Osmium Isotopic Evidence for Mesozoic Removal of Lithospheric Mantle Beneath the Sierra Nevada, California

Abstract

Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust.

A mantle melting profile across the Basin and Range, SW USA

“…Alternatively, melts derived in the asthenosphere could be contaminated by trace-enriched components in the thin overlying mantle lithosphere, thus affecting isotopic and incompatible trace element compositions without significantly disturbing major element components. These views are consistent with Nd isotopic values found in peridotite xenoliths from the area, which range from enriched to depleted compositions [Beard and Glazner, 1995], and Re-Os model ages, which range from Proterozoic to Phanerozoic [Lee et al, 2000].…”

“…There are several models for the causes of thinning of the mantle lithosphere beneath the eastern Sierras. These fall into two general categories: (1) shear or foundering/ delamination during Neogene extension [Jones et al, 1992;Ducea and Saleeby, 1996] and (2) thermal erosion or delamination during Mesozoic/Cenozoic subduction [Lee et al, 2000]. Better age constraints are needed to test whether one or both of these major tectonic episodes was the major cause of lithospheric thinning beneath the WGB/eastern Sierras.…”

“…Xenoliths from the central Sierras (Big Creek) record Mesozoic replacement of lithosphere by asthenosphere, based on thermobarometry, Os isotopic compositions, and Ca zonation in orthopyroxenes [Lee et al, 2000]. The timing of lithosphere replacement in the eastern Sierras is less clear, but mantle xenoliths from this region (some near the Big Pine volcanic field) record equilibration pressures that place them near the Moho (1.1 -1.9 GPa) and record higher temperatures (1100°-1200°C) than those beneath the central Sierras (900°-750°C) [Ducea and Saleeby, 1996;Lee et al, 2000]. Thus, the eastern Sierra xenoliths…”

Continental crust formation at arcs, the arclogite “delamination” cycle, and one origin for fertile melting anomalies in the mantle

“…There is some evidence that magmatism in continental arcs, such as the Sierra Nevada, may be episodic [89]. There is also some tentative evidence for multiple delamination events in the Sierra Nevada [43,90] and the Andes in South America [91]. It is even likely that delamination events in continental arcs are closely coupled with foreland fold-andthrust belts because removal of arc lower crust and lithospheric mantle provides room for continental basement to be underthrust beneath the arc [92].…”

Peridotite-melt interaction: a key point for the destruction of cratonic lithospheric mantle

“…This means that a dramatic physico-chemical change occurred in the lithospheric mantle beneath the North China Craton during the intervening time interval [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] . The behavior of the North China Craton is remarkably different from its counterparts elsewhere in the world, but similar to that in western America and Antarctic Peninsula [43,44] . This contradicts our previous understanding of lithospheric evolution.…”