DAVID M. SCOTFORD Miami University, Oxford, Ohio
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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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/6/1079/3428099/i0016-7606-80-6-1079.pdf by guest on 28 September 2021 1080 D. M. SCOTFORD—METASOMATIC AUGEN GNEISS, TURKEY 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/6/1079/3428099/i0016-7606-80-6-1079.pdf by guest on 28 September 2021 INTRODUCTION 1081 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/6/1079/3428099/i0016-7606-80-6-1079.pdf by guest on 28 September 2021 1082 D. M. SCOTFORD—METASOMATIC AUGEN GNEISS, TURKEY 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.