Distribution of Variscan I- and S-type granites in the Eastern Alps: a possible clue to unravel...

ELSEVIER Tectonophysics 272 (1997) 315-333

TECTONOPHYSICS I I I

Distribution of Variscan I- and S-type granites in the Eastern Alps: a possible clue to unravel pre-Alpine basement structures

A. Schermaier*, B. Haunschmid, E Finger

lnstitut fur Mineralogie der Universittit, HeUbrunnerstrafle 34, A-5020 Salzburg, Austria

Received 2 August 1995; accepted 5 June 1996

Abstract

Meta-granitoid rocks with Variscan intrusion ages have been investigated in the three large Austroalpine basement massifs of the Schladminger Tauern, the Seckau-B~Ssenstein Tauem and the Semmering-Raabalpen. The first two massifs are dominated by calc-alkaline I-type granitoids (mainly granodiorites/tonalites) and are thus similar to most other intra-Alpine Variscan granitoid massifs, while the Semmering-Raabalpen area is one of the rare examples of an intra-Alpine S-type batholith. This confirms previous concepts, according to which the Semmering-Raabalpen area is part of a separate Variscan terrane within the Austroalpine basement. It is suggested that this basement terrane already had some affinities to the thick mature crust, that is typical for the internal Moldanubian zone of the Variscides. The widespread occurrence of Carboniferous 1-type plutons suggests the presence of a less evolved or newly underplated Variscan lower crust in most other parts of the Eastern Alps. This kind of crust was probably typical for the former southern flank of the Variscan orogen.

Keywords: Eastern Alps; granite; Variscan granitoids; geochemistry; basement geology

1. Introduct ion

Typological features of granitoid magmas (Chap- pell and White, 1974, 1992; White and Chappell, 1983; Pearce et al., 1984; Barbarin, 1990) may greatly help to infer structures and processes that occur in the depth of an orogen (see e.g., Pitcher, 1982; Harris et al., 1984). In the Austrian section of the Variscan fold belt, for example, a significant regional duality of Carboniferous granite types was recognized (Finger and Steyrer, 1990): calc-alkaline, tonalite-granodiorite I-type plutons were found to

*Corresponding author. Fax: +43 662 8044-621; E-mail: fritz.finger@ sbg.ac.at

prevail in the Alps, i.e. along the former southern flank of the Variscan orogen, whereas the Moldanu- bian Variscan units north of the Alpine front turned out to be comparably much more rich in S-type, i.e. sedimentary source plutons. Neubauer (1991) and Schaltegger and Corfu (1995) suggested that the intra-Alpine 1-type plutons indicate a distinct zone of severe late Variscan transpressional and transten- sional tectonics with subsidence, and formed mainly as a consequence of thermal relaxation and decompression melting of the mantle. On the other hand, it has been speculated that the I-type plutons in the Alps may reflect the influence of a late Palaeozoic palaeo-subduction zone along the southern flank of the Variscan orogen (northward consumption of the

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved. PII S0040- 195 1 (96)00265-X

316 A. Schermaier et al. / Tectonophysics 272 ~ 1997) 315-333

Palaeotethys Ocean - - see Ziegler, 1986; Mercolli and Oberh~insli, 1988; Finger and Steyrer, 1990, 1991).

When proposing their hypothesis of a long, subduction-related I-type plutonic belt in the Alpine basement, Finger and Steyrer (1990) relied on a data set from the large Hohe Tauern I-type batholith (Fin- ger and Steyrer, 1988), and on additional chemical data available from many other late Palaeozoic granitoid massifs in the Southern, Eastern and Western Alps (see Bonin et al., 1993 and references therein). However, from a number of intra-Alpine Variscan granitoid massifs, little data were existent at that time. Particularly for the three large granitoid massifs between the Hohe Tauern and the Carpathians, i.e. the Semmering-Raabalpen massif, the Seckau- B6senstein and the Schladminger Tauern, there was no comprehensive geochemical record.

In this paper, we report new data from these three major Austroalpine granitoid massifs (see Fig. 1). Two of them, i.e. the Middle Austroalpine Schlad- minger and Seckau-B6senstein Tauern, turned out to be I-type dominated, broadly similar to the Hohe Tauern batholith and a number of Variscan I-type massifs in the Carpathians (Petrik et al., 1994). It may, therefore, be speculated that all these I-type granite terrains were situated in a related plate tectonic position during the Carboniferous, probably quite close to the southern margin of the Variscan orogen. However, the Lower Austroalpine Semmer- ing-Raabalpen massif exhibits significantly different, mainly S-type characteristics. Possible implications for the Variscan basement geology of the Eastern Alps are discussed.

2. Geological background

The classic tectonic subdivision of the Eastern Alps in Helvetic, Penninic, Lower, Middle, and Up- per Austroalpine nappe complexes (Tollmann, 1977) has been established to describe the Permo-Meso-

zoic evolution of this area. It constitutes thus no optimal basis for a tectonic classification of the Pre- Alpine basement. Main structures of the Variscan orogen may well have been obliquely disrupted during the break-up of the Alpine trough system. On one hand, this means that a Variscan tectonostratigraphic zonation may still be present laterally within the sin- gle Alpine tectonic belts. On the other hand, Variscan formations of the Helvetic, Penninic or Austroalpine basement could once have been situated in the same structural positions in the Variscan orogen as well.

Reconstructing the palinspastic arrangement of the Alpine basement formations during and prior to the Permian is one of the great tasks of present-day Alpine geology. Data may be won by different ap- proaches, e.g., from the study of Alpine deformation structures (calculation of displacement vectors - - Ratschbacher and Frisch, 1983; Neubauer, 1988a), or on the basis of lithostratigraphic comparisons (Sch6nlaub, 1979; Mercolli and Oberh~insli, 1988; Pfiffner, 1993). Studying granite typologies adds important new aspects to these comparisons, since information about the unexposed basement terranes may be gathered this way (Chappell et al., 1988).

Frisch and Neubauer (1989) introduced, for the first time, a tentative tectonostratigraphic concept for the evolution of the Variscan basement in the Eastern Alps. They argued that remnants of an important early Variscan ocean (Plankogel Ocean) are present, and that this ocean closed during the Carboniferous megacontinent collision of Gondwana and Laurasia, giving rise to the docking of different terranes: rock formations formerly situated south of this Plankogel Ocean were summarized to a Noric composite terrane (see inset b in Fig. 1). These formations are believed to have built up most of the South-Alpine, Upper and Middle Austroalpine basement (including the Schladminger and Seckau-B0senstein Tauern), but also some basement units of the Lower Aus- troalpine, particularly more to the west (e.g., the Bernina massif). The Lower Austroalpine basement

Fig. 1. Simplified geological map showing synoptically the major Alpine nappe units (after Tollmann, 1977) and the tectonostratigraphic subdivision of the pre-Alpine basement according to Frisch and Neubauer (1989). Shown in black are the major Variscan batholiths. G = Gleinalm batholith. Inset (a) shows the main structures of the Variscan fold belt in Central Europe, simplified after Franke (1989). Abbreviations: A M = Armorican Massif; B M --- Bohemian Massif; H = Harz; Mor = Moravikum; M C = Massif Central; RS =

Rheinisches Schiefergebirge; S W = Black Forest; V = Vosges. Inset (b) shows the model of Frisch and Neubauer (1989) for the Variscan evolution of the Eastern Alpine basement (symbols as in geological sketch map above).

A. Schermaier et al./Tectonophysics 272 (1997) 315-333

_

Regensburg

ALPINE NAPPE UNITS

~ Upper Austroalpine Unit Middle Austroalpine Unit

Lower Austroalpine Unit

Penninic and Helvetic Unit

PRE-ALPINE TERRANES (after Fdsch & Neubauer 1989)

~ Plankc~el terrane ~ - i Koriden terrane

' ~ Noric c~ml:~site terrane Pannonic and Wechsel tectonostratigraphic units

317

1& O ~" O A H U B | A N ~ " _. . . . . . . . . . .

Wien MOnchen )

@

!

Batholi th Seckau- Bernina BtJsenstein Batholith Batholith /

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b

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~ Plankooel suture zone Konden t / Foreland Africa

dispersion / / fold & thruSt ~ ; s . '/~ / v belt..t.flysc ~

S(E) Eady-

Middle Devonian

Late Carboniferous

(collisional stage)

318 A. Schermaier et al. / Tectonophysics 272 (1997) 315-333

units between Graz and Vienna, with the Semmer- ing-Raabalpen terrain, are considered by Frisch and Neubauer (1989) as continental crust north of this Plankogel Ocean.

Comparing the concept of Frisch and Neubauer (1989) with the tectonic situation in the western Eu- ropean Variscides (Matte, 1986), it is tempting to correlate the Plankogel suture with the French Mas- sif Central suture (see inset a in Fig. 1). This suture is commonly believed to separate southern Variscan terranes of the passive margin type (Vendee-Ceven- nes and Aquitaine-Montagne Noire terranes) from Armorican-Moldanubian terranes (active margins). If such a correlation to the western European section of the Variscan orogen is valid, most of the Lower Austroalpine basement between Graz and Vienna might belong to the Moldanubian realm sensu lato.

3. Granite typology of the Schladminger Tauern

The Schladminger Tauern are situated about 90 km southeast of Salzburg. They comprise large amounts of Variscan paragneisses and migmatites, and some metavolcanics (amphibolites, leptinites) of probable early Palaeozoic protolith age in the south (see Matura, 1980, 1987). The granitoids are mainly concentrated in the northern and eastern parts (see Fig. 2). They display a more or less pronounced gneissic texture, due to the Alpine overprint. Field observations suggest that most of the granitoids so- lidified originally as high-level plutons. Contacts with the migmatites are sharp and discordant, indi- cating that these were already cold upper crust when the granitoids intruded.

A review of the metamorphic history of the Schladminger Tauern area is given by Hejl (1984). K/Ar dating of muscovites from two pegmatites gave Carboniferous ages of 340 -4- 18 and 347:5 20 Ma. However, judging from the sample locations quoted in Hejl (1984), it seems possible that these pegmatites are related to the migmatites, and therefore older than the granitoids. In view of the generally cold discordant contacts, we believe that the granitoids of the Schladminger Tauern are mostly post-tectonic Variscan intrusions.

Applying the conventional petrographic and chemical criteria for granite classification (Chap- pell and White, 1974; Pupin, 1980; Eby, 1990), two

major groups of plutons may be distinguished in the Schladminger Tauern: (1) mainly granodioritic I-type granitoids and (2) leucocratic granites, that approach A-type granite composition in the sense of Eby (1990). Both groups include macroscopically different subtypes (Table 1).

A large pluton consisting of equigranular, fine- to medium-grained I-type granodiorite builds up the Krtigerzinken area. In the Kleins61ktal and Seewig- tal, porphyritic, medium- to coarse-grained I-type granodiorites are common. The A-type granites are especially widespread in the uppermost Schrder- bachtal, Etrachbachtal and Znachtal (see Fig. 2).

The 1-type granitoids of the Schladminger Tauern display a calc-alkaline granodioritic trend (Lameyre and Bowden, 1982), with biotite granodiorite as the dominant rock type, but also with some tonalitic and granitic end members (see Fig. 3). Only locally, some small dioritic and gabbroic bodies appear to be associated as well (e.g., in the Weil3priachtal).

The analysed granitoid samples cover a SiO2 range from 63 to 74% (Fig. 4a). The rocks may be generally described as high-K I-types in the sense of Pecerillo and Tylor (1976). Nevertheless, they have also remarkably high contents of Na20 (3.8-5.5 wt%), so that the K20/Na20 ratios are mostly below 1 (Fig. 4b). In Pearce-diagrams the rocks plot generally in the field of volcanic-arc granitoids. A characteristic trace element feature of the 1-type granitoids of the Schladminger Tauern is the high Ba (up to 1600 ppm) and Sr content (up to 900 ppm, see Table 4). The REE patterns are moderately enriched with (La/Lu)cH ~10-15 and display no or only slightly negative Eu-anomalies (Fig. 5a).

The accessory zircons of the I-type granitoids generally have large (110) prisms, while (100) prisms are small or absent. Terminations vary from (101) pyramids only to (101) = (211). Zircon morphologies are thus in a range that is typical of calc-alkaline I-type granitoid suites (Pupin, 1980).

The leucocratic group 2 granitoids are also metaluminous to weakly peraluminous. Their SiO2 contents vary between 72 and 77 wt% (Fig. 4a). Com- pared to the rare high-SiO2 I-types of group 1, they have clearly higher FeO/MgO ratios and lower A1203 and CaO contents.

The trace element spectra display distinct Rb and Th spikes (strong depletion of Ba), low Sr contents

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A. Schermaier et al./Tectonophysics 272 (1997) 315-333 321

Schladminger Batholith

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65 90 P

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/ A d 10 35

-q granodlorltes to granites (B~senstein-Spelkblchl type)

4" / ' ~ /'~ BB tonalltes to granodiodtes \ ~ m (RingkogeI-Pletzen type) ~ J¢- granites (Zinken type)

~ ~ augengneise (Seltenstalt type)

~ ~ e granite (K°ingraben type) \ \ ~ augengnelse ~ "v~ (Hochreichhart tyPe )

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\ ~\ \Q _ Mautemdorfer \ \ \ \ \ ~ ~ * tonalite(Peterfranzltype)

~ ~ ~ ~ , two-mica granites (~ Grobgneis

Fig. 3. QAP-diagrams (Strcckeiscn, 1976) with plots of granitoid types from the Schladminger Tauern, Scckauer-B6senstein Tauern yKfs and Semmering-Raabalpen area. Most plots are calculated from chemical analyses according to Mielke and Winkler 0979), "'Ab was generally assumed to be 0.15. Two analyses from the Raabalpen were taken from Wieseneder (1968).

322 A. Schermaier et al. / Tectonophysics 272 (1997) 315-333

A J C N K a S 4 1 ~ n g -Rl~balpen

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am= • & S i O 2 0.9 / , i ,

60 65 70 75 80

2.0 K20/Na20' ~ - / b

1.5 - - ~

0.5 • ==*== • • • / /

Bulk of Intra-AJpine Granitolds

0 , S i O 2 60 65 70 75 80

Fig. 4. (a) Plot of Mol (AI203/CaO + Na20 + K20) = A/CNK versus SiO2; (b) Plot of KzO/Na20 versus SiO2; fields according to data in Finger and Steyrer (1990), symbols as in (a).

and an enrichment of Y, Nb, U, Th and Ta relative to the I-types (see Tables 1 and 4 and Fig. 6a). REE- patterns feature high HREE contents, pronounced negative Eu-anomalies and only a slight enrichment of LREE relative to the HREE (see Fig. 5a). All these chemical signatures are well known from A-type granites (Eby, 1990). Furthermore, the rocks exhibit a zircon morphology (Table 1) typical of A-type granite suites (Pupin, 1980; Schermaier et al., 1992).

Unfortunately, it was not possible to find out during field work whether the A-type plutons of the Schladminger Tauern are older or younger than the I-types, because exposed contacts could not be found. We expect, however, that the A-types are generally younger and maybe of Permian age, just as in the Hohe Tauern batholith (Vavra and Hansen, 1991).

5oo !.

ai 100 ~-

~ I o [ - o 31

I

La CePr NdSrnEu GdTb Dy HoEr TmYb Lu

500

100

c- O e-

<")10 u O ne

500

100

c- o ¢-

~ 1 0

Q

n e

b ~

, i i , i L i , i ~ , i , i

La Ce Pr Nd SmEu GdTb Dy Ho Er TmYb Lu

C

i i , , i i i L J k i ~ J i

La Ce Pr Nd SmEu GdTb Dy Ho Er TmYb Lu

Fig. 5. Chondrite-normalized REE-patterns of representative granitoid types of the (a) Schladminger batholith, (b) the Seckau-Brsenstein batholith, and (c) the Semmering-Raabalpen batholith. Analyses and sample locations of plotted samples are listed in Table 4. For normalizing values see Wakita et al. (1971).


10

o 2:

(.) o

re 1

0,1

t

100 a~

5

3

(20 Rb Ba Th Ta Nb Ce ~ Zr Sm Y Yb

100

10

o -r.

o 1

0.4

b~

~ 2

I O I1

i i i i i i

R b , . Th r , c . . , Zr S t a y

100

r,~ 10 r,, 0 "I-

o

1

0.1 - ~ ' W 1 8 i i i , i , , i i i i i

K20 Rb Ba Th 1"- Nb Ce Hf Zr Sm Y YI)

Fig. 6. HORG-normalized geochemical patterns (Pearce et al., 1984) of representative granitoid types of the (a) Schladminger batholith, (b) the Seckau-Brsenstein batholith and (c) the Sem- mefing-Raabalpen batholith. Analyses and sample locations of plotted samples are listed in Table 4.

4. Granite typology of the Seckau-B/Jsenstein Tauern

The Seckau-Brsenstein Tauern are located in central Styria, ca. 50 km northwest of Graz, and form a NW-SE-elongated cupola, more than 500 km 2 large, between the rivers Enns and Mur. The Prls fault is a dextral strike-slip zone, which separates the B6sen- stein area from the Seckauer Alps (Metz, 1976).

Granitoid plutons make up ca. 90% of the Seckau-Brsenstein area (see Fig. 7). All of these are high-level plutons, with sharp and discordant contacts with intercalated Variscan micaschists and paragneisses (see also Metz, 1967, 1976). Serpen- tinites and amphibolites, probably with early Palaeo- zoic protolith ages, are also an important constituent of the local Variscan crust (Speik complex, Neubauer et al., 1989). In the north, the granitoids of the Seck- auer Tauern are overlain by the Rannach Group, a Permo-Skythian trangressive series, which consists mainly of coarse conglomerates and quarzites (Metz, 1976).

The whole Seckau-Brsenstein Tauern experienced Cretaceous greenschist-facies metamorphism (Frank, 1987). Many of the granitoids were subjected to considerable Alpine deformation as well.

Our data set (ca. 25 analysed samples) indicate that most granitoids of the Seckau-Brsenstein Alps form a coherent suite of I-type granitoids. Only a few of leucocratic granitoids seem to have a separate petrogenetic origin. For example, the granitoid of the Koingraben type and the Augengneise from Seit- enstaU display some affinities with A-type granites, while the Augengneise of the Hochreichhart type have features intermediate between S- and I-type (see Tables 2 and 4).

The 1-type suite of the Seckau-Btisenstein area comprises metaluminous to slightly peraluminous tonalites, granodiorites and some granites; the SiO2 contents of the analysed samples range from 62 to 75 wt%. Compared to the Schladminger Tauern, tonalites are more common. Large masses of tonalite are especially widespread in the western Seckauer Alps, between the B~'ntal and the Ringkogel-Plet- zen massif (see Fig. 7). The eastern Seckauer Tauern and the Btisenstein area host a number of leucocratic I-type granodiorites to granites, some with por-

j '

~ ~

*~,k

. I-

Gr

I I

I I I i.

I~~-

A4

sea,

men

"-~

T

ertia

ry to

Qua

tern

ary

~1

UPP

ER A

US

TR

OA

LP

INE

UN

IT

(foe

silif

erou

s P

aleo

zoic

form

atio

ns

I an

d qu

arlz

phyl

lites

) I

MID

DL

E A

usT

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LPI

NE

UN

IT

~ R

anna

chse

rie (

Per

mo-

Sky

thia

n)

Sec

kau-

B6s

enst

ein

Bat

holit

h

t+÷

+

~+

++

~o

ooo

eooo

o >°

°°°

gran

odio

rite

s to

gra

nite

s , r

.~,~

( B

6sen

stei

n-S

peik

bic

hi ty

pe)

tona

lites

to g

rano

diod

tes

¢ (R

ingk

ogeI

-Ple

tzen

type

) ~

;,

gran

ites

(Zin

ken

type

) I

Aug

engn

eise

(S

eite

nsta

ll ty

pe)

. "~

gran

ites

(Koi

ngra

ben

type

) I

Aug

engn

eise

(H

ochr

eich

hart

typ

e)

I

gran

itoid

s, u

ndiff

eren

tiate

d

pre-

gran

itic

coun

try

rock

s

I I

para

gnei

sses

and

mic

asch

ists

amph

ibol

ites

and

serp

entin

ites

(Spe

ik C

ompl

ex)

mic

asch

ist-

mar

ble

Com

plex

of W

SIz

er T

auer

n

a =

a n ~

" a o

a

~ .O

~J

~IK

U/G

III~

++

++~+

++++

+'at'-

s'+++

+++

? .9~

..~

"t

-" ~:

:"~""

+

++ +

~i~+

++

+

++

+

~Zin

ken.

4_

+ +

+ +

gel

x +

x x

x x~

_~_x

~

. -t

- -4

- -I

.~.,,

KN

ITrE

LF

EL

D .J

\.

( ,

,0.m

Fig.

7.

Geo

logi

cal

sket

ch m

ap o

f th

e Se

ckau

er-B

6sen

stei

n ar

ea,

com

pile

d ac

cord

ing

to m

aps

of M

etz

(196

7),

FIt~

gel a

nd N

euba

uer

(198

4).

Tec

toni

c in

terp

reta

uon

mai

nl 5,

ac

cord

ing

to N

euba

uer

and

Fris

ch (

1993

). C

lass

ific

atio

n of

dif

fere

nt g

rani

toid

ty

pes

is b

ased

on

des

crip

tion

s of

Met

z (1

976)

, Sc

harb

ert

(198

1)

and

fiel

d st

udie

s of

Sc

herm

aier

, 19

92.

7"

t~ I

Tab

le 2

Pe

trog

raph

ic a

nd c

hem

ical

cha

ract

eris

tics

of

the

Sec

kaue

r-B

6sen

stei

n gr

anit

oid

type

s; d

ata

sour

ces

for

Sr a

nd e

-Nd

mod

el i

niti

al c

alcu

lati

ons

Scha

rber

t (1

981)

, Fi

nger

et

al.

(199

2) (

for

abbr

evia

tion

s se

e T

able

1)

I-ty

pe s

uite

Subt

ype

Rin

gkog

el-P

letz

en

BiJ

sens

tein

-Spe

ikbi

chl

Zin

ken

Koi

ngra

ben

Seit

enst

all

Hoc

hrei

chha

rt

Mai

n ro

ck t

ype

tona

lite

gr

anod

iori

te

gran

ite

gran

ite

gran

ite

gran

odio

rite

R

ange

to

nali

te-g

rano

dior

ite

gran

odio

rite

-gra

nite

gr

anit

e gr

anit

e gr

anit

e-gr

anod

iori

te

gran

odio

rite

-gra

nite

G

rain

siz

e m

ediu

m

med

ium

to

coar

se

fine

to m

ediu

m

coar

se

coar

se

coar

se

Mes

osco

pic

equi

gran

ular

eq

uigr

anul

ar (

e.g.

Gr.

leuc

ocra

tic,

equ

igra

nula

r le

ucoc

rati

c, e

quig

ranu

lar,

po

rphy

riti

c po

rhyr

itic

aug

engn

eise

, st

ruct

ures

B

t~se

nste

in)

to

defo

rmed

var

iant

s oc

cur

auge

ngne

ise,

wit

h w

ith

Kfs

idi

ocry

sts

up

porp

hyri

tic

(e.g

. as

aug

engn

eise

, K

fs-i

dioc

ryst

s up

to

to s

ever

al c

m

Spei

kbic

hl)

som

etim

es

pegm

atit

es a

re c

omm

on

seve

ral

cm

wit

h pe

gmat

ites

Min

eral

ogic

al

Kfs

3-1

2%,

Qtz

K

fs 1

0-26

%,

Qtz

K

fs 2

5-27

%,

Qtz

K

fs 3

1% (

+

Kfs

25-

30%

K

fs 2

0%,

Qtz

37%

, co

mpo

siti

on

14-2

4%,

26-3

1%,

30-3

6%,

mes

oper

thit

ic),

Qtz

34%

, (s

omet

imes

pin

k co

lour

), Q

tz 3

2-35

%,

P1 s

ubid

iom

orph

, wit

h PI

wit

h os

cill

ator

y an

d so

me

PI w

ith

mag

mat

ic

PI s

how

s m

oder

ate

PI s

how

s m

oder

ate

PI s

how

s m

oder

ate

pron

ounc

ed o

scil

lato

ry

norm

al m

agm

atic

zo

ning

, fi

llin

g w

ith

fill

ing

of M

s m

icro

lite

s fi

llin

g of

Ms

mic

roli

tes

fill

ling

pre

dom

inan

tly

and

norm

al m

agm

atic

zo

ning

pr

edom

inan

tly

Ms

of M

s m

icro

lite

s, M

s zo

ning

m

icro

lite

s, M

s 1-

2%

1-2%

M

afic

min

eral

s B

t 12

-18%

( +

Rt,

B

t 3-

12%

( +

Rt,

B

t 4-5

%

Bt

~3

%

Bt

( +

Rt,

sag

enit

e)

Bt

~15

%

sage

nite

) sa

geni

te)

5-10

%

Acc

esso

ry

Zrn

, Ap

(abu

ndan

t),

Zrn

, Ttn

, :L

AIn

, Z

rn,

4-A

In,

+M

nz,

Ap,

Z

rn, T

tn,

Ap,

Ttn

, Z

rn,

AIn

(ve

ry

Zrn

, Ap,

Aln

(no

t so

m

iner

als

Ttn

(ab

unda

nt,

q-op

aque

s,

Grt

T

tn,

+op

aque

s, G

rt

+op

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s, C

zo/E

p,

com

mon

) A

p, T

tn,

com

mon

as

in

freq

uent

ly i

diom

orph

),

Czo

/Ep,

-I-

Chl

-I

-Chl

+

opaq

ues,

Czo

/Ep,

Se

iten

stal

l typ

e), T

tn,

AIn

(up

to

0.7

mm

),

Grt

, :l-

Chl

-I

-opa

ques

, C

al

opaq

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(fr

eque

ntly

(f

requ

entl

y), C

zo/E

p,

Hem

), E

p (s

omet

imes

-I

-Grt

, +C

hl

very

com

mon

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Chl

Zir

con

typo

logy

m

ainl

y S

13, S

12,

$7,

m

ainl

y G

, L

5, L

4, L

3,

mai

nly

G,

L5,

L4,

L3,

m

ainl

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, P1

, P2

, L

5,

mai

nly

$20,

P4,

P3,

m

ainl

y S

12, S

13,

S11

, $8

, S1

8, $

3, $

4, L

3,

P1.

$5,

$4,

$3,

SI0

, L

2, $

5, $

4, $

3, $

2, $

8,

$5,

S10,

S15

, P3

, $2

0,

S19,

$25

, S1

5, S

14,

S18,

$7,

$8,

$3

L4,

L5

$9,

$8

$7,

S13,

als

o S1

8, S

19,

S19,

S14

, $9

, $4

, L

4 P2

, S

I0,

PI.

$5

$20,

$25

(m

aybe

in

heri

ted)

SiO

2 72

-75%

A

/CN

K 1

.09-

1.13

K

20/N

a20

~ 1.

1 R

b/Sr

2.5

-3

Geo

chem

istr

y Si

O2

62-6

6%

SiO

2 65

-72%

Si

O2

76%

Si

Oz

72-7

5%

SiO

2 68

-70%

A

/CN

K 0

.94-

1.02

A

/CN

K 1

.02-

1.07

A

/CN

K 1

.02

A/C

NK

1.0

4-1.

07

A/C

NK

1.1

5 K

20/N

a20

0.4-

4).8

K

20/N

a20

0.6-

1.2

K20

/Na2

0 ~1

.1

K20

/Na2

0 1.

2-1.

4 K

20/N

a20

~1

Rb

40-1

00,

Rb

100-

140,

F

eO/M

gO ~

4.9

FeO

/MgO

4.5

-6

Rb/

Sr

~0.5

; B

a S

r 70

0-95

0 S

r 30

0-50

0 80

0-10

00

Ba

850-

1600

(se

e B

a 60

0-15

00(s

ee

(ana

lysi

s sa

mpl

e 11

, H

RE

E-e

nric

hed,

H

RE

E-e

nric

hed,

sa

mpl

es 7

and

8,

sam

ples

9 a

nd 1

0,

Tab

le 4

) pr

onun

ced

nega

tive

pr

onou

nced

neg

ativ

e T

able

4)

Tab

le 4

) E

u-an

omal

y E

u-an

omal

y

Rb/

Sr W

R a

ge

n.d.

n.

d.

354

-4- 1

6 m

.y.

(0.7

047)

43

2 +

16 m

.y (

0.71

158)

n.

d.

n.d.

Sr

i (T

320)

n.

d.

0.70

5-0.

708

0.70

5-0.

706

- n.

d.

n.d.

E

-Ndi

(T

320)

n.

d.

-0.9

0 to

-4

.7

n.d.

n.

d.

n.d.

n.

d.

,-.,. i

326 A. Schermaier et al./Tectonophysics 272 (1997) 315-333

phyritic textures (BOsenstein-Speikbichl type, see Fig. 7).

The petrographic and chemical parameters of the Seckau-B6senstein I-type suite are basically similar to the Schladminger I-types. In both cases a granodioritic trend (Lameyre and Bowden, 1982) is displayed in the Streckeisen-diagram (Fig. 3), and the rocks correspondingly have relatively high Na20, Ba and Sr contents (see Table 4), and a REE-pattern with (La/Lu)cH ~ 15-30 and weak Eu-anomalies (see Fig. 5b). Only some of the evolved I-types have more pronounced negative Eu-anomalies.

In the area around the Zinken peak (see Fig. 7), a leucocratic, fine- to medium-grained two-mica granite is widespread (Zinken granite after Scharbert, 1981; see Table 2). Considering the relatively low S7Sr/86Sr initial ratios of ca. 0.705, the only moderately negative Eu-anomaly and the presence of G-zircons (Pupin, 1980), this granite is interpreted as a highly evolved I-type, despite of the peraluminous composition. The low Y and HREE contents argue against an A-type origin.

The augengneiss of the Seitenstall <vpe (see Fig. 7 and Table 2), on the other hand, displays rather A-type characteristics, judging from the high FeO/MgO ratios, the Y and HREE enrichment and the zircon morphology (presence of P1-P5 zircons, see Pupin, 1980).

The metagranite of the Koingraben type is re- stricted to a ca. 5 km NW-SE-striking lamella in the westernmost Seckauer Alps (Fig. 7). A pre-Variscan age has been proposed for this granite, based on Rb/Sr isotope data published in Scharbert (1981). A characteristic feature of this coarse-grained, weakly peraluminous rock type is again high FeO/MgO (see Table 2). Some zircons display morphologies of A-type granites (P2, P3, P4 types, Pupin, 1980).

For the Zinken granite, a Rb/Sr WR age of 354 ± 16 Ma was obtained by Scharbert (1981). Based on these data, an Early Carboniferous age was commonly assumed for the Seckau-B6senstein granitoid terrane. However, the large masses of tonalites and granodiorites still await dating, and it may well be that they are significantly younger (i.e. later Carboniferous), just as the chemically very similar tonalites of the eastern Hohe Tauern (Cliff, 1981).

5. Granite typology of the Semmering- Raabalpen

The crystalline complex of the Semmering-Raab- alpen is located ca. 70 km southwest of Vienna and exposes ca. 600 km 2 of late Palaeozoic granitoids. The country rocks of the granitoids are mainly migmatitic paragneisses and micaschists of Variscan age (Neubauer and Frisch, 1993). Towards the east, the basement rocks are mostly overlain by Tertiary to Quaternary sediments, but they are believed to ex- tend in the subsurface at least to the area of Sopron in Hungary (see Fig. 8).

The Semmering-Raabalpen unit overlies the Waldbach and Wechsel Metamorphic Complexes, which both consist mainly of paragneisses and micaschists with intercalated greenschists, amphibolites, quartzites and some acidic orthogneisses of un- certain age (see Neubauer and Frisch, 1993). In the Semmering-Raabalpen and Wechsel units, Permo- Triassic cover sequences are widespread. An important constituent of these transgressives are also intercalated tufts and subvolcanics of Permian age (Neubauer, 1988b).

The rocks of the Semmering-Raabalpen batholith experienced Alpine metamorphism in greenschist facies, small areas in the southern regions even in amphibolite facies (Neubauer and Frisch, 1993).

From Rb/Sr WR age determinations of Scharbert (1990) and Peindl (1990), most granitoids are commonly interpreted as Early Carboniferous intrusions. Only for some smaller plutonic bodies in the south of the Semmering-Raabalpen area a Permian age is discussed.

The most widespread granite type in the Semmering-Raabalpen is the so-called Grobgneis, which occupies 90% of the Semmering-Raabalpen batholith (see Fig. 8). The Grobgneis is a coarse- grained porphyritic metagranite with K-feldspar id- iocrysts up to several centimetres (Schwinner, 1932; Wieseneder, 1968; Peindl et al., 1990; Table 3). A characteristic feature of the Grobgneis is a pronounced gneissic texture due to Alpine deformation. The Grobgneis forms sheet-like plutonic bodies with concordant contacts to the country rocks (Neubauer et al., 1992).

The Grobgneis samples that have been analysed during this study (10) all displayed typical features

NEUN

KIRC

HEN

~ Te

rtiar

y to

Qua

tern

ary s

edim

ents

'~']

U

PPER

AU

STR

OAL

PIN

E U

NIT

'] M

IDD

LE A

UST

RO

ALPI

NE

UN

IT

(Sie

ggra

ben

Co

mp

lex)

LOW

ER A

UST

RO

ALPI

NE

UN

IT

~KIN

DB

ERG

Perm

o-Tr

iass

ic co

ver

I I L

Sem

med

ng-R

aaba

lpen

Bat

holit

h

leuc

ocra

tic t

wo-

mic

a gr

anite

s,

~ flne-

to m

ediu

m g

rain

ed

BIR

K

Gro

bgne

is

pre-

gran

iUc c

ount

ry ro

cks

(mai

nly

mig

mat

ic p

arag

neis

sea

and

mic

asch

ists

)

Wec

hael

- an

d W

aldb

ach

Cry

stal

line

Com

plex

• ii~

! ~

~ P

EN

NIN

IC U

NIT

Hoc

hwec

hsel

i!'

PIN

KAFE

LD

~AR

TBER

G

20 k

m

Fig.

8. G

eolo

gica

l ske

tch

map

of

the

Sem

mer

ing-

Raa

balp

en m

etam

orph

ic c

ompl

ex a

ccor

ding

to

Neu

baue

r et

al.

(199

2).

SOPR

ON

S ....

.......

":" ~(

~ ...

... ~i

3 ~ ..

..

t~

~D

I

Tab

le 3

Pe

trog

raph

ic a

nd c

hem

ical

cha

ract

eris

tics

of

the

Sem

mer

ing-

Raa

balp

en g

rani

toid

typ

es (

see

also

Pei

ndl,

199

01 a

nd t

he M

aute

rndo

rfer

gra

nite

gne

iss

(see

als

o E

xner

, 19

901

S-t

ype

suit

e S

-typ

e

Subt

ype

Gro

bgne

is

Tw

o-m

ica

gran

ites

T

onal

ites

(P

eter

fran

zl-t

ype)

M

aute

rndo

rfer

gra

nite

gne

iss

Mai

n ro

ck t

ype

gran

ite

gran

ite

tona

l±re

gr

anit

e R

ange

gr

anit

e gr

anod

iori

te

gran

ite

tona

l±re

gr

anit

e G

rain

siz

e co

arse

fi

ne t

o m

ediu

m

fine

to

med

ium

co

arse

M

esos

copi

c po

rphy

riti

c, w

ith

Kfs

-idi

ocry

sts

up

leuc

ocra

tic

equi

gran

ular

eq

uigr

anul

ar :

t: po

rphy

riti

c st

rong

ly d

efor

med

por

phyr

itic

st

ruct

ures

to

10

cm,

usua

lly

defo

rmed

to

Aug

engn

eis

Aug

engn

eis

Min

eral

ogic

al

Kfs

28-

38%

(pe

rth±

tic m

icro

clin

e,

Kfs

35-

45c/

,, Q

tz 3

3-38

e~.

no K

fs,

Qtz

20-

30¢4

K

fs ~

35%

, Q

tz 3

5-39

%,

com

posi

tion

fr

eque

ntly

tw

inne

d),

Qtz

33

38%

PI

(al

bite

-oli

gocl

as)

wit

h P1

sho

ws

fill

ing

pred

omin

antl

y of

P1

wit

h m

oder

atly

til

ling

of

Ms

Ms

1-3%

m

oder

ate

till

ing

of M

s m

icro

lite

s.

Ms

mic

roli

tes.

Ms

1-3c

/, m

icro

lite

s M

s 1~

.%

Mar

ie m

iner

als

Bt

( iR

t, s

agen

ite)

4-1

2%

Bt

4-6c

~ B

t 15

-25c

/,. w

ith

char

acte

rist

ic

Bt

2-4%

re

dbro

wn

colo

ur,

som

e w

~ria

nts

cont

ain

Hbl

Zrn

, A

p, o

paqu

es,

Tin

, G

rt

Zrn

, A

p, o

paqu

es.

Ttn

Grt

, E

p.

(fre

quen

tly)

, ±

Chl

C

hl,

:LSt

p A

cces

sory

min

eral

s Z

rn,

Ap,

Ttn

, :l

:opa

ques

. ±

Mnz

Z

rn,

Ap,

±M

nz

(wit

h ri

m o

f A

p C

zo/E

p, G

rt (

freq

uent

ly),

+

Aln

), G

rt,

±T

ur,

iTtn

+

Cal

,iC

hl

Zir

con

typo

logy

m

ainl

y S1

2, S

13,

Sl4

, $7

, $8

, $9

. m

ainl

y L

2, $

2, $

7, L

I. L

2, $

6,

mai

nly

$7,

$2,

S12,

S11

. $8

, $3

, n.

d.

$2,

$3,

$4,

$5,

L5,

S18

, S1

9, $

20

S12,

L3,

$3,

$8

$6,

SI

Geo

chem

istr

y Si

O2

69-7

3%

SiO

2 70

74

%

SiO

2 62

%

SiO

2 74

%

A/C

NK

1.0

6-1.

15

A/C

NK

1.1

4-I.

18

A/C

NK

~ I

. 1

A/C

NK

1.1

5 K

20/N

a20

1-2,

Rb/

Sr

1~.,

K20

/Na2

0 1.

5.-2

.5.

Rb/

Sr 2

-8

K20

/Na2

0 ~

0.8

K20

/Na2

0 1.

5, R

b/S

r 1-

4 B

a 17

0-50

0, p

rono

unce

d ne

gati

ve

Eu-

anom

aly

Rb/

Sr

WR

age

s 33

8::t:

12

m.y

. (0

.707

1 )

326±

1 I

m.y

.(0.7

0681

34

3:L

20 (

0.70

571

n.d.

Sr

i (T

320)

0.

708-

0.71

0 0.

708-

0.71

3 0.

705-

0.70

6 0.

708

g-N

di (

T32

01

-5 t

o -6

.9

n.d.

n.

d.

n.d.

,,....

"-

N

t,o

Ln

I

Dat

a so

urce

s fo

r Sr

and

e-N

d m

odel

ini

tial

cal

cula

tion

s: P

eind

l (1

990)

. Sc

harb

ert

(199

01 a

nd F

inge

r et

al.

(199

21.

For

abbr

evia

tion

s se

e T

able

I.

Tab

le 4

R

epre

sent

ativ

e ch

emic

al a

naly

ses

from

the

mai

n ty

pes

of

gran

itoid

s o

f th

e S

chla

dmin

ger

bath

olith

, th

e S

ecka

u-B

Gse

nste

in b

atho

lith,

th

e S

emm

erin

g-R

aaba

lpen

bat

holit

h an

d th

e M

aute

mdo

rfer

gra

nite

gn

eiss

(M)

Sch

ladm

inge

r A

lps

Sec

kaue

r-B

Ose

nste

in A

Ips

Sem

mer

ing-

Raa

balp

en

M

Sam

ple

I 2

3 4

5 6

7 8

9 10

I 1

12

13

14

15

16

17

18

19

20

21

H

A

HA

H

A

HA

H

A

HA

A

S

AS

A

S

AS

A

S

AS

A

S

AB

A

B

AB

A

B

AB

A

B

AB

Fi

13

/92

14/9

2 4

7•9

2

28

•92

35

/92

37/9

2 2

01

92

47

/92

9/92

51

/92

50/9

2 3

91

92

2/

92

1/92

5/

92

30/9

2 1

41

92

15

/92

17/9

2 1

1/9

2

1018

8

SiO

2 68

.80

72.7

0 66

.80

76.3

0 72

.60

77.0

0 63

.00

65.8

0 65

.10

69.4

0 72

.10

76.0

0 74

.10

72.2

0 71

.00

70.2

0 71

.50

73.1

0 73

.20

62.1

0 74

.37

TiO

2 0.

35

0.21

0.

50

0.19

0.

29

0.07

0.

80

0.62

0.

50

0.46

0.

21

0.14

0.

25

0.19

0.

34

0.54

0.

28

0.19

0.

19

1.02

0.

19

A12

03

16.0

0 14

.80

16.1

0 11

.70

12

.90

12

.30

16.8

0 16

.10

17.4

0 15

.20

14.3

0 13

.20

13.4

0 13

.50

14.6

0 14

.40

14.0

0 14

.50

14.6

0 1

7.3

0

13.1

0 Fe

203

2.53

1.

46

3.49

2.

27

1.89

0.

89

4.87

3.

87

3.26

2.

89

1.32

1.

03

1.94

1.

54

2.67

3.

45

1.54

1.

89

1.55

5.

67

1.93

M

nO

0.05

0.

04

0.08

0.

03

0.05

0.

02

0.10

0.

07

0.07

0.

07

0.04

0.

03

0.04

0.

05

0.05

0.

07

0.03

0.

04

0.05

0.

10

0.04

M

gO

0.81

0.

50

1.08

0.

17

0.35

0.

14

1.9

8

1.5

4

1.41

0.

91

0.35

0.

19

0.29

0.

37

0.51

0.

92

0.40

0.

34

0.35

2.

21

0.53

C

aO

3.16

1.

72

3.07

0.

53

0.71

0.

49

4.14

4.

03

2.97

2.

03

0.93

0.

52

0.70

0.

90

1.30

2.

26

0.86

0.

52

0.66

4.

51

0.34

N

a20

5.

04

4.10

4.

16

3.52

3.

52

4.38

4.

46

4.38

5.

11

4.15

4.

04

4.18

3.

72

3.39

3.

12

3.35

3.

15

3.24

3.

40

2.98

3.

32

K2

0

1.98

3.

96

3.53

4.

50

4.99

4.

51

2.58

2.

05

3.01

3.

56

4.46

4.

73

5.09

4.

84

5.45

3.

61

5.15

5.

29

5.19

2.

35

4.98

P2

05

0.13

0.

07

0.18

0.

03

0.09

0.

02

0.29

0.

22

0.17

0.

19

0.13

0.

05

0.08

0.

13

0.15

0.

22

0.18

0.

27

0.24

0.

25

0.16

T

otal

98

.85

99.5

6 98

.99

99.2

4 97

.39

99.8

2 99

.02

98.6

8 99

.00

98.8

6 97

.88

100.

07

99.6

1 97

.11

99.1

9 99

.02

97.0

9 99

.38

99.4

3 98

.49

98.6

2 0.

90

1.45

0.

75

0.90

0.

85

0.95

1.

00

0.85

1.

15

1.65

0.

45

0.55

1.

40

0.95

1.

10

1.00

1.

05

0.95

1.

10

-

1.05

0.

99

1.01

1.

03

0.95

0.

95

0.96

1.

02

1.06

1.

09

1.02

1.

04

1.08

1.

09

1.06

1.

14

1.21

1.

18

1.11

1.

14

r,

LO

I 1.

30

A/C

NK

0.

99

Cr

bdl

Ni

6 C

o 6

Sc

4 V

28

C

u 2

Zn

62

Rb

55

bdl

6 7

4 bd

l 13

7

11

10

bdl

bdl

5 5

6 16

bd

l 2

bdl

36

- 4

6 3

3 2

9 8

8 6

2 2

3 4

5 6

3 2

5 13

-

3 7

3 5

2 10

10

7

6 3

2 5

4 5

8 4

3 3

14

- 3

7 5

4 3

11

5 5

6 3

2 6

4 10

7

3 2

4 11

-

13

48

12

16

bdl

80

68

40

33

bdl

bdl

14

12

18

33

12

11

12

100

- 2

7 2

2 1

8 3

6 7

1 3

1 5

2 5

2 2

2 15

-

101

55

34

26

10

80

75

84

51

17

17

34

38

55

70

47

52

36

119

31

58

93

177

330

325

101

44

119

106

211

125

141

253

198

169

263

319

317

102

267

Ba

500

1600

11

00

386

81

10

858

929

956

1210

51

7 35

0 43

4 13

7 48

8 23

3 20

2 20

9 16

3 35

8 14

8 S

r 85

8 G

a 19

L

i 19

T

a 0.

50

Nb

8.0

Hf

3.30

Z

r 17

5 y T

h 8.

5 U

2.

5 L

a 33

.0

Ce

63

Nd

27

Sm

4.

90

Eu

1.10

T

b 0.

60

Yb

3.30

L

u

629

507

32

37

43

825

991

864

504

84

31

72

70

I 15

164

95

41

65

374

- 15

20

18

19

22

23

20

27

20

18

15

19

14

18

18

14

13

15

20

19

11

25

10

31

5.

00

47.0

0 29

.00

43.0

0 38

.00

41.0

0 4.

00

9.00

75

.00

15.0

0 35

.00

16.0

0 65

.00

25

42

- 0.

50

1.00

0.

50

3.00

3.

00

1.0

0

1.0

0

0.50

1.

00

2.00

0.

50

0.50

! .

00

1.00

2.

00

1.00

1.

00

2.00

2.

00

- 8.

0 11

.0

12.0

16

.0

21.0

18

.0

11.0

11

.0

12.0

17

8.

0 15

.0

13.0

17

.0

19.0

15

.0

15.0

20

.0

19.0

13

2.

40

5.10

5.

20

4.80

6.

00

5.50

4.

10

5.70

5.

70

3.40

2.

60

6.60

2.

60

5.90

6.

00

2.20

2.

00

1.90

6.

60

- 83

22

3 19

8 16

6 10

1 27

5 20

8 23

1 21

4 11

8 87

21

3 81

23

2 23

9 89

71

71

32

2 11

4 7

11

56

40

89

24

- -

11

12

23

34

20

31

22

10

16

31

6.8

16

18

32

33

10

6.4

21

15

15

8.7

12

10

20

21

10

6.1

6.5

3.4

- 2.

4 4.

5 5.

3 10

14

.6

2.9

2.7

8.8

3.2

4.5

2.6

3.5

5.9

4.7

6.4

3.9

2.6

10.0

1.

3 11

19

.2

47.2

45

.7

31

14.7

64

.5

34.7

40

.6

47.6

25

.6

13.2

31

.5

19.8

43

.5

44.5

20

.7

8.6

12.6

20

.4

- 36

88

90

63

36

12

4 66

75

91

51

28

64

39

87

88

41

19

26

39

-

18

37

43

27

25

51

26

30

40

23

14

30

19

40

38

18

8 13

18

-

3.50

7.

10

8.90

5.

60

8.30

9.

40

5.10

5.

50

7.40

5.

00

3.20

6.

20

4.00

8.

70

7.90

3.

90

1.90

3.

10

3.40

-

0.80

1.

80

0.50

0.

60

0.50

2.

40

1.60

1.

20

1.10

0.

60

0.30

0.

90

0.30

1.

20

0.60

0.

80

0.50

0.

50

1.00

-

0.50

0.

80

1.10

0.

80

2.10

1.

20

0.60

0.

60

0.80

0.

60

0.50

0.

90

2.00

3.

00

0.90

0.

60

0.50

0.

50

1.30

-

2.00

1.

50

5.10

3.

70

8.90

2.

90

1.4

0

1.20

1.

61

1.40

2.

50

3.50

2.

00

3.00

2.

40

0.60

1.

00

1.50

1.

30

- 0.

31

0.26

0.

74

0.52

1.

29

0.38

0.

18

0.15

0.

18

0.17

0.

36

0.53

0.

28

0.42

0.

32

0.05

0.

14

0.19

0.

18

-

ix2

t.q

I

Maj

or e

lem

ents

: w

t% o

xyde

s; t

race

ele

men

ts a

nd r

are

eart

h el

emen

ts i

n pp

m.

RE

E,

Hf,

Ta,

Th,

U,

Sc,

Cr,

and

Co

wer

e de

term

ined

by

INN

A,

V a

nd N

i by

DC

P, G

a an

d L

i by

IC

P;

all

othe

r el

emen

ts w

ere

anal

ysed

by

conv

entio

nal

XR

F te

chni

ques

. A

bbre

viat

ions

: b

dl=

bel

ow d

etec

tion

lim

its;

L.O

.I.

= l

oss

on i

gniti

on;

A/C

NK

= m

ol A

I203

/CaO

+ N

a20

+

K20

. F

or o

ther

abb

revi

atio

ns s

ee T

able

1.

Sam

ple

loca

tions

. Sc

hlad

min

gerA

lps:

(1)

tona

lite,

for

est r

oad

E K

riig

erzi

nken

, 11

50 m

; (2)

gra

nodi

orite

, fo

rest

roa

d E

Krt

iger

zink

en,

1145

m;

(3)

porp

hyri

tic

gran

odio

rite

, fo

rest

roa

d ne

ar 'K

ohlu

ng'

1150

m,

Kle

ins6

1kta

l; (4

) gr

anit

e, l

ands

lide

Mos

eral

m,

1770

m,

Zna

chba

ch,

Wei

Bpf

iach

tal;

(5)

porp

hyri

tic

gran

ite,

Den

gght

itte,

Sc

hOde

rtal

; (6

) gr

anit

e, l

ands

lide

Wild

enka

rsee

, E

trac

htal

. Se

ckau

er-B

i~se

nste

in A

lps:

(7

) to

nalit

e, l

ands

lide

NE

Ple

tzen

, S

W H

ofal

m,

Inge

ring

tal;

(8)

tona

lite,

fo

rest

roa

d, W

Rin

gkog

el, n

ear

R0h

rmilc

hgra

ben,

G

aalg

rabe

n; (

9) p

orph

yriti

c gr

anod

iori

te,

Son

ntag

skar

W o

f G

r. B

Gse

nste

in;

(10)

po

rphy

ritic

gra

nodi

orite

, m

orai

ne S

W O

bere

Bod

enhU

tte,

1680

m, F

eist

ritz

grab

en;

(11)

gra

nite

(Z

inke

n ty

pe),

mor

aine

nea

r O

bere

Bod

enht

itte,

15

90 m

, Fei

stri

zgra

ben;

(1

2) g

rani

te, f

ores

t ro

ad S

E G

iegh

tibl,

Koi

ngra

ben;

(13

) po

rphy

ritic

A

ugen

gnei

s, n

orth

ern

part

Sei

tens

tallg

rabe

n.

Sem

mer

ing-

Raa

balp

en: (

14)

Gro

bgne

is, q

uarr

y o

f H

ader

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330 A. Schermaier et al./Tectonophysics 272 (1997) 315 333

of S-type granites (see Tables 3 and 4), i.e. peraluminous compositions and a limited SiO2 range between 69 and 73% (see also data in Peindl et al., 1990, Neubauer et al., 1992). The modal compositions of the samples is mostly granitic (see Fig. 3). Other chemical signatures of the Grobgneis are K20/Na20 ratios > 1 (Fig. 4b), high Rb/Sr ratios (I-4) and moderate to low Ba-contents (170-500). The REE- patterns (Fig. 5c) generally display deep negative Eu-anomalies, but relatively low (La/Lu)cH ratios (see also Kiesl et al., 1983). However. compared to A-type granite patterns (see Fig. 5a), the HREE-tails are steeper. The (Tb/Lu)cH ratios are ca. 4 5, versus 1-2 in the Schladminger A-type granites, for example (Fig. 5).

In the southern part of the Semmering-Raabalpen area (see Fig. 8), discordant stocks and dikes of fine- to medium-grained, Ieucocratic two-mica granites are widespread (Peindl, 1990, table 3). Peindl et al. (1990) and Neubauer et al. (1992) proposed that these two-mica granites intruded in two pulses, i.e. in the Upper Carboniferous (Waldbach and Hartberg type) and in the Permian (Stubenberg type - - see also Peindl et al., 1992). According to our data, the two supposed age groups are, however, not significantly different in their petrography and chemistry. The rocks are generally S-types. They have peralu- ruinous compositions with high K~O/Na20 (Fig. 4) and Rb/Sr ratios. The HREE and Y contents are significantly lower than in the Grobgneis. Judging from the generally smaller negative Eu-anomalies, the two-mica granites cannot be interpreted as more frac- tionated variants of the Grobgneis magma, but seem to represent an independent S-type granite suite.

The tonalite of the PeterfranJ-type (Peindl, 1990) is a fine- to medium-grained fairly peraluminous biotite tonalite with K20/Na20 ratios ~0.8 and high Rb/Sr ratios of 1-4. The rock is thus significantly different from the tonalites in the Schladminger and Seckau-B6senstein massifs and it is reminiscent of the rare S-type tonalites in the Southern Bohemian batholith (Schubert, 1989).

These tonalites of the Peterfranzl-type (as well as some small local gabbro bodies, e.g. at Birkfeld) previously gave rise to speculations that calc-alkaline Variscan plutonism was quite important in the Sem- mering-Raabalpen (Finger et al., 1992). However, from the now available data it appears at the Sem-

mering-Raabalpen batholith has many more genetic features in common with the Moldanubian Southern Bohemian batholith than with other I-type granite- dominated Variscan massifs in the Alps, such as the Schladminger and Seckau-B6senstein Tauern (see e.g. Fig. 4b).

6. Discussion and conclusions

The present investigation shows that the chemical records of the Middle Austroalpine Schladminger and Seckau-BOsenstein granitoid terrains are very similar. A distinct, calc-alkaline, I-type, tonalite-granodiorite suite with significantly high K20, Na20, Sr and Ba contents and intermediate Sr and Nd initial ratios is present in both areas. However, large amounts of chemical identical l-type granitoids are also known from the Penninic eastern Tauern Win- dow (Vavra, 1989; Finger et al., 1993; Haunschmid, 1993). Furthermore, leucocratic granite plutons with A-type affinities are present in all three areas. In view of this striking equivalence of granite plutonism, it seems valid to conclude that the Variscan lower lithosphere was largely of the same type in parts of the proto-Penninic and proto-Austroalpine basement.

Thus, it is possible that the granitoid terrains of the Seckauer-B6senstein, Schladminger and eastern Hohe Tauern were formerly situated in palaeogeo- graphic proximity, or at least in the same structural zone of the Variscan orogen, where I-type plutonism was common. Following the model of Finger and Steyrer (1990), a position quite close to the southern flank of the Variscan orogen would have to be assumed, where the Palaeotethys Ocean was subducted. A genetic relation of the Middle Aus- troalpine batholith to the palaeo-subduction zone of the Plankogel Ocean is unlikely, since they were part of the Noric terrane assembly, i.e. part of the passive and not of the active margin of the Planko- gel Ocean (see model of Frisch and Neubauer, 1989 in Fig. 1). Given that the Seckauer-B6senstein and Schladminger batholiths are subduction-related, an- other Variscan subduction zone, further to the south, would be required (e.g., the Palaeotethys subduction zone as suggested by Finger and Steyrer, 1990).

Unfortunately, magma sources and tectonic en- vironments for the I-type/A-type plutonism in

A. Schermaier et al. / Tectonophysics 272 (1997) 315-333 331

the Seckauer-B6senstein, Schladminger and Hohe Taueru cannot be identified with certainty on the basis of the present data set. On one hand, melting of subduction-modified mantle, combined with magmatic underplating, lower crustal hybridization and assimilation-/fractional crystallization (AFC) processes, may well result in such melts. Compara- ble I-type/A-type successions are quite common at evolved active continental margins (Eby, 1990). The A-type granites are mostly explained as be- ing derived from enriched lithospheric mantle in these cases. On the other hand, it has been pos- tulated that the I- and A-type granites may also form through poly-stage remelting of immature meta-igneous lower crust (e.g., in high-heat flow regions related to postcollisional extension), with- out any admixtures of mafic mantle melts (see the Lachlan Fold Belt example - - White and Chappell, 1983). Such a model has been applied to the eastern Hohe Tauern batholith by Vavra (1989). An effective mantle heat source (magmatic underplating) would be, nevertheless, required in this case as well. The fact that S-type granites are rare in the Schladminger and Seckauer-B6senstein Tauern, although the country rocks regularly contain a lot of paragneisses (i.e. suitable S-type source rocks) seems to favour the AFC model. However, in an AFC model, the magmatic mantle parents may still be either subduction-related (i.e. melts from a water-fluxed mantle wedge, Finger and Steyrer, 1988), or produced by decompression mantle melting in a non-subduction, extensional regime (Schaltegger and Corfu, 1995). These questions concerning the exact sources and genetic processes of the intra-Alpine I-type plutonism are largely unsolved, and their solution would require a far more detailed chemical and isotopic approach. What we can say at the moment, with some certainty, is that a chemically immature or newly underplated (basalt-fluxed?) Variscan lower crust was present in most parts of the Eastern Alps and that this kind of crust appears to be quite typical for the southern flank of the Variscan orogen.

The dominantly S-type character of the granitoids of the Semmering-Raabalpen indicates, on the other hand, that basement compositions and magma-forming processes were significantly different in this outermost eastern sector of the Alps and were mainly controlled by melting of more mature

lower crust (see also the isotope data in Table 3). This confirms the tectonostratigraphic concept of Frisch and Neubauer (1989), according to which a different Variscan terrane assembly is exposed in the Lower Austroalpine between Graz and Vienna. With the possible exception of a few early gabbros and tonalites, the Semmering-Raabalpen plutonism is definitely not of the Cordilleran type. It is thus seemingly unrelated to the palaeo-subduction zone of the Plankogel Ocean. Rather it is a product of the subsequent Variscan collision tectonics in the Carboniferous (see Fig. 1).

The granites of the Semmering-Raabalpen show that large Variscan S-type batholiths are not re- stricted to the extra-Alpine Moldanubian zone, but are also present in distinct parts of the Alps. How- ever, it would be incorrect to claim that S-type granites are particularly typical of the Lower Aus- troalpine, since the Bernina batholith, situated at the western end of the Eastern Alps (Fig. 1), hosts predominantly I-type and A-type granites (Rageth, 1984; Btichi, 1994). The Bernina batholith thus seems to have been part of the southern, I-type granite-dominated zone of the Variscan orogen, i.e. part of the Noric terrane assembly, as suggested by Frisch and Neubauer (1989). No further large granite terrains are exposed in the Lower Austroalpine between the Semmering and the Bernina. A small occurrence of a probably Variscan Lower Austroalpine granitoid south of Salzburg (Mauterndorfer granite gneiss, Exner, 1990, see Fig. 2) yields S-type characteristics (see also Table 3).

Unfortunately, it cannot be proved whether the same type of lateral west-east I-type/S-type granite zoning also exists in the Penninic, since no Penninic granitoids of Variscan age are exposed in eastern Austria. Nevertheless, the present-day distribution of intra-Alpine I- and S-type granites would be consis- tent with the previously very common assumption (e.g., Schwinner, 1933, 1951) that parts of the eastern end of the Alps are already built on Moldanubian basement.

Acknowledgements

This work has benefited from discussions with G. Frasl, J. Clemens, I. Broska, I. Petrik, E Neubauer and A. Schindlmayr. The authors would also like

332 A, Schermaier et al./Tectonophvsics 272 f 1997) 315-333

to thank two anonymous reviewers Ibr their stim- ulating comments. H.R Steyrer is thanked for his help in drawing the geological maps. G. Aigner and M. Dawoud are thanked for their assistance in the laboratory. This research has been supported by the Austrian grants OWP 69 (BMWF) and 9434 GEO (FWF).

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