Browsing by Subject "Metamorphism (Geology)--Texas--Van Horn Mountains"
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Item Structural evolution and metamorphism of mid-Proterozoic basement in the northwest Van Horn Mountains, Trans-Pecos, Texas(1987) Bristol, David Arthur, 1961-; Mosher, Sharon, 1951-Psammitic, pelitic and mafic schists in a NNW-trending horst in the NW Van Horn Mountains of west Texas show evidence of a multiphase, synmetamorphic, ductile Grenville-age deformation. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. Detailed mapping indicates that basement rocks exposed in the NW Van Horn Mountains underwent three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding episodes. The second phase of folding produced the dominant foliation and foliation intersection (S₁/S₂) lineation observed throughout the area. This deformation is inferred to have resulted from a large scale folding event, at or very near, peak metamorphic conditions, possibly associated with nappe formation. Garnet-biotite and garnet-hornblende Fe-Mg exchange thermometry indicate maximum temperatures of 640±50°C, corresponding to mid-amphibolite facies conditions, for this event. The intrusion of late syn- to post-orogenic pegmatite sills and dikes was widespread across the area. The associated pegmatitic fluids either facilitated or became the principal catalysts of a static recrystallization event. This event altered previously formed deformational fabrics and produced diagnostic strain-free textures in many minerals. The Grenville-age, polydeformational history recorded by the basement rocks of the NW Van Horn Mountains is similar to that noted in portions of the Llano Uplift of central Texas, about 650 km to the east. The strong similarity of deformational styles in these two areas suggests that large-scale folds with localized shear zones were produced in this region of the Grenville Province