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

Abstract

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, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb decrease, and ε 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 fl ux melting of mantle wedge that interacted to varying degrees during ascent with lower ε Nd and higher 87 Sr/ 86 Sr sub-Sierra Nevada continental mantle lithosphere. The younger volcanic rocks lack evidence for the involvement of a slab compo- GEOLOGIC SETTINGThe Sierra Nevada mountain range in eastern California (Fig. 1) has been the subject of intense investigation into the processes involved For permission to copy, contact editing@geosociety.org

A tale of two Walker Lane pull-apart basins in the ancestral Cascades arc, central Sierra Nevada, California

“…14), the convective mantle does indeed make its appearance, but only to the south and east, beneath the Coso volcanic field, where the north-migrating Mendocino triple junction has had sufficient time to affect the removal of continental mantle lithosphere. We would partially agree with Farmer et al (2013) that the lithosphere beneath the Sierra Nevada is being heated, but we argue that the lithosphere is degraded slowly, and that high-K 2 O volcanism marks the earliest stages of lithosphere degradation, not its removal, which comes much later (e.g., fig. 16).…”

“…2A). This yielded a 40 Ar/ 39 Ar age of 177 ± 7 ka (Farmer et al, 2013). We discovered a basalt feeder dike for this (sample Qdike 8-19-08; Table 1).…”

“…We discovered a basalt feeder dike for this (sample Qdike 8-19-08; Table 1). These mafic rocks form part of the regional suite of postsubduction magmas (Farmer et al, 2013), which have been interpreted as continental rift magmas (Lange and Carmichael, 1996;Cousens et al, 2011Cousens et al, , 2012John et al, 2015b;Putirka and Platt, 2012).…”

Pleistocene volcanism and shifting shorelines at Lake Tahoe, California

“…These compositions suggest that the magmas were produced by subduction-related melting that was later affected by conductive heating (Farmer et al, 2013). Putirka and Platt (2012) suggest that this type of volcanism is a passive response to extension and that degradation of the submantle root may continue up to 10 Ma after the cessation of subduction.…”

“…3). This change from arc to rift volcanism may have commenced as early as 3-4 Ma in the region (Farmer et al, 2013;Cousens et al, 2011).…”