STRONTIUM, NEODYMIUM AND LEAD ISOTOPIC EVOLUTION OF THE GRIZZLY PEAK MAGMATIC CENTER, COLORADO

The Grizzly Peak magmatic system of the Tertiary central Colorado volcanic field provides an opportunity to examine the interactions between an evolving large-volume high-silica magmatic center and a diverse variety of wallrocks. The magmatic system, centrally located in the Colorado Mineral Belt, is dominated by the ~600 km³, 34.3 Ma Grizzly Peak Tuff, preserved mostly as intracaldera ash-flow tuff zoned from 77 wt% SiO₂ at its base to 70 wt% SiO₂ at the exposed top of the unit. The tuff is intruded by 22 km³of resurgent plutons as young as 31 Ma, constituting the youngest and most voluminous episode of plutonism in the area of the caldera. Wallrocks to the magmatic center range from Proterozoic, including ~1800 Ma gneisses and ~1400 Ma Silver Plume-type granitoids, to Tertiary, including the 63-40 Ma Twin Lakes batholith. Previous radiogenic isotopic data for the tuff collected by Johnson and Fridrich (1990) led them to conclude that the Grizzly Peak magma was variably contaminated by nearby wallrocks.

Here we present new whole rock major, trace, and REE concentration and radiogenic isotope data for rocks from the Grizzly Peak magmatic center, as well as its nearby wallrocks, in order to evaluate long term geochemical patterns in the magmatic center, and the viability for wallrocks to influence the final compositions of the Grizzly Peak magmas. Preliminary Nd isotopic data for the tuff and adjacent wallrocks include εNd_{34 Ma} = -17.0 to -9.1. A sample of the middle Grizzly Peak Tuff subunit (εNd_{34 Ma} = -12.8) falls in the range (εNd_{34 Ma} = -14 to -12) measured by Johnson and Fridrich (1990). In contrast, a resurgent pluton was more radiogenic at εNd_{34 Ma} = -9.1; the large isotopic difference between the pluton and the tuff suggests that the two were likely not derived from the same magma body. Taken together, the isotopic data collected so far (including a sample of the adjacent 57 Ma Twin Lakes batholith with εNd_{57 Ma} = -8.9) suggest that the mechanism that produced the super-eruption of the Grizzly Peak Tuff may have been an excursion, in both volume and source, from local “background” magmatism that resulted in plutonic rocks such as the Twin Lakes batholith and the resurgent plutons. This is similar to isotopic shifts that have been observed in the slightly younger Latir volcanic field in northern New Mexico.