IGNEOUS ROCK COMPOSITIONS FROM BIG BEND RANCH STATE PARK AS A TEACHING TOOL IN PETROLOGY

A robust data set of ages, chemical and isotopic compositions, geologic maps, and petrography is available for Big Bend Ranch State Park (BBRSP). Geologically, BBRSP is a rugged volcanic terrane, although a spectacular section of Paleozoic sedimentary rocks is exposed around the Solitario. The park is located in Texas just to the NW of the better known Big Bend National Park. It is in the Chihuahuan Desert but is accessible to the public.

Using a published data set provides a way for students to discover petrogenetic relationships on their own by using a plotting program such as Excel. Volcanic rocks in the BBRSP region are sourced mainly from the Bofecillos vent, but also from distant vents to the north and from vents to the south in Mexico. Therefore, the volcanic pile exposed contains genetically related and non-related rocks. One exercise for the students is to have them plot the data in a series of variation diagrams to determine genetic relationships and to distinguish (possibly) between source areas.

Several concepts are reinforced with this type of exploration of a data set. For example: how trends between mafic and felsic rocks show up in a genetically related suite of rocks; how non-genetically related rocks may or may not show up on the same compositional trends; how trace-element ratio plots may discriminate between source regions when major-element chemistry does not. The idea is to give the students lectures or readings whereby the students are first introduced to the fundamentals of igneous differentiation and the use of X-Y plots to determine igneous processes that can produce compositional trends. The students are then given the data set to plot up and to use to produce hypotheses regarding the origins of the rocks, the relationships between the rocks, and the processes that might have produced one from another. A written report may be required.

A combination of a field trip and a related seminar-style class with limited laboratory and/or computer exercises provides for an extension of the proposed learning experience for undergraduate and graduate students. If the students complete required readings and class presentations before the field trip they are well-prepared. After the field trip, the exercises centering on chemical composition of the rocks are much more meaningful to them.