11-1 Variation Within the Terneros Creek Rhyolite at the Tascotal Mesa Fault, Big Bend, Texas

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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Abstract:

The Terneros Creek Rhyolite (Pgtc) is a series of peralkaline rhyolite domes surrounding the Solitario laccocaldera in the Big Bend Region of Texas. One example is located on MSU’s Dalquest Desert Research Station, which lies 5 km north of Solitario. The dome covers roughly 1 km² and is bisected by the east-west trending Tascotal Mesa Fault. This study characterizes the chemical composition and mineral content of this exposure of the Terneros Creek Rhyolite for whole-rock geochemistry (WDXRF), mineralogy, and microstructure (optical and electron microscopy with EDS).

The dome’s composition is similar throughout, varying most with proximity to the dome’s margins and its contact with the underlying unit. Samples from the dome's interior contained an abundance of euhedral arfvedsonite or scattered anhedral aegerine. Samples located further from the center but not quite on the margin exhibited Liesegang-like banding, appearing as concentric rings of iron oxide staining parallel to fractures. The banding is a widespread feature seen across the dome, except for areas near (~250 m) the contact. This is likely a late-stage hydrothermal staining, which is best resolved in hand sample and optical microscopy, and seems to be a submicron accumulation of iron oxide. 

Contacts and fault-adjacent rocks differ from the bulk of the dome. On the northwest edge of the dome, a contact exhibiting Pgtc emplacement into a black chert-bearing conglomerate with interbedded lacustrine limestone. The rhyolite at the contact included the conglomerate. Rhyolite fragments are coated with a rind of calcite and wollastonite, with rhyolite inclusions and crossed by calcite-filled fractures. Samples along the margin also contain a mineral with a composition consistent with krausite. Rhyolite adjacent to the Tascotal Mesa Fault is fractured and heavily weathered. Fault-adjacent rhyolite exhibits krausite-filled fractures, suggesting some fluid movement from the underlying sedimentary units. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8419

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