Structural petrology of the Val Verde tonalite, southern California

Abstract

Micrometric analyses of the Val Verde tonalite were made on samples taken along a line extending from the border of the intrusive to a point 5 miles out in the intrusive. An attempt was made to evaluate sampling errors and to determine variation in mineralogic composition and radioactivity. The border of the intrusive was found to be more sodic and higher in radioactivity than the central part, and this is thought to be due to assimilation of quartz-biotite schist. Inclusions in the tonalite are believed to be xenoliths of gabbro and quartz-biotite schist. Petrofabric analyses of the schist country rock and the tonalite indicate that : (1) the schist was deformed primarily by rotation of mineral grains about a horizontal axis; (2) during intrusion of the tonalite a gneissoid structure developed in it parallel to the contact with the country rock with a linear element lying in the plane of foliation and paralleling the dip; (3) later stresses directed parallel to the strike of the foliation caused rotation of mineral grains in the tonalite about an axis parallel to the dip. The orientation of the minerals in the tonalite is thus believed to be due to a combination of magmatic flow and post-magmatic deformation.