Composite radial cross section of the Lake City Caldera showing δ^(18)O contours: Supplement 5 from 'I. An ¹⁸O/¹⁶O Investigation of the Lake City Caldera, San Juan Mountains, Colorado. II. ¹⁸O/¹⁶O Relationships in Tertiary Ash-Flow Tuffs from Complex Caldera Structures in Central Nevada and the San Juan Mountains, Colorado' (Thesis)

Part I. 18O/16O analyses were made on 355 samples in and around the 11 by 14 km Lake City caldera, which formed 23 m.y. ago in response to the eruption of the rhyolitic Sunshine Peak Tuff. All of the major lithologies and hydrothermal alteration facies were analyzed, and a detailed δ18O map was made of the caldera and its surroundings. Intracaldera facies Sunshine Peak Tuff consists of three members interbedded with landslide debris and megabreccias shed into the caldera during eruption and collapse. Asymmetric resurgence within the Lake City caldera followed collapse and was accompanied by intrusion of a flat-topped, granitic magma centered in the resurgent dome. Ring magmatism produced dike-like intrusions along the northern ring fault and the Red Mountain-Grassy Mountain quartz latite ring dome on the eastern caldera margin. The caldera was emplaced into older Tertiary volcanic rocks and Precambrian granitic rocks. Based on analyses of outflow-facies samples and of the least altered intracaldera facies, we can demonstrate that the caldera-fill Sunshine Peak Tuff originally was isotopically very homogeneous, with an initial igneous δ18O value of +7.2 to +7.3. Thus, 18O depletions in the hydrothermally altered tuff could be compared without worrying about the complicating factor of different initial δ18O values. Nearly all the rocks within the caldera and outside the caldera within at least 3 km of the ring fault were altered by meteoric-hydrothermal fluids, and depleted in 18O down to values as low as δ18O = -3.1. Erosion has exposed the hydrothermally altered caldera-fill rocks and the upper contact of the altered resurgent intrusion in the western and central part of the caldera, providing about 2 km of vertical exposure. Because of post-alteration regional eastward tilting, the eastern part of the caldera has not been extensively eroded, and the original topography of the ring dome and the top of the caldera-fill rocks are locally preserved. This differential erosion from west to east furnishes a unique ...