Mantle lithosphere as a source of postsubduction magmatism, northern Sierra Nevada, California

Age, chemical, and isotopic data from late Cenozoic volcanic rocks in the northern Sierra Nevada, California (USA), from Lake Tahoe north to the southern end of the modern Cascades volcanic arc, were obtained to investigate the evolution of the upper mantle beneath this continental margin during the transition from active subduction to the opening of a slabless window, and to test the possibility that the foundering of mantle lithosphere proposed for the southern Sierra Nevada extended to the northern reaches of the mountain range. Our data are consistent with previous work in the region and illustrate that volcanism shifted from widespread intermediate composition magmatism to small volume, localized trachybasalts to trachyandesites ca. 3 Ma. Similar to southern Cascades volcanism, Sr-87/Sr-86 and Pb-206/Pb-204 decrease, and epsilon(Nd) increase in the older (older than 3 Ma) volcanic rocks with increasing proportions of a slab component, as measured by increasing (Sr/P)(N), where N is primitive-mantle normalized. We interpret these observations as evidence that the older volcanic rocks are subduction related and represent the products of basaltic melts derived from flux melting of mantle wedge that interacted to varying degrees during ascent with lower epsilon(Nd) and higher Sr-87/Sr-86 sub-Sierra Nevada continental mantle lithosphere. The younger volcanic rocks lack evidence for the involvement of a slab component in their generation, but have ranges of Nd, Sr, and Pb isotopic compositions similar to those of older volcanic rocks interpreted to have interacted to the greatest extent with the continental mantle lithosphere. However, the younger volcanic rocks have higher high field strength element (HFSE) and higher phosphorus abundances, and higher (La/Yb)(N), than their older counterparts, suggesting that they are not simply the products of smaller degrees of partial melting of the same mantle lithosphere involved in the older magmatism. The high HFSE and P contents were more likely controlled by metasomatic accessory carrier phases such as rutile and apatite, the stabilities of which control the abundance of these elements in melts produced from the lithospheric mantle after 3 Ma. One possibility is that the accessory phases were introduced to lithosphere during melt-wall-rock interaction associated with the older magmatic episode. These phases were then purged as a result of conductive heating of the remaining lithospheric mantle triggered by postsubduction upwelling of the sublithospheric mantle. Our data are consistent with lithospheric mantle serving as a melt reactor during the earlier subduction-related magmatism that was baked out during later conductive heating, a process that may be relevant to the production of immediately postsubduction magmatism along other continental margins.