Contrib Mineral Petrol (1995) 119:83-93 �9 Springer-Vertag 1995

Kurt Stiiwe - Roger Powell

PT Paths from modal proportions: application to the Koralm Complex, Eastern Alps

Received: 13 July 1993 / Accepted: 22 August 1994

Abstract Thermodynamic pseudosections portray those parts of a petrogenetic grid that arc relevant to a given bulk composition and the reactions appearing on them can therefore be used directly to infer the PT path that the rock followed. However, for many 'nor- mal' bulk compositions the use of pseudosections is hampered by the fact that they display only few large fields of high thermodynamic variance in the P T range of interest. Here it is discussed how modal information on reaction progress within these fields can be used to determine PT path information for thermodynamically high variant metamorphic assemblages. We use this information on reaction progress to contour pseudo- sections for modal proportions of minerals using the software package THERMOCALC. The approach is applied to di- tri- and quadrivariant assemblages from the Koralm complex in the eastern Alps. A PT path for these rocks is derived from modal considerations and compared with interpretations of mineral composition contours on the same pseudoscction and with conven- tional thermobarometry. It is shown that at least part of the complex must have cooled initially near isobari- cally from prevalent peak conditions around 700~ and 14 kbar before the rocks commenccd a Barrovian- type decompression path.

Introduction

Petrogcnctic grids are one of the most important tools in defining the pressure (P) and temperature (T) of

K. St/Jwe ( ~ ) Department of Geology and Geophysics, The University of Adelaide, Adelaide, SA 5005, Australia

R. Powell Department of Geology, Melbourne University, Parkville, Vic 3052, Australia

Editorial responsibility: V. Trommsdorff

formation of rocks and in infering P T paths that rocks followed during their tectonic evolution. For example, pelitic rocks can be described well in the system K20- FeO-MgO-A1203-SiO2-H20 (KFMASH), for which a petrogenetic grid is now well known (e.g. Powell and Holland 1990; Xu et al. 1994). The complexity of such grids may greatly be reduced by constructing pseudosections which portray only those parts of a full petrogcnetic grid that are relevant to a particular bulk composition (c.g. Dymoke and Sandiford 1992; Vance and Holland 1993; Xu ct al. 1994). However, the use of pseudosections may be limited because most rocks have bulk compositions that form mineral assemblages of high thermodynamic variance so that no new phases react in or out in these rocks, even for substantial changes of P and T. Bulk compositions with assem- blages of higher thermodynamic variance are therefore much more difficult to use for simple, qualitative PT estimates. As a consequence, geologists spend much of their time in the field searching for low-variance assem- blages so that reaction textures and appearance of phases may be used to estimate the conditions of formation. Additional information from high-variant assemblages may be obtained from information on reaction progress. Modal proportions and mineral compositions change in continuous reactions inside these fields (Thompson and Thompson 1976) and may, in principle, be used as an indicator for the PT path of a given rock (e.g. Thompson et al. 1982). While there have been a number of applications of this approach (Chamberlain 1986; Schneidermann 1990; Spear 1993, chapter 16), the direct use of mineral mode contours on pseudosections has been limited (although see: Brown and Skinner 1974; Saxena and Erikson 1983; De Capitani and Brown 1987). The calculation of informa- tion relating to modal reaction progress is now imple- mented in the software package THERMOCALC (Powell and Holland 1988) and it should, in theory, be possible to identify the formation conditions of well- equilibrated rocks by the modal abundance of their

84

minerals. In re-equilibrated rocks the sense of change of modal abundances will indicate the position of the .~ retrograde P T path. _x:

In order to assess the applicability of this logic, we apply it to "normal" metapelitic rocks from the Koralm crystalline in the eastern Alps. There, radiometric cool- ~8 ing ages have been interpreted to indicate rapid exhum- ation of the complex (Frank et al. 1983; Frank 1987) and an approximate P T path has been established by 16 Th/Sni and Jagoutz (1992). However, recent cooling rate studies show that the cooling history may, in part, be unrelated to exhumation (Ehlers et al. 1994). Therefore, 14 the nature of cooling, and therefore exhumation, his- tory remain unresolved, at least until depth-temper- ature relationships are better understood. Here we aim 12 to establish a more detailed P T path on modal grounds.

Modal proportions as a PI" indicator in KFMASH

Most pelitic and semipelitic rocks from the Koralm complex may be described in the system KFMASH. A revised petrogenetic grid for this system has recently been calculated by Xu et al. (1994). With foresight of the likely formation conditions of the rocks of the Koralm crystalline, we have extended their calculations to 22 kbar and 800~ (Fig. 1). We consider in our calcu- lations both the FeMg_l and the AI2Si_ i (Fe,Mg) 1 substitutions in mu, bi, chl, cd and the FeMg_ 1 substi- tution for ctd, gt and st (for abbreviations sec Table 1). The formulation of the compositional parameters, end member activities as well as the method of calculation are the same as those of Xu et al. (1994). All equilibria were calculated with the computer program THER- MOCALC (Powell and Holland 1988) with the ther- modynamic data set of Holland and Powell (1990, 1993, personal communication). In THERMOCALC, modal calculations are performed by calculating thc balanced reactions between the calculated composi- tions of the phases and the chosen bulk composition. For univariant reactions T H E R M O C A L C gives "en- try-mode" and "exit-mode" values referring to the mo- dal proportions of minerals on the low-T and high-T side of the reaction. Modal calculation in divariant fields is straightforward and is given directly as addi- tional output to the compositional parameters. In trivariant fields the composition of at least one phase must be fixed additionally to P and T to calculate the equilibria. To calculate the modes of the minerals an additional "bogus" phase is included in the calculation. A range of fixed compositions for the one phase is used and the required mode is the one for which the mode of the bogus phase equals zero. Mode output is in mole- percent normalised to one oxide so that the propor- tions may be viewed directly on compatibility dia- grams. In order to compare the values with the volumetric mode proportions these must bc divided by the molar volume and multiplied by the number of

10

J

%", /i/__}.. 2/~" (/ e 5~

:/ \

500 550 600 650 700

@

FMASH +H20

j f

I 750 ~

Fig. 1 Part of a petrogenetic grid for the system KFMASH with H20 in excess (extended to 22 kbar and 800 ~ C from Xu et al. 1994). Orthoamphibole and chloritoid equilibria are omitted. Within the P T space shown, orthoamphibole equilibria interfere only with the reactions st-bi-q -~ g-mu-ky and chl-q -+ ta-g-ky; chtoritoid-bear- ing equilibria interfere only with the reaction st-chl-q + g-ky and these do not affect the equilibria in the rocks of interest. Note that the reaction ta-chl-mu -+ bi-ky-g reaches almost but does not join the lower invariant point

oxides in the formula unit involved. We used average molar volumes of the different end members for each mineral (Table 1).

With the modal information for minerals as con- tours on pseudosections it should be possible to infer the P T conditions of formation of well-equilibrated metamorphic rocks by the intersection of the appropri- ate modal contours of all minerals in one rock on the pseudosection, in practice, rocks will not be available which show a range of well-equilibrated modes along a retrograde path. However, even small amounts of re- equilibration along such paths will indicate which mine- rals are growing and which are being consumed, thus indicating the nature of the P T path by the sense of change of the modal proportions.

In that the thermodynamic data used in calculating the K F M A S H system are not perfectly known and that there are difficulties regarding activity-composi- tion relations in some of the minerals (for example where Tschermak's substitutions are involved), the cal- culated modes are uncertain. However, akin to the identical problem in thermobarometry, the sense of change of mode with changing P T will be little affected

85

Table 1 A listing of the minerals and end members used in this study. The molar volumes and number of oxides that are given for each end member correspond to those of Holland and Powell (1990). Averaged molar volumes between different end members of each mineral are used if results of point counting are to be converted into mole percent. These averaged molar volumes are also listed

Mineral Formula Abbreviation V o l u m e Volmne Oxides Jbar- 1 used here

Staurolite st Mg-staurolitc MgaAll 8Siv.5048H 4 44.260 44.500 20.5 Fe-staurolite Fe4Al~sSiv.sO4sH 4 44.880 Garnet g Grossular Ca3AI2Si30 8 12.535 11.500 7 Pyrope Mg3AI2Si30 8 11.318 Almandine F%A12Si3() 8 11.511 Andradite Ca3Fe2Si30 8 13.204 Chlorite chl Clinochlore M g4(MgAI)Si2[SiA1]O 10(OH)a 21.090 21.000 8 Amesite Mg4(A12)Si2 [AI2] O 10(OH)8 20.920 Biotite bi Ann i t e KFeF%Si2[SiAI]O~ o(OH)2 15.432 15.000 7 Eastonite KMg(MgA1)Si2[A1] 2010(OH)2 14.751 Phlogopite KMgMg2Si2SiAIO 10(OH)2 14.964 Muscovite mu Muscovite KVA12 Siz [SiAl] O 1 o(OH)2 14.083 14.000 5 Margarite CaVAI2Si2A12010(OH)2 12.964 Celadonite KVMgAISizSi2Olo(OH)2 13.960 Na-phlogop. NaMg(Mg2)Si2[SiA1]O 1 o(OH)2 naph 14.450 Paragonite NaVA12Si2 SiA1010(O H)2 14.964 Alumosilicate as Kyanite A12SiO 5 ky 4.414 4.414 2 Sillimanite A12SiO s sill 5.003 5.003 Andalusite A12SiO 5 5.153 5.153 Quartz SiO 2 q 2.269 2.269 1

by the underlying uncertainties, even if the position of a particular mode in P T is uncertain. There are also minor errors introduced by using averaged molar vol- umes from different end members in one mineral.

If whole rock analyses are used to represent the composi t ion it must be ascertained that the equilibra- tion volume is much larger than any one grain size. For example, if garnets in a rock are growth-zoned, then those garnets should not contribute to the mode nor be included in the whole rock analysis. In well-equilib- rated rocks the "peak-mode" is representative of the whole rock composi t ion and may provide a good esti- mate of the peak metamorphic conditions, if little retro- grade mineral growth occurred. During cooling the equilibration volume contracts and it is important to compare modal changes with the size and bulk com- position of the equilibration volume at the time. The effective bulk composi t ion must be either nearing that of the bulk rock (at high T ) or nearing the composi t ion of a subset of phases of the peak assemblage (during cooling and contraction). This change of the bulk com- position will change the positions of the di- and trivariant fields on the pseudosection (Table 2), but will usually not change the topology of the pseudosection or the slope of the modal contours. In fact, even the positions of the modal contours shift substantially only for the phase towards which the bulk composit ion converges, while the numerical values of modal con- tours for thc other phases are much less affected. It should thcrefore be kept in mind that pseudosections

appropriate to whole rock composit ions may not neccessarly be a good indicator of the peak formation conditions, but that the sense of change of the modal proport ions will remain a good indicator of thc P T path on pseudosections contoured for bulk rock modes.

Geology of the Koralm Complex

The Koralm complex is one of the highest grade regions of the Eastern Alps that experienced the Cretaceous Eoalpine meta- morphic event. It forms therefore one of the key areas in understand- ing the Eoalpine metamorphism. It forms part of the Austroalpine nappe complex which was thickened and deformed during and after the Cretaceous event (Krohe 1987) forming a number of regionally important shear zones, for example the Plattengneis. The complex has a metamorphic history with signatures of pre-Alpine events (e.g. Frank et al. 1983; Frank et al. 1987) but it reached amphibolite to eclogite facies conditions at the Alpine metamorphic peak around 90 Ma (Morauf 1980, 1982; Miller and Frank 1983; Th6ni and Jagoutz 1992). Whilst exhumation has been considered to have been rapid (e.g. Frank et al. 1983), for example from the isotopic cooling history of the terrain (Morauf 1980; Th6ni and Jagoutz 1992), garnets and micas do overgrow the pervasive foliation that appa- rently formed during the cretaceous deformation, so that this inter- pretation is open to some discussion. Moreover, a thermometrically inferred cooling history for the terrain indicates a temperature-time history that is strongly concave towards the time axis with high cooling rates at high temperatures and slower cooling rates at low temperatures (Ehlers et al. 1994). This is markedly inconsistent with a cooling history that is the consequence of denudation (K. Stfiwe, unpublished data) and it is therefore not clear if the PT paths can be interpreted as the consequence of denudation alone. The ambiguities associated with the interpretations of the Cretaceous

86

Table 2 A listing of typical low- and high-variance assemblages from the Koralm crystalline complex. The first four assemblages are by far g-mu-q 4 the most common g-bi-mu-q 3 parageneses. [1 this study, g-bi-mu-ky-q 2 2 Wimmer-Frey (1984), g-ksp-mu-bi-ky-q 0 3 Kieslinger (1926), g-sl-bi-mu-ky-q 1 4 Heritsch and M6rtl (1977)] g-st-ctd-mu-ky-q 1

KFMAS peak KFMAS Secondary Rock type-name assemblage variance assemblage reference locality example

bi 1, 2 Plattengneis bi, mu 1, 2 Plaltengneis bi, mu 1, 2 Paramorphoseschiefer bi, mu 3 Zentraler gneisquarzit ctd, chl 3 Jankitzkogel

4 Jankitzkogel

tectono-thermal evolution of the terrain may be understood better if a P T path can be derived. Although an approximate P T path has been established from the eclogites (Th6ni and Jagoutz 1992) that occur as small lenses within the complex (Miller et al. 1988, 1990), the lack of low-variance assemblages and reaction textures in the metapelitic gneisses that form the bulk of the complex has largely hindered such work.

Petrography and modal information of pelitic gneisses

The rock types of the Koralm complex are dominated by pelitic and semipelitic gneisses with minor lenses of eclogite, marbles and ma- lics. The petrography of the pelites has been investigated in detail by a number of authors (e.g. Heritsch 1978; Wimmer-Frey 1984; Weber 1982) and been summarised by Frank et al. (1983). Here, we present a summary of the these studies. For details of the localities and description of the lithological units we refer to the extensive work by local authors (e.g. Beck Managetta 1980; Kleinschmidt and Ritter 1976; Kleinschmidt et al. 1984; Stfiwe 1994).

Most rocks in the Koralm complex are characterised by the trivariant Cretaceous peak assemblage g-bi-mu-q _+ ky but higher variance assemblages (g-mu-q) and lower variance assemblages (g- st-bi-mu-ky-q) are not uncommon (Table 2). Typical modal com- position of trivariant assemblages involve volumetric mineral ratios of the order of g : bi: mu : q = 10 : 15 : 25 : 50. Garnets have compli- cated zoning patterns and are texturally of at least two generations. Large porphyroblasls are prograde-zoned with narrow rims of retrograde inversion of thc zoning profile, while the small late-stage crystals preserve only retrograde zoning profiles (see Frank et al. 1983; Ehlers et al. 1994). Retrograde-zoned garnets range in core XMg between 0.3 (larger crystals) and 0,16 (smaller crystals) with rim Xug being lower by about 0.1. Large porphyroblasts have occasional core compositions with XMg at the low end of this range. The X w of biotite is generally around 0.4-0.5 (Ehters et al. 1994). The peak assemblages are intensely deformed. Growth of the small garnet generation, biotite and muscovite outlasted this deformation with generally 1 3% of the garnet having recrystallised or additionally grown post the deformation (Fig. 2b,c). In some highly deformed quadrivariant g-mu-q assemblages of the Plattengneis, biotite grows only in pressure shadows around garnet and it appears therefore that the trivariant assemblage g-bi-mu-q was only stabilised fi-om g-mu-q during deformation (Fig. 2a,b), while muscovite garnet and quartz were stable throughout the deformation history. Up to 10% late muscovite overgrows the foliation and the late garnets (Fig. 2e,g). One to two additional percent of late, statically grown biotite often overgrows the late-stage muscovites as a last reaction product (Fig. 2e,f). Muscovite also forms static seams between gar- net and kyanite whilst biotite grows between muscovite and garnet (Fig. 4c,g). Kyanite is destabilised during and after the deformation (Fig. 2c,g). Chlorite and staurotite are rare, but have been found as part of retrograde assemblages, in particular in the basal parts of the Koralm (Weber 1982). Retrograde sillimanite was found in one sample to overgrow all previously grown retrograde micas (Fig. 2h).

For the modal interpretations, bulk compositions of four sam- ples were obtained by XRF analysis and mineral modes were ob-

tained by point counting in thin section (Table 3). The samples include one from the Plattengneis with the assemblage g-mu-q plus biotite which only appeared during the delbrmation (48/92) and three samples from less-deformed rocks. Sample (26/92) contains abundant kyanite as part of the peak assemblage, sample (24/92) contains the trivariant peak assemblage g-bi-mu-q and sample (29/92) containing a trivariant peak assemblage g-bi-mu-q and ac- cessory amounts of retrograde sillimanite. Details of the mineralogy of each of these samples and details of the corresponding pseudosec- tions are discussed by Stfiwe (1994).

Conventional P T estimates

Because of the pervasive overprinting relationships of retrograde assemblages, P T estimates for the metamorphic peak conditions are difficult to obtain and many results appear to be mixed estimates. Temperatures of 590-600 ~ C have consistently been found for garnet rims in equilibrium with biotite in the Plattengneis (Frank et al. 1983; Wimmer-Frey 1984), and pressures around 14 kbar have been recorded from g-plag barometry in the same rocks. Temperatures of up to 700~ have been found by Ehlers et al. (1994) from equilibria of cores of garnets with retrograde zoning profiles with averaged biotite analyses. As calculated average PT calculations give poorly constrained temperatures ranging from 600~ to above 800 ~ average pressures were calculated u sing the likely metamorphic peak temperature of around 700 ~ C (Table 4). The calculations indicate equilibration of the rocks around 13.5 to 15 kbar with one c~ errors of about 2 kbar. For the eclogites, Miller (1990) derived peak pressures above 16 kbar at 580 630 ~ C. Retrograde assemblages at the base of the Koralm complex formed at 570 ~ C and 7 kbar (Weber 1982). A P T path derived from the eclogites by Th6ni and Jagoutz (1992) is consistent with these estimates and indicates near isother- mal decompression during the Cretaceous from probably above 15 kbar to 7 kbar at about 600-650 ~ C,

,=

Fig. 2a-h Photomicrographs of mineralogical features of meta- pelitic rocks from the Koralm complex: a a highly deformed quad- rivariant assemblage g-mu-q of the Plattengneis shear zone with trivariant biotite growth in pressure shadows around garnet during deformation, b as in (a) but showing recrystallisation (post deformation) of a deformed garnet mass (gtl) to small euhedral crystals (gt2). e statically recrystallised small garnet and kyanite crystals in a deformed rock. d fine-grained kyanite mass adjacent to garnet and biotite reaction texture between garnel and muscovite. e Late-stage muscovite growth in a trivariant deformed g-bi-mu-q paragenesis with last-stage biotite coronas around the late-stage muscovites, f: biotite corona around late-stage muscovite in a g-bi- mu-q paragenesis, g reaction texture between kyanite and garnet to form fine-grained muscovite, h as in (e) biotite overgrows muscovite in coronas. However, here very fine grained sillimanite overgrows the biotites as well. This indicates divariant production of aluminosilicate subsequent to retrograde muscovite and biotite growth

87

88

Table 3a d Bulk rock and modal composi t ion of the four samples selected for this study. Bulk rock analyses were obtained by X R F analysis, modal p ropor t ions were obta ined by point count ing in thin section, a Whole rock analyses from X R F analysis in FeO fbrm; b averaged and normal ised mole -propor t ions of K F M A S - c o m p o n c n t s of the pelitic gncisses; e point counted mineral volumes; d modal mineral propor t ions in mole percent normal ised to one molecule so that they may be plot ted directly on compatibil i ty diagrams. For the normalisat ion, volume percent from c are divided by averaged molar volumes of Table 1, multiplied with the number of oxides and normal ised to 100%. Note that the mineral p ropor t ions are from thin section and therefore assume that the equil ibrat ion volume has at least the size of the thin section. The data o fd may be compared directly with the modal contours on Fig. 5 but care must be taken in the considerat ion of pr imary and secondary par ts of the assemblage. (PG Plattengneis)

(a)

Samplc no. SiO 2 AlzO 3 F e O M n O M g O CaO N a 2 0 K20 TiO 2 P2Os Loss Total

24/92 68.06 14.33 5.14 0.08 1.83 1.16 2.24 2.55 0.75 0.32 1.57 98.04 29,/'92 58.92 19.73 6,98 0.18 2.76 1.02 1.56 4.24 0.99 0.16 2.90 99.44 26/92 62.99 18.08 6.69 0.10 2.45 0.84 1.77 3.35 0.91 0.09 2.77 100.04 48/92 68.58 15.46 4,70 0.07 1.68 t .20 1.95 2.97 0.72 0.12 1.85 99.30

(b)

Sample no. Rock type S K F M A S H K : F : M :A : S

24/;92: Biotite mica schist 91.91 3 : 5 : 3 : 1 0 : 7 9 29/92: Biotite mica schist 92.63 5 : 7 : 5 : 14 : 69 26/92: P G Kora lmsummi t 93.56 4 : 7 : 4: 12:73 48/92: P G Stainz 93.39 4 : 5 : 3 : 10: 78

K F M A S average pelite 4 : 6 : 4 : 11 : 75

(c)

Sample no. g ehl bi mu sill ky q plag ox tourm Total

24/92 102 - - 250 369 - - 181 501 3 - - 114 1520 29/92 122 - - 220 385 44 - - 399 14 1184 26/92 142 - - 192 416 - - - - 830 - - - - 1580 48/92 85 .... 224 93 - - 850 - - - - 1252

(d)

Sample no. g bi mu sill ky q Total

24,,"92 5 9 10 60 16 100% 29,,"92 15 20 27 3 - - 35 100% 26,,"92 13 13 21 - - 53 100% 48/92 9 19 6 66 100~

Pseudosections: compositional versus modal information

An appropriate pseudosection

A pseudosection of the petrogenetic grid of Fig. 1 ap- propriate to an average bulk composition of the four analysed samples is shown in Fig. 3a. A number of di- and trivariant fields appear in the pseudosection that show common assemblages for these rocks. These are the divariant fields st-g-bi-mu-q-H20; g-chl-bi- mu-q-H20 and g-bi-mu-sill-q-H20 as well as the trivariant spaces g-bi-mu-q-H20 and chl-bi-mu-q- H20. Note that some trivariant fields cover substantial parts of the P T range of interest for Barrovian-type metamorphism so that modal information within these

fields should provide more detailed information (Fig. 3b-h). For example, the trivariant field g-bi-mu-q covers, depending on bulk composition, about 250~ between 600 and 850 ~ C and at least 11 kbar between 6 and 17 kbar. Modal information helps to constrain P T details in this trivariant field. Rocks from the Kor- alto of approximately this bulk composition contain of the order of 10% garnet which is consistent with peak P T conditions around 14 kbar (Fig. 3c). in the same trivariant field muscovite modes decrease with pressure from about 40 to 30% for this bulk composition and biotite modes increase from zero to about 15%, The rare occurrence of retrograde staurolite or chlorite in- dicates that the low-pressure retrograde evolution must have evolved largely outside the stability field of these phases (Fig. 3b,d). This is consistent with a low-T

89

Table 4 Average P calculations (Holland and Powell 1988) of different samples from the western Koralm using the trivariant equilibrium g-bi-mu-q in the presence of H20. The calculations are performed for an assumed peak temperature of 700 ~ C. This assumption is supported by the best fit (%1,) liw this temperature. Average pressure estimates for higher and lower assumed temperatures show a dramatic increase in the error limits and the %ic Full analysis is only shown for one sample. The results of the other four calculations were obtained from equivalent assemblages in other thin sections and used the same set of independent reactions. End member abbreviations arc those used in THERMOCALC

Sample 26/92; analyses 26g18, 26mul 1, 26bi10 Mineral SiO 2 A120 3 FeO MnO MgO CaO Na~O K20 TiO 2 Total

gt 37.92 21.02 28.02 0.51 4.38 7.35 0.06 0.00 0.15 99.40 bi 36.59 18.29 16.60 0.03 11.07 0.00 0.15 8.80 2.37 93.91 mu 47.70 33.06 1.33 0.08 1.44 0.00 0.48 9.53 0.93 94.55

py alto phl ann east mu cel

a 0.0048 0.211 0.0554 0.0203 0.0411 0.591 0.0676 cy (a)/a 0.6209 0.15 0.3507 0.4759 0.38138 0.1 0.32856

Independent Reactions P(T) sd(P) a c~(a) b c In K crln K)

3 e a s t + 6 q - p y + p h l + 2 m u 11.2 4.34 30.33 5.77 0.00097 - 2.866 0.291 1.363 phl + east + 6 q - py + 2cel 13.9 2.75 81.10 3.46 0.00271 - 3.320 - 4.643 1.043 ann + east + 6q = alm + 2cel 14.1 2.16 35.00 3.53 0.01755 3.779 0.145 0.909

T - 700~ C; P = 13.9 kbar; r -- 2 . t ; . f= 0.5

Sample 29/92; analyses 29mul0, 29bi8, 29mu7:

Sample 29/92; analyses 29g2, 29bi5, 29mu6:

Sample 26/92; analyses 26g7, 26mu4, 26bi2:

Sample 24/92; analyses 2495, 24bi6, 24mu4:

T = 700"C;

T = 700"C;

T = 700"C;

T = 700~C;

P := 13.5 kbars; cy = 2.2; rsei t = 0.9

P -- 15.3 kbars; c~ = 2.7; csfi t = 1.2

P - 1 3 . 5 k b a r s ; ~ = 2 . 1 ; %Jt=0.6

P - 13.6 kbars; c~ = 2.2; %i~ = 0.6

evolution along a Barrovian-type decompression path involving simultaneous decompression and cooling (Fig. 3d) (ThiSni and Jagoutz 1992),

For more detailed predictions, in particular for the high-PT evolution around the peak conditions, we have selected a sample from the Plattengneis (48/92). In this sample some of the retrograde-grown micas over- grow rock volumes that are much larger than any one grain size of the peak assemblage (Fig. 2e) so that it appears to be well-equilibrated during the retrograde path of the Eoalpine event. Moreover, retrograde- zoned Eoalpine garnets have equilibrated with large volumes of the only other Fc-Mg phases in the assem- blage, the micas (Ehlcrs et al. 1994), so that a pseudosection for the whole rock composition is considered appropriate for modal and compositional interpretations (Figs. 4 and 5). In order to ascertain that the modally derived PT path for the sclected sample is not misinterpreted because of changcs of the equilibration volume during cooling, we have con- toured the pseudosection for both modal proportions of the minerals (Fig. 5) and isopleths (Fig. 4) and aim for an intergrated intcrpretation.

Compositional information on the pseudosection

Figure 4 shows the equilibrium XF, for mu, bi and g for the bulk composition of sample (48/92) in KFMASH. In the trivariant field g-bi-mu-q the Xve isopleths of garnet have a negative slope and increase in XFo to- wards low P and T. The XVe isopleths for biotite have shallow positive slopes with higher Xv,~ on the low-T high-P side. In the sample the XVe of garnet is generally below 0.8 and that of biotite generally around 0.4-0.5

so that this is consistent with little equilibration below 14 kbar and 600 ~ C (Fig. 4). Small euhcdral garnets are zoned with higher X w towards their rims constraining the PT path inasmuch as the isoplcths have to be crossed in the corresponding direction. In the divariant field g-bi-mu-chl-q the XFo contours change their slopes to be steep positive in order to accomodate the growth of chlorite. Within the quadrivariant fields g- mu-q and chl-mu-q compositional changes are minor being dominated by changes in Tschermak's substitu- tion and no contours are shown.

Modal information

Figure 5 shows the modal contours for the bulk com- position of the sample (48,/92) under discussion. In order to interpret this information we consider three possible end member PT paths. For each of these paths a mode-section is shown in Fig. 6. During pure decom- pression the trivariant assemblage g-bi-mu-q should have developed from the quadrivariant assemblage g- mu-q prior to the metamorphic peak, and, during the retrograde evolution, biotite growth should have oc- curred at the expense of muscovite and garnet (Fig. 6a). At low-P below 8 kbar sillimanite should become part of the assemblage. During pure cooling, no change of the modal proportions should have occurred, but be- low 600~ chlorite should grow at the expense of biotite with both garnet and biotite not only decreasing in mode but reacting completely out of the assemblage below 600 ~ C. During combined decompression and cool- ing with a dP/dT slope of lkbar/'15~ C the assemblage would have always been trivariant but garnet and mus- covite modes should continuously decrease during the

90

Fig. 3 A petrogenetic pseudosection (a) and modal mineral proportion (b-h) diagrams for an average bulk composition of the samples used in this study with SiO2: A1203 : MgO : FeO : K20 = 75 : 1 1:4: 6:4. This bulk composition is qualitalively representative of the majority of the rocks of the Koralm complex. The low- pressure PT evolution (two parallel arrows) indicates the approximate position of the retrograde evolution allowing the possibility of retrograde staurolite and sillimanite growth (see text)

550 600 650 700 750

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550 600 650 700 750

staurolite

" ( b ) , , , . . . . . i i i i i i i ~ i i i i , i i

700 750 550 600 650 700 750

700 750 550 600 650 700 750

]

retrograde path by continuously forming minor amounts of biotite and some staurolite at about 8 kbar and 600 ~ C.

In sample 48/92 the assemblage prior to deforma- tion appears to have been g-mu-q because biotite is only present in the pressure shadows around garnet (see above, Fig. 4a,b). These 2 3% bi were only being stabilised during the deformation (Fig. 4a,b). This is interpreted as evidence for the entering of the trivariant field g-bi-mu-q during decompression at temperatures above 620~ from much higher pressures, probably substantially above 18 kbar (Figs. 2, 5, stagcl). Garnet modes increase with lower-P in the quadrivariant field g-mu-q whilst they decrease with lower P in the

trivariant field g-bi-mu-q. It is therefore possible that the growth of 1 2% garnet during and after deforma- tion reflects the decompression from pressures well within the quadrivariant field g-mu-q. The amount of biotite that is produced during this decompression (1 3%) indicates that the trivariant field g-bi-mu-q is entered by only about 2-4 kbar from above 18 kbar to 14-16 kbar. It cannot be resolved if this decompression occurred near peak temperatures at around 700~ or around 610-620 ~ C prior to isobaric heating (stage 1 on Fig. 5). Equilibration at pressures of 14 16 kbar is in- dicated by the average P estimates (Table 4), by the P estimates of Wimmer-Frey (1984), as well as by 9 ]nodal percent garnet, which is the only phase that

91

560 580 600 620 640 660 680 700 720 740

Fig. 4 A KFMASH pseudosection appropriate to the bulk composi- tion S i ( ) 2 : A I 2 0 3 : M g O : F e O : K 2 0 = 7 8 : 1 0 : 3 : 5 : 4 which is ap- propriate to the selected Plattengneis sample (48/92). The contours are for the XF~ of garnet, biotite and muscovite. These contours can be compared directly with the results of thermobarometry and with the modal estimates shown in Fig. 5

stage of the P T path. Retrograde zoning of garnets developed on this part of the path and the amount of XFo-change between cores and rims of garnets is consis- tent with the amount of cooling suggested on modal grounds (see also Ehlcrs et al. 1994). The isobaric cool- ing is also indicated by the biotite composition which would increase rapidly beyond Xv~ = 0.5 if decom- pression continued at the elevated temperatures. Minor compression during cooling would also allow musco- vite growth and could produce the same zoning profiles in garnets but isopleths and modal contours intersect at too shallow angles to allow the observed Xv~ changes in garnet. The last mineral that grows in this sample is biotite as coronas on late-stage muscovites (Fig. 4e,f,h). This is best interpreted as isothermal decompression at 610 to 620 ~ C which will cause resorption of the minor amounts of chlorite formed in this divariant field and will cause biotite growth at the expense of newly form- ed muscovite (stage 3, Fig. 5).

No minerals in the sample have compositions cor- responding to growth below about 10 kbar and 600 ~ C which is consistent with the final part of the P T path being simultaneous decompression and cooling parallel to the mica contours in Fig. 3e,f (stage 3). Divcrsions from this path including somewhat higher and lower dP/dT slopes are indicated by the rare presence of sillimanite (Fig. 2h), staurolite (Weber 1982) and chlo- rite in parts of the complex. Isothermal decompression from around 600~C will produce staurolite and sub- sequently sillimanite in divariant reaction at 7-8 kbar and 4 5 kbar, respectively. The second isothermal de- compression period can therefore be infcrred to cover the P range from about 14 kbar to about 7-8 kbar.

Discussion and conciusion

Fig. 5 The same pseudosection as in Fig. 4 contoured lbr modal proportions of all minerals except quartz. Increasing biotite and decreasing muscovite contours ahnost coincide and are therefore labelled on one contour. Quartz modes may be found as the differ- ence between the sum of all modes at any P and T and 100%. Note that garnet modes increase in the quadrivariant field g-mu-q to lower pressures. Note also that the modal contours are more nar- rowly spaced in divariant fields than in trivariant fields. Peak condi- tions correspond to the average P estimates and an assumed peak temperature of 700~ (see text). The a r r o w s correspond to three stages of the retrograde evolution as discussed in the text

did not grow or get resorbed on the retrograde path. The compositional information of garnets is consistent with this interpretation of the peak conditions. Musco- vite is the next statically growing phase subsequent to deformation (Fig. 2c,c,f) with about 1 2% occurring in the sample. This indicates that the sample is unlikely to have decompressed further within the trivariant field g-bi-mu-q but cooled to enter the divariant field g-bi- mu-chl-q far enough to increase the modal amount of muscovite (stage 2, Figs. 5,6b). This is the second

This study shows that modal information may be an important and simple tool that can be applied to derive PT paths of metamorphic rocks. In particular for high-variance assemblages, modal mineralogical change may give important information for the sense of change of the metamorphic PT conditions. Indeed, we believe that modal information on pseudosections may be used to help in identifying the formation conditions of well-equilibrated rocks by the intersection of modal contours alone. In partially re-equilibrated rocks, as used here, this logic may be used to identify details of the P T path of a terrain.

In summary from above, the P T path of the Plat- tengneis shear zone of the Korahn compcx must be characterised by three stages. The first stage involved decompression fi'om above 18 kbar to 14 16 kbar at temperatures above 620~ reaching the metamorphic temperature peak around 14-16 kbar and 700 ~ C. The second stage involved near isobaric cooling at this pressure to around 600 _+ 20 ~ C. The third stage of the P T path follows a typical Barrovian evolution with

92

Fig. 6a-c Modal sections through Fig. 5 showing how the modal mineralogy of a rock of the bulk composition of the Plattengneis will change as a function of: a a P T path that involves only decompression after the metamorphic peak at 700 ~ C; b a P T path that involves only cooling from the metamorphic peak at 14 kbar; c a P T path that involvcs simultaneous cooling and decompression from peak conditions at 700 ~ C and 14 kbar

(a) -(3 2~

18

16

14

12

10

8

700~ (b) 14 kbar (c)

a second phase of isothermal decompression to around 600~ to 7 8 kbar followed by cooling during final exhumation. The first and second stages of the P T path are consistent with the thermal history established by Ehlers et al. (1994). The third stage of the PT path is consistent with that of Th6ni and Jagoutz (1992) and with the estimates for the low grade conditions of Weber (1982). The P T paths constructed on a modal basis for all other samples have similar features parti- cullary with respect to the third stage of the P T path. The isobaric cooling part of the path cannot always be discerned but has been recognised in some samples outside the Plattengneis. Regardless, the early P T evolution established for the Plattengneis has import- ant implications for interpretations of the tectonic his- tory of the region, which we will discuss briefly below.

The first stage of the P T path indicates that the metapelites experienced pressures at and possibly sub- stantially above 18 kbar. This indicates that their his- tory may be comparable to that of the eclogites and a common early high-P history of metapelites and eclogites is not unlikely (sce also: Neubauer 1991). The second stage of the PT path involved isobaric cooling after the temperature peak at around 700 ~ C. In fact, the presence of alkali feldspar in some of the assem- blages indicates that peak temperatures may have been locally even higher than 700~ at 14 kbar. This iso- baric cooling at the end of the decompression history is interpreted to coincide with the temperature-time his- tory established by Ehlers et al. (1994). They showed that the early cooling history of the rocks was extreme- ly rapid and decompression during this period is likely to have been insignificant so that the P T path describes essentially isobaric cooling. The rapid isobaric cooling is likely to have been the consequence of previous heating by a transient heat source so that it is independ- ent of the conductive processes in the crust as a whole. The third and last stage of the P T path involves a "nor- mal" evolution for Barrovian terrains with isothermal decompression prior to cooling during exhumation. All three stages are inferred to be part of the Eoalpine metamorphic cycle. This interpretation is based on the

similarity of the P T path to that of the cclogites, which are known to be of Eoalpine metamorphic age (Th6ni and Jagoutz 1992). It is also based on the intimate relationship of the early retrograde evolution with the Cretaceous Plattengneis deformation (Krohe 1987). Based on the isobaric cooling part of the P T path established above we suggest that care must be taken in interpreting a Cretaceous exhumation history of the Plattengneis from high-temperature radiometric ages of this age (e.g. Stiiwe and Sandiford 1994). It is possible that the Barrovian part of the path occurred at much later times. It is therefore suggested that details of the exhumation history will remain unclear until detailed low-temperature ages become available.

Pseudosections for common semipclitic bulk com- positions contoured for modal abundance of minerals show a number of important features. For example, the growth and resorption of biotite, muscovite, garnet and quartz is only pressure dependent above 600~ and about 6 kbar. The slope of these contours is governed by the isobaric boundary of the quadrivariant field g-mu-q with the adjacent trivariant field g-bi-mu-q. Care should therefore be taken in the correlation of observed growth of parageneses and temperature cha- nges at temperatures above 600 ~ C. With decreasing P, garnet modes increase in the quadrivariant field g-mu-q but decreasc in the trivariant field g-bi-mu-q.

Acknowledgements G. Xu, M. Sandiford, T. J. B. Holland and J. Arnold are thanked for a number of discussions with respect to the phase relationships in KFMASH. K. Ehlers is thanked for many discussions about the cooling history of the Koralm. W. Frank is thanked for the many hours in the field and the initial introduction ofK. S. to the Koralm. This work was supported by ARC. R. Abart and G. Hoinkes are thanked for their critical reviews.

References

Beck-Managetta P (1980) Die Koralpe. In: Der geolgische Aufbau ~sterreichs. Geol Bundesanst Wien, pp 386-392

Brown TH, Skinner BJ (1974) Theoretical prediction of equilibrium phase assemblages in multicomponent systems. Am J Sci 274:961 986

93

Chamberlain P (t986) Evidence for the repeated folding of isotherms during regional metamorphism. J Petrol 27:63-89

De Capitani C, Brown T (1987) The computation of chemical equilibrium in complex systems containing non-ideal solutions. Geochim Cosmochim Acta 51:2639 2652

Dymoke P, Sandiford M (1992) Phase relationships in Buchan facies series pelitic assemblages: calculations with application to an- dalusitc-staurolile parageneses in the Mount Lofty Ranges, South Australia. Contrib Mineral Petrol 110:121 132

Eblers K, Sti,iwe K, Powell R, Sandiford M, Frank W (1994) Ther- mometrically inferred cooling rates from the Plattengneis, Koralm region-Eastern Alps. Earth Planet Sci Lett 125:307-321

Frank W (1987) Evolution of the Austroalpine elements in the Cretaceous. In: Fl{igel HW, Faupl P (eds) Geodynamics of the Eastern Alps, Deuticke, Vienna, pp 379~06

Frank W, Esterlus M, Frey 1, Krohe A, Weber J (1983) Der relative Ablauf der Metamorphose geschichte yon Stub und Koralm kristallin und die Beziebung zum Grazer Paleozoikum. Jber ltochschulschwcrpunkt S15,4:229-236

Frank W, Kralik M, Schabert S, Th/Sni M (1987) Geochronological data from the eastern Alps. In: Flfigel HW, Faupl P (eds) Geo- dynamics of the eastern Alps, Deuticke, Vienna, pp 272 278

Heritsch H (1978) Die Metamorphose des Schiefergneiss-Glimer- schiefer-Komplexes der Koralpe, Steiermark. Mitt Naturwiss Ver Steiermark 108:19-30

Heritsch H, MiSrtl J (1977) Die Bildungsbedingungen eines Dis- then-Chloritoid-Staurolith-Granatglimmerschiefers lnit wesen- tlichem Paragonitgehalt vonder Rosshiitte, sfidliche Koralpc. Mitt Naturwiss Ver Steiermark 107:15-23

Holland TJB, Powell R (1990) An enlarged and updated inlernally consistent thermodynamic dataset with uncertainties and correlations: the system K20-NazO-CaO-MgO-MnO-FeO- Fe2OB-A120~-'I'iO2-SiO2-C-H2-O 2. J Metamorphic GeoI 8:89-124

Kieslinger A (1926) Geologie und Petrographic der Koralpe I-IX. Sitzungsber Akad Wiss Wien Math-Naturwiss KI Abt I: 135

Kleinschmidt G, Ritter U (1976) Geologisch petrographischer Auf- bau des Koralpenkristallins sfidlich von Soboth/Steiermark - KS, rnten (Raum Hiihnerkogcl-Laaken). Carinthia II, 166:57 91

Kleinschmidt G, Engel S, Kundrus KV, Wolf D (1984) Bericht 1980 fiber geologisch Aufnahmen im Kristallin der siidlichen Koralpe auf Blatt 205 St. Paul i.L.. Verh Geot Bundesanst 1981. Al16-120

Krohc A (1987) Kinematics of Cretaceous nappe tectonics in the Austroalpine basement of the Korahn region (eastern Austria). Tectonophysics 136:171-196

Miller C (1990) Petrology of the type locality eclogites from the Koralpe and Saualpe (Eastern Alps), Austria. Schweiz Mineral Petrogr Mitt 70:287-300

Miller C, Frank W (1983) Das Alter der Metamorphose yon Meta- basiten und Eklogiten in Kor- und Saualpe. Jber Hochschul- schwerpunkt S15 4:229-236

Miller C, Stosch HG, Hoernes S (1988) Geochemistry and origin of eclogites from the type locality Koralpe and Saualpe Eastern Atps, Austria. Chem Geol 67:103-118

Morauf W (1980) Die permische Differentiation und die atpidische Metamorphose des Granitgneises yon Wolfbcrg, Koralpe, SE Ostalpen, mit Rb-Sr und K-Ar Isotopenbestimmungen. Tscher- maks Mineral Petrogr Mitt 27:169-185

Morauf W (1982) Rb-Sr und K-Ar Evidenz fiir eine intensive al- pidische Beeinflussung der Paragcsteine in Kor und Saualpe. Tschermaks Mineral Petrogr Mitt 29:255-282

Nenbauer 1-' (1991) Kinematic indicators in the Koralm and Saualm eclogites, Eastern Alps. Zentralbl Geol Pal/iontol Tell 1:139 155

Powcll R, Holland TJB (1988) An internally consistent thermodyn- amic datasct with uncertainties and correlations. 3. Application to geobarometry worked examples and a computer program. J Metamorphic Geol 6:173-204

Powcll R, Holland TJB (1990) Calculated mineral equilibria in the pelite system KFMASH (K20-FeO-MgO-A1203-SiO2-H20). Am Mineral 75:367-380

Saxena SK, Eriksen G (1983) Theoretical computation of mineral assemblages in pyrolite and lherzolite. J Petrol 24:538-555

Schneidermann JS (1990) Use of reaction space in depicting poly- metamorphic histories. Geology 18:350 353

Spear FS (1993) Metamorphic phase equilibria and pressure-tem- perature-time paths. Mineral Soc Am Monogr, BookCrafters, Chelsea, Michigan, USA

Sttiwe K (1994) Some calculated thermodynamic pseudosections from the Plattengneis and other rocks from the Koralm crystalline complex, eastern Alps. Mitt Natur wiss Ver Steiermark (in press)

Stiiwe K, Sandiford M (1994). Lateral extrusion of the eastern Alps. The role of the mantle lithosphere. Tectonophysics (in press)

Thompson JB, Thompson AB (1976) A model system for mineral facies in pelitic schisls. Contrib Mineral Petrol 58:243 277

Thompson JB, Laird J, Thompson AB (1982) Reactions in am- phibole, greenschist and blueschist. J Petrol 23:1-27

Th6ni M, Jagontz E (1992) Some new aspects of dating eclogites in orogenic belts: Sm-Nd, Rb-Sr, and Pb-Pb isotopic results from the Austroalpine Saualpe and Koralpe lype locality Carinthia/Styria, southeastern Austria. Geochiln Cosmochim Acta 56:347-368

Vance D, Holland T (1993) A detailed isotopic and petrological study of a single garnet from the Gassetts Schist, Vermont. Contrib Mineral Petrol 114:101-118

Weber .I (1982) Untersuchungen zum Metamorphoseablauf der Metapelite im Wolfsberger Fenster und der Koralpenbasis. PhD thesis, Vienna Univ

Wimmer-F'rey I (1984) Gcftige und Metamorphose Untersuchungen am Plattengneis der zentralen Koralm, W-Steiermark. PhD the- sis, Vienna Univ

Xu G, Will T, Powell R (1994) A calculated petrogenetic grid for the system K20-FeO-MgO-A1203-SiO2-1t20 , with particular refer- ence to contact-metamorphosed pelites. J Metamorphic Geol 12:99 119