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
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 755oC 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 755oC 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.