PETROLOGY OF MEGABLOCKS IN A LARGE-SCALE PLIOCENE VOLCANIC DEBRIS AVALANCHE DEPOSIT IN THE NORTHERN SIERRA NEVADA, CALIFORNIA: INSIGHTS INTO MAGMATIC EVOLUTION AND PARTIAL COLLAPSE OF AN ANCESTRAL CASCADES STRATOVOLCANO

Cretaceous and older rocks in the northern Sierra Nevada in California are unconformably overlain by a widespread blanket of Miocene-Pliocene basaltic to andesitic lahar and fluvial deposits derived from parts of the ancestral Cascades volcanic arc. Source vents for the volcaniclastic debris, if they are still preserved, must lie east of the range-front fault zone defining the eastern margin of the tilted Sierran structural block. A massive volcanic debris avalanche deposit intercalated with Pliocene strata in the upper part of the succession records catastrophic collapse of parts of one of the ancient stratovolcanoes and occurs within paleochannels as much as 380 m deep that are exposed discontinuously over an area of ~ 400 km². Basaltic to dacitic megablocks within the deposit range from ~ 10 m to > 1.5 km across and are set within a chaotic, polymict volcaniclastic matrix. It is likely that most of the megablocks came from the source volcano and therefore may hold important clues to its petrogenetic evolution as well as possible causes of edifice collapse.

Chemical analyses of samples from 24 megablocks reveal SiO₂ contents ranging from 51 to 65 wt %. Major oxides generally show well-defined linear trends on Harker variation diagrams, indicating that most of the megablocks comprise a petrogenetically related suite. Such trends are inconsistent with fractional crystallization and instead suggest that magma mixing between mafic and felsic end-members played a major role in generating the array of compositions; abundant disequilibrium phenocryst assemblages support this interpretation. Five samples with the highest SiO₂ contents come from a series of distinctive pale gray, holocrystalline hornblende dacite megablocks that show very similar trace-element contents and are inferred to have been derived from a single felsic igneous unit within the source volcano. In one composite megablock the dacite shows a distinct chilled margin against overlying basaltic volcaniclastic material. In another megablock, basaltic volcaniclastic material underlain by dacite has been deformed into a laccolith-like antiformal structure. We infer that the dacite represents one or more hypabyssal bodies intruded into the source edifice, which may have oversteepened the side of the volcano, triggering large-scale failure.