Geology and dike swarms of the Homer Mountain area, San Bernardino County, California

Homer Mountain is located in the northern portion of the eastern Mojave Desert Province. Basement rock exposed in the 250 km2 study area consists of Precambrian large K-spar granitoid gneiss which is intruded by two Late Cretaceous peraluminous quartz monzonite plutons (Teutonia and Homer Mountain plutons). These plutons are similiar to the Mid Hills adamellite of the Teutonia batholithic complex of the New York Mountains. Developed within the eastern portion of the Precambrian basement, prior to the late Mesozoic plutonism, are a small number of thin, north-trending , east-dipping mylonitic zones. An early Tertiary or older system of generally northtrending altered tholeiitic quartz gabbro dikes are exposed in the central portion of the study area and intrude these basement rocks. Northwest- to northeasttrending conjugate leucocratic aplite dikes are interpreted to have been intruded along with scattered mineralized quartz veins during the final stage of Late Cretaceous plutonism. Late Cretaceous to early Tertiary (?) submarginally economic molybdenium-copper-tungsten mineralization in the western portion of the study area accompanied emplacement of the quartz veins along a northwest-trending shear zone and east-west-trending joints. Younger middle Tertiary age hydrothermal mineralization appears to have accompanied the intrusion of numerous east-west-trending felsic dikes. A similar pattern of mineralization associated with east-west-trending diking is developed in the New York Mountains and further west in the Cima Dome and Granite Spring area. During latest Oligocene time calc-alkaline basalt to quartz andesite flows were extruded onto the western portion of the study area and today cap the highest hills. These flows are associated with the volcanics of the southern Piute Range. A sample of the andesitic flows has yielded a K-Ar age of 27.7 ± 1.4 m.y.B.P. As these andesitic flows were being extruded , the Homer Mountain conglomerate, a cobble- to boulder-sized monolithic conglomerate with minor sandstone and volcanic interbeds, was being deposited in the eastern portion of the study area within a probable steep-walled graben. This conglomerate is intruded by alkalic rhyolitic breccia plugs which have yielded a latest Oligocene K-Ar age of 26.0 ± 1.7 m.y.B.P. The graben into which the Homer Mountain conglomerate was deposited was probably formed during the earliest phase of low-angle detachment faulting which was prevalent during the middle Tertiary in the present day lower Colorado River region. Also in the eastern portion of the study area, northtrending granophyric rhyodacite porphyry dikes and/or sills were intruded into the Precambrian and Mesozoic basement subparallel to the pre-Teutonia mylonitic zones. A sample of this granophyric dike has yielded a latest Oligocene K-Ar age of 27.2 ± 2.7 m.y.B.P. (See more in text)