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EDWARD D. GHENT Department of Geology, University of Calgary, Calgary 44, Alberta, Canada ROBERT G. COLEMAN U.S. Geological Survey, Menlo Par\, California 94025

Eclogites from Southwestern Oregon

ABSTRACT graphic, and geochronologic data on eclogite blocks from southwestern Oregon are available, Eclogite, high-grade , and am- but chemical data on bulk composition phibolite blocks occur within the Mesozoic and individual minerals are not so extensive as Otter Point Formation of southwestern Oregon those on comparable rocks in California. The and are inferred to have been tectonically purpose of this paper is to (1) report electron emplaced by eastward-directed overthrusting microprobe analyses of minerals and three involving Colebrooke Schist and serpentinite. new whole-rock chemical analyses on this Eclogite from southwestern Oregon is very eclogite, (2) interpret these data in light of similar in bulk chemistry and mineralogy to experimental and computed phase equilibria, the well-studied eclogite of California. and (3) point out certain regularities in the Calculations of phase equilibria at load chemistry of Group C eclogite (Coleman and 1 pressures of 7 and 10 kb and T = 400°C to others, 1965) from Oregon and California and 550°C suggest that many of the hydrates found to contrast this chemistry with that of asso- in eclogite could have been stable at very low ciated blueschist and volcanic rocks. H2O fugacities. The lack of and the presence of almandine-grossular set a FIELD RELATIONS AND maximum limit on H»0 fugacity for a given PETROGRAPHY Ps-T. Eclogite, blueschist, and blocks Chemically, Group C eclogite from Oregon occur within the Otter Point Formation (Fig. and California characteristically is nepheline 1). Mapping by Lent (1969) and Coleman normative and is enriched in normative (1972) has provided evidence that the Cole- relative to basaltic compositions. The brooke Schist is in tectonic contact with the present chemistry of this eclogite may be the Otter Point Formation and was transposed result of metasomatism in an ultramafic eastward by overthrusting (Fig. 1). The over- environment with low a Si02 and high a Ca, thrusting was facilitated by serpentinite and but outside the stability field of serpentine. produced a tectonic mélange beneath the The generally high jadeite content of thrust consisting of Otter Point Formation, clinopyroxene from Group C eclogite com- high-grade blueschist and eclogite blocks, pared with Group A and Group B eclogite is amphibolite, and low-grade blueschist blocks. largely a function of bulk rock chemistry. Later erosion dissected the Colebrooke thrust Crystallization under low a Si02 conditions sheet, leaving klippen of Colebrooke Schist stabilizes jadeite in clinopyroxene at lower Pe and serpentinite. Landsliding within the for a given T. underlying mélange has produced further mixing of tectonic blocks. INTRODUCTION The locations and mineralogy of eclogite The occurrence of isolated blocks of high- samples from southwestern Oregon are given grade blueschist, amphibolite, and eclogite in Table 1. The mineralogical subdivisions used within the Jurassic and Cretaceous eugeosyn-

clinal rocks of southwestern Oregon and the 1 Coast Ranges of California has puzzled geol- Eclogite is divisible into three groups based on mode of occurrence: Group A, inclusions in , , ogists for many years. Coleman and Lanphere or layers in ultramafic rocks; Group B, bands or lenses (1971) published new geological and geo- within gneissic terrains; and Group C, bands chronological data on these rocks and provided or lenses within high-P-low-T metamorphic rocks a review of previous literature. Field, petro- (Coleman and others, 1965).

Geological Society of America Bulletin, v. 84, p. 2471-2488, 5 figs., August 1973 2471

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Colîbrooke Schist —I3C million years I I Otter Point Formation Upoer Jurassic

grade E; num- shown.

Figure 1. Geologic map of part of southwestern high- and low-grade blueschist (after Coleman, 1972; Oregon showing distribution of blocks of eclogite, Lent, 1969).

in the table require some comment. It was Minerals which are not clearly of replace- assumed that , garnet, and ment origin are epidote, , phengite' represent the "primary" mineralogy of the some of the sphene, and some Mg-chlorite. eclogite. Other minerals are put into one of Epidote occurs as inclusions within garnet, three categories: (1) minerals showing obvious and sometimes these inclusions are accom- replacement relations; (2) principal minerals panied by chlorite, indicating possible replace- showing no obvious replacement relations; ment relations. In some rocks (56-69; 27-69-1), and (3) accessory and vein minerals. however, epidote is a principal constituent and The following criteria were used to indicate occurs in layers with omphacite and garnet replacement relations. The least ambiguous with no evidence of textural disequilibrium. criterion is that of pseudomorphism, but this Two different compositional ranges and criterion could only be applied to chlorite textural types of chlorite occur in several forming pseudomorphs after garnet. A second, samples (Table 1). Chlorite which shows no less certain criterion, is rimming of a primary evidence of textural disequilibrium is always mineral by other minerals for which a reason- optically positive and shows normal first-order able reaction relation might be inferred, for interference colors, suggesting a Mg-rich example, rutile included within sphene. variety (Albee, 1962). In the same samples, a

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TABLE 1. MINERALOGY OF ECLOGUE FROM SOUTHWESTERN OREGON

Minerals with no Minerals showing Accessory and possible replacement obvious replacement Sample no.* vein minerals relations relations

79-69A FeMg chlorite (garnet) phengite (v) sphene (rutile) quartz albite (v) glaucophane (?)(omphacite) epidote apatite sphene chalcopyrite pyri te

79-69 same same same

95-69-1 MgFe chlorite (garnet) phengite quartz sphene (rutile) epidote chalcopyri te glaucophane (?)(omphacite) sphene glaucophane

82-69-1 FeMg chlorite (garnet) phengite phengite sphene (rutile) sphene FeMg chlorite glaucophane (?)(omphacite) glaucophane sphene(?) MgFe chlorite

56-69 MgFe chlorite (garnet) sphene aragonite MgFe chlorite (omphacite) epidote sphene (v) glaucophane (green ) MgFe chlorite epidote (v) glaucophane (?)(omphacite) glaucophane albite (v) sphene (rutile) phengite

27-69-1 FeMg chlorite (garnet) epidote epidote (v) sphene (rutile) phengite glaucophane (?)(omphacite) glaucophane

Sample locations 79-69 White Rock Creek (SW*s, sec. 30, T. 30 S., R. 13 W.) Langlois quadrangle. Banded eclogite block 79-69A (40 ft x 40 ft) in stream bed. Specific gravity = 3.31. 95-69-1 Waterman Ranch (SWig, sec. 7, T. 30 S., R. 14 W.) Langlois quadrangle. Tectonic block (40 ft x 40 ft) on ridge containing layers of eclogite with glaucophane and garnet. Specific gravity = 3.23. 82-69-1 White Creek (SWk, sec. 30, T. 30 S., R. 13 W.) Langlois quadrangle, approximately 1,000 ft downstream from 79-69. Layered eclogite block (10 ft x 20 ft) in stream bed. Edges of mass show strong retrogression. Specific gravity • 3.49. 56-69 South Fork, Floras Creek (SW*s, sec. 1, T. 31 S., R. 13 W.) Langlois quadrangle. Banded eclogite block (50 ft x 50 ft) broken up and associated with extremely coarse-grained blue- schist. 27-69-1 Otter Creek (NWJs, sec. 5, T. 32 S., R. 13 W.) Langlois quadrangle. Small block in stream associated with blueschist. Specific gravity = 3.31 to 3.26.

'All samples contain omphacite, garnet, and rutile.

second chlorite forming pseudomorphs after the exception, and if there is a replacement garnet is usually optically negative and shows relation, the evidence is not clearcut. anomalous interference colors, suggesting a more iron-rich chlorite. Electron-microprobe ELECTRON-MICROPROBE ANALYSES analyses confirm the iron-rich character of the latter type of chlorite (Table 6). Introduction Phengite usually occurs in an intersertal Electron-microprobe analyses of garnet, texture with garnet and omphacite. Contacts clinopyroxene, glaucophane, phengite, and between phengite, garnet, and omphacite are chlorite are given in Tables 2 through 6 and smooth and regular, suggesting no replacement. Figure 2. Data were collected on an ARL More rarely, phengite occurs with chlorite in EMX electron microanalyzer, and the data pseudomorphous aggregates after garnet (82- were corrected following methods outlined 69), suggesting that garnet was not stable with by Bence and Albee (1968) and Albee and Ray this phengite. (1970). The analyses quoted represent the Glaucophane-omphacite textural relations averages of 15 to 40 spot analyses per sample. are ambiguous. Smooth, regular contacts be- Garnet. The summary of analytical data in tween the two minerals are the rule rather than Figure 2 indicates that garnet from Oregon

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2471/3417835/i0016-7606-84-8-2471.pdf by guest on 24 September 2021 Figure 2. Triangular diagrams featuring: (a) the associated with kimberlite pipes. Modified from Cole- end members grossular + andradite (Gr + An); man and others, 1965. (b) Triangular diagram featur- (Py) and almandine + spessartine (Aim + ing the end members jadeite (Jd); acmite (ac); and Spess). Open circles indicate average analyses of Oregon diopside + hedenbergite (di + he). Dashed line out- garnet; 1, approximate range of garnet from amphib- lines field of clinopyroxene from Group C eclogite. olite; 2, garnet from charnockite and ; 3, Solid circles indicate compositions of Oregon clino- garnet from Group B eclogite; 4, garnet from eclogite . in dunite and ; and 5, garnet from eclogite

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TABLE 2. AVERAGE MICROPROBE ANALYSES OF TABLE 3. AVERAGE ELECTRON MICROPROBE ANALYSES OF GARNET FROM OREGON ECLOGITE OMPHACITES FROM OREGON ECLOGITES

79-69A 82-69-1 95-69-1 79-69 56-69 76-69A 1Î2-69- 1 95-69-1 79-69 27-69-1 56-69

Si02 38.2 38.5 38.1 38.3 38.2 S102 55.0 55.2 54.9 55.5 55.9 54.8

TiO: n.d. 0.1 0.2 n.d. 0.1 T102 0.10 0.09 0.10 0.08 0.10 0.06

A120, 21.2 21.6 21.2 21.2 20.8 Al 203 8.2 8.5 7.9 8.0 10.3 9.0

Fe0 28.2 27.4 28.0 27.9 28.7 3.6 2.7 tot Fe20,* 7.9 2.3 8.0 7.1 MnO 1.6 0.8 2.1 1.5 0.9 FeO 1.8 5.9 2.1 2.7 2.9 5.5 MgO 1.8 3.6 2.0 1.9 2.7 MnO <0.04 <0.04 0.04 0.05 0.06 0.08 CaO 9.4 8.6 9.0 9.2 8.8 MgO 7.2 8.0 7.2 7.4 7.8 7.7 13.6 13.6 12.9 Total 100.4 100.6 100.6 100.0 100.2 CaO 13.5 13.0 13.6

Na20 7.3 6.5 7.3 7.1 7.1 7.5 Structural Formula Calculated on the Basis of 12 Oxygens Total 101.1 99.99 100.54 101.43 101.36 100.24 Si 3.02 3.01 3.01 3.03 3.03 Structural Formula on Basis of 6 Oxygens Al 1.98 1.99 1.98 1.98 1.94 and Cations Normalized to 4 Fe 1.86 1.79 1.85 1.85 1.90 Si 1.97 1.99 1.98 1.98 1.98 1.96 Mn 0.11 0.05 0.14 0.10 0.06 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.22 0.42 0.24 0.22 0.32 Al 0.35 0.36 0.34 0.34 0.43 0.38 Ca 0.80 0.72 0.76 0.79 0.74 Fe+Î 0.21 0.07 0.22 0.19 0.10 0.07 +2 0.16 End-Member Proportions Calculatec1 for Fe 0.05 0.18 0.06 0.08 0.09 all Iron in Ferrous State Mn 0.00 0.00 0.00 0.00 0.00 0.00 Almandine 62 60 62 63 63 Mg 0.39 0.43 0.39 0.39 0.41 0.41 Spassarti ne 4 2 5 3 2 Ca 0.52 0.52 0.50 0.52 0.51 0.49 Pyrope 7 14 8 7 11 Na 0.51 0.45 0.51 0.49 0.48 0.52 Grossular 27 24 25 27 24 End-Member Proportions Calculated Total 100 100 100 100 100 After Method of Banno (1959) Od 29.4 38.4 29.8 30.6 39.2 42.1 Additional Data on Minor Elements Ac 18.7 8.1 20.6 18.2 9.3 9.1 Cr20j < 0.03 < 0.03 . 0.04 < 0.03 n.d. Di + He 51.9 53.5 49.6 51.2 51.5 48.8 Na20 n.d. n.d. n.d. n.d. 0.03 'The ferrous and ferric-iron analyses were calculated n.d. = not determined. from the total analysis using total cations = 4 and the following two equations in two unknowns: {1) 2 x number of moles of Fe+t + 3 x number of moles of Fe* + £ of all other cationic charges = 12; (2) number of moles of Fe+2 + number of Fe+i B total number of moles of Fe. eclogite shows compositional overlap with those of Group C eclogite from California. Garnet is strongly zoned, with Mn and Ca metry is interpreted to be the result of cat- preferentially enriched in cores of grains, and ion ordering, whereas other omphacite of Fe and Mg enriched in the rims. Similar zoning C2/c space-group symmetry is interpreted to patterns have been observed in garnet from be disordered. Ordered omphacite occurs in other metamorphic rocks as well as eclogite Group C eclogite, and disordered omphacite (see Hollister, 1966; Dudley, 1969). occurs in eclogite inferred to be of higher Clinopyroxene. Clinopyroxene from Ore- temperature origin. No single-crystal data are gon eclogite has been recalculated to estimate available on omphacite from Oregon eclogite, the ferrous-ferric ratio (see note to Table 3 for but the ratios of acmite + jadeite/sum of method), and then the jadeite, acmite, and pyroxene molecules approach 0.5, suggesting diopside + hedenbergite molecules were com- that this omphacite is of the ordered variety. puted following the method outlined by Clinopyroxene shows zoning, particularly Banno (1959). They are similar in composition in Fe, Mg, and Al, with Fe generally enriched to clinopyroxene from California Group C in rims relative to cores and Mg and Al showing eclogite (Fig. 2). less regular behavior. If all Na zoning is assigned Clark and Papike (1968) have reviewed the to jadeite, there is a maximum range of 7 mol crystal chemistry of omphacite. They report percent in the jadeite content of a single grain. an omphacite having a P2 space-group sym- This is in contrast to zoning reported by metry and a restricted compositional range Essene and Fyfe (1967) of up to 25 mol per- such that the ratio acmite + jadeite/acmite cent jadeite in a single grain. + jadeite + diopside + hedenbergite + White Mica. Analyses of white mica from is about 0.5. This lower sym- six different samples are remarkably uniform

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79-69A 82-69- 1 95-69- 1 79-69 27-69-1 56-69

Si02 50.0 50.0 50.1 50.6 51.9 50.6

A120S 26.5 25.9 26.3 26.9 26.2 26.2

tìo2 0.3 0.1 0.3 0.4 0.2 0.2

Fe0 3.2 3.6 3.6 3.3 3.2 3.3 tot MnO <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 MgO 3.4 3.4 3.4 3.4 3.4 3.6

Cr20, <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 CaO 0.05 0.05 0.05 0.1 <0.03 0.1 BaO 0.3 O.B 0.8 0.1 0.4 n.d.

Na20 0.6 0.4 0.4 0.5 0.5 0.2

K20 10.3 10.2 10.4 10.5 10.4 10.4 F <0.1 <0.1 <0.1 <0.1 <0.1 n.d.

94.65 94.5 95.35 95.86 96.2 94.2 + H20 * 4.5 4.5 4.5 4.5 4.5 4.5

Total 99.15 99.0 99.85 100.3 100.7 98.7

Structural Formul a Calculated on Basis of 11 Oxyger.s Si 3.39 ) 3.41 3.39 3. 38 3.45 3.41 4.0 4.0 4. 0 4.0 4.0 AI 0.61 i 0.59 4-° 0.61 0. 62 0.55 0.59

AI 1.60 \ 1.49 1.,4 9 1. 50 1.50 1.50 Ti 0.02 1 0.01 0.02 0. 02 0.01 0. Ol Fe 0.18 f 0.18 0. ,18 0.18 0,.1 9 2.04 2.03 °'20 2.05 2. 04 2.03 2.06 Mn •• ( Mg 0.34 \ 0.35 0. 34 0. 34 0.34 0.36 Cr •• /

Ca •• \ 0. Ol Ba O.Ol / 0.02 02 O.Ol n.d. 0.98 0.96 0.97 0.97 0.95 0.92 Na 0.08 ( 0.05 0. 05 0. 06 0.06 0.02 K 0.89 ) 0.89 0. 90 0.90 0.88 0. 90

*Water content assumed. n.d. = not determined.

TABLE 5. AVERAGE MICROPROBE ANALYSES OF ALKALI

79-69A 82-69-1 95-69-1 79-69 27-69-1 56-69

Si02 56.7 57.0 56.2 56.8 57.7 56.4

T102 < 0.06 < 0.06 < 0.06 < 0.06 < 0.06 n.d. Al 1O3 10.8 10.3 8.4 11.5 11.1 10.0 FeO 13.0 11.9 15.5 13.9 11.2 13.6 TOT MgO 9.9 10.4 9.0 9.2 11.1 9.6 CaO 1.7 1.7 2.0 1.2 1.5 1.2

Na20 7.1 6.8 6.6 7.3 7.0 7.0

99.2 98.1 97.7 99.9 99.6 97.8

H20* 2.0 2.0 2.0 2.0 2.0 2.0

Total 101.2 100.1 99.7 101.9 101.6 99.8

Structural Formula Calculated on Basis of 23 Oxygens

Si 7.82) 7.901 7 97 7 79 7.84) 7.90) 8.0 8.0 " f 8.0 - i 8.0 8.0 8.0 Al 0.181 0.10 ì 0.031 0.21 1 0.16) o.ioi

Al 1.57) 1.58- 1.37 ) 1.65 j 1.62) 1.55) Fe 1.50 5• 5.10 1.38 »5.11 1.84(5.11 1.59Î5.12 1.27 j 5.14 1.59 >• 5.14 Mg 2.03) 2.15 J I.90; 1.88) 2.25) 2.00)

Ca 0.25/ 0.25) 30 0.181 0.22J 0.18) 2.15 2.07 °- Ì2.11 2J2 2.06 2.08 Na 1.90 i 1.82 ( 1.81 ( 1.94) 1.84( 1.90 t

'Water content assumed.

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(Table 4) and indicate that the mica is phengite is near 75 mol percent clinozoisite and 25 mol containing Si appreciably in excess of three percent pistacite. Zoning is common, with a atoms per formula unit and appreciable variation of as much as 6 mol percent pistacite octahedral Fe and Mg. Previous investigators in individual spot analyses. (for a comprehensive list, see Ernst and others, 1970, p. 159) have shown that white mica in ESTIMATION OF PHYSICAL low-grade metamorphic rocks is typically CONDITIONS OF phengite; however, analyses of phengite con- tained in eclogite are relatively rare (Bearth, Introduction 1959; Ernst, 1963). Literature on the interpretation of load No was detected in the present pressure (Ps), temperature (T), and fluid study, and the solid solution of paragonite in pressure (Pf) conditions of eclogite formation is phengite presumably was not the maximum extensive. We do not propose to review all of possible value. The greatest solid solution of this literature; however, a few salient points paragonite observed is about 8 mol percent, will be noted. which is comparable to that observed in First, if eclogite crystallizes within the , Franciscan blueschist (Ernst and others, 1970, extrapolated experimental data suggest it p. 159). cannot be in equilibrium with a fluid phase A notable feature of the Oregon phengite having the chemical potential of pure H20 is the high Ba content (Table 4) and the large (see, for example, Green and Ringwood, 1967; range (0.1 to 1.8 percent BaO) in a single Fry and Fyfe, 1969, 1971). sample. Second, at least some eclogite of alpine type Alkali Amphibole. Alkali amphibole from (Group C) has formed from crustal materials. Oregon eclogite is glaucophane on the basis The eclogitized pillow lavas described by of chemical analyses and optical properties Bearth (1959) provide compelling evidence. (Table 5). In addition, some eclogite from California, The range of Ca substituting for Na in the for example, from Crevison Peak (for locality X group is 0.18 to 0.30 cation per formula map, see Coleman and Lanphere, 1971), shows unit, a larger range than that reported by sharp, unfaulted contacts with layers of Himmelberg and Papike (0.18 to 0.21; 1969) metachert. for actinolite-glaucophane pairs and at the Third, oxygen-isotope studies on high-grade extreme range of Ca substitution in alkali blueschist and eclogite blocks in California amphibole coexisting with calcic amphibole and New Caledonia indicate that they reported by Klein (1969). The Oregon crystallized at higher temperatures than those analyses, however, are comparable to those obtained during the crystallization of lawsonite- reported by Ernst and others (1970, p. 68) for aragonite-bearing blueschist (Taylor and Cole- alkali amphibole which does not coexist with man, 1968). calcic amphibole. Fourth, the occurrence of pronounced Chlorite. Chlorite retrogressed from garnet compositional layering in much eclogite, in- has a rather limited range of Si, Al, and Mn cluding bands of unusual bulk composition content, but has a more variable Mg/Fe con- such as garnetite bands, suggests that some tent (Table 6). All of the chlorite shows chemical constituents were mobile at least on anomalous blue to brown interference colors, the scale of several millimeters and implies the and most are optically negative. Chlorite that presence of an active fluid medium during shows no obvious replacement of garnet is metamorphism. optically positive and shows normal inter- ference colors, suggesting a higher Mg/Fe Limits on Ps-Pf-T Conditions ratio than for chlorite of retrogressive origin In the following discussion, we have adopted (Albee, 1962). The more magnesian chlorite is as a working hypothesis that the Oregon rare and was not found in the microprobe eclogite crystallized at temperatures in the sections. range 400° to 550°C, as estimated from oxygen- Epidote. Partial electron-microprobe anal- isotope studies on associated high-grade blue- yses of epidote for Fe, Mn, Ca, and Al were schist from California and New Caledonia made for two samples (56-69 and 27-69-1). (Taylor and Coleman, 1968). Unfortunately, The composition of epidote from both samples 018/016 measurements on omphacite, garnet,

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TABLE 6. ELECTRON MICROPROBE ANALYSES OF CHLDRITES (RETROGRESSED FROM )

Sample no. 79-69A 82-69-1 95-69-1 79-69 <7-69-1 56-69

S102 26.4 26.5 27.0 26.7 27.0 n.d. TiOi 0.1 < 0.1 < 0.1 < 0.1 < 0.1 n.d. AU0, 19.2 19.7 19.5 19.7 18.9 19.6 28.2 26.0 24.6 25.0 26.0 23.5 Fe0tot MnO 0.6 0.4 0.5 0.5 0.5 n.d. MgO 13.7 15.4 16.3 16.3 15.9 17.1 F < 0.1 < 0.1 < 0.1 < 0.1 <: 0.1 n.d. + H20 * 12.0 12.0 12.0 12.0 12.0

Total 100.2 100.0 99.9 100.2 100.3

Structural Formulas Calculated on the Basis of 28 Oxygens Si 5.63) 5.59) 5.66 j 8.0 8.0 8.0 5'59( 8.0 5'68( 8.0 Al 2.37 J 2.41 I 2.34| 2.41 j 2.32 j

Al 2.45\ 2.481 2.47 2.45\ 2.36 \ Fe 5.03 4.581 4.31 , 4.37/ J 4'57(i2.0 Mn 0.11 > 11.96 0.07 > 11.97 0.09 1 11.96 0.09) 11.99 O.O5) Mg 4.35 1 4.84' 5.09 5.08» 4.98) Ti 0.02' .. /

Additional Data an Minor Elements

Cr20j < 0.03 0.04 < 3.03 < 0.03 0.05 n.d.

*water content assumed, n.d. = not determined.

and rutile do not provide good geothermom- fugacity at which the hydrous minerals remain eters. Quartz-rutile fractionation from some stable relative to their breakdown products eclogite (Vogel and Garlick, 1970) suggests (Tables 7 and 8)? Assuming that retrogressive that this pair might provide a good geo- reactions, such as replacement of garnet by thermometer. However, quartz is not abun- Fe-Mg chlorite, have net taken place at dant in California and Oregon eclogite. maximum Ps-T conditions and that epidote- In low-grade blueschist, the presence of rutile was a more stable pair than lawsonite- aragonite and jadeite + quartz indicates crys- sphene at these P,-T conditions, what is the tallization at relatively high Ps, at least 6 to 7 maximum possible H2O fugacity under which kb (for example, Taylor and Coleman, 1968). this eclogite crystallized? There are no independent estimates of load The estimates of maximum and minimum pressure for high-grade blueschist and eclogite; H2O fugacities can be looked upon only as however, a working hypothesis is that they order-of-magnitude estimates, since the experi- crystallized under load pressures at least as ments deal with end-member composition high as those of the associated low-grade blue- phases and not complex solid solutions. In schist. This hypothesis is consistent with the addition, some rather long extrapolations of distribution of tectonic blocks and ultramafic thermochemical and experimental data are nec- rocks in California and Oregon. In blueschist essary for several of the reactions. We did not , ultramafic rocks contain blocks of feel that ideal-solution approximations to the eclogite, high-grade blueschist, and amphib- behavior of the solid solutions were warranted olite; in terranes, such as the Sierra in this study. The estimates are of value in that Nevada foothills, they contain amphibolite they provide a model that can be tested against blocks but no blueschist or eclogite. other lines of evidence. In the calculations which follow we have Calculations of the minimum fn,o (XH,O, chosen temperatures of 400°, 500°, and 550°C assuming ideal mixing in the fluid) stability and load pressures of 7 kb and 10 kb. limits of hydrous minerals indicate that at We approached the problem in the following the Pa and T assumed, these phases are stable manner. If the hydrous minerals glaucophane, at values of XH3O as low as ~0.17 or less epidote, phengite, and Mg-chlorite crystal- (Table 8). lized in equilibrium with omphacite-garnet- The absence of lawsonite suggests XH„o rutile at Pf = Ps, what is the minimum H20 <0.3 (If the equilibrium, 12 lawsonite = 6

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Source of Reaction Equilibrium constant data

17210 1. chi = fo + 2en + sp + 4H20 4 log fHj0 -- + 29.50 + 0.32^-1) (3)(2)(1)(12)

-6775 0.125jP-l) 2. tr = 2di + 3en + q + H 0 + 6 9 + (4)(1)(12) 2 loS fH,0 =

. =12555 . 0.356(P-1) 3. 4Z0 + q = 5an + gr + 2M 2 f + 22-88 (5)0)0?) ">9 H20

-3836 + 7 36 + O^fdl 4. mu + q = Kf + sill + H20 ,Q f = (6)(1)(2)(12) 9 H20

= =8055 ,0.489 0.0028(P-1) 5. lws = an + 2Hz0 2 f + + (7)(1)(12) '°9 H20

,116330 + 38 36 , 0.236(M) 6. 31ws + sph = 2Z0 + rut + q + 5U „ 5 log fH;0 (1)(8)(2)(12) n2U

. -6602 + J 15 + 0.0479(P-1) 7. l/3mt + q = fs + 1/602 1/6 log f0s (9)(8)(1)

8. 11m + Si = py + rut + l/202 log K. lip - 57.041 - 0.0578(P-1) (8)(1)

9. 2po + S2 = 2py iG^ = -70790 + 69.04T (10)

10. 5ccp + S2 = bo + 4py ¿G° = -50730 + 56.95T (10)

11. 3Fechl* + 12q + 2mt - 6alm logK= =81188 + 104.939 +I431£!1 (11 )(1}(12) • + !2H20 + 02

12. 2Fechl* + 4q = 3alra + 18H20 18 logfH0= + 233.727 + SMfHl (11 )(1)(12)

*Fe chlorite composition chosen to balance the reaction/ (11) magnetite-quartz-fayalitc buffer, (12) iron-quartz- Fayalite buffer. Abbreviations : chi » Hg-chlorite/ fo - forsterite/ en = enstatite; spa spine J; tr = tremolite; di = diopside; q - quartz/ 2o » zopisite; an = anorthiter gr » grossular; mu = muscovite/ kf = k-feldspar/ sill " sillimanite; lsw = lawsonite; mt = magnetite/ fs B ferrosilite; ilm = ilmenite/ py = pyrite; po = pyrrhotite/ ccp = chalcopyrite/ bo = bornite; Fechl = iron chlorite/ aim = almandine. Source of data: 1. Calculated by B. D. Ghent. 7. Crawford and Fyfe (1965). 2. Zen (1972). 8. Robie and Waldbaum (1968). 3. Fawcett and Voder (1966). 9. Williams (1971). 4. Boyd (1959). 10. Barton and Skinner (1967). 5. Newton (1966). 11. Hsu (1968). 6. Eugster (1970). 12. Burnham and others (1969).

TABLE 8. ESTIMATES OF MINIMUM AND MAXIMUM XH IN FLUID BASED ON CALCULATIONS FROM EXPERIMENTAL PHASE EQUILIBRIA

400°C 400°C 500°C 500°C 550°C 550°C iction* 7,000 bars 10,000 bars 7,000 bars 10,000 bars 7,000 bars 10,000 bars X H20t X H20 X H20 X M X H20 X H20

1. >0.027 >C.K'. >0.088 >0.076 >0.150 >0.120 2. >0.001 >0.001 >0.005 >0.006 >0.012 >0.014 3. >0.001 >0.0005 >0.013 >0.001 >0.044 >0.004 4. >0.031 >0.015 >0.100 >0.054 >0.170 >0.089 5. 50.786 <0.309 n.s. n.s. n.s. n.s. 6. ¿0.853 <0.537 n.s. n.s n.s n.s 11. <0.088 <0.205 <0.292 <0.342 <0.490 <0.557 12. <0.006 —<0.00 4 <0.126 <0.080 <0.447 <0.286

*Refers to reactions on Table 7. t„

' not stable.

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zoisite + + 3 quartz + 21 H 0 2 Ps = 7kb [Newton and Kennedy, 1963] is considered, this estimate would be < ~0.4.) at 400°C and 10 kb, and <0.8 at 400°C and 7 kb. Lawsonite is not stable at temperatures of about 450°C and above for any reasonable H20 fugacity. If reaction 6 is considered, the stability field of lawsonite should be slightly more restricted (Table 8; Fig. 3), but the thermochemical data used here are apparently in disagreement with the earlier experiments of Crawford and Fyfe (1965). The presence of garnet rather than a com- positionally equivalent hydrous assemblage sets a maximum limit on XH.O for a given Ps-T. Since the garnet is dominantly al- mandine, experimental data on almandine stability provide a first approximation to estimates of maximum XH,O- Oxygen fugacity is likely to be no greater than that defined by the quartz-fayalite-magnetite buffer (see Figure 3. T-XH.O diagram at P8 = 7 kb for reac- next section). At 400°C, almandine is not tions 6 and 11 from Tables 8 and 9. At 400°C the stable for X o >0.09 (Fig. 3). At higher Hl lack of lawsonite + sphene indicates Xna0 < 0.85 temperatures, almandine has a larger XH,O (see arrow) and the presence oi" almandine indicates stability limit, XH,o <0.3 at 500°C and XH,O < 0.09 (see arrow). XH„o <0.5 at 550°C. The effect of pyrope solid solution in almandine is difficult to synthesize diopside-jadeite solid solutions at predict, since silica activity (a Si02) as well as moderate temperatures and H20 pressures up P3-PrT conditions affect the stability of to 5,000 atm with little or no crystallization of pyrope (Hsu and Burnham, 1969). Spessartine hydrates. Since the starting materials were solid solution in almandine should increase the glasses, the crystallization of pyroxene rather maximum XH,O stability limit of the garnet than hydrates could be due to more favorable solid solution; however, the low spessartine kinetics rather than to lower Gibbs free energy content (<5 mol percent, Table 2) should of the pyroxene. not appreciably affect the stability limits of Evidence from blueschist-facies rocks sug- the garnet solid solution. gests omphacite may be stable at low tem- The grossular component of the solid solu- peratures in the presence of an H20-rich phase. tion is likely to stabilize the garnet at higher Omphacite occurs in veins with lawsonite and XHJO- Evidence for this hypothesis is: (1) in drusy cavity fillings, suggesting that nearly pure grossular garnet is known to have omphacite could have crystallized in the crystallized under greenschist-facies conditions presence of an H20-rich phase (Essene and presumably with high XH„O (see Nicolas, Fyfe, 1967). 1966); (2) calculations of grossular + fluid Although hydrothermal experiments have stability relative to a wide variety of hydrates not demonstrated conversion of amphibolite indicate that grossular + fluid is stable relative or blueschist assemblages to eclogite assem- to these hydrates at relatively low temperatures blages, and vice versa at low temperatures, and and relatively high H20 fugacities; and (3) the effects of fo„, fco,, and other volatiles have garnet from the garnet and staurolite-kyanite not been fully investigated, the above cal- zones, which are dominantly almandine- culations and considerations are consistent with grossular, have been shown to be members experimental data reported by Yoder and of assemblages which crystallized at relatively Tilley (1962, Fig. 34); that is, amphibolite high XH,o (Ghent and DeVries, 1972). might be expected to be stable relative to There are few experimental data on the eclogite at ~500°C if the H20 pressure were stability of omphacite in the presence of an on the order of tens of bars only. H20-rich phase. Wikstrom (1970) was able to If fluids attending the production of Group

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C ecloglte from California and Oregon were TABLE 9. CHEMICAL ANALYSES OF ECLOGITIC ROCKS FROM SOUTHWESTERN OREGON low in H20 (low XH,O), then it is of interest Sample to inquire into the composition of the fluid. 56-69 79-69 95-69-1 Some of the possible fluid compositions are: no. Si0 47.94 48.17 (1) a fluid rich in C02; (2) a fluid rich in CH4 2 45.54 (for example, see French, 1966); (3) a fluid A1203 15.11 14.09 13.28 rich in NaCl-KCl (Fry and Fyfe, 1971); and Fe203 6.69 4.65 6.14 FeO 5.62 7.52 5.40 (4) a fluid rich in H20, the properties of which MgO 5.81 5.64 6.00 are not those of a separate H20-rich phase; 14.26 10.27 11.76 for example, the fluid occurs as a thin film CaO Na 0 3.35 3.78 4.29 along grain boundaries (see Greenwood, 1960). 2 K20 0.03 1.76 1.20 + Since the stability fields of sphene and clin- H2O 1.55 1.50 1.51

ozoisite are severely limited by the presence of H2O- 0.04 0.02 0.03 C02 in the fluid phase (Schuiling and Vink, Ti02 1.72 2.35 1.80 1967; Ernst, 1972; Ghent and DeVries, 1972), P2O5 0.12 0.23 0.15 a fluid equilibrated with these phases at low MnO 0.25 0.27 0.20 C02 0.07 0.02 0.01 XH,O cannot have high Xco2. Carbon- isotope data obtained on aragonite and calcite Total 100.16 100.04 99.94

from low-grade blueschist associated with CIPW Norm (Mole Percent) eclogite suggest that fluids present during Q 0.00 0.00 0.00 retrogressive metamorphism also had low OR 0.18 10.64 7.22 Xco2 (Taylor and Coleman, 1968). PL 47.46 41.38 37.89 Graphite has not been detected in Oregon (AB) 20.68 24.72 24.20 (AN) 26.78 16.66 13.69 eclogite and, consequently, CH4-rich fluids seem highly unlikely. NE 6.10 6.00 9.02 DI 35.63 27.04 35.74 Until experimental data on concentrated (WO) 17.82 13.52 17.87 salt solutions at high Pf-T are available, the (EN) 15.81 9.97 16.04 suggestion of Fry and Fyfe (1971) that fluids (FS) 2.01 3.56 1.83 equilibrated with solid NaCl and KC1 may HY 0.00 0.00 0.00 have produced very low XHao remains an OL 0.55 6.07 0.70 open question. (FO) 0.49 4.47 0.62 It is possible that no fluid phase was present (FA) 0.06 1.60 0.07 during the metamorphism of the ecolgite. Thus MT 7.17 4.97 6.54 it would be possible to have a fluid with low IL 2.46 3.35 2.55 AP 0.26 0.49 0.32 fiia0i but composed essentially of H20 (Ph,o=~ CC Pf < Ps)- The anhydrous-hydrous solid phases 0.18 0.05 0.03 in the assemblage could be interpreted as Analyst: Elaine Brandt. buffering fn„o at a low value. The occurrence of eclogite in contact with more hydrous ratios, rather than the incompatibility of assemblages implies sharp discontinuities in magnetite and omphacite at high pressure. fH.o and could be interpreted to mean that Coleman and Lee (1963, p. 275) have pointed the eclogite was metamorphosed in a system out, however, that Type III metabasalt as well closed with respect to H20 (Morgan, 1970). as Type IV eclogite and blueschist rarely Only a few experiments have been conducted in contains ilmenite, magnetite, and (or) hematite systems in which H20 was not present as a and contains pyrite instead. Thus, the scarcity phase (Greenwood, 1960), and obviously more of Fe-Ti oxides in blueschist terranes is not experiments of this type would be desirable. confined to eclogite alone. A careful search through the Oregon eclogite Scarcity of Ilmenite, Magnetite, and Hematite samples, using reflected-light microscopy and qualitative electron-microprobe analysis using Banno and Green (1968) reviewed the occur- nondispersive x-ray analysis, indicated the rence of magnetite in eclogite and concluded lack of primary Fe-Ti oxides in these rocks that it is commonly absent in eclogite from (excluding rutile) and the extreme rarity of blueschist terranes, and that this is due to sulfides. Pyrite and chalcopyrite have been chemical factors, including low Fe203/Fe0 identified, but it is possible that they have not

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equilibrated with the coexisting silicate. Both chemical analyses of Group C eclogite from pvrite and chalcopyrite have been partly California and Oregon ind icates that 11 out of altered to "limonite." 12 analyses are nepheline normative, and only We can suggest at least two possible sim- one eclogite analysis from Valley Ford reported plified reactions that limit the occurrence of by Bloxam (1959) is olivine normative (Fig. Fe-Ti oxides in these rocks (Table 7, reactions 5). Bloxam did not report the mineralogy of 7 and 8). Ferrosilite is an approximation to a this eclogite sample, but other eclogite from ferrous iron-bearing pyroxene solid solution. California and Oregon generally contains only If free-energy data on acmite and hedenbergite minute quantities of quartz. were available, more realistic reactions could A search through the literature of analyzed be written. Even though reaction 7 on Table 8 Mesozoic mafic volcanic rocks from California is only approximate, it indicates that for foa and Oregon showed thai: many are quartz of "pure" H20 (Miyashiro, 1964) in the Pf normative and only 3 oui: of 35 analyses are range 7 to 10 kb and T's of 400° to 550°C, the nepheline normative. Mafic blueschist from ferrosilite component of the pyroxene would Ward Creek (Coleman and Lee, 1963) and be oxidized to yield magnetite plus quartz elsewhere in California (Ernst and others, (for example, at 400°C, 7 kb the equilibrium 1970) is also rarely nepheline normative. f0. for "pure" H20 is about 10 exp —8 and At least two explanation; of these differences the equilibrium fo„ for reaction 7 is about 10 in bulk chemistry seem worthy of discussion: exp —37). Assuming the same Ps-T range, the (1) the olivine-nepheline-normative character presence of chalcopyrite instead of bornite + of the eclogite is a "primary" feature inherited pyrite and of pyrite + rutile instead of ilmenite from mafic volcanic rocks that were nepheline- sets limits on fs, and suggests, at 400°C and 7 normative spilitic basalt or alkali basalt prior -4 8 kb, fsa is between about 10 and 10~ . to being eclogitized; and (2) the olivine- nepheline-normative character is produced CHEMICAL during the , that is, by meta- somatism. Chemistry of Eclogite, Mafic Blueschist, and No extensive mafic volcanic of Mafic Volcanic Rocks from California and Mesozoic age which is chemically equivalent Oregon to the eclogite has yet been discovered in The chemistry of eclogite, mafic blueschist, California and Oregon. The K-Ar ages of the and Mesozoic mafic volcanic rocks from high-grade metamorphic rocks which occur California and Oregon has been reviewed as tectonic blocks are generally older than the several times during the past ten years (Cole- oldest fossils in the Franciscan Formation man and Lee, 1963; Bailey and others, 1964; (Coleman and Lanphere, 1971) and are Coleman and others, 1965; Ernst and others, approximately equivalent in age to the fossil- 1970; Coleman, 1972). On a plot by Kuno iferous parts of the Mariposa and Galice (Kuno, 1960; 1966, Fig. 4), all analyzed Formations of California and Oregon. The eclogite from California and three new few analvtical data available on mafic volcanic analyses of eclogite from Oregon (Tables 9, 10) rocks from the Mariposa and Galice Forma- plot in the field of alkali basalt. Coleman and tions indicate that they are not chemically Lee (1963, p. 293) and Ernst and others (1970, equivalent to the Group C eclogite. Thus, p. 208) pointed out that many California the present field, chemical, and geochronologic mafic volcanic rocks and mafic biuechist also evidence suggests that the eclogite blocks were plot in this field. Because of the spilitic nature not produced by simple isochemical meta- of many of the mafic volcanic rocks, this type morphism of either Franciscan or Galice and of plot is misleading with regard to original Mariposa mafic igneous rocks. Coleman and tvpe (for example, see Reed and Mor- Lanphere (1971) concluded that the source gan, 1971). for the high-grade metamorphic blocks in Coleman and others (1965, p. 501-502) southwestern Oregon lay to the west and that pointed out that Group C eclogite from both the tectonic blocks and the Colebrooke California is nepheline normative, whereas Schist were emplaced by eastward-directed Franciscan basalt is not as undersaturated with overthrusting. respect to silica. A review of the available Studies of the chemical t rends of spilitization

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Omphocite

48 50 SiO. WEIGHT PERCENT Figure 4. Variation diagram of NaaO + K2O versus (1960, 1966). Catoctin trend from Reed and Morgan, SÌO2 in weight percent. Line with dash-dot pattern 1971. Basalt analyses from Green and Ringwood, 1972. separates field of alkali and tholeiite basalt after Kuno trend from Coleman, 1967.

(Reed and Morgan, 1971, Figs. 4 and 5) sug- gesting that transformation of eclogite to gest that, although this process results in blueschist in the Zermatt area, Switzerland, rocks with alkali and silica values in the field was accompanied by increase in Mg (+H2O) of alkali basalt, strongly undersaturated rocks and decrease of Ca. are not produced. In addition, the general Coleman and others (1965, p. 501-502) trend of basaltic differentiation as plotted suggested that eclogitization of basaltic rocks on a quartz-forsterite-enstatite-diopside-nephe- similar to those in the Franciscan may have line-normative diagram is not toward rocks been accompanied by loss of silica. Enrichment enriched in nepheline and diopside as are the in Ca may also occur during this process (Fig. California and Oregon eclogites. 5). Since the eclogite blocks in California and Because early metasomatic hypotheses for Oregon are invariably associated with ser- the production of blueschist were not sup- pentinized ultramafic rocks, it is tempting to ported by adequate field, petrographic, and relate the eclogitization process to crystalliza- chemical data, recent workers in this field have tion in an environment influenced by the emphasized the chemical similarities between ultramafic rocks. mafic blueschist and Franciscan basaltic rocks During serpentinization, Ca is expelled (for example, Coleman and Lee, 1963). This from the ultramafic rocks and silica activity philosophy was also applied to eclogite, be- (a Si02) is less than that at quartz saturation cause its chemistry generally approximates (Barnes and others, 1967). Serpentinization that of basaltic rocks. A closer examination of has been shown to be related to metasomatic high-grade blueschist and eclogite, however, production of rodingite (Coleman, 1967), indicates chemical differences other than which is depleted in silica and enriched in Ca relative amounts of water. Where blueschist (Figs. 4 and 5). If serpentinization took place and eclogite are interlayered on the scale of at relatively high Ps, then fluids with high a millimeters, the chemical differences could be Ca and low a Si02 interacting with rocks of reasonably ascribed to metasomatism on this basaltic bulk composition might produce a scale. Retrogression of eclogite to blueschist metasomatized rock, relatively enriched in Ca may also involve metasomatic changes other and depleted in Si. It seems unlikely, however, than simple hydration. Bearth and Stern that eclogitization is simply related to ser- (1971) have presented analytical data sug- pentinization of ultramafic rocks. Serpentiniza-

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TABLE 10. QUANTITATIVE SPECTROGRAPHS ANALYSES (ppm)

Sample no. AG B Ba Be Cd Co Cr Cu La Mo N;> Ni Pb Sc

56-69 <2 <50 23 <2 <50 41 390 37 <70 <7 <20 88 <10 88 79-69 <2 <50 270 <2 <50 41 72 18 <70 <7 <20 50 <10 54 95-69-1 <2 <50 1,300 <2 <50 37 290 11 <70 <7 <20 52 <10 56 Average Group C 7.5 145 43 256 69 67 eclogite* 132 Average oceanic + 14 tholeiite 32 297 77 97 Average oceanic 1 498 25 67 36 alkali basalt' " 51 Average Catoctin 5 140 5C 77 67 24 2.5 37 41 35 spilite

*Coleman and others (1965). X = XRF value. ^Kelson and others (1968). 6 Reed and Morgan (1971). Analyst: J. C. Hamilton.

tion at crustal pressures would be restricted to considered to be metasomat:.c, probably in the a maximum temperature on the order of 500°C presence of a fluid phase. For example, the (Scarfe and Wyllie, 1967). Because of the high reaction olivine + silica = orthopyroxene was HjO vapor pressure of serpentine, serpentiniza- suggested by Bowen and Tuttle (1949). If tion at temperatures of 400° to 500°C requires rocks of basaltic composition were meta- relatively high fnao at equilibrium. Calcula- morphosed a: high Ps in contact with ultra- tions of equilibrium fe^o for serpentine, mafic rocks outside of the stability field of forsterite, and , when compared to data in serpentine, the magnesian olivine-magnesian Table 9, suggest that these H2O fugacities are orthopyroxene assemblage could buffer a Si02 too high to stabilize eclogite relative to in a fluid phase at values less than those in the amphibolite or blueschist. basaltic rocks. Silica could be lost from the Many ultramafic rocks are thought to basaltic rocks, and an eclogite could crystallize. undergo solid-state reactions during cooling Mafic igneous rocks within ultramafic masses and prior to serpentinization (for example, of California and Oregon do not show partial Loney and others, 1971). Some reactions in- conversion to eclogite assemblages; however, volve simple recrystallization, but others are this has been observed in the Swiss Alps

01 Hy Figure 5. Triangular diagrams featuring normative trend from Reed and Morgan, 1971. Average Francis- quartz (Qz), hypersthene (Hy), olivine (Ol), diopside can basalt from Coleman and Lee, 1963; Bailey and (Di), and nepheline (Ne). Field of eclogite from Cali- others, 1964; Ernst and others, 1970. Other basalt fornia and Oregon enclosed by solid line. Catoctin analyses from Green and Ringwood, 1972.

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TABLE 10 frantinueci;

Sn Sr V Y Zn Zr Ga Ge Yb Rb* Sr * K/Rb Ba/Sr Ni/Co Rb/Sr

<10 650 340 70 <200 110 28 <20 5 0.04 2.14 <10 58 340 70 <200 150 28 <20 7 4.66 1.22 <10 280 350 40 <200 100 25 <20 5 534 232 18.7 4.64 1.41 0.230

225 567 68 133 11 8 0.64 3.07

130 292 95 17 0.11 3.03

815 252 54 22 0.611 2.04

160 350 47 120 19 5 0.88 0.74

(Bearth, 1967). If the eclogitization occurs physical conditions of crystallization. The prior to serpentinization, then the eclogite discussion in the previous section indicates that must resist retrogression during the hydration formation of some Group C eclogite might not reactions occurring during serpentinization. be an isochemical process. As the bulk chemis- Actinolite-chlorite-talc rinds which are inferred try of the rocks now exists, however, eclogite to encase the high-grade tectonic blocks of Group C is more commonly nepheline (Coleman and Lanphere, 1971) may armor the normative than that of Groups A and B. In blocks from retrogression during serpentiniza- addition, Group B eclogite more commonly tion. We do not imply, however, that all contains more modal quartz than does Group Group C eclogite has formed by this type of C eclogite. If a Si02 were less than that mechanism. Nor do we imply that the final defined by the presence of quartz, the albite chemistry of a Group C eclogite is the result component of would break down of a single-stage process. to form the jadeitic component of clino- pyroxene solid solution at relatively lower Comparison of Chemistry of Eclogite from pressures for a given temperature (see Kushiro, Different Geological Environments 1969). Consequently, higher jadeite content Coleman and others (1965) separated eclogite of Group C clinopyroxene need not imply that into three groups on the basis of geological they crystallized at pressures as high as those environment. They suggested that there were of Group A and Group B eclogite. It would be also chemical differences between the different desirable to confirm this suggestion by experi- eclogite groups, with Group C eclogite gen- ments in systems more closely approximating erally being less mafic and richer in alkalies. natural systems, for example, Fe-bearing. Garnet from Group A eclogite tends to be highest in pyrope content, whereas clino- CONCLUSIONS pyroxene from Group C eclogite tends to be Eclogite from southwestern Oregon is very highest in jadeite content. If jadeite content similar in bulk composition to that found in of clinopyroxene and pyrope content of garnet the Franciscan Formation of California. are used as simple geobarometers, a contradic- Garnet and omphacite from Oregon eclogite tion results (Coleman and others, 1965, p. fall within the compositional field of those from 499-500). Several workers (Evans, 1965; California eclogite. Phengite, glaucophane, Essene and Fyfe, 1967; Banno, 1970) have and epidote coexisting with omphacite-garnet- explained the variation in pyrope content of rutile are chemically similar to those from eclogitic garnet as being due largely to different Type IV blueschist in California. temperatures of crystallization, rather than to Calculations of phase equilibria at Ps =7 and variations in Ps or control by bulk composition. 10 kb and T = 400° to 550°C suggest that The higher jadeite content of clinopyroxene many of the hydrates found in eclogite would from Group C eclogite is thought to be con- be stable to very low H2O fugacities, on the trolled more by bulk chemistry than by order of XH,O = 0.15 or less. Conversely, the

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presence of epidote-rutile instead of lawsonitc- schists in Sikoku: Geol. Soc. Japan Jour., v. sphene and the presence of almandine-grossular 65, p. 652-657. garnet suggests XHjo values below about 0.5. -— 1970, Classification of eclogites in terms of A mixture of omphacite-garnet and hydrates physical conditions of their origin: Physics could be in equilibrium at low XH,O- Increase and Planetary Interiors, v. 3, p. 405-521. Banno, S., and Green, D. H., 1968, Experimental in XH„O at constant Ps-T could result in an increase in the amounts of hydrate minerals; studies on eclogites: The roles of magnetite and acmite in eclogitic assemblages: Chem. that is, retrogression. Geology, v. 3, p. 21-32. Chemically, Group C eclogite from Oregon, Barnes, I., LaMarche, V. C., Jr., and Himmelberg, and California is different from associated G., 1967, Geochemical evidence of present- Mesozoic mafic igneous rocks and blueschist. day serpen tinization: Science, v. 156, p. Eclogitization of rocks of basaltic composi- 830-832. tion in an ultramafic environment could result Barton, P. B., Jr., and Skinner, B. J., 1967, Sulfide in an increase in Ca and a decrease in Si. If mineral stabilities, in Barnes, H. L., ed., Geo- metasomatism were involved in eclogitization, chemistry of hydrothermal ore deposits: New York, Holt, Rinehart, and Winston, p. 236- it would have to occur outside of the stability 333. field of serpentine. The eclogite would have Bearth, P., 1959, Über Eklogite, Glaukophan- to be preserved during subsequent ser- schiefer und metair.orphe Pillowlaven: pentinization. Schweizer. Mineralog. u. Petrog. Mitt., v. 39, The generally high jadeite content of p. 367-386. clinopyroxene from Group C as compared to 1967, Die der Zone von Zermatt- that of Groups A and B is largely a function Saas Fee: Beitr. Geologie Karte Schweiz, of bulk chemistry. Crystallization under low a neue Folge 132, 130 p. Si0 conditions stabilizes jadeite in clino- Bearth, P., and Stern, W., 1S'71, Zum Chemismus 2 der Eklogite und Glaukophanite von Zermatt: pyroxene at lower P for a given T, so that s Schweizer. Mineralog. u. Petrog. Mitt., v. 51, Group C eclogite need not have crystallized p. 349-359. at pressures as high as those attained during Bence, A. E., and Albee, A. L., 1968, Empirical crystallization of Group A and Group B correction factors for the electron micro- eclogite. analysis of silicates and oxides: Jour. Geology, v. 76, p. 382-403. ACKNOWLEDGMENTS Bloxam, T. W., 1959, Glau;ophane schists and W. G. Ernst and K. D. Watson of the associated rocks near Valley Ford, California: Am. Jour. Sei., v. 257, p. 95-112. Department of Geology, University of Califor- Bowen, N. L„ and Tuttle, O. F., 1949, The system nia at Los Angeles, and B. A. Morgan of the MgO-SiOi-HiO: Geol. Soc. America Bull., U. S. Geological Survey gave critical reviews v. 60, p. 439-460. of this paper. D. Johnson and B. Rutherford Boyd, F. R., Jr., 1959, Hydrothermal investiga- assisted in the preparation of samples for tions of amphiboles, in Abelson, P. H., ed., electron-microprobe work. E. D. Ghent Researches in geochemistry, v. 1: New York, received financial support from National John Wiley & Sons, Inc., p. 377-396. Research Council Operating Grant A-4379. Burnham, C. W., Holloway, J. R., and Davis, N. F., 1969, Thermodynamic properties of water to 1000°C and 10,000 bars: Geol. Soc. REFERENCES CITED America Spec. Paper 132, 96 p. Albee, A. L., 1962, Relationships between the Clark, J. R., and Papike, J. J., 1968, Crystal- mineral association, chemical composition and chemical characterization of omphacites: Am. physical properties of the chlorite series: Am. Mineralogist, v. 53, p. 840-868. Mineralogist, v. 47, p. 851-870. Coleman, R. G., 1967, Low-temperature reaction Albee, A. L., and Ray, L., 1970, Correction factors zones and alpine ultramafic rocks of California, for electron probe microanalysis of silicates, Oregon, and Washington: U.S. Geol. Survey oxides, carbonates, phosphates, and sulfates: Bull. 1247, 49 p. Anal. Chemistry, v. 42, p. 1408-1414. —— 1972, The Colebrooke Schist of southwestern Bailey, E. H„ Irwin, W. P., and Jones, D. L., 1964, Oregon and its relation to the tectonic evolu- Franciscan and related rocks and their signif- tion of the region: U.S. Geol. Survey Bull. icance in the geology of western California: 1339. California Div. Mines and Geology Bull., v. Coleman, R. G., and Lanphere, M. A., 1971, 183, 176 p. Distribution and age of high-grade , Banno, S., 1959, Aegerinaugites from crystalline associated eclogites, and from

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