RI-08 K-Ar Ages for Hornblende from Granites and Gneisses and for Basaltic Intrusives in Minnesota

Abstract

K-Ar ages have been determined for hornblende concentrates from granitic intrusive and metamorphic rocks from the Minnesota River valley, northeastern Minnesota, and an adjacent area in Ontario, which were involved in the Algoman orogeny. Nine determinations giving ages ranging from 2570 to 2770 m. y. have an average of 2670 + 30 m.y. This is significantly older than the age of about 2500 m.y. indicated by the Rb-Sr and K-Ar methods for biotite from these rocks. The age, however, agrees with the U -Pb ages of about 2700 m. y. for coexisting zircons. Mineral and whole-rock K-Ar ages for basic rocks that invaded the granitic and metamorphic rocks show two periods of intrusion in Minnesota--at 1600-1800 m. y. and at or before 2100 m. y. --in addition to the late Keweenawan igneous activity at about 1100 m. y.