UPPER CRUSTAL EVOLUTION OF A SMALL VOLUME CONTINENTAL MAGMATIC SYSTEM

Deciphering the history of large volume continental magmatic systems from exposed intrusive and extrusive igneous rocks is complicated by the fact that these systems likely involve overlapping episodes of magmatism, extensive wall rock melting and assimilation, and remobilization of solidified magmatic products at multiple crustal depths. Igneous rocks associated with small volume continental systems, in contrast, may provide clearer evidence of specific processes involved in their generation. For example, the small volume, short-lived (~1 m.y), Oligocene Never Summer igneous center in north-central Colorado is comprised of only two small epizonal plutons (< 50 km² of exposure) and <30 km³ of contemporaneous mafic to silicic composition volcanic rocks. The younger pluton (Mt. Cumulus stock) is a high silica (~77 wt.% SiO₂) granite porphyry that we interpret as a solidified melt that was characterized by a “gull-shaped” normalized rare earth element pattern and homogenous initial Sr (⁸⁷Sr/⁸⁶Sr(T)~0.715) and Nd (ε_Nd~-5 to -6) isotopic compositions. Comparisons to the isotopic and chemical compositions of lower crust xenoliths in nearby Devonian kimberlites indicate that the silicic melts were derived largely from “dry” partial melting of Precambrian mafic lower continental crust. Low initial water contents allowed these anatectic melts to ascend, with some additional feldspar removal in transit, to shallow crustal depths to form either shallowly emplaced plutons such as the Mt. Cumulus stock or to breach the surface as the topaz rhyolites present in Never Summer volcanic sequence. The older Mt. Richthofen stock is intermediate in composition but chemically and isotopically zoned (55 to 67 wt. % SiO₂, ε_Nd=-0.5 to -5.8). We interpret this pluton as being the product of simple mixing between a remobilized silicic igneous intrusive rock, similar to the Mount Cumulus stock, and underplated, mantle-derived, mafic magmas. Nd isotopic data preclude the possibility that “refreshing” of the Mt. Richthofen stock produced any of high silica igneous rocks found at the Never Summer igneous complex. Magmas in the upper reaches of the Never Summer magmatic system were silicic anatectic melts and mafic magma derived from greater lithospheric depths, and intermediate composition magmas generated in situ in the upper crust.