1. No category

META-PEGMATITES IN THE PRIJAKTPOLINIK COMPLEX (EASTERN ALPS)
Thomas REPOLUST 1)
Ralf SCHUSTER 2)
Georg HOINKES 1)
Christoph A. HAUZENBERGER 1)
1)
2)
Geological Survey (Geologische Bundesanstalt),
Neulinggasse 38, A-1030 Vienna, Austria
email: Ralf Schuster <[email protected]>
Geologische Bundesanstalt
100 km
Munich
Linz
section
2
1
03R37 Grt41
A
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.4
0.9
XFe/(XFe+XMg)
0.8
XFe
0.7
XMn
XMg
XCa
0.0
0
200
400
600 800 1000 1200 1400
distance [µm]
03R37 Grt82
Innsbruck
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.6
0.6
0.5
0.5
0.5
0.5
0.5
0.4
0.4
0.4
0.4
0.4
Bozen
Verona
Enns
SEMP
TriestMur
XFe
0.3
0.2
0.2
0.1
0.1
0.0
0.0
200
D
400
600
distance [µm]
03R37 Grt42
0.8
XFe
0.7
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.0
1000
0.0
800
E
XMg
XCa
0.1
0.0
0
200
400
03R37 Grt41
600 800 1000 1200 1400
distance [µm]
F
1
1000 µm
1000 µm
Villacher Alpe S
Zagreb
Nockberge
0.9
0.3
XMg
XCa
0
XFe/(XFe+XMg)
XMn
XMn
0.3
A
Traun
XFe/(XFe+XMg)
1.0
Fig. 3: Characterisation of garnet from sample 03R37 A) cut hand specimen of mylonitic
pegmatite, B) thin section, C) BSE-image with section lines, D-F) element distribution of
selected garnet crystalls. Explanation see text.
Graz
Niedere Tauern
C
1000 µm
1.0
1.0
0.1
Sopron
B
2 cm
0.2
Salzburg
03R37 Grt42
1
0.3
Vienna
Nördliche Kalkalpen
2
1mm µm
1000
The northeastern part of the Kreuzeck Mountains (Carinthia, Austria) is built up by the
Prijakt-Polinik Complex, which is part of the Koralpe-Wölz nappe system (Austroalpine
unit) according to SCHMID et al. (2004) (Fig. 1A, 1B). This Complex experienced preAlpine metamorphic imprints and an up to eclogite-facies eo-Alpine (Cretaceous)
metamorphic overprint. It consists of paragneisses and micaschists with intercalations of
retrogressed eclogites, amphibolites and minor orthogneisses. As in other Austroalpine units
deformed and partly mylonitised meta-pegmatites occur. According to Rb-Sr data by HOKE
(1990) they are Permian in age. In this study, two mylonitised meta-pegmatites from the
crests north of Mt. Polinik (2784 m) (Fig. 1C, Fig. 2) were investigated. The aim of the study
is to get information on the magmatic age of the rocks and the effects of the structural and
metamorphic overprint.
GRAZ
Institut für Erdwissenschaften, Karl-Franzens-Universität
Graz, Universitätsplatz 2, 8010 Graz, Austria
INTRODUCTION
N
UNI
Drau
Gail
PAL
Venedig
ADRIA
Geologische Bundesanstalt
C
03R36 Grt32a1
2
03R36 Grt32a2
B
Obervellach
Gmünd
Stall
Möllbrücke
Spittal a.d. Drau
Oberdrauburg
Greifenburg
Apulian plate
Southalpine unit
Austroalpine unit
Upper Austroalpine subunit
Drauzug-Gurktal nappe system
Ötztal-Bundschuh nappe system
Juvavic nappe system
Tirolic nappe system
Noric Nappe
Wölz-Koralpe nappe system
Bajuvaric nappe system
Silvretta-Seckau nappe system
GWZ exclusively Noric nappe
Lower Austroalpine subunit
Meliata-Hallstatt zone
1
2
A
1000 µm
1.0
1.0
0.9
0.9
0.8
0.8
0.8
0.8
0.7
0.7
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.4
0.3
0.3
0.3
0.3
0.2
0.2
0.2
0.2
0.1
0.1
0.0
0.0
1.0
XFe/(XFe+XMg)
0.9
0.6
XFe
0.5
XCa
XMg
Mauthern
0.0
0
200
400
03R36 Grt32a2
25km
Hermagor
SAMPLE LOCALITY
The samples were collected from the crests north of Mt. Polinik in the area of the 2nd and the
3rd Mörningkopf (Fig. 2). Sample 03R36 is an 0.5 m thick mylonitised pegmatite directly
from the crest at 2265m altitude (WGS83 E 13°10'23'', N 46°54'23''). The second sample
03R37 is a loose-block from below the rock walls west of the crest in 2130 m altitude
(WGS83 E 13°10'07'', N 46°54'24''). In the wall an about 1 m thick mylonitic pegmatite layer
is situated.
(WGS83 E 13°10'07'', N 46°54'24'')
600
800
distance [µm]
D
0.9
0.6
XFe
0.5
XMn
0.4
Fig. 4: Characterisation of
Garnet from sample 03R36
A) thin section, B-C) BSEimage with section lines, DE) element distribution of
selected garnet crystalls.
Explanation see text.
0.1
XCa
XMg
0.0
200
0
400
03R36 Grt32a1
600
800
distance [µm]
1000
E
Sm-Nd GEOCHRONOLOGY
For dating with the Sm-Nd method a whole rock powder and three garnet fractions of sample
03R37 were analysed. Except of fraction Grt1 (Fig.5A), which was represented by chips
from the central part of crystals without rims, the two other fractions (Grt2, Grt2b) were
represented by idiomorphic garnet crystals of 0.2-0.3 mm in size (Fig. 5B). From sample
03R36 also idiomorphic garnet crystals of 0.2-0.3 mm in diameter and feldspar were used.
With respect to the chemical zoning pattern of the garnet fraction, Grt1 contains large
percentages of core composition, whereas the other fractions contain large amounts of the
rims with XCa>XMg. The core shows lower Sm and Nd contents and a higher
147Sm/144Nd ratio with respect to the rims (core = 5.32; rim = 0.38-1.23) (Table 1).
Garnet-whole rock isochrons yielded a Permian age of 267±3 Ma for Grt1 and 239±18 Ma
for Grt2+Grt2b from sample 03R37 (Fig. 5C). For feldspar and garnet of sample 03R36 an
age of 248±3 Ma was calculated (Fig. 5D).
Fig. 2: A) Orthophoto and DHM showing the
sample localities along the crest between Mt.
Polinik and Polinik hut.
03R36
(WGS83 E 13°10'23'', N 46°54'23'')
Nd
Sm
[ppm] [ppm]
03R36 Fsp
0,14
0,03
03R36 Grt
0,41
0,82
03R37 WR
0,48
0,15
03R37 Grt1
0,13
1,12
03R37 Grt2
0,41
0,61
03R37 Grt2b 1,66
1,05
147
144
Sm/ Nd
143
144
Nd/ Nd
2Sd(m)
0.522
03R37
0.520
mylonitic pegmatite,
Prijakt-Polinik Complex
Kreuzeck Mountains
Carinthia
a
0.518
0.516
143
sample
2
67
Grt1
±
3
M
23
1
±
9
a
M
8
0.514
0,139
1,231
0,191
5,323
0,910
0,385
0,512017
0,513786
0,512201
0,521167
0,513332
0,512484
1,20E-05
5,75E-06
5,37E-06
9,96E-06
8,88E-05
7,17E-05
Grt2b
Grt2b
0.512
Io 0.51187 ± 1
Io 0.51190 ± 2
WR
0.510
A
1.0
2.0
Sm/
The meta-pegmatites are characterised by a mylonitic foliation (Fig. 3A). They consist of
quartz, albite-rich alkalifeldspar (Ab98), muscovite, tourmaline and almandine-rich garnet.
Quartz forms rods consisting of grains with a crystal preferred orientation and grain
boundary migration. The alkalifeldspar is present as fine-grained, totally recrystallised
layers and patches. Partly it is transformed into metamorphic white mica with a grain size of
about 1mm. These micas exhibit a significant paragonite component (Ms80, Pa20). Mica
fishes of up to 2 cm in diameter represent relics of the magmatic muscovite. Garnet is present
as idiomorphic crystals. They formed porphyroclasts during the mylonitic deformation. In
sample 03R37 (Fig. 3A, 3B, 3C) it is 0.2-3.0 mm in diameter and shows a chemical zoning
with cores characterised by homogeneous chemical composition (Alm66, Sps31, Grs2,
Prp1) and rims with increasing Fe and decreasing Mn contents (Alm71, Sps22, Grs5, Prp3)
(Fig. 3D, 3E, 3F). Fe and Mn show more or less continuous opposite trends in the outermost
0.1-0.4 mm. In some of the measured profiles (e.g. Fig 3E 03R37 Grt42) the Ca-content
shows an abrupt increase in the outermost 0.1 mm. In these rims the XCa is higher than
XMg. In sample 03R36 (Fig. 4A-E) garnet is up to 1.5 mm in diameter and exhibits
homogeneous chemical composition throughout the whole crystals (Fig. 4D, 4E). With
respect to the other sample the Mn content is somewhat higher and the Fe-content is lower
(Alm55, Sps42, Grs2, Prp1). The XCa is always higher than Xmg.
Nd
0.5136
mylonitic pegmatite,
Prijakt-Polinik Complex
Kreuzeck Mountains
Carinthia
0.5132
24
8
±
4
Fsp
0.2
6.0
C
Grt
0.5128
0.5116
0.0
5.0
4.0
03R36
0.5120
B
144
0.5140
0.5124
PETROLOGY
3.0
147
Nd/ 144Nd
Table 1: Sm-Nd data of analysed whole
rock, feldspar and garnet from samples
03R36 and 03R37.
0.0
143
Mt. Polinik (2784 m)
1000
Nd/ 144Nd
03R37
XFe/(XFe+XMg)
European plate (southern margin)
Fig. 1: A) Tectonic map of the Eastern Alps according to SCHMID et al. (2004). The red
square shows the area of Fig. 1B. B) Tectonic map showing the sample locality in the PrijaktPolinik Complex marked by the arrow. C) Eastern slopes of Mt. Polinik (2784 m).
Polinik hut
1.0
XMn
0.1
Penninic nappes
C
B
a
M
Fig. 5: Mineral seperates
and Sm-Nd isochron ages
of mylonitic pegmatites:
A) 0.2-0.3 mm sized garnet
fragments of sample 03R37
(Grt1), B) 0.2-0.3 mm sized
idiomorphic garnet of
sample 03R37 (Grt2a), C)
mineral isochrons of
sample 03R37, D) mineral
isochron of sample 03R36.
Explanation see text.
Io 0.51179 ± 2
0.4
0.6
147
Sm/
0.8
144
Nd
1.0
1.2
1.4
D
DISCUSSION AND CONCLUSION
With respect to the best defined age measured form the chips representing the core of the
garnet from sample 03R37 the pegmatites formed in Permian times at 267±3 Ma. This age is
in agreement with those published by HOKE (1990).
The age values measured on the idiomorphic garnets represent the age information of the
more Ca-rich rim composition. Their Early to Middle Triassic ages (239±18 Ma, 248±3) are
significantly lower than the formation age of the magmatic rock. Based on the chemical
zoning pattern the rims are influenced by postmagmatic processes. Most likely these
processes include overgrowth and diffusion which occurred in Permo-Triassic and/or
Cretaceous times.
REFERENCES
HOKE, L. (1990): The Altkristallin of the Kreuzeck Mountains, SE-Tauern Window, Eastern Alps - Basement Crust in a convergent plate Boundary Zone.
- Jb. Geol. B.-A., 133: 5-87, Wien.
SCHMID, S.M., FÜGENSCHUH, B., KISSLING, E. & SCHUSTER, R. (2004): Tectonic map and overall architecture of the Alpine orogen. - Eclogae
Geol. Helv., 97/1, 93-117.