DAVID M. SCOTFORD Miami University, Oxford, Ohio

Metasomatic Augen Gneiss in

Greenschist Facies, Western Turkey

Abstract: A roughly circular 8-sq km exposure of coarse orthoclase augen gneiss occurs with steeply dipping, narrowly__gradational contacts in phyllites and schists of greenschist facies metamorphic grade near Odemis, western Turkey. Structural, stratigraphic, or intrusive ex- planations of this occurrence of augen gneiss, a rock type typically associated with only the highest grade metamorphic rocks, in such a low-grade environment seem unsatisfactory. Petrographic evidence of the replacement of sodic plagioclase by potassium feldspar at the borders of the body points to a metasomatic origin in which the phyllites and schists have been converted to augen gneiss locally through the effect of steeply ascending fluids. Compositional and structural state determination of the potassium feldspar augen by optical and X-ray diffraction techniques, employing a least-squares cell refinement computer method, indicates that all the potassium feldspar specimens investigated lie in the orthoclase range, but range in structural state from barely monoclinic to just below the low sanidine limit, and have compositions which fall within 5 percent of 85 mole percent Or. Implications are drawn from the structural state of the potassium feldspar augen and the mineral paragenesis of gneiss and associated phyllites and schists as to the temperature environ- ment extent during the metasomatic event. A temperature of 500° C ± 50° is estimated. This lies within the 400° C to 550° C range suggested by Winkler (1967, p. 174) for the greenschist facies. Thus the augen gneiss appears to have been produced in the environment encompassed by that facies.

CONTENTS Introduction 1080 4. Potassium feldspar replacement of plagioclase Acknowledgments 1082 in plagioclase crystals 1089 Feldspar analytical methods 1082 5. Potassium feldspar replacement of plagioclase Sample preparation 1082 along edge of crystal 1089 X-ray diffraction methods 1082 6. Potassium feldspar replacing plagioclase on both Optical methods 1083 sides of a fracture 1090 Stratigraphy and petrography 1083 pja( Geographic and geologic setting 1083 e . Stratigraphy 1083 1. Augen gneiss and bordering phylhte . . . 1 Plate Phyllites and quartzose schists 1084 / Section Biotite, quartzite, and quartzose schists . . . 1084 Table Gneiss and augen gneiss 1084 1. Stratigraphic sequence north of major fault . . 1083 Quartzitic biotite schists 1085 2. Stratigraphic sequence south of major fault . . 1084 Structural description 1085 3. Measured modal analyses of the schist bordering Analytical results 1086 the augen gneiss 1084 Feldspar structural state 1086 4. Measured modal analyses of the augen gneiss Comparative petrography of the augen gneiss (Gi) 1085 and associated rock 1088 5. Summary of direct cell refinement of feldspars 1086 Conclusions 1090 6. Feldspar composition in percent of Or . . . . 1087 References cited 1093 7. Structural state of feldspar samples compared with b and c values of Wright and Stewart 1088 Figure 8. Structural state of feldspar by comparison of 2V 1. Index map of western Turkey 1080 values with Wright and Stewart's alkali ex- 2. Geologic map of area south of Odemis, Turkey 1081 change specimens 1088 3. b and c plot of Wright and Stewart with data 9. Calculated chemical analyses of the augen gneiss from Table 5 1087 and bordering schist 1090

Geological Society of America Bulletin, v. 80, p. 1079-1094, 6 figs., 1 pi., June 1969 1079

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INTRODUCTION lated problems remained unresolved after mapping was completed. The applicability of the structural state of The first problem involved the relative dis- potassium feldspar to geothermometry has placement along a major north-dipping, east- been suggested by several investigators (for striking fault (Fig. 2). The hanging wall on example, Steiger and Hart, 1967). In the the north side of the fault consists of well- present application an attempt is made to use crystallized staurolite zone schist, marbles, and such structural state data to elucidate the quartzite underlain by gneiss and augen gneiss, metamorphic environmental conditions in an whereas the foot wall to the south is composed unusual occurrence of a coarse potassium of phyllites and quartz-mica schist of low- feldspar augen gneiss in an upper greenschist metamorphic grade. Considering the fault to facies environment. be normal requires the higher grade meta- This study is an outgrowth of a field in- morphics to have been displaced downward vestigation made in west-central Turkey (Fig. and the low-grade rocks, upward. This ap- 1). A 198-sq km area of medium- to low-grade parently unrealistic conclusion seems to be metamorphic rocks including some hydro- supported by the relative movement as indi- thermal deposits containing sulfides of mercury cated by slickensides and the much lower and other metals was mapped by the writer on topographic level of the hanging-wall block a scale of 1:25,000 for the Mineral Research north of the fault, which includes the regionally and Exploration Institute of Turkey. Although extensive Little Menderes Valley. a broad solution to the structural configuration The second problem was encountered 4 km of the area was not difficult, two possibly re- south of the fault where a roughly circular 8-sq

A 0 K SEA

TZXRAUEAM S£A

Figure 1. Index map o£ western Turkey.

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Legend \M Zone of hydrothermal alteration

Alluvium

Landslide debris

Garnet, staurolile schisls, and quorlzites

Marble E Quartzite

Gneiss, and augen gneiss

Quartzitic biotite schists

Augen gneiss within phyllitic schists I Phyllitic schisls with quartz schist facies

\ Bedding 22 \ Fotialion / Lineation Contact

_. Gradational contact

^ i. Fault and dip direction

A. A Covered fault

/£ ft7 Fault with unknown direction of dip

IOOO 2000

Figure 2. Geologic map of area south of Odemis, Turkey. Scale in meters.

km exposure of coarse augen gneiss (Fig. 2) much lower grade rocks with no evidence of occurs in normal contact with greenschist facies structural disturbance at the contact, seemed rocks. The existence of augen gneiss, widely to be of unusual interest. If, before faulting, considered a product of the most intense this augen gneiss was actually a part of the metamorphic environment, surrounded by mass of gneiss and augen gneiss now exposed in

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the hanging wall to the north of the major specific gravity of 2.60 by dilution with N, fault, then a downward displacement of the N-Dimethylformamide (Hickling and others, hanging wall seems more reasonable. This 1961, p. 1502). After centrifugation for one interpretation would require the lower grade minute at 2000 rpm, as suggested by Schoen rocks to the south of the fault to have under- and Lee (1964, p. B155), the suspension was gone retrogressive changes not affecting the agitated with a glass rod to separate the small associated augen gneiss which remains as a ball of light material which commonly formed relict of the former higher grade environment. near the top of the liquid, and the centrifuga- If, however, the augen gneiss in the low- to tion was repeated for the same time. The heavy medium-grade terrain south of the major fault liquid and suspended light minerals were de- is a discrete body, it represents an unusual oc- canted onto filter paper. This procedure pro- currence and requires further explanation. duced a strong concentration of potassium-rich The problem was approached through feldspar, as indicated by the X-ray patterns, in petrographic study of the augen gneisses with which the only other mineral detectable was a particular emphasis on their potassium feld- small amount of quartz in a few samples. In spars. The optic angle was determined by the these samples, however, the quantity of quartz direct universal stage method, and the degree was too small to interfere with the feldspar of triclinicity, structural state, homogeneity, pattern. and potassium-sodium content was determined for each specimen using the methods described X-ray Diffraction Methods below. Normal X-ray diffraction patterns, using CuKtx radiation, were obtained from the feldspar concentrate to determine the degree ACKNOWLEDGMENTS of homogeneity of the feldspar and its sym- The Mineral Exploration and Research In- metry. The presence of the (002) albite peak at stitute of Turkey provided the author with about d = 4.030 A along with the potassium field transportation and a field assistant, Erol feldspar pattern is indicative of a perthitic Basarir, while he was in Turkey under a condition. The lack of this peak indicates a Fulbright Lecture Grant at Ege University homogeneous potassium-rich feldspar or a and a research grant from Miami University, perthite in which the albite phase is present in fames E. Bever, Robert R. Compton, William low concentration. C. Luth, and Hal T. Morris reviewed the The symmetry of the feldspar is indicated by manuscript and made suggestions for its im- the presence of the (131) reflection in the provement. triclinic microcline and its absence in the monoclinic orthoclase. For identification of microcline and orthoclase, the reflections be- FELDSPAR ANALYTICAL METHODS tween (201) at about 21° 26 and (132) at about 32° 26 were examined. If present, the (131) Sample Preparation reflection occurs at about 29.4° 26 as a separate In order to obtain a separation of potassium peak at a slightly higher angle than the (131) feldspar pure enough for X-ray study, the rock peak, or as a bifurcation of the (131) peak. was crushed and passed through a 200-mesh The degree of "triclinicity" of the microcline screen. The sample was allowed to settle for as defined by Goldsmith and Laves (1954, p. 5) two minutes in a beaker filled with water to a is indicated by the separation of (131) and depth of 10 cm and then decanted. The remain- (131) peaks. The measure of triclinicity, A, is ing sediment having a size range of 30 to 75 n 12.5[W(131) - 4131)]. However, if the amount was found to be small enough to allow separa- of microcline in the potassium feldspar is less tion of discrete particles of plagioclase from the than 25 percent, this measurement cannot be potassium-rich alkali feldspar and large enough made (Steiger and Hart, 1967, p. 93). to be caught on no. 1 filter paper following The sodium and potassium content of the centrifugation. The sediment was dried and feldspar was determined using Orville's (1958) washed with anhydrous acetone to eliminate method and his more recent determinative lumps. The sample was then separated into curves (Orville, 1967, p. 75). As most samples approximately four half a gram units, and each appeared on optical examination to be slightly unit placed in 10-ml polyethylene centrifuge perthitic, all samples were homogenized by tubes filled with bromoform adjusted to a heating at 900° C for 48 hours. The samples

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were then mixed with KBrOa and X-rayed. STRATIGRAPHY AND The 20 angle between the (101) reflection of PETROGRAPHY the KBrO3 internal standard and the (201) reflection of the homogenized feldspar was Geographic and Geologic Setting slow-scanned six times. The sodium-potassium The mapped area (Fig. 2) lies directly south content of the feldspar (Table 6) was de- of Odemis in west-central Turkey (Fig. 1). The termined using the appropriate curves of northern portion of the area is the flood plain Orville (1967, p. 75), depending upon the of the Little Menderes River having an eleva- monoclinic or triclinic symmetry of the speci- tion of about 100 m. To the south the topog- men. In addition, the composition versus cell raphy rises in largely deforested hills and moun- volume plot of Wright and Stewart (1968, Fig. tains reaching a maximum elevation of 1600 1) for orthoclase was used, as this plot takes the m. Jeep roads and foot paths provide access known structural state of the feldspar into to the entire area. account and also because most of the feldspars Geologically, the area lies within the com- proved to have anomalous cell dimensions. plex of crystalline rocks known as the northern To determine the unit-cell parameters of the Menderes Massif (Philippson, 1911, p. 82) feldspar, each sample was combined with which extends as an uplifted mountain mass in finely ground and annealed (48 hours at an easterly direction for several hundred 700° C) CaF2 (a = 5.4620). Three X-ray kilometers. powder patterns were run for each sample from low to high angles at a scan rate of 1/5° per Stratigraphy minute. The 20 angles were measured by using The rocks in the mapped area are readily the estimated center line from the upper tenth divided into two groups—those exposed to the of each peak and were corrected by comparison north of the major fault which marks the with the (111) and (220) X-ray reflection of boundary between the mountainous terrain to the CaF2 internal standard. Only values ob- the south and the lower hills which border the tained from those peaks which varied by less south side of the valley of the Little Menderes, than 0.03° 26 among the three runs and the and those exposed to the south of this fault in upper tenth of which covered 1° 20 or less were the more mountainous portion of the area. used in determining the unit-cell parameters. The northern group (Table 1) consists of a Between 12 and 14 such values were obtained clearly defined sequence of metamorphic rock for each sample in the range from 13° to 60° units overlying a basal gneiss of unknown 28. The mean values of the acceptable re- thickness. On the basis of the discovery of a flections were indexed using the methods of Lower Jurassic (Lias) algal-bearing unit in the Wright and Stewart (1968, Table 11), and the upper marble of the Menderes rocks and a unit cell parameters were determined using a similarity of sedimentary fades below this least-squares unit-cell refinement computer marble with rocks of known age, R. Brink- program originally written by Evans and others mann (1966, p. 612) suggested that the upper (1963), rewritten into FORTRAN IV by Apple- marble units and associated rocks are upper man and Handwerker, and supplied to the writer by David B. Stewart. The program was run on an IBM 360 computer. TABLE 1. STRATIGRAPHIC SEQUENCE NORTH OF Optical Methods MAJOR FAULT The optic angle of each feldspar was meas- Lithology Thickness ured by the direct-extinction universal stage method using thin sections cut from the rock Calc.-schist with marble and sample. One to four determinations, depending mica schist units 150 m ± upon the presence of usable crystals, were made Marble 10 to 15 m from each thin section. Each measurement was Garnet, staurolite schist and performed four times at each of the positions quartzite 75 to 100 m 45° off extinction. Marble 50 m Modal analyses were obtained by the point- Quartzites and mica schists 40 m count method and where in doubt, the Augen gneisses, lineated gneisses presence of potassium feldspar was verified by and other gneisses unknown sodium cobaltinitrite staining.

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Paleozoic to Jurassic, whereas the middle por- TABLE 3. MEASURED MODAL ANALYSES OF THE SCHIST tion of the section is lower Paleozoic and the BORDERING THE AUGEN GNEISS basal gneiss, Precambrian. He further stated that the metamorphism of the group took Specimen Number place in mid-Mesozoic time, basing his con- clusions on the existence of an unconformity Mineralogy 33 43 46 between the Menderes marbles and overlying Quartz 41 42 51 unmetamorphosed Cretaceous limestones. Plagioclase (Abso) 19 20 15 The stratigraphic sequence south of the Biotite 29 31 28 major fault (Table 2) constitutes a much less Garnet 11 6 Sericite 3 easily defined group whose relationship to those Tourmaline 1 north of the fault remains obscure. Unlike the Chlorite 3 sequence to the north, it contains no marble and is less highly metamorphosed. Because Total 100 100 100 these rocks relate directly to the problem of the origin of the augen gneiss with which they are associated, their petrography is described briefly below. Biotite, Quartzite, and Quartzose Schists These rocks occur as a facies included within TABLE 2. STRATJGRAPHIC SEQUENCE SOUTH OF the above unit. They are exposed in the south- MAJOR FAULT western portion of the area on the flank of the large anticline the axis of which lies south of Lithology Thickness the mapped area. These rocks apparently grade laterally into the phyllitic rocks described Quartzitic biotite schist more than 200 m above in the southeastern portion of the area. Biotite schist, augen gneiss The rock is a gray massive thin-bedded and other gneiss unknown Phyllite, quartzose schist quartzite to well-foliated gray quartz-mica schist. In addition to being coarser grained than the tan phyllite, it contains biotite, epidote, and garnet indicating a higher metamorphic grade. Phyllites and Quartzose Schists Gneiss and Augen Gneiss These rocks (Fi of Fig. 2) are exposed over a This unit (Gi on Fig. 2) is exposed over a wide area south of the major fault. They are roughly circular, approximately 8-sq km area typically tan, soft, fine-grained quartzose- in the central portion of the mapped area (Fig. phyllites with graphite locally present in thin 2). This surprising occurrence of high-grade layers parallel to the foliation or irregularly metamorphic rock within a low-grade terrain distributed. Some units are more coarsely would seem to necessitate a fault contact be- schistose and some quartzitic. tween the two; however, the writer could The mineralogy is typically quartz, pla- find no physical evidence of faulting at the gioclase, muscovite, limonite, graphite, and contact, and the map pattern does not seem chlorite, with kaolinite and hematite also to allow such an interpretation on geometric occurring in some places. Moreover the parts of grounds. The only evidence suggestive of this unit occurring near the augen gneiss also faulting is the common occurrence near the contain biotite and garnet (Table 3) indicating contact of a fine-grained quartzitic rock which a steep increase in metamorphic grade in the might represent a mylonitic zone, but this rocks adjacent to that body. interpretation is not convincing. Texturally, the rock consists of parallel The rock typically is a brown coarsely elongated angular quartz with small muscovite crystalline augen gneiss (PI. 1, fig. 2). The or graphite plates, or both, aligned between potassium feldspar augen range up to 3 cm in the quartz grains. In all probability the rock maximum dimension. The foliation resulting was originally a siltstone with some graywacke from biotite and small elongate quartz crystals lenses and is now only slightly metamorphosed. wraps around the augen. In some places the

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gneiss is without augen, and in others it shows and quartzose schists in the southern third of a strong mineral lineation. the area. Because, where observed, the The mineral composition of the augen gneiss lithologic contacts parallel the foliation, it is is indicated in Table 4. assumed that the foliation essentially parallels the bedding. As shown on the map (Fig. 2) Quartzitic Biotite Schists the foliation in the southern portion of the This unit (Fs on Fig. 2) lies above the tan area typically strikes easterly and dips 30° to phyllite and is exposed on the mountain south 40° to the north, whereas to the north the of Konakli. Because its upper contact is not attitude of the foliation is quite inconsistent present, its thickness is not known, but it and dips generally at a low angle. As shown by must exceed 200 m. It is a gray, moderately this foliation pattern, the central portion of the coarse-grained quartzitic schist containing area, which is 7 to 8 km wide and extends from quartz 40 percent, chlorite 30 percent, the east to the west border of the map, is un- muscovite 10 percent, potassium feldspar 20 deformed, except for minor low-amplitude percent, and a trace of biotite. The sutured, folding of the foliation. elongate quartz and chlorite parallel the The major fault, which cuts the northern foliation which bends around the somewhat portion of the area, is of regional, extent. It larger elliptical feldspar crystals. strikes between N. 70° E. and due east and dips about 45° to the north; its recognition is STRUCTURAL DESCRIPTION based on a wide (2 to 5 m) gouge zone and the The area consists of three major structural structural and stratigraphic discordance north units (Fig. 2). A major fault traverses the and south of the fault trace. The direction of northern portion in an approximately easterly displacement is somewhat problematical. Be- direction. To the north of this fault is a faulted cause the rocks of the hanging wall to the sequence of gneiss, quartzites, schists, and north are of a distinctly higher metamorphic marbles; to the south, in the middle portion grade (staurolite zone) than those of the foot of the area, is a thick sequence of phyllites, wall to the south (biotite zone), it could be schists, and quartzose schists, which is es- assumed that the hanging wall had moved up. sentially undisturbed except for minor low- However, a normal fault is suggested by the amplitude folds. In the southern portion of the fact that the hanging-wall block is topograph- area, the rocks form the northern flank of a ically lower than the foot wall, and slickensides large east-trending anticlinal fold. observed on the exposed fault trace seem to The presence of a major anticline, the axis of indicate an obliquely downward displacement which trends approximately east-west, lying for the hanging wall. to the south of the mapped area is indicated To the north of the major fault in the area by the attitude of the foliation in the schists of Yegenli, Kazanli, and Balabanli villages,

TABLE 4. MEASURED MODAL ANALYSES OF THE AUGEN GNEISS (Gi)

Specimen Number

Mineralogy 30 37 38 41 42 Quartz 47 37 45 49 45 Plagioclase (Abso) 13 tr 12 tr tr Orthoclase 24 31 21 29 31 Microcline 3 Muscovite 3 8 12 tr Biotite 9 8 4 19 Garnet 1 tr Sericite 4 22 11 Tourmaline tr Chlorite tr tr 6 5 Calcite 1 Total 100 100 100 100 100

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there are several apparently older faults with (specimens 30, 37, and 41) and from the orientation different from that of the major probably Precambrian augen gneiss underlying fault. Here the gneisses, marbles, quartzites, the staurolite-bearing crystalline rocks to the and schists which typically strike northeast north of the major east-west fault (specimens and dip northwest are cut by one northwest- 1, 2, and 9). Two sets of compositional data trending dip fault and two northeast-trending are quoted (Table 6) in percent of Or. One oblique faults. These faults are recognized group of data is derived from the calculated only on the basis of stratigraphic and structural cell volume using the plot of composition discontinuity. No physical evidence of the against volume of Wright and Stewart (1968, fault planes has been found. Because their fault Fig. 1). The other set of data is based on the planes may have been healed by later re- measurement of A 2d (201 feldspar-101 crystallization and because they do not extend KBrOs) and the application of Orville's (1967, across the major fault, all three faults are Fig. 8) curve for sanidine-high albite. The probably older than the major fault. difference in the compositional determinations Lineations are not uncommon particularly by the two methods is small (1 to 3 percent in the higher grade metamorphic rocks north Or) in three cases and large (7 to 8 percent Or) of the major fault. The lineation is a crinkling in three cases. The larger differences could have on the foliation surface of schist, the elongation resulted from the fact that the curves of Orville of quartz and feldspar crystals in the gneiss, are based on a feldspar series of higher struc- or, least commonly, a preferred mineral tural state than those analyzed or that the orientation. There is a fair consistency of analyzed feldspars have anomalous cell di- lineation orientation plunging at moderate mensions using the criteria of Wright and angles to the north-northwest. This orientation Stewart (1968, Fig. 2b), or both. As the error is reported by Schuiling (1962, p. 72) to be is not consistent, however, it is not understood regionally observable in the entire Menderes and may be analytical. Massif. He assigns great importance to it as The mean 2V determinations also are shown an indicator of a north-south elongation relat- on Table 6. A minimum of four and a maximum ing to an erogenic episode of pre-Hercynian of twelve determinations were made from thin age. sections cut from each sample. The maximum range of values in one thin section is 8.1°. Some ANALYTICAL RESULTS crystals show a variation in the value of 2V at different positions in the crystal of as much Feldspar Structural State as 6°. The range of mean 2V values lies within Table 5 is a summary of the direct unit cell that of the orthoclase polymorphic type data of potassium feldspar from the augen (Wright and Stewart, 1968, Table 3). The gneiss occurring within the phyllitic schists variation in 2V within the rock sample and TABLE 5. SUMMARY OF DIRECT CELL REFINEMENT OF FELDSPARS

No. of lines Standard used Speci- 8 Angle Error No. of lines men "(A)* *(A) r(A) Deg. Min. Volume (A8) of20t accepted 1 8.6011 12.9795 7.1929 116 1.230 721.61 ±0.97 0.02115 13/10 ±0.0140 ± 0.0050 ± 0.0024 ± 2.783 2 8.5757 12.9741 7.2120 116 2.570 720.97 ± 0.35 0.01869 14/10 ± 0.0032 ± 0.0042 ± 0.0024 ± 1.850 9 8.5545 12.9679 7.2003 115 59.070 718.02 ±0.52 0.01570 13/10 ± 0.0064 ± 0.0038 ±0.0016 ± 1.787 30 8.5441 12.9889 7.1855 116 1.289 716.63 ±0.46 0.01716 14/14 ± 0.0060 ± 0.0036 ±0.0017 ±2.016 37 8.5738 12.9809 7.1905 115 56.277 719.66 ±0.21 0.00852 12/ 9 ± 0.0026 ± 0.0021 ±0.0010 ± 0.971 41 8.5719 12.9535 7.2066 116 5.008 718.65 ±0.67 0.02068 14/ 9 ± 0.0082 ± 0.0033 ± 0.0032 ±3.941

* CuK a(\ = 1.54178A) radiation used at a room temperature of 22° C. t Unit weight used for all lines.

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TABLE 6. FELDSPAR COMPOSITION IN PERCENT OF OR BASED ON UNIT CELL VOLUME AND THE A29 (201 FELDSPAR—101 KBRO3) AND 2VX Cell Volume A20 Mean Specimen Method Method 2V* Range 1 86 87 61.0 60.1-61.9 2 85 86 60.0 59.2-60.8 9 80 82 60.1 59.4-60.8 30 78 86 57.8 55.8-60.6 37 82 89 51.5 50.3-52.7 41 80 88 60.7 57.7-65.8

crystal reflects principally an inhomogeneity of on Figure 3 for a particular sample plot, will structural state, although confined to the range show that all samples, except 2 and 30, have of structural state found in orthoclase, rather anomalous cell dimensions. According to than compositional variation. This is also the Wright and Stewart (1968, p. 71), such a con- strong implication of the feldspar unit cell data. dition is not uncommon and precludes the Figure 3 is a plot of the b and c cell dimen- use of the a dimension to indicate composition, sions from Table 5 derived by unit cell refine- but probably allows use of the cell volume for ment of the potassium feldspar specimens from this purpose. the augen gneiss and Precambrian gneiss on the Although all of the analyzed feldspars may be Wright and Stewart diagram (1968, Fig. 2b) described as orthoclase, a more precise defini- showing the relationship between these di- tion of their structural state is indicated on mensions and the composition and structural Table 7 which shows the potassium-rich feld- state of the alkali feldspars. The dimensions of spar with which the sample is most closely re- the rectangles representing the plots of a lated on the basis of its b and c plot (Fig. 3). feldspar sample indicate the standard errors The number preceding the type-feldspar name of the calculated lengths of b and c. A compari- is the relative ranking of structural state son of the plotted positions of the feldspar between high sanidine (1) and maximum samples on Figure 3, with the a dimensions microcline (10), as suggested by Wright and shown on Table 5 and the value for a indicated Stewart (1968, Table 8). The feldspars named

7.23 JMaximun mlcrocIlM 7.22 iSptnctr U 7.21 Sptnct— r107 8 0 .Benson 7.20 7.19 7.18 7.I7|0 7.16 ^° 7.15 7.14 7.13 7.12 7.11 7.10 12.95 13.00 13.05

Figure 3. b and c plot of Wright and Stewart (1968, Fig. 2b) with data from Table 5 of this paper added. The numbered rectangles show the positions of the analyzed feldspars (and their standard errors) as to structural state and composition and in comparison to alkali exchange feldspars of Wright and Stewart (shown as crosses).

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TABLE 7. STRUCTURAL STATE OF FELDSPAR SAMPLES AS Comparative Petrography of the Augen INDICATED BY COMPARISON WITH THE b AND c VALUES OF Gneiss and Associated Rocl( WRIGHT AND STEWART'S ALKALI EXCHANGE SPECIMENS The microscopic petrography of the augen Specimen Rank of Alkali Exchange gneiss (Gi on Fig. 2) and phyllite and schist Number Structural State Feldspar which border the gneiss, was studied in an 1 5 Benson attempt to establish whether or not a metaso- 2 8 Spencer "B" matic relationship exists between the two 9 7 Low Ab III + KC1 rocks. Evidence that the plagioclase in the 30 3 S62-34 schist has been converted to potassium feldspar 37 5 Benson 41* 8 Spencer "B" leading to the formation of orthoclase augen is seen in the schist sample from near the con- 'Specimen 41 plots between Spencer "B" and tact with the augen gneiss. Figure 4 is a Spencer "U," but is monoclinic so is assumed to be more photomicrograph of the schist showing pla- similar to Spencer "B." gioclase altered to potassium feldspar at the borders of the crystal. Similarly, Figures 5 and 6 show the conversion of plagioclase to potas- on Table 7 are those used by those authors as sium feldspar bordering fractures through the end members in their alkali exchange experi- plagioclase. This configuration of plagioclase ments. An indication of feldspar structural and potassium feldspar is believed to be state also is provided by optic angle measure- significant, as it seems to indicate that the ment. When the 2V values shown on Table 6 plagioclase in the bordering schist has under- are compared with those of the alkali exchange gone partial conversion to potassium feldspar feldspars of Wright and Stewart (1968, Fig. 4), by the introduction of potassium-bearing solu- similar, though not identical, levels of struc- tions along fractures and crystal interstices. In tural state are indicated to those obtained using the augen gneiss near the border with the the b and c cell dimensions (Table 8). The schist, relict plagioclase crystals are observable difference in the indicated structural state using within large orthoclase augen, clearly implying the two kinds of data probably results from that the augen resulted from the metasomatic the fact that the 2V measurements, which conversion of plagioclase. show a range of values within one sample or A comparison of the mineralogy of these even at different positions on the same crystal, rocks (Tables 3 and 4) allows the possibility do not give so reliable an averaging base as the that the augen gneiss resulted from metaso- X-ray powder data derived by separation of matism of the schist. A comparison of the many feldspar crystals from the rock sample. chemical compositions is shown on Table 9 in The variation in optic angle data provides the the form of a calculated oxide-weight percent valuable information, which would not be analysis of the augen gneiss and bordering apparent from the X-ray data alone, that the schist, based on the mean modal analyses of structural state varies over moderately narrow these rocks. To provide a picture of the limits among the feldspar crystals in a rock chemical exchanges which occurred if the sample and within a single crystal. rocks are related metasomatically, Earth's

TABLE 8. STRUCTURAL STATE OF FELDSPAR AS INDICATED BY THE COMPARISON OF 2V VALUES WITH WRIGHT AND STEWART'S ALKALI EXCHANGE SPECIMENS

Sample Mean Rank of Alkali Exchange Number 2V* Structural State Feldspar 1 61.0 5 SH 1070 (6)* 2 60.0 5 SH 1070 (6)* 9 60.1 6 SH 1070 (6)* 30 57.8 5 SH 1070 (6)* 37 51.5 4 Benson (5)* 41 60.7 5 SH 1070 (6)*

* Number in parenthesis refers to the rank of structural state based on b and c cell dimensions for comparison with Table 7.

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Figure 4. Potassium feldspar has replaced plagioclase at the periphery of the plagioclase crystals and along a fracture. From schist bordering augen gneiss.

Figure 5. Potassium feldspar replacing plagioclase along the edge of a crystal. The replacing fluid entered along fractures and crystal interstices.

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Figure 6. Potassium feldspar replacing plagioclase on both sides of a fracture. The plagioclase not close to the fracture is unaffected as is the quartz regardless of its proximity to the fracture.

(1948) rock standard cell was calculated from Because the data are based on only the few the mean weight percent analyses (Table 9). specimens avaikble, this calculation allows These are: only a crude measure of the comparative chemical composition of the two rocks, but is useful in indicating the elemental changes • Oi49.4(OH)10.6 augen gneiss which have occurred if it is assumed that the augen gneiss is a product of metasomatism and that the schist represents the unaffected rock. •O146.4(OH)i3.6 schist The algebraic addition of the two rock cells shows that Si and K plus small amounts of ++ TABLE 9. CALCULATED CHEMICAL ANALYSES IN PER- pe+++ Al and Ti have been added, while Fe , CENT OF THE AUGEN GNEISS AND BORDERING SCHIST Mg, Na, H, and a small amount of Ca have BASED ON MEAN MODAL ANALYSES been subtracted from the schist to form the augen gneiss. Augen gneiss Schist

Si02 74.36 67.97 CONCLUSIONS Ti02 0.60 0.09 A number of origins can be suggested for this A12O3 12.08 12.10 Fe2O3 1.09 0.52 occurrence of coarse augen gneiss completely FeO 2.49 8.82 surrounded by phyllites and schists of the MgO 0.99 2.88 greenschist facies. Any acceptable conclusion CaO 0.17 0.51 must adequately explain the apparently large K20 6.27 2.81 Na2O 0.84 1.93 discordance in apparent metamorphic grade H2O 0.92 1.28 between the augen gneiss and contact rocks. Total 99.81 98.91 The author favors a metasomatic origin for the augen gneiss, but before detailing this con-

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elusion will suggest others and will indicate his and in some places contains a strong lineation judgment as to their acceptability. caused by elongated augen, the lineation has a A structural interpretation is appealing low angle of plunge. It is obvious that the because it removes the problem of the dis- augen gneiss has been affected by a stress field crepancy in metamorphic grade most effec- during or following its recrystallization, as in- tively. If the major east-trending fault, which dicated by the augen shape of the feldspar separates the phyllites from the staurolite- metacrysts. bearing schist and marbles underlain by gneiss The possibility that the augen gneiss repre- to the north of the augen gneiss exposure sents an isokted portion of an otherwise meta- (Fig. 2), also divides the augen gneiss from its morphically retrogressed terrain must be con- associated rocks, it would leave the augen sidered. However, the stratigraphic dissimilar- gneiss as a klippe on top of the schist and ity of the rocks north and south of the major phyllite. This interpretation would require fault (for example, the kck of any carbonate merely that the fault surface become less steep units south of the fault) precludes the pos- in dip to the north to include the augen gneiss sibility that the rocks to the south are merely on the hanging-wall block and that the augen the retrogressed equivalents of those to the gneiss and the Precambrian basal gneiss, which north of the fault. Also, the mineralogical and includes some augen gneiss, are the same rock. chemical differences between the augen gneiss Two facts weaken, if not completely destroy, and the phyllites and schists are not easily this conclusion. The prominent gouge zone, attributable to difference in level of retrogres- which everywhere marks the position of the sion of originally similar rock (see Tables 3, 4, major fault, is not present between the augen and 9). Relict textures and pseudomorphic gneiss and rocks in contact with it. In a few relationships suggesting retrogressive develop- places along this contact, cataclastic textures ment from the augen gneiss have not been ob- are observable microscopically that could be served in the phyllites and schists. interpreted as mylonite, but more typically the A metasomatic origin for the augen gneiss contact is conformable and without evidence of by local alteration of the phyllite and schist, or movement. Additionally, the augen gneiss and possibly an earlier intrusive body within these Precambrian gneiss, although superficially rocks, appears to be the only hypothesis which simikr, are easily distinguished mineralogically cannot be ruled out or seriously weakened by and texturally. the geologic facts and for which there is sup- Interpreting the augen gneiss and associated porting petrographic and mineralogic evidence. rocks as part of the overturned limb of a nappe The narrowness of the gradation from the with the augen gneiss as a basal rock uncon- coarsely feldspathized gneiss into the finer tex- formably below the phyllites and schists does tured phyllites and schists, typically less than 1 not appear acceptable inasmuch as the straight- m (PI. 1, fig. 1), is unusual if caused by metaso- ness of the contact between the augen gneiss matism and must be attributed to steeply and the phyllites and schist (Fig. 2), where it ascending solutions, perhaps controlled by local crosses topographic relief, indicates a steep porosity differences. A steep-temperature orientation for this contact surface surrounding gradient also is implied by the narrowness of the augen gneiss body. the transitional zone. The steep contact also could suggest that the The assemblage of quartz, albite, biotite, augen gneiss intrudes into the phyllite and almandite, muscovite, and chlorite in the schist. Although small mafic intrusions are rocks bordering the augen gneiss places them in present in the phyllite and schist near the the quartz-albite-epidote-almandite subfacies, augen gneiss, it is mineralogically and tex- the highest grade subfacies of the greenschist turally difficult to interpret the augen gneiss facies (Fyfe and others, 1958, p. 224) of the as an igneous rock. The possibility does remain, Harrovian facies series. Phyllites, a few hundred however, that the augen gneiss was altered by meters more distant from the augen gneiss, lack metamorphism and metasomatism from a pre- garnet or biotite, or both, and are assignable to existing intrusive body. The possibility also the two lower subfacies of the greenschist suggests itself that the augen gneiss represents facies. The quartz, plagioclase (Abse), ortho- a diatreme intrusive of largely quartz-feld- clase, muscovite, biotite, garnet, chlorite as- spathic material of a plastic consistency. How- semblage of the augen gneiss is not an equilib- ever, although the foliation in the augen gneiss rium assembkge. The presence of orthoclase is typically steeper than that of the phyllite rather than microcline prevents it from being

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assigned to the greenschist facies, and the be related to a number of factors other than coexistence of orthoclase and muscovite and the temperature, but there is a possibility that the lack of sillimanite exclude it from the silliman- microcline-to-orthoclase inversion may be ite, almandite, orthoclase subfacies of the am- useful for this purpose where rising tempera- phibolite facies (Winkler, 1967, p. 111). What ture has been involved, as in metamorphic then can be stated concerning the P/T en- environments. In the present instance, the vironment of the metasomatic event which re- potassium feldspar appears to have been formed sulted in the formation of the augen gneiss? under rising temperature conditions as the Basing his estimate on the experimentally result of metasomatic alteration of plagioclase, determined upper stability limits of some of the at least in part. If the reasonable assumption is clay minerals and zeolites and the temperature made that the original sodic plagioclase was of and water vapor pressure of the reaction to a highly ordered structural state, then the produce the anhydrous aluminous silicates metasomatism involved an ion exchange and a characteristic of the amphibolite facies, Wink- heating of the feldspar, causing a degree of ler (1967, p. 174) assigned a 400° C to 550° C disordering of the Si and Al ions, and resulting temperature range to the greenschist facies and in a higher structural state. This is indicated by regarded the higher value as only slightly pres- the fact that the potassium feldspar in the sure dependent. If the upper portion of this augen gneiss is monoclinic (Fig. 3) and has temperature range is accepted as characteristic formed above the microcline-to-orthoclase in- of that of the rocks bordering the augen gneiss version temperature. Knowledge of the tem- during its recrystallization, the coarser crystal- perature of this inversion, therefore, would Unity and presence of orthoclase in the augen give a minimum value for the temperature of gneiss strongly suggests that a higher tempera- recrystallization of the augen gneiss. ture prevailed in that rock. Two lines of evi- Steiger and Hart (1967, p. 114) estimated dence, the mineral assemblage and the structural a temperature of 350° C to 400° C for the state of the potassium feldspar, perhaps provide transition on the basis of heat flow calcula- some indication of the maximum temperature tions where the transition has taken place as the environment of the augen gneiss. result of heating of the microcline in Pre- The fact that the chlorite and muscovite cambrian pegmatites by the intrusion of a have not reacted to produce staurolite in the Tertiary stock. Using the same material, augen gneiss would seem to indicate that a Wright (1967, p. 130) estimated the upper temperature of 540° C to 560° C + 15° has not stability temperature of microcline (Orgs) to been reached (Hoschek, quoted in Winkler, be 375° C + 50° on the basis of the evidence 1967, p. 179). Similarly, the absence of any of of homogenization of the orthoclase and the the A^SiOs polymorphs in the augen gneiss, application of the solvus determined syntheti- although the reactants quartz, biotite, and cally by Orville (1963). These values may be muscovite are present, appears to provide an considered as minimum temperatures of upper temperature limit. This reaction is metasomatism of the augen gneiss. strongly Pn2o as well as temperature de- The maximum temperature is more difficult pendent, ranging from about 570° C at low to estimate. As can be seen on Figure 3 and in pressure to 725° C at Pn2o = 5 kb (Winkler, Tables 7 and 8, the structural states of the 1967, p. 184). The presence of potassium feld- potassium feldspar from the augen gneiss spar would normally prevent this reaction, but (specimens 30, 37, and 41) show a range from the coexistence of potassium feldspar and very well-ordered (similar to Spencer B) to muscovite indicates that a disequilibrium re- less well-ordered orthoclase (similar to Benson). lationship exists between this feldspar and the If this variation in structural state is a function other silicates. of temperature difference within the body or of The structural state of the potassium feldspar varying degrees of reordering since cooling, augen in the augen gneiss may be useful in then the highest structural state should reflect estimating the temperature of recrystallization the maximum temperature. Additionally, the of that rock. As has been pointed out by composition of the alkali feldspar will influence Wright (1967, p. 134), the variety of structural the temperature of its inversion. Wright (1967, states encountered in plutonic rocks which have p. 132) has constructed a diagram showing the crystallized by cooling from a melt gives little temperature of the microcline-orthoclase trans- information as to temperature of crystalliza- formation as a function of composition based on tion, because the type of polymorph appears to the high albite-high sanidine solvus of Orville

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(1963) and plots of the Eldora specimens of literature. A metasomatic mechanism which Steiger and Hart (1967), assuming that they seems particularly applicable to this study is have passed through the solvus as a first-order suggested by the experimental treatment by transformation. Although the diagram is not Orville (1963) of alkali ion exchange between intended to be a quantitatively accurate phase feldspars and a vapor phase. It is found that diagram, it does indicate the influence of the alkali ratio in a vapor phase that coexists sodium-potassium composition on the tem- with two alkali feldspars is temperature de- perature of the inversion. The compositional pendent. A vapor phase that reaches equilib- range of the augen gneiss feldspar using both rium with two alkali feldspars at high tempera- cell volume and 2V (Table 6) is Or78 to Orgo, ture, on cooling replaces Na in feldspar suggesting a maximum inversion temperature structures with K. Thus, for example, volatiles of about 450° C. As two of the specimens (30 escaping from a cooling crystalline granitic and 37) from the augen gneiss have structural body into cooler country rock cannot reach states of the upper orthoclase range, yet below equilibrium in the country rock until the K in low sanidine, a higher maximum temperature solution is reduced by either direct crystalliza- is possible, perhaps 550° C. Thus, the best tion of potassium feldspar or by replacement of estimate of the temperature prevailing in the sodium feldspar by potassium feldspar. It is augen gneiss during metasomatism is a mini- probable that this process would be effective at mum of 400° C and a maximum of 550° C, 400° C, or less. The strong indications in the which conforms to the range of temperature present study are that metasomatic fluids rose suggested by Winkler for the greenschist steeply from a deeper source, resulting in the facies. It is probable that the bordering formation of the potassium feldspar augen by phyllites and schists were affected by tem- replacement of the sodium-rich feldspar in the peratures in the middle of this range while the phyllites and schists (Figs. 4, 5, and 6) and metasomatic event took place at temperatures possibly also by direct precipitation. toward the upper portion of the temperature Schoen and White (1967) have shown that range. pkgioclase is highly susceptible to potassium Essentially the same range of orthoclase metasomatism and produces potassium feldspar structural states occurs in the potassium where affected by geothermal waters, as ob- feldspars from the Precambrian gneiss north served in holes drilled through basaltic of the major fault as in the augen gneiss (G%, andesite in a thermal area near Steamboat Fig. 2). Very possibly, this indicates a reheat- Spring, Nevada, by the U.S. Geological Sur- ing of that rock sometime after its initial vey. The action of thermal fluids in causing the crystallization and perhaps contemporaneous metasomatism of the augen gneiss is equally as with that affecting the rocks south of the fault. acceptable a possibility as that of a vapor phase No attempt has been made to estimate the from a metasomatic source. In an area, such as pressure environment in any of the rocks in- western Turkey, where evidence of volcanic vestigated aside from that implied by their activity is abundant and the occurrence of assignment to the Barrovian facies series on the thermal springs is common, a mechanism of basis of their metamorphic mineral paragenesis. metasomatism involving a vapor phase or Alkali metasomatism has been widely treated thermal water, or both, would seem to be a in the descriptive and experimental petrologic reasonable one.

REFERENCES CITED Earth, T. F. W., 1948, Oxygen in rocks: a basis for petrographic calculations: Jour. Geology, v. 56, p. 50-60. Brinkmann, R., 1966, Geotektonesche gliederung von Werlanatolian: Neues Jahrb. Geologic U. Palaontol- ogie Monatsh., v. 10, p. 603-618. Evans, H. T., Appleman, D. E., and Handwerker, D. S., 1963, The least squares refinement of crystal limit cell with powder diffraction data by an automatic computer indexing method (Abs.): Am. Cryst. Assoc. Ann. Meet. Program, p. 42-43. Fyfe, W. S., Turner, F. J., and Verhoogen, J., 1958, Metamorphic reaction and metamorphic facies: Geol. Soc. of America Mem. 73, 259 p.

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Goldsmith, J. R., and Laves, F., 1954, The microcline-sanidine stability relations: Geochim. et Cosmochim. Acta, v. 5, p. 1-19. Hickling,N., Cuttitta, F., and Meyrowitz, R., 1961, N, N-dimethylformamide, a new diluent for bromo- form used as a heavy liquid: Am. Mineralogist, v. 46, p. 1502-1503. Orville, P. M., 1958, Feldspar investigations: Carnegie Inst. Washington Year Book, v. 57, p. 206-209. 1963, Alkali ion exchange between vapor and feldspar phases: Am. Jour. Sci., v. 261, p. 201-237. 1967, Unit cell parameters of the microcline-low albite and sanidine-high albite solid solution series: Am. Mineralogist, v. 52, p. 55-86. Philippson, A., 1911, Reisen und Forschungen im wesllichen Kleinassien: Gotha, Petermanns Geo- graphische Mitteilungen; ErgSnzangheft 172. Schoen, R., and Lee, D. E., 1964, Successful separation of silt-size minerals in heavy liquid: U.S. Geol. Survey Prof. Paper 501, p. B154-B157. Schoen, R., and White, D. E., 1967, Hydrothermal alteration of basaltic andesite and other rocks in drill hole GS-6, Steamboat Springs, Nevada: U.S. Geol. Survey Prof. Paper 575-A, p. B110-B119. Schuiling, R. D., 1962, On petrology, age, and structure of the Menderes migmatic complex (SW- Turkey): Mining Research and Exploration Inst, of Turkey Bull. 58, p. 71-84. Steiger, R. H., and Hart, S. R., 1967, The microcline-orthoclase transition within a contact aureole: Am. Mineralogist, v. 52, p. 87-116. Winkler, H. G. F., 1967, Petrogenesis of metamorphic rocks: New York, Springer-Verlag, 237 p. Wright, T. L., 1967, The microcline-orthoclase transition in the contact aureole of Eldorado Stock, Colorado: Am. Mineralogist, v. 52, p. 117-136. Wright, T. L., and Stewart, D. B., 1968, X-ray and optical study of composition and structural state of alkali feldspar: I. Determinations of composition and structural state from refined unit-cell parameters 2V: Am. Mineralogist, v. 53, p. 38-87.

MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 22, 1968 REVISED MANUSCRIPT RECEIVED DECEMBER 12, 1968

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Figure 2. Typical coarse phase of augen gneiss.

AUGF.X (iNKISS AM) BOKDKRINC; I'HYI.I.ITK

SCX)TF()RD, PLATI-: 1 Geological Society of America Bulletin, v. SO, no. 6

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