ZIRCON HAFNIUM ISOTOPE RECORD OF DEPLETED MANTLE CONTRIBUTIONS TO THE MESOZOIC SOUTHERN SIERRA NEVADA ARC, PART 2: ARC RIFT-QUENCHING (AR-Q)

Zircon hafnium isotope data (n=520) from plutonic-volcanic rocks of the Kern Plateau provide a near-continuous record of magmatic differentiation during the ~180-my lifespan of the southern Sierra Nevada arc. Gaps in this eHf record are filled by zircon xenocrysts from a 79±3 Ma diabase dike intruded through the Jurassic rift-graben. Temporal variation of εHf through the Mesozoic is dominated by a 40 e-unit spread at 150 to 145 Ma, signaling upwelling of depleted mantle (DM)-derived basalts, partial melts of ancient lower crustal rocks, isotopically enriched mafic magmas, and intermediate to felsic magmas. High (>+10) εHf zircon record intrusion of DM-derived magmas at ca. 240, 180, 150 and 100 Ma, challenging models of an enriched mantle lithospheric reservoir supplying the southeastern Sierra Nevadan arc. The marked εHf increase at ca. 150 Ma is interpreted to signal rifting, simultaneous with emplacement of the Independence dike swarm, which supports and extends existing models for deposition of Late Triassic-Middle Jurassic volcanic strata within an intra-arc rift graben.

Two eHf “pull-downs” flank the ca. 150 Ma rift event, both of which stabilize at approximately -10 εHf-units. 195-160 Ma zircon from the >700-1500 km² Sacatar Intrusive Complex (SIC) and coeval Tubatulabal volcanics vacillate around this average (-10±2), which may represent a maximum limit to crustal assimilation. Normal-to-transtensional extension along the rift-bounding Kern Plateau shear zone (KPsz) punctuated SIC emplacement, implicating extension as a driver of upper crustal pluton construction. After emplacement, and possible eruption of the ca. 147 Ma Osa Creek ring dike complex, there is no known upper crust plutonism in the Kern Plateau for ca. 50 m.y. Diabase xenocrysts, however, indicate DM-derived, presumably mafic magmas (zircon eHf ~+7), intruded the rift zone throughout this period, consistent with early stages of reconstruction of the crustal hot zone. Emplacement of silicic magmas into the upper crust of the Kern Plateau resumed at ca. 97 Ma. We interpret the gap in local upper crustal magmatism as indicating that extension, culminating in crust-scale rifting, evacuated the Jurassic melt column from top to bottom and suspended the local upper crustal record of arc magmatism for 50 m.y., a process we call arc rift-quenching.