A petrographic and geochemical study of Wildhorse Mountain, Brewster County, Texas



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Wildhorse Mountain is located just north of Big Bend National Park in the Terlingua Uplift area. The mountain was mapped and samples were studied to determine the shape of the intrusion and to elucidate the crysatllization history of the magma. The Wildhorse Mountain microsyenite hypabyssal intrusion has two distinct parts, the southwest portion is a plug and the northeast portion is a sill-like extension from the plug. Other intrusions in the mapped area, which range from alkali microsyenites to leucomicromonzo- diorites, and the Wildhorse intrusion are emplaced in the Upper Cretaceous Pen and Aguja Formations. The microsyenite contains multiphase feldspar together with ferroaugite and biotite phenocrysts in a groundmass of alkali feldspar, quartz, biotite and magnetite. Chemical analyses, modal analyses and its position on variation diagrams suggest the magma was a highly differentiated component of an alkaline rich suite of rocks. Detailed crystallographic studies show that feldspar crystallized in three phases: first plagioclase, then anorthoclase, and finally sanidine growing epitaxially, each phase mantling the previously the previously crystallized one. The three phases are optically distinct and exhibit a different "tracht" for each. Tracht is also dependent on grain size as crystal growth relationships vary from larger phenocrysts to microphenocrysts and groundmass feldspars All feldspar phases are compositionally zoned, as seen optically and determined by electron microprobe analyses. The phase relations suggests rapid crystallization of biotite, plagioclase and possibly early anorthoclase in a magma chamber, followed by reaction of biotite and meta-stable formation for the rest of the anorthoclase and sanidine during transportation and final emplacement of the mass at shallow depth. This crystallization sequence exemplifies the relatively high temperature low pressure conditions found in most of the intrusions of the Big Bend region.



Petrology, Texas, Brewster County, Geochemistry