Chemical Analysis Of Tertiary Volcanic Rocks, Central San Juan Caldera Complex, Southwestern Colorado

In conjunction with integrated mapping of the Oligocene central San Juan
caldera cluster, southwestern Colorado (USGS I-Map 2799, in press), all modern
chemical analyses of volcanic rocks for this area determined in laboratories of
the U.S. Geological Survey have been re-evaluated in terms of the stratigraphic
sequence as presently understood. These include approximately 700 unpublished
analyses made between 1986 and 2003, as well as all USGS analyses published
since 1965 when the widespread presence of regional welded ash-flow tuffs
erupted from large calderas was first recognized. All the analyses are assigned
unit identifiers consistent with those used for the new geologic map; quite a
few of these differ from those used on sample submittal forms and in prior USGS
publications.

[Summary provided by the USGS.]