Deep lithospheric dynamics beneath the Sierra Nevada during the Mesozoic and Cenozoic as inferred from xenolith petrology

Abstract

Peridotite xenoliths erupted in late Miocene basalts (~8 Ma) in the central Sierra Nevada sample a lithosphere that is vertically stratified in terms of age and thermal history. The deeper portions (~45-100 km) have asthenospheric osmium isotopic compositons and possess textural and chemical evidence for cooling from >1100° to 700-820°C. The shallower portions (<60 km) have unradiogenic Os isotopic compositions, which yield Proterozoic model ages, and contain orthopyroxenes that record temperatures as low as 670°C in their cores and heating up to 900°C on their rims. These observations suggest that the deeper xenoliths represent fragments of hot asthenosphere that upwelled to intrude and/or underplate the overlying Proterozoic lithosphere represented by the shallower xenoliths. The contrasting thermal histories between the shallow and deep xenoliths suggest that hot asthenosphere and cold lithosphere were suddenly juxtaposed, a feature consistent with the aftermath of rapid lithospheric removal or sudden intrusion of asthenospheric mantle into the lithosphere rather than passive extension. On the basis of regional tectonics and various time constraints, it is possible that this lithospheric removal event was associated with the generation of the Sierra Nevada granitic batholith during Mesozoic subduction of the Farallon plate beneath North America. Pleistocene basalt-hosted xenoliths record a different chapter in the geodynamic history of the Sierras. These xenoliths are relatively fertile, come from depths shallower than 45-60 km, are characterized by asthenospheric Os isotopic compositions, record hot equilibration temperatures (1000°-1100°C), and show no evidence for cooling. The strong contrast in composition and thermal history between the Pleistocene and late Miocene suites indicate that the post-Mesozoic lithospheric mantle, as represented by the latter, was entirely replaced by the former. The hot Pleistocene peridotites may thus represent new lithospheric additions associated with a post-Miocene lithospheric removal event or extension. High elevations, low sub-Moho seismic velocities, and the presence of fast velocity anomalies at 200 km depth may be manifestations of this event. If lithospheric removal occurred in the Mesozoic and Cenozoic, the observations presented here place constraints on the styles of lithospheric removal. In the Mesozoic, the lithospheric mantle was only partially removed, whereas in the Pliocene, the entire lithospheric mantle and probably the mafic lower crust were removed.