ORIGIN OF IGNEOUS FOLIATIONS IN THE GRAVEYARD POINT INTRUSION, OREGON

Differentiated mafic intrusions commonly contain rocks that are foliated owing to the preferred orientations of tabular crystals; however, the origin of these igneous textures remains poorly understood. They have been attributed to a variety of processes, including in situ crystal growth, sedimentation in a static or flowing magma, deformation in a crystal mush, and solution/precipitation in response to compaction. We evaluated these models by conducting a quantitative study of the shape-preferred orientations of plagioclase crystals in foliated rocks of the Graveyard Point intrusion, a 150-m-thick, differentiated tholeiitic sill in eastern Oregon.

Oversize thin sections were made from blocks cut from 7 oriented samples collected in a 20-m-thick sequence of foliated gabbro. One thin section was made in the horizontal plane of each block and sections were made in either one or both of the vertical planes. Mosaics of digital photographs were analyzed using Scion Image. Data on the sizes and orientations of plagioclase crystals were compiled on spreadsheets and analyzed graphically and numerically. The petrofabric data confirm the presence of a well-developed sub-horizontal foliation in most of the hand specimens. Results to date also suggest that crystal orientations may be weakly bimodal, possibly reflecting tiling structures related to simple shear in a flowing magma. Analyses of sections cut in the horizontal plane indicate the presence of a weak preferred orientation (lineation) in some specimens, which also may indicate flow.