RECORD OF SIERRAN ARC ACTIVITY IN VOLCANIC STRATA OF THE MOUNT MORRISON PENDANT, EAST-CENTRAL SIERRA NEVADA, CALIFORNIA: IMPLICATIONS FOR MAGMATIC TEMPO AND PALEOGEOGRAPHIC SETTING

The Mount Morrison pendant in east-central Sierra Nevada, California, contains pyroclastic and epiclastic strata that were emplaced during periods of high magmatic flux along the Mesozoic Sierran arc. The age of the entire sequence was originally thought to be Triassic-Jurassic based on general stratigraphic relations with adjacent pendants. The volcanic strata of the Mount Morrison pendant actually record two separate pulses of volcanism separated by a hiatus of more than 100 million years. Radio-isotopic U-Pb zircon dating of major pyroclastic units reveals a lower, Late Triassic section (218±4 Ma) and an upper, Middle Cretaceous section (102±1 Ma).

Products of an intermediate magmatic pulse in the Middle Jurassic, documented in other volcanic pendants of the Sierra Nevada, are not exposed in the field area. The hiatus between the two events may thus correspond to a period of local amagmatism or net erosion. Nonetheless, the rocks of the Mount Morrison pendant provide an opportunity to study and compare pulses of Sierran arc magmatism during the Mesozoic.

Stratigraphic data show a repeating trend in eruption sequence between the Late Triassic and Middle Cretaceous assemblages. In both cases, initial eruptions of volcanic breccia, likely dome-related, were followed by minor eruptions of ash leading up to a catastrophic caldera collapse event that emplaced high-silica ignimbrites. It is thought that these magmatic pulses may have been preceded by 15-25 m.y. episodes of very high contraction. Therefore, unique tectonic conditions that arose in the Late Triassic and Middle Cretaceous appear to be expressed in the volcanic record.

Facies analysis indicates a significant shift in paleogeographic setting between the Late Triassic and Middle Cretaceous events. This is illustrated in basal breccia textures. The Upper Triassic basal Breccia of Mammoth Rock contains juvenile rhyolite clasts with cuspate shapes, internal jigsaw fractures, and fluidal margins that indicate marine emplacement. In contrast, the basal Green Breccia of Duck Lake, interpreted to be Middle Cretaceous, has angular volcaniclasts that lack subaqueous textures. This supports previous findings that the Sierran arc was in a subaqueous setting in the Late Triassic, and that it was uplifted to the subaerial realm by the Middle Cretaceous event.