GEOCHEMISTRY AND PETROGENESIS OF THE TABLE MOUNTAIN LAVA FLOWS, GOLDEN, COLORADO

Intrusive and extrusive igneous rocks outside of Golden, Colorado provide insight into Colorado’s volcanic history during the early Paleocene. Four distinct shoshonite porphyry lava flows overlie the Cretaceous-Paleogene (K-Pg) boundary on North and South Table Mountains and are distinguished by their lateral continuity and modal mineralogy (Drewes, 2008). The lava flows are thought to be sourced from two intrusive complexes to the northwest: the Ralston Plug (a 2 km long by 1 km wide intrusive body associated with the Golden Fault), and a group of small dikes and sills related to a smaller intrusive plug farther to the southeast. In a petrographic study, Drewes (2008) found that all flows and intrusive units were porphyritic with plagioclase, olivine, and augite phenocrysts, set in a blocky groundmass containing interstitial plagioclase, pyroxene, olivine, sanidine and biotite. Drewes (2008) concluded that the mineral assemblage indicated a deep source for the initial basaltic magma, and that the alkalic groundmass composition is evidence for early fractionation of phenocrysts and the assimilation of a granitic crustal material during ascension prior to groundmass crystallization. Although only moderately supported by the mineral mode correlation, Drewes (2008) concluded that the southeastern plug system is the source of Flows 1 and 2, while Flows 3 and 4 stem from the large intrusive Ralston Plug.

This study aims to build upon the results of Drewes (2008) and will report whole rock geochemical data (ICP-MS and XRF) from a suite of samples collected from North and South Table Mountains as well as their potential source areas. These data will provide further evidence as to the source(s) of the four flows, as well as insight into the source and magmatic history of the Ralston intrusive complexes and the petrogenesis of the Table Mountain lava flows.

Drewes, Harald, 2008, Table Mountain shoshonite porphyry lava flows and their vents, Golden, Colorado: U.S. Geological Survey Scientific Investigations Report 2006-5242, 28 p.