ORIGIN OF AMCG BODIES IN THE HUDSON RIVER LINEAMENT: AN OUTRAGEOUS HYPOTHESIS UPDATED

The Hudson River valley outlines a NW-SE lineament in The Glen, North Creek, and Johnsburg quadrangles of the southeastern Adirondack Mountains. The lineament is at least 30 km long and 3-7 km wide, and it separates the SE Adirondack Allochthonous Block from the Siamese Ponds-Thirteenth Lake anorthosite-leucogabbro massif. The lineament consists of elongated, roughly elliptical bodies of Anorthosite-Mangerite-Charnockite-Granite (AMCG) surrounded by a thick marble. Most of the volume of the AMCG bodies is charnockitic and mangeritic with layers of anorthosite and metagabbro/leuconorite truncated at the contacts with surrounding marble. Dips of layers and lineations are steeply ENE to ESE in the lineament.

A unique layered ultramafic (UM) body described by Farrar (1976) is found in the lineament. This body may have major significance. McLelland et al. (2004) postulated that Adirondack anorthosite bodies were derived from aluminous magmas that originated in the mantle, rose to the base of continental crust, ponded and underwent fractional crystallization, with early formed UM bodies sinking back into the mantle. The Hudson River UM body may well prove that postulate. The problem then becomes one of explaining the mechanics that caused the UM body to become encased in marble along with the bodies of AMCG rock such as the Crane Mountain units mapped by Isachsen.

Subduction of carbonate rock from a forearc basin to the east and carried westward in the subduction zone as far as the base of the anorthosite domes, then squeezed upward along major shear zones at the leading edge of the Allochthonous Block would constitute a flowing stream of highly rheid marble. In a fashion similar to glacial plucking of bedrock at the surface, it is proposed that the flowing marble plucked AMCG bodies and the UM body from the base of the crust and carried them into their present positions.

The Ottawan compressional event between Laurentia and Amazonia may have resulted in some obduction of Amazonia over and affecting the allochthonous zone of the southeastern Adirondacks.