DETAILED MAPPING IN THE LOST MINE TRAIL AREA, BIG BEND NATIONAL PARK

The Lost Mine Trail (LMT) area of the Chisos Mountains lies just outside the north-eastern margin of the Pine Canyon Caldera (PCC). The South Rim Formation (SRF) in the high Chisos Mountains is thought to involve two separate and distinct peralkaline magmas, one producing the lava and tuffs of the Pine Canyon Rhyolite (PCR) and Boot Rock Members (BRM), and the other producing the lava and tuffs of the Emory Peak Rhyolite (EPR). Included in the SRF is a swarm of dikes that strike west-southwest from the north-eastern margin of the PCC toward extra-caldera vents at Casa Grande, Toll Mountain, and Emory Peak. Detailed field mapping at a 1:10,000 scale of the LMT area has recorded a more accurate spatial distribution of these dikes, as well as providing some new information on the structure and general geology of the area. Mass balance modeling combined with further trace element modeling has provided evidence for a co-magmatic relationship for the dikes on top of the LMT, and the Casa Grande Lava Dome. Pearce element ratios indicate that these dikes and the Casa Grande Lava Dome are related to the source magma for the PCR and BRM, and not the source magma of the EPR. The geologic map combined with the geochemical modeling provides evidence for a complex plumbing system that connects the PCC to the extra-caldera vent at Casa Grande. Ideas are also proposed about the structure and sequence of geologic events in the area.