THE VOLCANO-PLUTONIC CONNECTION AT THE MID-TERTIARY NEVER SUMMER IGNEOUS COMPLEX, NORTH CENTRAL COLORADO

Chemical and Nd, Sr isotopic data were obtained from various intrusive and extrusive igneous rocks at the ~28 Ma Never Summer igneous complex (NSIC), north-central Colorado to asses the genetic relationship between intermediate to silicic composition epizonal plutons and extrusive igneous rocks. The NSIC is a representative microcosm of other larger mid-Tertiary volcanic centers in the Rocky Mountain region, and exemplifies a rare locality where two epizonal plutons (the younger granitic Mt. Cumulus stock [MCS], and the older granodioritic Mt. Richthofen stock [MRS], consisting of the pluton body and a fine-grained border lithology), and a spatially related, contemporaneous suite of extrusive rocks (progressing from early mafic and intermediate compositions to the culminating Iron Mountain tuff [IMT]) are preserved. The genetic relationship between the intrusive rocks and the extrusive rocks of the NSIC remains unclear. Major- and trace-element, and isotopic data demonstrate that individual pumices at the base of the IMT are low silica rhyolites with eNd(0) ~-4, moderate negative Eu anomalies (Eu/Eu* ~ 0.5), and are moderately light rare Earth element (LREE) enriched (La/Yb_N ~ 19). In contrast, the granitic MCS has a predominant negative Eu anomaly (Eu/Eu* ~ 0.15), a low eNd(0) ~-6.8, and low LREE abundances (La/Yb_N ~ 6). The chemical characteristics of the MCS are similar to those of high-silica rhyolites worldwide, but demonstrate at this location that the stock cannot represent unerupted low-silica IMT, or a crystal mush from which the magma parental to this tuff was derived. The granodioritic MRS is more mafic (~65 wt. % SiO₂), but has a slightly lower eNd(0) ~ -5 and lacks the negative Eu anomaly of the IMT. The fine-grained border lithology, which ubiquitously surrounds the MRS’s margins, has a higher silica content than the stock itself, and unlike the stock, has a predominant negative Eu anomaly, similar to that of the tuff. This border lithology has a lower eNd(0) ~-6 than the tuff, making it unlikely to represent unerupted magma parental to the IMT, however, the existence of low-Si melt associated with the MRS suggests that liquids with chemical characteristics similar to the low-Si tuff could have been generated at relatively shallow crustal depths in this region by extraction from a granodioritic crystal mush.