GEOCHEMISTRY AND PETROGRAPHY OF THE JURASSIC METASEDIMENTARY

AND METAVOLCANIC UNITS OF THE ALABAMA HILLS AND SOUTHERN INYO

MOUNTAINS NEAR LONE PINE, CA

ABSTRACT

Jurassic metavolcanic and metasedimentary rocks outcrop in the Alabama Hills and southern Inyo Mountains near Lone Pine, CA.  The two outcrop areas lie approximately 10 kilometers apart on opposite sides of the right-slip Owens Valley fault system.  This research examined geochemistry and petrology of the two areas.

The Alabama Hills and Inyo Mountains vary in major oxide geochemistry, the former enriched in silica and potassium and the latter alumina, calcium, iron and magnesium.  Trace element content also varies, reflecting differences in source and depositional environment.  Field observations, and thin section petrology mimic the geochemical patterns.  Rocks from the Alabama Hills show restricted textures and mineralogy and are most likely the product of volcanic and volcaniclastic processes.  In contrast, Inyo Mountain rocks are more diverse lithologically, often distinctly layered and in thin section have textures and mineralogy reminiscent of sedimentary rocks.  Therefore, it appears the two rock units were deposited in differing environments, most likely separated geographically by 10’s of kilometers.  Comparisons of bulk rock chemistry and mineralogy utilizing ACF diagrams reveals that both areas did not experience regional metamorphism, but did experience similar degrees of contact metamorphism up to the albite-epidote hornfels facies.

There are two possible scenarios for the present geographic juxtaposition of the Alabama Hills and Inyo Mountains outcrops.  Both involve significant transport prior to the onset of Basin and Range faulting.  The first proposes that over 65 kilometers of late Cretaceous right-slip has occurred across the Owens Valley fault system.  While this hypothesis does account for the significant lithologic differences in the two areas studied, and perhaps some of the geochemical trends, it does not explain the similarity of metamorphic overprint.   The second theory envisions 20-30 kilometers of eastward transport of the Alabama Hills along a late Jurassic thrust fault.  This hypothesis is preferred.  It accounts for both the metamorphic overprint, as well as all observed major element trends.