EMPLACEMENT PROCESSES FOR BRECCIAS AT THE MOUNT YANA VOLCANIC CENTER, NORTHERN CALIFORNIA

The Yana Volcanic Center (YVC) is situated just south of Lassen Peak in California and includes an eroded stratovolcano that was active 2-3.5 Ma. Early construction of the YVC appears to have been primarily dominated by volcaniclastic breccias of basaltic-andesite and andesite compositions. Previous surveyors have only investigated these breccias at a general scale and a more detailed study is required to understand compositional variations and emplacement processes. Geological mapping, geochemistry, and petrology were used to investigate breccia lithologies and assist with stratigraphic correlation of breccia outcrops across the YVC. Results suggest four different breccia units based on textural classifications, erosional resistance, and dominant clast type. Among these groups are rare red breccia units appearing highly oxidized and eroded. These are described by being clast supported and poorly sorted with aphanitic clast of intermediate composition. Likely emplaced by brecciated andesitic lava flow cores. Common gray breccia units making up resistant cliff formers appear matrix-supported, non-welded, vesiculated, and contain scoria pebbles with dominant porphyritic-aphanitic clast of intermediate to mafic composition. Possibly emplaced by two separate debris flow events. Heavily eroded dark breccia units forming ridgelines represent highly vesiculated bomb clasts welded, poorly sorted, and clast supported with porphyritic clast of intermediate composition. Most likely emplaced by distal vent ejecta. Source vent breccia detail a localized unit appearing as eroded scoriaceous breccia bombs welded and poorly sorted with porphyritic clast intermediate in composition. Likely emplaced by source point vent ejecta. It appears breccias found in the vicinity do not all correlate to debris flow emplacement stories of YVC and instead depict different emplacement stories over the course of YVC active lifespan.