Alkali Amphiboles from the Blueschists of Cazadero., California 1

Transcription

1 Alkali Amphiboles from the Blueschists of Cazadero., California 1 by R. G. COLEMAN and j. j. PAPIKE U.S. Geological Survey, Menlo Park 94025, California; Washington, D.C. ABSTRACT Alkali amphiboles from Type III and Type IV metamorphic zones in blueschist facies rocks of Cazadero, California, and from comparable New Caledonian rocks have been characterized by X-ray crystallographic, optical, and chemical methods. The composition of any particular alkali amphibole is strongly controlled by the bulk composition of the host rock. Within the blueschist facies, metamorphic zones are not characterized by changes in amphibole composition. All the alkali amphiboles studied herein belong to the C2\m space group and complete miscibility between glaucophane and riebeckite has been demonstrated for the conditions prevailing during metamorphism in the Cazadero and New Caledonian blueschists. Linear relationships are found between unit-cell dimensions and variations in composition between glaucophane and riebeckite. The alkali amphiboles of glaucophane compositions belong to the high pressure-low temperature series, glaucophane Il-riebeckite. Limited miscibility of actinolite in glaucophane may be characteristic of blueschist facies metamorphism. INTRODUCTION PREVIOUS reports dealing with the Cazadero metamorphic sequence have detailed the general petrologic relationships and specifically related the garnets, pyroxenes, and carbonates to the metamorphism (Coleman & Lee, 1962, 1963; Lee, Coleman & Erd, 1963; Coleman, Lee, Beatty, & Brannock, 1965). The present paper relates the amphibole compositions and bulk composition of the host rock with the metamorphic types. This is not to be considered as a systematic study of alkali amphiboles although the accumulated data may provide a point of departure for such studies. The glaucophane schists of the Ward Creek area near Cazadero have been divided into three types representing apparent contrasts in metamorphic grade. This division into types is based mainly on textural features and, to a lesser extent, on changes in mineral assemblages as a convenient way of representing these various types on the geologic map. The three units are referred to as Type II (low grade), Type III (intermediate grade), and Type IV (high grade), and are considered to represent a progressive increase in pressure and temperature. A detailed description of these metamorphic types is given by Coleman & Lee (1963). Because Types II and III form narrow and irregular areas and Type IV rocks occur as isolated blocks, the actual field relationships are paradoxical, and the range in physical conditions may have been so small that the designation of low, intermediate, and high grades could be misleading when the reader compares the Cazadero area with metamorphic terrains such as the Barrovian 1 Studies of Silicate Minerals (5). Publication authorized by the Director, U.S. Geological Survey. Jooral of Petrology, Vol. 9, Pmrt 1, pp , 1968)

2 106 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES type. However, we shall retain our original designations of metamorphic types for the sake of continuity with previously published papers, leaving the question of pressure-temperature range still open. Glaucophane is abundant in the three metamorphic types from the Cazadero area and serves as a field guide to metamorphism because of the distinctive blue color it imparts to the schists. In fact, its ubiquitous distribution has given rise to the names glaucophane schist or blueschist facies. Amphiboles present within the Cazadero metamorphic rocks range in composition from glaucophane to riebeckite and in certain calcium-rich and sodium-poor rocks, actinolite coexists or may be the only amphibole present (Lee et al., 1966). For comparative purposes, glaucophanes from blueschists and eclogite from New Caledonia are included in this study. The New Caledonian rocks are blueschists similar to the Type IV rocks of Cazadero (Coleman, 1966, 1967). PETROLOGY OF THE AMPHIBOLE-BEARING ROCKS The blueschists of the Cazadero area which contain amphiboles can be divided into two main compositional groups: metabasalts and metasediments. Metabasalts, the most abundant, always contain glaucophane as an essential mineral. In Type II metabasalts, the glaucophane is most commonly associated with lawsonite, pumpellyite, aragonite, chlorite, and \M phengitic mica. In Type III metabasalts, the associated minerals are the same except for the change of the mica from \M to 2M and the appearance of the garnet in some metabasalts. There does not appear to be any noticeable change in the composition or optical character of the glaucophane from Type III to Type IV metabasalts. Glaucophanes from Type IV metabasalts form larger crystals than those glaucophanes from Types II and III and usually appear to be a darker color as a result of the increase in grain size. In Type IV metabasalts, glaucophane is commonly associated with epidote, phengitic mica, omphacite, and garnet. Crossite was identified in several specimens of low-grade Type II metabasalts, especially in veins or concentrated in the rims of pillow lavas. The mineral assemblages for analyzed amphiboles from metabasalts are given in the explanation to Table 1. The Ward Creek metasediments consist of metacherts, metashales, and metaironstones. Alkali amphiboles in these rocks have a much wider range in composition than that shown by the amphiboles in metabasalts. Metashales often contain abundant glaucophane-crossite associated with white mica and quartz. Increase in ferric iron over aluminium in the metacherts and meta-ironstones favors the formation of crossite-riebeckite in Type III rocks. Sodium-iron-rich amphiboles commonly are associated with stilpnomelane, garnet, quartz, deerite, and pyrite. The absence of metasediments of Type IV does not allow comparison to be made between composition and metamorphic grade. The mineral assemblages for analyzed amphiboles from Type HI metasediments of Ward Creek, and one from New Caledonia comparable to Type IV, are given in the explanation for Table 1.

3 FROM BLUESCHISTS, CALIFORNIA 107 Carbonate-rich metasediments of Type III often contain glaucophane and coexisting actinolite. Such a relationship suggests that a miscibility gap exists between glaucophane and actinolite (Lee et al., 1966). However, with increasing grade of metamorphism (Type IV), the amount of calcium in the glaucophane increases even in those rocks with only moderate amounts of calcium. MINERALOGY The basis for the mineralogical discussion is eleven new chemical analyses of amphiboles combined with four chemical analyses previously published (Table 1). For various reasons, not all of the analyses presented will be used in the more detailed discussions regarding optics, structure, and chemistry. Some of the analyses are not acceptable and for some amphiboles it was not possible to obtain reliable X-ray data. Optical properties The alkali amphiboles form two distinct groups according to their optical orientation and composition (Table 1). (1) The Mg-Al-rich glaucophanes from metabasalts all have b = Y with the extinction angle c A Z varying from 3 to 7 and 2V = The optic plane is parallel to (010) and axial dispersion is very weak (v > r) or not apparent. All of these amphiboles fall in the glaucophane compositional field (Fig. 1) according to the chemical classification of Miyashiro (1957). The pleochroic scheme of the glaucophanes is nearly identical even though considerable range in composition is encountered: X = < Y = lavender < Z = blue. (2) The Fe-rich group from the metasediments all have b = Z with the extinction angle YAC ranging from 0 to 8 but usually less than 3. These amphiboles do not exhibit the optical orientation of the igneous riebeckites (b = Z with CAX small). The optic axial angle is small in those amphiboles with intermediate iron contents such as 41-NC-62,but becomes very large in the high-iron types. The optic axial plane is normal to (010) and axial dispersion is strong (v > r). The pleochroic scheme of these crossites and riebeckites is similar, with the color intensity increasing with iron content; X = pale yellow < Y = blue < Z = purple. Chemically this group can be classified as crossite or riebeckite, depending on the amount of Fe +2 and Fe +3 substituting for Mg+ 2 or A1+ 3 (Fig. 1). The indices of refraction of both groups are related to the iron content and, in general, are higher with increasing iron content. Various attempts to plot indices against composition have had only moderate success (Miyashiro, 1957; Deer, Howie, & Zussman, 1963). Because of the dark color and strong dispersion of crossite and riebeckite, reliable data on the optical constants are not available and this lack has caused considerable confusion in relating composition and optics. Various plots using the new data in this paper and those from the literature were not successful in producing adequate correlations. Nevertheless, careful optical determinations including the indices of refraction can allow

4 108 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES TABLE 1 Chemical and optical properties of amphiboles Glaucophcute SiO, AJ.O, Fe,O. FeO MgO CaO Na,O K.O H,O + H.O- TiO, MnO F Others Less O for F Total Cu Co Ni Cr V Ca Sc Zr Sr Ba Zn Specific Gravity a ev Z*c IV Optic axial plane Z Y X Dispersion < *46 6-7" 32 II010 pale blue pale lavender < ±3 36 noio blue lavender * 45* II 010 pale blue pale lavender < < * II 010 pale blue pale liveoder K) < 4 < II 010 pale blue pale lavender * 39 noio blue lavender * 30 ;; 010 blue Uvender weak

5 FROM BLUESCHISTS, CALIFORNIA 109 TABLE 1 (cont.) Glaucophanc Crosslle-Riebeckile Aclinollte A1.O, Fe,O, FeO MgO CaO Na,O K,O H.O+ H,O~ TiO, MnO F Others Less O for F Total Cu Co Ni Cr V Ga Sc Zr Sr B* Zn Specific Gravity a V ZAC IV Optic axial plmne Z Y X Dispersion < blue lavender pale green-blue pale green-blue < < " X010 purple blue v>r weak < large X010 purple blue v>r strong " Urge ±010 deep purple dark blue pale yellow v>r strong n.dn.dn.d. 0-O incomplete ' large X010 purple blue pale yellow r>r strong OHM II 010 green pale green < large light green light green

6 110 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES EXPLANATION OF TABLE 1 1. Glaucophane (201-RGC-59-I) from metasediment associated with quartz, phengitic muscovite, and sphene; Type III blueschist, Ward Creek, Cazadero, California. Analyst, Leoniece Beatty, U.S. Geological Survey. (Other elements determined: Cr,O 3 004%, VjO, 003%, BaO %, ZnO 011%.) See Coleman & Lee, 1963, for whole rock analyses. 2. Glaucophane (4-CZ-) from metakeratophyre or metaspilite associated with jadeitic pyroxene and albite; Type IV blueschist, Valley Ford, California. Analyst, Leoniece Beatty, U.S. Geological Survey. (Other elements determined: ZnO 005%.) 3. Glaucophane (201-RGC-59-H) from metabasalt associated with lawsonite, garnet, phengitic muscovite, quartz, and sphene; Type III blueschist, Ward Creek, Cazadero, California. Analysts: S. Neil, A. Bettiga, L. Beatty, A. Chodos, W. W. Brannock, R. Stevens, U.S. Geological Survey. (Other elements determined: BeO 0%, Cr,O %, ScOj 0-01%, V,O 3 010%, ZrO, %, CoO 0%, CuO 0002%, NiO 0002%, ZnO 0-03%, BaO 0003%, PbO 0002%, SrO 002%.) See Lee et al., 1963, table 1 for analysis of coexisting garnet. 4. Glaucophane (51-CZ-59) from metabasalt associated with aragonite, quartz, pumpellyite, lawsonite, garnet, and sphene; Type III blueschist, Ward Creek, Cazadero, California. Analysts: Leoniece Beatty, A. C. Bettiga, W. W. Brannock, U.S. Geological Survey. See Lee et al., 1963, table 1, for analysis of coexisting garnet, and Coleman & Lee, 1962, table 3, for analysis of coexisting aragonite. 5. Glaucophane (1-CZ-59-A) from metabasalt associated with lawsonite, pumpellyite, jadeitic pyroxene, phengitic muscovite, sphene, and aragonite: Type III blueschist, Ward Creek, Cazadero, California. Analyst, Leoniece Beatty, U.S. Geological Survey. (Other elements determined: Cr 2 O, 004%, V,O 3 003%.) 6. Glaucophane (50-CZ-) from metabasalt associated with actinolite, garnet, epidote, phengitic muscovite, sphene, and rutile, with retrograde pumpellyite and chlorite; Type IV blueschist, Ward Creek, Cazadero, California. Analyst, Vertie C. Smith, U.S. Geological Survey. See Lee et al., 1966, for analyses of all coexisting minerals and the whole rock. 7. Glaucophane (55-CZ-59) from metabasalt associated with epidote, phengitic muscovite, sphene, rutile, and quartz; Type IV blueschist, Ward Creek, Cazadero, California. Analysts: S. T. Neil, A. C. Bettiga, L. B. Beatty, A. A. Chodos, W. W. Brannock, R. E. Stevens, U.S. Geological Survey. (Other elements determined: BeO 0020%, Cr a O, 0-08%, Sc,O 3 %, V,O 3 012%, Y.O, 0%, Yb,O %, ZrO, %, CoO 002%, CuO 0004%, NiO 003%, ZnO 016%, BaO 0%, SrO %.) See Coleman & Lee, 1963, for analysis of the whole rock. 8. Glaucophane (30-NC-62) from metabasalt associated with phengitic muscovite, garnet, and sphene: Ouegoa, New Caledonia. Compares with Type III blueschist, Ward Creek, Cazadero, California. Analysts: P. Elmore, S. Botts, G. Chloe, L. Artis, H. Smith, U.S. Geological Survey. (Other elements determined: P,O S 009%.) 9. 'Glaucophane' (36-NC-62) from metabasalt (eclogite) associated with omphacite, garnet, epidote, sphene, rutile, quartz, and white mica; Amos Stream, New Caledonia. Compares with Type IV blueschist, Ward Creek, Cazadero, California. Analysts: P. Elmore, S. Botts, G. Chloe, L. Artis, H. Smith, U.S. Geological Survey. See Coleman et al., 1965, for analysis of garnet, omphacite, and whole rock. 10. Crossite (41-NC-62) from metasediment associated with quartz, phengitic muscovite, epidote, garnet, and rutile. Ouegoa, New Caledonia. Compares with Type IV blueschist, Ward Creek, Cazadero, California. Analysts: P. Elmore, S. Botts, G. Chloe, L. Artis, H. Smith. (Other elements determined: P,O 6 006%.) 11. Crossite (38-CZ-59) from metasediment associated with quartz, garnet, and muscovite; Type III blueschist, Ward Creek, Cazadero, California. Analysts: Leoniece Beatty, A. C. Bettiga, W. W. Brannock.

7 FROM BLUESCHISTS, CALIFORNIA 111 approximate estimates of chemical composition using the diagram of Miyashiro (1957,fig. 3c). Chemistry Material used for analyses was carefully purified by a combination of centrifuging in heavy liquids and repeated passes through a magnetic separator. In Ho2FVzF."3Si8O22(0H)2 100 Ft" Ft"t Ug+Mn - -FERROGL V 1 1 CROSSITE 1 I I a "V MAGNES1ORB Fia. 1. Compositional variation of amphiboles using Miyashiro's (1957) classification. Numbers as from Table 1. most cases, samples consisted of 98 per cent or more amphibole. Optical inspection of the separates indicated contamination by small inclusions of garnet, epidote, and other associated heavy minerals. Major elements were determined by a combination of standard gravimetric techniques and X-ray fluorescence Explanation of Table 1 (com.) 12. Crossite (23-CZ-59) from metasediment associated with stilpnomelane, deerite, garnet, and pyrite; Type III blueschist, Ward Creek, Cazadero, California. Analysts: Leoniece Beatty, A. C. Bettiga, W. W. Brannock. See Coleman & Lee, 1963, for whole-rock analysis. 13. Riebeckite (25-CZ-59) from metasediment associated with quartz, garnet, acmite, deerite, and stilpnomelane; Type III blueschist, Ward Creek, Cazadero, California. Analysts: Leoniece Beatty, A. C. Bettiga, W. W. Brannock. See Lee et al., 1963, for analysis of coexisting garnet. 14. Actinolite (50-CZ-) from metabasalt associated with glaucophane, garnet, epidote, phengitic muscovite, sphene, and rutile, with retrograde pumpellyite and chlorite; Type IV blueschist, Ward Creek, Cazadero, California. Analyst, Vertie C. Smith, U.S. Geological Survey. See Lee et al., 1966, for analyses of all coexisting minerals and the whole rock. 15. Actinolite (2O1-RGC-59-D) from metasediment associated with quartz, stilpnomelane, garnet, crossite, and deerite; Type III blueschist, Ward Creek, Cazadero, California. Analysts: Leoniece Beatty, A. C. Bettiga. (Other elements determined: V,O 3 008%, BaO 0-05%, ZnO 0-12%.) See Lee et al., 1963, for analysis of coexisting garnet.

8 112 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES analysis. Minor elements were estimated by emission spectrography and are reported in parts per million (Table 1). Analyses were calculated to fit the amphibole formula X 2 Y i Z 8 O 2i (O l,f) i, where Z represents the tetrahedrally coordinated cations Si and Al; Y, the octahedrally coordinated cations Mg, Fe+ 2, Fe+ 3, Al, Mn, Ti; and X, the larger cations, Na, Ca, K. The Z group is made up to 800 by Al when Si is below the required amount. Large cations such as Na, Ca, and K are placed in the X group and, in most cases, this position made up to the required 200, except for samples 30-NC-62, 41-NC-62, and 25-CZ-59, for which the analyses were either in error, incomplete, or of an impure sample. Octahedral sites in the Y group should then sum up to 500, and except for the three samples mentioned, all analyses fall within the limits suggested by Phillips (1963) for good chemical analyses. The OH group, in most cases, does not come up to the required 20 for the formula and this may be due to the inherent difficulty in determining water in amphiboles (Borley, 1963, p. 371). However, Barnes (1930) has demonstrated that O~ 2 maybe substituting for OH in the structure where excess ferric ions are available to balance the substitution. For those analyses in Table 2 where OH is low, there is enough Fe+ 3 present to allow the substitution of O~ 2 for OH except in 30-NC-62 and this analysis is suspect on other grounds. These amphiboles contain only minor amounts of fluorine as compared with those described by Borley (1963). A further check of the analyses against other parameters can be made by calculating density and cell volumes against measured density and measured cell volumes. All those analyses that appear correct by other standards provide very close agreement between calculated and measured values (Table 2). The analyzed amphiboles are plotted on the chemical classification diagram adopted by Miyashiro (1957). The Cazadero alkali amphiboles range from glaucophane through crossite (subglaucophane of Miyashiro, 1957), to riebeckite (Fig. 1). Details of the distribution of minor elements between the alkali amphiboles and coexisting minerals will be discussed in a later publication. However, in general, the presence of minor elements in these amphiboles is in agreement with those reported from different environments (for instance, see Deer, Howie, & Zussman, 1963, p. 283). X-RAY CRYSTALLOGRAPHY Single crystal X-ray diffraction studies Single crystals from samples 51-CZ-59, 50-CZ-, 30-NC-62, 41-NC-62, and 23-CZ-59 were studied by X-ray precession camera techniques in order to establish the space group symmetry and to look for small-scale phase separations. The single crystals display diffraction symmetry 2jmC-/-, consistent with space groups Cm, C2, C2/m. Structural studies on alkali amphiboles by Whittaker (1949), Colville & Gibbs (1965), and Papike & Clark (1966) have presented

9 O *42' ? ' 905 TI >0 O r C m 00 o X O >r ti TI o z Si Al Other S tetrahedral Al Ti Fe"+ Mg Fe«+ Mn Other E octahedral Na Ca K Other large cation OH F O 1 " hydroxyl, fluorine Fe t+ /[Fe >+ +Al* + -l-ti] Fe'^IIFe^+Mg+Mn) Sp. Gr. (meal.) Density (calc.) gm. cm" 1 Gram formula weight a.k b.k r.a P, deg. v,(kr TABLE 2 Calculated formulae and unit-cell parameters of amphiboles* *36' ' IOS-IO *35' *35' ' ' 880 Formulae calculated on the basis of 24 (O, OH, F) from chemical analyzes given in Table 1 Fo+'/IFe+' + AI+' + Ti] is octahedral A1+". m at Penn State University (Paterno Lib) on March 5, O3 38' 895 Al +1 in the ratio

10 114 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES structure models consistent with C2\m symmetry and it is reasonable to assume this space group for our material. Because the crystals appear to be homogeneous with no apparent small-scale phase separations, our results are consistent with the idea of complete miscibility between glaucophane and riebeckite under the physical conditions of glaucophane schist formation. Powder X-ray diffraction studies The ten samples presented in Table 2 were studied by powder X-ray diffraction techniques. X-ray diffraction powder patterns were obtained both by film and diffractometer techniques, using Ni-filtered Cu radiation. For the films a standard diameter powder camera was used with the Wilson mounting technique (Wilson, 1949), and film measurements were corrected for shrinkage. The diffractometer patterns were prepared on a Norelco X-ray diffractometer using NaF as internal standard (a = A). The measurements were refined by a least-squares technique (Evans, Appleman, & Handwerker, 1963), to give the unit-cell parameters presented in Table 2. Standard errors on these parameters are less than ±0-1 per cent for a, b, and c; less than ±8 minutes for/? and less than 1 A 3 for the unit-cell volume. Four X-ray diffraction powder patterns have been calculated (Table 3) using a computer program written by D. K. Smith (1963) and revised by Mr. Cyrus Jahanbagloo, Geology Department, University of Minnesota. The calculations are based on the atomic positional parameters for glaucophane (Papike & Clark, 1966), and the chemical unit-cell contents and unit-cell parameters presented in Table 2. Calcium and sodium were assigned to the M 4 position, ferric iron and aluminium to the Mj. position, and ferrous iron and magnesium to the M x and M 3 positions. These calculated powder patterns should be useful in indexing X-ray diffraction powder patterns for alkali amphiboles and thus will be helpful in the initial stages of unit-cell refinement. Intensities and positions of the X-ray diffraction peaks show systematic variations with composition between glaucophane and riebeckite. The a and b unit-cell dimensions and the unit-cell volume show a linear increase withfe+^fe^+al+ti), and plots of these relationships, fitted by leastsquares technique, as well as one for 20 (310) CuKa vs. Fe +8 /(Fe +s +Al+Ti) are illustrated in Fig. 2. The latter curve is included because it may serve as a rapid analytical method. Scatter of points in the curves may be largely attributed to three effects; first, the Fe +2 -Mg substitutions which result in compositions off the join glaucophane-riebeckite; second, small differences in the degree of cation order; and third, the analytical error. All of these curves are quite similar to those of Ernst (1963a) for synthetic alkali amphiboles except for the one of the a axis where our curve is somewhat lower. This discrepancy may be due to incomplete ordering of the synthetic amphiboles. Our results demonstrate that the alkali amphiboles studied belong to the high pressure-low temperature series, glaucophane II-riebeckite.

11 FROM BLUESCHISTS, CALIFORNIA 115 a (A) _j i i i i b (I)! 0 17 t tto tlo foo to 0 t70 20(310) ««CuKo It 0 FIG. 2. Variation of unit-cell parameters with composition of glaucophaneriebeckite amphiboles; GL I, low pressure-high temperature polymorph; GLII, high pressure-low temperature polymorph. Parameters for synthetic GL I, GL II, and ricbeckite (Rb) are from Colville et al. (1966). Structural Interpretation The recently completed structural investigation of glaucophane II by Papike & Clark (1966) is pertinent to the interpretation of the unit-cell data presented above. This structural study shows that the high pressure-low temperature polymorph of glaucophane is characterized by a high degree of cation ordering as suggested by Ernst (1963a). The M 4 position is occupied by Na, M 2 is occupied by Al and Fe+ 3 with a mean metal-oxygen distance of 1-94 A, and Mj and

12 116 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES P > O fs O O V^ONOJ Nnw*<n0*f OnONt-vit*-«'tNA>ar»<>'np>^«P^O*^0»'nv}n-(>Qio'pooo- - 0 v-> dooo oior»*r-t o\«<oo>oos'pnoonsinfto i ir i»no9**t AXM<O 9r)--oj N'*i i- i 90G'f*nr)HomOi6<>iNh'«vo<n<n<n<nir"»Mnnr<riNMM w m I II P is s I! II r~oor-* n- ;8 c: S oor^ooogop^rnvp^poo^<^r^^^inr^\oooo^»o\ao*oi^*(*itnfnpoqr»'*iinq oo«^^ poo«^^^m ONONO\o^l-*^vp^»ntninin i *'*t-f»ir-ir*i»swc<mm -8*2' )<noo-ri'nnono«>ocf\yw(i)7yoy» 3r^Apo<bor4fnri'j'j'*'g'in\b*oooeibo\6\c 't-r' OMNh'00"rtf--N250ftff\05*iO(oa-nooffi>CMOoovoi»i-inffiN-(s«N mcim<nooob~v0* "* *n * r^* oo oo o\ o\ ^h ON ^? ON(S*W)**O^ g* o OO* N- ^ n^-i-np'*oa*or4oor^'*<n«n(nr^r>ioot»-in'*^ton 6oN^oo^5^T5ei*fp^^fs5o^*q\*if*^*f^^^»9^^MrpcHNNM(si2 0\Ot-dbONONO\ (Nf-i^t 1-0\0%0r-6\OOPp(N<*>r0 I *"'*'*w^»n«r-00000n0n0^pp o o <N p OQO MN-- i n M n t «* ^ N m Am d - P^^ ^-IM p r**nirnfcn(h^-c*i ini oon < rjp<n M< -no mi-w- ^-mir

13 FROM BLUESCHISTS, CALIFORNIA 117 (N (N (S (N *N fn «N + o\ ^ icnm i oe <* c* oo '' ir *os 00 i^oion- o '^ r r i M o o\»nin«oirtk-trin^' A I S?! II -n-nqnnn( i innp6<n(7i<o. "I

14 118 R. G. COLEMAN AND J. J. PAP1KEALKALI AMPHIBOLES M 3 are occupied by Mg and Fe+ 2 with a mean metal-oxygen distance of 2-09 A. Cation ordering of this type is consistent with the charge balance requirements and the minimization of volume and entropy required by high-pressure-low temperature formation conditions. The analyses presented in Table 2 are consistent with ordering of this type except for those of 51-CZ-59 and 5O-CZ-6O where the sum of Fe +3 +Al is considerably less than 20 so that the M 2 position would necessarily include some Mg and Fe +2. The most significant substitutions in the amphiboles under investigation appear to be between Fe+ 3 and Al in the M 2 sites and between Fe +2 and Mg in the M x and M 3 sites. PETROGENETIC RELATIONSHIPS The bulk composition of the host rock appears to exert the main influence on the kind of alkali amphibole that crystallizes during blueschist metamorphism. Glaucophane from metabasalts of the intermediate Type III schists is essentially the same as that from Type IV, and this fact indicates that the pressure-temperature variations, of unknown magnitude, developed during metamorphism, have had little influence on the glaucophane composition. The dependence of the alkali amphibole composition on the host rock bulk composition can be shown by comparing the ratios Fe+ 2 /(Fe +2 +Mg+Mn), and Fe+^Fe^-f-AF'+Ti) in the amphibole and in the rock (Fig. 3a, b). The strong linear correlation between these ratios established the relationship that the alkali amphibole species from blueschists are largely controlled by the composition of the host rock. The correlations are not merely a reflection of the dominance of alkali amphiboles because modal analyses demonstrate a range of 3 to 80 volume per cent alkali amphibole associated with other phases containing important amounts of Mg, Al, and Fe. In the metabasalts where Mg and Al predominate, glaucophane crystallizes, and in the metasediments where Fe+ 2 and Fe +3 predominate, crossite or riebeckite forms. Although sodium must bepresent, its activity has very little influence on determining the alkali amphibole species. Experimental work by Ernst (I9, 1961, 1962, 1963a) on the alkali amphiboles has provided guidelines relative to the probable stability fields for glaucophane and riebeckite. This work indicates that glaucophane, magnesioriebeckite, and intermediate members of this series are stable at most temperatures below 800 C. Riebeckite can only exist below about 500 C, arfvedsonite below about 700 C, and such minerals do not appear to be particularly pressure-sensitive; marked chemical and optical differences between igneous riebeckite-arfvedsonites and blueschist riebeckites suggest some pressure-temperature influence or differences in total bulk composition of the host rock. At higher pressures and lower temperatures (~ 3 kb, 300 C), Ernst (1963a) found that glaucophane II with a small unit-cell volume is stable, whereas at lower pressures, glaucophane I with a larger unit-cell volume is formed. The high-pressure polymorph, glaucophane II, can thus be expected to occur under pressure-temperature conditions comparable with those of the blueschist facies. It was shown earlier that all

15 FROM BLUESCHISTS, CALIFORNIA loor 119 o = ROCK Fe'"+AI + Ti FIG. 3a. Plot showing dependence of alkali amphibole composition on that of the host rock, using the ratio 100Fe+'/(Fe+'+Al [ll +Ti), based on atomic per cent. Fe"+Mg + Mn ROCK Fio. 3b. Plot showing dependence of alkali amphibole composition on that of the host rock, using the ratio 100Fe+7(Fe +t +Mg+Mn), based on atomic per cent. natural glaucophanes from the Cazadero blueschists have cell volumes comparable with glaucophane II. The low-pressure glaucophane I synthesized by Ernst has not yet been found in nature. From Ernst's data and earlier estimates by Miyashiro (1957) it would appear that the alkali amphiboles of riebeckite composition could be expected to form in a wide variety of geologic conditions, whereas the presence of glaucophane II characterizes the high pressure-low temperature conditions of the blueschist facies.

16 120 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES The occurrence of calcium in the alkali amphibole structure indicates that there may be partial miscibility between glaucophane and actinolite. The presence of actinolite coexisting with glaucophane in the Cazadero area has been verified by Lee et al. (1966), and in this occurrence, glaucophane contains 3-58 per cent CaO and actinolite 1-52 per cent Na 2 O. There are so few reliable analyses of coexisting glaucophane and actinolite that miscibility of actinolite in glaucophane as a function of geologic environment is not clear. There does appear to be more calcium in those glaucophanes from the higher grade blueschists, and such substitution could be a function of pressure-temperature conditions rather than FIG. 4. Triangular plot of Cazadero and New Caledonia alkali amphiboles and actinolites (closed circles) compared with analyzed amphiboles from other blueschist terrenes (open circles). Numbers same as those in Table 2. Tie-line connects coexisting glaucophane and actinolite. variations in bulk composition. Fig. 4 shows the possible compositional gap as it is now known from analyzed amphiboles of the blueschist facies. Iwasaki (1963) describes a miscibility gap of GlgeAc^ to Gl^Ac,;,, in the amphiboles from the schists of the Sanbagawa metamorphic belt of eastern Shikoku; more recently Klein (1966) has described an unusual 'riebeckite-tremolite' from metamorphosed iron formations of southwestern Labrador considered to be in the kyanite-staurolite zone of metamorphism. Further work is necessary before this problem can be clarified. Ernst (19636) has reviewed this question and suggests that experimental synthesis might establish the probable limits of miscibility between the alkali amphiboles and actinolite. SUMMARY Alkali amphiboles from the blueschists of Cazadero range in composition from glaucophane to riebeckite, including the intermediate crossite. These

17 FROM BLUESCHISTS, CALIFORNIA 121 amphiboles, although taken from rocks of apparently contrasting metamorphic grades, show no systematic compositional variation; however, the original rock type exerts a strong influence on the amphibole composition. Metabasalts contain mostly glaucophane with minor crossite and metacherts characteristically have crossite or riebeckite. Actinolite was found to coexist with glaucophane in the higher grade rocks or in the calcium-rich carbonate layers of the lower grade schists. The Cazadero glaucophanes have a structure similar to the high-pressure polymorph, glaucophane II, synthesized by Ernst (1963a). All of the blueschist alkali amphiboles were found to have C2\m symmetry and no structural discontinuities were found within the series from glaucophane to riebeckite for the Cazadero samples. Further work is needed to define the miscibility gap between the alkali amphiboles and actinolites, and the possibility of using the content of calcium in glaucophane as a possible temperature indicator should be investigated. The range in pressure-temperature conditions during metamorphism has been estimated to be 200 to 300 C with pressures of 6 to 9 kilobars (Coleman, 1966); the temperatures are determined by oxygen isotopic fractionation between coexisting metamorphic minerals and the pressures are then fixed by the calcitearagonite stability curve. Under these physical conditions, a complete range in composition from GlgoRieb^ to Gl^Rieb^, is stable, and the bulk composition of the original rock controls the composition. These estimates on the physical conditions of formation are in accord with the experimental data on alkali amphiboles. The compositional limits of alkali amphiboles within the blueschist facies now seems to be well understood; however, those schists transitional between greenschists and blueschists contain amphiboles whose compositional variations are not yet understood. ACKNOWLEDGEMENTS Many of the mineral separations were made by Donald E. Lee, and David B. Stewart was helpful in the early X-ray studies of the amphiboles. Terry Clark, Norman Page, and Margaret E. Hall provided considerable mineralogical support work to this study. We wish to thank Joan R. Clark for discussions concerning this research and Mr. Larry Finger, Geology Department, University of Minnesota, for help in the calculations of X-ray diffraction powder patterns. REFERENCES BARNES, V. E., Changes in hornblende at about 800 C. Am. Miner. 15, BORLEY, G. D., Amphiboles from the younger granites of Nigeria. Part I. Chemical classification. Mineralog. Mag. 33, COLEMAN, R. G., Glaucophane schists from California and New Caledonia. Proc. Pacif. Scl. Congr. 11th, Tokyo, Japan 4, 11 (abs.) Glaucophane schists from California and New Caledonia. Tectonophysics (in press).

18 122 R. G. COLEMAN AND J. J. PAPIKEALKALI AMPHIBOLES COLEMAN, R. G. & LEE, D. E., Metamorphic aragoaite in the glaucophane schists of Cazadero California. Am. J. Set., Glaucophane-bcaring metamorphic rock types of the Cazadero area, California. /. Petrology 4, BEATTY, L. B., & BRANNOCK, W. W., Eclogites and eclogites: their differences and similarities. Bull. geol. Soc. Am. 76, COLVILLE, A. A., & GIBBS, G. V., Refinement of the crystal structure of riebeckite. Spec. Pap. geol. Soc. Am. 82, 31 (abs.). ERNST, W. G., & GILBERT, M. C, Relationships between cell parameters and chemical compositions of monoclinic amphiboles. Am. Miner. 51, DEER, W. A., HOWIE, R. A., & ZUSSMAN, J., Rock-forming minerals, 2, Chain silicates. Longmans, Green & Co., Ltd. London, 379 pp. ERNST, W. G., 19. Stability relations of magnesioriebeckite. Ceochim. cosmochim. Ada 19, Stability relations of glaucophane. Am. J. Sci. 259, Synthesis, stability relations and occurrence of riebeckite and riebeckite-arfvedsonite solid solutions. /. Geol. 70, a. Polymorphism in alkali amphiboles. Am. Miner. 48, Petrogenesis of glaucophane schists. J. Petrology 4, EVANS, H. T., Jr., APPLEMAN, D. E., & HANDWERKER, D. J., The least-squares refinement of crystal unit cells with powder diffraction data by an automatic computer indexing method. Program and Abstracts, Am. Cryst. Ass. Mtg., Cambridge, Mass., Abs. E-10, 42. IWASAKI, MASAO, Metamorphic rocks of the Kotu-Bizan area, eastern Shikoku. /. Fac. Sci. Tokyo Univ. 15, 2, KLEIN, C., Jr., Mineralogy and petrology of the metamorphosed Wabush Iron Formation, southwestern Labrador. J. Petrology 7, LEE, D. E., COLEMAN, R. G., BASTRON, HARRY, & SMITH, V. C, A two-amphibole glaucophane schist in the Franciscan Formation, Cazadero area, Sonoma County, California. Prof. Pap. U.S. geol. Surv, 550-C, C COLEMAN, R. G., & ERD, R. C, Garnet types from the Cazadero area, California. /. Petrology 4, MIYASHIRO, A., The chemistry, optics and genesis of the alkali-amphiboles. J. Fac. Sci. Tokyo Univ. 11, 2, PAPIKE, J. J., & CLARK, J. R., Cation distribution in the crystal structure of glaucophane II, the high-pressure polymorph. A. Mtg. geol. Soc. Am (abs.). PHILLIPS, R., The recalculation of amphibole analyses. Mineral. Mag. 33, SMITH, D. K., A fortran program for calculating X-ray powder diffraction patterns. Report U.C.R.L. 7196, Lawrence Radiation Laboratory, Livermore, California, 72 p. WHITTAKER, E. J. W., The structure of Bolivian crocidolite. Ada crystallogr. 2, WILSON, A. J. C, Straumanis method of film-shrinkage correction modified for use without high angle lines. Rev. scient. Instrum. 20, 831.