ANOTHER LOOK AT THE ADIRONDACK ANORTHOSITE MASSIF

The Adirondack Anorthosite Massif is the most prominent feature in the Adirondack Mountains. Yet, its origin and structure has been controversial for the past 100 years. Some argue that it is an accumulation of magmatically differentiated plagioclase crystals that produced higher density mafic minerals at depth while others believe the anorthosite intruded in a liquid of plagioclase crystals rather than a differentiate.

Various shapes have been suggested for the anorthosite, a layered structure (Cannon 1937), a sill (Grant 1928), a batholith (Bowen 1917) and a tilted lens (Balk 1931). More recently Simmons (1964) determined the anorthosite massif to be a layered body 4 kms thick and having two roots extending downward to a depth of 10 kms. This model is based on a gravity low over the anorthosite with a slab shape and two negative local anomalies due to low density cylindrical roots.

Detailed gravity measurements by Mann over the anorthosite indicate the broad gravity low is a series of five gravity lows separated by intervening gravity highs. The gravity highs are interpreted as due to the thinning of anorthosite or presence of higher density rocks such as gabbro. The gravity low over the anorthosite is rimmed by a series of metagabbro bodies. In the northeast part of the Grenville in Quebec strong gravity positive anomalies are associated with anorthosite. Local discontinuous reflections within the anorthosite could be due to gabbroic horizons within the anorthosite (Hughes). The ambiguity in gravity interpretation and lack of a seismic expression of the anorthosite and the possibility of mafic bodies formed by differentiation suggest other models are possible.