MAGNETIC POLARITY CONSTRAINTS ON AGE OF PLIOCENE PLUTONS, NORTH CASCADES

Paleomagnetic results from four intrusive units (Lake Ann, Nooksack Cirque and Icy Peak plutons, and a dike in the latter two plutons) east of Mount Baker, Washington support no significant tilting or rotation since mid to early Pliocene. Hysteresis properties and Curie temperatures indicate that multi-domain magnetite is the predominate magnetic remnance carrier in each unit.

Magnetic polarity provides a simple check of radiometric ages for these units. Ages for all three plutons fall within normal polarity chrons. This agrees with normal polarity observed in 3.27 Ma Nooksack Cirque and 3.34 Ma Icy Peak plutons, corroborating their radiometric ages. However, the 2.75 Ma Lake Ann pluton magnetization is reverse polarity. We speculate, based on this magnetic polarity constraint and possible age dating errors, that the Lake Ann pluton is older than 3.04 Ma (C2An.1n). The dike has not been dated, but its reverse polarity magnetization and crosscutting relationship restrict its age to a reverse polarity chron younger than 3.11 Ma (C2An.2n). Determining magnetic polarity of more such units would compliment crosscutting relationships in constraining ages in such a complex area without dating each body.