Geologic evolution of Iron Mountain, central Mojave Desert, California

Abstract

Geologic mapping, structural analysis, petrologic study, and U-Pb geochronology at Iron Mountain, 20 km southwest of Barstow, California, place important constraints on the paleogeographic affinities of metasedimentary rocks in the area and provide new data to test Mesozoic and Cenozoic tectonic models for the central Mojave Desert. Rocks present at Iron Mountain include: Precambrian(?) and/or lower Paleozoic(?) miogeoclinal rocks, Middle Jurassic tonalite, Middle Jurassic Hodge volcanic series, Late Jurassic hornblende diorite, Cretaceous(?) peraluminous granite, and Cretaceous(?) granodiorite prophyry. Two phases of ductile deformation are present at Iron Mountain. The first phase, which penetratively deforms the miogeoclinal rocks, tonalite, and Hodge volcanic series, developed under amphibolite and greenschist facies metamorphic conditions. Fabric development in the Hodge volcanic series preceded emplacement of 151 ± 11 Ma granite. The second fabric deforms peraluminous granite and Late Jurassic hornblende diorite as well, and consists of spaced mylonitic shear zones. The shear zones predate emplacement of Late Cretaceous dikes (83 ± 1 Ma). The presence of probable miogeoclinal strata indicates that the boundary between allochthonous eugeoclinal rocks and parautochthonous miogeoclinal/cratonal rocks must lie north of Iron Mountain. The older amphibolite facies metamorphism and contractile deformation at Iron Mountain are interpreted to be part of a belt of Middle to Late Jurassic age deformation that runs northeastward through the Mojave Desert and forms the southern continuation of the east Sierran contractile belt. Newly recognized subvertical mylonitic shear zones of Cretaceous age at Iron Mountain have not been documented elsewhere in the central Mojave Desert. No significant Tertiary ductile deformation fabrics are present at Iron Mountain.