CORRELATION, AGE, AND TECTONIC IMPLICATIONS OF THE METAVOLCANIC ROCKS IN THE NORTHERN VERDI RANGE, SIERRA COUNTY, CALIFORNIA

The Verdi Range, just west of the California - Nevada border, is approximately 25 km north of Lake Tahoe. The crest of the northern Verdi Range consists of metamorphosed, volcanogenic, basaltic to andesitic breccia/conglomerate, sandstone, siltstone, and hypabyssal intrusives. Relict calcic plagioclase and augite phenocrysts are abundant. Sedimentological evidence indicates that the sedimentary rocks were deposited by subaqueous debris flows. Trace- and minor-element abundances indicate that the magma was calc-alkaline, and erupted in a predominantly oceanic-arc setting.

No radiometric or fossil age of the northern Verdi Range rocks is available. The southern crest of the Verdi Range consists of granitic rocks capped by Tertiary volcanics. The biotite 40Ar/39Ar age of the granitic rocks determined in this study is 96.64 +/-0.53 Ma. These rocks intrude (and thus provide a minimum age for) the northern Verdi Range metavolcanic rocks. Several other metavolcanic units occur in the region. The northern Verdi Range rocks were compared to each of these units, in hopes of establishing a likely age based on lithological, mineralogical and geochemical correlation.

The northern Verdi Range rocks are lithologically, depositionally, and compositionally very different from the Middle Jurassic Peavine Sequence continental-arc rocks (10 km NE, considered part of the Pine Nut Terrane), and they are mineralogically and geochemically different from the Lower Cretaceous Lacey Creek oceanic-arc rocks (25 km W). They are identical to the Middle Jurassic Tuttle Lake Formation oceanic-arc rocks (25 km W, considered part of the Northern Sierra Terrane). These relations 1) establish the northern Verdi Range metavolcanic rocks as part of the Tuttle Lake Formation; 2) establish a Middle Jurassic age for the northern Verdi Range metavolcanic rocks; 3) indicate that the northern Verdi Range is part of the Northern Sierra Terrane and not part of the Pine Nut Terrane; and 4) suggest a tectonic boundary between the northern Verdi Range and the Peavine Sequence, separating predominantly oceanic-arc related rocks (Northern Sierra Terrane) to the west and continental-arc rocks (Pine Nut Terrane) to the east.