MANTLE LITHOSPHERE AS SINK AND SOURCE OF CONTINENTAL ARC AND POST-ARC MAGMATISM

Cenozoic volcanism in the northern Sierra Nevada, CA, started at ~28 Ma and transitioned from calc-alkaline to alkaline composition at ~3 Ma when subduction of oceanic lithosphere ceased beneath the region. The calc-alkaline volcanic rocks are medium-K basalts to rhyolites and share similar chemical and isotopic characteristics with present-day volcanism in the southernmost Cascades. In particular, volcanic rock (Sr/P)_N, a measure of the slab-component involved in mantle melt production, increases with decreasing ⁸⁷Sr/⁸⁶Sr (T) and increasing e_Nd(T). We interpret these observation as evidence that calc-alkaline magmatism was subduction-related and that mafic melts generated by flux melting of the mantle wedge interacted with lower e_Nd(T) and higher ⁸⁷Sr/⁸⁶Sr (T) sub-Sierra continental mantle lithosphere. In contrast, < 3Ma volcanic rocks are predominately high-K trachybasaltic andesites. Compared to the lowest e_Nd(T)(~ -3), >3Ma volcanic rocks, the trachybasaltic andesites have higher HFSE contents, higher LREE abundances and (La/Yb)_N, higher wt.% P₂O₅, but similar Y and HREE contents and Nd and Sr isotopic compositions. The chemical differences between the <3Ma and >3Ma, low e_Nd(T) volcanic rocks are not the result of differences in the degree of partial melting of mantle lithosphere. Instead, the younger volcanic rocks are likely the products of melting of mafic veins and metasomatized mantle lithosphere, the incipient melting of which destabilized secondary, accessory mineral phases, including rutile, apatite and phlogopite but left residual garnet and clinopyroxene. We suggest that the mantle veining and metasomatism was associated with the older subduction related magmatism and that the mantle lithosphere served as a sink for components derived from infiltrating mafic melts. Once subduction ceased, reestablishment of a “normal” continental geotherm beneath this region heated and “baked out” the metasomatized and veined lithospheric mantle, resulting in the production of the post-subduction alkalic volcanism. While metasomatized continental mantle lithosphere is clearly involved in producing the spectrum of Cenozoic volcanic rock compositions observed in the northern Sierra, it may also play a role at other continental volcanic margins including the western margin of northern Mexico.