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Paleomagnetism of the Mount Stuart batholith revisited again: What has been learned since 1972?

Geology Department, Western Washington University, 516 High Street,
Bellingham, Washington 98225-9080

We have collected 20 new paleomagnetic sites from the Mount Stuart batholith and the adjacent Beckler Peak stock. Using thermal and low-temperature demagnetization, and rock-magnetic tests, we have found that the remanence in most of the batholith is carried by single-domain magnetite. The mean of the new Mount Stuart batholith sites is D = 354.2°, I = 46.2°, k = 87.2, 95 = 4.6°, N = 11, and is similar to those of the Beck and Noson (1972) and Beck and others (1981) studies. Examination of Ar geochronology of hornblende and biotite from the Mount Stuart batholith finds that the southern part of the batholith cooled through the blocking temperature of magnetite at 91 Ma. The northern part cooled through the blocking temperature of magnetite at 86 Ma, and of pyrrhotite at 83 Ma. From these combined results, we conclude that the paleomagnetic directions from the southern part of the batholith were acquired within 0.5 to 1.0 Ma of the time at which the AH barometers cooled below their closure temperature. Use of the AH barometry to establish paleohorizontal for these rocks is thus well justified. Correcting the Mount Stuart direction for the tilt indicated by the Aluminum-in-hornblende (AH) barometry, the mean becomes D = 355.9°, I = 50.6°. This result places the Mount Stuart batholith at a latitude of 31.3° ^+3.8°/_–3.4° N at 91 Ma. By identifying the carrier of remanence in the Mount Stuart batholith as being single domain (SD) magnetite, we have removed the major remaining uncertainty in interpreting the paleomagnetism of these rocks. The characteristic magnetization repeatedly found in the Mount Stuart rocks clearly supports the microplate tectonic model of Beck and Noson (1972), presently manifest as the Baja British Columbia (Baja-BC) hypothesis.