ROCK-MAGNETIC STUDY OF THE WALLOWA BATHOLITH

We studied the late Jurassic- early Cretaceous Wallowa batholith in the allochthonous Blue Mountains province of northeastern Oregon using paleomagnetic, rock magnetic and other methods. The objective of this work is to add to the existing paleomagnetic data (Wilson and Cox, 1980 and Housen, 2007) from the Jura-Cretaceous intrusives of the Blue Mountains to better assess estimates of rotation, and of paleolatitude. We used ~250 samples from the Wallowa and Sunset Butte intrusives provided by the authors of Zak et al. (2012). This abstract focuses on result from the Wallowa batholith, which comprises the Craig Mountain, Hurricane Divide, and Pole Bridge plutons.

Paleomagnetic methods included low temperature demagnetization with liquid nitrogen (LTD) and thermal demagnetization to remove thermal overprints in hopes that an original thermal remanent magnetization could be isolated. LTD removed much of the initial magnetization (NRM) from most specimens. Thermal treatment caused expansion and disintegration of many specimens, which precluded extraction of a magnetic component with high unblocking temperature that might have dated from emplacement. Most of the paleomagnetic results were noisy. Magnetic components that could be extracted were inconsistent within most sites, which thwarted calculation of useful mean directions. However, two sites in the southeastern part of the Craig Mountain pluton did have components with relatively high unblocking temperatures and consistent directions. Both site means are clockwise of the expected direction, but one was shallow (~28°), the other moderately steep (~60°).

To understand the inconsistency of results, representative materials were analyzed to estimate composition of magnetic mineral(s) and silicates. LTD results along with thermomagnetic curves, which have a sharp drop around 580°C, vote for multidomain magnetite as the main remanence carrier. Decomposition of isothermal remanence showed that most material had two populations of magnetite with different coercivity, with higher coercivity magnetite concentrated in feldspar. X-ray diffraction showed that what appeared to be biotite is hydrobiotite or vermiculite, which explains why samples expanded when heated, and hints that hydration of biotite may be tied to poor paleomagnetic results.