38-3 Petrogenesis of a mystery volcanic rock to rekindle ideas about volcanism in Texas

Session: Petrology and Geochemistry (Posters)

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Abstract:

This study focuses on an allochthonous volcanic rock from Bishop, Texas, with no evident outcrop. We propose to investigate this "mystery" rock to determine possible connections to volcanism in Texas. Volcanic activity in Texas can be traced back to the Paleogene (44 Ma) in the Trans Pecos region with some younger 27-million-year-old deposits of the Catahoula in southeast Texas. The rock exhibits a fine matrix with a porphyritic texture suggesting that it cooled rapidly at the surface while initiating flow banding. The source of volcanic rocks and their petrogenesis can be determined by their mineralogy and overall geochemistry. Secondary features of pedogenesis and alternations from water sources will be further studied, as the rock appears to have a weathered rind. Samples under thin section contain >70% SiOz and exhibit phenocrysts of sanidine, a high temperate potassium feldspar that can be recognized by Carlsbad twining and is relevant to extrusive rhyolitic rocks. A fine-grained matrix gives insight on how quickly cooled volcanic minerals have undergone stress and alteration. Other notable minerals in thin section include hematite and magnetite at < 5% which are secondary minerals found in volcanic rocks. Spherulites accounting to <55% have volcanic glass that was devitrified. Geiger counter values averaging 23.3 uR/h suggests some of the minerals are releasing some radiation. Specific gravity tests ranging ~2.53-2.60 suggests high content of silica glass, quartz, and other felsic minerals. Calderas of the Trans Pecos region are notable for their rhyolitic composition and violent history, perhaps the region is a good candidate for the mystery rock. Further studies on the petrography and geochemistry are planned for the next few months and will assist our interpretation of its petrogenesis.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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