APPLICATION OF THE CONODONT ALTERATION INDEX (CAI) TO THE MARBLE CANYON INTRUSION, SIERRA DIABLO, WEST TEXAS

The conodont color alteration index (CAI) has been well established using experimentally altered conodonts and comparing them to field indicators of heating history such as vitrinite reflectance and metamorphic mineral assemblages. Most CAI studies have been conducted on a regional scale using outcrop and subsurface well samples to determine the heating histories of various geologic settings, and comparing the results to known CAI values and interpreted temperatures. Only a few researchers have used contact metamorphic areas to conduct CAI studies, and none have been able to compare detailed igneous and metamorphic thermodynamic data to CAI indices from outcrop samples.

Marble Canyon in the Sierra Diablo of West Texas provides a nearly three-dimensional exposure of an igneous intrusion with associated contact metamorphism of surrounding Lower Permian limestones. An unknown factor at the beginning of this study was the shape of the intrusion in the subsurface. Detailed igneous and metamorphic thermodynamics were compared to CAI values collected from field samples around and through the contact metamorphic areole in order to calibrate the index to thermodynamically determined temperatures. Both vertical and horizontal sections were sampled for processing. Recovered conodonts had CAI values ranging from 1.5 to 6. A systematically detailed analysis of the Marble Canyon intrusion and an additional similar analysis of the neighboring Cave Peak intrusion could not be completed because of a rapid, unexpected sale of the Figure 2 Ranch, which resulted in loss of field access before the planned sampling program could be completed. However, sample locations and conodont recoveries were sufficient to create an isotherm map of Marble Canyon using CAI values. The result is a nearly concentric arrangement that follows the exposed portion of the intrusion. The data suggests that heat dissipated rapidly close to a stock-like intrusion, then much more slowly with increasing distance. Comparison of igneous and metamorphic thermodynamics and observed CAI indices support the expected relationship in most cases, but does not allow for direct calibration of the conodont alteration index due in part to changes in geothermal paleogradiants and a lack of higher index (6+) conodonts.