DIVERSE TRENDS OF DIKES OF THE COLUMBIA RIVER BASALT GROUP (CRBG) IN PRE-CENOZOIC HOST ROCKS IN NE OREGON AND W IDAHO

Dikes commonly follow joints, bedding, and foliation. Most dikes in the Wallowa batholith follow one or two joint sets. Zig-zag dikes follow both joint sets. Hawkins Pass (Eagle Cap quad) is near the south contact of the batholith. Dikes to the north in granitic rocks trend about N 25º W and N 60º E. Within 2 km south of the Pass, dikes in marble follow the same two trends. However, dikes in marble 3 to 5 km south of the Pass follow trends of about N 30º E and N 45º W. About 6 km to the east, dikes in greenstone follow one joint trend – N 20º W. In Idaho, north of junction of Rock Creek and Brownlee Reservoir (Henley Basin quad), dikes locally follow east-west bedding. About 19 km N 50º E, just NW of Iron Mountain (Monroe Butte quad), dikes in granitic rocks trend about N 30º W, as do joints. About 135 km to the NE, near junction of Slate Creek and Little Slate Creek (John Day Mountain quad), in the gneissoid suture zone between continental and oceanic crust, dikes trend about N 30º E.

Two points. Dike trends in pre-Cenozoic rocks differ vastly from trends in flows of the CRBG. Physical features of dikes vary depending on the host rocks. For example, dikes in shale and massive greenstone are very different.