Chapter 15: Two different lithosphere types in the Sierra Nevada, California

Abstract

Chemical and isotopic characteristics of plutons in the western United States reflect compositions and protoliths of subjacent source materials. A discontinuously exposed shear zone that extends along the length of the Sierra Nevada in California marks a boundary between two areas manifested geologically by wall-rock and roof-pendant lithologies of different ages, depositional environments, and structural histories. In addition, plutons on either side of the boundary have different chemical and isotopic compositions, which indicate that their source regions are of two fundamentally different lithosphere types. The western lithosphere type is called Panthalassan, whereas the eastern type is called North American. Isotopic investigations of plutons have defined an initial 87 Sr/ 86 Sr (Sr i ) = 0.706 line in each lithosphere type. However, δ 18 O more than +9 per mil in plutons with Sr i greater than 0.706 in the Panthalassan lithosphere indicates a significantly greater sedimentary component in the source materials for these plutons than for those plutons with similar Sr i but δ 18 O less than +9 per mil intruded into North American lithosphere. In contrast to the North American lithosphere, there is no evidence that a Proterozoic crystalline sialic basement exists where plutons have Sr i greater than 0.706 in the Panthalassan lithosphere. Instead, the plutons with Sr i greater than 0.706 intruded into Panthalassan lithosphere probably acquired that characteristic by assimilation of sediments derived from a Proterozoic sialic crust. Plutons with Sr i less than 0.706 have chemical and Nd isotopic characteristics that indicate time-integrated depletion in large ion lithophile elements in their source regions in the Panthalassan lithosphere relative to their sources in the North American lithosphere. The tectonic contact between the two lithosphere types may be the extension of the Sonora-Mojave megashear into northern California.