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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Plutonic and Metamorphic Complex

A poorly understood belt of gneissose crystalline rocks crops out in the western San Bernardino Mountains and along the south-central and southeast margins of the range. The dominant characteristic of these rocks is gneissose fabrics created by faint to conspicuous compositional layering of mafic-rich (dark-colored) and mafic-poor (light-colored) layers. Layering is developed at all scales, ranging from millimeter and centimeter lamination to layering on outcrop and hillside scales. Dark-colored layers are biotite-rich and typically are foliated; light-colored layers are quartzofeldspathic and texturally massive to foliated, depending on whether the fabric is equigranular or lenticular. Mafic-rich layers generally are granodioritic to tonalitic; mafic-poor quartzofeldspathic layers mainly are biotite-bearing granodiorite, but include monzogranite, tonalite, and less common quartz-poor rock that is quartz monzodioritic. Mappable bodies of plutonic rock occur throughout the gneiss terrane (Matti and others, 1992b). The crystalline complex locally is traversed by low-angle shear zones, and the rocks are highly fractured.

Granitic rock .Granitic-rock outcrop (granodiorite to tonalite) in the San Bernardino Mountains; pencil is about 6 inches (15 cm) long. Photo by J.C. Matti, USGS.

 

Polished slab
This is a polished slab of gneissose and foliated rock similar to rock in the plutonic and metamorphic complex of the southern margin of the San Bernardino Mountains. It is not clear whether rocks of this type in the San Bernardino Mountains are deformed Mesozoic granitoids or older Proterozoic metasedimentary or metaigneous rocks. This example probably is granodioritic to monzogranitic in composition (image source: Marble and Granite, Inc )

These rocks traditionally have been interpreted as a Precambrian gneiss complex intruded by Mesozoic plutons (Dibblee, 1975, 1982b; Rogers, 1967, who compiled the work of Dibblee, 1964b,c, 1967a,b, and unpublished). However, the origin and age of the gneissose terrane are problematical. Without question, gneissose fabrics of the crystalline complex reflect metamorphic deformation under dynamothermal conditions. However, it is not clear if all of these gneisses formed during Precambrian metamorphism of Precambrian protoliths. For example, the gneissose rocks locally are intermingled with bodies of calcite marble, calc-silicate rock, metaquartzite, and garnetiferous biotite-sillimanite schist (Miller, 1979) that may well be Paleozoic in age, in which case the enclosing gneissose rocks also may be high-grade Paleozoic metasedimentary rocks or even deformed and metamorphosed older Mesozoic granitic rocks. To reflect these relations, Matti and others (1992b) interpret this terrane as a metamorphic and plutonic complex that largely was created in Mesozoic time, even though it contains tracts of Precambrian and Paleozoic metamorphic rocks. By this interpretation, some of the gneissose rocks are deformed and metamorphosed granitic bodies of Mesozoic age.


Continue to Upper Cenozoic Sedimentary Rocks

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