AL-IN-HORNBLENDE BAROMETRY OF THE WALLOWA BATHOLITH, WALLOWA TERRANE, OREGON: IS TEMPERATURE A FACTOR IN DEPTH OF INTRUSION CALCULATIONS?

Edward D.Lane and Peter D.Crowley
Department of Geology,
Amherst College
Amherst MA 01002

The Cretaceous Wallowa batholith, a quartz diorite plutonic complex lies within the Wallowa Terrane, the easternmost accreted terrane in northeast Oregon. The Wallowa batholith intruded the Wallowa Terrane after its accretion to North America. However, paleomagnetism (Wilson and Cox, 1980) from the Wallowa batholith is discordant with that of Cretaceous North America indicating a post-accretion rotation of the Wallowa Terrane. The paleomagnetism is compatible with either a 60° rotation about a vertical axis or a 25° down to the west tilt.

Wallowa batholith quartz diorites are composed primarily of plagioclase feldspar, alkali feldspar, quartz, biotite, hornblende, magnetite, and titanite. Plagioclase and hornblende are commonly zoned. Plagioclase is complexly zoned with An40 cores and An22 rims. The cores of hornblende are slightly more aluminous than the rims.

Al-in-hornblende pressures have been determined from 104 samples to constrain post Cretaceous rotations of the Wallowa Terrane. Hornblende from the Wallowa batholith is relatively actinolitic with Al content varying from <4.5 to >7 weight %. This corresponds to a range in Al-in-hornblende pressures of 2.9 kbars. Al-in-hornblende depths of intrusion vary from approximately sea-level to 6.5 km, with the greatest depths along the NW margin of the batholith, and the shallowest depths near the center. The pattern of hornblende pressures is not consistent with a simple down to the west tilt of the entire Wallowa Terrane. This pattern suggests that temperature may be an important factor in hornblende composition and pressure variation an artifact. Hornblende-plagioclase temperatures and Fe-Mg distribution between hornblende and biotite will be determined to investigate the effect of temperature variation on depth calculated by Al-in-hornblende barometry.