Hornblendes Formed During Progressive Metamorphism of Amphibolites, Northwest Adirondack Mountains, New York

A. E. J. ENGEL University of California, La Jolla, Calif. CELESTE G. ENGEL U.S. Geological Survey, La Jolla, Calif.

Hornblendes Formed During Progressive Metamorphism Of Amphibolites, Northwest Adirondack Mountains, New York

Abstract: Hornblendes in amphibolite interlayers increasing grade of metamorphism include in- in the paragneiss of the northwest Adirondack creases in Ti, Na, K, Cr, V, and Sc. Decreases occur Mountains undergo systematic changes in color, in the amounts of Mn, Zn, OH + F + Cl, and in composition, and density during progressive the ratios Fe2Os/FeO and Fe/Mg. metamorphism from almandine-amphibolite to Density of the hornblendes increases from 3.260 hornblende-granulite facies. In contrast, indices of to 3.278 with the increasing grade of metamor- refraction of the hornblendes remain about con- phism. stant. These changes in the hornblendes with increasing In the almandine-amphibolite facies the am- T and P, although well denned, are less pronounced phibolite layers have the bulk composition of a than those measured in biotites and garnets of the saturated basalt and consist of bluish-green horn- enclosing paragneiss. Large variations in the physi- blende, andesine, and quartz. As these layers are cal and chemical properties of hornblendes in traced into the hornblende-granulite facies, their metamafic rocks reconstituted above the epidote- composition undergoes a progressive change to amphibolite facies appear to be induced principally that of an olivine basalt with brownish-green by critical changes in the bulk composition of the hornblende, clinopyroxene and orthopyroxene, total rock, and not by the regional gradients in and calcic andesine as major constituents. T, P, or by changes in kind, or composition, of the Compositional changes in the hornblendes with coexisting minerals.

CONTENTS

Introduction 1500 of Ca to Na + K in hornblendes from Acknowledgments 1500 amphibolites of the Emeryville and Colton Occurrence and properties of hornblende-bearing areas, New York 1507 rocks 1500 Hornblendes 1502 Table Introduction 1502 1. Average compositions of least altered amphibo- Chemical composition 1503 lite rocks in the gneiss-amphibolite complex, Color and pleochroism 1507 Emeryville and Colton areas, New York 1502 Density 1508 Indices of refraction 1508 Facing Comparison with metamorphic hornblendes from 2. Chemical composition and indices of refraction other terranes 1509 of hornblendes from least altered amphibo- Variations in properties of hornblendes induced by lite rocks, Emeryville area, New York . . regional metamorphic gradients 1510 3. Chemical composition and indices of refraction References cited 1513 of hornblendes from sericitic and biotitic amphibolite rocks, Emeryville area, New 1504 Figure York 1. Sketch map of the northwest Adirondack 4. Chemical composition and indices of refraction Mountains showing relations of the major of hornblendes from amphibolite rocks, rock types 1501 Colton area, New York 2. Variations in the chemical composition of horn- 5. Chemical composition and indices of refraction blendes plotted as a function of increasing of hornblendes from biotitic, garnetiferous, grade of metamorphism between Emery- and retrograded amphibolite rocks, Colton ville and Colton, New York 1504 area, New York 1505 3. Plot in atoms per unit cell showing the relations 6. Chemical analyses of hornblendes from amphi-

Geological Society of America Bulletin, v. 73, p. 1499-1514, 3 figs., December 1962

1499

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bolites in the Edwards and East Edwards compared with properties of hornblendes areas, New York 1506 from amphibolites in the Emeryville- 7. Average chemical composition, index of refrac- Colton region, New York 1510 tion, density, and color of hornblendes Comparison of average pyroxene-bearing am- from least altered amphibolite rocks in the phibolite and constituent hornblende, Emeryville and Colton areas, New York . 1508 Colton, New York, with similar rock and 8. Average properties of hornblendes from am- constituent hornblendes from the Lafit phibolites in the Black Hills, South Dakota, Mountains, Sudan, Africa 1512

Colton, New York, the gneiss-amphibolite com- INTRODUCTION plex is reconstituted to the hornblende-granulite The hornblendes discussed in this paper are facies. Most of the classical features of progres- major constituents of amphibolites and py- sive metamorphism are apparent in the rocks roxene-bearing amphibolites formed in the between these two areas (Engel, 1949; Engel upper-amphibolite and hornblende-granulite and Engel, 1953; 1958; 1960a; 1960b; 1962; facies of regional metamorphism, in the north- Engel and others, 1961). west Adirondack Mountains, New York. The Geological thermometers indicate tempera- physical and chemical properties of these horn- tures of metamorphism of at least 525° C at blendes, especially chemical composition, in- the southwest (Emeryville) end of the belt. dices of refraction, density, and color, reflect Northeast near the perimeter of the Adiron- the properties of the enclosing rocks and the dack massif (Colton) temperatures of meta- systematic regional variations in temperature, morphism were at least 625° C (Engel and pressure, and composition during regional Engel, 1958, p. 1383-1387). The depth of burial metamorphism. of the gneiss-amphibolite complex during metamorphism appears to have been at least 5 miles ACKNOWLEDGMENTS at Emeryville and at least 7 miles at Colton. This study has been possible because of the Possibly the depth of burial was as great as financial support of the United States Geologi- 10-15 miles. During metamorphism, partial cal Survey, the National Science Foundation pressure of water in the rocks appears to have (Earth Sciences Grant 14177), and the Uni- approached load pressures. In the Colton area versity of California. A. F. Buddington, the paragneiss (metagraywacke?) appears to Michael Fleischer, and David Wones kindly have been partially molten during the highest read the manuscript and offered constructive grade of metamorphism and to have released comments, which we appreciate. some anatectic, alkali-silicate magma (Engel and Engel, 1960b). The amphibolite inter- OCCURRENCE AND PROPERTIES OF layers probably remained as solid bodies re- HORNBLENDE-BEARING ROCKS constituted 50° or more below temperature of The amphibolite rocks, from which horn- incipient liquification (Engel and Engel, 1962). blendes under discussion are derived, form thin The amphibolite rocks at Colton are deficient sheets and lenses within the major Adirondack in H2O, K, Ti, F, and Cl compared with those paragneiss (Engel and Engel, 1958; 1960a). at Emeryville. These changes in composition Field relations, lithologic features, composition, of the amphibolite rocks between Emeryville origin, and progressive metamorphism of the and Colton are accompanied by a marked re- amphibolite rocks are discussed in detail in a duction in the ratio Fe2Og/FeO and by an separate, but complementary paper (Engel and appreciable enrichment in Ca and Mg. Con- Engel, 1962). The paragneiss and amphibolite, sequently, the chemical compositions of the together with lenses of granite, comprise an amphibolites reconstituted during metamor- elongate belt that strikes northeast across the phism are those of a typical saturated basalt at Grenville lowlands of the northwest Adiron- Emeryville and an olivine basalt at Colton dack Mountains (Fig. 1). The southwest por- (Table 1; see also Engel and Engel, 1962). tion of this gneiss-amphibolite complex, well The mineralogical changes in the amphibolite exposed at and southwest of Emeryville, New rocks, with increased temperature of meta- York (Fig. 1), is metamorphosed to the upper morphism, are marked and systematic. The amphibolite facies. Northeastward from Emery- average mineral composition of amphibolites ville, the grade of metamorphism increases at Emeryville that are devoid of retrograde progressively for about 35 miles to the edge of effects and granitic interpenetrations is (in the central Adirondack massif. There, near volume per cent): hornblende 69; plagioclase

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TABLE 1. AVERAGE COMPOSITIONS OF LEAST ALTERED 19; quartz 9; and ilmenite 2. At Colton the AMPHIBOLITE ROCKS IN THE GNEISS-AMPHIBOLITE average of similarly unaltered amphibolites is: COMPLEX, EMERYVILLE AND COLTON AREAS, NEW YORK plagioclase 36; hornblende 31; clinopyroxene 19; orthopyroxene 11, and ilmenite 2 (Table Emeryville area Colton area 1). Important deviations from these composi- (seven samples) (nine samples) Mean Mean tions appear wherever granites are emplaced Mean dev. Mean dcv. in and along amphibolite rock, or where the amphibolites are cut by shear zones. MODES, VOLUME PER CENT Interaction of amphibolite with granite re- Quartz 8.6 4.8 0.1 0.02 sults in enrichment of the amphibolite in Plagioclase 18.5 4.5 35.6 4.6 alkalis, halogens, alumina, and silica so that Hornblende 68.6 3.3 31.3 11.0 biotite, chlorite, sericite, garnet, scapolite, and Clinopyroxene .9 .8 19'2\ 6 0* Orthopyroxene 11. 3J quartz increase at the expense of plagioclase and Opaque minerals 2.1 l'i 2.0 .7 pyroxenes (at Colton) and hornblendes and Biotite .6 .3 .2 .1 plagioclase (at Emeryville). Sheared amphibo- Sphene .4 .2 lites commonly contain biotite, chlorite, and .3 .2 .3 .2 Others* sericite, which replace pre-existing minerals. CHEMICAL ANALYSES Although the origin of the Emeryville- SiO2 48.20 1.43 47.89 .91 Colton amphibolites remains conjectural, most 1.89 .39 1.56 .17 TiO2 of them could have formed by reconstitution 14.45 .81 14.63 .35 A1203 of basaltic sills or flows. This interpretation Fe2O3 3.50 .45 1.85 .58 FeO 10.53 .73 11.20 .84 is not entirely convincing, for some amphibo- MnO .25 .02 .25 .02 lites less than an inch thick are entirely con- MgO 6.62 .41 7.41 .24 cordant and persist for 100 or more feet along CaO 10.25 .71 11.54 .48 the strike. This and other features discussed Na2O 1.94 .45 2.19 .14 K20 .96 .08 .58 .12 in succeeding sections indicates that these H2O+ 1.31 .06 .72 .19 thinner amphibolites, or all of them, may have H2O- .01 .01 .03 .02 formed either by metamorphic differentiation P205 .18 .03 .14 .03 Total Fe or by metasomatic addition of mafic elements as Fe2Os 15.22 1.22 14.27 1.05 into certain sheared beds of paragneiss. TRACE ELEMENTS (ppm) HORNBLENDES B <20 <20 Ba 100 22 72 22 Introduction Co 44 1 45 3 Cr 201 61 353 71 The abundance of hornblende in least Cu** 47 15 34 25 granitized and sheared amphibolites is inversely Cu 73 19 61 43 Ga 14 2 13 proportional to the grade of metamorphism. La <50 <50 Thus, hornblende is the dominant mineral in Mo < 10 < 10 the amphibolites at Emeryville (Table 1), but, Ni" 53 10 59 14 with increasing grade of metamorphism, clino- Ni 61 4 60 9 Pb" 8 3 7 3 pyroxenes and ultimately orthopyroxenes ap- Sc 56 2 57 4 pear at the expense of hornblende. Eskola Sr 226 36 164 19 (1952, p. 140) has argued that hornblendes V 344 49 318 24 are not present in granulite per se, but this con- Y 59 13 45 2 Yb 7 6 clusion is unwarranted. The granulite facies of Zn" 155 23 151 30 rocks are formed over a range of T and PI and Zr 145 33 104 14 Pf. In the mafic rocks under discussion horn- C.I.P.W. NORMS blende persists at least in the lower ranges of Quartz .38 physical conditions encountered in the granu- Orthoclase 5.67 3.45 lite facies, although it may commonly be un- Albite 16.40 18.03 Anorthite 27.88 28.36 Diopside 17.87 23.03 * Mean deviation of total pyroxene content in rock Hypersthene 21.38 8.54 f Chiefly apatite, chlorite, calcite, sericite, and zircon Olivine t ( 11.29 ** Colorimetric analyses; analyst, H. L. Nieman. All Ilmenite 3.65 3.01 other trace-element analyses are quantitative spectro- Magnetite 5.08 2.69 Apatite .44 .34 graphic analyses by Nancy M. Conklin and R. G. Havens.

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stable in the medial and upper ranges of tem- tinguish the hornblendes in "least-altered" peratures and rock pressures. pyroxene-bearing amphibolites from those in The chemical composition, structural for- variously injected and retrograded amphibo- mula, density, and index of refraction of horn- lites. This is because both the brownish-green blendes in the Emeryville-Colton region are hornblende and the pyroxene formed during listed in Tables 2, 3, 4, 5, 6, 7, and 8. Data on the apical stage of regional metamorphism at hornblendes at Emeryville and at Colton are Colton may be replaced by a green hornblende given in Tables 2, 3, 4, and 5, respectively. more nearly in equilibrium during the waning Table 6 lists the properties of several horn- conditions of metamorphism. Samples A 65 blendes in the amphibolites at two intermediate and A 8 are examples of these green hornblendes areas, Edwards and East Edwards, just south- that have replaced pre-existing brownish-green west and northeast of the isograd marking the hornblende and pyroxene (Table 5). Both the appearance of orthopyroxene in the amphibo- physical and chemical features of A 65 and A 8 lite rocks (Fig. 1). Comparison of hornblendes are more like the hornblendes from Emeryville from opposite sides of this isograd indicates the than like those in the least-altered amphibolite important changes in the minerals in amphibo- at Colton. In scattered, late metamorphic shear lites at essentially the lowermost limits of the zones these green hornblendes, with plagioclase hornblende-granulite facies (Table 6). more sodic than that typical at Colton, are the The mean (average) properties of the horn- dominant minerals. Locally, they, in turn, are blendes at Emeryville and Colton are grouped replaced by chlorite, serpentine, and albite. in Table 7 to aid rapid comparisons and to Perhaps it is desirable to emphasize that the point up the major differences in hornblendes textural, structural, and lithologic features in from the opposite ends of the belt of progres- these areas indicate that most, or all, of the sive metamorphism. Similarly, the average green hornblendes are not formed synchronous- compositions of the amphibolite rocks from ly with the brownish-green hornblende- which the hornblendes at Emeryville and pyroxene association, but in a later, more Colton are obtained have been listed in Table 1. hydrous environment of decreased T and rock Chemical and modal analyses of each of the load. In the gneiss-amphibolite complex, PH,O rocks from which the hornblendes have been appears to be a function of T and PI during separated are published in the complementary metamorphism. discussion of the amphibolite rocks (Engel and The metamorphism of the amphibolite rocks Engel, 1962). Sample numbers in the two to hornblende-granulite gneisses at Colton is studies are identical. All chemical analyses of clearly reflected in changes in the amount and hornblende are by Celeste G. Engel except as the properties of the constituent hornblendes. follows: A 2, A 3, A 10, A 14, A 15, and A68 The most obvious changes are: (1) the abrupt are by M. Seerveld. We are also deeply in- decrease in amount of hornblende with in- debted to D. L. Maynes for performing partial creased T and P of metamorphism; (2) color of duplicate analyses of AE 317, AE 326, AE 337, hornblendes, which changes from bluish-green AE 338, Z 104, and AE 358. at Emeryville, in the amphibolite facies, to The hornblende in the hornblende-andesine brownish-green at Colton, in the hornblende- amphibolites at Emeryville is a bluish-green, granulite facies (Table 7), (3) the density of calciferous amphibole, prismatic in form. Only the hornblendes, which increases from ^3.260 the hornblendes from the least-altered am- at Emeryville to ^-3.278 at Colton; and (4) phibolites in the Emeryville area are described chemical composition of the hornblendes, in Table 2, following the presentation of rock which undergoes a clearly discernible but not data (Engel and Engel, 1962). Properties of extreme change (Fig. 2). hornblendes from the altered amphibolites at Emeryville are listed in Table 3. Actually this Chemical Composition separation seems desirable for the amphibolite The changes in chemical composition of the rock at Emeryville but not for most constit- hornblendes with increasing grade of meta- uent hornblendes. With but one exception morphism include: (1) increases in concentra- (A 14), the hornblendes from both least- tions of Ti, Na, K, F, Cr, V, Sc, and probably altered and incipiently altered amphibolites at Co and Ni; and (2) decreases in amounts of Emeryville are essentially identical in composi- OH + F + Cl, Mn, Zn, and in the ratios tion, color, optics, density, and crystal struc- Fe2O3/FeO and FeO and Fe/Mg. The con- ture and are discussed together in this paper. centrations of Si, Al, and Ca remain nearly In the Colton area it is necessary to dis- constant throughout the gradient in tempera-

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ture of metamorphism of 100° C or more. The TiC>2 content of the hornblendes in- The lack of significant variations in con- creases with increasing grade of metamorphism centrations of Si and Al and hence in the ratio from 1.3 (weight per cent) at Emeryville to Al/Si is in contradiction to most generaliza- 2.4 at Colton. In this respect the hornblendes tions and predictions in the literature. The in- are analogous to the biotites in the enclosing

CaO/IO / I 500 „.

450

1 Miles 1 Miles

A A I t A I

Figure 2. Variations in the chemical composition of hornblendes plotted as a function of increasing grade of metamorphism between Emeryville and Colton, New York

crease in the ratio Mg/Mg + Fe, although gneiss which show a parallel increase in TiO2 real, is smaller than commonly assumed. CaO from 3.2 to 5.2 between Emeryville and Colton is nearly constant in all the hornblendes at (Engel and Engel, 1960a, Table 15, p. 25). This about 11.5 weight per cent, irrespective of increase in TiOz content in hornblende and variations in physical conditions. biotite is not accompanied by an increase in The low ratios of Fe2Os/FeO and their TiOz content of the enclosing rock. In fact, in systematic decrease from an average af 0.36 at the amphibolites, the total TiOa may decrease Emeryville to 0.23 at Colton clearly distin- slightly with increasing grade of metamor- guishes these hornblendes from the so-called phism, and the concentration of ilmenite, the oxyhornblendes and basaltic hornblendes which other important titanium-bearing mineral in have formed at equivalent or higher tempera- the amphibolites, remains about constant. tures but much lower pressures. What happens, in essence, is that the amount of

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hornblende is halved but its content of TiO2 quite as marked as that in K (Table 7). Potas- doubles. sium also is expelled from the amphibolite rock Amounts of both Na and K in the horn- system (Engel and Engel, 1962). It has been blendes increase with increasing grade of noted that the associated large cation Ca re- metamorphism. The increment in Na is not mains essentially constant in the hornblendes

TABLE 5. CHEMICAL COMPOSITION AND INDICES OF REFRACTION OF HORNBLENDES FROM BIOTITIC, GARNETIFEROUS, AND RETROGRADED AMPHIBOLITE ROCKS, COLTON AREA, NEW YORK

A 65 A8 A 68 A9

SiO2 42.24 42.51 42.57 42.42 TiO2 1.73 1.90 2.41 2.01 A1203 12.09 11.92 11.76 10.20 Fe2O3 5.15 5.05 4.10 4.30 FeO 11.43 14.01 12.60 17.73 MnO .31 .36 .18 .18 MgO 10.51 9.03 10.55 6.97 CaO 11.62 11.13 11.51 12.01 Na2O 1.39 1.55 1.37 1.50 K2O 1.53 .81 1.39 1.40 H2O+ 1.54 1.56 1.50 1.43 H2O- .00 .01 .02 .00 P205 .06 .03 .02 n.d F .29 n.d .17 .31 Cl .17 n.d .02 .12 Total 100.12 99.87 100.17 100.58 0 -F .16 .07 .16 Total 99.96 100.10 100.42 Total Fe as Fe2Os 17.85 20.25 18.10 24.00 Fe2O3/FeO .45 .36 .33 .24 Fe/Mg 1.97 2.65 1.99 4.00 «z 1.670 1.670 1.669 1.674 KS 1.691 1.692 1.691 1.696 nz— nx .021 .022 .022 .022 Density 3.268 3.266 3.268 3.328 Purity 98 + 99 99 100 STRUCTURAL FORMULAE K .292 .153 .264 .273 (W) Na . .404 2.57 .452 2.40 .396 2.51 .445 2.70 Ca 1.870 1.796 1.847 1.978 Mg 2.366 2.027 2.356 1.596 (X) Fe++ 1.435 3.84 1.764 3.84 1.578 3.96 2.279 3.90 Mn .038 .045 .022 .023 Al .487 .525 .459 .371 (Y) Fe+^+ .581 1.19 .572 1.31 .461 1.19 .497 1.10 Ti .125 .214 .271 .231 Si 6.348 6.408 6.381 6.524 (Z) Al 1.652 8.00 1.592 8.00 1.619 8.00 1.476 8.00 OH 1.542 1.566 1.498 1.465 F .137 1 . 72 .080 1.58 .150 1.66 Cl .042 .004 .030 X +Y 5.03 5.15 5.15 5.00

A 65—Hornblende from amphibolite with 58 per cent plagioclase, 4 per cent K feldspar, and no pyroxene A 8—Hornblende from amphibolite with 68 per cent hornblende, 23 per cent plagioclase, and no pyroxene A 68—Hornblende from typical pyroxene-plagioclase amphibolite in which plagioclase is sencitic A 9—Hornblende from pyroxene-plagioclase amphibolite with 11 per cent garnet

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of the region. Figure 3 illustrates the increase ville is 1.31, whereas at Colton it is only 0.72. in size of the large cation group Ca + Na + K, The ratio Fe2O3/FeO in the amphibolite caused by the increase in Na and K with in- rocks at Emeryville is 0.33. At Colton this creasing grade of metamorphism. ratio is 0.17. A similar dehydration and reduc- The decrease in OH + F + Cl and in the tion have occurred in the enclosing biotitic ratio Fe2O3/FeO in the hornblendes with in- paragneiss (quartz - biotite - oligoclase - garnet creasing T and P reflects analogous changes in gneiss), where both the constituent biotites and the parent amphibolite rocks (Table 1). In the the host rock also decrease in OH + F + Cl total rock the average H2O content at Emery- and in the ratio Fe2O3 /FeO with increasing

TABLE 6. CHEMICAL ANALYSES OF HORNBLENDES FROM AMPHIBOLITES IN THE EDWARDS AND EAST EDWARDS AREAS, NEW YORK

Edwards area East Edwards area (Group 2) (Group 2A) A 19 A 21 A 22 AED 399 AED 403

SiO2 43.16 43.43 43.01 42.73 43.40 TiO2 1.71 1.59 2.30 2.38 2.43 A12O3 11.63 11.56 11.85 12.05 11.67 Fe203 4.48 3.70 3.60 3.75 3.10 FeO 12.70 13.68 14.09 14.36 12.60 MnO .25 .31 .18 .20 .18 MgO 10.17 9.97 8.96 8.74 10.69 CaO 11.53 11.72 11.70 11.23 11.43 Na20 1.37 1.28 1.44 1.44 1.45 K20 .91 .81 1.36 1.36 1.21 H20 + 1.63 1.65 1.64 1.69 1.43 H2O- .02 .02 .01 .02 .04 F .22 .18 n.d n.d n.d Cl .02 .12 n.d n.d n.d Total 99.80 100.02 100.14 99.95 99.63 O - F .09 .10

Total 99.71 99.92 Total Fe as Fe2O3 18.58 18.89 19.25 19.70 17.09 Fe203/FeO .35 .27 .26 .26 .25 STRUCTURAL FORMULAE K .172 .153 .259 .260 .230 (W) Na .395 2.42 .370 2.25 .418 2.56 .418 2.49 .418 2.48 Ca 1.849 1.880 1.879 1.807 1.832 Mg 2.269 2.224 2 .002 1.956 2.383 4 (X) Fe -*" 1.590 3.89 1.712 3.97 1.766 3.79 1.804 3.79 1.576 3.98 Mn .031 .038 .022 .025 .022 Al .524 .549 .545 .557 .553 (Y) Fe+++ .503 1.22 .415 1.14 .405 1.21 .422 1.25 .348 1.17 Ti .192 .178 .258 .268 .273 Si 6.467 6.508 6 .452 6 .423 6.496 (Z) Al 1.533 8.00 1.492 8.00 1.548 8.00 1.577 8.00 1.504 8.00 OH 1.627 1.647 1.639 1.692 1.425 F .103 1.73 .085 1.76 Cl .004 .029 X +Y 5.11 5.11 5. 00 5. 04 s'is

Samples A 19 and A 21 from hornblende-andesine amphibolites with up to 11 per cent quartz, traces of clinopyroxene and biotite. No orthopyroxene in these rocks; andesine sericitic Sample A 22 from an amphibolite without either orthopyroxene or sericite that occurs almost on the orthopyroxene isograd. Samples AED 399 and AED 403 from two pyroxene-andesine amphibolites northeast of and across orthopyroxene isograd from A 19 and A 21

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grade of metamorphism (Engel and Engel, less than half its abundance at Emeryville, but 1960a). it is the only mineral in the system capable of An important result of the progressive de- incorporating sizeable amounts of K, H2O, Ti, crease in OH + F + Cl in hornblende and F, and Cl in its crystal structure. The P and T biotite is the very obvious lack of fit of the conditions at Colton seem to be very near the chemical composition of these minerals with upper limits of the stability field for hornblende their conventionally defined structural for- in these rocks. Reconstitution at more elevated mulae (Table 7). The hornblendes in the T and P commonly results in complete dissolu- Colton area are invariably 20 per cent deficient tion of hornblende and in the further release of in the structural requirements for hydroxyl water, alkalis, and halogens from the rock,

2.2

• Hornblendes, Emeryville area A Hornblendes, Colton area 2.0 Emeryville area

Colton area O I L /•l 1.9 o 334* o ^342 O - -

338 1.8 1.8

0.6 0.7 0.8 Na + K Figure 3. Plot in atoms per unit cell showing the relations of Ca to Na + K in hornblendes from amphibolites of the Emeryville and Colton areas, New York

plus halogens. This is coupled with the pre- either as highly volatile fluids or in an incipient viously noted progressive increase in total anatectic magma. value of the large cation structural position Color and Pleochroism (Table 7). Possibly K2O replaces H2O or K+ replaces H+ in charge, but not position. Proba- Hornblendes from amphibolites at Emery- bly some H and O are lost from the rock sys- ville are consistently bluish green, whereas tem. those in the amphibolites of the Colton area The changes in H2O, F, Ti, Cl, K, and Na are brownish green. Pleochroism is as follows: in the hornblendes with increasing T and P hornblendes in Emeryville amphibolites, X = are of particular interest with regard to the yellow green, Y = green, and Z = bluish upper limits of the field of stability of horn- green; hornblendes in Colton amphibolites, blendes and changes in the total rock system. X = light brownish yellow, Y = yellow The abrupt decrease in amount of hornblende brown, and Z = brownish green. These dif- and quartz with increasing grade of metamor- ferences are very consistent except in the phism of the amphibolites has, as its counter- "altered" amphibolites at Colton. Therein the part, the increase in amounts of clinopyroxene hornblendes appear as a replacement of and orthopyroxene and plagioclase (Table 1). pyroxenes, especially where the rocks at Colton The plagioclase also becomes more calcic. In are cut by shear zones or invaded by granite or effect, hornblende and quartz are replaced by pegmatite. At these places, pyroxenes are two pyroxenes and a more calcic plagioclase partially or wholly replaced by hornblende and (Table 1). This means that, at Colton, the the rock is retrograded to a hornblende- amount of hornblende is not only reduced to andesine-quartz assemblage very like that in

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TABLE 7. AVERAGE CHEMICAL COMPOSITION, INDEX the Emeryville area. The secondary hornblende OF REFRACTION, DENSITY, AND COLOR OF HORNBLENDES in these retrograded amphibolites at Colton FROM LEAST ALTERED AMPHIBOLITE ROCKS IN THE tends to revert in color and in other physical EMERYVILLE AND COLTON AREAS, NEW YORK and chemical properties toward the Emery- Mean deviations of the various oxides and elements are ville-type hornblende. Samples A 65 and A 8 listed alongside the chemical analyses. illustrate this type of hornblende (Table 5). Similar retrograde changes have been observed Emeryville area Colton area in hornblendes in metamafic rocks from most 7 samples 10 samples other granulite terraines. Mean Mean Mean dev. Mean dev. Density The hornblendes in the amphibolites at SiO 42.46 .54 42.17 .34 2 Colton are denser than those at Emeryville by Ti02 1.32 .05 2.39 .15 A1203 12.53 .51 12.22 .33 about 0.018. Most hornblendes from the am- Fe2O3 4.94 .22 3.28 .48 phibolites at Emeryville have a density near FeO 13.71 .43 14.42 .48 3.260 (Table 2). Hornblendes in the Colton MnO .31 .01 .17 .01 MgO 9.04 .59 9.42 .17 amphibolites have an average density of about CaO 11.64 .17 11.46 .06 3.278 (Table 4). Na20 1.32 .05 1.51 .08 These data were suggested by initial de- K20 1.02 .08 1.41 .14 terminations with the micropycnometer but H 0 + 1.83 .03 1.50 .08 2 have been definitely established and refined P205 .05 .02 .06 .02 F .11* .04 .17t .02 by measurements of relative densities in tall Cl .03* .00 .03t .01 columns of identical heavy liquids. The con- Total Fe sistent increase in density of hornblendes be- as F'e2Os 20.17 .70 19.30 .40 tween Emeryville and Colton accompanies FezOa/FcO .36 .015 .23 .039 an increase in Na + K and Ti, a decrease in ++ Fe + + and to TRACE ELEMENTS (PPM) Fe/Mg, Fe / *. tal OH in the Ba 86 32 106 40 hornblendes. Co 47 3 60 2 The two hornblendes with very different Cr 249 95 737 116 higher densities are samples A 14 and A 9. Cu** 13 5 These are respectively, a sericitic, Fe-rich Cu 17 6 7 4 Ga 18 2 19 2 amphibolite at Emeryville and a garnet- Ni 57 11 76 13 bearing amphibolite at Colton. Both rocks and Pb" 6 1 7 3 constituent hornblendes are uncommon. The Sc 77 2 109 10 hornblendes have an abnormally high total Fe Sr 46 12 42 12 V 527 66 811 81 content and a correspondingly high Fe/Mg, Y 72 22 118 22 features consistent with the recorded densities Yb 9 2 13 of 3.323 and 3.328. •Ln" 216 32 163 21 Zr 97 26 75 8 Indices of Refraction Density 3.260 .004 3.278 .. The nx and nx indices of refraction of the hornblendes show remarkably little variation INDICES OF REFRACTION from 1.67 and 1.69 (Tables 2-7). In several ex- "j: 1.668 1.670 HZ 1.689 1.692 amples, as much variation exists between the nz — nx .021 .022 indices of grains taken from a cubic inch of one homogeneous-appearing sample as there is be- PLEOCHROISM tween grains taken from samples 30 miles apart X yellow green light brownish yellow (^ .003). Certainly the measured variations y green yellow brown in chemical composition of the hornblendes be- z blue green brownish green tween Emeryville and Colton do not impart readily measured changes in index of refraction. Presumably the variations in composition im- * Mean derived from four determinations t Mean derived from seven determinations part mutual compensation in indices. ** Colorimetric analyses; analyst, H. L. Nieman. All The complexities inherent in useful correla- other trace-element analyses are quantitative spectro- tions between indices of refraction and composi- graphic analyses by R. G. Havens. tion of hornblendes are becoming increasingly

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apparent (Engel, 1959, p. 973; Wilkinson, blende with either more than 2.5 or less than 1961, p. 349). Conventional determinations of 1.5 units of OH+F+C1. Curiously, there is optical properties of hornblendes are no sub- very little published comment regarding these stitute for a chemical analysis. discrepancies in the amounts of water and halogens in amphiboles. Many determinations COMPARISON WITH METAMORPHIC of H2O are made with the Penfield tube, using HORNBLENDES FROM OTHER a flux of lead oxide. The PbO is strongly del- TERRANES iquescent. Undoubtedly, some analyses with Comparisons of the hornblendes from the high-water content are due to a wet flux, amphibolites in the Emeryville-Colton region whereas most analyses that report less than 1.0 with hornblendes from other metamafic rocks weight per cent f[2O reflect too-low roasting and analyzed in other laboratories are compli- temperatures and an incomplete yield. cated by two major problems. One of these re- Enumerations of other analytical errors sults from the several sources of error inherent could be expanded to include all elements de- in conventional analytical work, as well as termined. The essential point is that until, and errors induced by impure samples. The other unless, the precision and accuracy of mineral problem stems from the fact that mineralogical analyses are improved, especially between studies are rarely complemented by adequate laboratories, it will be impossible to consider data on the composition, field occurrence, and seriously many of the comparisons presently origin of the total rock. attempted. The inaccuracies of rock analyses have been Although the facts cited above are occasion- pointed up by the statistical studies of the ally recognized, much recent mineralogical standard rocks G-1 and W-l (Fairbairn and work and most attempts to circumvent con- others, 1953; Stevens and others, 1961). ventional analytical procedures have employed Errors in analyses of minerals are undoubtedly even less precise techniques. A deplorable ex- larger, principally because of the more asym- ample is the use of the emission spectrograph in metric distribution of elements in many min- the analysis of major elements in minerals. erals. The very high concentrations of Fe in Almost without exception this analytical work hornblendes illustrates this fact. If conven- is too fragmentary and, in many instances, it tional analytical methods are employed, in- verges on the useless because of the imprecise accuracies in the determination of total Fe are method used. X-ray fluorescence analyses also directly reflected in complementary errors have been tested for both rock and mineral in Al 20s. Moreover, errors in the determina- analyses (Chodos and Engel, 1961a; 1961b). tion of total Fe are matched, if not exceeded, The technique employed results in partial by the errors made in determining FeO. The analyses that require the use of working curves result is a serious discrepancy in the important based upon conventional analytical techniques. ratio Fe2O3/FeO, for conventionally Fe2O3 is Some of the partial analyses obtained by X-ray obtained by difference; that is, the amount of fluorescent methods are very precise, but about FeO is determined directly and subtracted 10 per cent of the numbers are of marginal from total Fe. What is left is calculated as value. Frequent checks by gravimetric methods Fe2Os in the hornblendes. are desirable. Large errors in opposite directions have been Unfortunately, too few mineralogists and produced by very different causes. Low de- geologists who use analytical data either do the terminations of FeO are frequently due either analyses themselves, or understand the limita- to the difficulties encountered in getting all the tions and potentials of the methods involved. mineral into solution or to oxidation of addi- This separation between geologist and analyst tional iron during analyses. Some abnormally is sufficiently great that the analyst is not told high values for FeO have resulted from the of the special problems he must expect from fact that the number depends upon a precise the extreme range of elements. In effect, the colorimetric titration. analyst may be using a technique devised to The determination of plus H2O in minerals analyze granites and basalts for minerals with like hornblende is also a source of important very different concentrations and configura- error. The conventionally employed structural tions of the elements. It is natural that analyti- configuration of hornblendes requires essen- cal complications result, and too frequently tially two units of OH+F+C1. Yet it is quite these difficulties cannot be resolved, either with common to find published analyses of horn- the remaining sample or in the available time.

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It is possible at present to evaluate the pre- pelitic schists. The Black Hills terrane is one of cision of analyses relative to such factors as typical regional metamorphism and has been analyst, technique, and time. This means that studied in some detail by Noble and Harder internal comparisons can be undertaken of (1948) and by Dodge (1942). The grade of trends, or variations, in composition of the metamorphism increases progressively from a hornblendes with: (1) degree of metamorphism, biotite zone through a garnet and staurolite (2) composition of host rock, and (3) related zone. The amphibolite interlayers show a pro- geological features; but the absolute amount of gressive change from epidote-bearing amphib- many elements remains equivocal as do comparisons with data from other laboratories. TABLE 8. AVERAGE PROPERTIES OF HORNBLENDES The lack of really good mineral standards, in FROM AMPHIBOLITES IN THE BLACK HILLS, SOUTH this instance a hornblende standard, makes it DAKOTA, COMPARED WITH PROPERTIES OF HORN- especially difficult to evaluate absolute de- BLENDES FROM AMPHIBOLITES IN THE EMERYVILLE- terminations of HaO+j FeO, Fe2Os, and COLTON REGION, NEW YORK AUOs. It appears possible to measure rather Black Hills, Adirondack accurately the total H and O in hornblendes South Dakota Mountains, by, respectively, isotope dilution techniques Lead, NE. Lead, New York and the mass spectrometer. These appear to be S. Dakota S. Dakota Colton the next steps necessary to verify, or correct, Epidote- Almandine- imeryville Horn- the data on H, O, and the ratio Fe2O3 obtained amphi- amphi- Mmandine- blende in hornblendes by conventional gravimetric bolite bolite amphibolite granulite techniques. Measurement of absolute amounts fades* faciest facies facies (400°C) (500°C) (525°C) (625°C) of Al in hornblendes, pyroxenes, and thus the

ratio Al/Si, remains a challenging problem that Si02 44.95 45.09 42.46 42.17 must be solved before many of the presently Ti02 .43 .56 1.32 2.39 attempted conjectures can be evaluated. A12O3 13.73 13.30 12.53 12.22 Analogous problems exist in comparisons, Fe2O3 1.85 1.84 4.94 3.28 FeO 12.53 13.55 13.71 14.42 between laboratories, of optical data, color, MnO .21 .25 .31 .17 density, and the other measurable, meaningful MgO 11.22 10.13 9.04 9.42 properties of minerals. CaO 11.15 11.67 11.64 11.46 Na2O 1.18 .97 1.32 1.51 VARIATIONS IN PROPERTIES OF K2O .25 .44 1.02 1.41 H2O+ 1.99 1.92 1.80 1.50 HORNBLENDES INDUCED BY RE- F .03 ,10 .11 .17 GIONAL METAMORPHIC GRADIENTS Cl .01 tr .03 .03 Hornblende frequently is referred to as a Total Fe "sponge" capable of, and actually undergoing, asFe2O3 15.77 16.89 20.17 19.30 wide variations in composition with changing X pale yellow yellow yellow brownish green green green yellow conditions of regional metamorphism (Ram- brownish berg, 1952, p. 65). These comments appear Z blue green blue green blue green green valid with reference to common hornblendes nt 1.661 1.664 1.668 1.670 formed in rocks of highly diverse composition, Density 3.19 3.20 3.260 3.278 recrystallized over a wide range of T and P. TRACE ELEMENTS (ppm) The hornblendes extracted from the amphib- Ba 9 18 86 106 olites of the Emeryville-Colton region offer a Co 31 45 47 60 striking contrast, being relatively unresponsive Cr 102 129 249 737 Cu 9 10 17 7 to the recorded regional gradients in T, P, Ni 85 92 57 76 andX. Sc 29 33 77 109 This fact prompted the authors to suggest to Sr 5 31 46 42 B. Raychaudhuri that he complement the V 185 244 527 811 Adirondack study by investigating horn- Y 12 11 72 118 Zn 104 123 216 163 blendes from amphibolite rocks having about Zr 17 32 97 75 the same composition as those at Emeryville, but reconstituted at lower temperatures and * Black Hills samples 1, 15, 13 (Raychaudhuri, 1958, pressures. In the Lead area, Black Hills, South Ph.D. thesis, Calif. Inst. Technology) Dakota, as in the northwest Adirondacks, the t Black Hills samples 69, 70, 71, 72 (Raychaudhuri, amphibolites form interlayers in Precambrian 1958, Ph.D. thesis, Calif. Inst. Technology)

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olites to garnetiferous hornblende-andesine- range of T and P that essentially spans the quartz amphibolites of the almandine amphib- amphibolite facies of metamorphism (B.Ray- olite facies (B. Raychaudhuri, 1958, Ph.D. chaudhuri, 1958, Ph.D. thesis, Calif. Inst. Tech- thesis, Calif. Inst. Technology). The range in nology).1 Columns 3 and 4 of Table 8 are partic- T seems to be on the order of 125° C, from ularly significant in that they show a compari- -> 400° C in the biotite zone to ^ 525° C in son of the average features of hornblendes from the almandine amphibolite zone. Thus, studies amphibolites in identical metamorphic zones, of the Black Hills amphibolites and constituent in the Black Hills and Adirondack terranes. hornblendes offer one means of evaluating the The average Black Hills hornblende formed in compositional behavior of hornblende in ap- amphibolite rock at the almandine-amphibolite proximately the same mafic rock type over a grade of metamorphism contains slightly more composite terrane and range of regional Si, Al, and Fe and less K and Ti than its metamorphic conditions, extending from the Adirondack counterpart. But these same small epidote amphibolite facies (about 400° C at differences also exist in the composition of the Lead, South Dakota) to the hornblende- host rocks in the two regions. granulite facies (about 625° C at Colton, New Apparently hornblendes from these metamafic York). rocks are not extremely sensitive to wide varia- Because Raychaudhuri's data on the Black tions in T and P, or to the kinds and amounts Hills hornblendes are extremely pertinent to of associated mineral phases. On the other hand, this discussion they are summarized in Table 8, moderate changes in chemical composition of with average properties of the hornblendes the host rock do seem to be reflected in ap- from the Emeryville-Colton amphibolites. propriate changes in composition of the con- Comparisons are facilitated because the horn- stituent hornblende. If the Adirondack studies blendes used by Raychaudhuri in his work and are considered individually it is noteworthy all of those in the Emeryville-Colton area have that the hornblendes are less responsive to been reanalyzed by both wet and X-ray systematic changes in T and P of meta- fluorescent techniques using the Adirondack morphism than are garnet, biotite, and plagio- hornblendes as standards (Chodos and Engel, clase feldspar in the enclosing paragneiss. 1961b). No serious discrepancies between the The question occurs, are the Black Hills and two groups of analyses were found. Adirondack examples typical? Search of the The characteristics of "average" horn- literature leaves the question unanswered. The blendes from the Black Hills and Adirondack pyroxene-bearing amphibolites at Colton and regions listed in Table 8 indicate the mean their constituent hornblendes have almost exact characteristics of: (1) three hornblendes from duplicates in the granulite terranes of North epidote-bearing amphibolite rocks near Lead Africa (Howie, 1958, Tables 1, and 2), as in the Black Hills sequence (epidote-amphib- shown in Table 9. Other similar hornblendes olite facies), (2) five hornblendes from amphib- and host rocks in the hornblende-granulite olite rocks north of Lead, South Dakota facies seem to exist in the major charnockitic (almandine-amphibolite facies), (3) seven horn- and granulite terranes of the world (Parras, blendes from the amphibolites at Emeryville, 1958; Engel and Engel, 1962). northwest Adirondack Mountains (almandine- There are innumerable published reports of amphibolite facies), and (4) 10 hornblendes hornblende-andesine-quartz amphibolites very from the pyroxene-bearing amphibolites at like those in the Black Hills and in the vicinity Colton, New York (hornblende-granulite of Emeryville. Many of these amphibolites facies). The data in Table 8, from opposing ends appear in belts of progressive regional meta- of the belt of progressive metamorphism in the morphism, but adequate data on geological Black Hills, coupled with analyses of horn- occurrence, composition of the host rock, and blendes at intermediate sites in the Black Hills properties of constituent hornblendes do not terrane may be summarized as follows: al- exist. Studies of geologists at the University of though there are significant differences in Tokyo on amphibolites of the Abukuma composition of Black Hills hornblendes, there are no statistically verifiable, systematic varia- 1 The differences that appear in the average specimens tions in chemical composition, indices of re- of columns 1 and 2, Table 8, are due to the small sample fraction, density, or lattice constants that may size and to rather large in-group variations. Data on be correlated with progressive metamorphism intermediate sites indicate that no systematic, regional of the Black Hills amphibolites, formed over a trend exists.

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TABLE 9. COMPARISON OF AVERAGE PYROXENE- Plateau are of interest in this respect (Shido, BEARING AMPHIBOLITE AND CONSTITUENT HORN- 1958; Miyashiro, 1958; Shido and Miyashiro, BLENDE, COLTON, NEW YORK, WITH SIMILAR ROCK 1959). The Abukuma terrane appears to be AND CONSTITUENT HORNBLENDES FROM THE LAFIT most complex however, and some of the rocks MOUNTAINS, SUDAN, AFRICA are either polymetamorphic or so complicated by structural elements and granite that Amphibolite rock Hornblende studies are not entirely definitive. Investigations of hornblendes in metamafic Colton, Lafit Colton, Lafit rocks in the Ukrainian Shield by Siroshtan and N. Y. Mtns. N. Y. Mtns. Polovko (1959) include a discussion of compositional changes related to parent rock type and CHEMICAL CHEMICAL O'Hara (1961) has recently published com- ANALYSES ANALYSES plete mineralogical and chemical compositions (WEIGHT (WEIGHT of an amphibolite rock and a constituent horn- PERCENT)* PER CENT)t blende essentially identical to those at Emery- Si02 47.89 50.84 42.17 41.96 ville. There is also the work of Wilcox and Ti02 1.56 1.04 2.39 2.10 Poldervaart (1958) in North Carolina, Ward 14.63 14.36 12.22 13.06 A1203 (1959) and Rosenzweig and Watson (1954) in Fe2O3 1.85 3.98 3.28 3.20 FeO 11.20 9.64 14.42 14.12 Pennsylvania, Subramaniam (1956) in Madras, MnO .25 .19 .17 .16 Buddington (1952) and Buddington and MgO 7.41 5.17 9.42 8.98 Leonard (1953) in the Adirondacks, Eskola 11.54 10.66 11.46 11.72 CaO (1952) and Parras (1958) in Finland. These Na2O 2.19 2.85 1.51 1.46 K20 .58 .41 1.41 1.48 studies offer interesting, but frequently contra- H20+ .72 .26 1.50 1.71 dictory observations regarding variations in Total Fe the properties of mafic rocks and of constituent as Fe2Os 14.29 14.69 19.30 18.89 hornblendes in metamorphic terrances. Fe2O3/FeO .17 .41 .23 .23 One of the most widely discussed variations in composition of metamorphic hornblendes is MODAL that of Al and Si, or in effect, the Al/Si ratio. ANALYSES (VOLUME Commonly, Al is inferred to replace succes- PER CENT) sively larger amounts of Si in the tetrahedral Quartz .1 7.0 position of hornblendes with increasing tem- Plagioclase 35.6 38.0 perature of metamorphism, and the excess Al is Hornblende 31.3 20.2 thought to spill over into the (Mg, Fe)s posi- Clinopyroxene 19.2 17.4 tion (Ramberg, 1952, p. 65; Siroshtan and Orthopyroxene 11.3 13.1 Opaque minerals 2.0 2.6 Polovko, 1959, p. 54; Seitsaari, 1956, p. 43; Others .5 1.7 Harry, 1952; Foslie, 1945, p. 87-98; and DeVore, 1955, p. 159-161). The crystal struc- NORM ture of hornblende seems to permit this change, (C.I.P.W. and as Ramberg notes, scattered observations SYSTEM) may be so interpreted (1952, p. 65). It is clear, Quartz 1.61 however, from inspection of Table 8, that Orthoclase 3.45 2.45 neither the Adirondack studies nor those of Albite 18.50 24.10 Anorthite 28.36 25.16 Raychaudhuri in the Black Hills support this Diopside 23.03 21.97 interpretation for hornblendes in amphibolitic Hypersthene 8.54 16.35 rocks. The data on Abukumba hornblendes Olivine 11.29 compiled by Shido (1958) also indicate no Magnetite 2.69 5.78 Ilmenite 3.01 2.01 change in Al with advancing metamorphic Apatite .34 .37 grade. Very possibly the significant variations in Al in metamorphic hornblendes are related to distinct variations in Al in the host rocks. * Chemical compositions for amphibolite rock from Increases in Ti content of hornblende with Colton, N. Y., represent average composition of nine samples of pyroxene-bearing amphibolite rocks increasing grade of metamorphism are noted ^ Chemical compositions for hornblende from Colton, by Shido (1958, p. 183), Ramberg (1952, p. N. Y., represent average composition of 10 constituent 69), and other investigators. This change clearly hornblendes exists in the Adirondack hornblendes and ap-

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pears to be characteristic wherever adequate Ti the OH+F+C1 group clearly decrease (Table is available in the rock system. Increases in Ti 7). It is notable that average data from other content commonly are inferred to be a princi- terranes show the same trend; most meta- pal reason for the change in color of horn- morphic hornblendes formed in rocks of the blendes from blue green to brownish green hornblende-granulite facies are water-deficient with increasing grade of metamorphism. It is and have Fe2O3/FeO less than O.3. Analyses apparent, however, that there is no simple re- of hornblendes in amphibolite rocks of inter- lation between the amount of Ti and the color mediate metamorphic grade commonly show of hornblendes2 (Barnes, 1930; Deer, 1938; much higher ratios of FezOs/FeO and enough Wilkinson, 1961, p. 340-341). H2O+ to satisfy the structural formula. The Problems of accurately evaluating the oxida- occasional statement that Fe2Os/FeO in horn- tion state of Fe and the amount of hydroxyl blendes increases with increasing grade of meta- in hornblendes have already been noted. Sev- morphism appears to have confused the char- eral recent studies have raised further questions acteristics of this mineral formed at elevated regarding the role of O and H during regional T and P with the so-called oxyhornblendes or metamorphism (Chinner, 1959; Greenwood, basaltic hornblendes. These have very high 1960). For reasons already stated it seems un- Fe2Oa/FeO but are hydroxyl deficient as realistic to attempt comparisons between the might be expected from the near-surface en- existing data from diverse terranes on Fe2C>3/ vironmental conditions in which they have FeO and the H2O+ in hornblendes or their formed (Wilkinson, 1961). host rocks. The Adirondack studies suggest Several investigators have either inferred or that both gneisses and amphibolites and the affirmed that a large increase in Mg/Fe in constituent micas and amphiboles are progres- hornblendes occurs with increasing grade of sively stripped of H (and O?) with increasing metamorphism. Ramberg has noted however grade of metamorphism. The Fe2O3/FeO and that appropriate variations in bulk composition of the host rock must be involved (1952, p. 2 Many samples of hornblende contain ilmenite or 159). Short of these changes there is less change sphene either as inclusions in the hornblende or as in Mg/Fe in hornblendes than in the biotites attached grains. It is not clear whether some, or all, of and garnets formed in associated pelitic rocks the Ti in the inclusions has exsolved from solid solution with the hornblende after the apical conditions of meta- (Engel and Engel, 1960, p. 18-46). Again, the morphism. This and analogous features in the mineral data suggest that hornblende is a responsive assemblage plague all investigators as to the partitioning "sponge" only when the metamorphism in- of elements between "coexisting" phases. volves major changes in rock composition.

REFERENCES CITED Barnes, V. E., 1930, Changes in hornblende at about 800° C: Am. Mineralogist, v. 15, p. 393-417 Buddington, A. F., 1952, Chemical petrology of some metamorphosed Adirondack gabbroic, syenitic and quartz syenitic rocks: Am. Jour. Sci., Bowen Volume, p. 37-84 Buddington, A. F., and Leonard, B. F., 1953, Chemical petrology and mineralogy of hornblende in northwest Adirondack granitic rocks: Am Mineralogist, v. 38, p. 891-902

Chinner, G. A., 1959; The effect of varying oxygen content in a natural rock sequence: Carnegie Geophys. Lab. Ann. Rept., 1958-1959, p. 104-106 Chodos, A. E., and Engel, Celeste G., 1961a, Fluorescent X-ray spectrographic analyses of amphibolite rocks: Am. Mineralogist, v. 46, p. 120-133 1961b, Fluorescent X-ray spectrographic analyses of amphibolite rocks and constituent hornblendes, p. 401-413 in Advances in X-ray analysis: New York, Plenum Press, v. 4, 526 p. Deer, W. A., 1938, The composition and paragenesis of the hornblendes of the Glen Tilt complex, Perth- shire (Scotland): Mineralog. Mag., v. 25, p. 56-74 DeVore, G. W., 1955, The role of adsorption in the fractionation and distribution of elements: Jour. Geolo- gy, v. 65, p. 159-189 Dodge, T. A., 1942, Amphibolites of the Lead area, northern Black Hills, South Dakota: Geol. Soc. Amer- ica Bull., v. 53, p. 561-584 Engel, A. E. J., 1949, Studies of cleavage in the metasedimentary rocks of the northwest Adirondack Moun- tains, New York: Am. Geophys. Union Trans., v. 30, p. 767-784

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