Metavolcanic rocks and their origin,Southern Adirondack mountains,New York

One of the problems associated with massif-type anorthosite in high grade metamorphic terranes is the absence of anorthositic extrusives. However, if anorthosite formed by crystal segregation, volcanic equivalents of the final fluid differentiate might occur. Massive, stratiform hornblende-garnet granitic gneisses occur between two metasedimentary sequences in asynclinorium south of the main Adirondack anorthosite massif and north of the Thirteenth Lake anorthosite dome. The granitic gneisses are divided into three stratigraphic units on the basis of mineralogy. The entire stratigraphic configuration has been traced 70 kilometers laterally, and the interpretation of the literature indicates that it may extend for over 100 kilometers. Earlier workers considered the granitic gneisses to be sill-like intrusives. The following evidence suggests a volcanic origin for these gneisses:

1. A thin (3 to 5 meters) but continuous zone of cale-silicates and quartzite occurs within the sequence of granitic gneisses at the same horizon for some 25 kilometers laterally and does not appear to be truncated nor intruded by the gneisses. This observation virtually precludes an intrusive origin.
2. 2V, of plagioclase (An_24,25) in granulated, augen-shaped zones in the granitic gneisses variesfrom 109° to 120°, whereas 2V, for groundmass plagioclase (also An_{20, 25}) is 84° to 88°. According to the data of Slemmons, the 2V of the granulated augen is characteristic of volcanic (high temperature) plagioclase and that of the groundmass is characteristic of plutonic (low temperature) plagioclase.
3. The great lateral extent of the granitic gneisses suggests that the materials were deposited as volcanic ash or pyroclastics.
4. The stratigraphic coherency of metasedimentary sequences and granitic gneisses indicates that the gneisses are an integral part of the stratigraphy.

A jotunite-mangerite-charnockite suite of intrusives in this area is related to the main anorthosite massif. Since plagioclase augen or their remnants exist in both the granitic gneisses and in the intrusive suite, it is possible that the granitic gneisses and the intrusives were derived from the same magma. It has been suggested that differentiation of the anorthosite suite occurs at considerable depth whereas differentiation of the same magma at shallow depth produces the rapakivi-laboradorite porphyry association of the Fennoscandian region. If the stratiform granitic gneisses and the anorthosite suite in the Adirondacks are comagmatic, differentiation of the magma must have occurred at moderate depth and the parental magma may have been granodioritic and thus more silicic than presumed by most workers.