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Glaucophane Schists and Eclogites Near Healdsburg, California

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Glaucophane Schists and Eclogites Near Healdsburg, California BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 67, PP. 1563-1584. 4 FIGS., 1 PL. DECEMBER 1956 GLAUCOPHANE SCHISTS AND ECLOGITES NEAR HEALDSBURG, CALIFORNIA BY IRIS Y. BORG ABSTRACT Glaucophane schists, hornblende rocks, and eclogites are intimately associated within the Franciscan formation of the Healdsburg quadrangle. Discontinuity of megascopic structures and rapid variation in rock type indicate that the schists have undergone con- siderable disturbance since formation. However, weak trends conforming to the regional structure of the Franciscan formation can still be discerned in the metamorphic rocks. Most of the metamorphic rocks are derived from basic igneous rocks. The parents of the pumpellyite-Iawsonite-glaucophane schists are dense aphanitic volcanic rocks termed greenstones. The quartz-rich rocks show close chemical affinities with Franciscan cherts and arkosic wackes. Eclogites bearing almandite garnet and acmitic diopside-jadeite are mineralogically atypical with reference to eclogites found elsewhere. It would seem that at one time they were more extensively developed in the area, for several existing rock types appear to have formed from them by retrograde processes. Retrograde products are members of the albite-epidote-amphibolite, greenschist, and glaucophane schist facies. Some members of the first two groups have been subsequently modified by the crystallization of glauco- phane. Final products in such rocks are chlorite-glaucophane schists with remnant horn- blende, and muscovite-chlorite-glaucophane schists with remnant pyroxene. Conditions accompanying the development of the eclogite are unknown. Although serpentinite is associated with the group, the genesis of the schists and eclogite appears to be unrelated to it. The similarity in chemical composition between the schists and unaltered basaltic rocks and sediments of the formation suggests that the metamorphism was not accompanied by metasomatism. CONTENTS TEXT Page A. Eclogite 1569 Pase B. Chloritized eclogites 1575 Introduction 1564 C. Pyroxene-chlorite rocks with and Acknowledgments 1564 without epidote 1575 Distribution of rock types 1564 D. Garnet-pyroxene-hornblende rocks. 1575 Structure 1566 E. Pyroxene-hornblende rocks 1576 Description of exposures 1566 Group V. Rocks in which glaucophane, Statistical analysis of schistosity and linea- epidote, and/or micas are important tion 1566 constituents 1576 Petrography 1567 Relations within and between Groups IV Group I. Greenstones 1567 and V 1576 Group II. Schists in which glaucophane and Chlorite bands containing nodules of actin- lawsonite are important constituents. 1567 olite 1579 A. Pumpellyite - lawsonite - glaucophane Summary of field and petrographic relations!'. 1580 schist 1567 Petrogenesis 1581 B. Garnet-lawsonite-glaucophane schist. 1567 References cited 1582 Group III. Schists in which quartz and a member of the glaucophane-riebeckite series are important constituents 1569 A. Lawsonite - glaucophane - quartz ILLUSTRATIONS schist 1569 B. Crossite-quartz schist 1569 F'sure Pase Group IV. Rocks in which pyroxene and/or 1. Sketch map of portion of Healdsburg hornblende are important constitu- quadrangle 1565 ents 1569 2. Areal distribution of rock types 1565 1563 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/12/1563/3431640/i0016-7606-67-12-1563.pdf by guest on 01 October 2021 1564 BORG—GLAUCOPHANE SCHISTS AND ECLOGITES, HEALDSBURG, CALIF. Figure Page Table Page 3. Equal-area projections of poles of foliation 2. Optical properties of pyroxenes 1570 and lineation 1566 3. Pyroxene analyses 1571 4. Composition of garnets occurring in eclo- 4. Garnet analysis 1572 gites and glaucophane-bearing rocks. ... 1573 5. Per cent of end members in garnets of Plate Facing Page eclogites and glaucophane-bearing rocks. 1573 1. Photomicrographs of altered eclogite and 6. Analyses of eclogites 1574 glaucophane schists 1574 7. Resume of mineral assemblages in py- TABLES roxene- and hornblende-bearing rocks... 1577 Table Page 8. Retrograde metamorphism of eclogites.... 1578 1. Analyses of glaucophane schists 1568 9. Minerals of the glaucophane schists 1579 INTRODUCTION was undertaken, for profitable discussions and for critically reading the manuscript. Drs. A. In the past 5 years there has been an in- Pabst, C. M. Gilbert, and L. Weiss also con- creased interest in glaucophane schists and the tributed valuable comment and technical aid. petrogenetic problems they pose. The glauco- The work was made possible by a Genevieve phane schists of the California Coast Ranges McElenery Fellowship. The Department of are particularly perplexing because of their Geology of the University of California gener- sporadic occurrence within an otherwise un- ously paid for all but one analysis. metamorphosed geosynclinal suite, the Francis- • can formation (Taliaferro, 1943; Reed, 1933). DISTRIBUTION or ROCK TYPES The arkosic and lithic wackes, shales, cherts, and basic igneous rocks constituting the bulk of The area studied surrounds the Junction the formation rarely grade into the schists. School and is characterized by low relief. Out- Commonly areas containing metamorphic rocks crops of metamorphic rocks, usually less than 20 are a few hundred feet in dimension and are feet in diameter, dot the countryside. At no surrounded by unaltered sediments. Serpen- place within the area are schists continously tinized peridotite is rarely far distant. exposed, nor do they grade into Franciscan The purpose of this investigation is to supply sediments and igneous rocks. Except for the data concerning field, petrographic, mineralogi- serpentinite body, it is not possible to map cal, and chemical relations in one typical area. boundaries of rock types. Rapid change in The area chosen is within a belt of glaucophane lithology and orientation of megascopic struc- schists occurring in the Healdsburg quadrangle, tural features indicate that many of the out- 65 miles northwest of San Francisco. A sketch crops do not reflect the mineralogy or attitude map of the area surrounding the largest series of of underlying rock. The outcrop map (Fig. 2) continuous outcrops of the metamorphic rocks shows the distribution of five rock types: in the quadrangle is shown in Figure 1. (1) Serpentinite, in places somewhat sheared, References to rocks found within the Healds- forms a continuous belt trending N. 60°-70° W. burg quadrangle may be found in almost every in general conformity with the foliation of the account of the glaucophane schists in Cali- adjacent schists and with the regional structure fornia. The first discussion of the Healdsburg (Cf. Fig. 1). In part the serpentinite flanks a localities is that of Nutter and Barber (1902). topographic depression along which Gealey The most important work in the region was mapped a fault. There are also scattered, iso- done by Gealey (1951), who mapped the quad- lated outcrops of serpentinite not visibly related rangle, and by Switzer, who described the to the main mass. mineralogy of the schists (Switzer, 1951) and (2) Strongly lineated lawsonite-glaucophane- the associated eclogites (Switzer, 1945). quartz schist veined with quartz is the most extensively developed metamorphic rock. It is ACKNOWLEDGMENTS confined to the northern and northeastern sectors of the area. The writer is particularly indebted to Dr. (3) Greenstone is also plentiful although not F. J. Turner under whose direction the project limited to any one part of the area. It is concen- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/12/1563/3431640/i0016-7606-67-12-1563.pdf by guest on 01 October 2021 DISTRIBUTION OF ROCK TYPES 1565 Ouaternory Tertiary Knoxville fm oc Greenstone ^5 I E Gabbro,diabase1 o Ss, shale, chert c S Metamorphic rks 'o 2 Serpentine FIGURE 1.—-SKETCH MAP OF PORTION or HEALSDBURG QUADRANGLE Showing location of area investigated (modified from Gealey, 1951) "00 x =greenstone A =lawsonite-gloucophone- quartz schist • =eclogites and chloritized eclogites ° = hornblende- and pyroxene-bee •ocks FIGURE 1.—AREAL DISTRIBUTION or ROCK TYPES Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/12/1563/3431640/i0016-7606-67-12-1563.pdf by guest on 01 October 2021 1566 BORG—GLAUCOPHANE SCHISTS AND ECLOGITES, HEALDSBURG, CALIF. trated in two belts that trend N. 60°-70° W. Outcrops within the westernmost belt are ad- jacent to and on both sides of the serpentinite body. Within this belt most outcrops are green- stone in contrast to the eastern belt which con- tains other rock types as well. (4) Eclogite occurs in six outcrops nearly aligned along a line trending N. 50° W. (broken line of Fig. 2) subparallel to and east of the main serpentinite body. (5) Outcrops of hornblende and pyroxene rocks with and without garnet are closely associ- ated with the eclogite and similarly aligned, though not so narrowly limited in distribution. They are related to the eclogite by diaphthoritic processes. Two contrasting elements emerge from the structural picture afforded by the outcrop map. Superficially there is general impression of chaos resulting from wide petrographic variety of rocks exposed and rapid variation between adjacent outcrops. Nevertheless, the main rock types tend to outcrop predominantly in limited belts whose N. 60°-70° W. trend is parallel to that of the regional structure. In spite of the capricious nature of the metamorphic rocks, the effects of subsequent structural disturbance, and the probability that many of the rock masses
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