitic composition (the granitic alaskite), followed by differentiation within the magma chamber and eruption of quartz latite and dacite (the marginal plagioclase-rich rocks). The presence of abundant Na2O in the alaskite bodies is a problem associated with other Grenville feldspathic rocks and may represent enrichment during diagenesis of tuff or hydrothermal alteration prior to metamorphism.

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