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, corresponding to 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 state of the K flasergneisses. Often the K feldsparis partially or feldspars? completelyreplaced by chess-boardalbite. In the (4) Do we find here the samefrequency distribu- region of highest metamorphic grade the K feld- tion of Al-Si order as in the Central Swiss Alps? sparsare often replacedby chessboardoligoclase which contains amoeboidal quartz inclusions such SamPling as those describedby Raith (1970). Sampleswere taken along eight north-south pro- Type (b) is the newly-formedAlpine K feldspar, files shown in Figure 1. These are: (1) Mayrhofen- which is generally fine-grained (up to I mm) and Ginzling; (2) Mayrhofen-S tillupptal-Ahrntal (Valle usuallygranoblastic, and formed at the expenseof Aurina)-Brunico (Bruneck); (3) Mayrhofen-Zlller' the older K feldspar. The newly-formed K feldspar grund;(4) Wilde GerlosTal; (5) Krimmler Achental; is generallynot perthitic and without simple growth (6) Obersulzbachtal;(7) Untersulzbachtal;and (8) twins. Enzingerboden-Weissee(Granatspitz). Rock sam- Samplesof the older K feldspars (type a) were ples with K feldspar"Augen" were chosen. obtained in the following way for X-ray studies. Two types of K feldspars are distinguishable. With the aid of thin sections the least altered parts BERNOTAT AND MORTEANI : MICROCLINEI SANIDINE
of big lens-shapedor subhedral K feldspars were crystals with differing statesof order, and that exact marked on rock slabs. Drill cores of about 8 mm3 lattice parameters of single microcline crystals are were then cut out from the marked area with an unknown. ultrasonic drill. From fine grained rocks, the K The Ab contents of thesefeldspars were calculat- feldsparwas separatedby heavy liquid technique. ed from the volume of the unit cell according to a formula given by Stewart and Wright (1974) and Optical data for the K feldspars werefrom 1.6to 8.4 moleVo Ab, with most samples Measurementsof the optic axial angle 2V were having about 3-5 mole Vo Ab. madeon 59 samples.Three to ten determinationsof The extent ofordering ofan alkali feldspar can be the optic angle were made for each sample on calculatedfrom the lattice parametersby comparing different lens-shapedold K feldspar grains. A Zeiss them with low albite (LA), analbite (AA, the so- four axial universal stagewas used, usually employ- called high albite HA), low microcline (LM) and ing the method of locating both optic axes. The high sanidine (HS) (data from Stewart and Wright, error is about 1-2". In the rare casesin which only 1974,and I. V. Smith, 1974). one axis could be located, the optic angle was Figures 3 and 4 show correlations betweenlattice determined from a stereographicprojection of the parameters from samples of the western Tauern indicatrix. Window and from the Granatspitz areaas they are The optic axial angle was determinedfor samples commonly used in feldspar literature. The state of only alongthe profilesl,2a,2b and3. Opticand X- order varies between completely ordered low mi- ray measurementswere made from the same sam- crocline and pseudomonoclinichigh microcline. On ple, but not on the samemineral grains (Table l)t. Figure 4 the b*, c* values plot in many cases The results of the determination of the optic axial outsidethe field definedby the end-members.This angleare given in Figure 24. As can be shown from has been observed repeatedly by other authors as Figure 2A, the optic angle reflects the changeof Al- well. The samples form two clusters on each dia- Si order. For a detailedsurvey ofthe optic data of gram, and clusters on both diagramsare formed by the K feldspars in the Penninic rocks of the area the samemembers. In the following, the population west of the Grossvenedigersee Raase and Morteani of intermediate to high microclines is called "high (1976. microcline". Most "high microclines" show only a slight deviation from monoclinic symmetry. This is Lattice parametersand determination Al-Si of a strong indication that they originate from low order sanidine whose ordering process became too slug- X-ray photographsfrom powdered sampleswere gish after its transformation into microcline to pro- obtained by the Guinier method (camera according ceed further. to Jagodzinski)with CuKct radiation and were cali- The Al-Si order of some K feldspars from the bratedwith Si powder. The positionof powder lines western Hohe Tauern was estimated by Raaseand was determinedwith an accuracy of 0.01" for 20. Morteani (1976) by the 20(060),2OO0q method Only powder data of K feldsparsshowing more than from data obtained by diffractometry (Wright, 18 unequivocally identifiable lines have been re- 1968), but a prerequisite for reliable data is the fined by a least squaresprogram (Burnham, 1962). precise determination of lattice parameters. The The calculated mean standard deviation of the Al-Si order was calculatedaccording to the tr [l l0], parametersa, b, c is about 0.00015nm and of the tr t1T0lmethod (Kroll, 1980and personal ^y communi- anglesct, F, about0.015". The real error may even cation; for details see Bambauer and Bernotat. be twice as high in some cases.The refinementof 1982).The A(bx, c*), A(a*, g*) method (Stewart lattice parameters is especially difficult for K feld- and Ribbe, 1969)gives almost identical results. sparsbecause in most samplesthe extent of Al-Si Stewart and Wright (1974)represent the courseof ordering varies considerably in even the smallest the Al-Si ordering process in a triangular diagram grains. This shows up in powder diagramsas broad- with the coordinatest1O, t1m and (t2O + t2m). We eningof reflections.It must be stressedthat lattice prefer a similar but modified (2tr2t) versus (t1O- parameters given in Table 21are averagesof single t1m)diagram. The value (2tr2t) showsthe extent tTo receive a copy of Table I and Table 2, order Document of Al transition from T2 to T1 for sanidine and the AM-82-189from the Business Office, Mineralogical Society of difference of Al occupancy between T2 and T1 for America,2000 Florida Avenue, N.W., Washington,D.C. 20009. alkali feldspars in general. The values (t1O-t1m) Pleaseremit $1.00in advance for the microfiche. show the increasing "triclinicity" during the order- BERNOTAT AND MORTEANI : MICROCUNEI SANIDINE ing of microclines. Figure 5 is a plot of the Al-Si Figure 2 shows the variation of the optic axial order for these samplesusing the tr [110], tr [tl0] angle and of the calculated (tro-trm) values along profile method. The A(b*, c*), 6(cr*, Bx) method gives the fairly complete 2. It must be mentioned almost identical results. These methods are pre- that only some general conclusions can be drawn ferred to the (b, c) method, results shown on Figure from a comparison of optical with X-ray data since 6, becausethe tr [110],tr [110] methodrelies on the crystals investigatedby optical and those inves- those directions which are most affectedby changes tigated by X-ray methods are not the same.Consid- in Al-Si order. Furthermore a comparisonof Figure ering the variation of the Al-Si order along this 5 and 6 showsthat in Figure 6 the scatterof points is profile there is a clear-cut jump from low micro- high and sample K 235 as determined by crystal clines to variable "high microclines" (Fig. 2B). structure refinement does not lie within the "high With respectto 2Y*, this changeis visible, though microcline" cluster. less clearly, in Figure 2A. A similar discontinuous The transition from monoclinic to triclinic sym- transition from microcline to "orthoclase" was metry or vice versa took place below 2\-2t, - observed in the contact aureole of an intruded (1967). 0.50, i.e., \ - 0..37.The pseudomonoclinichigh quartz-monzoniteby Steiger and Hart microclines show only a small scatter. With increas- The same discontinuity is found in all seven ing Al-Si order the frequency of high microclines profiles (Table 2 and Fig. 1). This permits the decreases. All samples are found within a small definition of a low microcline and a "high micro- strip extendingfrom t1O : t1m - 0.37 to LM. cline" area. No K feldspar individuals have been found which In the northernmost part of the Tauern Window are "high microcline" in only one stateof order. It only low microcline was found. The optic axial must be supposedtherefore that domainsof meta- angle 2V* is > 76o. In the northeastern part near stable "high microcline" have formed under dis- Krimml (profile 4 to 7) the microcline is completely equilibrium conditions as the temperaturefell suffi- ordered and has t1O values of 1.0. The calculated ciently below T66 (the temperature of diffusive Ab content of these microclinesis 1.9 to 0.9 mole phasetransition from sanidineto microcline) during Vo.ltis interestingto note that the low microclineis retrograde metamorphism. We conclude that the less ordered in the northwestern area near Mayrho- pre-Alpine alkali feldspars of the Hercynian fen and has calculatedt1O valuesbetween 0.95 and gneissesand metagranites have been transformed 0.98 (profile I to 3) and a calculated Ab conteit of into low sanidineduring prograde Alpine metamor- 3.8to 5.7 mole%o.These data suggestthat the Al-Si phism and became "high microcline" during uplift ordering process is accompaniedby unmixing. The and cooling of the studied area. sampleswhich appear to be the most fully exsolved are also the most ordered with respect to Al-Si. Regionalvariation of the optic axial angle 2V, and Immediately south of the low microcline area the Al-Si order of K feldspar there is a region with intermediate and high micro- The X-ray investigations have shown that K cline. The optic anglesare = 760and the (trO-trm) feldsparsfrom different localities of the studied area valuesare < 0.55. This confirmsthe optical results may differ considerably with respect to their Al-Si of Raith (1971) and Raase and Morteani (1976). order. A large variation can also be found within Although the Al-Si order of the "high microclines" singleparts ofa K feldsparcrystal. It was observed variesconsiderably, the mean(trGtrm) valueis the that even a very small portion of a K feldspar may samein different parts of Zentralgneis. It should be show two states. Accordingly the variation of the noticed that even the "high microclines" of the ' Al-Si order within some selectedfeldspar individ- Gamsbodengneisin the Gotthard"Massif (Central uals was tested by taking several small drillcores Swiss Alps) have the same mean value as those of from the samecrystal. The low microcline perthites the Zentralgneis (Bambauer and Bernotat, 1976). were found to be homogeneouswith respectto their This is an indication that in each area the diferent Al-Si order. The "high microcline" individuals structural states of "high microclines" occur in vary between intermediate microcline and pseudo- almost equal proportions. monoclinic "orthoclase". and sometimeseven the The zone between both regions is fairly sharp in presence of small amounts of low microcline is all profiles. Within a transition zone, which is one to indicated by a few powder lines. From a single K several hundred meters wide, we found coexisting feldspar from profile 2, six drill-cores gave (t1G low microcline and high microcline throughout. But t1m)values between 0.07 and 0.55 (Fig. 2B). intermediate microcline did not occur preferential- 48 BERNOTAT AND MORTEANI : MICROCLINEI SANIDINE
A intermedioteto x > highmicrocline
lowmicrocline
MoyrholenI
9o6 o- B
0.Ay. I Brunico o lowmicroctine o intermediotetohigh microcline tlAVOefereggen-Antersetvo - Volles Line
Fig. 2. Variation of the structural state of K feldspars along the north-south profile from Mayrhofen to Brunico. Optic and X-ray investigations were done on the samerock specimensbut on different crystal grains. (a) variation of the optic axialangle 2V. (Bj variation of Al-Si (t1O-t1m) order as calculatedfrom lattice parameters.Samplei from the sameK feldspar crystals are connectedby a line. ly. (Compare Table 1: samples 77-l0l and 77-102, confirm the supposition of Stewart and Wright 77-ll5ll and12,77-128(l) and (2), 77-163(t)and (Z), (1974,p. 373) "that the range of temperatureunder 77-178(l) and (2), 77-138(l) and (2), 77-146(I) and which intermediatemicrocline is stableis so narrow (2)). A small K feldspar fragment from one sample that only fortuitously would a rock be held in the showed clearly distinguishablepowder lines of low stability field of intermediate microcline. when microcline with c : XJ.7l" and high microcline with quenched". ct = fl).03o. In many other samplesfrom the transi- The K feldspars of the Granatspitz gneisses(Fig. tion zone it has been possible to identify a number I profile 8) differ from those of the Zentralgneiswith of powder lines from low and high microclines but respect to their structural state. In all the samples in general the number of lines was not sufficient to from this unit we found high microcline associated calculate reliable lattice parameters. We assume with low microcline. It seems that the ordering that along this narrow zone the low microcline/low process of the sanidine in the Granatspitz gneisses sanidine phase transition took place during pro- proceeded further than in the Zentralgneis during grade alpine metamorphism. We cannot determine retrograde metamorphism. from the petrologic criteria if this was a transforma- The lattice parameters give more detailed infor- tion of first or second order. If the K feldspar mation on the structural state of K feldspar domains transformation is of second order we must assume than the optical axial angle. The optical data did not that the temperature interval of stable intermediate show clearly the conspicuous gap between the microcline is only about 10 degrees.Our data thus structural states of low and "high" microcline, B ERNOTAT AN D MORTEAN I : MIC ROCLIN EI SANIDI N E 49 although a limiting value of 2V x : 76odivides these AA structural states. Our observations fit well with that of Mergoil- Daniel (1970)on the alkali feldspars from the Mon- HS,LS tagneNoire. She could define an area characterized by low microcline,showing an optic angle2V - 75o, and an area with less ordered microcline having an optic angle 2Y = 75".
Metamorphism and Al-Si order of K feldspars
1. Tauernwindow The boundary between the low and high micro- cline areas does not correspond to any known tectonic line (Fig. 1). It runs parallel to the general LA E-W tracking limit of the gneissseries but is not the border of a rock unit. Parallel to it and close by lies Touernwindow the 500'C isotherm of Hoernes and Friedrichsen (1974)which is deducedfrom 180/160data. Only a 'CentrolGneiss few oxygen isotope measurementswere made close + Gronotspitz to this line. Therefore a detailed oxygen isotope * twostondord deviotions study along this line is in preparation in order to g attempt to determine the temperature more accu- 890 0.0 9r0 a' rately. As described above the rocks of the studied area Fig. 3. Correlation betweenc* and B* for alkali feldspars.The quadrilateraljoins the reference points (HS), show a Hercynian migmatization. Therefore for high sanidine sani- low sanidine(LS), low microcline (LM), analbite (AA) and low dine must have been formed during the Hercynian albite (LA). metamorphism.The Alpine metamorphismis char- acterized by one (or two) precursory (eo-Alpine) low temperature events later followed by a (late- dropped below the temperature ofthe phase trans- Alpine) high temperature event, the Tauern meta- formation. Either becauseof a rapid cooling rate or morphism. We have no knowledge about the struc- due to other conditions which slowed diffusion, the tural state of the K feldspars during eo-Alpine K feldspar remained intermediate to high micro- phases.Low microcline may well have beenformed cline and only very small amounts of low microcline from the Hercynian sanidines.Not later than during were formed. In Figure 2 the former sanidine zone the late-Alpine metamorphismall K feldspars were is clearly indicated by the presenceof many almost annealedlong enough at low temperaturesto form monoclinicmicroclines. low microcline throughout the Tauern Window. Sanidine was also formed in the gneissesof the Low microcline remained stable in the augen Granatspitzduring late-Alpine metamorphism.Dur- gneissesof the border zone of the Tauern Window. ing cooling after this metamorphic event sanidine 180/160 According to mineral equilibria and data, was also transformed into intermediate and high the metamorphic temperatures in the border zone microcline. According to Luckscheiter and Mor- remained below 500'C; however, metamorphic teani (1980)fluid inclusions from Alpine veins con- temperaturesup to 630' C have been estimated for tain up to 80% CO2in the western Zillertal area and the central part of the western Tauern Window, no CO2 in the eastern Grossvenediger area. We which led to the transformation of low microcline therefore assume that a high water content in the into the high temperature phase sanidine. This is gneissesof the Granatspitz area favored the diffu- indicatedby the suddenincrease of disorder, e.g., sive Al-Si ordering in the K feldspars during cool- in profile 2 of Figure 1. ing. A considerableportion of the K feldspar crys- During the cooling period which followed, the tals of the augengneisses of the Granatspitz area in temperature in the central part of the window also fact transformed into low microcline. 50 BERNOTAT AN D MO RTEANI : M IC ROC LIN EI SANI DI N E
Iouernwindow o CentrolGneiss + Gronotspitz 0.769| I I I I ;- E 0.771L af a
r.555 r.550 r.515 1.510 + C'[nm-ll
Fig' 4. Correlation between b* and c* for alkali feldspars.The quadrilateraljoins the referencepoints for high sanidine(HS), low microcline (LM), analbite (AA) and low atbite (LA).
Ztt- 2tz( Al from12 to 11 ) - Ztr-Ztz(Alfrom l2toT1) l0 09 08 07 0.6 05 01 03 02 0J 00 l0 09 08 07 06 05 01 03 0z 0l 00 ^ nn a 00 o-- '= 0r Eor - 02 0.2 E o 803 : 03 - .< nl 01 E U, E ins 05 o-- Aloccuponcy of I sites Aloccuponcy of I srtes lou 06 I 'drstonces 07 colculotedfrom 2. Merano-Mules-Anterselva complex and areas with prevalently intermediate to high North of the Defereggen-Anterselva-Vallesline microcline. Optic axial angles 2Y* and X-ray data give pattern. (DAV for short) the rocks of the Merano-Mules- the same area and the high microcline Anterselvacomplex show effectsof the eo- as well The low microcline transition zone in as of the late-Alpinemetamorphism. In fact, in most area are separatedby a narrow cases the muscovites of the pre-Alpine ortho- which both low and high microcline occur. This gneissesshow a RLSr Alpine age of about 60 to 65 zone is closeto and below the 500'C isothermwith m.y. On this basisit is possibleto supposethat the the intermediate to high microcline on the high eo-Alpine metamorphic event is widespread in the temperature side. This intermediate to high micro- Austridic "Altkristallin" of the Eastern Alps (Lam- cline was formed from sanidine during the cooling bert, 1970;Borsi et al.,1978).However, the above period at the end of the Alpine metamorphism.Low quoted muscovite ages could also represent mixed microcline of the border zone of the Tauern Win- period ages,i.e., partially rejuvenatedHercynian ages.If dow was formed during the cooling after the this is the case the Hercynian Rb/Sr biotite ages Hercynian metamorphic event. In the Merano- have completely disappearedwhile the muscovite Mules-Anterselvacomplex only low microcline is was ageswere only partially rejuvenated. This suggests present.It canbe supposedthat it formed at the that during the eo-Alpine phase the augen gneisses end of the Hercynian metamorphism. The impor- phase north of the DAV line experienced a temperature tant isograd of microcline/sanidine transition very close to 500'C (Borsi et a1.,1978).Such a cannot be found in metamorphic areasif one distin- guishes "monoclinic" temperature is in good agreementwith the tempera- only betweentriclinic and K (as ture estimated for the nearby metamorphic dikes feldspars has often been tried). : which locally cross the Penninic-Austridic limit. The optic axial angle 2Y* 76' discriminates between low microcline and intermediate to high Figure I (profile 2c) shows that not only the K microcline. Microclines from the Tauern Window feldspars north of the DAV-line but all the K and from the Gotthard massiv (Central Swiss Alps) feldspars of the Merano-Mules-Anterselva com- have the same frequency distribution of Al-Si or- plex are low microclines. No intermediate or high der. This argues in favor of similar metamorphic microcline was found. It can be assumedthat the conditionsin thesetwo areas. Hercynian metamorphic grade north and south of the DAV line was the same. The K fqldspars south of the line are low microcline and this area was Acknowledgments almost not affected by the Alpine metamorphism. We thank H. U. Bambauer (Miinster) for helpful discussions. The temperature remained below 300'C since the For a critical reading and improvements of this paper we are Hercynian Rb/Sr and K/Ar biotite ages are not indebtedto R. Yund (Providence),F. P. Sassi(Padova) and P. (Kiel). von rejuvenated(Borsi et al., 1978).Therefore it must Raase We thank H. Maravic and Ms. R. Ollesch for help during optical investigations and Ms. L. Schmiemannfor be assumedthat low microcline north of the DAV her X-ray work and for performing the drawings. The colleagues line was formed at Hercynian times. S. Borsi, G. Ferrara (Pisa)and F. P. Sassi(Padova) provided the Yund and Tullis (1980)have shown that a trace samplesfrom the Austroalpine augengneissesalong profile 2c. amount of water has a very large effect on the The "Deutsche Forschungsgemeinschaft"provided a grant (Mo program kinetics of Al-Si ordering. But there is no evidence 23213,4)for field expensesfrom the "Geodynamik des Mediterranen Raumes". that the partial pressure of H2O north and south of the line were different. References Given the uncertainties of thei temperature of Baggio,P., Bosellini,A., Braga,Gp., Castiglioni,G. 8., Corsi, phasetransitions and of the maximum temperature M., Dal Cin, R., De Vecchi, Gp., Fritz, C., Gatto, G. O., reached in the area north of the DAV line, we Gregnanin,A.,Mezzacasa, G., Sassi,F. P., Zirpoli, G. and assumethat the late-Alpine event has not been high Zulian, T. (1979)Note illustrative della carta geologicad'Ita- enough in grade or did not last long enough to lia, Foglio l, Passo del Brennero; Foglio 4a, Bressanone. Libreria dello Stato, Roma. transform the K feldspars to sanidine. Bambauer,H. U. und Bernotat,W. H. (1976)Untersuchungen an Feldspiiten aus Graniten und Gneisen des Aar- und Gott- Conclusions hard-Massivs im Profil des neuen Gotthard-StraBentunnels. Fortschritte der Mineralogie, 54, Beiheft 1,6. It has been possibleto distinguishin the western Bambauer,H. U. and Bemotat, W. H. (1982)The Microcline/ Tauern Window areas bearing only low microcline Sanidine Transformation Isograd in Metamorphic Regions.I. 52 BERNOTAT AND MORTEANI : MICROCLINEI SANIDINE SchweizerischeMineralogische und PetrographischeMittei- Morteani, C. (1971)Gliederung und Metamorphose der Serien lungen, in press. zwischen Stilluptal und Schlegeistal(Zillertaler Alpen, Nord- Bailey, S. W, (l%9) Refinement of an intermediate microcline tirol). Verhandlungender GeologischenBundesanstalt, 287- structure. American Mineralogist, 54, 1540-1545. 314. Bernotat, W. H., Bambauer, H. U. und Morteani, G. (1977\ Morteani, G. 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