Amertcan Mineralogist, Volume 67, pages 43-53, 1982 The microcline/sanidinetransformation isograd in metamorphic regions: Western Tauern Window and MeraneMules-Anterselva complex (Eastern Alps) W. H. BBnNorer I n st it ut fiir M in er alo g i e, U niv er sit cit M iins t er Correnstra$e 24. D4400 Miinster. Germany euo G. MonreeNr Institut fiir Angewandte Geophy sik, P etrologie u. Lag erstdttenfors chung Tec hnis c he U niv ersit rit B erlin StraJ3edes 17. Juni 135, D-1000 Berlin 12, Germany Abstract Lattice parameters and optic axial angles of alkali feldspars from the Penninic augen gneisses of the western Tauern Window and from the Austroalpine Merano-Mules- Anterselva unit south of it have been determined along eight N-S profiles. Parallel and somewhatbelow the 500'C isotherm of the syn- to post-kinematic metamor- phism of Tertiary age the K feldsparsshow an abrupt changein their optic axial angle and their Al-Si order. This line separatesa northern area showing exclusively low microclines (2Y*> 76oand 0.95 < (trO-trm) < 1.0)from a southernarea showing "high microclines" (2V* < 76" and 0.0 < (tlo-ttm) < 0.55).The border line representsthe isothermat which the diffusive phase transformation microcline/sanidineoccurred during prograde Alpine metamorphism. The Merano-Mules-Anterselva complex is characterized by low grade Hercynian metamorphismbelow 5fi)'C. In spite of the fact that north of the Defereggen-Anterselva- Valles line the Alpine metamorphism seemsto have exceeded 500'C all investigated K feldspars of the complex are low microclines. Introduction ied the optic axial angles of the K feldspars in the Penninic rocks of the central and western Tauern The temperature of the diffusive phase transfor- Window. They foundlhat in a zone along the border mation between microcline and sanidine (Toir) has of the Tauern Window, which showsa greenschist not been determined experimentally becauseof the facies metamorphism, the K feldspars are low mi- very slow kinetics of Al-Si ordering. This tempera- croclines, whereas in the highest-grademetamor- ture can be estimated from a careful study of the phic central part of the Window, the K feldsparsare structural state of alkali feldspars and their relation intermediate microclines to "orthoclases". By to metamorphic temperatures estimated by inde- comparing tsgTt0g data from Hoernes and Fried- pendent methods. A discussion of the available richsen(1974) they deduceda temperatureof500'C information on the temperature of this transforma- for the microcline/sanidinephase transition. tion is given in a separatepaper by Bambauer and The Penninic rocks of the Tauern Window belong Bernotat(1982). to the deepest tectonic unit of the East Alpine A temperature-inducedboundary between a low tectonic pile. They consist of phyllites, schists, microcline area and an intermediate to high micro- ortho- and paragneissesand representthe basement cline area was found in the rocks of greenschist over which the Austroalpine nappes moved to the facies of the Central Swiss Alps (Bambauer and North. The Tauern Window is surrounded on the Bernotat, 1976; Bernotat and Bambauer, 1980). western,southern and easternsides by rocks of the According to Frey et al. (1976) Alpine metamor- Austroalpine "Altkristallin". The Penninic zone phism reachesapproximately 400"C in this area. comprisestwo main lithological and tectonic units: Raith (1971)and Raaseand Morteani (1976)stud- the "7-entralgneis" and the "Schieferhiille". The 0003-ux)v82l0102-(M3$02.00 43 BERNOTAT AND MORTEANI : MICROCLINEI SANIDINE "Zentralgneis" consists mostly of augen and flaser oritic bodies of Oligocene age (Borsi et al., 1978a, gneisses and metatonalites.In these well foliated 1978b)in the complex which is subdivided by some gneisses, one to two cm K feldspar crystals or important west-east trending tectonic lines (Sassiet crystal aggregates or chessboard albites are al., 1976).The most important one is the Defereg- sheathedby white mica and set in a finer ground- gen-Anterselva-Valles line (DAV line), as recently mass quartz, of plagioclase,K feldspar, biotite and demonstratedby Borsi et al. (1978)and Sassier a/. light mica. The metatonalites range in their chemi- (1e78)(Fie. 1). cal composition between granitic and dioritic, with The pre-Alpine metamorphic history of this com- granodioritic composition most frequent. Although plex is characterizedby a high grade Caledonianas they have often maintained magmatic textures, well as a low grade Hercynian metamorphic event primary their magmatic minerals underwent com- (SassiandZanfenai,1972; Borsi er al., 1978).The ',Schie- plete recrystallizationin Alpine times. The breakdown of kyanite and staurolite to sericite -r -r ferhiille" is a complicated sequenceof gneisses, chlorite chloritoid (Sassi and Zanfenai, 1972: amphibolites,schists, marbles and phyllites gener- Sassie/ al., 1974)indicates temperatures lower than ally overlying the "Zentralgneis" cores (for details 550'C. North of the DAV line the Austroalpine see e.9., Morteani, 1974). The "Zentralgneis" is rocks show not only a pre=Alpinebut also an Alpine polymetamorphic with not only Alpine but also a metamorphic history. According to Borsi et al. Hercynian and perhaps a Caledonian metamorphic (1978)Rb-Sr ages of muscovites and biotites sug- history; an intense pre-Alpine metamorphism is gest that Alpine temperatures did not significantly -r revealed by the migmatitic structures (Morteani, exceed 500 50'C north of the DAV line. Maxi- 1974). All Penninic rocks of the western Tauern mum temperatures were probably-reached in the Window were affected by a pervasive recrystalliza- Upper Eocene-Lower Oligocene.They suggestthat tion during Alpine metamorphism.In some rocks of the Alpine temperaturesprobably never exceeded the amphibolite facies a relic mineral paragenesisof 300"Csouth of the DAV line. an earlier greenschistfacies is found, indicating that the Alpine metamorphism was a two stage event, Statementof problem with the earlier one being lower in grade (Morteani, In the western Tauern Window, the line separat- 1971;Morteani and Raase, 1974).A temperature ing the low microcline area from the intermediate differenceof about 200'C between these two stages microcline and "orthoclase" area is found in a probable. seems The thermal climax of the Alpine region without postmetamorphic Alpine faulting. It metamorphismwas reachedin the secondstage; to can be supposed,therefore, that the areal distribu- which radiometricmica cooling agesof 35-25 m.y. tion of the diferent structural states of the K are related(Raith et a1.,1978;Satir, 1975).In the feldspar has remained unchanged since the Alpine Alpine metamorphism the highest degree of meta- metamorphismproduced it. In the northern part of morphism, correspondingto the lower amphibolite the western Tauern Window the transition can be facies, was reachedin the western Zillertal Alps. observedin very homogeneousaugen gneiss series The gradual decrease of metamorphic grade from of granitic composition.Because the gneisseshave the center to the margin of the Tauern Window is the samechemical and mineral composition and the marked by isograds of the anorthite content of sametectonic history, we must conclude that the plagioclase coexistingwith epidotein mica schists, adjacent areas with microclines of different struc- and by the biotite isograd (Morteani and Raase, tural state were causedby a thermal gradient. 180/160 The 1974;Hoernes, 1973). data of Hoernesand region is therefore very suitablefor a detailed X-ray Friedrichsen (1974)indicate temperatures of up to study of the Al-Si ordering process in K feldspars. 600'C in the western Zillertal Alps and less than In this paper the structural state is studied in more 500"C at the northern border of the Tauern Win- detail than by Raase and Morteani (1976) and by dow. Bernotat et al. (1977). In particular, the following The Austroalpine unit south of the Tauern Win- questionsare considered: dow is called the MeraneMules-Anterselva com- (1) Is the transition from low microcline to high plex (Baggio et al., l97g). It consists of para- microcline in the Tauern Window abrupt, and there- gneisses, marbles, amphibolites and granitic fore caused by a well defined isotherm, or does it gneisses,partly developedas augen gneisses.As form a rather broad zone? shown in Figure 1, there are also granitic to granodi- (2) Does the transition line really occur in the BERNOTAT AND MORTEANI : MICROCLINE| SANIDINE 45 S$ottPnltlil rcl0km . \e\ * ( $t \${ t a1 ; -o = PI SO U T H E RN ALP S Fig. l. Distribution of low microcline and of intermediate to high microcline along the profiles I to 8. Filled dots are low microclines,open dots are intermediateto high microclines. A and B are the Penninicunits; A = upper and lower Schieferhi.ille,B : Zentralgneis.CandDarerocksoftheMerano-Mules-Anterselvacomplex;Q=prevalentlymicaschists,D=Augengneisses.E: granites to granodiorites of Oligocene age. M.Z. is the Matrei Zone. D.A.V. is the Defereggen-Anterselvaline and P.L. the Periadriatic line. southwesternpart of the Zentralgneissas Raaseand Type (a) is the older, probablymagmatically grown, Morteani (1976) supposed?From 180/160data a deformed and recrystallized Hercynian K feldspar temperaturehigher than 500"Ccan be deduced. which is generally perthitic, and usually twinned (3) Was the influenceof Alpine metamorphismin according to the Carlsbad or Baveno law. This the MeraneMules-Anterselva complex strong feldsparforms lensesup to 10mm in the augenand enough to change the structural