THREE RIVERS STOCK AND PALISADES TUFF: CORRELATING INTRUSIVE SOURCE WITH IGNIMBRITE SHEET IN THE TECTONICALLY DISRUPTED SIERRA BLANCA IGNEOUS COMPLEX, NEW MEXICO

The Sierra Blanca Igneous Complex (SBIC) consists of a late Eocene to Oligocene volcanic pile (Sierra Blanca Volcanics) intruded by numerous dikes, plugs and three stocks (Rialto, Bonito Lake and Three Rivers stocks). New 1:24,000 geologic mapping, whole rock geochemistry, microprobe analyses and ^40/39Ar dates allow us to correlate quartz syenite of the Three Rivers stock (TRS) with recently recognized ignimbrite, the Palisades Tuff (PT), and to identify the stock as the probable intrusive root of an ancient caldera. PT (28.67±0.07 Ma, n=2) is exposed in a rotated block in the Godfrey Hills 11 km west of TRS. PT consists of 100+ m of welded ignimbrite dipping roughly 25° NE with variable thickness and preservation of basal deposits, vitrophyres, devitrified core and top. Maximum aspect ratios of fiammi are ≥50:1 and the phenocryst assemblage is san, bio, aug, minor plag and sparse qtz. Four analyses reveal the ignimbrite is trachytic in composition, slightly more mafic at top than bottom (normally zoned). The TRS contains three quartz syenite bodies 15 to 30 km² that are dated at 28.95±0.18 to 28.24±0.19 Ma. These syenites are porphyritic containing large Kspar in a groundmass of Kspar, minor qtz and plag, cpx, bio, and Na-amph. Importantly, these syenites contain relict augite with aegirine augite rims. Geochemically, PT ignimbrite closely resembles TRS quartz syenite in major/trace elements. Microprobe analyses show that PT augites and TRS relict augites are practically identical. Other known stocks and ignimbrites in the SBIC region do not correlate in age, composition or mineralogy like the PT-TRS association. We conclude that explosive eruption of quartz syenite melt in the proto-TRS produced an inferred 7-km-wide caldera and the PT ignimbrite. Subsequently, small bodies (≤12 km² total) of alkali granite (27±2 Ma) intruded the residual quartz syenites. The granites are the source of geochemically similar rhyolites that partially surround the stock. Tectonic uplift and erosion of the TRS since ~27 Ma has removed caldera-fill ignimbrite and obvious collapse structures, although roof pendants of older volcanics are preserved. Assuming the top of the stock crystallized at 3 km depth, total offset between Palisades Tuff and Three Rivers caldera across the faulted Godfrey Hills-Sierra Blanca boundary is roughly 5 km.