NEW CA-TIMS AND LA-ICP-MS ZIRCON U-PB GEOCHRONOLOGY OF THE GRIZZLY PEAK MAGMATIC CENTER, CO: CONFOUNDING CHRONOLOGY OF A CLASSIC CALDERA

Previous field and geochemical work on the Grizzly Peak magmatic center, CO, identified four stages of activity that follow the classic resurgent cauldron model: 1) precaldera ring fractures and rhyolitic diking owing to tumescence over a pre-eruptive magma chamber; 2) eruption of the ~600 km³ Grizzly Peak Tuff (GPT) as a single cooling unit and collapse of a ~230 km² caldera; 3) intrusion of a resurgent pluton; 4) post-resurgent intrusions associated with weak mineralization. We use new field work and high-precision zircon U-Pb geochronology (chemical abrasion-isotope dilution-thermal ionization mass spectrometry [CA-TIMS] and laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]) to test past interpretations of the Grizzly Peak caldera.

Field work identified possible cooling breaks in multiple locations in the GPT, suggesting it may be the result of multiple eruptions. In the southern third of the mapped caldera, we did not observe caldera margins or in-place intracaldera tuff. Thus, the caldera may be much smaller than 230 km². An extracaldera dike previously interpreted as precursory to the GPT yields a weighted mean LA-ICP-MS age of 66.4 ± 0.5 Ma. Pre-caldera dikes may exist, but previous geochemical correlations are nonunique. The lowest stratigraphic GPT subunit yields preliminary CA-TIMS ages on single zircons suggesting crystallization ca. 34.9 Ma, ~100 ka older than our previously reported CA-TIMS weighted mean age of 34.78 ± 0.03 Ma for the middle GPT. This raises the possibility the GPT accumulated from successive magma pulses. Age data for the resurgent pluton contradict previous interpretations of its internal growth and its relationship to the GPT. CA-TIMS data for the oldest mapped phase of the pluton are indistinguishable from the middle GPT (34.84 ± 0.05 Ma), and data for the younger mapped phase indicate it crystallized 250–450 ka prior to the middle GPT eruption (35.13 ± 0.07 Ma). No samples, including those mapped as post-resurgent, are resolvably younger than the GPT in our preliminary CA-TIMS data. This suggests GPT-related magmatism may have spanned <0.5 Ma and ceased after the last ignimbrite eruption. The new age data, field work, and lack of distal GPT outcrops suggests the Grizzly Peak system may have been smaller, more localized, and more complex than previously thought.