FIELD TESTS OF COLOR INDEX DETERMINATION IN PLUTONS WITH THE STRABOTOOLS MOBILE APP

Color index (CI), the area proportion of dark minerals in a plutonic rock, correlates strongly with bulk composition. CI is easy to observe but difficult to estimate with either precision or accuracy. Petrology instructors are well aware of this, but in a recent field quiz six experienced petrologists gave estimates ranging from 5 to 30 on one clean outcrop of granodiorite. To help, we have developed a mobile app, StraboTools, that incorporates a CI tool, and have conducted field tests of its usability, precision, and accuracy.

We collected data in two areas of well-exposed plutonic rocks in California: Cretaceous granodiorites of the Tuolumne Intrusive Suite (TIS) along Tioga Road in Yosemite National Park and the Jurassic Sage Hen Flat Granite (SHF) in the White Mountains. Both areas have compositional and modal datasets for comparison. We took iPad photos of random shaded outcrops (freshly broken roadcuts in the TIS; natural faces in SHF) and made CI determinations after letting eyes adapt to dim conditions. Findings include: (1) There is significant operator bias; person-to-person results vary systematically by as much as 20%. (2) Outcrop variability is significant, with coefficients of variation typically 5-20%. (3) Data correlate well with previous determinations made by point-counting slabs or thin sections, although the mobile determinations have greater variability. (4) Data from the TIS show a well-defined west-to-east decrease, matching the known pattern. At SHF, 40 CI determinations reveal well-defined concentric zonation that matches previously mapped textural zonation, and concentric grain-size variation shows up well in the grain-size tool that is under development.

Rapid field determination of CI can be a useful mapping tool, but further development is needed. We are currently assessing the use of lightweight ring lights to insure even illumination and are developing a calibrated photo set that should help to eliminate inter-operator variability.