EPISODIC SIERRAN ARC MAGMATISM EXPRESSED IN VOLCANIC STRATA OF THE MOUNT MORRISON PENDANT, EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

The magmatic tempo of the Mesozoic Sierran arc is expressed in six kilometers of pyroclastic and epiclastic strata in the Mount Morrison pendant in east-central California. These rocks, which have been tilted subvertically and faulted out of their original arrangement, are the extrusive products of arc-wide high-flux events that were also responsible for the most voluminous additions to the Sierra Nevada batholith. Stratigraphic sections, whole rock and zircon trace element geochemical data, and seven new zircon U-Pb ages from the Mount Morrison pendant provide insight into stages of arc evolution at ~37.6°N.

Radioisotopic U-Pb zircon dating of major pyroclastic units reveals that the pendant contains discrete stratigraphic packages from the Late Triassic (218±4 Ma), Middle Jurassic (172±2 Ma), and Early Cretaceous (102±2 Ma). The Jurassic section overlies the Triassic and Cretaceous sections along a fault contact, a relation that prevents specific spatial correlation without tectonic restoration. Nonetheless, these dates correspond to the three major pulses of Sierran arc magmatism, and indicate that high-flux events in the crust manifest as ignimbrite eruptions. These geochronologic data largely agree with those from other nearby pendants, affirming that magma production was episodic at this latitude.

These magmatic pulses may have been preceded by episodes of strong crustal contraction. Regeneration of these unique tectonic conditions at discrete time intervals in the Mesozoic appear to be expressed in the volcanic record. Stratigraphy of individual packages points to trends in eruption sequence, with minor eruptions of ash and lava leading to catastrophic caldera-collapse events and the emplacement of monotonous, high-silica ignimbrites.

Similarities in age and trace element geochemistry suggest correlation of the sections in the Mount Morrison pendant with the Ritter Range and Saddlebag Lake pendants to the north. Marine facies are here documented in the Mount Morrison pendant in correlation with similar rocks found in the adjacent pendants. Primary volcanic textures and sedimentary lithologies suggest that marine, or at least subaqueous, conditions may have persisted at this latitude longer than previously thought.