A six-point whole-rock isochron of 1032 ± 45 (Iσ) m.y. has been established for the Canopus diorite-monzonite pluton in the Hudson Highlands of New York (41°20′ N.-73°55′ W.). The Canopus pluton intrudes Precambrian gneisses of hornblende granulite metamorphic grade that formed during a dynamothermal event about 1150 m.y. ago, based on previously reported zircon data (Long and Kulp, 1962).

Structural studies indicate intrusion was accompanied by right-lateral strike-slip faulting along the ancestral Ramapo fracture system (Ratcliffe, 1971), which was contemporaneous with a period of F2 folding in the gneisses along a N. 40° E.-trending fold system. The pluton is thought to have been emplaced during a period of strike-slip faulting and the F2 folding episode.

K-Ar and Rb-Sr mineral ages range from 700 m.y. to 524 m.y. in the igneous rocks. The lower ages are found near the fracture zone west of Canopus Hollow on the east side of the pluton and may have been produced by reactivation of the fault zone in Phanerozoic time, or may be a consequence of proximity to a region affected by Paleozoic metamorphism.

Phlogopite from a phlogopite-talc calcitic marble shear zone in diopside calcite marbles has a K-Ar age of 893 ± 10 m.y. This phlogopite establishes a minimum age for cataclasis and retrogression along the western mylonite zone at Canopus Hollow that is consistent with the Rb-Sr whole-rock age of the pluton, and suggests that this marble is of Precambrian age as stated by Berkey and Rice (1919).

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