COMPOSITIONAL STRATIGRAPHY THROUGH A MULTICYCLIC, VIGOROUS AND DIVERSE CALDERA CENTER: THE SAN LUIS CALDERA COMPLEX, SAN JUAN VOLCANIC FIELD, COLORADO

The San Luis caldera complex located in the NW corner of the Central Caldera cluster of the Oligocene San Juan volcanic field, Colorado consists of three overlapping caldera cycles, each associated with a voluminous ash-flow tuff and numerous lavas, constituting the Rat Creek, Cebolla Creek and Nelson Mountain cycles. From oldest to youngest (27.1 to 26.8 Ma), the tuffs are: the ~250-km³ Rat Creek Tuff, the ~250-km³ Cebolla Creek, and the >500-km³ Nelson Mountain Tuff. The Rat Creek Tuff and Nelson Mountain Tuff are zoned from a crystal-poor (~5%) high-silica rhyolitic base to a crystal- rich (40%) dacitic top. In contrast, the middle tuff, the Cebolla Creek Tuff, is mafic dacite with little or no compositional zonation. Each of the three tuffs is associated with diverse pre- and post-caldera lavas cropping out mainly within the calderas. Lavas of the San Luis complex represent a wider compositional spectrum than the three tuffs, ranging from andesite to high-silica rhyolite compared to a lack of andesitic compositions in the tuffs. Nearly all eruptive units of the San Luis caldera complex have been resampled to investigate the petrogenetic history through this well exposed multicyclic caldera system. Here we report on new major and trace element analyses and on some mineral compositional data. In general, one can summarize the following: 1) there are some compositional trends that progressively change from the oldest to the youngest caldera cycle and that are essentially silica independent (except the high-silica end); for example, at a given MgO content, magmas are richest in silica in the rocks of the Rat Creek caldera cycle and are poorest in the rocks of the Nelson Mountain caldera cycle with rocks of the intervening Cebolla Creek cycle in between; 2) particular differences exist in chemical trends of tuffs that are strongly compositional zoned as compared to the nearly unzoned Cebolla Creek Tuff and all lava series; for example, dacitic components of the Rat Creek and Nelson Mtn. Tuff contain about two times higher Zr concentrations than all lavas and the Cebolla Creek Tuff; 3) high-silica rhyolite regardless whether erupted as tuff or as lavas share commonalities that are distinct from all other magmas of the San Luis complex but that are typical for high-silica rhyolites elsewhere such as low Ba/Rb of 2 to 0.2.