GABBROS AND RELATED ROCKS 



57 



something that should be determined experimentally. 

 From the chemical nature of the rock, however, it 

 seems not at all unlikely that the conversion of a 

 minute fraction of the serpentine to magnesium-silicate 

 gel may perhaps aid the intrusion by acting as a lubri- 

 cant. 



Age 



The age of the serpentine bodies in the California 

 Coast Ranges has been variously placed by geologists 

 who have worked in different parts of the region. In 

 addition, serpentine was formei-ly treated as an inte- 

 gral part of the Franciscan group, much as the green- 

 stones are treated, chiefly because it is particularly 

 common in areas of rocks assigned to the Franciscan 

 group. 



Taliaferro (1943b, p. 153) has noted serpentine in 

 the late Upper Jurassic Knoxville formation, but 

 Anderson (1945, p. 956-957) maintained that the ser- 

 pentine masses in the Knoxville formation were all 

 plastically injected. Yates and Hilpert (1946, p. 239) 

 have shown that Knoxville sedimentary rocks contain 

 detrital serpentine and are also invaded by serpen- 

 tinized peridotite, and they conclude that there were 

 serpentine intrusions of at least two ages. Near Wil- 

 bur Springs, in southern Colusa County, in the Lower 

 Cretaceous Paskenta formation there are extensive 

 sedimentary beds hundreds of feet thick consisting al- 

 most wholly of serpentine detritus. These are most 

 easily explained by the extrusion of serpentine onto 

 the sea floor during Early Cretaceous time. Still 

 younger rocks have been invaded by serpentine else- 

 where in the Coast Ranges (Clark, B. L., 1935, p. 

 1060, 1074; Bailey, 1942, p. 150-151; and Eckel and 

 Myers, 1946, p. 94) . 



Injections of serpentine into the younger rocks of 

 the Coast Ranges are generally believed to be plastic 

 injections, but the intrusions into the older rocks are 

 commonly believed to represent intrusions of molten 

 magma. Because the writers believe that it is likely 

 most of the older serpentine bodies were injected plas- 

 tically as serpentine, rather than as peridotitic magma, 

 the time of intrusion is believed to be dependent upon 

 orogenic forces rather than the time when molten 

 magma was available. On this hypothesis, once the 

 serpentine is formed at depth it is available for in- 

 trusion at any time thereafter, although, of course, 

 through geologic periods the thickening of the over- 

 lying cover by sedimentation makes its injection from 

 the substructure to high levels increasingly less likely. 

 However, previously injected serpentine may be later 

 remobilized, and in the New Almaden area there is 

 some evidence that this has taken place. 



686-671 O 63 5 



GABBROS AND RELATED BOCKS 



The intrusive body lying along the main divide 

 south of Mount Umunhum and shown on the map of 

 the district as serpentine contains perhaps as much 

 gabbroic rock as serpentine. The gabbroic rocks are 

 partly serpentinized themselves and are so intricately 

 intermixed with the normal serpentines that mapping 

 them as a separate unit, if it could have been done at 

 all, would have required more time than it seemed to 

 be worth. Because these gabbroic rocks are of rather 

 small extent, they have not been studied in any great 

 detail. 



Areas underlain by the gabbroic rocks have a dis- 

 tinctive appearance. They generally have a rather 

 subdued topography, are covered with a moderately 

 dense growth of brush in which manzanita predomi- 

 nates, and are overlain by a deep reddish soil. Large 

 rounded boulders of relatively fresh rock protrude 

 from the soil here and there, and, particularly on gen- 

 tle slopes, smaller boulders and pebbles tend to form 

 a surficial pavement. Natural exposures of rock in 

 place are scarce, but some good exposures are af- 

 forded by road cuts. Here the gabbroic rocks may 

 be seen to vary widely in both texture and mineral 

 content within a few feet or even inches. Some 

 patches are fine grained, others are coarse grained, 

 and still others have a pegmatitic texture and contain 

 large poikilitic crystals of pyroxene or hornblende. 

 The rocks range in color from white to deep green or 

 nearly black, depending on the amount of feldspar, 

 intermediate varieties being speckled or mottled. A 

 single exposure may show nearly all these variations, 

 and in some places the rocks are distinctly banded, 

 different minerals being concentrated in either layers 

 or concentric zones. In most exposures they are also 

 cut by autoinjection dikes, which tend to be somewhat 

 more feldspathic than the average gabbro. 



The principal minerals that can be identified with 

 a hand lens are feldspar, pyroxene, hornblende, ser- 

 pentine minerals, and mafic accessories, such as mag- 

 netite, chromite, and picotite. A limited amount of 

 microscopic study showed that the original feldspars 

 were calcic labradorite and bytownite, and that the 

 other common minerals were olivine, augite, and horn- 

 blende. Virtually all the original minerals have been 

 altered in varying degrees. The plagioclase is not 

 highly altered in most sections; in some, however, it 

 is sausuritized, and in others fine-grained secondary 

 albite forms a "groundmass" between the larger grains. 

 The serpentine minerals antigorite and serpophite 

 have replaced the olivine to such an extent that only 

 a few residual cores of olivine were noted. A few 



