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Southern California Geology

 

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Mesozoic Batholithic rocks

In latest Paleozoic and Mesozoic time, the ancient continental margin of North America now exposed in the San Bernardino Mountains was subjected to successive intrusions of granitic rock called plutons that coalesced regionally to form a vast regional plutonic complex or batholith . These plutonic rocks can be grouped into two packages: (1) older latest Paleozoic and Mesozoic plutonic and volcanic rocks, and (2) younger Mesozoic plutonic rocks.

Older Mesozoic plutonic and volcanic rocks .--These include:

  • granitoid rocks of Permian and Jurassic age: These plutonic rocks are quartz-poor to quartzose alkalic and potassic rocks that occur in two main areas: (1) adjacent to the Mill Creek strand of the San Andreas Fault in the south-central part of the range, where a megaporphyritic hornblende-biotite monzogranite has a U/Pb intrusive age of about 215 million years (Triassic) (Frizzell and others, 1986); (2) in the north-central and northwest part of the range, where alkalic hornblende monzonite has yielded an 40Ar/39Ar minimum age of about 214 million years (Cameron, 1981). The Triassic granitic rocks are lithologically and chemically distinct from younger Mesozoic granitoids, with the hornblende monzonite in the north part of the range representing a distinctive suite of alkalic rocks that developed along the continental margin of western North America during early Mesozoic time (Miller, 1977a,b, 1978; Smith, 1982a,b).
  • Jurassic hypabyssal (shallow crustal) plutonic rocks and volcanic rocks: Jurassic plutonic rocks occur locally in the central San Bernardino Mountains, and these may be related to a hypabyssal dike complex that crops out in the same region. The plutonic rocks are dioritic, quartz dioritic, and tonalitic in composition, and yield 40Ar/39Ar minimum ages of 126.73.5 Ma and 148.13.1 Ma and model Ar-retention ages of about 156 to 158 Ma (Cameron, 1981, p. 334). The hypabyssal dike complex was mapped and described by Richmond (1960) and Smith (1982a,b), who concluded that it was emplaced at shallow crustal levels within a northwest-trending fault zone that developed in prebatholithic rocks. Cameron (1981, p. 177-179) observed textural and compositional similarities between andesitic components of the dike complex and tonalitic, dioritic, and quartz dioritic shallow-level Jurassic plutonic rocks that he and Richmond (1960) mapped north of Big Bear Lake; Cameron proposes that the dike complex and the shallow-level dioritic rocks may be coeval and comagmatic. If Cameron is correct, then the dioritic rocks and the hypabyssal dike complex in the north-central San Bernardino Mountains may be part of the Late Jurassic Independence dike swarm of the Owens Valley region that James (1989) has recognized elsewhere in southern California.
  • Granitic-rock outcrop Granitic-rock outcrops in the San Jacinto Mountains; this scene typifies a geomorphic landscape underlain by plutonic rocks of granitic composition. Photo by J.C. Matti, USGS, June, 1978.

     

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

     

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

Younger Mesozoic plutonic rocks .--These include granitoid rocks of Cretaceous age that crop out extensively throughout the San Bernardino Mountains. In the east part of the range, the granitoid rocks typically are monzogranitic in composition and are biotite-bearing; in the west part of the range, comparable granitoid rocks are granodioritic to monzogranitic in composition and are biotite- and hornblende-biotite bearing.

A polished slab A polished slab of a texturally massive, equigranular granitic rock similar to late Mesozoic granitic rock in the San Bernardino Mountains. This example probably is monzogranitic in composition (image source: Marble and Granite, Inc )

 

A polished slab This is a view of a polished slab of a slightly foliated equigranular granitic rock similar to Late Mesozoic rock in the San Bernardino Mountains. This example probably is monzogranitic in composition (image source: Marble and Granite, Inc )

 

polished slab of a texturally massive, equigranular granitic rock Click on this thumbnail to view a polished slab of a texturally massive, equigranular granitic rock similar to late Mesozoic granitic rock in the San Bernardino Mountains. This example probably is granodioritic in composition (image source: Marble and Granite, Inc )
Most workers group these rocks within a single undifferentiated unit (quartz monzonite [qm] of Dibblee, 1964a,b, 1967b,c; Cactus Granite of Vaughan, 1922, and Guillou, 1953); however, detailed studies like those of MacColl (1964) and Miller (1987; Miller and others, 1998, 2000) show that discrete phases and plutonic bodies can be differentiated within the monzogranitic terrane. Locally these include muscovite-garnet granite, granodiorite, tonalite, and alaskite bodies.
Continue to Plutonic and Metamorphic Complex

Selected References
 
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