FRANCISCAN GROUP 



45 



been effective in "triggering" the metamorphic re- 

 action. 



The origin of the glaucophane-bearing metamorphic 

 rocks is even more obscure, partly because these ap- 

 pear to be derived from many of the rocks of the 

 Franciscan group rather than from a single rock type 

 or favorable bed and partly because of their erratic 

 distribution. The largest masses in the district are 

 associated with both greenstones and sedimentary 

 rocks of the Franciscan group ; some border serpen- 

 tine intrusions but many others do not. More than 

 50 bodies, many of them less than 20 feet long, are 

 isolated outcrops surrounded by unaltered graywacke 

 or siltstone. The majority of these small isolated 

 masses are probably in shear zones, but such an en- 

 vironment cannot be demonstrated for all of them. 

 Nevertheless, the most striking feature of the glau- 

 cophane-bearing rocks is their close areal relationship 

 to shear zones, faults, or structural "knots." 



In texture the glaucophane-bearing rocks are in 

 general more distinctly foliated than the hornblende- 

 bearing rocks, and although some show no obvious 

 mineral orientation, many others show both planar 

 and linear parallelism. The foliation is parallel to 

 the original bedding, where the relation can be de- 

 termined, but it has been seen in only a few outcrops. 

 The textures of the rocks are thus of little help in as- 

 certaining the metamorphic process by which they 

 were formed. 



No chemical analyses of the glaucophane rocks of 

 the New Almaden district are available, but several 

 analyses of similar rocks from elsewhere in the Coast 

 Ranges are given in an excellent paper by Smith 

 (1906, p. 183-242). He concluded, on the basis of 

 these analyses, that no new materials, except possibly 

 water, had been added in the formation of the glau- 

 cophane rocks. Taliaferro (1943b, p. 178-179), on 

 the other hand, cites analyses to prove the introduc- 

 tion of material during the metamorphism of a chert 

 to a quartz-glaucophane rock. He apparently does 

 not take into account, however, the shaly parting lay- 

 ers, which are excluded from the analyses of chert 

 but were doubtless incorporated into the glaucophane 

 schist. It does seem likely, however, that in the for- 

 mation of some glaucophane-bearing rocks of the Cali- 

 fornia Coast Ranges small amounts of soda and other 

 materials have been introduced, and that other con- 

 stituents have been lost. On the whole, however, the 

 rocks are not appreciably more sodic than the rocks 

 of the Franciscan group from which most of them 

 were derived; they could easily have been formed by 

 a reconstitution of the elements in the original rock 

 accompanied by a little metamorphic diffusion. 



Despite the schistose appearance and relatively com- 

 plete recrystallization of the glaucophane-bearing 

 rocks, the presence of stilpnomelane, epidote, albite, 

 and chlorite indicates that they are probably the equiv- 

 alent of the more usual assemblages of the greenschist 

 facies and represent relatively low-grade metamor- 

 phism (Turner, F. J., 1948, p. 99-100). They appear 

 to have formed in the New Almaden district along 

 tectonic zones. This distribution may have resulted 

 from the development of high-pressure areas during 

 deformation, the generation of heat by shearing, the 

 availability of solutions, or by several of these com- 

 bined. In places the glaucophane rocks are adjacent 

 to serpentine masses, but as these serpentine bodies 

 are of the type thought to have been squeezed into 

 their present position as serpentine (p. 54-57), they 

 were probably incapable of any pronounced thermal 

 or hydrothermal metamorphism. In many more places 

 in the district the glaucophane-bearing rocks do not 

 lie adjacent to serpentine, and it would appear un- 

 likely that they have any direct relationship to these 

 rather impotent intrusives. 



The origin of the less common crocidolitic meta- 

 cherts is not known. In areal distribution these rocks 

 are comparable to the glaucophane-bearing rocks, and 

 it seems likely that they have a common origin. The 

 presence of aegirine, however, suggests a moderately 

 high temperature of formation. 



AGE OP THE FRANCISCAN GROUP 



Fossils diagnostic of the age of the Franciscan group 

 in the New Almaden area have been found only in the 

 limestone, which on the basis of Foraminifera has 

 been dated as early Late Cretaceous (Cenomanian), 

 see page 24-25. The next younger rocks in the district 

 are also Late Cretaceous in age, but we know neither 

 the part of the Late Cretaceous they represent nor 

 their stratigraphic relation to the Franciscan, because 

 they are in fault contact with this group. 



The time span represented by the Franciscan group 

 elsewhere in the State is imperfectly known for many 

 reasons. Few fossils have been found. Correlations 

 over a wide area are based on similarity of lithology, 

 and it is possible that rocks assigned to this group in 

 different areas were deposited at different times. The 

 base of the formation is nowhere exposed; the upper 

 contact is controversial. Taliaferro (1943b, p. 190- 

 202, 208-212) cites several localities where beds as- 

 signed to the Franciscan group grade upward into 

 fossiliferous Knoxville (Upper Jurassic) shales, with 

 which cherts and greenstones are interbedded. On 

 the basis of these relations, and previous fossil dis- 

 coveries which he carefully summarizes, Taliaferro 



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