39Ar ages

ELSEVIER

Tectonophysics 290 (1998) 111–135

Variscan vs. Alpine tectonothermal evolution of the Southern Carpathian orogen: constraints from 40 Ar=39 Ar ages R.D. Dallmeyer a , F. Neubauer b,Ł , H. Fritz c , V. Mocanu d a

Department of Geology, University of Georgia, Athens, GA 30602, USA of Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria c Department of Geology, University of Graz, Heinrichstr. 26, A-8010 Graz, Austria d Department of Geology, University of Bucharest, Vuia Traiana 6, RO-70139 Bucharest, Romania b Department

Received 27 May 1997; accepted 20 November 1997

Abstract 40 Ar=39 Ar hornblende and muscovite ages indicate that basement units exposed within all major Alpine nappes of the Southern Carpathian orogen were penetratively deformed and metamorphosed during ‘late’ Variscan (Carboniferous) tectonothermal events. Subsequent exhumation resulted in cooling from ca. 500ºC (hornblende K–Ar retention temperature) to ca. 375ºC (muscovite K–Ar retention temperature) between ca. 320 and 295 Ma. Late Variscan exhumation of middle crustal levels is interpreted to have resulted from intracontinental contraction and regional uplift subsequent to Variscan continental collision between Gondwana-derived tectonic elements (e.g., Africa) and Laurussia. Muscovite ages within mylonite zones suggest that late Variscan shear zones were probably associated with a regional shear system that developed along the southern margins of Laurussia. Alpine metamorphic effects within the Southern Carpathians are generally nonpenetrative, and are only significant within narrow zones of retrogression that are interpreted to have formed during initial attenuation of Variscan crust associated with formation of the Jurassic Severin rift.  1998 Elsevier Science B.V. All rights reserved.

Keywords: exhumation; cooling; tectonics; detachment faulting

1. Introduction Several workers have suggested that exhumation of metamorphic crust within orogenic belts is largely accomplished through late stage orogenic extension (e.g., Platt, 1986; Dewey, 1988; Ratschbacher et al., 1989). Regional-scale exhumation of medium-grade metamorphic crust is most often achieved by simple extensional processes (e.g., Malavieille, 1993). Ł Corresponding

author. Tel.: C43 662 8044 5401; Fax: C43 662 8044 621; E-mail: [email protected]

Several other mechanisms have been proposed to contribute to local exhumation of medium-grade metamorphic sequences within orogenic belts. These include: (1) exhumation and surface erosion of upper continental plate rocks during thrust translation over crustal-scale ramps (e.g., England and Thompson, 1984); (2) exhumation of footwall continental plate crust through footwall thrust propagation (e.g., Rodgers, 1995); (3) exhumation associated with tectonic unloading of footwall successions of continental crust by extension and gravitational collapse of an overthickened orogenic wedge (Platt, 1986;

0040-1951/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 0 0 6 - 7

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Dewey, 1988); and (4) gravitational collapse resulting from breakoff and removal of mantle portions of the subducted lithosphere (e.g., Platt and Vissers, 1989; Andersen et al., 1990). These tectonic differences are of no particular importance within the European Variscides where late-stage, intracontinental exhumation of high-grade metamorphic basement was related to very different processes including thrusting (e.g., Ledru et al., 1989; Dallmeyer et al., 1992; Matte et al., 1992; Fritz et al., 1996), extension (e.g., Malavieille, 1993), and strike-slip-related extension (e.g., Krohe, 1996). Results of recent field work and collaborative 40 Ar=39 Ar dating in the Southern Carpathian orogen has enabled calibration of the Variscan tectonothermal evolution. These new data require significant revision of previous interpretations of the tectonothermal evolution of the Southern Carpathian orogen, and provide constraints for regional Paleozoic exhumation and surface uplift following Variscan orogenic events.

2. Geological setting The Southern Carpathian orogen is comprised of a structural succession of metamorphic basement nappe complexes which are internally separated by variably metamorphosed intercalations of late Paleozoic and Mesozoic ‘cover’ sequences (e.g., Burchfiel, 1976; Kra¨utner et al., 1981, 1988; Sandulescu, 1984; Kra¨utner, 1993; Berza and Iancu, 1994). These were palinspastically derived from regions situated between the European plate (Moesian platform) and the Meliata–Mures–Vardar oceanic domain (e.g., a western extension of Tethys: Fig. 1a). Basement rocks exposed within the Southern Carpathian orogen comprise three major nappe complexes. Traced structurally upward these include (Fig. 1b): (1) the Danubian nappe complex (Cadomian granitoids, and medium- to high-grade metamorphic sequences); (2) the Getic nappe complex (mainly medium-grade metamorphic sequences); and (3) the Supragetic nappe complex (mainly

Fig. 1. (a) Generalized regional map illustrating the tectonic setting of the Southern Carpathian orogen relative to the Alpine–Carpathian mountain system.

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113

Fig. 1 (continued). (b) Generalized tectonic map of the Southern Carpathian orogen showing locations for samples collected for 40 Ar/39 Ar dating. The Danubian nappe complex is exposed within the Danubian window which is surrounded by the Getic and Supragetic nappes.

medium-grade metamorphic sequences). The Danubian nappe complex is locally structurally separated from the Getic nappe complex by tectonic units of the exotic Severin nappe. These include Jurassic rift deposits, ophiolite fragments, Early to early Late Cretaceous deep-water successions and Late Cretaceous synorogenic flysch sequences (e.g., Burchfiel, 1976; Sandulescu, 1984; Gherasi and Hann, 1990). Sedimentary successions have been interpreted to record Jurassic separation of an originally amalgamated Danubian=Getic continental basement. The chronology of assembly of the present nappe architecture generally resembled that of the Austro–

Alpine nappe complex of the internal Eastern Alps, and the Eastern and Western Carpathians. It resulted from mid- to Late Cretaceous continental collision and associated nappe assembly (e.g., Burchfiel, 1980; Sandulescu, 1984; Dallmeyer et al., 1996). In detail, the Danubian nappe complex comprises several Alpine nappes (Berza et al., 1994). Tectonically lower nappes are exposed in southern sectors of the South Carpathians and consist of medium-grade metamorphic sequences (Lainici–Paius Group) intruded by locally discordant granitic plutons. Structurally higher Alpine Danubian nappes include the Dragsan amphibolite group which is also intruded

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by granitoids (Berza and Iancu, 1994). The Dragsan amphibolite is tectonically juxtaposed with Ordovician to Early Carboniferous, low-grade metasedimentary units along ductile shear zones (Kra¨utner et al., 1981, 1988; Kra¨utner, 1993; Berza and Iancu, 1994). Structural evolution of the upper Danubian nappes must have at least partially included Variscan tectonic phases because Jurassic successions locally stratigraphically overlie all crystalline nappe units. Retrogression within greenschist facies metamorphic conditions occurred along tectonic boundaries of the Alpine structural units within the Danubian window (Ratschbacher et al., 1993; Berza and Iancu, 1994; Neubauer et al., 1997). Previous geochronological results reported from the Danubian basement sequences include (Liegois et al., 1996): (1) a 777 š 3 Ma U–Pb upper intercept zircon age for augengneiss associated with the Dragsan amphibolite group; (2) a 825 š 156 Ma Sm–Nd whole-rock age for the Dragsan amphibolite; (3) several U–Pb zircon ages for intrusive granitoids within the Lainici–Paius metamorphic sequences (610 š 30 Ma: by Gru¨nenfelder et al., 1983; reinterpreted by Liegois et al. to represent concentrates from two distinct plutons with U–Pb upper intercept zircon ages of 588 š 5 Ma and 582 š 7 Ma); and (4) an upper intercept zircon age of 567 š 3 Ma for the Tismana granitoids. K–Ar ages reported for whole-rock samples and concentrates of amphibole, muscovite and biotite range between ca. 550 and 70 Ma (Gru¨nenfelder et al., 1983; Kra¨utner et al., 1988; Ratschbacher et al., 1993). Together the previously available radiometric results have been interpreted to record the effects of penetrative Cadomian=Baikalian (Late Precambrian) tectonothermal activity which has been variably overprinted by retrogressive Variscan (late Paleozoic) and=or Alpine orogenesis (e.g., Kra¨utner et al., 1988; Balintoni et al., 1989). The Getic nappe largely comprises mediumgrade metamorphic sequences (locally eclogite-bearing paragneisses and micaschists intruded by pegmatites) and minor granites (Kra¨utner et al., 1988; Iancu and Maruntiu, 1994a,b). Published radiometric results include an upper intercept, 1100–1000 Ma U–Pb zircon age for a gneiss, and a lower intercept of 310 Ma U–Pb zircon age (Pavelescu et al., 1983). A discordant granite yielded a U–Pb zircon age of

ca. 350 Ma (Stan et al., 1992). K–Ar mineral ages range between 350 and 70 Ma (Gru¨nenfelder et al., 1983; Pavelescu et al., 1983; Kra¨utner et al., 1988; Ratschbacher et al., 1993). The Supragetic nappe includes medium-grade metamorphic basement sequences that have been partially retrogressed along distinct ductile shear zones (Berza et al., 1994; Iancu and Maruntiu, 1994a,b; Pana and Erdmer, 1994). Low-grade sequences include fossiliferous Cambrian to Silurian, and Late Devonian to Early Carboniferous successions in northwestern sectors (Kra¨utner et al., 1988; Berza et al., 1994). 3. Analytical methods Representative samples were collected within generally nonretrogressed, medium-grade metamorphic rocks from all major basement units exposed within the Southern Carpathian orogen. Multigrain mineral concentrates and a whole-rock phyllonite sample were prepared for 40 Ar=39 Ar analysis. Sample locations are indicated in Fig. 1b. The samples included: (1) three hornblende concentrates (samples 1, 2, 3) and a muscovite concentrate (sample 4) from the Danubian basement rocks; (2) four hornblende (samples 5A, 6A, 8A, 9) and four muscovite concentrates (samples 5B, 6B, 7B, 8B) from five locations within the Getic nappe complex; (3) a whole-rock phyllonite (sample 10) from a ductile shear zone near the structural contact between the Supragetic nappe from the Getic nappe cover (Hann and Balintoni, 1988); and (4) two hornblende (samples 11A, 13A) and four muscovite concentrates (samples 11B, 12, 13B, 14) from the Supragetic nappe complex. The 40 Ar=39 Ar analytical techniques are described in Appendix A. The sample locations and descriptions of the petrographic characteristics of the dated samples are listed in Appendix B. The 40 Ar=39 Ar analytical data of hornblende concentrates are listed in Table 1. 36 Ar=40 Ar vs. 39 Ar=40 Ar isotope-correlations for hornblende concentrates are given in Table 2. Analytical data for incremental-heating experiments on muscovite concentrates are presented in Table 3, those of the whole-rock phyllonite in Table 4. The 40 Ar=39 Ar results are portrayed as age spectra in Figs. 2–9.

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Table 1 40 Ar=39 Ar analytical data for incremental-heating experiments on hornblende concentrates from structural units comprising the Southern Carpathian Orogen, Romania Release temp. (ºC)

40 Ar=39 Ar a

36 Ar=39 Ar a

37 Ar=39 Ar b

39 Ar

% of total

%40 Ar non-atmos. c

36 Ar

Ca

%

Apparent age (Ma) d

Danubian ‘Parautochthon’ Sample 1: J D 0.0097771: 620 62.24 720 38.11 760 45.73 785 43.50 800 43.49 815 42.06 825 41.13 835 40.36 850 40.01 865 40.12 885 40.03 905 40.13 925 38.93 Fusion 38.32 Total 41.27

0.07545 0.01017 0.00796 0.00368 0.00269 0.00234 0.00125 0.00202 0.00155 0.00242 0.00138 0.00183 0.00361 0.00155 0.00379

1.347 2.383 3.707 3.265 3.350 3.280 3.207 3.282 3.308 3.246 3.263 3.382 4.404 4.616 3.347

1.91 2.15 1.75 2.19 9.24 14.22 12.22 10.60 10.94 10.05 5.72 9.65 7.36 2.01 100.00

64.34 92.60 95.49 98.09 98.78 98.97 99.71 99.16 99.50 98.86 99.62 99.31 98.15 99.75 98.26

0.49 6.37 12.66 24.13 33.90 38.15 69.82 44.19 57.95 36.54 64.17 50.26 33.14 80.76 45.26

596.1 š 2.7 535.1 š 1.3 642.3 š 0.6 629.7 š 1.3 633.4 š 0.7 616.7 š 0.7 609.0 š 0.8 596.4 š 0.5 593.8 š 0.6 591.8 š 1.1 594.7 š 0.4 594.4 š 0.9 573.6 š 0.5 573.9 š 1.0 601.3 š 0.8

Sample 2: J D 0.009631: 610 15.72 710 15.20 740 15.76 765 17.04 790 17.41 815 17.75 840 18.06 865 18.25 890 18.63 915 19.04 Fusion 19.43 Total 17.26

0.00458 0.00147 0.00229 0.00174 0.00087 0.00095 0.00109 0.00134 0.00146 0.00253 0.00367 0.00179

0.681 0.754 0.847 0.898 1.045 1.185 1.340 1.595 1.259 1.651 7.218 1.184

8.90 7.91 12.60 9.95 10.85 12.95 14.31 10.22 2.47 8.90 0.94 100.00

91.70 97.50 96.09 97.38 98.98 98.92 98.78 98.50 98.20 96.74 97.38 97.37

4.04 13.95 10.04 14.06 32.77 33.90 33.52 32.44 23.47 17.76 53.54 22.85

234.5 š 0.8 240.8 š 0.2 245.7 š 0.3 267.6 š 0.3 277.1 š 0.4 282.0 š 0.4 286.2 š 0.2 288.4 š 0.4 293.0 š 0.4 294.8 š 0.3 303.2 š 0.2 270.8 š 0.4

Sample 3: J D 0.009165: 610 223.72 0.48504 710 59.28 0.17682 735 17.54 0.00881 760 18.02 0.01039 785 17.90 0.00123 810 19.28 0.00350 835 19.21 0.00106 860 20.10 0.00105 885 20.36 0.00157 910 19.84 0.00059 945 21.87 0.00700 980 20.396 0.00224 1020 21.15 0.00260 Fusion 21.43 0.00041 Total 21.37 0.00852 Total without 610–835ºC, 1020ºC – fusion

4.759 0.890 2.723 2.608 0.283 0.305 0.516 0.087 0.115 0.102 0.170 0.288 0.414 0.451 0.518

0.16 2.49 5.26 5.45 4.67 7.64 6.88 5.28 10.32 15.04 17.79 9.16 5.85 4.03 100.00 57.58

36.10 11.97 86.37 84.09 98.07 94.74 98.55 98.46 97.74 99.13 90.58 96.83 96.50 99.57 92.90

0.27 0.14 8.41 6.83 6.27 2.37 13.21 2.25 1.99 4.67 0.66 3.49 4.34 29.72 5.12

1001.8 š 19.0 113.7 š 1.8 234.9 š 0.3 234.9 š 0.3 269.2 š 0.5 279.3 š 0.5 288.7 š 0.3 300.7 š 0.4 302.3 š 0.3 299.0 š 0.2 301.0 š 0.4 300.0 š 0.7 309.4 š 1.0 322.4 š 0.9 287.5 š 0.5 300.5 š 0.4

a Measured. b

Corrected for post-irradiation decay of 37 Ar (35.1 day 1=2-life). (36 Aratmos. ) (295.5)]=40 Artot. . tot. d Calculated using correction factors of Dalrymple et al. (1981); two sigma, intralaboratory errors. c [40 Ar

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Table 1 (continued) 37 Ar=39 Ar b

39 Ar

2.349 4.661 7.057 7.676 8.712 8.925 8.881 10.840 9.663 8.606 8.795 8.902 8.849 8.851

0.15583 0.13596 0.04965 0.01324 0.00486 0.00474 0.00424 0.00310 0.00536 0.00584 0.00325 0.00295 0.00343 0.00428 0.00729

Sample 8A: J D 0.010265: 620 51.35 0.07804 710 17.38 0.02193 765 21.73 0.02855 795 22.23 0.01433 820 20.02 0.00595 840 19.869 0.00558 850 19.50 0.00357 860 19.30 0.00326 870 19.20 0.00269 885 19.25 0.00356 905 19.26 0.00296 930 19.22 0.00351 955 19.53 0.00273 Fusion 19.89 0.00253 Total 19.96 0.00537 Total without 620–795ºC, 955ºC – fusion

Release temp. (ºC)

40 Ar=39 Ar a

36 Ar=39 Ar a

%40 Ar non-atmos. c

36 Ar

1.39 2.04 1.04 0.92 3.57 15.50 12.98 8.59 6.00 8.58 15.41 11.24 12.73 100.00 78.31

25.01 56.32 46.73 70.04 96.94 95.79 96.30 99.78 97.62 99.37 98.14 97.66 98.18 95.00

0.43 3.97 3.79 8.48 53.70 46.72 50.25 96.12 63.16 85.89 67.00 61.71 66.47 61.03

254.1 š 5.0 205.4 š 12.2 221.7 š 14.1 269.4 š 2.9 328.5 š 1.1 320.1 š 0.6 316.5 š 0.5 319.7 š 0.5 321.4 š 0.6 319.6 š 0.8 318.5 š 0.2 320.7 š 0.7 335.0 š 0.5 316.9 š 1.0 319.3 š 0.5

2.550 17.996 6.446 6.765 7.754 8.049 7.700 7.628 7.977 7.585 7.714 7.740 7.436 7.066 7.614

1.32 0.39 1.22 1.37 3.77 15.53 13.21 4.41 3.68 4.70 12.70 15.49 12.16 10.06 100.00

43.06 48.68 48.50 85.98 96.07 96.32 96.84 98.49 91.35 94.71 98.29 98.67 97.96 96.64 95.45

0.45 3.60 3.53 13.90 43.37 46.15 49.42 66.89 25.96 35.34 64.49 71.29 58.98 44.95 52.07

540.9 š 36.0 576.6 š 20.0 228.2 š 2.2 342.8 š 1.4 332.9 š 1.3 331.1 š 0.3 325.9 š 0.9 334.8 š 1.8 333.1 š 1.3 333.8 š 1.4 331.7 š 1.2 327.7 š 1.2 335.6 š 0.7 334.8 š 2.2 332.0 š 1.7

1.039 3.607 6.223 8.689 8.489 8.363 8.365 8.331 8.173 8.238 8.120 8.315 8.477 7.996 8.103

1.52 1.53 1.01 1.19 3.59 10.90 15.40 13.43 8.74 7.76 7.92 13.00 9.28 4.72 100.00 80.74

55.24 64.33 63.45 84.06 94.59 95.05 98.00 98.45 99.26 97.94 98.82 98.06 99.33 99.45 96.30

0.36 4.47 5.93 16.49 38.78 40.73 63.70 69.56 82.78 62.93 74.73 64.51 84.45 86.11 63.64

461.2 š 6.2 196.4 š 2.5 239.7 š 2.6 318.2 š 1.5 322.1 š 0.8 321.1 š 0.5 324.7 š 0.8 323.0 š 0.7 323.9 š 0.7 320.8 š 0.8 323.5 š 0.6 320.5 š 0.8 329.2 š 0.5 335.0 š 0.6 323.1 š 0.6 322.6 š 0.6

% of total

Ca

%

Apparent age (Ma) d

Getic nappe complex Sample 5A: J D 0.010292: 620 58.68 0.14953 710 20.75 0.03192 760 27.07 0.05069 790 22.24 0.02462 815 19.91 0.00441 840 19.59 0.00520 855 19.25 0.00481 865 18.75 0.00307 880 19.29 0.00416 895 18.86 0.00273 915 19.02 0.00357 940 19.25 0.00392 Fusion 20.09 0.00362 Total 20.01 0.00727 Total without 620–815ºC, and fusion Sample 7A: J D 0.010070: 620 80.52 710 75.48 750 27.49 790 24.06 810 20.84 840 20.66 855 20.21 865 20.46 880 21.21 895 21.21 915 20.29 940 20.28 970 20.63 Fusion 20.86 Total 21.54

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117

Table 1 (continued) Release temp. (ºC)

40 Ar=39 Ar a

36 Ar=39 Ar a

Sample 9: J D 0.008875: 610 471.55 1.48734 710 104.86 0.26267 740 31.95 0.01010 770 33.15 0.01192 825 33.25 0.01159 850 22.09 0.00299 880 24.09 0.00940 905 21.49 0.00062 930 21.68 0.00074 965 22.08 0.00314 1000 19.37 0.00161 1040 19.29 0.00449 1090 18.29 0.00455 Fusion 19.86 0.00404 Total 28.88 0.02318 Total without 610–825ºC, 1000ºC – fusion

37 Ar=39 Ar b

39 Ar

% of total

%40 Ar non-atmos. c

36 Ar

Ca

%

Apparent age (Ma) d

4.146 8.979 4.970 0.736 1.762 2.324 1.677 1.244 1.332 3.726 2.695 0.841 1.033 0.802 2.082

0.94 1.81 7.05 2.61 6.45 11.45 10.18 10.17 17.29 9.05 3.15 6.32 8.54 5.00 100.00 58.13

6.86 26.66 91.89 89.53 90.11 96.82 89.00 99.58 99.45 97.13 98.63 93.45 93.08 94.29 93.10

0.08 0.93 13.39 1.68 4.14 21.14 4.85 54.26 48.71 32.30 45.44 5.10 6.18 5.40 23.60

456.6 š 70.6 401.8 š 4.7 418.9 š 0.6 422.0 š 1.9 425.8 š 2.2 314.0 š 0.4 314.6 š 2.1 313.9 š 0.2 316.1 š 0.9 315.0 š 2.9 282.9 š 1.6 267.8 š 0.7 254.0 š 0.3 277.5 š 0.3 324.0 š 1.8 314.8 š 1.2

Sample 11A: J D 0.010185: 620 94.49 0.24705 710 28.86 0.04498 740 26.03 0.03122 765 27.08 0.03651 790 25.61 0.02564 815 20.36 0.00755 835 19.63 0.00497 850 19.36 0.00408 860 19.08 0.00203 870 19.30 0.003525 885 19.53 0.00475 905 19.32 0.00293 930 18.68 0.00191 Fusion 19.79 0.00284 Total 21.85 0.01236 Total without 620–790ºC, and fusion

6.347 3.852 5.782 6.616 7.589 7.030 7.230 7.183 6.892 6.984 7.038 6.834 6.772 8.540 6.948

2.30 4.46 1.67 1.02 1.58 11.97 8.30 11.09 10.56 11.24 6.00 6.12 15.15 8.54 100.00 80.43

23.28 55.00 66.32 62.10 72.77 91.79 95.45 96.72 99.73 97.49 95.68 98.33 99.87 99.20 92.37

0.70 2.33 5.04 4.93 8.05 25.33 39.57 47.86 92.39 54.02 40.33 63.40 96.63 81.78 55.76

366.0 š 4.9 271.0 š 3.8 293.1 š 4.3 286.3 š 4.6 314.9 š 2.2 315.7 š 0.4 316.3 š 0.6 316.2 š 0.9 320.8 š 0.9 317.6 š 1.1 315.6 š 1.0 320.3 š 0.4 315.1 š 0.8 330.5 š 1.0 316.5 š 1.1 317.0 š 0.8

Sample 13A: J D 0.010325: 620 136.74 0.33561 710 29.34 0.05158 760 21.68 0.02081 790 20.62 0.00827 815 19.40 0.00410 835 19.22 0.00351 850 18.91 0.00285 860 18.95 0.00361 875 19.28 0.00382 890 19.12 0.00388 910 18.82 0.00307 935 18.93 0.00299 960 19.47 0.00339 Fusion 20.27 0.00370 Total 20.14 0.00674 Total without 620–760ºC, 960ºC – fusion

4.914 2.792 6.792 7.371 7.139 7.1601 7.202 7.162 7.395 7.132 7.266 7.113 7.305 7.619 7.109

0.63 1.85 1.20 2.25 11.31 15.27 11.17 8.71 5.28 7.13 11.28 12.60 8.67 2.64 100.00 85.01

27.76 48.80 74.13 90.99 96.69 97.57 98.57 97.38 97.20 96.98 98.26 98.32 97.83 97.601 95.93

0.40 1.47 8.88 24.25 47.41 55.56 68.64 53.99 52.66 50.04 64.46 64.72 58.53 56.05 55.23

598.1 š 10.0 249.2 š 5.2 278.1 š 2.1 320.8 š 0.5 320.7 š 0.5 320.7 š 0.6 318.8 š 0.6 316.0 š 0.6 320.5 š 0.7 317.4 š 0.8 316.6 š 0.6 318.4 š 0.3 325.2 š 0.8 336.8 š 1.5 319.8 š 0.8 318.8 š 0.6

Supragetic nappe complex

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Table 2 36 Ar/40 Ar vs. 39 Ar/40 Ar plateau isotope correlations for hornblende concentrates from structural units comprising the Southern Carpathian orogen, Romania Sample

Isotope correlation age (Ma) a

40 Ar=36 Ar

intercept b

MSWD

Increments included (ºC)

Total 39 Ar (%)

Calculated 40 Ar=39 Ar plateau age (Ma)

Danubian ‘Parautochthon’ 3 299.5 š 0.4

316.6 š 4.2

2.01

860–980

57.58

300.5 š 0.4

Getic nape complex 5A 317.0 š 0.9 8A 319.8 š 0.6 9 313.9 š 0.4

303.6 š 9.2 309.4 š 6.7 294.6 š 9.4

1.31 1.57 1.01

840–940 820–930 850–965

78.31 80.74 58.13

319.3 š 0.5 322.6 š 0.6 314.8 š 1.2

Supragetic nappe complex 11A 316.8 š 0.5 13A 316.5 š 0.6

296.7 š 8.2 334.3 š 7.9

1.11 1.02

815–930 790–935

80.43 85.01

317.0 š 0.8 318.8 š 0.1

Calculated using the inverse abscissa intercept (40 Ar=39 Ar ratio) in the age equation. a Inverse ordinate intercept. b Table 1.

4. Results 4.1. Introduction All hornblende concentrates were prepared from samples of amphibolite and diorite which are well recrystallized and which do not show retrogression. The hornblende concentrates display variable spectra of age discordance. The relatively small-volume increments evolved at low experimental temperatures display considerable variation in apparent ages. These are matched by fluctuations in apparent K=Ca ratios which suggest that experimental evolution of argon occurred from compositionally distinct, relatively non-retentive phases. These could be represented by: (1) very minor, optically undetectable mineralogical contaminants in the concentrates; (2) petrographically unresovable exsolution or compositional zonation within constituent amphibole grains; (3) minor chloritic replacement of amphibole; and=or (4) intracrystalline inclusions. Most intermediateand high-temperature gas fractions display little intrasample variation in apparent K=Ca ratios, suggesting that experimental evolution of gas occurred from compositionally uniform intracrystalline sites. The muscovite concentrates display variably discordant apparent age spectra. Apparent K=Ca ratios are very large with considerable associated analytical uncertainties. Consequently they are not shown with the apparent age spectra in Figs. 4, 6 and 9. The

ratios display minor and non-systematic intrasample variations suggesting that experimental evolution of gas occurred from compositionally uniform intracrystalline sites. The whole-rock sample (10) displays an internally discordant apparent age spectrum. Variable and relatively young apparent ages are recorded in the smallvolume increments evolved at low experimental temperatures. These are also marked by relatively small and systematically increasing apparent K=Ca ratios. The intermediate- and high-temperature portions of the analysis record similar apparent ages. Apparent K=Ca ratios systematically decrease in highest temperature portions of the analysis. Although the sample is composed primarily of very fine-grained, syn-kinematic white-mica, systematic intrasample variations in K=Ca ratios suggest that several other phases probably contributed gas at various stages in the wholerock analysis. Relative to white-mica these appear to have included: (1) a more non-retentive phase with relatively low apparent K=Ca ratio; and (2) a more refractory phase with a relatively low apparent K=Ca ratio. Mineralogical characteristics and observed modal variations suggest that these phases are chlorite and plagioclase feldspar, respectively. 4.2. Danubian units A hornblende concentrate was prepared from a sample of undeformed diorite exposed within the

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Table 3 40 Ar=39 Ar analytical data for incremental-heating experiments on muscovite concentrates from structural units comprising the Southern Carpathian orogen, Romania Total 39 Ar (%)

Non-atmos. 40 Ar (%) c

36 Ar

0.018 0.004 0.013 0.007 0.007 0.011 0.007 0.008 0.006 0.004 0.004 0.008 0.030 0.007

1.23 2.74 3.11 2.16 2.54 4.64 12.48 12.46 13.29 14.98 15.24 13.86 1.29 100.00 69.82

46.58 79.87 86.94 85.04 82.02 92.38 95.88 96.38 96.93 97.37 97.16 98.68 99.60 94.86

0.02 0.02 0.06 0.02 0.01 0.06 0.07 0.08 0.08 0.06 0.06 0.25 3.18 0.13

106.4 š 1.8 118.3 š 0.4 176.3 š 0.9 236.8 š 0.6 261.3 š 1.4 297.2 š 0.4 305.0 š 0.4 300.4 š 0.2 297.3 š 0.2 294.3 š 0.2 294.7 š 0.1 294.2 š 0.3 300.5 š 1.4 284.2 š 0.3 296.0 š 0.2

Sample 5B: J D 0.01035: 490 19.74 0.01402 550 18.69 0.00620 585 18.08 0.00254 620 18.5 0.00203 650 17.97 0.00227 685 18.81 0.00174 715 17.76 0.00197 750 17.66 0.00196 785 17.67 0.00216 820 17.79 0.00223 850 17.90 0.00192 925 17.95 0.00131 Fusion 18.00 0.00140 Total 17.91 0.00234 Total without 490–620ºC, 925ºC – fusion

0.039 0.011 0.006 0.008 0.003 0.004 0.006 0.005 0.006 0.004 0.006 0.004 0.006 0.006

1.43 4.47 5.10 5.24 10.96 10.00 10.26 10.52 12.10 9.54 7.87 6.75 5.78 100.00 71.24

79.00 90.18 95.82 96.65 96.23 97.08 96.69 96.70 96.36 96.26 96.79 97.81 97.67 96.15

0.08 0.05 0.06 0.11 0.04 0.06 0.08 0.06 0.08 0.05 0.09 0.08 0.136 0.07

269.4 š 1.2 289.4 š 0.9 296.9 š 0.4 298.7 š 0.6 296.4 š 0.4 296.3 š 0.3 294.5 š 0.7 293.0 š 0.7 292.2 š 0.3 293.7 š 0.5 296.9 š 0.6 300.6 š 0.5 300.9 š 0.4 295.1 š 0.5 294.6 š 0.5

Sample 6: J D 0.010322: 480 19.80 0.02007 540 17.51 0.00450 575 17.42 0.00176 600 17.94 0.00511 645 18.27 0.00441 680 17.99 0.00230 710 17.99 0.00167 740 18.03 0.00238 805 18.07 0.00191 825 18.18 0.00203 845 18.28 0.00214 880 18.18 0.00206 Fusion 18.20 0.00144 Total 18.10 0.00273 Total without 480–645ºC, and fusion

0.005 0.010 0.015 0.011 0.007 0.003 0.006 0.003 0.007 0.006 0.016 0.013 0.029 0.009

1.63 4.17 3.90 3.66 7.14 12.55 8.41 8.00 9.45 9.86 14.95 13.47 2.80 100.00 76.69

70.02 92.37 96.99 91.56 92.84 96.19 97.22 96.06 96.84 96.67 96.52 96.62 97.64 95.56

0.01 0.006 0.24 0.06 0.04 0.04 0.10 0.04 0.10 0.08 0.20 0.17 0.54 0.12

241.3 š 1.6 278.4 š 0.7 289.9 š 0.6 282.6 š 0.3 291.0 š 0.6 296.4 š 0.3 299.3 š 0.7 296.6 š 0.3 299.4 š 0.3 300.7 š 0.4 301.8 š 0.8 300.6 š 0.5 303.8 š 1.2 296.1 š 0.5 299.4 š 0.5

Release temp. (ºC)

40 Ar=39 Ar a

36 Ar=39 Ar a

37 Ar=39 Ar b

Ca

(%)

Apparent age (Ma) d

Danubian ‘Parautochthon’ Sample 4: J D 0.009293: 480 14.03 0.02535 540 9.13 0.00620 570 12.71 0.00560 605 17.75 0.00897 640 20.45 0.01243 675 20.86 0.00537 710 20.67 0.00286 745 20.23 0.00246 775 19.89 0.00205 805 19.58 0.00172 850 19.65 0.00187 875 19.32 0.00085 Fusion 19.58 0.00026 Total 19.29 0.00301 Total without 480–710ºC, and fusion Getic nappe complex

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Table 3 (continued) Release temp. (ºC)

40 Ar=39 Ar a

Sample 7B: J D 0.009855: 480 21.39 540 20.55 610 19.98 625 20.16 645 19.72 680 19.53 710 19.70 740 19.70 775 19.73 810 19.72 850 19.84 890 19.81 Fusion 19.54 Total 19.78 Total without 480–540ºC

36 Ar=39 Ar a

37 Ar=39 Ar b

Total 39 Ar (%)

Non-atmos. 40 Ar (%) c

36 Ar

Ca

(%)

Apparent age (Ma) d

0.01638 0.00618 0.00380 0.00397 0.00208 0.00170 0.00221 0.00291 0.00275 0.00271 0.00274 0.00272 0.00185 0.00278

0.007 0.008 0.006 0.004 0.004 0.004 0.004 0.004 0.003 0.004 0.008 0.007 0.004 0.005

0.80 2.97 5.18 8.45 11.92 13.38 10.64 9.47 6.85 7.39 8.80 6.11 8.04 100.00 96.23

77.35 91.09 94.36 94.15 96.85 97.41 96.65 95.61 95.85 95.92 95.89 95.91 97.18 95.85

0.01 0.03 0.04 0.02 0.05 0.07 0.04 0.04 0.03 0.04 0.08 0.07 0.06 0.05

272.5 š 1.2 305.4 š 0.5 307.5 š 0.6 309.4 š 0.5 311.1 š 0.4 309.9 š 0.3 310.2 š 0.7 307.2 š 0.6 308.3 š 0.8 308.3 š 0.6 310.0 š 0.7 309.6 š 0.4 309.5 š 0.6 309.0 š 0.5 309.4 š 0.5

0.01532 0.00451 0.00250 0.00163 0.00151 0.00122 0.00165 0.00059 0.00129 0.00146 0.00101 0.00051 0.00028 0.00158

0.010 0.009 0.002 0.002 0.002 0.002 0.003 0.002 0.004 0.003 0.003 0.003 0.002 0.003

1.38 4.00 6.69 9.47 9.75 10.04 7.82 6.64 9.01 9.00 11.65 10.95 3.60 100.00

65.32 88.24 93.85 96.00 96.22 96.84 95.72 98.43 96.60 96.20 97.38 98.65 99.27 96.01

0.02 0.05 0.025 0.04 0.04 0.05 0.06 0.10 0.09 0.06 0.09 0.15 0.20 0.07

149.2 š 1.7 174.3 š 1.2 196.1 š 0.4 202.1 š 0.5 197.8 š 0.2 193.9 š 0.2 190.4 š 0.2 194.2 š 0.2 189.9 š 0.2 191.1 š 0.2 194.9 š 0.2 197.9 š 0.3 207.1 š 0.3 194.0 š 0.3

Sample 11B: J D 0.010172: 480 17.68 0.01461 540 18.11 0.00301 575 18.31 0.00119 605 18.40 0.00165 640 18.41 0.00085 675 18.39 0.00088 710 18.63 0.00156 745 18.59 0.00104 780 18.77 0.00091 820 18.88 0.00165 860 18.86 0.00169 900 18.89 0.00119 Fusion 21.40 0.01105 Total 18.60 0.00188 Total without 480–605ºC, 900ºC – fusion

0.005 0.008 0.010 0.005 0.005 0.005 0.007 0.012 0.006 0.012 0.008 0.005 0.005 0.007

2.34 6.53 6.34 10.55 12.94 11.00 7.98 4.82 6.63 8.42 13.93 6.75 1.76 100.00 65.73

75.55 95.07 98.05 97.31 98.61 98.56 97.50 98.32 98.53 97.40 97.33 98.11 84.71 96.99

0.01 0.07 0.23 0.09 0.16 0.16 0.12 0.30 0.18 0.19 0.13 0.12 0.01 0.15

229.7 š 1.5 291.1 š 0.3 302.6 š 0.5 301.8 š 0.4 305.7 š 0.3 305.2 š 0.4 305.8 š 0.3 307.6 š 0.4 311.0 š 0.5 309.3 š 0.3 308.8 š 0.8 311.5 š 0.5 305.2 š 0.9 303.9 š 0.5 307.4 š 0.4

Sample 8B: J D 0.010105: 480 13.07 540 11.38 575 12.11 605 12.22 635 11.92 665 11.59 695 11.51 730 11.43 775 11.37 810 11.50 845 11.59 880 11.63 Fusion 12.13 Total 11.71 Supragetic nappe complex

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Table 3 (continued) Release temp. (ºC)

40 Ar=39 Ar a

Sample 12: J D 0.009555: 540 11.34 560 12.35 590 12.90 620 13.12 655 14.08 690 15.00 720 14.55 750 14.29 780 14.44 815 14.82 850 15.35 885 15.92 Fusion 16.52 Total 14.43

36 Ar=39 Ar a

0.00237 0.00108 0.00163 0.00214 0.00196 0.00178 0.00230 0.00133 0.00083 0.00053 0.00051 0.00037 0.00072 0.00126

Sample 13B: J D 0.010272: 490 18.10 0.01762 550 16.82 0.00311 585 17.54 0.00217 620 18.26 0.00253 655 18.22 0.00179 685 18.15 0.00128 715 18.18 0.00138 745 18.23 0.00190 780 18.26 0.00128 820 18.34 0.00166 885 18.35 0.000844 890 18.36 0.00077 Fusion 18.36 0.00059 Total 18.20 0.00165 Total without 490–585ºC, 855ºC – fusion Sample 14: J D 0.010305: 480 11.16 540 10.06 575 10.23 610 10.47 645 10.73 680 10.95 810 10.95 845 11.05 780 11.20 815 11.62 850 11.80 Fusion 11.79 Total 10.94

0.00951 0.00273 0.00100 0.00080 0.00194 0.00156 0.00096 0.00088 0.00113 0.00097 0.00095 0.00046 0.00148

37 Ar=39 Ar b

Total 39 Ar (%)

Non-atmos. 40 Ar (%) c

36 Ar

(%)

Apparent age (Ma) d

0.009 0.004 0.009 0.011 0.014 0.004 0.008 0.004 0.004 0.007 0.008 0.006 0.005 0.007

4.93 9.12 2.33 3.45 6.21 8.60 11.33 9.39 10.48 10.30 9.71 5.79 8.35 100.00

93.78 97.36 96.22 95.14 95.86 96.46 95.29 97.21 98.27 98.91 98.99 99.28 98.68 97.30

0.10 0.11 0.15 0.14 0.19 0.06 0.09 0.08 0.14 0.35 0.44 0.41 0.18 0.19

174.5 š 1.9 196.2 š 0.7 202.2 š 0.6 203.1 š 0.6 218.8 š 0.4 233.4 š 0.2 224.3 š 0.4 224.8 š 0.3 229.3 š 0.3 236.4 š 0.7 244.4 š 0.6 253.6 š 0.6 261.0 š 0.3 227.0 š 0.5

0.018 0.006 0.012 0.005 0.004 0.005 0.003 0.004 0.003 0.006 0.003 0.004 0.005 0.005

1.05 3.24 2.92 7.60 12.82 11.01 8.68 9.31 8.46 7.99 11.58 8.81 6.55 100.00 65.86

71.22 94.51 96.32 95.88 97.07 97.89 97.73 96.90 97.90 97.30 98.61 98.73 99.02 97.27

0.03 0.05 0.15 0.06 0.05 0.11 0.06 0.05 0.07 0.10 0.11 0.14 0.25 0.09

224.4 š 1.3 272.9 š 0.2 288.7 š 0.4 298.3 š 0.3 301.1 š 0.3 302.4 š 0.4 302.3 š 0.4 300.7 š 0.5 304.0 š 0.4 303.6 š 0.4 307.6 š 0.4 308.1 š 0.4 308.8 š 0.5 301.3 š 0.4 301.8 š 0.4

0.008 0.008 0.005 0.023 0.006 0.006 0.008 0.021 0.008 0.007 0.010 0.024 0.011

2.76 7.56 11.18 6.53 10.82 9.81 8.45 13.71 9.66 9.11 7.49 2.93 100.00

74.77 91.91 97.06 97.71 94.62 95.75 97.35 97.61 96.98 97.49 97.59 98.82 95.93

0.02 0.08 0.15 0.78 0.09 0.10 0.22 0.64 0.19 0.20 0.30 1.45 0.30

148.7 š 1.1 164.1 š 0.7 175.7 š 0.3 180.8 š 0.3 179.6 š 0.6 185.1 š 0.4 188.1 š 0.3 190.1 š 0.4 191.5 š 0.4 199.2 š 0.5 202.3 š 1.0 204.6 š 1.3 185.3 š 0.5

a Measured. b

Corrected for post-irradiation decay of 37 Ar (35.1 day 1=2-life). 36 40 tot. ( Aratmos. )(295.5)]= Artot. . d Calculated using correction factors of Dalrymple et al. (1981); two sigma intralaboratory errors. c [40 Ar

Ca

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Table 4 40 Ar=39 Ar analytical data for incremental-heating experiments on a sample of whole-rock phyllonite from within the thrust contact between the Getic and Supragetic nappe complexes, Southern Carpathian orogen, Romania Release temp. (ºC)

40 Ar=39 Ar a

36 Ar=39 Ar a

Sample 10: J D 0.010173: 440 15.28 0.01292 470 14.87 0.00054 510 18.03 0.01845 550 21.83 0.00365 585 18.23 0.00059 615 17.24 0.00079 650 17.01 0.00045 695 16.87 0.00015 720 16.98 0.00058 755 17.48 0.00183 790 17.77 0.00192 825 18.04 0.00438 Fusion 18.42 0.00347 Total 17.79 0.00206 Total without 440–585ºC, 790ºC – fusion

37 Ar=39 Ar b

0.149 0.00054 0.01845 0.00365 0.00059 0.00079 0.00045 0.00015 0.00058 0.00183 0.00192 0.00438 0.00347 0.00206

Total 39 Ar (%)

Non-atmos. 40 Ar (%) c

36 Ar

1.84 1.24 2.62 10.33 9.00 8.98 8.35 12.79 17.13 11.70 8.53 4.09 3.39 100.00 58.95

75.05 99.01 69.87 95.16 99.04 98.63 99.20 99.72 98.96 96.88 96.79 92.82 94.41 96.63

0.31 12.55 0.48 2.58 2.57 1.44 1.04 2.32 0.70 0.21 0.35 0.37 0.35 1.39

Ca

(%)

Apparent age (Ma) d 199:0 š 1:0 251:8 š 1:0 217:5 š 1:1 345:9 š 0:3 304:2 š 1:3 287:7 š 0:7 285:6 š 0:5 284:9 š 0:6 284:6 š 0:4 286:7 š 0:6 290:8 š 0:3 283:7 š 0:4 293:8 š 1:8 290:4 š 0:6 285:7 š 0:5

a Measured. b

Corrected for post-irradiation decay of 37 Ar (35.1 day 1=2-life). 36 40 tot. ( Aratmos. )(295.5)]= Artot. . d Calculated using correction factors of Dalrymple et al. (1981); two sigma intralaboratory errors. c [40 Ar

Tismana valley (location 1). It is characterized by an internally discordant 40 Ar=39 Ar age spectrum. Age variations are matched by fluctuations in apparent K=Ca ratios (Fig. 2). The intermediate-temperature gas fractions record systematically decreasing apparent ages (ca. 640–610 Ma). The 835–905ºC increments generally record similar apparent ages which range between ca. 596 and 591 Ma. These probably are geologically significant, and probably date postmagmatic cooling of the late Proterozoic magmatic suite. A hornblende concentrate (sample 2) was also prepared from an amphibolite sample collected within the Dragsan Group exposed in southwesternmost sectors of the Danubian unit (Fig. 3). Although no significant variations in K=Ca ratios are displayed in the intermediate- and high-temperature portions of the analysis, apparent ages systematically increase from ca. 235 to ca. 295 Ma (except fusion). This type of internal discordancy has been described for hornblende within crystalline terrains affected by several metamorphic episodes (e.g., McDougall and Harrison, 1988). In the present setting the discordance may reflect incomplete rejuvenation

of intracrystalline argon systems that initially cooled through appropriate argon retention temperature following Variscan orogenesis. Sample 3 is from a nonretrogressed Dragsan amphibolite exposed in the Jiu valley proximal to a pre-Alpine shear zone. Apparent 40 Ar=39 Ar ages systematically increase from 113:7 š 1:8 to 300:0 š 0:7 Ma (Fig. 3). 36 Ar=40 Ar vs. 39 Ar=40 Ar isotope correlation of the 860–980ºC heating increments yield an age of 299:5 š 0:4 Ma. The inverse ordinate intercept (40 Ar=36 Ar) is close to the present-day atmospheric ratio, and indicates no significant intracrystalline contamination with extraneous argon components. The ca. 300 Ma isotope correlation age is considered geologically significant and is interpreted to date last cooling through temperatures required for intracrystalline argon retention. Harrison (1981) suggested temperatures of ca. 500 š 25ºC are appropriate. A muscovite concentrate prepared from a basement gneiss sample collected within the Polatistea valley (sample 4). The concentrate displays an internally discordant age spectrum (Fig. 4) in which apparent ages systematically increase throughout lowtemperature portions of the analysis. The intermedi-

R.D. Dallmeyer et al. / Tectonophysics 290 (1998) 111–135

Fig. 2. 40 Ar=39 Ar apparent age and apparent K=Ca spectrum of a hornblende concentrate from the Tismana diorite of the lower Danubian nappe complex. Analytical uncertainties (two sigma, intralaboratory) represented by vertical width of bars. Experimental temperatures increase from left to right.

ate- and high-temperature increments record similar apparent ages which define a plateau of 296:0 š 0:2 Ma. The character of spectra discordance displayed by the muscovite concentrate is identical to that which has been described for partially rejuvenated muscovite in other polymetamorphic settings (e.g., Dallmeyer and Takasu, 1992). Comparison of the present discordant muscovite spectrum from results in other areas suggest that initial cooling through argon retention temperatures (ca. 375 š 25ºC: e.g., Cliff, 1985; Blanckenburg et al., 1989) occurred at ca. 296 Ma. This appears to have been followed by slight Mesozoic rejuvenation (<10%) of intracrystalline argon systems. 5. Getic nappe complex Two hornblende concentrates were prepared from amphibolite samples (5A and 8A) collected within

123

southwestern exposures of the Getic nappe (north of Turnu Severin close to the Danube). Both concentrates record well-defined intermediate- and hightemperature plateau ages of 319:3 š 0:5 Ma and 322:6 š 0:6 Ma (Fig. 5). Plateau isotope-correlations are also well-defined and yield ages of 319:0 š 0:5 and 322:6 š 0:6 Ma. A hornblende concentrate from amphibolite collected from the Bahna klippe (Getic nappe; sample 9) displays a discordant 40 Ar=39 Ar apparent age spectrum with relatively old apparent ages recorded in low temperature increments (Fig. 5). No significant variation in K=Ca ratios is observed. The 850–965ºC increments yield a well-defined isotope correlation age of 314:8 š 1:2 Ma age that is interpreted to be geologically significant. Two muscovite concentrates were prepared from schist samples collected at the same localities (5B and 8B). That from sample 5B records a well-defined plateau age of 294:6 š 0:5 Ma (Fig. 6). That from sample 8B yielded a discordant pattern where 40 Ar=39 Ar apparent ages increase from 149:2 š 1:7 Ma to 207:1 š 0:3 Ma. These spectral characteristics are interpreted to reflect variable Mesozoic (Early Jurassic) thermal rejuvenation at ca. 200 Ma. A muscovite concentrate from a gneiss sample collected at location 6 (NW of Turnu Severin) is characterized by an internally concordant age spectrum (Fig. 6) which defines a plateau age of 309:5 š 0:5 Ma. A gneiss sample was collected (location 7) in the vicinity of a regional ductile shear zone along the tectonic boundary between the Getic and Supragetic nappes in northwestern sectors of the Sebes Mountains (nonretrogressed schist of the Sebes–Lotru Series). The muscovite concentrate from this sample records a 299:4 š 0:5 Ma plateau age (Fig. 6). 6. Shear zone between Getic and Supragetic units A whole-rock sample of phyllonite was collected at location 10 within a shear zone proximal to the structural base of the Supragetic unit (Hann and Balintoni, 1988; Hann, 1995). Low-temperature gas fractions are characterized by considerable variations in apparent age and relatively low apparent K=Ca ratios. These are attributed to gas evolved from constituent chlorite. The intermediate- and high-tem-

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Fig. 3. 40 Ar=39 Ar apparent age and apparent K=Ca spectrum of two hornblende concentrates from metamorphic rocks in the Danubian unit. Data plotted as in Fig. 2.

perature gas fractions are characterized by higher apparent K=Ca ratios and are interpreted to represent gas evolved from constituent, very fine-grained

white-mica (Fig. 7). These record generally similar apparent ages which define a plateau of 285:7 š 0:5 Ma. 7. Supragetic nappe

Fig. 4. 40 Ar=39 Ar apparent age spectrum of a muscovite concentrate prepared from the Danubian unit. Data plotted as in Fig. 2.

Hornblende concentrates have been prepared from two samples of amphibolite collected within exposures of the Supragetic nappe exposed in the Fagaras and Cibin Mountains, respectively (11A, 13A). The two concentrates display similar internally discordant age spectra as a result of low-temperature experimental evolution of gas from compositionally variable phases. Intermediate- and high-temperature gas fractions display minor intrasample variations in apparent K=Ca ratios and define similar plateau ages of 317:0š0:8 Ma and 318:7š0:6 Ma (Fig. 8). 36 Ar=40 Ar isotopic correlations of the plateau data are welldefined (MSWD <2.0), with inverse ordinate intercepts (40 Ar=36 Ar ratios) only slightly larger than the

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125

Fig. 5. 40 Ar=39 Ar apparent ages and apparent K=Ca ratios from hornblende concentrates from the Getic nappe complex. Data plotted as in Fig. 2.

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Fig. 6. 40 Ar=39 Ar apparent age spectra of muscovite concentrates from gneisses and schist of the Getic nappe complex. Data plotted as in Fig. 2. 40

Ar=36 Ar ratio in the present-day atmosphere. Using the inverse abscissa intercepts (40 Ar=39 Ar) ratios in the age equation yields plateau isotope correlation ages similar to the calculated values. The plateau ages are considered geologically significant and are interpreted to date the last cooling through temperatures required for intracrystalline argon retention.

Fig. 7. 40 Ar=39 Ar apparent age spectrum of a whole-rock phyllonite of a shear zone that separates the Getic and Supragetic nappe complexes. Data plotted as in Fig. 2.

Muscovite concentrates from samples of schist collected from the same two exposures (11B, 13B) and from two additional sites (12, 14) within Fagaras and Sebes–Lotru Mountains have been prepared. Samples 11B and 13B record well-defined plateau ages of 307:4 š 0:4 Ma and 301:8 š 0:4 Ma (Fig. 9). These are interpreted to date the last cooling through appropriate argon retention temperatures. Slightly younger apparent ages are recorded at lowermost experimental temperatures and suggest partial (<10%) Mesozoic rejuvenation of intracrystalline argon systems. A muscovite concentrate was prepared from a mylonitic quartzite collected at Bilea Cascada in the northern sectors of the Fagaras Mountains (12). The mylonite is from a shear zone associated with a penetrative retrogression overprint within greenschist-grade metamorphic conditions. The concentrate is characterized by a very discordant age spectrum which suggests extensive Mesozoic rejuvenation of intracrystalline argon systems which had initially cooled through appropriate closure temperatures some time prior to ca. 250 Ma. A muscovite concentrate was also prepared from a more retrogressed mylonitic schist collected

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at location 14 proximal to a ductile shear zone (basal thrust of the Supragetic nappe) within the Sebes– Lotru Mountains. This is characterized by an internally discordant spectrum which suggests extensive Mesozoic rejuvenation of intracrystalline argon systems which had initially cooled through appropriate argon retention temperatures some time prior to ca. 200 Ma. 8. Tectonic implications The regional age distribution of the new Ar=39 Ar whole-rock and mineral ages from the Southern Carpathians is summarized in Fig. 10. The data suggest that four distinct tectonothermal events are recorded. These include: (1) local evidence of Cadomian tectonothermal activity in the lower Danubian nappe complex; (2) regional evidence for ‘late’ Variscan (Carboniferous) penetrative tectonothermal activity; (3) local ductile shearing close to the base of the Supragetic nappe; and (4) minor thermal overprint and retrogression during a regionally nonpenetrative overprint at ca. 200 Ma (ca. Triassic=Jurassic boundary according to calibrations of Gradstein et al., 1994). The last event affected portions of the Getic and Supragetic units. In addition, local, nonpenetrative tectonic reworking of basement occurred during Late Cretaceous nappe assembly. This has not been not been recorded in the samples analysed in the present study because these were generally collected within unretrogressed basement rocks except samples 8, 12, 14. Hornblende and muscovite ages from nonretrogressed, medium-grade metamorphic sequences suggest that the regionally penetrative high-temperature mineral assemblages and associated ductile structural elements displayed within structural units of the Danubian nappe complex and Getic and Supragetic nappes are related to late Paleozoic (‘late’ Variscan) tectonothermal activity. No record of pre-Variscan tectonothermal activity has been found except for the Tismana granitoids of the Danubian nappe complex. A systematic relationship between hornblende cooling ages (dating ca. 500ºC), and muscovite cooling ages (ca. 375ºC) has been observed within all regional medium-grade metamorphic basement sequences in the Southern Carpathians (Fig. 11). These similar ‘late’ Variscan mineral ages con-

40

Fig. 8. 40 Ar=39 Ar apparent age spectra and apparent K=Ca ratios from hornblende concentrates of the Supragetic nappe complex. Data plotted as in Fig. 2.

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Fig. 9. 40 Ar=39 Ar apparent age spectra of muscovite concentrates from of the Supragetic nappe complex. Data plotted as in Fig. 2.

strain a regional exhumation of metamorphic sequences that was contemporaneous with localized deposition of Late Carboniferous terrestrial sedimen-

Fig. 10. Generalized tectonostratigraphy of the Southern Carpathian orogen and variations in 40 Ar=39 Ar muscovite and hornblende ages.

tary successions exposed within both the western Danubian nappe complex and along the western boundary between the Getic and Supragetic nappes (Fig. 12). These sedimentary sequences occur only along distinct zones that may represent localized, fault-bounded sedimentary basins that developed in the vicinity of evolving tectonic boundaries between both the Danubian and Getic nappes, and the Getic and Supragetic nappes (Fig. 1b, Fig. 12a). The Late Carboniferous sequence in both basins initiated with deposition of Westfalian B=C terrestrial sediments (Kra¨utner et al., 1981; Sandulescu, 1984; Nastaseanu, 1987). Following time-scale calibrations of Harland et al. (1990) and Gradstein and Ogg (1996) the Westfalian B=C boundary corresponds to ca. 310–300 Ma. These relationships suggest that cooling and associated exhumation of medium-grade basement sequences likely was contemporaneous with sediment deposition in associated sedimentary basins. This suggests a tectonic linkage between subsidence along faults and exhumation of metamorphic sequences. Sedimentary clasts within Late Carbonif-

R.D. Dallmeyer et al. / Tectonophysics 290 (1998) 111–135

Fig. 11. Late Variscan cooling paths based on Ar thermochronology recorded within various basement complexes of the Southern Carpathian orogen. Note significant difference in timing of cooling and exhumation between the Danubian and Getic=Supragetic basement units, and the contemporaneous formation of terrestrial sedimentary basins.

erous units indicate erosion of low- to medium-grade metamorphic sequences with lithologic characteristics generally similar to those presently exposed in portions of the Southern Carpathian orogen (Nastaseanu, 1987). Furthermore, distinct differences in the chronology in post-metamorphic cooling are recorded in the Supragetic=Getic units (320 Ma) and the Danubian units (ca. 300 Ma). Late- to post-orogenic exhumation of metamorphic sequences and contemporaneous subsidence of terrestrial basins may be explained by two different tectonic processes: (1) within contractional settings where terrestrial basins form as peripheral foreland basins (similar to Recent setting of intra-continental tectonics in the Pamir–Tienshan region, where thrusts reach the surface in front of intra-continental subduction zones (e.g., Molnar and Lyon-Caen, 1989; Strecker et al., 1995); or, (2) as post-orogenic collapse basins that form above uprising metamorphic core complexes (similar to the Basin and Range Province in western North America: e.g., Davis and Lister, 1988; Malavieille, 1993; or to the Norwegian Caledonides: Seguret et al., 1989; Chauvet and Dallmeyer, 1992). The first possibility is considered most appropriate for the Southern Carpathian orogen because of the general contractional tectonic setting which has been

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widely documented between Europe and Gondwana in the late Paleozoic (e.g., Arthaud and Matte, 1977; Muttoni et al., 1996). Also, Permian tectonic activity on steep shear zones is locally recorded in Southern Carpathians. These may have resulted from orogenic polarity from Danubian units (later cooling) towards Getic=Supragetic units (earlier cooling; Fig. 12a). Retrogressive shearing is recorded by the ca. 286 Ma whole-rock phyllonite age (Fig. 12b). This shear zone comprises part of a regional system of mainly dextral shear zones that affected the entire circumAtlantic Variscides during intracontinental deformation following collision between Gondwana and Laurussia (e.g., Arthaud and Matte, 1977; Muttoni et al., 1996). This has been documented in other portions of the Variscan orogen exposed in central Europe (e.g., Echtler and Malavieille, 1990; Cassard et al., 1993; Brandmayer et al., 1995). The origin of the Permian shear zone exposed in the Southern Carpathians may have been closely linked with the final exhumation of metamorphic sequences and the continued subsidence within Late Carboniferous sedimentary basins which were continuously evolving in the Late Carboniferous and the Permian. Late Variscan cooling and exhumation (between 330 and 300 Ma) of medium-grade metamorphic sequences are commonly observed in structurally higher tectonic levels an internal structural basement units of the Alpine–Carpathian belt. These include the Southalpine units of the Alps (Hammerschmidt and Sto¨ckhert (1987), the Austroalpine units of the Eastern Alps (Frank et al., 1987; Dallmeyer et al., 1996), Western Carpathians (Cambel and Kral, 1989; Krist et al., 1992; Maluski et al., 1993), Eastern Carpathians (Kra¨utner, 1988; our own unpublished results) and Apuseni Mountains (Dallmeyer et al., 1994). This suggests that late Variscan cooling regionally affected central and southern external portions of the Variscan orogen. The localized ca. 200 Ma old retrogressive overprint observed within the Southern Carpathian orogen would have been coeval with the rapid rifting in the Severin trough between the Danubian and Getic realms (Burchfiel, 1976; Sandulescu, 1984). Rifting may have been associated with a non-penetrative thermal overprint of basement units that had initially cooled following ‘late’ Variscan tectonic events. Other zones of retrogression likely were as-

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Fig. 12. Late Variscan and Mesozoic tectonic evolution of the Southern Carpathians orogen constrained by the new ages. For explanation, see text.

sociated with Early Jurassic subhorizontal ductile shear zones which developed in association with attenuation of the crust during Early Jurassic extension of Variscan crust (Fig. 12c). The most penetrative

40 Ar=39 Ar

mineral

overprint appears to have occurred proximal to the present structural base of the Getic nappe. This suggest that a Cretaceous thrust may have developed along a reactivated Jurassic extensional shear zone,

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and that portions of the Getic nappe may represent an Early Jurassic extensional allochthon that was structurally inverted during Cretaceous contraction (Fig. 12d). The Cretaceous thermal overprint associated with nappe assembly apparently was minor and did not result in penetrative deformation of the basement. This is consistent with recent studies elsewhere in the Alpine–Carpathian system that described only localized Alpine ductile fabrics (e.g., Ratschbacher et al., 1993; Neubauer et al., 1997). The new 40 Ar=39 Ar ages record large-scale cooling and exhumation of metamorphic crust following Variscan orogenic events. The Alpine tectonic record appears to be limited to discrete, intermediate- or low-temperature ductile shear zones developed in basement rocks. Pre-Variscan tectonothermal events were apparently restricted to emplacement of Cadomian plutons within lower (southern) Danubian successions.

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40 Ar=39 Ar

ages were calculated from corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and Ja¨ger (1977). Intralaboratory uncertainties have been calculated by statistical propagation of uncertainties with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are ca. š1.25– 1.5% of the quoted age. Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39 Ar released within each temperature increment. A ‘plateau’ is considered to be defined in the ages recorded by two more contiguous gas fractions (with similar apparent K=Ca ratios) each representing >4% of the total 39 Ar evolved (and together constituting >50% of the total quantity of 39 Ar evolved) are mutually similar within a š1% intralaboratory uncertainty. Analyses of the MMhb-1 monitor indicate that apparent K=Ca ratios may be calculated through the relationships 0.518 (š0.00005)ð(39 Ar=37 Ar)corrected for the TRIGA reactor and 0.505 (š0.003)ð(39 Ar=37 Ar)corrected for the Ford Reactor. Plateau portions of the analyses have been plotted on 36 Ar=40 Ar isotope correlation diagrams. Regression techniques followed the methods of York (1969). A mean square of the weighted deviates (MWSD) has been used to evaluate isotopic correlations.

Acknowledgements The paper benefitted from discussions with Tudor Berza, Viorica Iancu, Marin Seclaman (all Bucharest), and Patrick Ledru (Orle´ans). We gratefully acknowledge help during field work by AnaVoica Bojar, Marin Seclaman, and Sorin Udubasa. We appreciate suggestions by two anonymous reviewers. Work has been supported by grants from the Tectonics Program of the National Science Foundation to R.D.D. (EARTH-9316042) and from the Austrian Research Foundation to F.N. (P8652-GEO). Appendix A.

40

Ar/39 Ar analytical methods

The techniques used during 40 Ar=39 Ar analysis of mineral concentrates generally followed those described by Dallmeyer and Takasu (1992). Optically pure (>99%) mineral concentrates and sized whole-rock powders were wrapped in aluminium foil packets, encapsulated in sealed quartz vials, and irradiated in the TRIGA Reactor at the U.S. Geological Survey in Denver. Variations in the flux of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 hornblende (Sampson and Alexander, 1987). The samples were incrementally heated until fusion in a doublevacuum, resistance-heated furnace following methods described by Dallmeyer and Gil-Ibarguchi (1990). Measured isotopic ratios were corrected for total system blanks and the effects of mass discrimination. Interfering isotopes produced during irradiation were corrected using factors reported by Dalrymple et al. (1981).

Appendix B. Sample descriptions B.1. Danubian unit Location 1: Tismana valley, ca. 3 km west of Tismana monastery. Metagranodiorite with a coarse-grained magmatic, but metamorphically overprinted fabric without preferred orientation of minerals. Magmatic biotite includes apatite inclusions, fine ore minerals along margins, and is partly replaced by chlorite. Amphibole is optically zoned with brown cores and green rims. Large sphene grains are of magmatic origin. Plagioclase forms large subeuhedral crystals, their cores are largely replaced by fine clinozoisite š chlorite. Location 2: Ponicova valley, 1 km north of Dubova village (west of Orsova at the Danube). Hornblende-bearing gneiss with plagioclase augens which is surrounded by fine-grained biotite. Hornblende forms large anhedral crystals with quartz inclusions. Old large quartz grains of the matrix are strained, and are partly replaced by undulose subgrains. Biotite is dark brown, and is partly replaced by chlorite. Location 3: Jiu valley between Petrosani and Bumbesti, roadcut ca. 900 m south of the confluent with the Poltistea valley. Amphibolite with a two-stage fabric. Stage I plagioclase is heavily transformed into a mixture of clinozoisite, sericite, and epidote. In contrast, stage I amphiboles are well preserved, anhedral, and contain only rare inclusions (quartz, sphene=rutile). Further constituents are sphene, often containing cores of rutile or opaque minerals, and chlorite; the latter occurs as product of limited retrogression. Location 4: Jiu valley of Petrosani, roadcut ca. 3.7 km to the south of the confluent with the Poltistea valley. Deformed,

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coarse-grained tonalite with a few shear bands. Plagioclase porphyroclasts (ca. 3–4 mm in diameter) are nearly completely replaced by sericite and zoisite, K-feldspar, too. About 2 mm long white-mica flakes are kinked and folded. Biotite is dark brown in the center, partly replaced by chlorite, and includes fine aggregates of sphene=rutile. Quartz forms large, undulose crystals which are strongly recrystallized along the rims (core– mantle texture). Cores of these quartz grains are transformed into equidimensional, blocky subgrains. Minor constituents are epidote, chlorite, opaque minerals, apatite, and zircon.

B.2. Getic unit Location 5: Rovesti valley, western tributary to the Olt within the Sebes–Lotru Mountains; ca. 1.5 km west of the lower contact of the Getic basement and low-grade cover. Sample 5A: Biotite plagioclase amphibolite with unzoned minerals and a well-annealed single stage metamorphic fabric. Sample 5B: Strongly foliated, coarse-grained schist with mica layers alternating with quartz–feldspar–garnet layers. Large white-mica grains are often deformed to mica fishes that are replaced by fine biotite along margins. Garnet is stretched and partly replaced by biotite and chlorite. Large biotite grains are often recrystallized containing fine opaques and=or sphene=rutile exsolutions. K-feldspar forms large crystals that are surrounded by quartz–feldspar–garnet layers. Quartz grains are large and serrated along margins. Location 6: Road along the Danube, ca. 700 m to the west of the Iron Gate dam west of Turnu Severin. Sample 6A: Well-recrystallized and annealed, fine-grained amphibolite with a perfectly developed foliation, and strong hornblende lineation. Main constituents are plagioclase, quartz, and amphibole. Amphibole is optically unzoned, and contains rare large quartz inclusions. Plagioclase is zoned, twinned, and comprises rare sericite=clinozoisite inclusions as alteration products of plagioclase. Biotite includes linear exsolution of opaque minerals, and is rarely partly chloritized. Accessories are sphene, opaque minerals, apatite, and rutile. Sample 6B: Entirely annealed, well-foliated gneiss. Quartz forms coarse crystals with sutured boundaries. A few, tiny garnet grains occur within quartz. Well-recrystallized plagioclase, K-feldspar, white-mica, and biotite are other major constituents. Biotite is sometimes replaced by chlorite, opaques, rutile=sphene. Location 7: Sebes valley south of Deva within the Sebes– Lotru Mountains, southern edge of Martinec village. Heavily overprinted, mylonitic micaschist that display S–C fabrics, and shear bands due to the vicinity to the overlying Supragetic nappe. Muscovite forms large mica fishes and flakes, that partly contain fine opaque exsolution lamellae parallel to the cleavage, and fine sericite along margins. Quartz occurs, together with rare plagioclase, in layers, is undulose and bear irregular boundaries. Biotite is partly chloritized, and includes fine opaque minerals along boundaries. Location 8: Bahna valley northeast of Orsova, ca. 1 km to the west of Ciresu village, respectively ca. 4 km to the north of Bahna village.

Sample 8A: Entirely recrystallized and annealed, mediumgrained amphibolite with cracks cross-cutting the foliation. Homogeneous brown-green hornblende contains large quartz, plagioclase, and sphene (C rutile in cores) inclusions, and has straight to lobate grain boundaries. Plagioclase is homogeneously composed. Rare biotite contains sphene=rutile exsolutions, and is locally replaced by chlorite. Further phases are epidote and opaques. Sample 8B: Gneiss with predominant plagioclase and quartz, subordinate K feldspar, and biotite and muscovite. Muscovite is intergrown with biotite. Garnet forms fine anhedral grains, that are partly transformed into biotite. Minor constituents are chlorite, opaque minerals, rutile, apatite, and zircon. Location 9: Road from Cerna valley to Baie de Arame, Marasesti village, ca. 8 km west of Baie de Arame. Well-recrystallized hornblende gneiss. Main constituent is plagioclase that is partly strained as evidenced by kinked twin boundaries. Grain boundaries are straight to serrate. Clinozoisite and sericite forms alteration products after plagioclase. Green hornblende has lobate boundaries, and sometimes contains large plagioclase inclusions. Quartz is undulose. Clinozoisite, opaque minerals, apatite, and zircon are accessories. Clinozoisite also occurs in some narrow veins that cross the penetrative foliation.

B.3. Shear zone between the Getic and Supragetic units Location 10: Southern edge of Rasinari village near Sibiu. Mylonitic sericite quartzite. Strongly foliated phyllonite with layers variable in grain size. Layers with very fine quartz and subordinate sericite dominate. Quartz is partly elongated (aspect ratio ca. 2 : 1). Some coarser layers contain well-recrystallized, polygonal quartz (with diameters about ten times larger than in fine-grained layers) with sutured grain boundaries, some calcite, sericite, chlorite, and very minor albite.

B.4. Supragetic unit Location 11: Road across Fagaras Mountains to the north of Curtea de Arges, close to Capris village north of the dam. Sample 11A: Medium-grained amphibolite (grain size ca. 1 mm), granular, nearly unfoliated fabrics with patches of equidimensional, well-annealed hornblende. Brown-green hornblende possesses straight grain boundaries, is optically unzoned and of brown-green color. Rims of fine hornblende grains often occur around plagioclase. Well-recrystallized plagioclase and subordinate quartz, sphene, opaques and apatite are further constituents. Sample 11B: The quartzitic schist displays a two-step fabric; a layering is visible by alternating white-mica and quartz layers. Quartz grains are elongated parallel layers and entirely annealed (textural stage 1). Ellipsoidal garnet grains are also parallel with long axis to mica layers. Garnet bears quartz inclusions and is not retrogressed. Plagioclase which is nearly entirely transformed to sericite due to cataclastic overprint along shear bands and due to mica fish formation and intrafolial folding (textural stage 2).

R.D. Dallmeyer et al. / Tectonophysics 290 (1998) 111–135 Location 12: Fagaras Mountains, Bilea valley; Trans-Fagaras road above Bilea Cascades chalets. Micaschist with strong retrogression. Main constituents are muscovite fishes, undulose quartz, and chlorite. Large mica fishes of the first textural generation are marginally recrystallized to sericite. Location 13: Olt valley south of Sibiu, roadcut at Ciineni Mari village, southern edge. Sample 13A: A strongly foliated amphibolite with cracks across the foliation. Hornblende is optically slightly zoned (green cores and dark green rims) and bears some quartz and rare biotite inclusions. Generally xenomorphic biotite is aligned parallel to the foliation, and contains sphene exsolutions along boundaries. Plagioclase, quartz, opaques, and apatite are minor phases. Sample 13B: Entirely recrystallized and annealed, well-foliated micaschist. Essential constituents are quartz, plagioclase (oligoclase), muscovite, biotite, and garnet. Quartz is annealed with straight to slightly curved grain boundaries. Garnet is sometimes stretched but entirely in equilibrium with surrounding minerals and contains biotite, muscovite, and quartz inclusions. Biotite occurs in two textural types. Biotite from the mica matrix is not recrystallized and never transformed into chlorite. A few grains from cross-grown biotite are nearly completely altered into chlorite C ilmenite C other opaque minerals. Location 14: Sebes valley south of Sebes, 1 km north of the dam and ca. 1 km south of Capilna village. Porphyroclastic protomylonitic gneiss with fine-grained, strongly foliated fabric and shear bands. Quartz forms heavily elongated grains which bear fine recrystallized quartz grains along boundaries. Plagioclase forms porphyroclasts, and augen when occurring with quartz. White-mica forms porphyroclasts and is often recrystallized along margins. Biotite occurs in fishes, too. Rarely occurring garnet is stretched and partly replaced by a fine sericite=chlorite mixture.

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