MID-MIOCENE CALDERAS IN THE REVEILLE RANGE, NEVADA

The Reveille Range in central Nevada provides a unique window into the mid-Miocene geology and volcanology of the central Great Basin during the “ignimbrite flare up.” Although faulted along its margins, the interior of the range is relatively undeformed thus preserving volcanic centers. The Reveille Range contains three mid-Miocene caldera complexes. Previous studies have identified the Goblin Knobs caldera (25.6 Ma) in the central Reveille Range and the caldera of northern Reveille Range (25.3 Ma) to the north. These calderas are completely filled with intracaldera tuffs. This study has identified an older third caldera located in the southern part of the Reveille Range and the southern margin of the Goblin Knobs Caldera. The newly discovered caldera in the southern Reveille Range, herein named the Pyramid Spring caldera, contains intracaldera tuff, pumice-rich outflow sheets, and volcaniclastic sedimentary moat units dipping to the north off a possible resurgent dome. Crystal rich rhyolite domes occur along the eastern and northeastern margin of the caldera. An 8 km long composite dacite dike (Fang Ridge) is also associated with the caldera. Moat rocks include pyroclastic flows, debris flow breccias, and fine to coarse grained immature sandstones that contain lithic fragments up to 5 cm in size. Of the three calderas in the Reveille Range, the Pyramid Spring caldera is the only one that is associated with outflow tuffs. Three cooling units were identified; the middle is welded and contains fiamme up to 15-20 cm in size. The outflow tuffs of the Pyramid Spring caldera are abruptly cut by the Goblin Knobs caldera. Besides the stratigraphic truncation, the Goblin Knobs caldera margin was identified by a zone of hydrothermal alteration (silicification) and mineralization and a section of outflow from the Pyramid Spring caldera that has slumped (back tilted) into the Goblin Knobs caldera. Correlation, age and geochemistry of Pyramid Spring caldera tuffs and domes will be determined by paleomagnetic, 40Ar/39Ar, and geochemical studies.