TECTONIC SETTING OF THE C. 1060-1030 MA OTTAWAN-AGED LYON MT. GRANITE, HOST TO THE IRON OXIDE-APATITE DEPOSITS IN THE ADIRONDACKS OF NORTHERN NEW YORK STATE

Understanding the tectonic setting of ores and related rocks is key to developing holistic models of ore generation. Iron oxide-apatite deposits are found throughout the eastern Adirondack Highlands of northern New York State. These deposits are intimately related to bodies of the Lyon Mt. Granite (LMG), whose age and origin continue to be extensively debated.

The Skiff Mt. body of the LMG, found 15 km W of Ticonderoga, preserves magmatic textures and contains zircon with bright CL cores truncated by dark oscillatory zoned rims. Zircon cores can be embayed and inclusions are mostly found along the core-rim boundary. Ion microprobe dating of rims provides a date of c. 1063 Ma, which is interpreted as the body’s magmatic crystallization age and confirms LMG intrusion occurred during the 1090-1030 Ma Ottawan orogeny.

Monazite from a migmatitic paragneiss outcrop 2 km W of Ticonderoga have U-Th-total Pb dates of 1070-1045 Ma that record a drop in monazite Y content from about 14,000 to 2500 ppm. This indicates garnet growth during this time. Monazite continues to record younger ages at low Y levels until c. 1010 Ma where subsequent monazite dates records progressively higher Y concentrations, indicating garnet breakdown. Garnet breakdown is, in part, associated with fluids based on textures. Zircon from late to syn-kinematic leucosome, dated by LA-ICP-MS, indicate crystallization and deformation ending at about 1047 Ma.

Immediately adjacent to the paragneiss is a garnet amphibolite. Thermodynamical modelling of this rock in the MnNCFMASHTO chemical system produces a model where observed mineral abundances and composition best match metamorphic conditions at about 755^oC and 5 kbar. These conditions would result in extensive paragneiss melting.

LMG intrusion has widely been interpreted to occur during collapse of the Ottawan orogeny, beginning at about 1070-1050 Ma. Collapse should accompany decompression, but preliminary paragneiss thermodynamical models indicate that decompression at 755^oC results in garnet consumption. An alternative model is that a rise in temperature during a prograde path produced paragneiss melting and garnet growth during 1070-1045 Ma. This alternative model suggests that the LMG are crustal melts associated with peak Ottawan metamorphism.