11 Thermal History and Metamorphism of the Adirondacks in the Context of the Broader Grenville Orogenic Cycle II

Monday, 14 March 2005
1:00 PM - 2:10 PM

SOUTHEASTERN ADIRONDACK ANORTHOSITIC SILLS AND DIKES WITH INCLUDED ULTRAMAFIC LENSES

Paper number: 11-3
Program: THEME SESSIONS
Time: 1:40 PM – 2:00 PM

Authors

Abstract

A concentration of more than 20 anorthositic sills and dikes north of Warrensburg, NY, is not closely associated with an anorthosite massif. The two largest sills (Moon Mtn. and Kelm Pond) include Mg-Al-rich ultramafic lenses.

The 50 m by 5 km Kelm Pond is layered anorthositic gabbro to gabbroic anorthosite with an ultramafic layer comprising phlog+oliv+spinel-bearing hornblende clinopyroxenite. Cpx in anorthosite is augite with 1% Al2O3; in ultramafic, diopside with 10% Al2O3. Much of this ultramafic has been retrogressed to serpentinite.

The 100 m by 7 km Moon Mtn. layered anorthosite to gabbroic anorthosite has an ultramafic lens comprising phlog+oliv+spinel-bearing hornblende orthopyroxenite. The Moon Mtn. has been contaminated by B, with tourmaline throughout and kornerupine in the ultramafic.

Perhaps these bodies are remnants of a massif-size magma which intruded to a shallower level, leaving behind these small bodies, with ultramafic lenses dragged from below, and contaminated by B-rich metasediments the magma passed through.