New Mexico Geological Society Annual Spring Meeting — Abstracts

Volcanic dikes are prominent in many western states. Petrologists have studied dike composition and texture, and have measured rock melting and crystalization in the laboratory, but have had no independent way to measure past temperatures at actual sites.

Where dikes cut through clastic rocks that contain primary sedimentary organic matter, heat causes changes that can be measured and calibrated to temperature by laboratory studies through the range from 50 to 600 °C, permitting determination of maximum lithotemperatures through the contact zone.

About 70 miles east of Farmington, New Mexico, near Dulce, where a dike 7 m thick cuts strata mapped as the cretaceous Lewis Shale, apparently unweathered samples from a road cut contained 1-2% organic grains consisting of about 60% vitrinite, 30% inertinite, and 10% fragments of cuticle and pollen. Rare polymaceralic grains indicate that part of of the kerogen is recycled coal.

The measured reflectance (oil immersion) of the least-altered vitrinite ranges from 0.45% beyond 8 m from the dike to more than 4% within 40 cm of contact. These values indicate maximum lithotemperatures from 150°C to higher than 550ºC. The plot of reflectance (or temperature) versus distance from the contact (normalized as percentage of dike thickness) for this dike falls within comparable plots for several dikes near Spanish Peaks, Colorado. The temperatures are 50-100 degrees higher than temperatures predicted by a thermal model for 1200-degree magma intruded "instantly" into shale. This small difference could result from use of wrong values for magma, fluid and rock properties in the model, but if the magma had moved far beyond the point sampled or to the surface, contact zone temperatures these dikes conditions of would have been mush higher. I conclude that these dikes were emplaced rapidly at or near modeling conditions of an "ideal body".