New Mexico Geological Society Annual Spring Meeting — Abstracts

The margin of a mid-Tertiary volcano-tectonic structure, the Emory cauldron, lies in the northern Black Range. Kneeling Nun Tuff (the ashflow tuff of the Emory cauldron) crops out in a massive section ( 300 m) 2 km south of latitude 33° 15' 00" N. The stratigraphic sequence outside the structural caldera wall includes: Rubio Peak Formation, andesite lava and agglomerate, 335 m thickt initial Sr-B7/Sr-86 of 0.7061; unconformity; porphyritic andesite of North Fork Palomas Creek, intrusive lava (interpreted as the defluidized residue of the Emory cauldron magma). initial Sr-87/Sr-86 of 0.7076; Mimbres Peak Formation, rhyolite tuff, breccia, intrusive lava and volcaniclastic deposits (interpreted as moat and ringfracture deposits of the Emory cauldron), bedded deposits 165 m thick, initial Sr-87/Sr-86 of 0.7141; unconformity; andesite of McKnight Mountain, lava and volcaniclastic deposits, 140 m thick; disconformity; Caballo Blanco Tuff, rhyolite ash-flow. 110 m thick, initial Sr-87/Sr-86 of 0.7135; disconformity; andesite of Poverty Creek, lava and lahar deposits, 235 m thick, initial Sr-87/Sr-86 of 0.7060; unconformity; rhyolite of Little Mineral Creek, lithic tuff and flow-banded lava, 120 m thick, initial Sr-87/Sr-86 of 0.7121; tuff of Diamond Creek, bedded ash-flows, 320 m thick. initial Sr-87/Sr-86 of 0.7332; unconformity; rhyolite of Franks Mountain, intrusive lava plugs; Bearwallow Mountain Formation, basalt and basaltic andesite lavas, 55 m thick, initial Sr-87/Sr-86 of 0.7071.

The northern Black Range is a homocline, dipping 5-40° NW. W- to NW-striking normal faults (dip 52-68° S or SW; slickenslides plunge 46-58° SE) show greatest stratigraphic separation in Rubio Peak Formation, suggesting they formeddur'ing collapse of the Emory cauldron, about 34 Ma. N- to NE-striking normal faults (dip 74-85° NW or SE; slickenslides plunge 71-82° NE or SW) cut W to NW faults and probably formed during post-20 Ma Basin and Range extension.

XRF analyses show sympathetic variation of K₂O, antipathetic variation of MgO, CaO, TiO₂, Fe₂O₃(T), Al₂O₃ and P₂O₅ and invariant behavior for Na₂O, plotted against SiO₂. K₂O + Na₂O vs SiO₂, A-F-M and An-Ab-Or plots suggest a subalkalic calc-alkalic classification. INAA analyses show sympathetic variation of Rb, Cs, light REE, Th and U, antipathetic variation of Sr, Ba, Sc, V, Co and Zn, if plotted against SiO₂. Chondrite-normalized REE patterns for Rubio Peak, Poverty Creek and Bearwallow Mountain Formations have negative slopes, no Eu anomalies and REE increase with time, whereas North Fork Palomas Creek, Mi mbres Peak, Caballo Blanco, Little Mineral Creek and Diamond Creek Formations have negative slopes and negative Eu anomalies. Trace elements, REE patterns and initial Sr-87/Sr-86 suggest partial melting of volatile-rich upper mantle as a source for Rubio Peak, Poverty Creek and Bearwallow Mountain Formations, lower crust for North Fork Palomas Creek, Mimbres Peak, Caballo Blanco and Little Mineral Creek Formations and upper crust for tuff of Diamond Creek.