EVALUATING VOLCANIC-PLUTONIC CONNECTIONS IN THE GRIZZLY PEAK CALDERA: EVIDENCE FROM ISOTOPE GEOCHEMISTRY, ELECTRON MICROSCOPY, AND HIGH-PRECISION GEOCHRONOLOGY

The Grizzly Peak caldera in the Sawatch Range, Colorado, formed following the ~34.91–34.78 Ma eruption of the Grizzly Peak Tuff (GPT), a crystal-rich compositionally variable rhyolite associated with several stages of plutonic magmatism. Previous work showed the GPT has a highly evolved isotopic signature relative to nearby Cenozoic rocks; it was interpreted as the product of fractional crystallization of a large volume of mantle-derived basalt that assimilated 20–40% Proterozoic crust (Johnson and Fridrich, 1990). However, the relations of pre- and post-caldera intrusions to the GPT were mostly limited to field interpretations and major- and trace-element geochemistry.

New whole rock Sr, Nd, and Pb isotope geochemistry, electron microprobe analyses (EMPA), and high-precision zircon U-Pb geochronology reveal the complexity of the Grizzly Peak magmatic system. Initial εNd (εNd_i = -12.4 to -13.5), ²⁰⁶Pb/²⁰⁴Pb_i, and ²⁰⁸Pb/²⁰⁴Pb_i ratios in the GPT largely agree with previous work. However, whole rock ⁸⁷Sr/⁸⁶Sr_i (0.710216–0.716405) is more radiogenic than published fiamme analyses, especially in the middle and upper GPT subunits. Alkali feldspar EMPA data for the middle and upper GPT yield compositional groups with Or_0-12, Or_70-75, and Or_85-95. We interpret these as two populations: volcanic sanidine autocrysts and plutonic (Proterozoic?) alkali feldspar xenocrysts. The lower GPT subunit, which is 26–236 ka older than the middle subunit, yields no plutonic feldspar but has high-F biotite comparable to biotite associated with the Henderson porphyry Mo deposit.

Intrusive rocks bracket the GPT in age (35.13–34.82 Ma) and isotopic compositions (εNd_i = -10.3 to -15.0; ⁸⁷Sr/⁸⁶Sr_i = 0.708902–0.731755). A geochemically diverse “post-resurgent” stage (55–73 wt% SiO₂) is isotopically as crustal and variable as other Grizzly Peak magmas and yields autocrystic zircon U-Pb ages indistinguishable from the GPT. Unlike other Grizzly Peak units, the post-resurgent rocks yield abundant ~1.4 Ga xenocrysts. Altogether, the data are compatible with a caldera model that calls for multiple batches of magma—with varied size, composition, storage depths, and wallrock assimilation—that were broadly derived from a source that was evidently untapped by other Sawatch Range magmatism during the Late Cretaceous–Cenozoic.