PETROLOGY OF BLACK HILLS AND DAGGER MOUNTAIN INTRUSIONS, BIG BEND REGION: DETERMINING EMPLACEMENT STYLES FROM MAGMATIC FOLIATIONS AND COMPOSITIONS

SE Brewster County in the Big Bend region contains forcefully emplaced laccoliths in the Black Hills (BH; Dahl and Lambert, 1986) and passively emplaced sills flanking Dagger Mountain (DM). BH laccoliths and DM sills intruded the same level of crust at about the same time. Mapping at 1:10,000-scale, thin-section petrography, and published data show differences in texture and composition appear to control emplacement mechanism. BH and DM intrusions are part of the Trans-Pecos Igneous Province, a belt of subduction-related Cenozoic igneous rocks that stretches across Mexico and West Texas (Ewing, 1991). BH contains three shallow alkaline intrusions that display pervasive inward dipping flow foliations assumed to be subparallel to original pluton margins (Dahl, 1984). Similar magmatic foliations parallel intrusive margins in Henry Mountains, Utah forcefully-emplaced laccoliths (Saint- Blanquat et al., 2006). DM sills examined do not contain flow structures and do not strongly deform their well-exposed country rock. BH intrusions are dated at 22 Ma (K-Ar on plagioclase feldspar; Dahl and Lambert, 1986); DM sills are 32 Ma ( ⁴⁰Ar/³⁹ Ar on monzonite; Morgan and Shanks III, 2008). BH laccoliths intruded Cretaceous Santa Elena Limestone and Del Rio Clay. DM sills intruded slightly younger Cretaceous Boquillas Formation limestone and calcareous shale. BH trachyte, nepheline trachyte, and phonolite contain sanidine and nepheline phenocrysts, plus aegerine- augite, sodalite, natrolite, and zeolites. (Dahl, 1984). DM quartz diorite, anorthosite, gabbro, leuciteolite, nephelinite, and leucite gabbro contain major minerals hornblende, leucite, plagioclase feldspar, and nepheline, and minor minerals orthopyroxene, clinopyroxene, biotite mica, potassium feldspar, untwined feldspar, and quartz. Mafic minerals comprise 30 to 80 percent of DM samples. Henry Mountains porphyritic diorite and quartz diorite contain plagioclase and hornblende phenocrysts, plus clinopyroxene, magnetite, titanite, and apatite.(Saint – Blanquat et al., 2006). More viscous felsic magmas appear to preferentially form flow foliations and laccoliths.