GeolCarp_Vol67_No2_121

www.geologicacarpathica.com

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

, APRIL 2016, 67, 2, 121—132

doi: 10.1515/geoca-2016-0008

The Schwarzhorn Amphibolite (Eastern Rätikon, Austria):

an Early Cambrian intrusion in the Lower

Austroalpine basement

NILS-PETER NILIUS

1,2

, NIKOLAUS FROITZHEIM

, THORSTEN JOACHIM NAGEL

FRANK TOMASCHEK

and ALEXANDER HEUSER

Steinmann-Institut, Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany; nilius@geowi.uni-hannover.de

Present adress: Institut für Geologie, Leibniz Universität Hannover, Callinstraße 30, D-30167 Hannover, Germany

Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark

(Manuscript received May 8, 2015; accepted in revised form December 8, 2015)

Abstract: The Alpine nappe stack in the Penninic-Austroalpine boundary zone in the Rätikon (Austria) contains a 4

×1 km

tectonic sliver of meta-diorite, known as the Schwarzhorn Amphibolite. It was deformed and metamorphosed in the
amphibolite facies and is unconformably overlain by unmetamorphic Lower Triassic sandstone, indicating pre-Triassic
metamorphism.  Cataclastic  deformation  and  brecciation  of  the  amphibolite  is  related  to  normal  faulting  and  block
tilting during Jurassic rifting. Zircon dating of the Schwarzhorn Amphibolite using LA-ICP-MS gave a U-Pb age of
529+9/—8 Ma, interpreted as the crystallization age of the protolith. Geochemical characteristics indicate formation of
the  magmatic  protolith  in  a  supra-subduction  zone  setting.  The  Cambrian  protolith  age  identifies  the  Schwarzhorn
Amphibolite as a pre-Variscan element within the Austroalpine basement. Similar calc-alkaline igneous rocks of Late
Neoproterozoic to Early Cambrian age are found in the Upper Austroalpine Silvretta Nappe nearby and in several other
Variscan  basement  units  of  the  Alps,  interpreted  to  have  formed  in  a  peri-Gondwanan  active-margin  or  island-arc
setting.

Keywords: Rätikon, Lower Austroalpine, Arosa Zone, pre-Alpine basement, Jurassic rifting, U-Pb zircon geochronology.

Introduction

The  present  study  concerns  the  Schwarzhorn  Amphibolite,
a sliver of meta-diorite exposed in the Penninic-Austroalpine
boundary  zone  in  the  Tilisuna  area,  NE  Rätikon  (Fig.  1).
Field  relations  suggested  a  pre-Mesozoic  age  for  the  meta-
morphic overprint and in consequence also for the protolith
(Nagel  2006).  It  was  therefore  interpreted  as  part  of  the
Variscan  basement  of  the  Lower  Austroalpine  nappes.  So
far, the reported protolith ages in Lower Austroalpine base-
ment units are Carboniferous and Permian (Spillmann & Bü-
chi  1993;  von  Quadt  et  al.  1994).  Here,  we  determine  the
protolith  age  of  the  Schwarzhorn  Amphibolite  and  the  tec-
tonic setting in which its magmatic precursor was emplaced.

Geological outline

A stack of Lower Penninic to Upper Austroalpine nappes

is  exposed  along  the  Penninic-Austroalpine  boundary  in
eastern  Graubünden  (Switzerland)  and  adjacent  Vorarlberg
(Austria).  The  structurally  deepest  position  is  occupied  by
Lower Penninic Bündnerschiefer and flysch units, the highest
by Upper Austroalpine crystalline basement with its locally
preserved  Mesozoic  sedimentary  cover.  The  trace  of  this
boundary is characterized by an eastward embayment of the
Penninic  into  the  Austroalpine;  the  Prättigau  half-window
(Fig. 1  inset).  Imbricated  along  the  Penninic-Austroalpine

boundary, thin nappes and slices of Middle Penninic, Upper
Penninic and Lower Austroalpine origin are exposed.

South of the Prättigau half-window, Upper Penninic and

Lower  Austroalpine  nappes  exhibit  remnant  Jurassic-age
structures of the non-volcanic passive continental margin of
the Adriatic continent. Kinematics and architecture of Early
to Middle Jurassic rifting and the subsequent opening of the
Piemont-Ligurian  Ocean  have  been  extensively  studied  in
the  last  decades  (Eberli  1988;  Froitzheim  &  Eberli  1990;
Froitzheim  &  Manatschal  1996;  Handy  1996;  Mohn  et  al.
2010,  2011).  Penninic  and  Austroalpine  nappes  in  SE
Graubünden  were  assigned  to  their  former  positions  on  the
margin.  The  Platta  and  Malenco  Nappes  display  the  ocean-
continent transition. The Err Nappe represents a distal rem-
nant  of  the  Adriatic  passive  margin,  characterized  by
westward  (oceanward)  dipping  normal  faults,  whereas  the
Bernina  Nappe  occupied  a  more  proximal  position  on  the
margin (Froitzheim & Manatschal 1996; Mohn et al. 2011).

From Middle Jurassic to Cretaceous times, stacking of the

Austroalpine nappes was the result of the closure of the Me-
liata Ocean, southeast of Adria (Neubauer et al. 2000; Mis-
soni  &  Gawlick  2011),  and  the  following  intracontinental
shortening.    This  first  orogenic  event  led  to  mainly  W-  to
NW-directed  thrusting.  It  was  followed  by  the  Eocene  clo-
sure  of  the  Penninic  oceanic  basins  between  Europe  and
Adria which resulted in the formation of the present day Pen-
ninic-Austroalpine  nappe  stack  (Froitzheim  et  al.  1996;
Müller et al. 1999). During this Early Tertiary collision, the

123

CAMBRIAN SCHWARZHORN AMPHIBOLITE IN LOWER AUSTROALPINE BASEMENT

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Austroalpine nappes were thrust towards the north over the
Penninic units (Ring et al. 1989; Froitzheim et al. 1994).

The nappe stack in the Tilisuna area

The Tilisuna area (Figs. 1 & 2) is characterized by a north-

dipping nappe stack of Middle Penninic to Upper Austroal-
pine  units,  starting  in  the  South  with  the  Middle  Penninic
Sulzfluh Nappe (von Seidlitz 1906; Tollmann 1970; Burger
1978; Biehler 1990; Nagel 2006). It represents the sedimen-
tary cover of the Briançonnais continental spur and compri-
ses mainly Upper Jurassic platform carbonates, discordantly
overlain  by  thin,  often  reddish,  pelagic  marls  (Couches
Rouges)  of  Late  Cretaceous  to  Paleogene  age  (Allemann
1952).

To the north follows the Arosa Zone, comprising units de-

rived  from  the  Piemont-Ligurian  Ocean.  It  is  divided  into
three  subunits,  from  bottom  to  top:  (1)  the  lower  mélange,
consisting of a clay matrix with embedded slivers of Middle
Penninic as well as Upper Austroalpine origin (von Seidlitz
1906; Stahel 1926); (2) the Cenomanian—Turonian Verspala
flysch  (Oberhauser  1983);  (3)  the  upper  mélange,  compri-
sing a matrix of mainly serpentinite and clasts of Upper Pen-
ninic and Lower Austroalpine origin (Nagel 2006).

To the North, the upper mélange is overlain by the ca.

4 km long and 1 km thick Schwarzhorn slice (Fig. 3). It fol-
lows  the  general  SE—NW  strike  in  the  area  from  the  Gam-
padels valley in the southeast to Grünes Eck in the northwest
and builds the peak of Schwarzhorn (Fig. 1). The dominating
lithology, making up two thirds of the Schwarzhorn slice, is
the Schwarzhorn Amphibolite. The mineral composition re-
flects  a  dioritic  to  quartzdioritic  magmatic  predecessor
which  experienced  amphibolite  facies  overprint.  The  fabric
of  the  Schwarzhorn  Amphibolite  shows  strong  variations
and  reaches  from  strain-free  domains  where  primary  mag-
matic  structures  are  preserved  to  amphibolites  with  a  well-
developed  foliation.  A  pre-Mesozoic  age  of  this
metamorphism  is  indicated  by  the  much  lower,  anchizonal
overprint of the Mesozoic sedimentary cover (Ferreiro Mähl-
mann & Giger 2012).

The sedimentary cover of the basement rocks is preserved

on  the  northern  and  eastern  margins  of  the  Schwarzhorn
slice. Despite the strong deformation, typical pre-, syn-, and
post-rift  sedimentary  successions  can  be  identified  (Fig.  4).
In the Gampadels valley, the cover of the meta-diorite starts
with  Scythian  quartzites  and  sandstones,  followed  by  dolo-
mites  and  claystones  of  probably  Late  Triassic  age  (Nagel
2006).  This  pre-rift  succession  is  discordantly  overlain  by
strongly deformed Late Jurassic to Cretaceous post-rift sedi-
ments (von Seidlitz 1906; Furrer 1985). Sediments related to
the  syn-rift  stage  (Early  to  Middle  Jurassic)  are  scarce.
A small outcrop northeast of the Schwarzhorn shows a well
bedded  breccia  with  sedimentary  components  derived  from
the  pre-rift  sequence,  and  greenish  granite  components
(Nagel  2006).  The  breccia  rests  on  cataclastic  basement
rocks and is covered by post-rift sediments.

Along the northern and southeastern margins of the

Schwarzhorn slice, two cataclastic fault zones are attributed
to Jurassic rifting. Therefore the Schwarzhorn slice is inter-

preted  as  an  eastward  tilted  block  of  the  Jurassic  passive
margin,  bounded  by  two  westward  dipping  normal  faults
(Fig.  4)  (Nagel  2006).  Such  faults  are  also  known  from  the
Lower  Austroalpine  Err  Nappe  further  south  (Eberli  1988;
Froitzheim  &  Manatschal  1996).  Based  on  these  observa-
tions,  Nagel  (2006)  attributed  the  Schwarzhorn  slice  to  the
Lower  Austroalpine  nappe  system  as  formerly  proposed  by
Cadisch (1923), while other authors treated it as part of the
Arosa Zone (Richter 1958; Biehler 1990; Ferreiro Mählmann
1994).

The Walser slice overlies the Schwarzhorn slice and repre-

sents gneissic and granitic basement with remnants of Meso-
zoic cover. It is in a similar structural position as the Lower
Austroalpine  Bernina  Nappe  in  SE  Graubünden  (Nagel
2006).  The  southern  (SMZ)  and  the  northern  Mittagsspitz
Zone  (NMZ)  follow  to  the  north.  They  comprise  Mesozoic
sedimentary rocks. The SMZ is in a similar structural posi-
tion  as  the  Allgäu  Nappe  of  the  Northern  Calcareous  Alps,
whereas the NMZ is connected with the Lechtal Nappe. The
NMZ  represents  the  inverted  sediment  cover  of  the  Phyllit-
gneiss  Zone,  a  Variscan  basement  unit  connected  with  the
Silvretta Nappe.

Samples and analytical methods

Samples of Schwarzhorn Amphibolite were collected

along  the  northwestern  and  the  southern  flank  of  the  peak
Schwarzhorn (Fig. 1). We sampled a representative suite to
cover  nearly  undeformed,  well  foliated  and  catalastic  do-
mains respectively (Table 1). Petrological, geochemical and
geochronological  analyses  were  carried  out  at  facilities  of
the Steinmann Institute, Bonn.

Rock samples were petrographically investigated by thin-

section  microscopy  and  electron-microprobe  analysis
(EMPA).  The  microprobe  analyses  of  main  and  accessory
minerals  were  carried  out  on  a  JEOL  8200  Superprobe.
Whole rock major and trace element contents were measured
by X-ray fluorescence (XRF) on Li

-fluxed fusion discs

(PANalytical-Axios spectrometer). Volatile-free mass pro-
portions were recalculated to 100%.

For U-Pb zircon geochronology, a large aliquot of sample

SH1  was  crushed,  milled  and  sieved  before  applying  mag-
netic and heavy-liquid separation techniques. The processing
of ca. 10 kg of meta-diorite yielded 407 zircon crystals. Op-
tically  clear,  inclusion-  and  crack-free  zircon  was  hand-
picked  under  the  binocular  microscope  and  mounted  in
epoxy resin. After polishing down to half section, cathodolu-
minescence  imaging  was  performed  to  reveal  the  internal
textures.  Established  methods  for  U-Pb  data  acquisition
(Kooijman  et  al.  2012)  were  adapted  for  application  at  the
Steinmann  facilities.  Laser  ablation  inductively  coupled
plasma mass spectrometry (LA-ICP-MS) analyses were car-
ried out using a Resonetics RESOlution M50-E 193nm exci-
mer-laser  coupled  to  a  Thermo  Scientific  ELEMENT  XR
SF-ICP-MS. A comprehensive list of instrument settings and
analytical strategy is provided in the online supplement (S1)*.
In short, the gas blank was recorded for 27 seconds followed
by the ablation for 30 seconds with a laser spot size of 33 µm

* Only in an electronical version on www.geologicacarpathica.com

125

CAMBRIAN SCHWARZHORN AMPHIBOLITE IN LOWER AUSTROALPINE BASEMENT

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Table 2: LA-ICP-MS U-Pb zircon data for Schwarzhorn Amphibolite sample SH1. (Sampling coordinates: E 784329, N 212563, Swiss
Grid CH1903).

reference material (Wiedenbeck et al. 1995, 2004). Raw-data
was  processed  using  the  reduction  scheme  VizualAge
in  Iolite  2.5  (Paton  et  al.  2011;  Petrus  &  Kamber  2012).
The  stacking  of  measured  signals  in  Iolite  revealed  an  in-
stability  of  element  ratios  during  the  first  6  seconds,
and  this  time  interval  was  excluded  from  data  regression.
Furthermore,  a  few  reversely  discordant  analyses  were  re-
jected.

With correction for the monitored isobaric interference of

204

Hg on

204

Pb, the signal was indistinguishable from the

background. The amount of common-Pb was generally
found insignificant (f206<0.5 %), as expected for the given

zircon  free  of  inclusions  or  cracks,  and  no  correction  for
common-Pb  was  applied.  Within-run  reproducibility  of  the
primary reference material has been propagated into the final
data,  provided  in  Table  2.  Data  was  plotted  and  population
ages calculated with the help of the Isoplot plugin for Excel
(Ludwig  2012).  Concerning  accuracy  and  systematic  error
estimation, the Plešovice zircon (online supplement S2, S4*)
returned  a  concordia  age  of  339.6±1.7  Ma  (internal  error)
which corresponds within an error of 1% to the accepted ID-
TIMS age (337.13±0.37 Ma, Sláma et al. 2008). A systema-
tic error of 1.5 % was propagated by quadratic addition into
the final age of the unknown population.

* Only in an electronical version on www.geologicacarpathica.com

Analysis

Name

[ppm]

206

238

Measured Isitopic Ratios

Ages [Ma]

[cps] [cps]

f206%

207

Pb/

235

206

Pb/

238

rho

207

Pb/

206

207

Pb/

235

U 2

206

Pb/

238

U 2

A2-1

154

101791 1005020

0,02

0,691

0,018

0,0853 0,0014 0,29

0,0581

0,0014

533

528

4 A4-1

163

36 9

108624

1060394

0,17 0,715 0,019

0,0864 0,0015 0,03 0,0595

0,0016

547 11 534 9

5 A6-1 108 17 5 72726

699785 0,05 0,713 0,025

0,0875 0,0019 0,40 0,0587

0,0019

546 15 541 11

6 A6-2

191

37 9

124990

1241142

0,11 0,696 0,023

0,0851 0,0015 0,22 0,0588

0,0018

536 14 526 9

7 A10-1

260

68 16

160118

1684224

0,10 0,691 0,021

0,0850 0,0015 0,59 0,0587

0,0019

533 13 526 9

8 A14-1

182

34 9

119063

1178368

0,06 0,692 0,022

0,0853 0,0016 0,12 0,0584

0,0018

534 13 528 9

9 A14-2

240

64 17

157383

1554820

0,12 0,705 0,021

0,0860 0,0016 0,05 0,0590

0,0017

541 13 532 9

10 A16-1

349 94 23

233913

2266224 -0,05 0,708 0,018

0,0875 0,0015 0,57 0,0579

0,0009

543 11 541 9

14 A21-1 148 37 9 96632

963624 0,00 0,686 0,027

0,0852 0,0015 0,41 0,0579

0,0020

529 16 527 9

15 A21-2

190 58 14

129794

1240818 -0,22 0,688 0,018

0,0878 0,0015 0,38 0,0565

0,0012

531 11 542 9

16 A23-1

149 23 8 125803

975491

-0,07

0,928

0,040

0,1090 0,0032 0,34 0,0612

0,0021

665 21 667 19

17 A24-1

171 44 10

113027

1120017 -0,06 0,682 0,020

0,0854 0,0014 0,16 0,0575

0,0015

527 12 528 8

18 A24-2

533 84 20

362734

3511258

0,01 0,708 0,018

0,0876 0,0014 0,62 0,0584

0,0010

543 11 542 8

19 B2-1

319

62 15

213538

2110332

0,07 0,693 0,018

0,0854 0,0014 0,56 0,0585

0,0011

534 11 528 9

20 B3-1

245

60 15

167897

1653688

0,15 0,701 0,033

0,0850 0,0026 0,51 0,0591

0,0024

539 20 526 15

24 B9-1

297

41 10

184894

1998783

0,33 0,654 0,017

0,0794 0,0012 0,19 0,0597

0,0013

511 11 493 7

25 B9-2

165

32 8

112077

1123529 -0,02 0,677 0,021

0,0846 0,0014 0,13 0,0577

0,0017

524 13 523 8

26 B10-1

241 56 14

163616

1641622

0,16 0,692 0,021

0,0844 0,0015 0,30 0,0591

0,0016

533 13 522 9

27 B17-1 129 26 6 89307 881753 -0,15 0,681 0,023

0,0863 0,0015 0,08 0,0569

0,0019

527 14 534 9

28 B18-1

261 58 14

173084

1792597

0,06 0,667 0,019

0,0828 0,0020 0,45 0,0580

0,0015

518 12 512 12

29 B18-2

323 77 18

218249

2221950

0,17 0,683 0,018

0,0834 0,0016 0,13 0,0590

0,0017

528 11 517 10

30 B24-1

203 51 13

140911

1403605

0,29 0,708 0,024

0,0854 0,0016 0,31 0,0603

0,0021

546 16 528 10

34 B25-1

195 55 14

140549

1350262 -0,10 0,712 0,021

0,0890 0,0016 0,34 0,0577

0,0014

545 12 549 10

35 C2-1

325

46 11

208555

2249225

0,20 0,639 0,018

0,0787 0,0016 0,46 0,0585

0,0013

501 11 488 9

36 C2-2

262

51 13

186185

1814030 -0,06 0,701 0,017

0,0875 0,0016 0,55 0,0578

0,0011

539 10 541 9

37 C10-1

251 58 14

175751

1736505

0,03 0,694 0,018

0,0859 0,0018 0,38 0,0583

0,0012

535 11 532 10

38 C11-1

213 62 15

147916

1479372

0,32 0,684 0,026

0,0824 0,0016 0,32 0,0601

0,0021

529 16 510 9

39 C11-2

207 62 15

142073

1431375

0,17 0,689 0,022

0,0846 0,0015 0,20 0,0592

0,0019

531 13 524 9

40 C11-3

448 68 15

307679

3107344

0,02 0,698 0,016

0,0863 0,0015 0,23 0,0583

0,0012

537 10 534 9

44 C17-1

224 70 17

161215

1553728 -0,01 0,717 0,020

0,0885 0,0018 0,40 0,0584

0,0014

548 12 547 11

45 C17-2

588

127 29

416099

4080341 -0,05 0,699 0,014

0,0871 0,0016 0,50 0,0578

0,0009

538 8 539 9

46 C17-3

1000

193 44

726030

6949284 -0,10 0,713 0,014

0,0888 0,0016 0,49 0,0577

0,0009

547 8 548 9

47 C20-1

209 59 14

145774

1453427

0,08 0,701 0,029

0,0859 0,0021 0,70 0,0587

0,0017

538 17 531 12

48 C21-1

151 39 10

112705

1053666 -0,27 0,719 0,023

0,0913 0,0018 0,09 0,0567

0,0016

549 13 563 11

49 C30-1

145 37 9 110397

1012740 -0,06 0,753 0,021

0,0926 0,0014 0,13 0,0586

0,0015

570 12 571 8

50 C30-2

285 52 14

227132

1997377 -0,37 0,778 0,020

0,0984 0,0019 0,49 0,0570

0,0012

584 11 605 11

54 D13-1

144 35 9 99907

1013060

0,27 0,716 0,025

0,0859 0,0018 0,51 0,0602

0,0017

548 15 531 11

56 D16-1

164 38 10

121026

1147556 -0,02 0,727 0,024

0,0894 0,0015 0,06 0,0584

0,0019

554 14 552 9

57 D16-2

537 81 19

374857

3771058 -0,10 0,669 0,013

0,0846 0,0015 0,24 0,0570

0,0010

520 8 523 9

58 D25-1

207 64 17

159101

1453573

0,14 0,681 0,028

0,0885 0,0018 0,46 0,0596

0,0021

527 17 547 11

59 D25-2

255 41 11

217504

1796744 -0,40 0,805 0,019

0,1015 0,0019 0,16 0,0573

0,0013

599 10 623 11

60 D25-3

363 66 18

278781

2554336 -0,08 0,761 0,019

0,0937 0,0017 0,45 0,0586

0,0011

574 11 577 10

64 D25-4

949

209 46

641998

6678438

0,01 0,656 0,013

0,0823 0,0012 0,21 0,0576

0,0010

512 8 510 7

65 D26-1

240 70 17

178788

1688468 -0,18 0,730 0,023

0,0916 0,0020 0,43 0,0575

0,0015

556 14 565 12

66 E12-1

254 69 16

174194

1787184

0,18 0,709 0,024

0,0858 0,0017 0,48 0,0595

0,0017

543 14 530 10

67 E12-2

328 70 17

219082

2302962

0,08 0,662 0,017

0,0820 0,0017 0,41 0,0581

0,0012

515 10 508 10

68 E12-3

291 60 14

193126

2038157

0,33 0,678 0,021

0,0814 0,0018 0,58 0,0600

0,0015

525 13 504 11

69 E15-1

218 61 14

153793

1523332 -0,15 0,693 0,026

0,0873 0,0018 0,56 0,0570

0,0016

534 15 539 11

70 E15-2

216 37 9 151327

1508035 -0,06 0,707 0,026

0,0877 0,0023 0,46 0,0578

0,0018

542 16 542 14

74 E16-1

182 42 10

128497

1260898

0,08 0,684 0,022

0,0845 0,0017 0,63 0,0585

0,0014

529 13 523 10

75 E16-2

750 93 22

528203

5182833 -0,04 0,701 0,014

0,0870 0,0014 0,35 0,0579

0,0009

539 9 538 8

76 E18-1

276 88 22

197123

1898917

0,13 0,734 0,016

0,0882 0,0015 0,24 0,0595

0,0013

559 10 545 9

Bold marked analyses are used for age calculation by the TuffZirc algorithm.

f206% denotes the fraction of

206

Pb that is common

206

Pb and is calculated with f206% = (

207

Pb/

206

measured

207

Pb*/

206

Pb*) / (

207

Pb/

206

common

207

Pb*/

206

Pb*)*100.

126

NILIUS, FROITZHEIM, NAGEL, TOMASCHEK

and HEUSER

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Petrography and mineral chemistry

The meta-diorite of the Schwarzhorn slice mainly consists

of  hornblende,  plagioclase  and  quartz  as  main  constituents
and  rutile,  epidote,  apatite  and  rare  zircon  as  accessory  mi-
nerals. The textural range of meta-diorite samples comprises
unstrained  domains  with  preserved  primary  magmatic  fab-
rics,  domains  with  well-developed  amphibolite-facies  folia-
tion, as well as cataclastic zones.

Unfoliated meta-diorite with a primary magmatic texture

is  characterized  by  large  isometric  plagioclase  and  horn-
blende.  Plagioclase  is  strongly  sericitizied  and  shows
polysynthetic twinning. Zonation and twinning are also com-
mon  in  hornblende.  It  can  be  divided  into  core  (horn-
blende 1)  and  rim  domains  (hornblende 2).  The  rims  are
yellow-greenish  and  show  distinct  pleochroism.  The  cores
appear  dark  brown  to  almost  opaque.  Small  inclusions  of
quartz and rutile are common in core domains but absent in
rims. A magmatic amphibole
composition  is  not  directly
preserved but high density of
rutile  inclusions  indicates
a Ti-rich  precursor.  Quartz
veins  and  up  to  1mm  large
grains  make  up  ca.  10 %  of
the rock.

moderately

foliated

rocks,  plagioclase  is  com-
pletely  recrystallized  into
smaller grains (Fig. 5a). The
anorthitic component of pla-
gioclase ranges from 10 % to
40 %  (Table  3).With  increa-
sing  foliation,  the  opaque
and  inclusion-rich  core  do-
mains  of  hornblende  1  are
replaced  by  inclusion-free,
acicular grains of hornblende 2. All ana-
lysed    amphiboles  are  magnesio-horn-
blende  and  no  chemical  distinction
could  be  made  between  core  and  rim
domains (Table 4). Chlorite and epidote
interfinger  with  hornblende  in  rim  do-
mains (Fig. 6a).

In rocks with cataclastic overprint,

hornblende  is  almost  completely  re-
placed by iron-rich chlorite. Brittle fault
zones  are  characterized  by  fractured
components  of  plagioclase  and  epidote
and  a  phyllitic  matrix  of  mainly  chlo-
rite, biotite (Phl

31-55

) and some pumpel-

lyite.

Plagioclase

shows

common

seritization and is the main constituent.
It  is  recrystallized  in  grains  of  up  to
0.3 mm size in unfractured domains. In
brittle  fault  zones,  the  Ca-plagioclase
has  been  altered  to  albite  (An

0.2-12

)

(Fig. 7) and epidote (saussuritization)
(Figs. 5b and 6b).

Whole-rock geochemistry

Results of XRF analyses of the Schwarzhorn Amphibolite

are presented in Table 5 and Figure 8. The total alkali versus
silica plot of Wilson (1989) indicates a dioritic composition
of the protolith (Fig. 8a). A relatively large scatter of SiO

contents is observed and the composition ranges from SiO

−

poor diorite (55 wt. %) to quartz diorite (68 wt. %). Accor-
ding to the ternary diagram of Mullen (1983), all samples are
within the island-arc tholeiite field, except for sample SH3
which plots due to its low TiO

content at the transition be-

tween  island-arc  tholeiite  and  calc-alkali  basalt  (Fig.  8b).
A supra-subduction  zone  setting  is  also  suggested  by  the
Ti/Zr  diagram  of  Pearce  et  al.  (1982)  where  samples  SH1,
SH2, and SH8 reflect arc lava compositions (Fig. 8c). In the
AFM  diagram  and  the  FeO

/MgO vs. SiO

diagram, the

Schwarzhorn Amphibolite shows a calc-alkaline differentia-
tion trend (Fig. 8d, e).

SH8

Sample

M13

M14

M15

M16

M17

M18

M28

SiO

45.31

45.65

43.46

45.25

45.84

48.01

46.19

43.69

44.94

44.48

TiO

0.49

0.54

0.45

0.48

0.36

0.51

0.49

0.51

0.47

12.86

12.69

14.50

13.38

12.76

10.77

12.21

15.78

14.26

14.61

FeO 12.11

12.22

13.00

12.61

12.26

11.55

12.32

13.46

12.86

12.99

MnO 0.32

0.27

0.23

0.22

0.12

0.23

0.25

0.24

0.23

MgO 12.91

12.48

11.48

12.35

12.69

13.86

12.84

11.16

11.94

11.48

CaO 11.66

11.70

11.66

11.65

11.91

11.62

11.57

11.51

11.48

11.96

O 1.39

1.28

1.48

1.36

1.20

1.16

1.36

1.63

1.41

1.27

O 0.28

0.32

0.37

0.39

0.26

0.30

0.27

0.40

0.29

0.06

0.03

0.06

0.10

0.05

0.03

0.08

0.00

0.01

Total

97.05 96.79

96.34

97.25

97.56

97.24

97.94

97.62

97.48

Cations p.f.u.

Si 6.60

6.66

6.42

6.58

6.66

6.91

6.71

6.35

6.52

6.48

Ti 0.05

0.05

0.06

0.05

0.04

0.06

0.05

0.06

0.05

Al 2.21

2.18

2.52

2.29

2.18

1.83

2.09

2.70

2.44

2.51

Fe 1.48

1.49

1.61

1.53

1.49

1.39

1.50

1.63

1.56

1.58

Mn 0.04

0.03

0.01

0.03

Mg 2.81

2.72

2.53

2.68

2.75

2.97

2.78

2.42

2.58

2.49

Ca 1.82

1.83

1.84

1.82

1.85

1.79

1.80

1.79

1.78

1.87

Na 0.39

0.36

0.42

0.38

0.34

0.32

0.38

0.46

0.40

0.36

K 0.05

0.06

0.07

0.05

0.07

0.05

Cr 0.01

0.00

0.01

0.00

0.01

0.00

Total (f.p 23 O²)

15.45 15.40

15.50

15.43

15.38

15.33

15.39

15.50

15.42

Table 4: Representative electron microprobe analyses of amphibole in weight % and
cations p.f.u.

SH3

SH8

Sample

M11

M10

M11

M20

M21

M22

M23

M24

SiO

71.32

66.86 59.47

61.82

58.01

61.94

58.34

57.99

58.48

66.47

58.14

64.10

TiO

0.00

0.00 0.00

0.01

0.00

0.00 0.02 0.01

0.01

0.05

19.97

22.01 25.86 24.22

26.51

23.93

26.57

26.76 26.79 21.73

26.52

23.46

FeO 0.03

0.03

0.00 0.00

0.01

0.02

0.01 0.06 0.00

0.05

0.00

MnO 0.00

0.00

0.00 0.04

0.00

0.02

0.00

0.03 0.00 0.00

0.02

0.00

MgO 0.00

0.00

0.02 0.00

0.00

0.03

0.00

0.00 0.01 0.00

0.00

0.01

CaO 0.08

2.75

7.25 5.50

8.32

5.44

8.34

8.58 8.41 2.56

8.55

4.60

O 10.39

10.18

7.07 8.28

6.81

8.08

6.85

6.66 6.72

10.04

6.52

8.78

O 0.12

0.12

0.09 0.09

0.05

0.12

0.06

0.07 0.06 0.08

0.08

0.12

0.00

0.03 0.01

0.00

0.01

0.00

0.00 0.01 0.00

0.00

0.01

Total

101.91

101.94 99.79 99.96

99.72

99.57 100.19 100.10 100.56 100.90

99.87 101.11

Orthoclase 0.69

0.64

0.53 0.52

0.30

0.67

0.36

0.38 0.36 0.46

0.45

0.68

Anorthite

0.22

12.37 35.74 26.57

40.09

26.05

39.91

41.18 40.48 11.82

40.84

21.43

Albite

99.09

86.99 63.73 72.91

59.60

73.28

59.73

58.44 59.16 87.72

58.71

77.89

Table 3: Representative electron microprobe analyses of feldspar in weight % oxides and molar
proportions

127

CAMBRIAN SCHWARZHORN AMPHIBOLITE IN LOWER AUSTROALPINE BASEMENT

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Zircon U-Pb geochronology

The zircon crystals of the Schwarzhorn Amphibolite

appear  under  the  binocular  microscope  inclusion  free  and
mostly  colourless,  some  are  slightly  yellowish.  The  size
of the zircons ranges from 50 to 200 µm. Grains show typi-
cally a short to medium prismatic morphology with a width-
to-length ratio of 1:1 to 1:2, and rarely 1:3. Crystal faces are
mostly  subhedrally  developed.  CL  images  (Fig.  9)  reveal
that  the  internal  texture  is  dominated  by  broad  oscillatory
zoning  patterns  as  typically  found  in  zircons  precipitated
from melts (e.g. Corfu et al. 2003). Oscillatory domains are
commonly  surrounded  by  a  thin  (1  to  5  µm)  bright-CL
rim  discordantly  crosscutting  oscillatory  zoning  patterns
(Fig. 9).

The results of the U-Pb analyses of the Schwarzhorn Am-

phibolite are listed in Table 2 and shown in Figure 10. Single

spot

206

Pb/

238

U ages range between ca. 488 and 667 Ma with

a main population at about 530 Ma. Multiple spot analyses
of rim and core domains of single zircon grains either did not
yield satisfactory results or did not comprise significant age

Fig. 6. Backscattered electron images of the Schwarzhorn Amphibolite. A – Moderately foliated amphibolite with hornblende 1 and rim of
hornblende 2. B – Plagioclase of the cataclastic sample SH3 shows saussuritization. Amphibole is replaced by chlorite.

Fig. 7. Results of feldspar microprobe analyses. Rectangles reflect
analyses from cataclastic rock units and indicate that plagioclase
has been altered to a more albitic composition. Dots reflect analyses
from the moderately foliated sample SH8.

Table 5: Major- and trace-element analyses of Schwarzhorn Am-
phibolite. Oxides are in [wt. %].

Sample

SH1 SH2 SH3 SH8

SiO

60,01

62,89

68,71

54,27

16,49 15,42 13,35 17,20

6,96 5,76 4,18 7,10

MnO

0,14 0,09 0,07 0,13

MgO

4,05 4,08 2,31 6,92

CaO

3,18 2,32 3,66 6,04

3,90 4,90 3,57 3,40

1,07 0,92 0,77 0,88

TiO

0,42 0,61 0,21 0,59

0,06 0,11 0,05 0,08

0,04 0,06 0,03 0,03

L.O.I.

2,95 2,70 2,21 2,53

Sum

99,27 99,86 99,12 99,17

Mg#

54 58 52 66

Trace Elements (ppm)

- 2 3 3

190 110 129 110

8 16 20 16

16 16 8 26

Cr 53

164

- - 4 -

17 37 18 31

14 13 13 16

Hf 1

16 18 10 23

>1000 628 483 920

- - - -

1 3 - 2

4 7 9 6

15 18 10 57

12 15 18 14

34 22 20 21

21 17 11 23

0 4 2 1

274 192 333 407

2 3 3 2

2 4 4 3

107 112 55 159

17 12 10 19

Y 15

64 54 33 84

44 86 44 44

Mg# = (MgO/MgO+FeO)*100

130

NILIUS, FROITZHEIM, NAGEL, TOMASCHEK

and HEUSER

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

differences. Whereas the oldest age indicates some inheri-
tance, the youngest ages tend to be discordant (Fig. 10) and
hence are interpreted as influenced by lead-loss during
a later tectono-thermal event.

The large scatter of the dates, in combination with a rela-

tively  continuous  distribution  between  minimum  and  maxi-
mum  ages  (Fig.  10),  impedes  the  selection  of  a  population
which  represents  the  protolith  age  of  the  intrusion.  To  pre-
vent a subjective bias of the population, we used the TuffZirc
algorithm  of  Ludwig  and  Mundil  (2002).  It  primarily  ex-
cludes  analyses  with  anomalously  high  errors  and  seeks  to
find  a  representative  population  of  ranked

206

Pb/

238

U dates

which yield a probability of fit >0.05. The medium age of
this population is reported with an asymmetric 95% confi-
dence error. The resulting population consists of 28 analyses
with a

206

Pb/

238

U-age of 529+9/—8 Ma (Fig. 10 inset).

Discussion

Whole rock geochemical and U-Pb zircon geochronologi-

cal  analyses  of  the  Schwarzhorn  Amphibolite  reveal  that
the  protolith  formed  in  a  supra-subduction  zone  setting
(Fig. 8b—e) in the Early Cambrian. Meta-magmatic rocks of
Neoproterozoic  to  Ordovician  age  are  commonly  found  in
Alpine basement units and are attributed to an active margin
setting  north  of  Gondwana.  In  the  Austroalpine,  pre-
Variscan magmatites are reported from meta-gabbros in the
Ötztal basement with 530—521 Ma reflecting MORB affinity
(Miller & Thöni 1995) and from basement units south of the
Tauern  Window  with  590−450  Ma  (Schulz  et  al.  2004;
Schulz 2008). Neoproterozoic to Cambrian intrusives of the
latter, rather reflect subduction related magmatism, whereas
Ordovician magmatites are characterized by acid magmatism
from crustal sources. An early stage of an active margin set-
ting is attributed to a 590 Ma N-MORB-type basite, which is
followed  by  more  abundant  volcanic  arc  basalts  of  early
Cambrian  age  (Schulz  et  al.  2004).  Similarly,  the  Silvretta
Nappe was divided into the Neoproterozoic to Lower Cam-
brian  “older  orthogneiss”  and  the  Ordovician  “younger  or-
thogneiss” (Maggetti & Flisch 1993; Poller 1997; Müller et al.
1994, 1995, 1996). Thus, the protolithic age of 529+9/—8 Ma
of  the  Schwarzhorn  Amphibolite  would  correspond  to  rock
units regarded as “older orthogneisses” or “gneiss-amphibo-
lite  complexes”.  Although  older  orthogneisses  summarize
a heterogeneous  group  of  rock  units,  ranging  from  granitic
through  intermediate  to  mafic  and  ultra-mafic  gneisses  and
amphibolites,  they  are  geochemically  characterized  by
a calc-alkaline  affinity.  In  contrast,  younger  othogneisses
show a more alkaline affinity. The reported intrusion ages of
older  orthogneisses  (Table  6)  indicate  two  phases  of  in-
creased magmatic activity in the Late Proterozoic and Early
Cambrian, related to a complex active margin setting on the
northern margin of Gondwana (von Quadt 1992; Kounov et
al. 2012; von Raumer et al. 2003, 2013). The first evidence
of  island  arc  magmatism  is  given  by  the  609±3  Ma  meta-
diorite and a 568±6 Ma calc-alkaline orthogneiss (Schalteg-
ger  et  al.  1997;  Müller  et  al.  1995).  Between  535  Ma  and
520 Ma, oceanic plagiogranites infer the evolution of either

back-arc or fore-arc basins but also contemporary island arc
magmatism,  as  indicated  by  tholeiitic  island  arc  basalts,
calc-alkaline  intermediate  rocks  and  an  S-type  granitoid
(Müller  et  al.  1995,  1996;  Poller  1997;  Schaltegger  et  al.
1997). This is in accordance with the geochemical characte-
ristics of the Schwarzhorn Amphibolite which suggest a su-
pra-subduction  zone  setting,  and  in  this  context  most
probably in an island arc setting. Subsequently the Neopro-
terozoic to Cambrian back arc or fore arc basins were closed
and  the  island  arc  and  micro-continents  at  the  Gondwana
margin were involved into an Ordovician to Silurian oroge-
ny as indicated by S-type granitoids and gabbroic intrusions
in  a  collisional  belt,  namely  the  “younger  orthogneiss
units”(Liebetrau 1996; Poller 1997).

The dominant amphibolite facies overprint (M1) of

the  Schwarzhorn  meta-diorite  may  be  either  related  to
a vaguely  constrained  high-pressure  event  between  470  Ma
and 530 Ma suggested by Poller (1997), or more probably to
the  Variscan  orogenic  cycle.  Ladenhauf  et  al.  (2001)  dated
metamorphic domains in zircons from eclogite of the Silvretta
nappe  at  351±22  Ma  (U-Pb  SHRIMP),  similar  to  Sm-Nd
isochrons  for  eclogites  of  the  Ötztal  basement  (Miller  &
Thöni  1995).    Unfortunately,  the  bright-Cl  “metamorphic
rims”,  crosscutting  the  growth-zoned  Schwarzhorn  meta-
diorite  zircons,  were  too  narrow  for  conventional  LA-ICP-
MS analyses.

Cataclastic zones of the Schwarzhorn Amphibolite com-

prise  a  low-temperature  greenschist-facies  mineral  assem-
blage  (M2).  This  is  expressed  by  the  lack  of  biotite,  the
alteration of Ca-plagioclase to albite and epidote and the re-
placement of amphibole by iron-rich chlorite. Coal and clay
petrological studies have demonstrated that Alpine metamor-
phism did not exceed high-grade diagenetic conditions in the
Tilisuna area (Ferreiro Mählmann & Giger 2012). Hence, we
interpret  the  cogenetic  low-temperature  greenschist  facies
metamorphism  and  cataclastic  faulting  as  a  reflection  of
Jurassic rifting.

We note that the preservation of rift-related structures

clearly  indicates  that  the  Schwarzhorn  slice  is  part  of  the
Lower Austroalpine nappe system (Nagel 2006). While “older
orthogneisses”  are  particularly  well  documented  from  the
Upper  Austroalpine,  the  Cambrian  protolith  age  of  the
Schwarzhorn Amphibolite provides the first evidence for the
presence  of  similar  rocks  in  the  Lower  Austroalpine.  To
date,  the  rare  geochronological  data  from  the  Lower  Aus-
troalpine  basement  rocks  comprises  mostly  Paleozoic  ages
linked to Variscan tectonics, which intruded in a poly-meta-
morphic basement (von Quadt et al. 1994). Great petrologi-
cal  and  geological  similarities  between  the  Schwarzhorn
Amphibolite  and  the  southward  located  “Gabbrozug  Arosa-
Davos-Klosters”  (Streckeisen  1948)  of  the  Lower  Austroal-
pine  Dorfberg  Nappe  suggest  a  similar  age  of  the  Dorfberg
Nappe basement.

Conclusion

1. U-Pb zircon geochronology of the Schwarzhorn Am-

phibolite yields a magmatic protolith age of 529+9/—8 Ma.

131

CAMBRIAN SCHWARZHORN AMPHIBOLITE IN LOWER AUSTROALPINE BASEMENT

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

2. The magmatic precursor of the Schwarzhorn Amphibo-

lite formed in a supra-subduction zone setting at the northern
margin of Gondwana.

3. The rock is part of the Variscan basement of the Lower

Austroalpine with close similarities to the Upper Austroal-
pine “older orthogneiss” of the Silvretta Nappe.

4. It became a tilted fault block of the distal passive mar-

gin during Jurassic rifting, and was incorporated in an imbri-
cate stack of tectonic slivers during Cretaceous and Tertiary
thrusting.

Acknowledgements:  The  authors  would  like  to  thank  Nils
Jung for the careful preparation of thin sections and sample
mounts.  Further,  Radegund  Hoffbauer  and  Kathrin  Faßmer
are  thanked  for  XRF  analysis,  and  Jiří  Sláma  for  providing
a split of the Plešovice zircon reference material. Finally, the
authors gratefully appreciate the constructive reviews of Jür-
gen von Raumer and Albrecht von Quadt, which were a great
help. This is contribution No. 28 from the DFG-funded LA-
ICP-MS laboratory of the Steinmann-Institut Bonn.

References

Allemann F. 1952: Die Couches Rouges der Sulzfluhdecke im

Fürstentum Liechtenstein. Eclogae Geol. Helv. 45, 264—298.

Biehler D. 1990: Strukturelle Entwicklung der Penninisch-Ostalpinen

Grenzzone am Beispiel der Arosa Zone im Ost-Rätikon
(Vorarlberg, Österreich). Eclogae Geol. Helv. 83, 221—239.

Burger H. 1978: Arosa- und Madrisa-Zone im Gebiet zwischen

Schollberg und Verspala (Osträtikon). Eclogae Geol. Helv. 71,
255—266.

Cadisch J. 1923: Geologie der Weissfluhgruppe. Beitr. Geol. Karte

der Schweiz NF 49, 50.

Corfu F., Hanchar J.M., Hoskin P. & Kinny P. 2003: Atlas of zircon

textures. In: Hanchar J.M. & Hoskin P.W.O. (Eds.): Zircon.
Rev. Mineral. Geochem. 53, 469—500.

Eberli G.P. 1988: The evolution of the southern continental margin

of the Jurassic Tethys ocean as recorded in the Allgäu Forma-
tion of the Austroalpine Nappes of Graubünden (Switzerland).
Eclogae Geol. Helv. 81, 175—214.

Ferreiro Mählmann R. 1994: Zur Bestimmung von Diagenesehöhe

und beginnender Metamorphose-Temperaturgeschichte und
Tektogenese des Austroalpins und Südpenninikums in Vorarl-
berg und Mittelbünden. Frankfurter Geowiss. Arb. C14,
1—498.

Ferreiro Mählmann R. & Giger M. 2012: The Arosa zone in Eastern

Switzerland: oceanic, sedimentary burial, accretional and oro-
genic very low- to low-grade patterns in a tectono-metamor-
phic mélange. Swiss. J. Geosci. 105, 203—233.

Froitzheim N. & Eberli G. 1990: Extensional detachment faulting in

the evolution of a Tethys passive continental margin, Eastern
Alps, Switzerland. Geol. Soc. Amer. Bull. 102, 1297—308.

Froitzheim N. & Manatschal G. 1996: Kinematics of Jurassic rifting,

mantle exhumation, and passive-margin formation in the Aus-
troalpine and Penninic nappes (eastern Switzerland). Geol.
Soc. Amer. Bull. 108, 1120—1133.

Froitzheim N., Schmid S.M. & Conti P. 1994: Repeated change

from crustal shortening to orogen-parallel extension in the
Austroalpine units of Graubünden. Eclogae Geol. Helv. 87,
559—612.

Froitzheim N., Schmid S.M. & Frey M. 1996: Mesozoic paleogeo-

graphy and the timing of eclogite facies metamorphism in the

Alps: a working hypothesis. Eclogae Geol. Helv. 89, 81—110.

Furrer H. 1985: Field workshop on Triassic and Jurassic sediments

in the Eastern Alps of Switzerland. Mitt. Geol. Inst. ETH
Zürich 248, 1—81.

Handy M.R. 1996: The transition from passive to active margin tec-

tonics: a case study from the Zone of Samedan (eastern Switzer-
land). Geol. Rundsch. 85, 832—851.

Kooijman E., Berndt J. & Mezger K. 2012: U-Pb dating of zircon

by laser ablation ICP-MS: recent improvements and new in-
sights. Eur. J. Mineral. 24, 5—21.

Kounov A., Graf J., von Quadt A., Bernoulli D., Burg J.P., Seward

D., Ivanov Z. & Fanning M. 2012: Evidence for a “Cadomian”
ophiolite and magmatic-arc complex in SW Bulgaria. Gondwa-
na Res. 212—213, 257—295.

Ladenhauf C., Armstrong R.A., Konzett J. & Miller C 2001: The

timing  of  the  pre-alpine  HP-metamorphism  in  the  Eastern
Alps:  constraints  from  U-Pb  SHRIMP  dating  of  eclogite  zir-
cons  from  the  Austro-Alpine  Silvretta  nappe.  Geol.  Paläont.
Mitt. Innsbruck. 25, 131.

Liebetrau V. 1996: Petrographie, Geochemie und Datierung der

“Flüelagranitischen Assoziation” (sog. Jüngere Orthogneise)
des Silvrettakristallins, Graubünden – Schweiz. Dissertation
University Freiburg (Switzerland), 1—233.

Ludwig K.R. 2012: Isoplot 3.75. A geochronological toolkit for Mi-

crosoft Excel. Berkeley Geochronology Center Spec. Publ. 5,
1—75.

Ludwig K.R. & Mundil R. 2002: Extracting reliable U-Pb ages and

errors from complex populations of zircons from Phanerozoic
tuffs. Geochim. Cosmochim. Acta 66, Supplement 1, 463.

Maggetti M. & Flisch M. 1993: Evolution of the Silvretta Nappe.

In: von Raumer J.F. & Neubauer F. (Eds.): Pre-mesozoic geo-
logy in the Alps. Springer, Berlin, 469—484.

Miller C. & Thöni M. 1995: Origin of eclogites from the Austroal-

pine Ötztal basement (Tirol, Austria): geochemistry and Sm-
Nd vs. Rb-Sr isotope systematics. Chem. Geol. 122, 199—225.

Missoni S. & Gawlick H.J. 2011: Jurassic mountain building and

Mesozoic-Cenozoic geodynamic evolution of the Northern
Calcareous Alps as proven in the Berchtesgaden Alps (Germa-
ny). Facies 57, 137—186.

Miyashiro A. 1974: Volcanic rock series in island arcs and active

continental margins. Amer. J. Sci. 274, 321—355.

Mohn G., Manatschal G., Müntener O., Beltrando M. & Masini E.

2010: Unravelling the interaction between tectonic and sedi-
mentary processes during lithospheric thinning in the Alpine
Tethys margins. Int. J. Earth Sci. 99, 75—101.

Mohn G., Manatschal G., Masini E. & Müntener O. 2011: Rift-re-

lated inheritance in orogens: a case study from the Austroal-
pine nappes in Central Alps (SE-Switzerland and N-Italy). Int.
J. Earth Sci. 100, 937—961.

Mullen E.D. 1983: MnO/TiO

: a minor element discriminant

for basaltic rocks of oceanic environments and its implications
for petrogenesis. Earth Planet. Sci. Lett. 62, 53—62.

Müller B., Klötzli U. & Flisch M. 1994: Dating of the Silvretta Older

Orthogneiss intrusion: U-Pb-zircon data indicate cadomian
magmatism in the Upper Austroalpine realm. J. Czech Geol.
Soc. 39, 74—75.

Müller B., Klötzli U. & Flisch M. 1995: U-Pb and Pb-Pb zircon da-

ting of the older orthogneiss suite in the Silvretta nappe, eas-
tern Alps: Cadomian magmatism in the upper Austro-Alpine
realm. Geol. Rundsch. 84, 457—465.

Müller B., Klötzli U.S., Schaltegger U. & Flisch M. 1996: Early

Cambrian oceanic plagiogranite in the Silvretta Nappe, eastern
Alps: geochemical, zircon U-Pb and Rb-Sr data from garnet-
hornblende-plagioclase gneisses. Geol. Rundsch. 85, 822—831.

Müller W., Dallmeyer R.D., Neubauer F. & Thöni M. 1999: Defor-

mation-induced resetting of Rb/Sr and

Ar/

Ar mineral sys-

132

NILIUS, FROITZHEIM, NAGEL, TOMASCHEK

and HEUSER

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

tems in a low-grade, polymetamorphic terrane (eastern Alps,
Austria). J. Geol. Soc. London 156, 261—278.

Nagel T. 2006: Structure of Austroalpine and Penninic units in the

Tilisuna area (Eastern Rätikon, Austria): implications for the
paleogeographic position of the Allgäu and Lechtal nappes.
Eclogae Geol. Helv. 99, 223—235.

Neubauer F., Genser J. & Handler R. 2000: The Eastern Alps: Re-

sult of a two-stage collision process. Mitt. Österr. Geol. Ges.
92, 117—134.

Oberhauser R. 1983: Mikrofossilfunde im Nordwestteil des Un-

terengadiner Fensters sowie im Verspalaflysch des Rätikon.
Jb. Geol. Bundesanst. Wien 126, 71—93.

Paton C., Hellstrom J., Paul B., Woodhead J. & Hergt J. 2011: Iolite:

Freeware for the visualisation and processing of mass spectro-
metric data. J. Anal. Atom. Spectrometry 26, 2508—2518.

Pearce J.A. 1982: Trace element characteristics of lavas from de-

structive plate boundaries. In: Thorpe R.S. (Ed.): Andesites:
Orogenic Andesites and Related Rocks. John Wiley & Sons,
525—548.

Petrus J.A. & Kamber B.S. 2012: VizualAge: a novel approach to

laser ablation ICP-MS U-Pb geochronology data reduction.
Geostand. Geoanal. Res. 36, 247—270.

Poller U. 1997: U-Pb single zircon study of gabbroic and granitic

rocks of Val Barlas-ch (Silvretta nappe, Switzerland). Schweiz.
Mineral. Petrogr. Mitt. 77, 351—359.

Richter M. 1958: Über den Bau der nördlichen Kalkalpen im Rä-

tikon. Zeitschr. Deutsch. Geol. Ges. 110, 307—325.

Ring U., Ratschbacher L., Frisch W., Biehler D. & Kralik M. 1989:

Kinematics of the Alpine plate margin: structural styles, strain
and motion along the Penninic-Austroalpine boundary in the
Swiss-Austrian Alps. J. Geol. Soc., London 146, 835—849.

Schaltegger U., Nägler T.F., Corfu F., Maggetti M., Galetti G. &

Stosch H.G. 1997: A Cambrian island arc in the Silvretta
nappe: constraints from geochemistry and geochronology.
Schweiz. Mineral. Petrogr. Mitt. 77, 337—350.

Schulz B. 2008: Basement of the Alps. In: McCann T. (Ed.): Geolo-

gy of Central Europe. Precambrian and Paleozoic, vol 1. Geol.
Soc., London, 79—83.

Schulz B., Bombach K., Pawlig S. & Brätz H. 2004: Neoproterozoic

to Early-Palaeozoic magmatic evolution in the Gondwana-
derived Austroalpine basement to the south of the Tauern
Window (Eastern Alps). Int. J. Earth Sci. 93, 824—843.

Sláma J., Košler J., Condon D.J., Crowley J.L., Gerdes A., Hanchar

J.M.,  Horstwood  M.S.A.,  Morris  G.A.,  Nasdala  L.,  Norberg
N.,  Schaltegger  U.,  Schoene  N.,  Tubrett  M.N.  &  Whitehouse
M.J. 2008: Plešovice zircon – A new natural reference material
for  U-Pb  and  Hf  isotopic  microanalysis.  Chem.  Geol.  249,
1—35.

Spillmann P. & Büchi H.J. 1993: The Pre-Alpine Basement of the

Lower Austro-Alpine Nappes in the Bernina Massif (Grisons,
Switzerland; Valtellina, Italy). In: von Raumer J.F. & Neu-
bauer F. (Eds.): Pre-Mesozoic Geology in the Alps. Springer,
457—467.

Stahel A.H. 1926: Geologische Untersuchungen im nordöstlichen

Rätikon. Dissertation, Universität Zürich, 1—82.

Streckeisen A. 1948: Der Gabbrozug Klosters-Davos-Arosa.

Schweiz. Mineral. Petrogr. Mitt. 28, 195—214.

Tollmann A. 1970: Der Deckenbau der westlichen Nordkalkalpen.

Neu. Jb. Geol. Paläont. Abh. 136, 80—133.

von Quadt A. 1992: U-Pb zircon and Sm-Nd geochronology of mafic

and ultramafic rocks from the central part of the Tauern Win-
dow (eastern Alps). Contrib. Mineral. Petrol. 110, 57—67.

von Quadt A., Grünenfelder M. & Büchi H.J. 1994: U-Pb zircon

ages from igneous rocks of the Bernina nappe system (Grison,
Switzerland). Schweiz. Mineral. Petrogr. Mitt. 74, 373—382.

von Raumer J.F., Stampfli G.M. & Bussy F. 2003: Gondwana-

derived microcontinents: The constituents of the Variscan and
Alpine collisional orogens. Tectonophysics 365, 7—22.

von Raumer J.F., Bussy F., Schaltegger U., Schulz B. & Stampfli

G.M. 2013: Pre-Mesozoic Alpine basements – their place in
the European Paleozoic framework. Geol. Soc. Amer. Bull.
125, 89—108.

von Seidlitz W.1906: Geologische Untersuchungen im östlichen

Rätikon. Ber. Naturf. Ges. Freiburg i.B. 16, 232—366.

Wiedenbeck M., Alle P., Corfu F., Griffin W.L., Meier M., Oberli

F., von Quadt A., Roddick J.C. & Spiegel W. 1995: Three nat-
ural zircon standards for U-Th-Pb, Lu-Hf, trace element and
REE analyses. Geostand. Newslett. 19, 1—23.

Wiedenbeck M., Hanchar J.M., Peck W.H., Sylvester P., Valley J.,

Whitehouse M., Kronz A., Morishita Y. & Nasdala L. 2004:
Further characterisation of the 91500 zircon crystal. Geostand.
Geoanal. Res. 28, 9—39.

Wilson M. 1989: Igneous Petrogenesis. Unwin Hyman, London,

1—466.

133

CAMBRIAN SCHWARZHORN AMPHIBOLITE IN LOWER AUSTROALPINE BASEMENT

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Laboratory & Sample Preparation

Laboratory name

Steinmann-Institut, Rheinische Friedrich Wilhelms-Universität Bonn

Sample type/mineral

Meta-Diorite/ igneous and inherited zircons

Sample preparation

conventional mineral separation (SPT), 1 inch resin mount, 1

µm polish

Imaging / characterization

CL, Jeol 8200 Superprobe

Acquisition date & sequence name 2014-01-30_Zircon2.seq

Laser Ablation System

Make, model & type

Resonetics Resolution M-50E 193nm excimer

Ablation cell & volume

Laurin Technic two-volume cell, effective volume 1-2 cm

Laser wavelength

193 nm

Pulse width

5 ns

Fluence

~ 9 J/cm

(5mJ, 100%T)

Repetition rate

10 Hz

Spot size

33 µm

Sampling mode / pattern

single spot analyses

Carrier gas

100% He, 650 ml/min He

Background collection

27 s

Ablation duration

30 s

Wash-out delay

8 s (post-ablation) + 60 s (after 3 cleaning pulses, 58 µm)

ICP-MS Instrument

Make, model & type

Thermo Scientific Element XR single collector SF-ICP-MS

Sample introduction

via conventional tubing, no squid

RF power

1250 W

Ar gas flows

cooling: 16 l/min, auxiliary: 0.80 l/min, sample: 1.28 l/min,

Detection system

single collector secondary electron multiplier (counting mode)

Masses measured

202, 204, 206, 207, 208, 232, 238

Integration time per peak

4 x 40 ms (202, 204, 207), 4 x 10 ms (206, 208, 232), 10 x 4 ms (238)

Integration time per reading

650 ms (65 s / 100 runs)

Sensitvity

7430 cps/ppm U (238 on NIST SRM 612 at measurement conditions)

Dead time

2 ns (dead time correction applied)

Data Processing

Calibration strategy

91500 used as primary reference material, Plešovice used as secondary
reference materials.

Reference material info

91500 (1065.4 ± 0.6 Ma, Wiedenbeck et al. 1995, 2004)
Plešovice (337.1 ± 0.4 Ma, Sláma et al. 2008)

Data processing package used / correction
for LIEF

Iolite (Paton et al. 2011) with DRS VizualAge 2013.02 (Petrus & Kamber
2012). Isoplot (Version 3.75) (Ludwig 2012) was used for plotting and
calculation of weighted mean ages.

Mass discrimination

Standard-sample bracketing with ratios normalized to primary reference zircon
91500.

Common-Pb correction, composition and
uncertainty

Amount of common-Pb was insignificant (f206 < 0.5%), no correction applied.

Uncertainty level & propagation

Ages are quoted at 2 sigma absolute, within-run reproducibility and age
uncertainty of reference material are propagated. A systematic error of 1.5 % is
propagated by quadratic addition into the final population age.

Quality control / validation

Plešovice: 339.6 ± 1.7 Ma (2σ, MSWD of concordance = 0.24 and probability
of concordance 0.63).

Table S1: LA-ICP-MS U-Th-Pb zircon dating methodology, Steinmann-Institut, Bonn, Germany.

134

NILIUS, FROITZHEIM, NAGEL, TOMASCHEK

and HEUSER

GEOL

EOL

EOLOGICA CARPA

OGICA CARPA

OGICA CARPATHICA

THICA

THICA, 2016, 67, 2, 121—132

Table S2: LA-ICP-MS U-Pb isotopic data for Plešovice zircon reference material.

Table S3: LA-ICP-MS U-Pb isotopic data for 91500 zircon reference material.

Fig. S4. Resulting concordia diagram for the measured Plešovice zircon reference material.

Analysis

Name

[ppm]

206

238

Measured Isotopic Ratios

Ages

[Ma]

[cps] [cps]

f206%

207

Pb/

235

206

Pb/

238

rho

207

Pb/

206

207

Pb/

235

U 2

206

Pb/

238

U 2

91500_1

113851 532304

-0,15

1,887

0,052

0,1835

0,0028 0,19

0,0744

0,0023

1075

1086

2 91500_2

81 30 15

110964 531960

-0,17 1,839 0,049

0,1793 0,0028

0,46

0,0734 0,0021 1058 17

1063 16

12 91500_3

79 31 15

106394 512589

-0,03 1,837 0,048

0,1791 0,0028

0,33

0,0745 0,0022 1058 17

1062 15

13 91500_4

76 29 15

102194 492913

0,26 1,896

0,050

0,1784 0,0031

0,51

0,0768 0,0020 1078 18

1058 17

22 91500_5

77 29 14

105570 514728

-0,02 1,814 0,044

0,1771 0,0029

0,26

0,0742 0,0020 1049 16

1051 16

23 91500_6

87 32 16

117969 578614

0,15 1,848

0,041

0,1764 0,0027

0,15

0,0755 0,0018 1062 15

1047 15

32 91500_7

83 30 15

118569 574915

-0,02 1,846 0,045

0,1787 0,0027

0,05

0,0745 0,0019 1061 16

1060 15

33 91500_8

82 30 15

115577 563905

0,21 1,858

0,048

0,1776 0,0028

0,44

0,0762 0,0021 1065 17

1054 16

42 91500_9

81 31 16

114453 557910

-0,05 1,820 0,047

0,1781 0,0031

0,19

0,0741 0,0021 1051 17

1056 17

43 91500_10 76 29 15

108282 525510

-0,01 1,835 0,054

0,1791 0,0033

0,39

0,0747 0,0023 1060 21

1062 18

52 91500_11 79 29 15

112688 550066

0,09 1,867

0,049

0,1795 0,0031

0,45

0,0756 0,0020 1068 18

1064 17

53 91500_12 81 32 15

116645 562994

-0,06 1,854 0,041

0,1803 0,0028

0,24

0,0745 0,0019 1067 14

1069 15

62 91500_13 82 31 15

118953 575344

-0,03 1,857 0,046

0,1801 0,0028

0,33

0,0747 0,0020 1065 16

1067 15

63 91500_14 81 30 15

117556 568638

0,00 1,856

0,048

0,1793 0,0031

0,19

0,0748 0,0022 1064 17

1063 17

72 91500_15 79 29 15

111564 550225

0,06 1,825

0,044

0,1767 0,0032

0,39

0,0748 0,0019 1053 16

1049 17

73 91500_16 83 31 16

117969 572438

0,00 1,861

0,055

0,1798 0,0030

0,22

0,0749 0,0023 1065 20

1066 16

78 91500_17 79 30 15

111448 537763

-0,02 1,869 0,044

0,1806 0,0027

0,05

0,0749 0,0020 1069 16

1070 15

f206% denotes the fraction of

206

Pb that is common

206

Pb and is calculated with f206% = (

207

Pb/

206

measured

207

Pb*/

206

Pb*) / (

207

Pb/

206

common

207

Pb*/

206

Pb*)*100.

Analysis

Name

[ppm]

206

238

Measured Isotopic Ratios

Ages

[Ma]

[cps] [cps]

f206%

207

Pb/

235

206

Pb/

238

rho

207

Pb/

206

207

Pb/

235

U 2σ

206

Pb/

238

U 2σ

Plesov_1 706

289611 4576425 0,08

0,4032

0,0087 0,05393

0,00086 0,28

0,05390

0,00095

343,8

6,3

338,6

5,3

Plesov_2 730 79 13

305525 4846501 0,00

0,3973

0,0088 0,05388

0,00082 0,30

0,05323

0,00094

339,5

6,3

338,3

5,0

Plesov_3 818 84 13

352652 5639379 -0,02 0,3928

0,0081 0,05356

0,00074 0,30

0,05299

0,00080

336,3

5,9

336,3

4,6

Plesov_4 737 80 13

321238 5108126 0,04

0,3994

0,0077 0,05394

0,00082 0,26

0,05352

0,00079

341,1

5,6

338,7

5,0

Plesov_5 818 98 15

360948 5706533 -0,01 0,4027

0,0087 0,05466

0,00078 0,09

0,05322

0,00093

343,4

6,3

343,0

4,7

Plesov_6 841 87 14

368157 5920307 -0,03 0,3949

0,0085 0,05384

0,00088 0,38

0,05299

0,00090

337,8

6,2

338,0

5,4

Plesov_7 774 83 13

339123 5392473 -0,15 0,3935

0,0083 0,05441

0,00093 0,45

0,05212

0,00086

336,8

6,0

341,5

5,7

Plesov_8 854 99 16

367928 5869538 0,05

0,4026

0,0076 0,05414

0,00081 0,22

0,05366

0,00078

343,4

5,5

339,9

5,0

f206% denotes the fraction of

206

Pb that is common

206

Pb and is calculated with f206% = (

207

Pb/

206

measured

207

Pb*/

206

Pb*) / (

207

Pb/

206

common

207

Pb*/

206

Pb*)*100.