Dikes


Metasomatically altered diabase dikes cross cut undeformed serpentine in large roadcuts along Highway 41.  These dikes are probably feeder dikes where dunite has been replaced by serpentine (Page, 1972).  Serpentine near the dikes is blue-green in color while light brown serpentine is not in contact with the dikes.  The dikes consist mainly of chlorite-actinolite-prehnite with some relict clino-pyroxene.  The white color of the dike is due to fractures and vugs of prehnite, magnesite, dolomite, calcite, and barite (Cloos & Apted, 1979).  This is indicative of fluids present during metasomatism causing the hydration of peridotite to serpentine.  These altered dikes are rodingites.  Rodingites commonly consist of prehnite and hydrogarnets (Coleman, 1967).
     The formation of minerals is influenced by the composition of fluids and the composition of inclusions.  Reactions show dunite was altered to serpentine and olivine was replaced (Cloos & Apted, 1979).
a) 2Mg2SiO4 + 2H2O = Mg3Si2O5(OH)4 + MgOaq
b) 2Mg2SiO4 + 3H2O = Mg3Si2O5 (OH)4 + Mg(OH)2

Other possible reactions in the rodingite dikes are (Cloos & Apted, 1979):
15CaMgSi2O8 + 9CaAl2Si2O8 + 2NaAlSi3O8 + 16H2O + 5MgO ®
      CPX              Ca-PLAG       Na-PLAG
                     9Ca2Al2Si3O10(OH)2 + 3Ca2Mg5Si8O22(OH)2 + Mg5Al2Si3O10(OH)8 + Na2O
                            Prehnite                      Actinolite                     Serpentine
 

4CaAl2Si2O8 + 2CaMgSi2O6 + Mg3Si2O5(OH)4 + 5H2O ®
  Ca-PLAG          CPX              Serpentine
                     3Ca2Al2Si3O10(OH)2 + Mg5Al2Si3O10(OH)8 + 2SiO2
                          Prehnite                       Chlorite

Border zone reaction:
3Mg2SiO4 + 4H2O + SiO2 ® 2Mg3SiO5(OH)4
Olivine                                    Serpentine
 
 


Plexus of dikes along Highway 101 in California.

More Pictures


References:

Bailey, E.H., Blake, M.C., Jr., and Jones, David L., On-Land Mesozoic Oceanic Crust in California Coast Ranges: U.S. Geol. Survey Prof. Paper 700-C, p. C70-C81

Page, Ben M., Ophiolites and Subduction Complex, Cuesta Ridge and Vicinity:Geol. Soc. America Bulletin, v. 83, p. 957-972

Chipping, David H., 1987, Geologic History of San Luis Obispo County: The Geology of San Luis Obispo County, p. 115-123

Cloos, Mark, and Apted, Mick, 1979, Guidebook to the Southern Coast Ranges Geology and Plate Tectonics, p. 17-35


Back
Home

Created by:  Mollie Laird
Last Updated:  December 2003