EARLY MESOZOIC VOLCANICLASTIC ROCKS OF AGNEW MEADOWS, EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

In the Ritter Range pendant of the east-central Sierra Nevada, volcaniclastic strata of the Koip sequence in Agnew Meadows have a northwest strike and dip steeply to the northeast, allowing a transect through early Mesozoic volcanic arc stratigraphy. Strata exposed around Agnew Meadows include ~220 Ma rhyolitic tuff and tuff breccia. The tuff contains broken phenocrysts (< 2 mm) and fiamme (< 2 cm) and is likely a pyroclastic density current deposit. Predominantly matrix-supported pumice clasts (< 1.5 m) constitute the tuff breccia, which may have formed from collapse of the carapace of a sub-aqueous dome. These rhyolitic deposits are deformed and altered; however, we look past those secondary processes to characterize the original volcanic facies and magmatic products.

The tuff and tuff breccia clasts are crystal-poor and porphyritic with an average of 11% ß-quartz and plagioclase phenocrysts. The ß-quartz phenocrysts are mostly fragmented with few subhedral to euhedral grains (< 2.6 mm), and ~30% of the intact crystals are embayed. The plagioclase grains are approximately equally fragmented and subhedral to euhedral grains (< 1.8 mm). Harker diagrams show whole-rock compositions are similar, while the breccia clasts are relatively more silica-rich: the tuff samples average 72.4% SiO₂ by weight while the breccia clasts average 76.7%. Negative covariation between silica and fluid-immobile elements (Al₂O₃, TiO₂, Zr) indicate the breccia clasts are more fractionated than the tuff. Chondrite-normalized REE patterns are light REE-enriched with small negative europium anomalies and slight depletion in the middle to heavy REE.

Similar physical and chemical characteristics are noted in other Triassic tuffs in the eastern Ritter Range and Mount Morrison pendants as well as in the granite of Lee Vining Canyon, a pluton of the late Triassic Scheelite Intrusive Suite. Observations of the physical, chemical, temporal, and spatial homogeneities between various Sierran intrusive and extrusive rocks suggests that volcaniclastic rocks and plutons were coeval and probably cogenetic. If early Sierran arc plutons and volcanic deposits had similar origins, then at least some of the plutons must be fossil magma chambers.