VARIATIONS IN MAGMA SOURCE REGIONS DURING LARGE-SCALE CONTINENTAL EXTENSION, DEATH-VALLEY REGION, WESTERN UNITED-STATES

Extension in the Death Valley region proceeded in two phases: a rapid extensional phase between approximately 12 and 5 Ma and a slow phase between 5 Ma and the present. During the early rapid phase the Nd isotopic composition of rift-related mafic igneous rocks changed from high epsilon(Nd) (approximately - 1) values at 12 Ma to low epsilon(Nd) (approximately - 10) values at 5 Ma. This is opposite to what would be expected if the shift in the source of rift magmas was in response to simple thinning of the lithosphere, as has been suggested by previous models. The Nd and Sr isotopic variations suggest that the sources of the rift-related magmas changed from a source with possibly a relatively large asthenospheric component during the early rapid phase of extension to enriched lithospheric mantle during later slow extension. Assuming predominantly vertical magmatic transport, the Nd isotopic trend observed in the Central Death Valley volcanic field may indicate a relationship between rates of extension and variation in magma sources. The high epsilon(Nd) (depleted) component in the early magmas may reflect greater ease of penetration of the lithosphere by asthenospheric magmas while extension was rapid, while later enriched basalts were derived from overlying, old lithospheric mantle once extension slowed. Alternatively, a model with lateral flow of a fluid crustal layer during early extension may have forced asthenosphere melts derived from source regions under the Sierra Nevada eastward, interposing them between enriched lithospheric mantle and upper crust. Subsequently, late rift magmas would be derived mainly from the underlying enriched lithospheric mantle. Many previous estimates of mantle addition to continental crust have taken depleted mantle values as end-member estimates for the mantle component. However, the lithospheric mantle has isotopic and trace element characteristics similar to the continental crust. Accordingly, mantle addition in continental rifts may be much larger than previously estimated.