Franciscan blueschist-facies metaconglomerates, Diablo Range, California: Discussion and reply Discussion

T. W. BLOXAM Department of Geology, University College of Swansea, Singleton Park, Swansea, Wales SA2 8PP, United Kingdom

I was interested to read a further account of Franciscan Contrary to the impression created by Fyfe and Zardini (1967) blueschist-facies metaconglomerates by Piatt and others (1976) and Piatt and others (1976), rocks of this type were identified as that contains information similar to that presented by Fyfe and metaconglomerates by me in 1955. The Angel Island and related Zardini (1967). outcrops of metaconglomerate (Ransome, 1894, p. 197; Bloxam, Ransome (1894) noted the presence of glaucophanized clasts in 1955; 1960, p. 561) are frequently jadeite-bearing and form lenses what he considered to be unmetamorphosed Franciscan conglom- (possibly channel-type deposits) within jadeite-bearing meta- erates on Angel Island (San Francisco Bay). He found no difficulty graywackes; the latter also constitute the finer-grained matrix be- in ascribing an intraformational origin for these conglomerates be- tween blueschist clasts in the metaconglomerates. cause the blueschist pebbles were thought to have been derived An important part was also ascribed to metasomatising solutions from shallow-level Franciscan contact metamorphic zones as- derived from the serpentinization of adjacent ultramafic bodies sociated with ultramafic bodies. These conclusions were also ac- (Taliaferro, 1941, 1943; Crittenden, 1951; Bloxam, 1960, 1966; cepted by Taliaferro (1943). Fyfe and Zardini, 1967). Reply

JEREMY B. PLATT* J. G. LIOU Department of Geology, Stanford University, Stanford, California 94305 BEN M. PAGE

We appreciate the additional references supplied by T. W. nately, our metaconglomerate localities in the Diablo Range do not Bloxam illustrating the controversy relating to the interpretation of provide evidence bearing on these hypotheses. In the localities we blueschist pebbles in Franciscan conglomerates. In our preoccupa- studied, no mafic igneous material or serpentinite is exposed tion with the Diablo Range, we regrettably overlooked the fact that nearby except as disconnected blocks in mélanges. In the case of Bloxam (1955, 1960) had recognized in situ metamorphism of serpentinite blocks, the ultramafic protoliths were probably conglomerates on Angel Island in San Francisco Bay. Franciscan elsewhere at the time of serpentinization, judging from the lack of conglomerates have received little intensive study and generally discernible alternation in the surrounding mélange material. The have been only briefly described, as in the papers just cited. We pétrographie features of the metaconglomerates, although indicat- hope other readers will bring to our attention additional articles ing the presence of a fluid phase during metamorphism, do not re- and localities that may provide further information. solve whether these features reflect local migratory or more wide- As a note of caution, neither Bloxam's conclusions concerning spread and perhaps subtle metasomatic processes nor do they indi- the in situ metamorphism of the Angel Island conglomerate nor our cate what the source of such solutions might be. similar conclusion regarding the Diablo Range conglomerates COMBINED REFERENCES CITED should be construed to suggest that the issue of pre- or postdepo- Bloxam, T. W., 1955, Glaucophane schists and associated rocks at two sitional metamorphism of blueschist-bearing conglomerate is set- Californian localities [Abs.]: Geol. Soc. America Bull., v. 66, p. tled. Each occurrence must be examined on its own merits. In some 1644-1645. _ cases, it will be difficult to rule out the possibility that, for example, 1960, Jadeite-rocks and glaucophane-schists from Angel Island, San predepositional blueschist-facies detritus has contributed to Fran- Francisco Bay, California: Am. Jour. Sci., v. 258, p. 555—573. ciscan conglomerates that have been overprinted by a later episode 1966, Jadeite-rocks and blueschists in California: Geol. Soc. America of blueschist-facies metamorphism after deposition. Bull., v. 77, p. 781-786. Bloxam's last paragraph brings up the possibility that Crittenden, M. D., Jr., 1951, Geology of the San Jose—Mount Hamilton metasomatising solutions may have played a role in the develop- area, California: California Dept. Nat. Resources., Div. Mines Bull. 157, 74 p. ment of blueschist-facies minerals. Bloxam (1955, 1960, 1966) and Fyfe, W. S., and Zardini, R., 1967, Metaconglomerates in the Franciscan other writers cited by him reported instances where blueschist- Formation near Pacheco Pass, California: Am. Jour. Sci., v. 265, p. facies minerals in Franciscan metasediments show pronounced 819-830. spatial relationships to contacts with mafic igneous rocks or ser- Piatt, J. B., Liou, J. G., and Page, B. M., 1976, Franciscan blueschist-facies pentinite. Earlier workers mistakenly thought such relations be- metaconglomerates, Diablo Range, California: Geol. Soc. America tokened contact metamorphism, but this is virtually precluded by Bull., v. 87, p. 581-591. modern knowledge of the P-T conditions favoring blueschist-facies Ransome, F. L., 1894, The geology of Angel Island: California Univ. Pubs. minerals. Bloxam (see especially his 1966 paper) suggested that Geol. Sci., v. 1, p. 193-240. blue amphibole in metasedimentary rocks of Angel Island is Taliaferro, N. L., 1941, Geologic history and structure of the central Coast Ranges of California: California Dept. Nat. Resources, Div. Mines localized near diabase because metasomatism related to the mafic Bull. 118, p. 119-163. intrusion had created a favorable chemical environment. He also 1943, Franciscan-Knoxville problem: Am. Assoc. Petroleum referred to the possibility that fluids involved in serpentinization Geologists Bull., v. 27, p. 109-219. may play a part where ultramafic bodies are present. Unfortu- MANUSCRIPT RECEIVED(DISCUSSION) BY THE SOCIETY JUNE 3, 1976 * Present Address: Electric Power Research Institute, 3412 Hillview Avenue, P. O. MANUSCRIPT RECEIVED (REPLY) BY THE SOCIETY JUNE 28, 1976 Box 10412, Palo Alto, California 94303 MANUSCRIPTS ACCEPTED AUGUST 24, 1976 Printed in U.S.A. Geological Society of America Bulletin, v. 88, p. 480 March 1977, Doc. no. 70314.

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