New Mexico Geological Society Annual Spring Meeting — Abstracts

North-northeast extension and bimodal magmatism during early Rio Grande rifting is recorded by the emplacement histories of two suites of dikes and associated plugs at Faulkner Canyon. Whole-rock geochemistry and 40Ar/39Ar data suggest that alkalic trachybasalts (34.7 +/- 1.1 Ma) and trachydacites (35.96 +/- 0.09 Ma) were emplaced first and were followed by the emplacement of two petrogenetically distinct groups of calc-alkaline magmas consisting of basaltic andesite and andesite. Major and trace element compositions from these two suites indicate that Ne-normative trachybasalts (Nb/Ba = 0.057-0.026) were likely derived from the asthenosphere and evolved to Qtz-normative trachydacitic compositions by assimilation and fractional crystallization (AFC) of mid-lower crust. Calc-alkaline magmas (Nb/Ba = 0.006-0.013), however, were likely sourced from an enriched lithospheric mantle source and evolved independently. Spatial zonation of hypabyssal units suggest that magmas were emplaced by a series of laccolithic bodies that may have been elongated from a previously existing zone of structural weakness possibly associated with the Texas Lineament. Although previous works suggest that early Rio Grande rift extension was oriented generally E-W during hypabyssal emplacement, localized stresses from the intrusions initially deformed the overlying Eocene Palm Park Formation into a broadly folded E-W trending anticline and sub-parallel joints that were filled by alkaline and calc-alkaline magmas.