MIDDLE TO LATE JURASSIC U-PB AGES OF INTERMEDIATE TO SILICIC, METAVOLCANIC ROCK OF THE PEAVINE SEQUENCE, NEAR RENO, NEVADA: IMPLICATIONS FOR REGIONAL CORRELATION WITHIN THE MESOZOIC CORDILLERAN ARC

Jurassic metavolcanic rocks are discontinuously exposed within roof pendants in a north trending belt along the eastern edge of the Sierra Nevada between Gardnerville, Nevada (NV) and Susanville, California (CA). They form part of the Peavine sequence. Roof pendant stratigraphy is grossly similar being dominated by upright, proximal-facies, volcanic rock of intermediate (lower in the section) to silicic composition (top of the section) with only minor, interbedded, coarse-grained, clastic rock. Hypabyssal intrusives are common. All these pendants lie outboard of postulated Late Jurassic or Cretaceous strike-slip faults suggesting they originated several hundred km’s south of their current location.

Prior to this study, the ages of these Peavine sequence exposures were only loosely defined as post-Triassic and pre-Cretaceous based on field relations and/or limited Rb-Sr data. ‘Pre’-volcanic basement is not preserved in the pendants. To gain a better understanding of the age and possible correlation of Peavine sequence volcanic rock, samples were collected from the northern (Dooley Creek, CA), central (Peavine Peak, NV) and southern (Genoa Peak, NV) parts of the exposure belt. The Peavine and Genoa Peak samples were obtained from the top of the roof pendant section, while the Dooley Creek sample represents the lower, intermediate section. U-Pb zircon ages from all three roof pendant samples were found to be similar (ca. 163 Ma) and fall near the boundary of the Middle and Late Jurassic (2012 GSA Timescale). Rhyolite from Genoa Peak provided the oldest age (163.8 ± 2.4 Ma) while silicic tuff at the summit of Peavine Peak yielded the youngest age (162.9 ±2.5 Ma). Andesite at Dooley Creek provided an age of 163.4 ± 2.3 Ma.

The new U-Pb ages indicate Peavine sequence magmatism took place within a very short time interval. This pulse of volcanism postdates mafic to andesitic magmatism in the Northern Sierra (NST) west of the study area. However, it is partially age-equivalent to Oxfordian (ca. 163 – 160 Ma) volcanic rock in the Mount Jura sequence (MJS) of the Eastern Mesozoic Belt near Taylorsville, CA, 75 km to the northwest. The MJS unconformably overlies Lower Jurassic volcanic and plutonic rocks of the Eastern Mesozoic Belt and the Peavine sequence may share a similar relationship with Lower to Middle Jurassic rocks of the NST.