GEOCHEMICAL TRANSITION FROM MIOCENE-PLIOCENE TO QUATERNARY ARC VOLCANISM IN THE NORTHERN SIERRA NEVADA, NORTHERNCALIFORNIA

Miocene-Pliocene volcanism around Lake Tahoe, California/ Nevada, part of the Southern Ancestral Cascade Arc, ceased at around 3 Ma as the southern edge of the subducting Juan de Fuca plate migrated north of the region. As a result of continuing subducting plate migration, arc volcanism continued north of Lake Tahoe but now ends at the modern LassenVolcanic Centre. The Sierra Nevada between Lake Tahoe and Lassen, or North Sierra Segment (NSS), includes remnants of Ancestral Cascade volcanic rocks, including lava flow complexes, intrusions, and landslide/debris flow deposits. Within the southern NSS is the 4-6 Ma Sagehen Volcanic Centre (SVC), located between Truckee and Sierraville, that includes similar rock types. Mafic Tahoe arc lavas appear to include an older, subduction modified lithospheric mantle source component which is not common in Lassen arc rocks. The goal of this work is to investigate how Miocene-Pliocene magma sources and/or volcanic processes transition at the north end of the Sierra Nevada between Tahoe and Lassen. Lava samples from the NSS include calc-alkaline basalts to dacites, rare low-K tholeiite basalts (LKT), mildly alkaline basalts with within-plate (WIP) trace element patterns, and the Lovejoy flood basalt. LKT and WIP compositions are also found at Lassen but not in the Tahoe region. All calc-alkaline and LKT lavas exhibit normalized incompatible element patterns with negative Nb, Ta and Ti anomalies, and prominent positive Pb, Sr and Ba anomalies, whereas WIP basalts have only small negative Nb anomalies. The calc-alkaline rocks of the NSS cover the range of Sr, Nd and Pb isotopic compositions observed at both Lassen and Tahoe. The basalts to andesites of the SVC cover the almost same range in isotopic compositions as the entire NSS. The mafic NSS rocks show a scattered isotopic trend with latitude from Lassen (87Sr/86Sr < 0.704) to Tahoe (87Sr/86Sr 0.7045-0.705) but no trend is seen in Pb isotope ratios. Compared to Lassen, mafic NSS rocks show much greater variation in trace element ratios (e.g., Ba/La, La/Nb, Th/La), similar to the large variations observed at Tahoe. Mafic NSS volcanic rocks are derived from more heterogeneous mantle sources than Lassen lavas, and we propose that sub-Sierra Nevada lithospheric mantle remains an important component in magmatism south of the modern Cascades.