U-Pb Zircon Geochronology and Geochemical Characterization of Pre-Alpine Metagranitoids and Metasediments of the Seckau Nappe System, Eastern Alps

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U-Pb zircon geochronology and geochemical characterization of pre-Alpine metagranitoids and metasediments of the Seckau Nappe System, Eastern Alps

DISSERTATION / DOCTORAL THESIS zur Erlangung des akademischen Grades / to obtain the academic degree of Doktor der Naturwissenschaften (Dr. rer. nat.)

vorgelegt von / submitted by Magdalena Mandl, MSc

NAWI Graz Geocenter, Institute of Earth Sciences University of Graz, Austria

Gutachter / Reviewers: Univ.-Prof. Mag. Dr. Walter Kurz Ao.Univ.-Prof. Mag. Dr.rer.nat. Christoph Hauzenberger

Graz, Jänner, 2020

Acknowledgements

First of all, I would like to thank my supervisors Walter Kurz and Christoph Hauzenberger for their unlimited and ongoing support, many valuable discussions, guidance and advice during all stages of this research.

My gratitude extends to Harald Fritz for his scientific inputs, reviews and enthusiasm for this project. I would like to thank Urs Klötzli for introducing me to the world of U-Pb zircon geochronology and for many helpful discussions. I am grateful to Dorothee Hippler for her assistance to handle the laser ablation-multi collector-inductively coupled plasma mass spectrometer (LA-MC-ICPMS). I want to thank Ralf Schuster for his constructive comments that helped to improve the first part of this manuscript. Many thanks to Fritz Finger for insightful discussions and to Stefan Pfingstl for providing some of his samples for U-Pb age dating.

I also would like to thank all the staff at the Institute of Earth Sciences for their constant support during the last years.

My gratitude extends to all my colleagues and friends at the Institute of Earth Sciences. Special thanks to Jenny, Gerald, Philip, Pevo, Dennis, Jule, Zsofi, Angie, Isaline, Tom, Marcus and Dominik for a good working atmosphere, team spirit and support. Jenny, it was a great pleasure to share the office with you. Thank you for a good mood every day, good ideas, comments and interesting, not only scientific, discussions!

Finally, I want to thank my family and friends whose constant support helped me to achieve this goal.

Last but not least, I would like to express my sincere thanks to those who have helped me to finish this doctoral thesis and I have not mentioned here by name.

Many thanks to all of you!

i Abstract

New petrographic, geochemical and geochronological data from paragneisses and metagranitoids of the Seckau Complex allow deeper insights into the pre-Alpine continental crust of the Eastern Alps. Based on these data a new subdivision of the crystalline basement of the Seckau Complex, into the following three lithostratigraphic sub-units, is proposed. (1) The Glaneck Metamorphic Suite is mainly composed of fine-grained paragneisses (and mica schists) that yield U-Pb zircon ages between 572 ± 7 Ma and 559 ± 11 Ma. These pre-Cambrian paragneisses are the host rock for the two subsequent intrusion events represented by plutonic bodies of the Hochreichart Plutonic Suite and the later emplaced Hintertal Plutonic Suite. (2) The Hochreichart Plutonic Suite is mainly composed of less to strong deformed granites and granodiorites indicating a peraluminous character (A/CNK = 1.08-1.51), a Rb/Sr ratio > 45, a significant negative Eu anomaly as well as U-Pb zircon ages in the range from 508 ± 9 Ma to 486 ± 9 Ma. These late Cambrian to Early Ordovician rocks are not very common in the Eastern Alps and reflect most likely the re-attachment event of the Cadomian Arc to Gondwana. (3) The Hintertal Plutonic Suite mainly comprises slightly to strongly deformed intermediate to acidic intrusion bodies. They display a meta- to peraluminous character (A/CNK = 0.88-1.23), a magmatic fractionation trend with Rb/Sr ratios ≤ 0.45 and U-Pb zircon ages in the range between 365 ± 11 Ma and 343 ± 12 Ma. Based on petrographic and geochemical characteristics the Hintertal Plutonic Suite can be further subdivided into (a) the Pletzen Pluton and (b) the Griessstein Pluton. The Pletzen Pluton is composed of intermediate to acidic intrusion bodies that are mainly meta- to weakly peraluminous in composition (A/CNK = 0.88 to 1.10) and show no significant Eu anomaly. Large-sized titanite crystals and green- colored biotite minerals, observed under a microscope, characterize this sub-unit. In contrast, the predominantly acidic rocks of the Griessstein Pluton indicate a more fractionated melt with a peraluminous character (A/CNK= 1.10 to 1.23) and a weakly developed Eu anomaly. The intrusive rocks of the Pletzen Pluton can be related to the subduction process of the Paleotethys prior to the collision event of Gondwana and Laurussia. The rocks of the Griessstein Pluton are considered to represent the fractionation product from the more primitive Pletzen melt. In addition to comprehensive investigations of the Seckau Complex its sedimentary cover, the Rannach Formation, was also examined. The detrital zircon age spectrum of sedimentary rocks from the Rannach Formation indicates a wide range of ages including Neoarchean to Paleoproterozoic (e.g., 2063 ± 30 Ma), late Neoproterozoic (555 ± 7 Ma), Cambrian (508 ± 7 Ma), Ordovician (457 ± 7 Ma), Late Devonian/early Carboniferous (358 ± 5 Ma) and Permian (293 ± 6 Ma) ages. These U-Pb zircon ages display predominantly the same ages as observed from different basement units of the Seckau Complex. Only Ordovician and Permian zircon ages are not documented in the rocks of the Seckau Complex and suggest more regional sources.

ii Zusammenfassung

Neue petrographische, geochemische und geochronologische Daten von Paragneisen und Metagranitoiden des Seckau-Komplexes ermöglichen tiefe Einblicke in die voralpidisch gebildete kontinentale Kruste der Ostalpen. Basierend auf diesen neuen Daten kann das kristalline Grundgebirge des Seckau-Komplexes in die nacholgenden drei lithostratigraphischen Einheiten gegliedert werden. (1) Die Glaneck Metamorphic Suite besteht vorwiegend aus feinkörnigen Paragneisen (und Glimmerschiefern) die ein U-Pb-Zirkonalter zwischen 572 ± 7 Ma und 559 ± 11 Ma aufweisen. Diese präkambrischen Paragneisse bilden den kristallinen Rahmen für die nachfolgend eindringenden Plutonite der Hochreichart Plutonic Suite und der Hintertal Plutonic Suite. (2) Die Hochreichart Plutonic Suite besteht hauptsächlich aus leicht bis stark deformierten Graniten und Granodioriten, die einen peraluminösen Charakter (A / CNK = 1,08-1,51), ein Rb/Sr Verhältnis > 45, eine negative Eu- Anomalie sowie U-P Zirkonalter zwischen 508 ± 9 Ma und 486 ± 9 Ma zeigen. Diese spät- kambrischen bis früh-ordovizischen Gesteine sind selten in den Ostalpen zu finden und spiegeln wahrscheinlich die Wiederanbindung des Cadomischen Bogens an Gondwana wider (3) Die Hintertal Plutonic Suite umfasst hauptsächlich leicht bis stark deformierte Intrusivgesteine mit einer intermediären bis sauren Zusammensetzung. Diese Gesteine weisen einen meta- bis peraluminösen Charakter (A /CNK = 0,88 bis 1,23), einen Fraktionierungstrend mit Rb/Sr Verhältnissen ≤ 45 sowie U-Pb Zirkonalter im Bereich von 365 ± 11 Ma bis 343 ± 12 Ma auf. Aufgrund petrographischer und geochemischer Merkmale kann die Hintertal Plutonic Suite weiter in den (a) Pletzen Pluton und (b) den Griessstein Pluton unterteilt werden. Der Pletzen Pluton setzt sich aus intermediären bis sauren Intrusivkörpern zusammen, die einen meta- bis leicht peraluminösen (A / CNK = 0,88 bis 1,10) Charakter sowie keine Eu-Anomalie aufweisen. Diese Gesteine zeigen typischerweise große Titanite und Biotite mit einer grünen Eigenfarbe unter dem Lichtmikroskop. Die überwiegend sauren Gesteine des Griessstein Plutons repräsentieren stark fraktionierte Schmelzen, die einen peraluminösen Charakter (A / CNK = 1,10 bis 1,23) und eine nur schwach ausgeprägte Eu-Anomalie zeigen. Die Intrusivgesteine des Pletzen Plutons können mit dem Subduktionsereignis der Paleotethys infolge der Kollision von Gondwana und Laurussia in Verbindung gebracht werden. Die Gesteine des Griessstein Plutons repräsentieren vermutlich fraktionierte Schmelzen der primitiveren Pletzen Schmelze. Neben umfassenden Untersuchungen des Seckau-Komplexes wurde auch dessen sedimentäre Bedeckung, die Rannach Formation, untersucht. Das Altersspektrum der detritären Zirkone der sedimentären Gesteine der Rannach Formation umfasst neoarchäische bis paläoproterozoische (z.B.: 2063 ± 30 Ma), spät-neoproterozoische (555 ± 7 Ma), kambrische (508 ± 7 Ma), ordovizische (457 ± 7 Ma), spät-devonische/früh-karbonische (358 ± 5 Ma) und permische (293 ± 6 Ma) Alter. Diese U-Pb Alter spiegeln dieselben Alter wider, wie sie in den verschiedenen Einheiten des Seckau- Komlexes beobachtet wurden. Einzig ordovizische und permische Zirkonalter wurden in den Gesteinen des Seckau-Komplexes nicht dokumentiert und lassen auf ein regionaleres Liefergebiet schließen.

iii Table of contents

Acknowledgements ...... i Abstract ...... ii Zusammenfassung...... iii

1 Introduction ...... 1 1.1 Objectives of this study ...... 1 1.2 Geological overview ...... 3 1.2.1 Seckau Complex ...... 5 1.2.2 Rannach Formation ...... 6 1.3 References ...... 6

Chapter 1: Pre-Alpine evolution of the Seckau Complex (Austroalpine basement/ Eastern Alps: Constraints from in-situ LA-ICP-MS U-Pb zircon geochronology Abstract ...... 10 1 Introduction ...... 11 2 Geological setting ...... 12 2.1 Published geochronological data from the Seckau Nappe ...... 16 2.2 Published geochronological data from surrounding units ...... 17 3 Methods ...... 18 3.1 X-ray fluorescence (XRF) ...... 18 3.2 LA-MC-ICP-MS ...... 19 3.3 Sm-Nd ...... 20 4 Lithological units ...... 21 4.1 Paragneisses and micaschists (Glaneck Metamorphic Suite) ...... 21 4.2 Metagranitoids (Hochreichart Plutonic Suite, Hintertal Plutonic Suite) ...... 21

5 Geochemistry of metagranitoids ...... 25 6 Ub-Pb zircon ages ...... 27 6.1 Precambrian paragneisses and migmatitic paragneisses (Glaneck Metamorphic Suite) .. 30 6.2 Late Cambrian-Early Ordovician metagranitoids (Hochreichart Plutonic Suite) ...... 31 6.3 Late Devonian-early Carboniferous metagranitoids (Hintertal Plutonic Suite) ...... 33 6.3.1 Metagranitoids with S-type signatures (Griessstein Pluton), including pegmatitic-, leucogranitic- and granitic dikes ...... 33 6.3.2 Metagranitoids with I-type affinity (Pletzen Pluton) ...... 35 7 Lithostratigraphic subdivision of the Seckau Complex ...... 38 7.1 Glaneck Metamorphic Suite ...... 38 7.2 Hochreichart Plutonic Suite ...... 38 7.3 Hintertal Plutonic Suite ...... 39 7.3.1 Griessstein Pluton (Lithodeme)...... 39 7.3.2 Pletzen Pluton (Lithodeme) ...... 39 8 Discussion ...... 40 8.1 Evolution of the Seckau Complex ...... 40 8.2 Provenance and paleogeographic position of the Glaneck Metamorphic Suite ...... 41 8.3 Tectonic evolution during the early Paleozoic ...... 42 8.4 Evolution during the Variscan tectonometamorphic event ...... 43 9 Conclusions ...... 45 10 Acknowledgment ...... 46 11 References ...... 46

Chapter 2: Geochemistry of granitoids from the Seckau Complex (Eastern Alps): a key for revealing the pre-Alpine evolution in the Eastern Alps Abstract ...... 54 1 Introduction ...... 55 2 Geological setting ...... 57

3 Analytical methods ...... 59 4 Sample description ...... 60 4.1 Hochreichart Plutonic Suite ...... 60 4.2 Hintertal Plutonic Suite ...... 63 4.3 Dikes ...... 64 5 Whole-rock major, trace and rare earth elements ...... 66 5.1 Hochreichart Plutonic Suite ...... 68 5.2 Hintertal Plutonic Suite ...... 71 5.2.1 Pletzen Pluton of the Hintertal Plutonic Suite ...... 72 5.2.2 Griessstein Pluton of the Hintertal Plutonic Suite ...... 73 5.3 Dikes ...... 74 6 Discussion ...... 77 6.1 Geochemical evolution of the different magmatic suites of the Seckau Complex ...... 77 6.1.1 Hochreichart Plutonic Suite ...... 77 6.1.2 Hintertal Plutonic Suite ...... 77 6.1.3 Origin of dikes...... 81 6.2 Paleogeographic setting of the Seckau Complex ...... 81 7 Conclusions ...... 83 8 References ...... 83

Chapter 3: U-Pb zircon geochronology and sedimentary provenance of detrital zircons from sedimentary rocks of the Rannach Formation, Eastern Alps Abstract ...... 90 1 Introduction ...... 90 2 Geological setting ...... 91 2.1 Rannach Formation...... 92 3 Methods ...... 93 4 Sample location/sample description ...... 93

5 Geochemistry ...... 97 6 Detrital zircons ...... 98 6.1 U-Pb zircon ages ...... 99 7 Discussion ...... 105 7.1 Potential source areas ...... 105 7.1.1 “Internal Source” ...... 105 7.1.2 “External Source” ...... 105 7.2 Tectono-metamorphic events ...... 106 8 Conclusions ...... 107 9 References ...... 108

Discussion and general conclusions...... 113

Appendix ...... 115 Appendix A ...... 115 Digital Appendix (DVD) ...... 115

1 Introduction The understanding of magmatic, metamorphic and tectonic processes of the Austroalpine Unit (Eastern Alps) has developed during the last decades rapidly, but there are still many open questions about its internal structure and plate tectonic evolution. To gain more insight into todays’s structure of the Eastern Alps and to decipher the complex history of the different nappe systems within the Austroalpine Nappe System, new geological data from less discussed units e.g., from Variscan domains within the Austroalpine Unit, are necessary. Variscan domains are generally widespread in Western and Central Europe (e.g., Iberian Massif, Armorican Massif, Massif Central, Rheinisches Schiefergebierge, Bohemian Massif) and represent the Variscan orocline of Europe. This Variscan orogen results from the collision of Gondwana and Laurussia and has formed the supercontinent Pangea during Paleozoic times (Matte, 1986; Franke, 2000). Geochronological data from the southern part of the Variscan basement are, in contrast to the well-studied plutons of the northern and central European Variscan units, quite rare. Although the Alpine orogeny reworked the southern part of the Variscan basement, pre-Variscan and Variscan fragments are still preserved in different tectonic positions within the Alps, e.g., the Austroalpine Unit comprising e.g., the Schladming Nappe System, the Seckau-Bösenstein Unit and the Semmering-Raabalpen Unit (e.g., Schermaier et al., 1997, Schmied et al., 2004, Mandl et al., 2018, Pfingstl et al., 2015). For this reason it was decided to investigate the Seckau Complex and its sedimentary cover, the Rannach Formation, as part of the Silvretta-Seckau Nappe System.

1.1 Objectives of this study The present study predominantly focuses on the petrographic, geochemical and geochronological characterization of different metagranitoids from the crystalline basement of the Seckau Complex and on the identification of detrital zircon age spectra from various metasedimentary rocks of the Rannach Formation. The aim of this study is to get a better understanding of the magmatic evolution of the Seckau Complex and to identify potential source areas of its sedimentary cover unit. Investigations of the Seckau Complex and the Rannach Formation should help to get more information of the pre-Variscan and Variscan crust to contribute deciphering the complex pre-Alpine history of the Austroalpine Unit and provide arguments for Alpine and pre-Alpine paleogeographic reconstructions.

1 This thesis is divided into (1) a general introduction part, including the main objectives of this study and a general geological overview of the study area, (2) three main chapters (described below) as well as (3) a general discussion- and conclusion part

Chapter 1 deals with the magmatic and geochronological evolution of the Seckau Complex. U- Pb zircon ages of metagranitoids have been determined by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) to identify the crystallization age of various intrusion bodies. Based on additional petrographic and geochemical data, a new subdivision of the Seckau Complex into three lithostratigraphic sub- units (suites) is proposed. This chapter comprises a peer-reviewed paper entitled "Pre- Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constrains from in-situ LA-ICP-MS U/Pb zircon geochronology" by Mandl, M., Kurz, W., Hauzenberger, C., Fritz, H., Klötzli, U. & Schuster, R. is published in Lithos, 296-299 (2018), 412-430 and can be found in the Appendix A on page 116-134.

Chapter 2 describes the geochemical characteristics of the different plutonic suites of the Seckau Complex and discusses their geochemical evolution and spatial relationship based on whole rock major, trace and rare earth elements determined by XRF (X-Ray Fluorescence) and ICP-MS (Inductively Coupled Plasma-Mass Spectrometry), respectively.

In Chapter 3 various U-Pb zircon ages from different metasedimentary rocks of the Rannach Formation, the sedimentary cover of the Seckau Complex, were determind and interpreted as crystallization age of potential source domains, to clarify the age spectra of detrital zircon grains and identify less revealed internal and external provenances.

The Appendix consists of two parts: Appendix A presents the published manuscript "Pre- Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constrains from in-situ LA-ICP-MS U/Pb zircon geochronology" by Mandl, M., Kurz, W., Hauzenberger, C., Fritz, H., Klötzli, U. & Schuster, R. and a Digital Appendix comprises all U-Pb analysis determined by LA-MC-ICP-MS as well as zircon images used for zircon age dating.

2 1.2 Geological overview Alpine basement units located in south and central Europe build a complex nappe stack resulting from continuous convergence of the African and European plates since Late Jurassic to Cretaceous times. The Alps are formed by two orogenic collision events that record the closure of ocean basins in the Mediterranean region. (1) The Eo-Alpine event (Early Cretaceous to early Late Cretaceous) is related to the closure of the Meliata-Hallstatt Ocean and affects predominantly the eastern part of the Alps. (2) The subsequent Alpine event (Late Cretaceous to Paleogene) is caused by the subduction of the Piemont-Ligurian Ocean and the Valais Ocean (Alpine Tethys oceans) and mainly affects the western part of the Alps (e.g., Froitzheim et al., 1996; Schmid et al., 2004; Schuster, 2004). The Alps can generally be subdivided into four main tectonic units (1) the Helvetic (2) the Penninic (3) the Austroalpine and (4) the South Alpine (Fig. 1). The South Alpine, positioned south of the Periadriatic Line, forms a south-directed fold and thrust belt and is characterized by the absence of an Alpine metamorphic overprint. In contrast, Helvetic, Penninic and Austroalpine nappes, located north of the Periadriatic Line, thrust towards the European foreland (towards north and west) and show various grades of Alpine metamorphic imprints. Each of these three superunits consists of multiple nappe systems which together form a complex nappe stack, comprising Helvetic nappes at its base, Penninic nappes in the middle and the Austroalpine Unit on the structural highest position (e.g., Froitzheim et al., 2008). The Austroalpine Unit originated from the former northern continental margin of Adria and can be subdivided into the Lower Austroalpine Unit and the Upper Austroalpine Unit. The Lower Austroalpine Unit (adjacent to the Penninic Ocean) was subducted together with the Penninic Ocean towards the south, below the Upper Austroalpine Unit, during the Alpine orogenic event. Therefore crustal material occurs only locally on the base of the Lower Austroalpine Unit e.g., at the northern margin of the Tauern Window (e.g., Radstätter Tauern) and the Wechsel nappes. The Upper Austroalpine Unit escaped Alpine metamorphism in an upper plate position and its crystalline basement units were formed by the previous Eo-Alpine collision event.

Fig.1. Schematic map of the Alps comprising the four main tectonic units (modified after Pfiffner, 2009).

The Upper Austroalpine Unit is widely exposed in the Eastern Alps and is built up, coming from the north, by Mesozoic sedimentary sequences (Northern Calcareous Alps) Paleozoic metasediments and metavolcanics (Greywacke Zone), and crystalline basement units subdivided (from bottom to top) into (1) the Silvretta-Seckau Nappe System, (2) the Koralpe- Wölz Nappe System, (3) the Ötztal-Bundschuh Nappe System and (4) the Drauzug-Gurktal Nappe System (Fig. 2). Most of the crystalline basement units are covered by Permian to Mesozoic sedimentary sequences and occupied a position close to the Meliata-Hallstatt Ocean before Alpine tectonics (e.g., Schmid et al., 2004; Schuster et al., 2001, 2004). The Silvretta-Seckau Nappe System, the structurally lowermost Upper Austroalpine Nappe System, comprises e.g., the Languard and Campo-Sesvenna-Silvretta nappes west of the Tauern Window and e.g., the Schladming, the Seckau and the Semmering nappes further to the east (Schmid et al., 2004). The Seckau Nappe consists of several pre-Variscan/Variscan basement units e.g., the Seckau Complex, the Amering Complex and the Speik Complex. The sedimentary cover of the Seckau Complex is summarized as Rannach-Formation (Metz, 1967, Flügel and Neubauer, 1984; Faryad and Hoinkes, 2001; Gaidies et al., 2006; Pfingstl et al., 2015).

Fig.2. Overview maps showing the tectonic subdivision of the Austroalpine Unit based on Schmid et al. (2004). The study area is marked with a black rectangle. Abbreviations: BN: Bundschuh Nappe, DZ: Drauzug, GP: Graz Paleozoic, GWZ: Greywacke Zone, GN: Gurktal Nappe, NCA: Northern Calcareous Alps, Ötz: Ötztal Nappe, Sil: Silvretta Nappe, SN: Seckau Nappe, TW: Tauern Window, Wz: Wölz Nappe.

1.2.1 Seckau Complex The Seckau Complex, as part of the Silvretta-Seckau Nappe System, basically assembles pre- Cambrian paragneisses (locally migmatized), (garnet-) micaschists and amphibolites (Glaneck Metamorphic Suite) that were intruded by peraluminous late Cambrian to Early Ordovician plutons (508 ± 9 Ma and 486 ± 9 Ma) and meta- to peraluminous Late Devonian to early Carboniferous intrusive bodies (365 ± 11 Ma to 343 ± 12 Ma) (Mandl et al., 2018; Metz, 1976). Together these plutons, now observed as metagranitoids, make up about 90 % of the crystalline basement (Schermaier et al., 1997) and comprise predominantly granites, granodiorites and minor intermediate to basic rocks. They follow the calc-alkaline fractionation trend after Irvine and Baragar (1971) and reflect the geochemical signature of

5 an active continental margin (volcanic-arc granites after Pearce et al. (1984)). These basement units represent parts of the pre-Variscan and Variscan lithosphere that was affected during the Eo-Alpine orogenic event by a low grade metamorphic overprint, indicating upper greenschist facies conditions (520°C at 0.9 Ga ; Faryad and Hoinkes, 2001).

1.2.2 Rannach Formation The Rannach Formation represents the metasedimentary cover of the Seckau Nappe and overlies the northern part of the Seckau Complex and the northwestern part of the Bösenstein area (Schmid et al., 2004). The Rannach Formation is characterized by a thickness of more than 1000 meters and comprises coarse-grained conglomerates at its base, followed by pebbly sandstones and fine grained clastics on the top (Pfingstl, 2013, Master Thesis; Metz, 1967). The sedimentary cover of the Seckau Nappe experienced a middle to upper greenschist facies Eo-Alpine metamorphic overprint, similar to the Seckau Complex (Faryad and Hoinkes, 2001).

1.3 References

Faryad, S.W., Hoinkes, G., 2001. Alpine Chloritoid and Garnet from the Hochgrössen Massif (Speik Complex, Eastern Alps). Mitteilungen der Österreichischen Geologischen Gesellschaft 146, 387-396.

Flügel, H.W., Neubauer F.R., 1984. Geologische Karte der Steiermark 1:200.000. Geologische Bundesanstalt, Wien.

Franke, W., 2000. The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution. In: Franke, W., Haak, V., Oncken, O., Tanner, D. (Eds.), Orogenic Processes: Quantification and Modelling in the Variscan Belt. Geological Society, London, Special Publication 179, 35-61.

Froitzheim, N., Plasienka, D., Schuster, R., 2008. Alpine tectonics of the Alps and Western Carpathians. In: McCann, T. (Ed.), The Geology of Central Europe Volume 2: Mesozoic and Cenozoic, Geological Society of London, pp. 1141-1232.

Froitzheim, N., Schmid, S.M., Frey, M., 1996. Mesozoic paleogeography and the timing of eclogite-fazies metamorphism in the Alps: A working hypothesis. Eclogae Geologicae Helvetiae 891 (1), 81-110.

6 Gaidies, F., Abart, R., De Capitani, C., Schuster, R., Connolly, J.A.D., Reusser, E., 2006. Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometry. Journal of Metamorphic Geology 24, 451- 475.

Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences 8, 523-548.

Mandl, M., Kurz, W., Hauzenberger, C., Fritz, H., Klötzli, U., Schuster, R., 2018. Pre-Alpine evolution oft he Seckau Complex (Austroalpine basement/Eastern Alps): Constraints from in- situ LA-ICP-MS U-Pb zircon geochronology. Lithos, 296-299, 412-430.

Matte, P., 1986. Tectonics and plate tectonics model fort he Variscan belt of Europe. Tectonophysics 126, 329-332.

Metz, K., 1967. Geologische Karte der Republik Österreich 1:50.000, Blatt 130-131 Oberzeiring-Kalwang. Geologische Bundesanstalt, Wien.

Metz, K., 1976. Der Geologische Bau der Seckauer und Rottenmanner Tauern. Jahrbuch der Geologischen Bundesanstalt 119, 151-205.

Pearce, J., Harris, N., Tindle, A., 1984. Trace elements discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956-983.

Pfiffner, O., A., 2009. Geologie der Alpen. 1st ed., Haupt, Bern.

Pfingstl, S., 2013. Tektonische und metamorphe Entwicklung des Seckauer Kristallins. Karl- Franzens University of Graz, Master Thesis.

Pfingstl, S., Kurz, W., Schuster, R., Hauzenberger, C., 2015. Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe System (Eastern Alps). Austrian Journal of Earth Sciences 108 (1), 172-185.

Schermaier, A., Haunschmid, B., Finger, F., 1997. Distribution of Variscan I- and S-type granites in the Eastern Alps: a possible clue to unravel pre Alpine basement structures. Tectonophysics 272, 315-333.

Schmid, S.M., Fügenschuh, B., Kissling, E., Schuster, R., 2004. Tectonic map and overall architecture of the Alpine orogen. Eclogae Geologicae Helvetiae 97, 93-117.

Schnabel, W., 1980a. Permomesozoikum und Paleozän in den zentralen Ostalpen: Zentralalpin (Mittelostalpin), und Oberostalpin, westlicher Teil (Abb.). In: Oberhauser, R. (Ed.), Der geologische Aufbau Österreichs, Springer, Wien, New York, p. 346.

Schnabel, W., 1980b. Permomesozoikum in den zentralen Ostalpen : Zentralalpin (Mittelostalpin), Oberostalpin incl. Zentralalpine Gosau mit Eozän, mittlerer und östlicher Teil (Abb.). In : Oberhauser, R. (Ed.), Der geologische Aufbau Österreichs, Springer, Wien- New York, pp. 406-407.

7 Schuster, R., 2004. The Austroalpine crystalline units in the Eastern Alps. Berichte des Instituts für Erdwissenschaften, Karl-Franzens-Universität Graz, ISSN 1608-8166, Band 9.

Schuster, R., Scharbert, S., Abart, R., Frank, W., 2001. Permo-Triassic extension and related HT/LP metamorphism in the Austroalpine - Southalpine realm. Mitteilungen der Gesellschaft der Geologie- und Bergbaustudenten in Österreich 45, 111-141.

Chapter 1 |

Pre-Alpine evolution of the Seckau Complex (Austroalpine basement / Eastern Alps): Constraints from in-situ LA-ICP-MS U-Pb zircon geochronology

Magdalena Mandl1*, Walter Kurz1, Christoph Hauzenberger1, Harald Fritz1, Urs Klötzli2 and Ralf Schuster3

1 University of Graz, Institute of Earth Sciences, NAWI Graz Geocenter, Heinrichstraße 26, 8010 Graz, Austria 2 University of Vienna, Department of Lithospheric Research, Althanstrasse 14, 1090 Vienna, Austria 3 Geologische Bundesanstalt, Neulinggasse 38, 1030 Vienna, Austria

Keywords: Seckau Complex; Eastern Alps; U/Pb geochronology; Late Cambrian – Early Ordovician magmatism; Late Devonian – Early Carboniferous magmatism

9 Abstract The Variscan European Belt is a complex orogen with its southern margin partly obscured by Alpine tectonics and metamorphism. This study focuses on one of the units, the Seckau Complex, that constitute the southern part of the Variscan European Belt in the Eastern Alps in order to clarify its origin, age and lithostratigraphy. The magmatic and geochronological evolution of this Complex in the northwestern part of the Seckau Nappe (as part of the Austroalpine Silvretta-Seckau Nappe System) was investigated by zircon U-Pb dating of paragneisses and metagranitoids coupled with petrological and geochemical data. This reveals the distinction of three newly defined lithostratigraphic/lithodemic sub-units: (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite. The Glaneck Metamorphic Suite is mainly composed of fine-grained paragneisses that yield U-Pb zircon ages in the range between 2.7 Ga and 2.0 Ga, as well as concordia ages from 572 ± 7 Ma to 559 ± 11 Ma. All of these ages are interpreted as detrital zircon ages originating from an igneous source. The paragneisses are the host rock for the large volumes of metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. The Hochreichart Plutonic Suite comprises highly fractionated melts with mainly S-type characteristics and late Cambrian to Early Ordovician U-Pb zircon ages (508 ± 9 Ma to 486 ± 9 Ma), interpreted as magmatic protolith ages. The Hintertal Plutonic Suite is composed of metagranitoids with Late Devonian to early Carboniferous (365 ± 11 Ma and 331 ± 10 Ma) protolith ages that intruded during an early phase of the Variscan tectonometamorphic event. The metagranitoids of the Hintertal Plutonic Suite define a magmatic fractionation trend, seen in variable Rb/Sr ratios. On this base they can be further subdivided into (a) the Griessstein Pluton characterized by S-type metagranitoids and (b) the Pletzen Pluton distinguished by intermediate to acidic metagranitoids with I-type affinity. The detrital zircon age spectra suggest a Neoproterozoic ancestry of the Glaneck Metamorphic Suite, which was located west of the Arabian Nubian Shield, probably next to the Trans-Saharan Belt. The early Paleozoic evolution of the recent Seckau Complex shows similarities to basement units of the Southalpine Unit, parts of the Austroalpine Unit as well as the Tatric and Veporic units of the Central Western Carpathians.

10 1 Introduction The European Variscan belt, a large scale orogen resulting from the collision of Gondwana and Laurussia, assembled crustal material mainly from North Gondwana derived microcontinents, e.g., Avalonia, Cadomia, Armorica, the Intra-Alpine terranes (von Raumer et al, 2002). To the north of the Alpine orogenic front the Variscan orogen has been studied intensively in several regions, such as the Massif Central (e.g., Faure et al., 2009) and the Bohemian Massif (Finger et al., 2007; Franěk et al., 2011; Franke et al., 2017; Friedl et al., 2004; Kroner and Romer, 2013; Kroner et al., 2016; Schulmann et al., 2005), but there are still many open questions with respect to the internal structure and plate tectonic evolution. The southern margin of the Variscan orogen in Central Europe is incorporated within the Alpine orogenic belt. It is highly obscured by reworking during Alpine tectonics and our knowledge about its setting and structure is poor, as there is still limited information from Variscan domains within the Austroalpine Unit of the Eastern Alps. Given the fact that the Variscan orogen needs to be understood as a whole, it was decided to more detail investigate the Seckau Complex, which represents one of these Variscan domains within the Eastern Alps. The Seckau Complex represents a piece of pre-Variscan/Variscan continental crust reworked during Alpine orogeny. Together with other basement units (e.g., Speik Complex, Amering Complex) and remnants of transgressively overlying Permo-Triassic metasediments they build up the Seckau Nappe as part of the Austroalpine Silvretta-Seckau Nappe System (sensu Schmid et al., 2004). Until now little attention was given to this complex due to its presumably monotonous composition and structure. According to available geological maps (Flügel and Neubauer, 1984; Metz, 1967) the Seckau Complex comprises migmatitic paragneisses, (garnet-) micaschists and amphibolites that were intruded by different types of granitoids (Metz, 1976). Despite the lack of precise geochronological ages the granitoids as well as the metamorphic imprint of the rocks were interpreted to be Carboniferous in age and related to the Variscan orogenic event (e.g., Schermaier et al., 1997). This study presents new U-Pb zircon and Sm-Nd ages on paragneisses and metagranitoids as well as geochemical data on metagranitoids of the Seckau Complex. This new data give a better understanding of the pre-Variscan and Variscan tectonic evolution of this piece of continental crust and allow correlation to other Gondwana derived fragments within and outside of the Alpine orogenic belt.

11 2 Geological setting The Alpine orogen developed from two continental and two oceanic realms existing in Late Jurassic to Cretaceous times (Fig. 1a). The tectonically lowermost units are derived from the former continental margin of Europe and they appear along the northern front of the orogenic belt (Helvetic nappes) and within tectonic windows (Subpenninic nappes). The Penninic nappes above represent remnants of the Penninic (Alpine Tethys) Ocean and continental fragments therein. The Austroalpine Unit, separated by the Periadriatic fault from the Southalpine Unit, both derived from the Adriatic continental margin, thrusted the Penninic units. The Austroalpine nappes were formed since the early Cretaceous and represent the northwest vergent pro-wedge of the Alpine orogen, whereas the Southalpine Unit was formed as south vergent retro-wedge since Oligocene time. Tectonic slices derived from the Neotethys oceanic realm occur within the Austroalpine nappe pile and are referred as Meliata Unit (e.g., Schmid et al., 2004). The Austroalpine Unit is part of the AlCaPa (Alpine-Carpathian-Pannonian) Megaunit, which continues into the Western Carpathians (e.g., Neubauer et al., 1995). It forms a complex nappe stack being subdivided into a Lower Austroalpine and an Upper Austroalpine Subunit (Fig. 1b), both generally consisting of pre-Alpine basement, Paleozoic metasediments and Mesozoic metasediments. With respect to the Late Jurassic/Cretaceous paleogeographic arrangement the Lower Austroalpine Subunit derived from the northern/northwestern continental margin of the Adriatic Plate towards the Penninic Ocean. After the deposition of Triassic pre-rift shelf sediments this area was affected by Jurassic rifting and the Lower Austroalpine Subunit was formed by nappe stacking during middle Late Cretaceous to Eocene times (Froitzheim et al., 2008; Schmid et al., 2004). These nappes appear in Eastern Switzerland (Err-Bernina Nappe System) in the frame of the Tauern Window (e.g., Radstadt Nappe System) and at the eastern margin of the Eastern Alps (Semmering-Wechsel Nappe System).

Fig. 1. Overview maps (a) showing the paleogeographic origin of the main tectonic units of the Alps and (b) the tectonic subdivision of the Austroalpine Unit based on Schmid et al. (2004). The study area is marked with a dashed rectangle. Abbreviations: AC: Amering Complex, BM: Bösenstein Massif, BN: Bundschuh Nappe, DZ: Drauzug, FA: Fischbacher Alps, Gl: Gleinalm, GP: Graz Paleozoic, GWZ: Greywacke Zone, GN: Gurktal Nappe, Kor: Koralpe, NCA: Northern Calcareous Alps, Ötz: Ötztal Nappe, Rf: Rennfeld, Sa: Saualpe, Sc: Schladming Nappe, Sil: Silvretta Nappe, SN: Seckau Nappe, SpC: Speik Complex, TW: Tauern Window, Wz: Wölz Nappe.

13 The overlying Upper Austroalpine Subunit represents a nappe pile, which mainly formed during the Eo-Alpine event (as term for Early Cretaceous to early Late Cretaceous tectonics). It is built up, coming from the north, by Mesozoic sedimentary sequences of the Northern Calcareous Alps (comprising the Bajuvaric, Tirolic-Noric and Juvavic nappe systems), Paleozoic metasediments and metavolcanics of the Greywacke Zone and crystalline basement units that are mostly covered by Upper Carboniferous-Permian to Mesozoic metasediments. The tectonically lowermost element of the crystalline basement units is the Silvretta-Seckau Nappe System, which shows a greenschist to epidote-amphibolite facies Eo-Alpine metamorphic imprint. It is overlain by the Koralpe-Wölz Nappe System which experienced a Permian metamorphic imprint and an Eo-Alpine metamorphic overprint reaching upper- greenschist, amphibolite or eclogite facies conditions. The Ötztal-Bundschuh Nappe System above shows lithological similarities to the Silvretta-Seckau Nappe System and the overlying Drauzug-Gurktal Nappe System, which additionally contains larger portions of Paleozoic metasediments holds the topmost tectonic position. The latter two nappe systems show an upright metamorphic zoning reaching epidote-amphibolite facies conditions in the structurally lowermost parts (Schuster et al., 2004). The Seckau Nappe as part of the Silvretta-Seckau Nappe System covers a triangle shaped area extending in the northwest from the Bösenstein Massif and Seckauer Tauern (part of Niedere Tauern) to the Fischbacher Alps in the east over nearly 100 km (Pfingstl et al., 2015). Towards the south it extends to the northern part of the Koralpe ridge. The northwestern part in the Bösenstein Massif and Seckauer Tauern is built up by the Seckau Complex which is dominated by paragneisses (partly migmatitic), (garnet-) micaschists and minor amphibolites and quartzites. Therein different metagranitoids, derived from quartz monzodiorites, granodiorites as well as granites and some pegmatites intruded (Fig. 2). The well-foliated paragneisses and the partly deformed metagranitoids generally strike NE - SW with dip angles from 20 to 75° towards northeast. The southeastern part extending from the Koralpe ridge towards the Gleinalm ridge and the Fischbacher Alps is more rich in paragneisses and denoted with different terms (e.g., Amering Series; Becker, 1979) but the boundary to Seckau Complex is not well constrained. The Speik Complex, mainly composed of amphibolites and serpentinites with a few eclogites, is situated in a hanging-wall position. According to Neubauer and Frisch (1993) it represents a pre-Silurian ophiolite.

Fig. 2. Simplified geological map of the Seckau Complex north (-east) of the Mur valley (compiled from Metz, 1967; Moser, 2015) showing the locations of samples taken for U-Pb zircon age dating. Dikes intruded within paragneisses and metagranitoids are marked with a highlighted star. Abbreviations: Ghpt: Geierhaupt, Gr.Grst: Grosser Griessstein, Pltz: Pletzen, Rk: Rosenkogel, SZ: Seckauer Zinken.

These basement units are transgressively overlain by Permian to Lower Triassic clastic metasediments summarized as Rannach Formation (Flügel and Neubauer, 1984, after "Rannachserie" from Metz, 1967). The contact between the Rannach Formation and the basement of the Seckau Complex is strongly overprinted by the formation of distinct shear zones, indicating extensional fabrics that are characterized by conjugate sets of top-to-the WNW and top-to-the-ESE sense of shear. These shear zones are related to the Late Cretaceous exhumation of the Seckau Complex (Pfingstl et al., 2015). The metamorphic conditions in the Seckau Nappe show a slight increase of metamorphic grade towards the south. In the northwest the transgressive Permian to Lower Triassic metasediments are characterized by the assemblage muscovite + chloritioid + chlorite + quartz, indicating upper greenschist facies conditions (520°C at 0.9 Gpa) (Faryad and Hoinkes, 2001), whereas in the south amphibolite facies conditions had been reached (550-600°C at 0.9-1.0 GPa) (Faryad and Hoinkes, 2003).

15 2.1 Published geochronological data from the Seckau Nappe

Based on Rb-Sr whole rock (~ 350 Ma), white mica ages (~ 330 Ma) and the low-grade Eo- Alpine metamorphic overprint of the overlying Permian to Mesozoic metasediments, the granitoids and the metamorphic imprint of the Seckau Nappe were generally thought to be related to the Variscan orogenic event (Scharbert, 1981; Schermaier et al., 1997). In particular, Rb-Sr muscovite ages from the area around Bodenhütte (eastern central Seckauer Tauern) yielded Variscan ages of about 330 Ma (Scharbert, 1981). Another Rb-Sr whole rock age of 432 ± 16 Ma from an orthogneiss argues for a pre-Variscan, possibly Ordovician intrusion age (Scharbert, 1981). A regression line calculated from whole rock analyses of granites, granodiortites and quartz monzodiorites yielded an age value of 517 ± 16 Ma with an initial 87Sr/86Sr ratio of 0.7047 ± 0.0016 (Pfingstl et al., 2015). U-Pb zircon data from the Seckau Nappe in the Gleinalm-Rennfeld region indicate a two- stage pre-Alpine magmatic/metamorphic evolution. During a first magmatic/metamorphic event in the Ordovician and Silurian the paragneisses and amphibolites in this area were intruded by tonalities as well as diorites and affected by local migmatization. The second anatectic event occurred between 360 and 350 Ma and produced trondhjemites by partial melting of the amphibolites. This event can be related to the intrusion of the Rennfeld tonalites between 367 and 353 Ma (Neubauer et al., 2003). Plagioclase-quartz-gneisses from the core of the Gleinalm yield a Rb-Sr whole rock isochron of 518 ± 50 Ma (Frank et al., 1976) and an U-Pb zircon age of > 500 Ma indicating magma formation documented by Haiss (1990). For ultramafic rocks of the Speik Complex a Sm-Nd errorchron of 745 ±44 Ma, an isochron age of 554 ± 37 Ma and a Re-Os errorchron age of 550 ± 17 Ma were described by Melcher and Meisel (2004). Additionally, an 40Ar/39Ar amphibole age of about 390 Ma was measured on a retrogressed eclogite from the Bösenstein Massif (Faryad et al., 2002). Timing of the peak of Eo-Alpine metamorphism in the Seckau Nappe is not constrained with age data and also for the cooling through greenschist facies conditions the data are scarce. Three Rb-Sr biotite ages are in the range of 70 to 77 Ma (Scharbert, 1981) and Rb-Sr biotite- whole rock ages determined by Pfingstl et al. (2015) range from 86 to 76 Ma. These Rb-Sr biotite ages are interpreted to date cooling below 300 ± 50°C. Apatite from metagranitoid samples from the summit and valley areas of the Seckau Tauern yielded fission track ages of approximately 60 Ma and 44 Ma, respectively (Hejl, 1997). These fission track data, including confined track length distributions reflecting the thermal

16 history, indicate that after fast cooling during Late Cretaceous times the Seckau Nappe stayed at nearly constant temperature in the order of 80°C until the late Oligocene, followed by a phase of faster cooling and a phase of slow cooling during Miocene times. The fission track data exclude any post-Cretaceous metamorphic overprint within the Seckau Nappe. During the whole Cenozoic the highest portions of the Seckau Nappe therefore resided at a depth of less than 3000 meters at temperatures much cooler than the conditions of anchimetamorphism (Hejl, 1997).

2.2 Published geochronological data from surrounding units

In the Silvretta Nappe, representing the westernmost part of the Silvretta Nappe System, so- called "Older Orthogneisses" and "Younger Orthogneisses" are distinguished. The "Older Orthogneisses" represent an inhomogeneous group comprising ultramafic, mafic, intermediate and felsic rocks with mainly calc-alkaline affinity, which are Neoproterozoic to early Cambrian in age. Different types of orthogneisses, including metadiorite, metatonalite, flasergabbro and garnet-hornblende-plagioclase gneiss yielded zircon ages in the range of 609 to 526 Ma (Müller et al., 1995, 1996; Poller, 1997; Schaltegger et al., 1997). The crystallization age of 473 ± 5 Ma for a pegmatoid gabbro (Schaltegger et al., 1997) agrees well with fine-grained metagabbros displaying an age of 467 ± 14 Ma (Poller, 1997). The "Younger Orthogneisses" (also known as "Flüela-Granite association") comprise mainly granitic rocks showing a more alkaline geochemical signature (Liebetrau, 1996; Maggetti and Flisch, 1993; Müller et al., 1995, 1996; Streckeisen, 1928). They are Ordovician to Silurian in age and based on U-Pb zircon ages in the range between 450 and 420 Ma (Flisch, 1987; Grauert, 1969; Liebetrau et al., 1996). It shall be mentioned that these zircon ages from the Silvretta Nappe were analyzed using different methods including U-Pb single zircon measurements and 207Pb/206Pb zircon evaporation ages. Based on similar geochronological ages, comparable rock types, structures and an analogous polymetamorphic history the basement of the Ötztal-Bundschuh Nappe System of the Austroalpine Unit (Flisch, 1987) as well as the Strona-Ceneri Zone of the Southalpine Unit (Zurbriggen et al., 1997) experienced a similar pre-Variscan evolution as the Silvretta Nappe. E.g., metagabbros from the Ötztal Nappe yielded Sm-Nd mineral isochrone ages in the range between 530 and 520 Ma (Miller and Thöni, 1995) and the Winnebach migmatite gives a conventional U-Pb zircon age of 490 ± 9 Ma (Klötzli-Chowanetz et al., 1997).

17 In the Austroalpine basement units to the south of the Tauern Window (being part of several nappe systems) two major pre-Variscan magmatic evolution lines are identified based on isotopic data and Pb-Pb single zircon ages. The older subduction-related evolution resulted in Ediacarian (590 Ma) protoliths of eclogitic amphibolites, Cambro-Ordovician (550-530 Ma) hornblende-plagioclase gneisses and Ordovician (480-450 Ma) orthogneisses as well as meta- porphyroids. The younger evolution line is characterized by Silurian tholeiitic and alkaline within-plate basalt-type rocks yielding ages around 430 Ma (Schulz et al., 2004). To the north the Seckau Nappe is tectonically overlain by the Greywacke Zone, which contains slices of paragneisses and amphibolites indicating a tectonothermal event between 520 and 495 Ma (Neubauer et al., 2002). The Tatric and Veporic units of the Western Carpathians represent eastern parts of the AlCaPa Megaunit. From these units a late Cambrian to Early Ordovician (525-470 Ma) and an Ordovican (480-440 Ma) magmatic event is reported. Cores of zircons mainly yield Ediacarian U-Pb zircon ages (638-549 Ma) but also older cores were found (3.4-2.0 Ga) (Putis et al., 2008). Further, there are Cambrian zircon ages related to a metamorphic overprint. An early Variscan magmatic event determined by Late Devonian to early Carboniferous zircon ages in the range of 370-340 Ma (Gaab et al., 2005; Putis et al., 2008) is common within these units.

3 Methods 3.1 X-ray fluorescence (XRF) Geochemical whole-rock analyses for major and trace elements were obtained by a Bruker Pioneer S4 X-ray fluorescence (XRF) spectrometer at the Institute of Earth Sciences, NAWI Graz Geocenter, University of Graz, Austria. Samples were prepared as fused glass discs using 1 g of the dried powdered sample material and 7 g of lithium tetraborate (Fluxana). The glass discs were fused at ~1300 °C with a semi-automatic VAA-2 fusion machine from HD Electronic. Loss on ignition (LOI) was determined by heating the powdered sample material for more than 1 h at 1025 °C. For the calibration of the Bruker Pioneer S4 about 80 international reference materials were used. Routinely the international reference standards AC-E, MA-N and GSP-2 were measured, among others, as external standards to ensure analytic accuracy.

18 3.2 LA-MC-ICP-MS Zircon 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios were determined by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) techniques (Košler, 2008) using a New Wave 193 nm ArF excimer laser coupled to a Nu Plasma II mass spectrometer at the NAWI Graz Central Lab for Water, Minerals and Rocks, NAWI Graz Geocenter, Austria. Zircons for U-Pb age dating were separated using standard techniques. For LA-MC-ICP-MS analysis hand-picked zircons were embedded in epoxy resin, grinded and polished to the half of their thickness and cleaned in 2% HNO3 for a few minutes. Combined back-scattered electron (BSE) and cathodoluminescence (CL) images were taken from all zircons to document their internal structure. Zircons were ablated with a repetition rate of 7-10 Hz, a scan speed of 10 µm s-1 and a maximum laser energy of 5.0 to 5.5 J cm-2. The laser beam was constantly focused on the sample surface and the length of the scan line varied between 50 and 150 µm with a diameter of 10-15 µm. Helium 5.0 (0.7 l min-1) was used as carrier gas and 0.75 l min-1 argon was admixed upstream the ICP plasma torch. 238U and 232Th were measured with the Faraday cups, Pb and Hg masses were detected in the discrete ion counting channels, but data were not corrected for Hg or common Pb because measured intensities were extremely low and corrections did not have a noticeable influence on the ages. For the calculation of the 238U/235U ratio the commonly recommended value of 137.88 was used (Steiger and Jäger, 1977). The measurements were performed for a maximum of 130 s, including 30-35 s background signals, prior the total zircon ablation time of around 100 s. An integration time of 1 s was taken for all analyses. Plešovice zircon (Sláma et al., 2008) was used as primary standard with an internal repeatability of better than 1%. The external reproducibility of the secondary standards 91,500 (Wiedenbeck et al., 1995) and/or M257 (Nasdala et al., 2008) and/or Gj-1 (Jackson et al., 2004) and/or Mud Tank (Black and Gulson, 1978) was within 3%. Isotopic ratios were calculated using the MS Excel spreadsheet LamTool U-Th-Pb version VI (Košler et al., 2008; Košler and Klötzli, unpublished). In order to avoid potential surface contamination the signals from the first few laser passes were rejected and untypical Pb- peaks, from Pb-rich inclusions, were removed in the LamTool spreadsheet. To control the fractionation effect the same signal length was taken for each zircon measurement per sample. Final age calculations and concordia diagrams were made using Isoplot 3.75

19 (Ludwig, 2012). After the exclusion of outliers, concordia ages indicate the weighted average of all analyses. All plotted concordia ellipses are quoted at the 2σ level.

3.3 Sm-Nd Garnets of sample MM3 were separated by standard methods of crushing, grinding, sieving, magnetic separation and were hand-picked from sieve fractions of 0.2–0.3 mm under the binocular microscope at the University of Graz. Chemical preparation was performed at the Geological Survey of Austria in Vienna and at the Department of Lithospheric Research at the University of Vienna, following the procedure described by Sölva et al. (2005). Before decomposition garnets were cleaned in distilled water and acetone and leached in 6 N HCl at 100 °C for a few hours. For dissolution about 150 mg of whole rock powder and 40 mg (grt2) and 65 mg (grt1) for garnet were used. Element concentrations were determined by isotope dilution using a mixed 147Sm/150Nd spike. Total procedural blanks are ≤300 pg for Nd and Sm. All Sm and Nd ratios were analyzed using Re double filaments with a ThermoFinnigan® Triton TI TIMS at the Department of Lithospheric Research at the University of Vienna. The La Jolla standard yielded 143Nd/144Nd = 0.511846 ± 1 (n = 20, 2σm) on the Triton TI. Errors of 1% were determined for the 147Sm/144Nd ratios based on interactive sample analysis and spike recalibration. Ages were calculated with the software ISOPLOT/Ex (Ludwig, 2003) using the Sm-decay constant of 6.54 × 10−12 a−1.

20 4 Lithological units 4.1 Paragneisses and micaschists (Glaneck Metamorphic Suite) Metapsammites and metapelites, mainly paragneisses (Fig. 3a, b) and (garnet-) micaschists (later classified as Glaneck Metamorphic Suite), constitute the crystalline basement into which large volumes of granitoids – now observed as metagranitoids (Fig. 3c-f) - intruded. The paragneisses generally strike from north-west to south-east and dip to the northeast with a dip angle varying between 20 and 75 degrees. The strongly foliated paragneisses are dominated by fine-grained quartz and feldspar and up to 1.5 mm thick quartz layers, as well as coarser grained quartz clasts and blocky quartz. Further they contain biotite, garnet, opaque phases as well as minor white mica, chlorite and partly epidote. Garnet is tiny (~0.1 mm) but frequent and often idiomorphic. The paragneisses often show indications of migmatization and are transected by granitic dikes. Four paragneiss samples, one from the Ingeringgraben close to Praterbrücke (MM4), two located in the Alpinpark Steinmühle, a crag 2 km west of Seckau (MM6 and MM7b) and one exposure several tens of meters below the top of the Rosenkogel (MM16) were investigated (Fig. 2, Table 1). Sample MM7b, exposed in the western part of the Alpinpark Steinmühle, represents a migmatitic paragneiss.

4.2 Metagranitoids (Hochreichart Plutonic Suite, Hintertal Plutonic Suite)

Slightly to strongly deformed metagranitoids (Fig. 3c-f) with SiO2 compositions ranging from intermediate to acidic form the major part of the Seckau Complex (Schermaier et al., 1997). Thirty-one samples of these individual intrusion bodies, including pegmatitic-, leucogranitic- and granitic dikes, were systematically sampled from all over the study area (Fig. 2, Table 1). The different intrusives, mainly metagranites and metagranodiorites, and one monzodiorite gneiss, can be generally subdivided into two groups based on petrographic observations. The first group (in the following chapter summarized as Hochreichart Plutonic Suite, Fig. 3c, d) comprises predominantly strongly foliated two-mica granite gneisses e.g., SP79, MM32, MM33 and MM10 as well as white mica-dominated granite gneisses with only minor amounts of biotite, e.g., MM27, SP79. This group comprises also discordantly intruded dikes Fig. 3c). The two-mica gneisses generally consist of quartz, K-feldspar, partly saussuritisized plagioclase, biotite with orange to brownish pleochroism, white mica and sporadic secondary chlorite (Fig. 4a, b).

21 Table 1 Sample list and localities of rocks analyzed for U-Pb geochronology. Abbreviations: bt: biotite, hbl: hornblende, qz: quartz, wm: white mica.

Sample Lithology Locality Latitute Longitude MM4 Paragneiss Ingeringgraben 47°17'11.7" N 14°40'40.7" E MM8 Pegmatitic dike (discordant dike) Alpinpark Steinmühle 47°17'01.4" N 14°45'31.6" E MM6 Paragneiss Alpinpark Steinmühle 47°17'03.0" N 14°45'34.7" E MM16 Paragneiss Rosenkogel 47°17'41.8" N 14°33'09.7" E MM7b Paragneiss (migmatitic) Alpinpark Steinmühle 47°17'01.4" N 14°45'31.6" E SP79 Granite gneiss Hochreichart 47°21'48.7" N 14°40'53.9" E MM32 Granite gneiss Kerschkern 47°23'18.5" N 14°37'08.1" E MM2 Granite gneiss Vorwitzgraben 47°17'59.5" N 14°41'49.1" E MM27 Granite gneiss Mitterkogel 47°21'57.3" N 14°47'43.5" E SP78 Wm-granite gneiss Antonikreuz 47°21'59.8" N 14°47'34.6" E MM13 Granite Rosenkogel 47°17'43.0" N 14°33'08.5" E MM1 Granite gneiss Vorwitzgraben 47°17'59.5" N 14°41'49.1" E MM28 Granodiorite gneiss Alpsteigerhütte, Mitterkogel 47°22'12.6" N 14°48'19.8" E MM66 Bt-granodiorite gneiss Herrschaftskranz 47°19'35.2" N 14°40'20.0" E MM33 Granite gneiss Stellmauer 47°23'35.4" N 14°36'33.6" E MM10 Granite Hochreichart 47°21'42.0" N 14°41'08.0" E MM57 Granite Gaalgraben 47°16'27.0" N 14°37'27.0" E MM5 Pegmatitic dike (discordant dike) Alpinpark Steinmühle 47°17'03.0" N 14°45'34.7" E MM3 Leucogranitic dike (discordant dike) Vorwitzgraben 47°17'59.5" N 14°41'49.1" E SP54 Granodiorite gneiss Untere Bodenhütte 47°21'23.0" N 14°47'12.1" E MM54 Granite (discordant dike) Rosenkogel 47°17'39.0" N 14°33'10.0" E MM14 Granite gneiss (concordant dike) Rosenkogel 47°17'41.8" N 14°33'09.7" E MM20 Granite gneiss Triebener Törl/ Knaudachtörl 47°23'18.4" N 14°32'04.8" E MM22 Granite Knaudachtörl 47°22'56.8" N 14°32'32.3" E MM15 Granite gneiss (concordant dike) Rosenkogel 47°17'41.8" N 14°33'09.7" E MM12 Hbl-bt qz monzodiorite gneiss Roßbachgraben, Gaal 47°16'56.4" N 14°33'40.7" E SP58 Granodiorite Schmähtaschen 47°20'56.4" N 14°48'32.6" E MM17 Granite gneiss Schleifgraben, St Johann 47°21'39.2" N 14°29'41.1" E MM19 Bt-granodiorite gneiss Schleifgraben, St Johann 47°21'44.9" N 14°29'26.2" E MM18 Bt-granite Schleifgraben, St Johann 47°21'43.5" N 14°29'25.0" E MM65 Granodiorite Tiertal 47°20'56.8" N 14° 38' 0.1" E MM55 Bt-granodiorite Hintertal, Lanzalm 47°16'52.0" N 14°35'22.0" E SP81 Bt-granodiorite gneiss Sundlsee 47°19'34.3" N 14°37'39.4" E MM70 Granite gneiss Sonntagkogel 47°20'55.7" N 14°36'08.7" E MM56 Granodiorite Hintertal, Tuscherriegel 47°17'09.0" N 14°35'24.0" E MM23 Bt-granite gneiss Gamskogel 47°21'56.7" N 14°32'54.8" E

Fig. 3. (a, b) Paragneisses of the Glaneck Metamorphic Suite, (a) Ingeringgraben and (b) Alpinpark Steinmühle. (c, d) Granite gneiss (MM1, MM2) and granite (MM13) of the Hochreichart Plutonic Suite including a leucogranitic dike (MM3) of the Hintertal Plutonic Suite. (e, f) Granite gneiss (MM20) and granodiorite (MM65) of the Hintertal Plutonic Suite.

The second group (in the following chapter classified as Hintertal Plutonic Suite) generally shows similar textures and mineral assemblages except for a fine-grained concordant granitic dike (MM14), which contains additional garnet of presumed metamorphic origin. The granitic dike, mentioned before, intruded the paragneisses at the Rosenkogel, but is also part

23 of the above mentioned Hintertal Plutonic Suite. This unit, however, generally contains intermediate rocks and metagranites dominated by biotite and minor white mica and accessory apatite as well as metamorphic epidote, zoisite and rarely staurolite. In the field this group is typically characterized by less deformation, blocky appearance and the absence of intruded dikes (Fig. 3e-f). Olive-green to brownish colored biotite as well as up to 1 mm large titanite crystals (see Fig. 4c, d) are common and a crucial criteria to characterize this suite.

Fig. 4. (a) Representative photomicrograph of granite gneiss MM2 from the Hochreichart Plutonic Suite, crossed polarizers. (b) Transmitted light photomicrograph of granite gneiss MM2, showing orange to brownish colored biotite. (c) Representative photomicrograph of granite gneiss MM17 of the Hintertal Plutonic Suite, crossed polarizers. (d) Transmitted light photomicrograph of granite gneiss MM17 showing olive-green colored biotite together with large titanite grains. Abbreviations: bt: biotite, ksf: K-feldspar, pl: plagioclase, qz: quartz, st: staurolite, ttn: titanit, wm: white mica.

24 5 Geochemistry of metagranitoids The results of chemical analyses of metagranitoids of the Seckau Complex are presented in Table 2. They were classified following the total alkali versus silica (TAS) plot for plutonitic rocks (Middlemost, 1994). The majority of the individual intrusives are granitic or granodioritic in their composition, only sample MM12 plots into the quartz monzodiorite field (Fig. 5a). All of these metagranitoids indicate calc-alkaline chemical composition with a predominant peraluminous character (i.e. molar proportions of Al2O3/ (CaO + Na2O + K2O) > 1) (Zen, 1988). The fractionation of Rb and Sr, a tool to identify petrogenetic relationships, was used to illustrate evolutionary trends within the metagranitoids (Clark and Černŷ, 1987).

Based on the binary discrimination of Rb/Sr vs. Al2O3 the metagranitoids of the Seckau Complex can be chemically subdivided into two groups (Fig. 5b). The first group, classified as Hochreichart Plutonic Suite, comprises highly fractionated metagranitoids with Rb/Sr > 0.45, whereas the second group, classified as Hintertal Plutonic Suite, yields a magmatic fractionation trend with Rb/Sr ratios in the range between 0.09 and 0.45.

Fig. 5. (a) TAS classifications diagram (total alkali vs. silica) for plutonitic rocks after Middlemost (1994) and (b) discrimination plot Rb/Sr vs. Al2O3 reflecting the chemical distinctions between metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. Blue colored triangles indicate metagranitoids of the Hochreichart Plutonic Suite. Those of the Hintertal Plutonic Suite are shown by green square symbols: Darker green squares symbols represent metagranitoids of the Pletzen Pluton, wheras those of the Griessstein Pluton are shown by light green square symbols and black crosses (granitic dikes). Abbreviations: D: Diorite, G: Gabbro, Mz: Monzonite.

25 Table 2 Chemical analyses of the metagranitoids from the Hochreichart Plutonic Suite (HrPS) and the Hintertal Plutonic Suite (HtPS), which is subdivided into the 1 Griessstein Pluton (GrstP) and the Pletzen Pluton (PltzP). Major oxides are given in weight percent and rubidium (Rb) and strontium (Sr) values in parts per million [ppm]. 2 Abbreviations: bt: biotite, hbl: hornblende, qz: quartz, wm: white mica; Lith. Unit: Lithodemic Unit. 3 4

Sample Lithology Lith. Unit Type SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Rb Sr Sum SP79 Granite gneiss HrPS S-type 71.24 0.391 14.00 2.85 0.047 0.72 0.95 3.46 4.03 0.152 123 82 99.14 MM32 Granite gneiss HrPS S-type 72.77 0.415 13.59 3.32 0.040 0.74 0.74 3.33 3.31 0.173 91 64 99.78 MM2 Granite gneiss HrPS S-type 71.20 0.475 16.22 3.33 0.051 0.97 2.05 3.64 3.50 0.154 121 171 102.19 MM27 Granite gneiss HrPS S-type 75.47 0.182 13.96 1.72 0.037 0.35 0.42 5.04 1.50 0.144 64 81 99.72 SP78 Wm-granite gneiss HrPS S-type 74.76 0.210 13.08 2.12 0.026 0.36 0.25 3.47 3.35 0.123 141 58 98.93 MM13 Granite HrPS S-type 77.84 0.247 12.28 1.28 0.016 0.35 0.49 4.93 1.56 0.046 32 69 99.56 MM1 Granite gneiss HrPS S-type 70.84 0.458 15.62 3.12 0.046 0.89 2.01 3.56 3.54 0.146 117 175 100.78 MM28 Granodiorite gneiss HrPS S-type 67.77 0.598 14.79 4.69 0.062 1.34 1.20 2.51 4.19 0.172 160 68 99.66 MM66 Bt-granodiorite gneiss HrPS S-type 68.46 0.558 15.04 4.16 0.048 1.80 0.71 2.50 4.25 0.141 178 67 99.07 MM33 Granite gneiss HrPS S-type 71.79 0.382 14.53 2.89 0.038 0.67 0.44 3.05 4.78 0.148 143 65 99.86 MM10 Granite HrPS S-type 72.55 0.404 13.83 3.31 0.055 0.90 0.60 3.59 3.32 0.155 100 81 99.95 MM57 Granite HrPS S-type 75.89 0.126 12.68 1.13 0.015 0.14 0.36 3.63 4.70 0.036 133 64 99.11 SP54 Granodiorite gneiss HtPS, GrstP S-type 68.45 0.498 15.25 3.27 0.055 1.04 2.03 4.06 2.73 0.215 94 308 98.95 MM54 Granite (discordant dike) HtPS, GrstP S-type 72.33 0.232 14.62 1.82 0.039 0.43 1.31 4.09 3.53 0.086 75 357 99.02 MM14 Granite gneiss (concordant dike) HtPS, GrstP S-type 72.21 0.416 15.42 1.93 0.030 0.68 1.45 5.74 0.97 0.096 36 195 99.65 MM20 Granite gneiss HtPS, GrstP S-type 73.24 0.247 14.40 1.71 0.043 0.48 1.19 3.98 3.85 0.080 112 264 99.89 MM22 Granite HtPS, GrstP S-type 73.24 0.242 13.90 1.93 0.058 0.62 1.30 3.50 4.15 0.097 102 337 99.71 MM15 Granite gneiss (concordant dike) HtPS, GrstP S-type 73.30 0.383 14.95 1.73 0.013 0.75 0.91 5.52 1.33 0.122 38 136 99.86 MM12 Hbl-bt qz monzodiorite gneiss HtPS, PltzP I-type 53.71 1.030 17.54 9.04 0.179 4.51 6.80 3.16 2.22 0.389 67 762 99.26 SP58 Granodiorite HtPS, PltzP I-type 63.58 0.840 16.19 5.04 0.074 1.38 3.65 3.41 2.95 0.378 74 455 99.05 MM17 Granite gneiss HtPS, PltzP I-type 68.41 0.625 15.06 3.53 0.070 1.41 1.75 3.82 3.95 0.180 104 416 99.76 MM19 Bt-granodiorite gneiss HtPS, PltzP I-type 64.21 0.681 16.90 4.39 0.096 1.84 3.05 3.85 3.30 0.245 96 558 99.62 MM18 Bt-granite HtPS, PltzP I-type 70.00 0.427 14.56 3.03 0.055 1.03 2.27 3.80 3.51 0.150 89 434 99.61 MM65 Granodiorite HtPS, PltzP I-type 62.73 0.719 16.77 4.73 0.079 1.86 4.22 4.13 2.56 0.269 74 733 98.73 MM55 Bt-granodiorite HtPS, PltzP I-type 63.08 0.703 16.94 4.53 0.074 1.80 4.30 4.27 2.47 0.258 69 756 98.99 SP81 Bt-granodiorite gneiss HtPS, PltzP I-type 63.96 0.701 16.41 4.34 0.072 1.66 3.66 4.15 2.57 0.246 84 640 99.03 MM70 Granite gneiss HtPS, PltzP I-type 71.02 0.300 14.44 2.26 0.050 0.79 1.70 3.78 3.85 0.114 94 365 99.10 MM56 Granodiorite HtPS, PltzP I-type 64.15 0.702 16.14 4.49 0.076 2.06 4.25 3.77 2.27 0.256 60 700 98.89 MM23 Bt-granite gneiss HtPS, PltzP I-type 67.73 0.532 15.61 3.34 0.063 1.29 2.67 4.19 2.98 0.183 79 534 99.43

Although samples SP54, MM20, MM22 as well as the granitic dikes MM54, MM14 and MM15 follow the trend of the Hintertal Plutonic Suite (shown in Figure 5b with light green square symbols and crosses, respectively), petrographic criteria, like the dominance of muscovite, orange to brownish-colored biotite, indicate a similarity with the Hochreichart Plutonic Suite. Thus, for an additional distinction of the Hintertal Plutonic Suite the classification scheme SiO2 vs. A/CNK (molar proportions of Al2O3/ (CaO + Na2O + K2O)) (Chappell and White 1974, 2001) was applied. The aforementioned samples (SP54, MM20, MM22, MM54, MM14, MM15) predominantly reflect S-type affinity and were classified as Griessstein Pluton, whereas the metagranitoids, reflecting I-type characteristics were classified as Pletzen Pluton. Petrographic and geochemical data described by Schermaier et al. (1997) also indicate a separate petrogenetic origin for their I-type granitoids and leucocratic granitoids.

6 Ub-Pb zircon ages Zircon grains from the paragneisses and from different metagranitoids of the Seckau Complex show a distinct appearance. Zircons separated from the paragneisses are typically well rounded and range in size between 80 and 200 µm. By inspection with a binocular microscope they mostly appear colorless comprising few cracks and inclusions of mainly quartz and apatite. In cathodoluminescence (CL) images (Fig. 6) zircons show rounded shapes with internal magmatic zoning and overgrowing rims. The cores are interpreted as detrital zircons whereas the rims are of metamorphic or co-magmatic origin. Zircons from metagranitoids are typically euhedral long-prismatic with a width to length ratio of 1:3 and 1:4. These zircons are generally colorless, some are slightly brownish. The grain sizes usually is 100 and 250 µm. Some zircons comprise apatite, quartz and feldspar inclusions. The zircon’s internal structure is dominated by oscillatory zoning, a common feature in zircons precipitated from melts (e.g., Corfu et al., 2003). External rims are rare but when present they are only a few micrometers thick and generally homogeneous. U-Pb zircon ages from paragneisses and individual metagranitoids fall into three age clusters; for summary see Table 3. The sample localities are shown in Fig. 2 and Table 1.

Fig. 6. Representative CL-images of zircon crystals of the Glaneck Metamorphic Suite, the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite including corresponding 206Pb/238U ages (± 2σ). The white colored tracks indicate the laser track used for U-Pb dating.

28 Table 3 Analytical data of U-Pb zircon dating for paragneisses of the Glaneck Metamorphic Suite (GlMS) and for metagranitoids of the Hochreichart Plutonic Suite (HrPS) and the Hintertal Plutonic Suite (HtPS). Abbreviations: bt: biotite, hbl: hornblende, qz: quartz, wm: white mica; Lith. Unit: Lithodemic Unit.

Sample Lithology Lith. Unit Age and 2σ error Locality MM4 Paragneiss GlMS 572 ± 7 Ma (n= 26) Ingeringgraben MM8 Pegmatitic dike (discordant dike) GlMS 562 ± 33 Ma (n=4) Alpinpark Steinmühle MM6 Paragneiss GlMS 560 ± 10 Ma (n= 29) Alpinpark Steinmühle MM16 Paragneiss GlMS 559 ± 11 Ma (n= 17) Rosenkogel MM7b Paragneiss (migmatitic) GlMS 505 ± 11 Ma (n= 26) Alpinpark Steinmühle SP79 Granite gneiss HrPS 508 ± 9 Ma (n= 39) Hochreichart MM32 Granite gneiss HrPS 502 ± 9 Ma (n= 34) Kerschkern MM2 Granite gneiss HrPS 502 ± 3 Ma (n= 24) Vorwitzgraben MM27 Granite gneiss HrPS 500 ± 8 Ma (n= 18) Mitterkogel SP78 Wm-granite gneiss HrPS 498 ± 7 Ma (n= 36) Antonikreuz MM13 Granite HrPS 497 ± 8 Ma (n=15) Rosenkogel MM1 Granite gneiss HrPS 497 ± 6 Ma (n= 21) Vorwitzgraben MM28 Granodiorite gneiss HrPS 493 ± 10 Ma (n= 17) Alpsteigerhütte, Mitterkogel MM66 Bt-granodiorite gneiss HrPS 489 ± 8 Ma (n= 32) Herrschaftskranz MM33 Granite gneiss HrPS 488 ± 11 Ma (n= 19) Stellmauer MM10 Granite HrPS 488 ± 9 Ma (n=21) Hochreichart MM57 Granite HrPS 486 ± 9 Ma (n= 20) Gaalgraben MM5 Pegmatitic dike (discordant dike) HtPS. GrstP 365 ± 11 Ma (n=12) Alpinpark Steinmühle MM3 Leucogranitic dike (discordant dike) HtPS. GrstP 361 ± 13 Ma (n=10) Vorwitzgraben SP54 Granodiorite gneiss HtPS. GrstP 358 ± 8 Ma (n= 23) Untere Bodenhütte MM54 Granite (discordant dike) HtPS. GrstP 349 ± 8 Ma (n= 23) Rosenkogel MM14 Granite gneiss (concordant dike) HtPS. GrstP 348 ± 14 Ma (n= 10) Rosenkogel MM20 Granite gneiss HtPS. GrstP 346 ± 10 Ma (n= 15) Triebener Törl/ Knaudachtörl MM22 Granite HtPS. GrstP 343 ± 7 Ma (n= 22) Knaudachtörl MM15 Granite gneiss (concordant dike) HtPS. GrstP 331 ± 10 Ma (n= 19) Rosenkogel MM12 Hbl-bt qz monzodiorite gneiss HtPS, PltzP 365 ± 8 Ma (n= 26) Roßbachgraben, Gaal SP58 Granodiorite HtPS, PltzP 360 ± 11 Ma (n= 15) Schmähtaschen MM17 Granite gneiss HtPS, PltzP 359 ± 8 Ma (n= 18) Schleifgraben, St Johann MM19 Bt-granodiorite gneiss HtPS, PltzP 356 ± 9 Ma (n= 15) Schleifgraben, St Johann MM18 Bt-granite HtPS, PltzP 352 ± 10 Ma (n= 22) Schleifgraben, St Johann MM65 Granodiorite HtPS, PltzP 352 ± 6 Ma (n= 33) Tiertal MM55 Bt-granodiorite HtPS, PltzP 350 ± 8 Ma (n= 29) Hintertal, Lanzalm SP81 Bt-granodiorite gneiss HtPS, PltzP 349 ± 8 Ma (n= 24) Sundlsee MM70 Granite gneiss HtPS, PltzP 349 ± 7 Ma (n= 23) Sonntagkogel MM56 Granodiorite HtPS, PltzP 346 ± 6 Ma (n= 31), Hintertal, Tuscherriegel MM23 Bt-granite gneiss HtPS, PltzP 343 ± 12 Ma (n= 10) Gamskogel

29 6.1 Precambrian paragneisses and migmatitic paragneisses (Glaneck Metamorphic Suite) Zircons from the paragneisses yield the oldest analyzed U-Pb ages within the study area (Figs. 7a-f, 8). A paragneiss sampled in the Ingeringgraben (MM4) provides a concordia age of 572 ± 7 Ma (n= 26; Fig. 7b) and the paragneisses from the Alpinpark Steinmühle (MM6) as well as the area around the top of Mount Rosenkogel (MM16) display slightly younger concordia ages of 560 ± 10 Ma (n= 29; Fig. 7d) and 559 ± 11 Ma (n= 17; Fig. 7e), respectively. Occasionally these paragneisses comprise older zircon grains indicating two concordia ages of 2075 ± 21 Ma (n= 20) and 2710 ± 50 Ma (n= 4), respectively (Fig. 7a). These Ediacarian, Paleoproterozoic and Neoarchean zircons have U/Th ratios of exceeding 0.1, recommending a magmatic origin (Rubatto, 2002). Few analyses of one pegmatite (MM8), discordantly crosscutting the paragneiss (located at Alpinpark Steinmühle) show a concordia age of 562 ± 33 Ma (n= 4; Fig. 7c). One migmatitic paragneiss (MM7b), located in the western part of the Alpinpark Steinmühle, yields a concordia age of 505 ± 11 (n= 26; Fig. 7f). A Kernel density plot (Vermeesch, 2012) of all concordant zircon ages measured in the paragneisses is displayed in Fig. 8.

Fig. 7. (a-f) U-Pb Concordia diagrams showing zircon age data from paragneisses of the Glaneck Metamorphic Suite and one intruded pegmatitic dike. Error ellipses are given at the 2σ level.

Fig. 8. Combined Kernel density estimation plot and histograms for zircons within the paragneisses of the Glaneck Metamorphic Suite.

6.2 Late Cambrian-Early Ordovician metagranitoids (Hochreichart Plutonic Suite) Felsic metagranitoids mainly situated in the central part of the Seckau Complex represent highly fractionated melts with a Rb/Sr ratio exceeding 0.45 and show S-type characteristics (Chappell and White 1974, 2001). Zircons yield late Cambrian to Early Ordovician ages in the range between 508 ± 9 Ma and 486 ± 9 Ma (Fig. 9a-l). These metagranitoids are defined as Hochreichart Plutonic Suite. Three granite gneisses, one located close to the top of Mount Hochreichart (SP79), a second exposed on the ridge between Mount Zwölferköpfl and Mount Kerschkern (MM32), and another situated in the valley Vorwitzgraben (MM2) yield concordant U-Pb zircon ages of 508 ± 9 Ma (n= 39; Fig. 9a), 502 ± 9 Ma (n= 34; Fig. 9b) and 502 ± 3 Ma (n= 24; Fig. 9c), respectively. The granite gneiss sample, a few tens of meters below the top of Mount Mitterkogel (MM27), displays a concordia age of 500 ± 8 Ma (n=18; Fig. 9d) and the white mica-bearing granite gneiss exposed in the area of the Antonikreuz (SP78) yields a concordia age of 498 ± 7 Ma (n= 36; Fig. 9e). A granite sampled from the intrusive body a few meters below the top of Mount Rosenkogel (MM13) displays a similar concordia age of 497 ± 8 Ma (n= 15; Fig. 9f), similar to a granite gneiss from the Vorwitzgraben (namely MM1: 497 ± 6 Ma; n= 21; Fig. 9g). A granodiorite gneiss located close to Alpsteigerhütte, some hundreds meters below the summit of Mount Mitterkogel (MM28), provides a slightly younger concordia age of 493 ± 10 Ma (n= 17; Fig. 9h); a biotite granodiorite gneiss situated in the area around Herrschaftskranz (MM66) shows a concordia age of 489 ± 8 Ma (n= 32; Fig. 9i) and a granite gneiss exposed at the Stellmauer (MM33) yields a concordia age of 488 ± 11 Ma (n= 19; Fig. 9j). Another granite gneiss, located a few tens of meters below the top of

31 Mount Hochreichart (MM10), displays a concordia age of 488 ± 9 Ma (n=21; Fig. 9k). The youngest Cambrian to Early Ordovician granite is situated in the Gaalgraben valley (MM57) with a Concordia age of 486 ± 9 Ma (n= 20; Fig. 9l).

Fig. 9. (a-l) Concordia diagrams of U-Pb zircon age data from late Cambrian to Early Ordovician metagranitoids of the Hochreichart Plutonic Suite. Error ellipses are given at the 2σ level.

32 6.3 Late Devonian-early Carboniferous metagranitoids (Hintertal Plutonic Suite) Intermediate to acidic rocks, indicating a magmatic fractionation trend seen in variable Rb/Sr ratios, cover an age span between 365 ± 11 to 343 ± 12 Ma (Hintertal Plutonic Suite) (Figs. 10a-h, 11a-k). The only exception is a granitic dike (MM15) with a concordant U-Pb zircon age of 331 ± 10 Ma (n= 19; Fig. 10h), which intruded concordantly into the paragneisses of the Rosenkogel area. One pegmatitic dike (MM5), one leucogranitic dike (MM3) and two granitic dikes (MM54 and MM14) also yield U-Pb zircon ages within this cluster. These Late Devonian to early Carboniferous metagranitoids of the Hintertal Plutonic Suite can be subdivided, based on petrographic observations, into the Griessstein Pluton, indicating mainly S-type characteristics, and the Pletzen Pluton yielding generally I-type affinity (Chappell and White, 1974, 2001).

6.3.1 Metagranitoids with S-type signatures (Griessstein Pluton), including pegmatitic-, leucogranitic- and granitic dikes A granodiorite gneiss (SP54) sampled near to the Untere Bodenhütte shows a concordia age of 358 ± 8 Ma (n= 23; Fig. 10a). From granites exposed in the area around Knaudachtörl (MM20 and MM22) concordia ages of 346 ±10 (n= 15; Fig. 10b) and 343 ± 7 Ma (n= 22; Fig. 10c) were determined. A pegmatitic dike (MM5), intruded into the paragneisses located at the Alpinpark Steinmühle (MM6), yields a concordia age of 365 ± 11 Ma (n= 12; Fig. 10d) and a leucogranitic dike (MM3) that crosscuts the granite gneisses (MM2 and MM1) outcropping in the Vorwitzgraben displays a concordia age of 361 ± 10 Ma (n=10; Fig. 10e). The latter is in good accordance with a Sm-Nd garnet age of 374 ± 7 Ma (MM3; Figs. 12a-c, 13, Table 4). Garnets from this leucogranitic dike are hypidiomorphic and up to 1 mm in diameter. In the back scattered electron (BSE) image (Fig. 12c) they show a relatively homogeneous core and some darker overgrowth. The core is characterized by high FeO and MnO contents indicating a magmatic origin (Fig. 12b, c). Towards the core margins slight growth zoning with increasing MnO as well as decreasing FeO and MgO at constant CaO (not visible in the back scattered electron image) can be observed. The rims showing lower FeO, MnO and MgO but higher CaO contents are interpreted as metamorphic overgrowth. As the rims are less than 100 µm in thickness, irregularly shaped and less magnetic, it was possible to exclusively select fragments from the magmatic garnet’s core for dating. The zoning of the garnets is very similar to that from meta-pegmatites within the Ötztal Nappe described by Thöni and Miller (2004). 33 Three fine-grained granitic dikes, one discordant (MM54) and two concordant (MM14 and MM15), intruded into the paragneisses at Rosenkogel (MM16), yield concordia ages of 349 ± 8 Ma (n= 23; Fig. 10f), 348 ± 14 Ma (n= 10; Fig. 10g) and 331 ± 10 Ma (n= 19; Fig. 10h), respectively. Some of the Griessstein Pluton metagranitoids additionally comprise zircons with older cores reflecting predominantly late Cambrian to Early Ordovician ages (e.g., MM5: 517 ± 28 Ma (n= 6), MM14: 503 ± 55 Ma (n= 3), SP54: 505 ± 8 Ma (n=15), MM 3: 499 ± 23 Ma (n= 6), MM20: 495 ± 47 Ma (n= 3), MM54: 476 ± 15 Ma (n= 10)).

Fig. 10. Concordia plots of U-Pb zircon ages (a-c) from Variscan S-types metagranitoids and (d-h) from several dikes of the Hintertal Plutonic Suite. Error ellipses are given at the 2σ level.

34 6.3.2 Metagranitoids with I-type affinity (Pletzen Pluton) For the most basic rock, a hornblende-biotite quartz monzodiorite gneiss, sampled in the Roßbachgraben (MM12), a concordia age of 365 ± 8 Ma (n= 26; Fig. 11a), representing the oldest crystallization age of this suite, was determined. From the area around Schmähtaschen (SP58) a granodiorite yields a concordia age of 360 ± 11 Ma (n= 15; Fig. 11b) and the granodioritic to granitic rocks situated in the Schleifgraben (MM17, MM19, MM18) give concordia ages of 359 ± 8 Ma (n= 18; Fig. 11c), 356 ± 9 Ma (n= 15; Fig. 11d) and 352 ± 10 (n= 22; Fig. 11e), respectively. Two granodiorites, one from the area of the Tiertal (MM65), another exposed in the Hintertal, close to the Lanzalm (MM55), show concordia ages of 352 ± 6 Ma (n= 33; Fig. 11f) and 350 ± 8 Ma (n= 29; Fig. 11g), respectively. A granodiorite gneiss, occuring in the area of the Sundlsee (SP81), gives a concordia age of 349 ± 8 Ma (n= 24; Fig. 11h). A granite gneiss close to the top of Mount Sonntagkogel (MM70) indicates a concordia age of 349 ± 7 Ma (n= 23; Fig. 11i); the granodiorite situated in the area Hintertal, Tuscherriegel (MM56) yields a concordia age of 346 ± 6 Ma (n= 31; Fig. 11j) and the granite gneiss, located in the area around Mount Gamskogel, (MM23) shows a concordia age of 343 ± 12 Ma (n= 10; Fig. 11k). Only few zircons of the Pletzen Pluton metagranitoids contain older cores reflecting late Cambrian to Early Ordovician ages (e.g., SP58: 492 ± 20 Ma (n= 6), MM18: 488 ± 17 Ma (n= 8), SP81: 483 ± 19 Ma (n=5), MM65: 475 ± 15 Ma (n= 6), MM23: 465 ± 24 Ma (n= 4)). The late Cambrian to Early Ordovician aged zircon cores from the metagranitoids of the Hintertal Plutonic Suite suggest assimilation processes with material of the Hochreichart Suite.

Fig. 11. (a-k) Concordia plots of U-Pb zircon ages from Variscan I-types metagranitoids of the Hintertal Plutonic. Error ellipses are given at the 2σ level.

Fig. 12. Representative images of a magmatic garnet within the leucogranitic dike MM3: (a) crossed polarizers, (b) parallel polarizers and (c) back scattered electron (BSE) picture. Abbreviations: grt: garnet; pl: plagioclase; qz: quartz.

Fig. 13. Samarium-Neodymium isochron diagram for sample MM3 representing a leucogranitic dike of the Griessstein Pluton (Hintertal Plutonic Suite) located in the Vorwitzgraben. Data points for whole rock (WR) and two garnet fractions (grt1 and grt2) are given as error ellipses at the 2σ level.

Table 4 Samarium-Neodymium isotopic data for sample MM3 representing a leucogranitic dike of the Griessstein Pluton (Hintertal Plutonic Suite) located in the Vorwitzgraben.

Sample Material Sm [ppm] Nd [ppm] 147Sm/144Nd 143Nd/144Nd 2σ(m) Age [Ma] MM3 WR 2.944 0.761 0.1563 0.512446 0.000003 MM3 grt1 1.891 1.137 0.3634 0.512956 0.000009 MM3 grt2 1.838 1.167 0.3838 0.513001 0.000008 374 ± 7

37 7 Lithostratigraphic subdivision of the Seckau Complex According to the petrographic, geochemical and geochronological data presented above a subdivision of the Seckau Complex into three lithostratigraphic/lithodemic sub-units (suites) is proposed: (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite. The Hintertal Plutonic Suite is further subdivided into two lithodemes (a) Griessstein Pluton (b) Pletzen Pluton (Fig. 14). The proposed classification scheme follows the North American Commission of Stratigraphic Nomenclature (2005) that defined a lithodeme as a body of intrusive, pervasively deformed or highly metamorphosed rock, generally non-tabular and lacking primary depositional structures, that is characterized by lithic homogeneity. A suite is considered to be next higher in rank, comprising two or more associated lithodemes of the same class (metamorphic or magmatic). It has to be mentioned that there are no specific rules on how to define a lithodemic unit until now. Although geochronological ages and geochemical data were used as additional information for this subdivision, the distinction is predominantly based on field observations including the modal rock compositions, texture and structure.

7.1 Glaneck Metamorphic Suite The Glaneck Metamorphic Suite comprises predominantly fine-grained, biotite dominated and garnet bearing paragneisses which partly show indications of migmatization, (garnet-) micaschists and minor amphibolites. Zircons of the paragneisses are well rounded, 80 to 200 µm in size and colorless. Their cores indicate a detrital origin whereas some rims may have formed during a metamorphic overprint. U-Pb zircon ages of the cores cluster in the Neoarchean (~2.7 Ma), Paleoproterozoic (~2.0 Ga) and Ediacarian (572 ± 7 Ma to 559 ± 11 Ma). Zircons from a migmatitic paragneiss crystallized in the late Cambrian (505 ± 11 Ma). Rocks of the Glaneck Metamorphic Suite occur predominantly in the marginal areas of the Seckau Complex (Fig. 14).

7.2 Hochreichart Plutonic Suite The Hochreichart Plutonic Suite is composed of different types of granites and granodiorites as well as granite- and granodiorite gneisses containing more than 67 wt% SiO2. These rocks crystallized from highly fractionated silica melts (Rb/Sr > 0.45) without a significant fractionation trend indicating a S-type affinity (Chappell and White, 1974, 2001). U-Pb zircon ages from this suite are late Cambrian to Early Ordovician (508 ± 9 Ma and 486 ± 9

38 Ma). The Hochreichart Plutonic Suite is predominantly found in the central part of the Seckau Complex.

7.3 Hintertal Plutonic Suite The Hintertal Plutonic Suite comprises intermediate to acidic intrusions including hornblende-biotite-quartz monzodiorite gneisses, granodiorites and granites as well as granodiorite- and granite gneisses. These rocks indicate a magmatic fractionation trend with Rb/Sr ratio varying generally from 0.09 to 0.45, reflecting S- and I-type characteristics (Chappel and White, 1974, 2001). U-Pb zircon ages are Late Devonian to early Carboniferous (365 ± 11 Ma and 343 ± 12 Ma) and reflect an early phase of the Variscan tectonometamorphic event. Rocks belonging to this unit cover predominantly the north- eastern and south-western part of the Seckau Complex. The Hintertal Plutonic Suite can be subdivided into (a) the Griessstein Pluton and (b) the Pletzen Pluton.

7.3.1 Griessstein Pluton (Lithodeme) The main lithologies of the Griessstein Pluton are different types of granites and granite gneisses with a predominant S- type geochemical signature as well as fine-grained leucogranitic and pegmatitic dikes. Based on U-Pb zircon ages the intrusion of this unit crystallized in the Late Devonian to early Carboniferous (365 ± 11 Ma and 343 ± 7 Ma). Metagranitoids belonging to the Griessstein Pluton are predominantly situated in the northwestern and north-eastern part of the Seckau Complex adjacent to the metasediments of the Permian to Lower Triassic Rannach Formation.

7.3.2 Pletzen Pluton (Lithodeme) The Pletzen Pluton comprises generally intermediate to acidic intrusion sequences of e.g., hornblende-biotite quartz monzodiorite gneisses and slightly to strongly deformed granodiorites and granites reflecting I-type affiliation. Rocks of the Pletzen Pluton are characterized by greenish colored biotite and frequent, up to 1 mm sized, titanite crystals. They crystallized during Late Devonian to early Carboniferous (365 ± 8 Ma to 343 ± 12 Ma) and occur predominantly in the north-eastern and south-western part of the Seckau Complex.

Fig. 14. Simplified geological map of the Seckau Complex north (-east) of the Mur valley (compiled from Metz, 1967; Moser, 2015) showing the proposed subdivision into the Glaneck Metamorphic Suite (brownish colored), Hochreichart Plutonic Suite (blue colored) and Hintertal Plutonic Suite (Griessstein- and Pletzen Pluton; green colored). Dikes are marked with a highlighted star. Abbreviations: Ghpt: Geierhaupt, Gr.Grst: Grosser Griessstein, Pltz: Pletzen, Rk: Rosenkogel, SZ: Seckauer Zinken.

8 Discussion Petrographic observations as well as new geochemical and geochronological data from the Seckau Complex document a polyphase evolution of this part of pre-Alpine basement incorporated into the Austroalpine nappe stack. This chapter presents a brief summary of the evolution of the Seckau Complex, followed by a discussion of the paleogeographic implications. Particular attention is given to the source areas for the detrital material and the paleogeographic setting during the intusion of the individual magmatic suites in the late Cambrian to Early Ordovician and during the Variscan orgenic cycle.

8.1 Evolution of the Seckau Complex Paragneisses and micaschists of the metasedimentary/metavolcanic Glaneck Metamorphic Suite developed from clastic sediments with source areas containing predominantly Ediacarian (572 ± 7 Ma to 559 ± 11 Ma) but also some Neoarchean (~2.7 Ga) and Paleoproterozoic (~2.0 Ga) magmatitic rocks. Some of the Ediacarian-aged zircon grains contain inherited cores, which were not measured because of their small sizes, but are also assumed to be Neoarchean or Paleoproterozoic in age. With respect to the youngest detrital

40 zircons (559 ± 11 Ma) a maximum sedimentation age of at least parts of the Glaneck Metamorphic Suite in the Ediacarian can be expected. A potential source for detrital zircons is the North Gondwana margin from which voluminous late Neoproterozoic granitoids are known (e.g., Johnson et al., 2011). During late Cambrian to Early Ordovician times (508 ± 9 Ma to 486 ± 9 Ma) the paragneisses of the Glaneck Metamorphic Suite were intruded by S-type metagranitoids of the Hochreichart Plutonic Suite. This may also have triggered migmatization of distinct domains of the paragneisses, as indicated by an U-Pb zircon age of 505 ± 11 Ma (MM7b). The maximum sedimentation age of detrital zircons and the timing of Hochreichart Plutonic Suite intrusions as well as related migmatization also constrain the timing of amphibolite facies metamorphism within the Glaneck Metamorphic Suite between 559 ± 11 Ma and 505 ± 11 Ma. Moreover, both the Hochreichart and Hintertal Plutonic Suites were not affected by post- magmatic high-grade metamorphic overprint. In the Late Devonian to early Carboniferous (365 ± 11 Ma to 331 ± 10 Ma) S-type and I-type granitoids of the Hintertal Plutonic Suite intruded. They reflect a magmatic evolution related to an early Variscan tectonic event. Subsequent to magmatism, but most likely still during the Variscan event, the Seckau Complex got welded with the neighboring Ammering Series and Speik Complex. Subsequently and after a phase of erosion Permo-Mesozoic sediments (Rannach Formation) were deposited onto the basement units. Finally, in Late Cretaceous times during the Eo-Alpine event the upper part (Middle Triassic to Early Cretaceous), the Seckau magmatic and metamorphic basement and its Permian to Lower Triassic were subducted southeastward (Froitzheim et al., 2008) and experienced a pressure dominated metamorphic overprint at greenschist to amphibolite facies conditions (Faryad and Hoinkes, 2001, 2003). The Seckau Nappe sheared off from the downgoing plate and was exhumed during the Late Cretaceous to Eocene. The related cooling history during exhumation can be reconstructed from Rb-Sr bioite (Pfingstl et al., 2015; Scharbert, 1981) and apatite fission track data (Hejl, 1997).

8.2 Provenance and paleogeographic position of the Glaneck Metamorphic Suite Vast occurrence of Ediacarian zircons and minor presence of older (Neoarchean and Paleoproterozoic) zircon grains in the metasedimentary Glaneck Metamorphic Suite need to be discussed in terms of the paleo-geographic origin of this part of the Austroalpine

41 basement. Generally it is accepted that the continental crust appearing in the Alpine realm, including the AlCaPa Megaunit and Austroalpine Unit, respectively, derived from Gondwana fragments (e.g., von Raumer et al., 2002; von Raumer and Neubauer, 1993). Archean cratonic material as dated in zircon cores of the Glaneck Metamorphic Suite appears all over Gondwana, except in the Arabian Nubian Shield, that comprises predominantly Neoproterozoic crust (e.g., Johnson et al., 2011). Based on the presence of Neoarchean zircon cores, the lack of zircon ages related to the Grenville orogenic event at around 1.0 Ga (Linnemann et al., 2004) and the absence of metamorphic ages around 600 Ma as typical for the Mozambique Belt it is suggested that the ancestry of the Glaneck Metamorphic Suite in the Neoproterozoic was to the west of the Arabian Nubian Shield, probably next to the Trans- Saharan Belt (Fig. 15a).

8.3 Tectonic evolution during the early Paleozoic According to Neubauer (2002) and Candan et al. (2015) late Neoproterozoic to early Cambrian magmatism on the Gondwana margin evolved in the back of a retreating subduction zone. Sediment transport across future Cadomian terranes between Morocco and the Oman suggests that the North Gondwana margin was largely intact at that time. Subsequently, in middle to late Cambrian times, the North Gondwana margin was successively segmented and small rift basins and extensional allochthons evolved accompanied by the emplacement of tonalitic to gabbroic plutons and high temperature metamorphism (Fig. 15b). Oceanic remnants from this period present in the Alpine orogenic belt include the 496 ± 6 Ma old Chamrousse ophiolite (External Massif in the French Alps) (Ménot et al., 1988; von Raumer and Stampfli, 2008) and the 504 ± 2 Ma old metagabbro in the Brianconnais Unit (Penninic domain) (Sartori et al., 2006). In the Austroalpine Unit similar ages were derived from metagabbros and tonalities from the Silvretta Nappe and Ötztal Nappe (Miller and Thöni, 1995; Müller et al., 1996; Schaltegger et al., 1997). At the same time the Hochreichart Plutonic Suite intruded and a high temperature event caused local migmatization of the paragneisses in the surrounding Glaneck Metamorphic Suite. This assumption is based on an U-Pb zircon age of 505 ± 11 Ma from a migmatitic paragneiss (MM7b). The latter age is in good agreement with an U-Pb zircon age of 490 ± 9 Ma, determined for the migmatization event of paragneisses in the Ötztal Nappe (Kloetzli-Chowanetz et al., 1997).

42 With respect to the available data the basement of the Southalpine Unit parts of the Austroalpine Unit (e.g., Ötztal-Bundschuh and Silvretta-Seckau Nappe systems) as well as the Tatric and Veporic basement of the Western Carpathians share a similar history. Interestingly, the Late Ordovician (ca. 460 Ma) magmatism that appears in the Southalpine, Brianconnais, many of the Austroalpine units, and in the Veporic Unit is rarely recorded in the data of this study of the Seckau Complex (e.g., Gaab et al., 2005; Neubauer et al., 2002; Putis et al., 2008; von Raumer et al., 2013). This suggests that the Silvretta Complex escaped magmatism during the opening of the Paleotethys (von Raumer et al., 2002; von Raumer and Stampfli, 2008).

8.4 Evolution during the Variscan tectonometamorphic event From the Silurian onwards the composite Hunic Terrane drifted from the Gondwana margin northwards. Following the paleogeographic reconstruction by von Raumer et al. (2013) the composite Hunic Terrane assembled smaller subterranes (Armorica, Ligeria, Galatia) that amalgamated between approximately 380 and 350 Ma before they finally attached to Laurussia (Fig. 15c). The second major intrusion event within the Seckau Complex took place during that time in the Late Devonian to early Carboniferous. Melting and assimilation processes of continental crust formed already during late Cambrian to Early Ordovician times and are well documented by the occurrence of inherited zircon cores showing such ages. The contemporaneous crystallization of the intermediate to acidic Pletzen Pluton and Griessstein Pluton with I-type and S-type affinities, respectively, indicates a complex setting at an active continental margin and arc magmatism. Within the Alpine orogenic belt Late Devonian to early Carboniferous magmatism and metamorphism are documented also in the southeastern part of the Seckau Nappe within the Gleinalm area (363-350 Ma tonalite and trondhjemite; Neubauer and Frisch, 1993) and towards the east in the Kaintaleck Nappe (Handler et al., 1999) of the Silvretta-Seckau Nappe System. Further it plays an important role in the Tatric and Veporic nappes of the Central Western Carpathians (Broska et al., 2013; Putis et al., 2008). In contrast, several units in the western part of the Austroalpine Unit (e.g., Ötztal Nappe, Silvretta Nappe) show a different Variscan evolution with eclogite facies metamorphism in the Late Devonian to early Carboniferous (e.g., 362 Ma gabbroic eclogites in the Ötztal Nappe, Miller and Thöni, 1995)

43 and a high-grade metamorphic peak in the Visean at approximately 340 Ma (Thöni, 1999). This evolution is much more similar to the Moldanubian Unit in the foreland of the Alps.

Fig. 15. Paleogeographic position of peri Gondwana terranes during (a) Precambrian (ca. 570 Ma; modified after Balintoni et al., 2010; Linnemann et al., 2004, 2008), (b) early Ordovician (around 480 Ma; modified after Franke et al., 2017) and (c) Late Devonian/early Carboniferous (around 350 Ma; modified after Franke et al., 2017). The assumed position of the recent Seckau Complex is marked with a light grey star. Abbreviations: Am: Armorica, B: Bohemia, F: Franconia, Ib: Iberia, PA: Palaeo-Adria, PT: Palaeotethys, T: Thuringia, TS: Trans- Saharan Belt.

44 9 Conclusions Based on lithological differences visible in the field and geochemical characteristics and U- Pb zircon ages the Seckau Complex can be subdivided into three lithostratigraphic/lithodemic sub-units:

(1) The metasedimentary Glaneck Metamorphic Suite mainly consists of paragneisses and minor micaschists. Cores from rounded zircons present in the paragneisses represent the oldest analyzed zircons of the Seckau Complex with ages clustering in the Neoarchean (ca. 2.7 Ga) and Paleoproterozoic (ca. 2.0 Ga). The majority of the analyzed zircons is Ediacarian in age, ranging between 572 ± 7 Ma and 559 ± 11 Ma. The youngest detrital zircon ages indicate that at least parts of the Glaneck Metamorphic Suite show an Ediacarian maximum sedimentation age.

(2) U-Pb zircon ages between 508 ± 9 Ma and 486 ± 9 Ma constrain the late Cambrian to Early Ordovician intrusion age of highly fractionated S-type metagranitoids within the Hochreichart Plutonic Suite.

(3) Late Devonian to early Carboniferous U-Pb zircon ages indicate crystallization of the Hintertal Plutonic Suite during an early phase of the Variscan tectonometamorphic event. Variable Rb/Sr ratios from the rocks in this suite define a fractionation trend. The suite can be further subdivided into the S-type granitoids of the Griessstein Pluton, including granitic and pegmatitic dikes, and I-type granitoids of the Pletzen Pluton.

The detrital zircon data and the evolution of the Seckau Complex provide constraints on its paleogeographic position during several orogenic cycles:

(1) Based on the detrital zircon ages a Neoproterozoic ancestry of the Glaneck Metamorphic Suite to the west of the Arabian Nubian Shield is suggested, probably next to the Trans-Saharan Belt (Fig. 15a).

(2) The early Paleozoic evolution of the recent Seckau Complex shows similarities to other basement units of the Alps (e.g., Southalpine Unit, other parts of the

45 Austroalpine Unit) as well as the Tatric and Veporic units of the Central Western Carpathians.

(3) During the Variscan tectonometamorphic cycle the Seckau Complex as part of the eastern Silvretta-Seckau Nappe System shared an evolution with the Tatric and Veporic units of the Central Western Carpathians, dominated by intermediate I-type and S-type magmatism, probably representing an arc-type setting. This differs from the evolution of Austroalpine units west of the Tauern Window (e.g., Silvretta and Ötztal Nappe).

10 Acknowledgment This study was supported by the University of Graz by providing a four years doctorate position for M. Mandl. We appreciate the constructive comments by Franz Neubauer and one anonymous reviewer and the editorial handling by chief editor Xian-Hua Li. We would like to thank Stefan Pfingstl for providing some of his samples for U-Pb zircon age dating. Monika Horschinegg and Wencke Wegner (University of Vienna) are thanked for their help with chemical separation and measurements of the Sm-Nd isotopic data.

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Chapter 2 |

Geochemistry of granitoids from the Seckau Complex (Eastern Alps): a key for revealing the pre-Alpine evolution in the EasternAlps

53 Abstract Recent studies revealed that the calc-alkaline metagranitoids of the Seckau Complex are part of both (1) a late Cambrian to Early Ordovician and (2) a Late Devonian to early Carboniferous (early Variscan) intrusive complex. The older rocks of the Hochreichart Plutonic Suite reflect S-type affinity and are peraluminous with A/CNK ratios (molar proportions of Al2O3/ (CaO + Na2O + K2O)) in the range between 1.08 and 1.51. They are characterized by a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, FeOt and MnO with increasing SiO2. Trace elements show a negative fractionation trend of Rb and Ba against

SiO2, and a constant Sr content. Pyrolite-normalized diagrams display positive Rb, U, K, Pb, Nd and Sm anomalies and strong negative Ba, Nb, Ta, Sr, P and Ti anomalies. Chondrite- normalized rare earth element (REE) plots display a slight enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) as well as negative Eu 87 86 anomalies ((Eu/Eu*)N = 0.15- 0.77). The whole-rock initial Sr/ Sr ratios of these rocks vary between 0.7056 to 0.7061. The early Variscan rocks of the younger Hintertal Plutonic Suite can be subdivided into (a) the meta- to peraluminous granodioritic suite of the Pletzen Pluton and (b) the peraluminous granitic suite of the Griessstein Pluton. The Pletzen Pluton shows typical magmatic fractionation trends for most of the major oxides and trace elements plotted against SiO2. Trace elements display a positive fractionation trend of Rb and a negative trend of Sr. Pyrolite-normalized spider diagrams show a positive anomaly of K, Pb, Nd and Sm and negative anomalies of Nb, Ta, P and Ti. On a chondrite-normalized diagram metagranitoids are strongly enriched in LREE and show no significant negative Eu anomaly. Metagranitoids of the Griessstein Pluton have a more peraluminous character and similar major and trace elements fractionation trends compared to the Pletzen Pluton. However, the contents in SiO2, major and trace elements clearly point towards a more evolved melt with generally lower

TiO2, Al2O3, MgO and CaO values and higher K2O content. Metagranitoids of the Griessstein Pluton are additionally characterized by a strong positive K and Pb anomaly and by negative Nb, Ta, P and Ti anomalies on a pyrolite-normalized diagram and a very slight negative Eu anomaly of about 0.81 on a chondrite-normalized REE plot. Initial 87Sr/86Sr values from rocks of the Pletzen Pluton and the Griessstein Pluton vary between 0.7051 - 0.7061 and 0.7054 - 0.7063, respectively and suggest the same magmatic source for both units.

54 These new geochemical data suggest that the early Variscan granitoids of the Seckau Complex were part of a magmatic arc along the southern Bohemian active continental margin that was related to the subduction of the Paleotethys, prior to the final collision of Gondwana and Laurussia.

1 Introduction Variscan belts formed through long and diachronous convergence and the collision of Gondwana fragments with Laurussia that finally amalgamated during the Carboniferous. Individual orogenic phases are known to expand from Lower/Middle Devonian to Upper Carboniferous and were associated with the emplacement of numerous granitoids, which are heterogenous in terms of age, geochemistry, source and regional distribution (Finger et al., 1997). The European Variscides extend from Poland to southern Iberia with extensive exposures in the Bohemian Massif (e.g., Franke 2000; Franke and Zelazniewiez, 2002; Finger et al., 2007), the French Massif Central (e.g., Santallier et al., 1994), the Armorican Massif (e.g., Ballèvre et al., 1994) and the Iberian Massif (e.g., Gόmez Barreiro et al., 2006, 2007). The Alpine orogeny reworked the southern part of the Variscan orogeny, but Variscan fragments are still preserved and occur in different tectonic positions in the Alps e.g., the Aar Batholith (Helvetic Unit), the Hohe Tauern Batholith (Penninic Unit) and numerous granitoids belonging to the Austroalpine Unit e.g., the Schladming Batholith, the Seckau-Bösenstein Batholith and the Semmering Raabalpen Batholith (e.g., Schaltegger, 1990, Schermaier et al., 1997, Eichhorn et al., 2000, Schmied et al., 2004, Mandl et al., 2018). Along the northern front of the Alps Cetic granitoid boulders represent early Variscan granitoids embedded in the Ultrahelveticum and the Rheno-Danubian Flysch Belt (Fig. 1; Finger et al., 1997; Frasl and Finger, 1988). Further to the east, Variscan granitoids mainly occur in the Tatric and Veporic units of the Western Carpathians and correlate with granitoids of the Austroalpine Unit (Neubauer, 1994, Broska and Uher, 2001). In contrast to the well-studied plutons of the northern and central European Variscan orogeny only few data exist from the reworked intra-Alpine Variscan granitoids belonging to the southern part of the Variscan orogeny. Granitoids of the Seckau Complex (Eastern Alps), now observed as metagranitoids, were generally considered to be part of the intra-Alpine Variscides. Recent studies, however,

55 revealed that the metagranitoids of the Seckau Complex are part of both a late Cambrian to Early Ordovician and Late Devonian to early Carboniferous (early Variscan) intrusive complex (Mandl et al., 2018). While the older intrusives comprise primarily peraluminous granitoids with S-type characteristics, the early Variscan granitoids represent meta- to peraluminous granitoids, which represent mostly I-type characteristics. Based on the geochronological results of Mandl et al. (2018) new geochemical major, trace and rare earth element data were obtained in order (1) to better understand and constrain the evolution of Variscan magmatism and magmatic differentiation of the Seckau Complex and (2) to characterize the older late Cambrian to Early Ordovician plutonic suite of the Seckau Complex.

Fig. 1. Simplified geological sketch-map showing the distribution of Variscan granitoids of central Europe (modified after Finger et al. 1997, Finger et al., 2003, Franěk et al., 2011) Abbreviations: AB: Aar Batholith, BB: Bernina Batholith, BM: Bohemian Massif, CBB: Central Bohemian Batholith, HTB: Hohe Tauern Batholith, SB: Schladming Batholith, SkB: Seckau-Bösenstein Batholith, SRB: Semmering Raabalpen Batholith, RS: Rheinisches Schiefergebirge, SBB: Southern Bohemian Batholith

56 2 Geological setting The European Alps are the result of a continuous convergence of the African and European continental plates since Cretaceous times. Several oceanic basins and microcontinents were involved in this collision event at different times (Schmid et al., 2004). The Eastern Alps, defined as the area east of a line from Lake Constance (Bodensee) to Lake Como, are the product of two orogenic collision events: (1) the Eo-Alpine event and (2) the Alpine event (Froitzheim et al., 1996). The Eo-Alpine event, as term for Early Cretaceous to early Late Cretaceous, is related to the closure of the Meliata-Hallstatt Ocean. The subsequent Alpine event (Late Cretaceous to Paleogene) is caused by the subduction of the Alpine Tethys oceans (Piemont-Ligurian Ocean and Valais Ocean), due to the collision of Europe and Adria (Froitzheim et al., 1996; Schmid et al., 2004; Schuster, 2004). The result of plate tectonics in the Eastern Alps is a complex nappe stack that can be subdivided into three structural units (1) Helvetic realm and sub-Penninic nappes, representing the former southern European continental margin, (2) Penninic nappes, comprising oceanic (Piemont-Ligurian Ocean and Valais Ocean) and continental crust (e.g., Brianconnais) and (3) Austroalpine nappes, representing continental crust from the northern part of former Adria (Fig. 2a; Schuster, 2003; Schmid et al., 2004; Froitzheim et al., 2008). The uppermost Austroalpine Unit can be further subdivided into a Lower Austroalpine and an Upper Austroalpine Subunit (Schmid et al., 2004). The Lower Austroalpine Subunit is derived from the northern continental margin of Adria towards the Piemont-Ligurian Ocean. After deposition of Triassic pre-rift shelf sediments this area was affected by Jurassic extension and by nappe stacking processes due to the opening and closure of this oceanic realm from the Late Cretaceous to Eocene. The Upper Austroalpine Subunit, widely exposed in the Eastern Alps, is mainly shaped by the Eo- Alpine collision event. It comprises Paleozoic and Mesozoic cover units at the northern rim of the Eastern Alps (Northern Calcareous Alps) and Paleozoic metasediments and metavolcanics of the Greywacke Zone. In the southern part, pre-Alpine crystalline basement units such as (from bottom to top) the Silvretta-Seckau-, the Koralpe-Wölz-, the Ötztal- Bundschuh- and Drauzug-Gurktal Nappe Systems are mostly covered by Late Carboniferous- Permian to Mesozoic metasediments (Schmid et al., 2004; Froitzheim et al. 2008).

Fig. 2. (a) Overview map showing the paleogeographic origin of the main tectonic units of the Alps after Schmid et al. (2004). The dotted square indicates the location of the map shown in Fig. 2b. (b) Simplified geological map of the Austroalpine basement units to the east of the Tauern Window (modified after Mandl et al., 20018). The study area, part of the Seckau Nappe System, is marked with a dotted rectangle.

58 The Seckau Nappe, as part of the Silvretta-Seckau Nappe System, extends from the Bösenstein Massif and the Seckauer Tauern in the north to the Fischbacher Alps in the east. There it is cut off by the sinistral, east-west trending Trofaiach fault and continues in the Troiseck-Floning Nappe (Neubauer, 1988). In the south, it is represented by the Ameringkogel and in the west it is confined by the Pöls fault system, a dextral strike slip fault zone that also separates the Bösenstein Massif from the Seckauer Tauern (Fig. 2b) (Metz, 1976; Schermaier et al., 1997). The Seckau Nappe comprises several pre-Alpine basement complexes: Seckau Complex, Amering Complex, Speik Complex as well as Permian to Lower Triassic metasediments summarized as Rannach Formation (Metz, 1967; Flügel and Neubauer, 1984; Faryad and Hoinkes 2001; Gaidies et al., 2006; Pfingstl et al., 2015). The Seckau Complex contains pre-Variscan/Variscan basement units dominated by paragneisses (partly migmatitic) and micaschists into which large volumes of meta- to peraluminous granitoids intruded. Together, the granitoids make up about 90% of the Seckau Complex (Schermaier et al., 1997). Recent studies within the Seckau Complex revealed a more distinct subdivision of the Seckau Complex based on petrological, geochemical and geochronological data (Mandl et. al., 2018) (Fig. 3). The Glaneck Metamorphic Suite comprises paragneisses with U-Pb zircon ages in the range from Neoarchean (ca. 2.7 Ga) to Ediacaran (ca. 559 Ma) but shows its distinct frequency peak between 572 ± 7 Ma and 559 ± 11 Ma. Highly fractionated peraluminous granites of the Hochreichart Suite indicate a magmatic event between 508 ± 9 Ma and 486 ± 9 Ma that may also have caused migmatisation of distinct domains in the paragneisses. The Hintertal Plutonic Suite displays a second intrusion event ranging from 365 ± 8 Ma to 343 ± 12 Ma and comprises meta- to peraluminous granodiorites (Pletzen Pluton) as well as peraluminous granites (Griessstein Pluton). Granitoids of the Pletzen Pluton are characterized by olive-green to brownish colored biotite, large titanite crystals

(with an average grain-size of 200-500 μm), variable SiO2 contents and a magmatic fractionation trend seen in variable Rb/Sr ratios. Granitoids of the Griessstein Pluton display high SiO2 values and are defined by the dominance of muscovite in contrast to biotite (Mandl et al., 2018).

3 Analytical methods Major oxides and selected trace element concentrations (Rb, Sr, Zr and Ba) of whole-rock samples were determined by a Bruker Pioneer S4 X-ray fluorescence (XRF) spectrometer on

59 fused glass discs at the NAWI Graz Geocenter - Institute of Earth Sciences, Petrology and Geochemistry, University of Graz, Austria (described in Mandl et. al., 2018). The reproducibility of major elements was better than 1% and for trace elements within 3-5%. International standards e.g., AC-E, MA-N, GSP-2 were routinely measured to ensure analytic accuracy. Trace elements and rare earth elements were determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500) at the Institute for Analytical Chemistry, University of Graz, Austria. For this method, about 50 mg of powdered sample were placed in a Teflon beaker and dissolved in a concentrated nitric (HNO3) and hydrofluoric (HF) acid mixture with a volumetric ratio of 1:2. The uncapped beaker were placed on the hot plate at 100°C for 30 min, then tightly capped and returned to the hot plate at 180°C for at least 48 h.

After dissolution and evaporation the samples were extracted with 7.5 N HNO3 and diluted 1500 times. The analytical results for standards GSP-2, JR-2, BCR-2, BHVO-2 were for most elements within the specified uncertainty.

4 Sample description Sample locations of the Seckau Complex metagranitoids are provided in Figure 3 and Table 1. Based on field observations and petrographic characteristics the granitoid intrusions of the Seckau Complex, now mostly observed as metagranitoids, can be distinguished into two plutonic suites.

4.1 Hochreichart Plutonic Suite The first suite, termed Hochreichart Plutonic Suite, comprises old granite gneisses and slightly deformed granites as well as subordinate amounts of granodioritic gneisses with U- Pb zircon ages in the range between 508 ± 9 Ma and 486 ± 9 Ma (Mandl et al., 2018). These plutons predominantly occur in the northern, northeastern- and southern parts of the Seckau Complex.

Table 1 Sample list and localities from metagranitoids and different dikes of the Seckau Complex.

Sample Lithology Locality Latitute Longitude MM1 Granite gneiss Vorwitzgraben 47°17'59.5" N 14°41'49.1" E MM2 Granite gneiss Vorwitzgraben 47°17'59.5" N 14°41'49.1" E MM10 Granite Hochreichart 47°21'42.0" N 14°41'08.0" E MM13 Granite Rosenkogel 47°17'43.0" N 14°33'08.5" E MM27 Granite gneiss Mitterkogel 47°21'57.3" N 14°47'43.5" E MM28 Granodiorite gneiss Alpsteigerhütte, Mitterkogel 47°22'12.6" N 14°48'19.8" E MM32 Granite gneiss Kerschkern 47°23'18.5" N 14°37'08.1" E MM33 Granite gneiss Stellmauer 47°23'35.4" N 14°36'33.6" E MM36 Ms-granite gneiss Grieskogel 47°22'41.6" N 14°38'37.4'' E MM57 Granite Gaalgraben 47°16'27.0" N 14°37'27.0" E MM58 Granite gneiss Hämmerkogel 47°20'36.4'' N 14°45'11.6" E MM59 Granite gneiss Hämmerkogel 47°20'35.3" N 14°45'15.1" E MM60 Granite Goldlacke 47°20'22.6" N 14°45'13.1" E MM61 Granite Seckauer Zinken 47°20'31.4" N 14°44'34.4" E MM62 Granite Seckauer Zinken 47°20'20.9" N 14°44'11.1" E MM64 Granite Schwaigerhöhe/Tagwart 47°20'25.5" N 14°46'35.3" E MM66 Bt-granodiorite gneiss Herrschaftskranz 47°19'35.2" N 14°40'20.0" E MM68 Granite Herrschaftskranz 47°19'28.0" N 14°40'05.5" E SP62 Granite gneiss Vorwitzgraben 47°18'01.0" N 14°41'51.4" E SP78 Wm-granite gneiss Antonikreuz 47°21'59.8" N 14°47'34.6" E SP79 Granite gneiss Hochreichart 47°21'48.7" N 14°40'53.9" E MM12 Hbl-bt qz monzodiorite gneiss Roßbachgraben, Gaal 47°16'56.4" N 14°33'40.7" E MM17 Granite gneiss Schleifgraben, St Johann 47°21'39.2" N 14°29'41.1" E MM18 Bt-granite Schleifgraben, St Johann 47°21'43.5" N 14°29'25.0" E MM19 Bt-granodiorite gneiss Schleifgraben, St Johann 47°21'44.9" N 14°29'26.2" E MM23 Bt-granite gneiss Gamskogel 47°21'56.7" N 14°32'54.8" E MM55 Bt-granodiorite Hintertal, Lanzalm 47°16'52.0" N 14°35'22.0" E MM56 Granodiorite Hintertal, Tuscherriegel 47°17'09.0" N 14°35'24.0" E MM65 Granodiorite Tiertal 47°20'56.8" N 14° 38' 0.1" E SP58 Granodiorite Schmähtaschen 47°20'56.4" N 14°48'32.6" E SP59 Qz monzodiorite Untere Bodenhütte 47°20'56.4" N 14°48'32.6" E SP80 Bt-granodiorite gneiss Sundlsee 47°19'41.7" N 14°37'26.2" E SP81 Bt-granodiorite gneiss Sundlsee 47°19'34.3" N 14°37'39.4" E MM20 Granite gneiss Triebener Törl/ Knaudachtörl 47°23'18.4" N 14°32'04.8" E MM21 Granite gneiss Triebener Törl/ Knaudachtörl 47°23'18.4" N 14°32'04.8" E MM22 Granite Knaudachtörl 47°22'56.8" N 14°32'32.3" E MM52 Granite gneiss Triebener Törl/Sonntagskogel 47°23'28.3" N 14°31'13.1" E MM53 Granite gneiss Triebener Törl/Sonntagskogel 47°23'28.1" N 14°31'15.0" E SP53 Granite gneiss Untere Bodenhütte 47°21'23.0" N 14°47'12.1" E SP54 Granodiorite gneiss Untere Bodenhütte 47°21'23.0" N 14°47'12.1" E MM3 Leucogranitic dike Vorwitzgraben 47°17'59.5" N 14°41'49.1" E MM5 Pegmatite dike Alpinpark Steinmühle 47°17'03.0" N 14°45'34.7" E MM14 Granite dike Rosenkogel 47°17'41.8" N 14°33'09.7" E MM15 Granite dike Rosenkogel 47°17'41.8" N 14°33'09.7" E MM51 Pegmatite dike Herrschaftskranz 47°19'43.7" N 14°40'16.3" E MM54 Granite dike Rosenkogel 47°17'39.0" N 14°33'10.0" E MM69 Granite dike Herrschaftskranz 47°19'34.3" N 14°40'20.3" E SP61 Pegmatite dike Alpinpark Steinmühle 47°17'01.7" N 14°45'31.1" E

Fig. 3. Simplified geological map of the Seckau Complex (modified after Mandl et al.,) to the northeast of the Mur valley showing the sample location of the metagranitoids. Abbreviations: Ghpt: Geierhaupt, Gr.Grst: Großer Griessstein, Pltz: Pletzen, Rk: Rosenkogel, SZ: Seckauer Zinken.

The granite gneisses are mainly exposed in the area of Mount Geierhaupt (MM32, MM33, and MM36), Mount Hochreichart (MM10, SP79), Mount Seckauer Zinken (MM58, MM59) and Mount Mitterkogel (MM27, MM28 and SP78). These granite gneisses usually display S- type affinity and predominantly consist of quartz, K-feldspar, plagioclase (partly saussuritisized), muscovite, biotite (orange colored) and sporadic chlorite. Epidote/zoisite, garnet, calcite, zircon, apatite, titanite and opaque phases are common as accessory minerals. The dominance of muscovite instead of biotite and an inequigranular texture can be observed in most samples. Large K-feldspar idiocrysts, up to several cm in size, as well as quartz aggregates occur in a fine-grained groundmass of equigranular quartz and feldspar (Fig. 4a, b). Shear bands of fine-grained muscovite and biotite often penetrate the gneisses. Muscovite-bearing granite gneisses (e.g., SP78), exposed in the area of Mount Mitterkogel differ from adjacent granite gneisses due to the absence of biotite. Undeformed to slightly deformed granites, situated in the area around Mount Seckauer Zinken (MM60, MM61, MM64, MM68), appear in the field as felsic rocks compared to the slightly darker granite gneisses in this area. They are usually medium to coarse-grained with equigranular textures and consist of quartz, feldspar and minor mica, mostly muscovite. In the southern region of the Seckau Complex, in the Vorwitzgraben, granite gneisses are characterized by pronounced penetrative foliation, medium-grained mica, up to a few millimeters, and the dominance of 62 biotite instead of muscovite. These granite gneisses (e.g., MM1, MM2) are mostly composed of approximately 35-40% quartz, 20-28% K-feldspar, 20% plagioclase, 15-20% biotite (brownish to orange colored) and around 5% muscovite. Large-sized quartz grains (up to 1 mm) are mainly characterized by undulatory extinction, lobate and irregular shaped grain boundaries and the formation of subgrains suggest bulging as the main dynamic recrystallization mechanism. Subgrain rotation recrystallization fabrics also occur but are more rare (Passchier and Trouw, 2005).

4.2 Hintertal Plutonic Suite The Late Devonian to early Carboniferous (365 ± 8 Ma to 343 ± 12 Ma) Hintertal Plutonic Suite can be subdivided into (a) the Pletzen Pluton representing metagranitoids with I-type characterictics (sample: MM12, MM17, MM18, MM19, MM23, MM55, MM56, MM65, SP58, SP59, SP80 and SP81) and (b) the Griessstein Pluton, fine-grained metagranitoids with S-type affinity (sample: MM20, MM21, MM22, MM52, MM53, SP53 and SP54) (Mandl et. al., 2018). The Pletzen Pluton makes up about 90% of the Hintertal Plutonic Suite and predominantly covers the central, western and some southern parts of the Seckau Complex, while the Griessstein Pluton only occurs in small domains close to the Hochreichart Plutonic Suite. The Hintertal Plutonic Suite can be clearly distinguished from the Hochreichart Plutonic Suite and comprises besides granitic rocks predominantly granodiorites and quartz monzodiorites. In the field most of these rocks appear blocky, coarse-grained and weakly foliated. They are dominated by plagioclase, quartz, K-feldspar, biotite (olive- to moss-green colored), muscovite and large titanite crystals with an average grain-size of 200-500 µm (Fig. 4c, d). Biotite is usually more common than muscovite and feldspar is mostly completely altered to sericite but its original shape is usually still preserved. Feldspar also shows poikiloblastic textures in some cases. The most basic rock within this suite a coarse-grained hornblende-biotite bearing monzodiorite gneiss, located in the Roßbachgraben (MM12) comprises about 31% biotite, 25% hornblende, 22% quartz and 21% feldspar (Fig. 4e, f). Slightly deformed granitoids display an equigranular texture, composed of intensively sericitized feldspar and olive-green colored biotite (e.g., MM55 and MM65: 60% plagioclase, 20% quartz, 14-8% biotite, 10-5% K-feldspar). Strongly deformed granitoids show an inequigranular texture and less and smaller sized plagioclase (e.g., MM56). Biotite-granite gneisses located in the Schleifgraben and around Mount Gamskogel (e.g., MM18, MM23)

63 are characterized by a mineral content of about 38% plagioclase, 32% quartz, 21% K-feldspar and approximately 7% biotite. The predominantly granitic rocks of the Griessstein Pluton, represented by samples MM20, MM21, MM52, MM53, SP53 and SP54, can be clearly distinguished from the Pletzen Pluton due to their fine-grained texture, their dominance of K-feldspar and muscovite as well as the absence of olive- to moss-green colored biotite and large titanite crystals (Fig. 4g, h).

4.3 Dikes Granitic, leucogranitic and pegmatitic dikes intruded into paragneisses and some metagranitoids are frequently observed in the Seckau Complex. They are summarized in a group named “dikes” and are discussed separately because they may have interacted with their host rocks, predominantly paragneisses. Fine-grained granitic dikes penetrate paragneisses, exposed at the Mount Rosenkogel, concordantly (MM14, MM15) and discordantly (MM54). Another granitic dike (MM69) was sampled at the Herrschaftskranz within a biotite granodiorite gneiss and a leucogranitic dike (MM3) intruded within a granite gneiss, situated in the Vorwitzgraben. Pegmatitic dikes crosscut paragneisses mostly discordantly and were represented by samples MM5 and SP61, located in the Alpinpark Steinmühle, and by sample MM51, located at the Herrschaftskranz.

Fig. 4. Representative photomicrographs of (a, b) granite gneiss MM27 from the Hochreichart Plutonic Suite, (c, d) biotite granite MM18 and (e, f) hornblende-biotite quartz monzodiorite gneiss MM12, both representative of the Pletzen Pluton (Hintertal Plutonic Suite) and (g, h) granite gneiss MM20 of the Griessstein Pluton (Hintertal Plutonic Suite). Photomicrographs on the left-hand side are shown with crossed polarizers and those on the right-hand side are mapped under parallel polarizers. Abbreviations: bt: biotite, hbl: hornblende, kfs: K- feldspar, ms: muscovite, pl:plagioclase, qz: quartz.

65 5 Whole-rock major, trace and rare earth elements Major, trace and rare earth element (REE) composition of forty-eight metagranitoids (including granitic, leucogranitic and pegmatitic dikes) are listed in Table 2-4 and plotted in Figs. 5 to 10. Major elements as well as Rb and Sr from 28 samples (MM1, MM2, MM10, MM12, MM13, MM14, MM15, MM17, MM18, MM19, MM20, MM22, MM23, MM27, MM28, MM32, MM33, MM54, MM55, MM56, MM57, MM65, MM66; SP54, SP58, SP78, SP79, SP81) were taken from Mandl et al. (2018) and Pfingstl et al. (2015). All metagranitoids of the Seckau Complex follow the calc-alkaline fractionation trend (Irvine and Baragar, 1971) and were classified following the TAS (total alkali vs. silica) diagram for plutonitic rocks after Middlemost (1994). Metagranitoid rocks of the Hochreichart Plutonic Suite (HrPS) predominantly fall in the granite field, whereas metagranitoids of the Hintertal Plutonic Suite (HtPS), subdivided into the Pletzen Pluton (PltzP) and the Griessstein Pluton (GrstP), range in their composition from quartz monzodiorite to granodiorite to granite (Fig. 5a).

Fig. 5. (a) TAS (total alkali vs. silica) classification diagram (Middlemost, 1994) and (b) A/NK vs. A/CNK plot (Shand, 1943) for metagranitoids (including granitic-, leucogranitic- and pegmatitic dikes) of the Hochreichart Plutonic Suite (HrPS) and the Hintertal Plutonic Suite (HtPS) comprising the Pletzen Pluton (PltzP) and the Griessstein Pluton (GrstP).

66 Granites of the Hochreichart Plutonic Suite show a consistently peraluminous character (Shand, 1943) and reflect S-type affinity (Chappell and White, 2001) with A/CNK values

(molar proportions of Al2O3/ (CaO + Na2O + K2O)) in the range between 1.08 and 1.51 and

A/NK values (molar proportions of Al2O3/ (Na2O + K2O)) ranging from 1.15 to 1.73. Metagranitoids of the Hintertal Plutonic Suite demonstrate a meta- to peraluminous trend with generally lower A/CNK values ranging from 0.88 to 1.23 and A/NK values from 1.34 to 2.31 (Fig. 5b). On the (Y + Nb) vs. Rb tectonic discrimination diagram for granites (Pearce et al., 1984) the metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite plot predominantly in the volcanic-arc granites field (VAG), reflecting geochemical signatures of an active continental margin (subduction-related setting). Only few samples are marginally located in the syn-collisional granites field (syn-COLG; MM63, MM36) and in the within- plate granite field (WPG; MM33, SP79, MM32, SP59), respectively (Fig. 6). However, most likely these samples also reflect a VAG setting.

Fig. 6. Geotectonic discrimination diagram (Rb vs. (Y+Nb)) for granitoids rocks (Pearce et al., 1984) of the Seckau Complex showing the fields of volcanic-arc granites (VAG), syn-collisional granites (syn-COLG), within-plate granites (WPG) and ocean-ridge granites (ORG). Abbreviations: HrPS: Hochreichart Plutonic Suite, HtPS: Hintertal Plutonic Suite, comprising the Pletzen Pluton (PltzP) and the Griessstein Pluton (GrstP).

67 5.1 Hochreichart Plutonic Suite

Binary major oxide vs. SiO2 variation diagrams for peraluminous metagranitoids of the Hochreichart Plutonic Suite show typical fractionation trends for most of the major oxides.

There is a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, FeOt and MnO with increasing

SiO2, whereas Na2O and K2O indicate no significant fractionation trend as seen in Fig. 7. The metagranitoids generally show high contents of SiO2 (67.77-77.84 wt%), Na2O (1.84-5.08 wt%) and K2O (1.50-5.43 wt%). Their TiO2 values vary from 0.06 to 0.60 wt%, Al2O3 ranges from 12.28-16.22 wt%, Fe2O3 (total Fe) content is between 1.02 to 4.69 wt% and MnO values are in the range between 0.02-0.06 wt%. MgO values vary between 0.12 and 1.80 wt% and CaO and P2O5 content ranges from 0.21 to 2.05 wt% and from 0.04 to 0.17 wt%, respectively.

Selected trace elements often display negative correlations plotted against SiO2 (Fig. 8). Large ion lithophile elements (LIL) e.g., Rb, Ba and transition metals such as Sc, V and Cr, predominantly display a negative correlation with SiO2, whereas high field strength elements (HFSE e.g., Y, Zr, Nb) show no significant fractionation trend or display only a slight decrease against SiO2. Rb values are usually high and range between 91 and 229 ppm, with exception for sample MM13 (32 ppm) and MM27 (64 ppm), Sr content is low and more or less constant with an average of about 100 ppm and Ba values vary from 186 to 956 ppm. Sc content is between 2 and 12 ppm, V values range from 2 to 66 ppm and Cr content varies between 1 and 35 ppm. Y and Zr values range from 5 to 58 ppm and 40 to 262 ppm, respectively and Nb content is low between 4 and 17 ppm (Table 2). On the pyrolite-normalized (Mc Donough and Sun, 1995) spider diagram the metagranitoids of the Hochreichart Plutonic Suite show consistent trace element patterns with enrichment in Rb, U, K, Pb, Nd, Sm and strong depletion in Ba, Nb, Ta, Sr, P and Ti as well as relatively flat distribution patterns in HREE (Fig. 9a). On the chondrite-normalized (Mc Donough and Sun, 1995) REE diagram these rocks display an enrichment in LREE relative to HREE with

(La/Yb)N of 2.23- 25.06 [(La/Sm)N = 1.95-4.01; (Dy/Yb)N = 0.8-1.73], and moderate negative

Eu anomalies with (Eu/Eu*)N values of 0.15- 0.77 (calculated after Taylor and McLennan,

1985; (Eu/Eu*)N = EuN/√(SmN*GdN)) (Fig. 9b). A slight negative Ce anomaly can only be observed in sample MM32.

68 Table 2 Representative whole-rock major (wt%) and trace element compositions (ppm) for metagranitoids of the Hochreichart Plutonic Suite.

Lable MM1 MM2 MM10 MM13 MM27 MM28 MM32 MM33 MM36 MM57 MM58 MM59 MM60 MM61 MM62 MM64 MM66 MM68 SP62 SP78 SP79

SiO2 (wt%) 70.84 71.20 72.55 77.84 75.47 67.77 72.77 71.79 74.90 75.89 69.55 70.12 72.60 71.11 71.10 73.90 68.46 75.28 69.22 74.76 71.24

TiO2 0.46 0.48 0.40 0.25 0.18 0.60 0.42 0.38 0.20 0.13 0.37 0.31 0.17 0.23 0.24 0.20 0.56 0.06 0.45 0.21 0.39

Al2O3 15.62 16.22 13.83 12.28 13.96 14.79 13.59 14.53 13.22 12.68 15.00 14.61 14.23 14.71 15.09 14.09 15.04 13.62 15.31 13.08 14.00

Fe2O3 3.12 3.33 3.31 1.28 1.72 4.69 3.32 2.89 1.88 1.13 2.37 2.43 1.43 1.70 2.36 1.15 4.16 1.02 3.24 2.12 2.85 MnO 0.05 0.05 0.06 0.02 0.04 0.06 0.04 0.04 0.03 0.02 0.05 0.05 0.02 0.04 0.04 0.02 0.05 0.03 0.06 0.03 0.05 MgO 0.89 0.97 0.90 0.35 0.35 1.34 0.74 0.67 0.47 0.14 0.91 0.85 0.39 0.58 0.55 0.31 1.80 0.12 0.97 0.36 0.72 CaO 2.01 2.05 0.60 0.49 0.42 1.20 0.74 0.44 0.32 0.36 1.07 1.60 0.70 1.05 1.31 0.21 0.71 0.27 1.69 0.25 0.95

Na2O 3.56 3.64 3.59 4.93 5.04 2.51 3.33 3.05 1.84 3.63 4.64 3.75 5.08 3.74 4.33 3.07 2.50 3.49 3.55 3.47 3.46

K2O 3.54 3.50 3.32 1.56 1.50 4.19 3.31 4.78 5.43 4.70 3.30 3.87 3.04 4.27 2.99 4.96 4.25 4.62 3.44 3.35 4.03

P2O5 0.15 0.15 0.16 0.05 0.14 0.17 0.17 0.15 0.11 0.04 0.17 0.16 0.08 0.16 0.10 0.13 0.14 0.07 0.09 0.12 0.15 LOI 0.54 0.59 1.23 0.53 0.90 2.35 1.35 1.15 1.31 0.40 1.62 1.22 1.13 1.54 0.96 0.94 1.42 0.72 0.73 1.04 1.13 Total 100.78 102.19 99.95 99.56 99.72 99.66 99.78 99.86 104.36 99.11 99.04 98.97 98.88 99.12 99.07 98.97 99.07 99.30 98.94 98.93 99.14 Li (ppm) 36.80 36.86 23.13 13.29 13.37 41.65 24.69 24.48 17.01 10.05 30.06 45.95 32.74 18.31 43.72 15.19 43.92 7.45 37.80 21.63 21.34 Sc 7.08 7.18 7.48 3.30 3.73 12.11 8.27 7.19 3.78 2.21 4.67 4.37 3.66 4.19 5.66 2.44 8.40 3.03 5.79 2.12 6.92 V 42.43 43.68 28.43 11.74 12.82 66.13 23.90 24.19 12.42 6.34 36.41 28.74 11.31 17.55 19.33 9.70 51.82 2.49 41.11 12.09 24.66 Cr 23.94 23.81 13.77 4.40 5.28 35.08 9.92 10.01 4.36 2.86 5.95 4.97 2.74 2.48 2.89 4.20 27.90 1.46 24.92 5.44 9.60 Co 47.34 61.02 26.14 51.44 23.95 31.96 31.80 33.41 32.83 58.69 35.78 30.02 52.09 34.91 29.77 37.03 45.83 38.63 48.17 32.61 4.46 Ni 13.15 13.38 6.72 4.57 2.55 18.85 4.66 3.53 3.04 1.30 2.19 1.71 1.10 0.84 1.20 1.32 14.15 0.58 10.29 1.99 5.20 Cu 10.74 11.00 2.28 2.02 16.74 13.75 19.29 15.01 4.75 4.66 1.28 3.23 10.78 1.62 4.19 0.53 2.89 0.52 6.65 6.01 4.66 Zn 57.55 56.15 30.68 18.70 22.95 70.78 71.73 0.24 0.13 0.22 0.44 0.66 0.32 0.21 0.64 0.09 0.38 0.28 52.45 18.55 30.97 Ga 17.66 17.87 18.47 13.57 17.62 21.38 17.71 21.54 20.25 17.88 20.18 20.03 19.82 20.83 23.83 22.51 22.76 21.61 17.87 16.32 17.30 Rb 116.70 120.90 100.20 32.00 64.20 159.90 90.70 143.30 191.90 133.40 140.90 156.10 117.40 130.60 159.30 228.90 177.80 191.60 122.50 141.00 123.00 Sr 174.60 171.40 80.60 68.90 81.10 68.30 63.80 64.50 31.50 64.00 130.10 241.00 74.90 213.30 116.10 57.00 66.80 58.00 128.20 58.30 81.90 Y 35.01 33.23 23.20 35.61 29.61 26.15 57.82 52.05 7.05 27.22 17.74 18.30 19.28 20.53 25.53 5.07 30.23 17.40 27.57 9.73 56.85 Zr 182.40 194.76 242.88 199.92 98.16 243.24 198.00 193.92 141.84 66.50 125.20 114.60 118.20 119.70 98.90 90.00 176.40 39.50 166.20 109.70 262.20 Nb 8.67 8.43 9.83 5.09 6.46 12.45 9.79 10.75 5.93 3.54 12.19 11.09 12.43 11.33 16.81 14.32 12.37 10.58 12.19 8.54 10.00 Cs 4.55 4.34 3.13 0.55 5.14 6.86 2.40 4.59 4.15 1.23 7.56 6.55 3.29 2.43 8.27 4.33 4.80 1.80 4.74 3.40 3.37 Ba 955.78 890.81 762.31 238.22 186.36 769.62 678.32 749.81 297.96 271.38 522.96 681.63 410.09 924.45 261.42 500.60 623.56 365.24 554.69 314.43 732.26 La 25.68 24.61 32.75 17.19 16.22 38.95 72.20 22.34 9.31 9.43 21.95 22.01 20.04 41.46 19.14 15.84 21.32 9.29 17.78 6.25 39.03 Ce 43.09 45.76 63.57 41.07 34.65 63.81 60.92 63.06 27.27 31.94 70.47 61.38 47.78 80.51 48.49 52.86 64.94 20.49 44.86 18.48 82.34 Pr 5.99 5.90 7.95 4.66 4.21 8.97 15.73 6.59 2.81 2.87 5.43 5.62 4.89 9.79 5.12 3.92 5.84 2.44 4.93 1.79 9.50 Nd 24.81 24.48 32.95 18.81 17.34 36.35 64.71 26.80 11.36 10.94 20.78 21.86 18.88 36.05 19.65 14.60 22.77 9.04 20.76 7.24 39.71 Sm 6.11 5.95 8.24 4.84 4.88 8.53 15.16 7.10 2.27 3.02 4.05 4.44 4.33 6.45 4.68 2.81 4.86 2.43 5.01 1.62 9.24 Eu 1.24 1.17 1.00 0.43 0.54 1.23 1.72 0.99 0.35 0.16 0.88 0.89 0.57 1.05 0.61 0.22 0.65 0.22 0.65 0.19 1.05 Gd 3.99 3.83 4.70 3.16 3.00 4.96 8.55 7.50 1.72 3.31 3.60 4.02 3.96 5.29 4.72 2.09 4.46 2.44 3.00 0.95 6.09 Tb 0.72 0.71 0.74 0.67 0.60 0.77 1.40 1.32 0.26 0.65 0.51 0.57 0.59 0.70 0.79 0.23 0.72 0.45 0.57 0.18 1.15 Dy 5.90 5.75 4.78 6.42 5.20 5.20 10.20 8.98 1.52 4.51 2.86 3.33 3.50 3.70 4.66 1.07 4.89 2.97 4.40 1.54 9.27 Ho 1.14 1.11 0.80 1.33 1.01 0.87 1.88 1.94 0.30 0.97 0.57 0.63 0.66 0.69 0.88 0.18 1.07 0.59 0.88 0.34 1.86 Er 2.41 2.35 1.62 3.08 2.32 1.76 4.01 5.64 0.85 2.76 1.59 1.70 1.78 1.80 2.30 0.46 3.08 1.67 1.92 0.86 4.13 Tm 0.44 0.44 0.30 0.63 0.48 0.32 0.74 0.79 0.12 0.40 0.22 0.22 0.24 0.23 0.31 0.06 0.43 0.24 0.34 0.18 0.76 Yb 2.49 2.52 1.81 3.76 3.00 1.97 4.34 5.54 0.91 2.87 1.70 1.61 1.69 1.69 2.20 0.43 2.99 1.83 2.24 1.26 5.11 Lu 0.39 0.39 0.29 0.52 0.46 0.31 0.70 0.81 0.14 0.43 0.28 0.25 0.25 0.26 0.35 0.07 0.45 0.27 0.31 0.20 0.77 Ta 1.56 1.83 1.24 1.39 1.15 1.38 1.40 1.41 1.40 1.56 2.42 1.80 2.44 2.45 2.45 3.21 1.85 2.32 2.10 2.31 0.87 Pb 16.97 16.19 8.49 4.52 5.90 5.84 39.48 16.96 4.91 21.89 13.28 18.19 18.51 23.33 12.31 10.53 8.35 25.04 12.07 5.62 15.62 Th 8.86 9.44 13.02 15.16 6.35 10.74 9.84 11.52 3.32 8.34 8.67 9.98 10.34 17.14 12.31 10.32 10.38 7.48 9.16 3.61 13.64 U 2.65 2.81 1.51 1.61 2.18 1.74 2.54 8.76 0.57 2.46 4.12 5.99 2.93 3.81 5.45 3.47 3.41 2.55 2.72 1.41 3.22 A/CNK 1.17 1.20 1.31 1.15 1.31 1.36 1.31 1.32 1.40 1.08 1.14 1.10 1.10 1.16 1.18 1.31 1.51 1.21 1.21 1.34 1.19 A/NK 1.61 1.66 1.46 1.25 1.41 1.71 1.50 1.43 1.49 1.15 1.34 1.41 1.22 1.36 1.46 1.35 1.73 1.27 1.60 1.40 1.39

(Eu/Eu*)N 0.77 0.75 0.49 0.34 0.43 0.58 0.46 0.41 0.55 0.15 0.70 0.64 0.42 0.55 0.40 0.27 0.42 0.28 0.51 0.47 0.43

(La/Yb)N 7.00 6.64 12.32 3.11 3.67 13.41 11.29 2.74 6.92 2.23 8.76 9.27 8.07 16.69 5.91 25.06 4.85 3.45 5.39 3.37 5.19

Table 3 Representative whole-rock major (wt%) and trace element compositions (ppm) for metagranitoids of the Hintertal Plutonic Suite. The samples on the left side (MM12-SP81) are part of the Pletzen Pluton and the samples on the right are part of the Griessstein Pluton.

Lable MM12 MM17 MM18 MM19 MM23 MM55 MM56 MM65 SP58 SP59 SP80 SP81 MM20 MM21 MM22 MM52 MM53 SP53 SP54

SiO2 (wt%) 53.71 68.41 70.00 64.21 67.73 63.08 64.15 62.73 63.58 59.14 65.54 63.96 73.24 73.07 73.24 70.40 69.11 70.30 68.45

TiO2 1.03 0.63 0.43 0.68 0.53 0.70 0.70 0.72 0.84 1.01 0.66 0.70 0.25 0.28 0.24 0.37 0.46 0.38 0.50

Al2O3 17.54 15.06 14.56 16.90 15.61 16.94 16.14 16.77 16.19 17.06 15.57 16.41 14.40 14.27 13.90 14.49 14.68 14.83 15.25

Fe2O3 9.04 3.53 3.03 4.39 3.34 4.53 4.49 4.73 5.04 6.47 4.13 4.34 1.71 1.93 1.93 2.51 3.13 2.40 3.27 MnO 0.18 0.07 0.06 0.10 0.06 0.07 0.08 0.08 0.07 0.13 0.09 0.07 0.04 0.05 0.06 0.04 0.06 0.04 0.06 MgO 4.51 1.41 1.03 1.84 1.29 1.80 2.06 1.86 1.38 2.58 1.60 1.66 0.48 0.56 0.62 1.24 1.69 0.98 1.04 CaO 6.80 1.75 2.27 3.05 2.67 4.30 4.25 4.22 3.65 4.89 2.95 3.66 1.19 1.13 1.30 0.95 1.10 1.24 2.03

Na2O 3.16 3.82 3.80 3.85 4.19 4.27 3.77 4.13 3.41 3.58 4.44 4.15 3.98 3.96 3.50 3.77 3.59 4.14 4.06

K2O 2.22 3.95 3.51 3.30 2.98 2.47 2.27 2.56 2.95 2.45 2.56 2.57 3.85 3.79 4.15 3.76 3.74 2.86 2.73

P2O5 0.39 0.18 0.15 0.25 0.18 0.26 0.26 0.27 0.38 0.32 0.23 0.25 0.08 0.10 0.10 0.13 0.17 0.17 0.22 LOI 0.69 0.95 0.78 1.06 0.85 0.57 0.72 0.66 1.23 1.13 0.82 1.03 0.67 0.67 0.67 1.54 1.42 1.61 1.10 Total 99.26 99.76 99.61 99.62 99.43 98.99 98.89 98.73 99.05 99.02 98.85 99.03 106.47 106.31 99.71 99.21 99.13 99.09 98.95 Li (ppm) 21.69 29.95 20.52 30.40 33.44 31.21 25.24 38.75 47.68 94.66 35.89 43.68 25.57 34.88 20.75 27.69 35.81 31.77 43.05 Sc 24.09 9.24 6.50 9.40 6.69 9.74 8.49 7.47 8.45 23.61 9.64 7.92 4.50 4.78 3.90 4.63 5.44 2.42 4.02 V 213.85 55.83 48.46 73.79 53.20 77.84 84.82 77.70 77.97 136.51 64.78 70.95 15.38 19.88 24.74 34.92 43.93 29.64 40.19 Cr 72.70 12.31 11.87 15.30 11.78 16.11 25.23 16.07 3.54 7.81 13.23 13.86 3.44 4.03 5.58 12.95 16.20 5.46 5.39 Co 42.68 33.15 49.48 40.23 42.73 38.60 41.34 31.34 42.81 30.15 6.63 7.31 24.07 40.70 27.34 19.18 23.57 3.07 47.15 Ni 21.77 4.70 4.21 5.79 4.28 5.52 10.06 5.58 2.97 5.52 4.60 5.10 1.27 1.54 1.99 2.51 3.18 2.11 2.49 Cu 44.65 2.37 1.55 3.16 4.09 4.89 7.44 5.96 3.81 8.66 2.59 4.32 1.36 3.00 1.06 0.79 1.10 1.23 1.12 Zn 147.03 59.71 44.24 84.47 67.33 0.97 0.86 0.91 86.45 107.71 91.75 75.84 37.68 0.52 36.41 0.31 0.52 23.65 53.16 Ga 21.70 19.49 18.23 20.67 18.51 23.39 21.30 22.20 20.08 20.54 16.91 18.86 18.38 19.75 16.10 18.60 20.25 17.28 17.67 Rb 66.90 104.30 88.90 95.50 78.50 69.00 59.70 73.80 73.50 76.70 74.50 83.60 111.50 118.40 102.10 95.30 107.50 107.40 93.90 Sr 761.50 416.00 434.00 558.30 534.10 756.00 699.90 733.30 455.40 521.60 531.20 639.60 263.50 247.20 336.50 165.70 208.60 164.20 307.70 Y 36.05 33.98 19.54 25.75 23.98 25.26 18.40 19.53 30.81 48.59 32.57 27.01 19.09 21.51 9.28 9.40 19.46 8.33 17.25 Zr 255.60 177.36 165.48 241.44 191.64 215.30 183.90 207.70 308.30 223.90 215.90 223.40 114.96 156.12 125.64 128.40 149.60 165.90 208.90 Nb 10.90 16.77 10.16 12.67 10.72 11.72 10.84 11.24 11.73 9.73 10.50 11.77 9.02 10.20 4.30 6.52 10.19 11.63 11.39 Cs 1.49 2.49 1.61 1.69 2.54 2.04 1.23 1.81 3.78 6.36 2.46 2.09 2.63 3.52 2.35 4.91 3.28 3.27 2.65 Ba 1220.82 868.37 904.24 922.83 1122.30 1015.40 929.18 1085.02 1569.07 1077.97 1101.26 883.09 1054.71 1064.91 961.89 982.63 958.14 617.11 902.87 La 44.37 93.95 39.34 26.45 38.93 38.25 43.70 25.77 83.74 97.71 66.39 59.22 24.36 24.63 27.80 22.88 28.14 17.04 20.92 Ce 83.23 163.15 71.28 63.08 73.10 103.63 113.73 97.24 138.32 136.47 114.52 111.76 51.66 60.77 54.40 53.41 66.20 53.23 78.46 Pr 9.53 16.35 7.20 7.27 8.01 10.09 10.00 7.32 19.36 21.46 13.92 11.79 5.57 5.94 5.27 5.28 6.87 3.82 5.31 Nd 40.86 59.56 26.89 30.90 31.98 39.47 36.56 29.73 78.73 87.21 55.06 44.54 22.08 22.50 19.42 19.62 26.18 15.07 21.88 Sm 9.39 10.65 4.96 6.93 6.59 7.30 5.96 5.67 14.51 17.07 10.42 8.00 4.81 4.73 3.09 3.32 4.94 2.66 4.56 Eu 2.21 1.73 1.11 1.56 1.49 1.75 1.42 1.41 2.92 3.34 2.02 1.65 0.94 0.90 0.88 0.94 1.19 0.45 0.89 Gd 5.33 6.18 2.91 3.96 3.81 6.42 4.98 5.03 7.57 9.32 5.87 4.68 2.88 4.44 1.88 2.76 4.39 1.53 2.77 Tb 0.86 0.86 0.43 0.64 0.58 0.85 0.62 0.68 1.00 1.38 0.88 0.69 0.48 0.65 0.23 0.33 0.60 0.22 0.42 Dy 6.45 5.92 3.13 4.83 4.30 4.71 3.40 3.77 5.95 9.15 5.90 4.74 3.49 3.87 1.53 1.76 3.56 1.50 2.94 Ho 1.23 1.06 0.60 0.89 0.79 0.92 0.65 0.73 1.02 1.64 1.09 0.89 0.64 0.75 0.29 0.34 0.70 0.29 0.54 Er 2.71 2.38 1.37 1.86 1.69 2.44 1.84 1.93 2.07 3.35 2.30 1.89 1.29 1.89 0.67 0.82 1.90 0.59 1.17 Tm 0.51 0.45 0.26 0.35 0.31 0.31 0.24 0.25 0.33 0.53 0.38 0.33 0.22 0.23 0.13 0.10 0.24 0.11 0.21 Yb 3.12 2.76 1.62 2.07 1.90 2.08 1.68 1.67 1.97 3.33 2.37 2.12 1.27 1.50 0.88 0.68 1.65 0.80 1.43 Lu 0.50 0.45 0.26 0.32 0.29 0.31 0.24 0.25 0.28 0.49 0.35 0.32 0.21 0.24 0.16 0.10 0.25 0.11 0.22 Ta 0.92 1.83 1.73 1.76 1.64 1.54 1.83 1.38 1.50 1.02 0.84 0.86 0.97 1.49 0.82 0.63 1.16 1.03 2.10 Pb 21.17 19.64 13.98 10.33 16.06 17.69 14.62 15.40 16.23 16.79 17.98 18.39 23.15 35.94 15.51 10.20 10.26 4.11 10.29 Th 5.72 19.19 12.30 5.99 8.66 9.30 12.62 6.69 17.92 18.62 11.73 12.22 10.98 14.27 9.17 6.52 12.47 6.04 11.22 U 1.05 4.75 2.17 3.07 2.37 2.26 3.19 2.28 2.77 2.65 2.27 2.64 2.19 2.75 2.41 0.97 3.85 1.54 2.85 A/CNK 0.88 1.10 1.03 1.09 1.04 0.97 0.99 0.97 1.05 0.98 1.01 1.01 1.12 1.13 1.10 1.21 1.23 1.22 1.14 A/NK 2.31 1.43 1.45 1.71 1.54 1.75 1.87 1.75 1.84 2.00 1.54 1.71 1.34 1.34 1.36 1.41 1.48 1.50 1.58

(Eu/Eu*)N 0.95 0.65 0.89 0.91 0.91 0.78 0.80 0.81 0.85 0.81 0.79 0.82 0.77 0.60 1.11 0.95 0.78 0.68 0.76

(La/Yb)N 9.67 23.11 16.48 8.68 13.89 12.49 17.65 10.51 28.84 19.95 19.07 18.97 13.04 11.12 21.52 22.86 11.57 14.47 9.95 70

Fig. 7. Major elements vs. SiO2 (wt%) variation diagrams (wt%) for metagranitoids of the Seckau Complex. Abbreviations:HrPS: Hochreichart Plutonic Suite, HtPS: Hintertal Plutonic Suite, PltzP: Pletzen Pluton, GrstP: Griessstein Pluton.

5.2 Hintertal Plutonic Suite Meta- to peraluminous metagranitoids of the Hintertal Plutonic Suite show a negative fractionation trend for most major oxides plotted against SiO2, as the metagranitoids of the

Hochreichart Plutonic Suite, but display a larger variation in SiO2 and have a positive correlation of K2O with SiO2 (Fig. 7). Trace element variation diagrams (Fig. 8) show clear differences in their distribution pattern of predominantly LIL elements in contrast to metagranitoids of the Hochreichart Plutonic Suite. Rb indicates a positive fractionation trend, Sr a negative one and Ba content is more or less constant with an average of about 1060 ppm.

71 Sc, V and Cr indicate negative fractionation trends as well as Y and Zr. Nb is more or less constant with an average of about 12 ppm at SiO2 values < 70 wt% and then decreases with increasing SiO2. Based on field observations, petrographic, geochemical and geochronological criteria, the metagranitoids of the Hintertal Plutonic Suite can be subdivided into (a) the Pletzen Pluton and (b) the Griessstein Pluton (Mandl et al., 2018). Consequently, metagranitoids of the Hintertal Plutonic Suite are discussed separately with regard to the aforementioned groups.

5.2.1 Pletzen Pluton of the Hintertal Plutonic Suite The intermediate to acidic rocks of the Pletzen Pluton display a meta- to peraluminous character and display low A/CNK contents ranging from 0.88 to 1.10 compared to the Griessstein Plutonic Suites (Fig. 5b). On binary variation diagrams of major oxides and trace elements versus SiO2 (Fig. 7, 8) these samples display the highest contents in TiO2 (0.43-1.03 wt%), Al2O3 (14.56-17.54 wt%), MgO (1.03-4.51 wt%), CaO (1.75-6.80 wt%), P2O5 (0.15-

0.39 wt%), Fe2O3 (3.03-9.04 wt%), MnO (0.06-0.18 wt%), Sr (416-762 ppm), Sc (7-24 ppm),

V (48-214 ppm), Y (18-49 ppm) and Zr (165-308 ppm) as well as the lowest contents in SiO2

(53.71-70.00 wt%), K2O (2.22-3.95 wt%) and Rb (60-104 ppm) of all analyzed samples from the Hintertal Plutonic Suite. Na2O values are high and range between 3.16 and 4.44 wt%. Barium and Nb are nearly constant with a mean of 1058 ppm and 12 ppm, respectively (Table 3). On the pyrolite-normalized (McDonough and Sun, 1995) spider diagram a negative Nb, Ta, P and Ti anomaly are developed (Fig. 9c). In the chondrite-normalized (Mc Donough and Sun, 1995) REE distribution diagram (Fig. 9d) the metagranitoids are enriched in LREE relative to HREE with (La/Yb)N ratios of 8.68 to 28.84 [(La/Sm)N = 2.38-5.51;

(Dy/Yb)N = 1.27-1.97] and are characterized by the absence of a significant Eu anomaly with

(Eu/Eu*)N values in the range between 0.65 and 0.95.

Fig. 8. Selected trace elements (ppm) vs. SiO2 (wt%) variation diagrams for metagranitoids of the Seckau Complex. Abbreviations: HrPS: Hochreichart Plutonic Suite, HtPS: Hintertal Plutonic Suite, PltzP: Pletzen Pluton, GrstP: Griessstein Pluton.

5.2.2 Griessstein Pluton of the Hintertal Plutonic Suite Metagranitoids of the Griessstein Pluton have a consistently peraluminous character (A/CNK= 1.10-1.23; Fig. 5b) and display mineralogical and geochemical similarities to S- type metagranitoids of the Hochreichart Plutonic Suite. Major oxides of the Griessstein

Pluton usually indicate the lowest TiO2, Al2O3, MgO, CaO, P2O5, FeOt and MnO values, the highest K2O content and similar Na2O content for all samples of the Hintertal Plutonic Suite

(Fig. 7). They are characterized by moderate to high concentrations of SiO2 (64.48-

73.24 wt%), Na2O (3.50-4.14 wt%) and K2O (2.73-4.15 wt%). Their TiO2 content ranges

73 between 0.24 and 0.50 wt%, Al2O3 values range from 13.90 to 15.25 wt%, Fe2O3 content is between 1.71 and 3.27 wt%, MnO is constant between 0.04 and 0.06 wt%, MgO varies from

0.48 to 1.69 wt%, CaO between 0.95 and 2.03 wt% and P2O5 values range from 0.08 to 0.22 wt%. Metagranitoids of the Griessstein Pluton display the highest Rb values (94-118 ppm) and the lowest Sr (164-337), Sc (2-5 ppm), V (15-44 ppm), Cr (3-16 ppm),Y (8-22 ppm) and Zr (115-209 ppm) values of the Hintertal Plutonic Suite and usually overlap with samples from the Hochreichart Plutonic Suite (Fig. 7 and 8, Table 3). On the pyrolite-normalized multi-element diagram (Fig. 9e) after McDonough and Sun (1995) the Griessstein Plutonic displays a strong negative Nb, Ta, P, and Ti anomaly as well as a positive K and Pb anomaly. The chondrite-normalized REE distribution patterns (Mc Donough and Sun, 1995) from metagranitoids of the Griessstein Pluton are similar to the Pletzen Suite, but slightly less enriched in REE. The (La/Yb)N ratio varies between 9.95-22.86 [(La/Sm)N = 2.87-5.63;

(Dy/Yb)N = 1.14-1.80]. A minor negative Eu anomaly with an average of (Eu/Eu*)N = 0.81 is developed (Fig 9f).

5.3 Dikes Granitic, leucogranitic and pegmatitic dikes vary widely in their major and trace element composition (Table 4). Partly they define a seperate trend as seen in the SiO2 vs K2O and

SiO2 vs Rb bivariate diagrams (Fig. 7 and 8). On pyrolite-normalized and chondrite- normalized multi-element diagrams (McDonough and Sun, 1995) the dikes fall into two distinct groups. The first group contains pegmatitic dikes penetrating the paragneiss exposed in the Alpinpark Steinmühle (MM5 and SP61) and at the Herrschaftskranz (MM51). Further, one leucogranitic dike (MM3) and one granitic dike (MM69) intruded within the metagranitoids of the Hochreichart Plutonic Suite. The samples are dominated by a positive Rb, U, K and Pb anomaly as well as a negative Nb and Ti anomaly (Fig. 9g). On the chondrite-normalized (Mc Donough and Sun, 1995) REE diagram (Fig. 9h), LREE and HREE patterns are relatively constant with (La/Yb)N ranging from 1.51 to 3.52. A negative Eu anomaly with

(Eu/Eu*)N values vary from 0.57 to 0.92, with exception of sample MM3, which has a positive Eu anomaly. The second group comprises fine-grained, small-sized granitic dikes (MM14, MM15, MM54) that intruded into the paragneiss at Mount Rosenkogel. A few meters to the west of these dikes a huge granitic intrusive body, represented by sample MM13 (Hochreichart Plutonic Suite), also penetrates the paragneiss. This group is characterized by a negative Nb, P and Ti 74 anomaly (Fig. 9g), an enrichment in LREE relative to HREE with (La/Yb)N of 12.35-19.72, and a negative Eu anomaly with (Eu/Eu*)N = 0.39-0.59 (Fig. 9h, Table 4).

Table 4 Representative whole-rock major (wt%) and trace element compositions (ppm) for granitic (MM14, MM15, MM54, MM69), leucogranitic (MM3) and pegmatitic dikes (MM5 and SP61, MM51).

Lable MM3 MM5 MM14 MM15 MM51 MM54 MM69 SP61

SiO2 (wt%) 75.31 78.27 72.21 73.30 75.56 72.33 75.82 74.92

TiO2 0.04 0.15 0.42 0.38 0.06 0.23 0.07 0.08

Al2O3 13.92 13.16 15.42 14.95 13.97 14.62 12.93 14.23

Fe2O3 0.44 0.83 1.93 1.73 0.60 1.82 0.56 1.00 MnO 0.04 0.01 0.03 0.01 0.03 0.04 0.01 0.03 MgO 0.08 0.29 0.68 0.75 0.09 0.43 0.13 0.33 CaO 0.77 0.83 1.45 0.91 0.49 1.31 0.41 1.65

Na2O 2.84 3.30 5.74 5.52 3.85 4.09 2.98 4.42

K2O 5.85 1.91 0.97 1.33 4.62 3.53 5.73 1.26

P2O5 0.10 0.06 0.10 0.12 0.11 0.09 0.10 0.11 LOI 0.39 0.98 0.70 0.85 0.45 0.54 0.36 0.80 Total 109.57 103.09 99.65 102.36 107.57 99.02 99.09 98.95

Li (ppm) 5.67 8.23 24.69 29.08 9.70 24.49 7.77 7.22 Sc 2.45 2.90 6.11 4.62 2.81 3.95 2.35 2.04 V 1.67 3.92 26.76 23.02 2.35 12.11 3.24 7.85 Cr 1.33 1.43 5.37 5.45 1.93 2.00 1.74 7.98 Co 42.26 39.31 40.11 126.68 41.12 42.50 36.26 50.13 Ni 1.39 1.12 6.35 3.28 0.82 1.13 1.12 3.89 Cu 1.63 4.12 5.65 2.24 0.40 0.74 3.51 7.37 Zn 8.87 17.99 26.26 0.28 18.71 0.80 0.23 12.72 Ga 14.86 12.51 16.39 19.10 16.81 19.98 14.79 14.64 Rb 122.20 39.20 35.70 38.10 114.90 74.90 134.80 30.70 Sr 113.70 206.80 194.80 136.40 53.90 357.30 94.30 266.50 Y 10.45 42.47 18.91 12.85 13.57 20.27 13.35 11.60 Zr 43.56 56.88 288.72 281.40 56.40 110.70 19.60 25.00 Nb 1.64 4.96 10.69 9.13 5.80 7.02 4.33 4.27 Cs 4.23 0.35 1.47 0.87 1.57 1.21 2.50 0.44 Ba 698.75 1031.90 247.54 384.56 377.01 1178.94 404.22 182.56 La 3.66 6.83 39.62 27.76 7.08 30.72 4.14 3.93 Ce 4.40 9.98 84.30 65.04 12.72 59.84 9.69 7.47 Pr 0.79 1.90 9.38 7.22 1.70 7.65 1.10 1.21 Nd 3.18 7.86 36.12 27.47 6.67 28.73 4.22 4.72 Sm 0.78 2.37 7.49 5.10 1.87 5.64 1.32 0.99 Eu 0.67 0.67 0.82 0.58 0.29 1.01 0.40 0.17 Gd 0.60 2.08 3.93 4.02 1.23 4.87 1.58 0.61 Tb 0.15 0.56 0.57 0.49 0.27 0.70 0.33 0.16 Dy 1.48 5.86 3.77 2.50 2.32 3.90 2.32 1.69 Ho 0.33 1.26 0.64 0.47 0.42 0.73 0.46 0.35 Er 0.80 2.78 1.36 1.19 0.95 1.94 1.26 0.94 Tm 0.19 0.50 0.24 0.14 0.20 0.24 0.18 0.22 Yb 1.38 2.70 1.46 0.96 1.37 1.69 1.30 1.77 Lu 0.24 0.40 0.23 0.14 0.22 0.24 0.19 0.29 Ta 1.18 1.12 1.45 3.31 1.36 1.39 1.39 2.10 Pb 23.89 8.51 8.28 7.87 26.57 24.07 41.18 11.23 Th 1.47 13.93 23.09 15.84 4.67 9.49 2.98 4.97 U 1.83 2.78 2.21 2.18 4.06 1.75 2.58 5.81 A/CNK 1.12 1.46 1.18 1.23 1.14 1.13 1.09 1.22 A/NK 1.26 1.76 1.47 1.42 1.23 1.39 1.16 1.65

(Eu/Eu*)N 2.98 0.92 0.46 0.39 0.57 0.59 0.85 0.68

(La/Yb)N 1.80 1.72 18.49 19.72 3.52 12.35 2.17 1.51

Fig. 9. Pyrolite-normalized trace element distribution patterns and chondrite-normalized REE patterns for metagranitoids of the Seckau Complex: (a, b) for the metagranitoids of the Hochreichart Plutonic Suite, (c, d) for metagranitoids of the Pletzen Pluton of the Hintertal Plutonic Suite, (e, f) for metagranitoids of the Griessstein Pluton of the Hintertal Plutonic Suite and (g, h) for granitic, leucogranitic- and granitic dikes.

76 6 Discussion

6.1 Geochemical evolution of the different magmatic suites of the Seckau Complex The spatial relationship of the different plutonic bodies, with small volumes of Late Devonian/early Carboniferous Griessstein Pluton occuring between the Late Devonian/early Carboniferous Pletzen Pluton and the late Cambrian/Early Ordovician Hochreichart Suite, points to two explanation models: (1) the Griessstein Pluton represents a more evolved Pletzen Pluton composition or (2) the Griessstein Pluton is the result of contamination of the Pletzen Pluton by Hochreichart Suite wall rock. Therefore we discuss the chemical characteristics of the different plutonic suites and use Rhyolite-MELTS (Gualda et al., 2012) to compare observed compositions with calculated ones.

6.1.1 Hochreichart Plutonic Suite Metagranitoids of the Hochreichart Plutonic Suite are chemically evolved and display relatively low initial 87Sr/86Sr values between 0.7056 and 0.7061 (Table 5) although the geochemical characteristics place this suite to a S-type source. These metagranitoids show generally high Rb values and a low Sr content resulting in a high Rb/Sr ratio, typically for an evolved granitic magma. Negative fractionation trends of several major and trace elements indicate the crystallization of zircon, apatite, Fe-Ti-oxide, feldspar and biotite with cooling. The presence of a negative Eu anomaly, seen in chondrite-normalized plots (Fig. 9b), indicates the crystallization and fractionation of plagioclase from the granitic source.

6.1.2 Hintertal Plutonic Suite Chemical fractionation of the Pletzen Pluton is clearly marked by major oxides and trace element trends vs. SiO2 (Figs. 7 and 8). Most rocks of the fractionated Griessstein Pluton show similar decreasing TiO2, Al2O3, MgO, CaO, P2O5, FeOt, MnO trends and increasing

K2O values vs. SiO2 but have lower or higher values compared to the Pletzen Pluton samples, respectively (Fig. 7, 8 and 10). LIL elements such as Cs and Rb are enriched in the Griessstein Pluton compared to the Pletzen Pluton and Sr, HFS elements (e.g., Y, Zr, Ce) as well as transition metals (e.g., Sc, V, Cr, Ni) are enriched within the rocks of the Pletzen Pluton (Fig. 7, 8 and 10). The similar geochemical behavior of K and Rb is also a good indicator to demonstrate fractionation effects on granitic melts (Cerńy et al., 1982;

77 Quemeneur and Lagache, 1999; London, 2008). The correlation of e.g., K/Rb ratio vs. Sr (Fig. 10i) suggests that the studied granitoids of the Griesssstein Pluton are generally more evolved than rocks of the Pletzen Pluton and thus most likely have been derived from the same source. Interestingly sample MM22 and MM52 deviate in this variation diagrams from the trend and might indicate wall rock contamination. Based on the equation of Watson and Harrison (1983) whole-rock zircon saturation temperatures (TZr) were calculated for both plutons of the Hintertal Plutonic Suite. Calculated

TZr of the parental Pletzen Pluton range from 765 to 833°C with an average of 793°C. The Griessstein Pluton indicates insignificant lower temperatures ranging from 764 to 814°C with an average of 788°C. During cooling and fractionation, zircon crystallized from the melt incorporating yttrium and heavy rare earth elements. This is seen in decreasing yttrium and REE with fractionation. At the same time, Fe-Ti oxide and apatite crystallized from the melt producing the observed negative anomalies in pyrolite normalized spider plots (Fig. 9). Initial 87Sr/86Sr ratios from rocks of the Pletzen Pluton vary between 0.7051 and 0.7061 and initial 87Sr/86Sr data of rocks from the Griessstein Pluton are in the range between 0.7054 and 0.7063.

Table 5 Calculated initial 87Sr/86 Sr for metagranitoids of the Hochreichart Plutonic Suite (HrPS) and the Hintertal Plutonic Suite (Pletzen Pluton (HtPS, PltzP) and Griessstein Pluton (HtPS, GrstP)). Rb and Sr isotopic data were taken from Pfingstl et al. (2015) and the average age of the respective granitoids were calculated from U-Pb zircon ages represented in Mandl et al. (2018).

87 86 87 86 87 86 87 86 Sample Lith. Unit Rb Sr Rb/Sr Rb/ Sr ( Sr/ Sr)WR 2SE ( Sr/ Sr)353 ( Sr/ Sr)496 SP62 HrPS 119 130 0.91 2.6421 0.724216 0.000004 0.7056 SP79 HrPS 122 86 1.42 4.1077 0.735010 0.000004 0.7061 SP58 HtPS, PltzP 73 470 0.16 0.4490 0.708325 0.000004 0.7061 SP59 HtPS, PltzP 75 540 0.14 0.4005 0.708124 0.000004 0.7061 SP80 HtPS, PltzP 75 550 0.14 0.3934 0.707748 0.000004 0.7058 SP81 HtPS, PltzP 90 647 0.14 0.4022 0.707143 0.000004 0.7051 SP53 HtPS, GrstP 108 168 0.64 1.8612 0.714706 0.000004 0.7054 SP54 HtPS, GrstP 91 318 0.29 0.8290 0.710423 0.000004 0.7063

In this study the Griessstein Pluton is considered being the product of a fractionation process from the Pletzen Pluton where probably only small amounts of wall rock or a SiO2-rich melt were assimilated. Major element calculations derived from Rhyolite-MELTS demonstrate that at isobaric crystallization conditions of 0.3 GPa, 20-30% of the Pletzen melt (calculated

78 with an average composition of the samples MM17, MM18, MM19, MM23, MM55, MM56, MM65) needs to crystallize in order to produce a composition close to the Griessstein Pluton (calculated with an average composition of the samples MM20, MM21, MM22, MM52, MM53, SP53) (Table 6). The Griessstein Pluton, however, only accounts for approximately 10% of the total Hintertal Plutonic Suite. Either it is just the result of fractionation of small volumes of the original plutonic source or the Griessstein Pluton intruded at shallower crustal levels and large parts of it were eroded during the subsequent Variscan uplift and post- Variscan continental rifting. Parts of these granitoids can probably be recovered in the Permian clastic metasediments of the Rannach Formation. Very similar crystallization ages from rocks of the Griessstein Pluton (358 ± 8 Ma to 343 ± 7 Ma) and the Pletzen Pluton (365 ± 8 Ma to 343 ± 12 Ma) (Mandl et al., 2018) support the assumption that rocks of the Griessstein Pluton are the product of a fractionation process from the more primitive Pletzen melt.

Table 6 Major element calculation with Rhyolite-MELTS (Gualda et al., 2012). Abbreviations: PltzP: Pletzen Pluton, GrstP: Griessstein Pluton.

average average calculated PltzP composition GrstP composition 30 % crystallized [wt%] [wt%] [wt%]

SiO2 65.76 71.56 70.20

TiO2 0.63 0.33 0.72

Al2O3 16.00 14.43 14.24

Fe2O3 4.00 2.27 1.97 MnO 0.07 0.05 0.00 MgO 1.61 0.93 0.53 CaO 3.22 1.15 2.18

Na2O 3.97 3.82 3.28

K2O 3.01 3.69 4.08

P2O5 0.22 0.12 0.00

Fig. 10. Variation diagrams. (a-h) SiO2 (wt%) vs major oxides (wt%) and trace elements (ppm). (i) K/Rb ratio vs trace elements (ppm) for metagranitoids of the Pletzen Pluton and the Griessstein Pluton as well as for pegmatitic and granitic dikes.

A possible assimilation of Hochreichart granitic rocks into the marginal portions of the Pletzen intrusives was tested in order to explain the differences between the two plutons of the Hintertal Suite. By using average major and trace element compositions of Hochreichart and Pletzen plutonic rocks it was not possible to obtain the measured composition of the

Griessstein Pluton. Assuming an assimilation of 10 % of the Hochreichart Pluton, lower SiO2 and Rb values as well as higher Sr, V, Cr and Nd values would be required to reproduce the chemical composition of the Griessstein Pluton. Alternatively, also the paragneiss of the Glaneck Metamorphic Suite was tested as contaminant for the Pletzen intrusives. Again, the observed chemical composition of the Griessstein Pluton could not be reproduced.

80 6.1.3 Origin of dikes Based on chondrite normalized spider plots two distinct groups of dikes can be distinguished. Dikes of group 1 intrude the paragneiss of the Glaneck Metamorphic Suite (MM5 and SP61) but also the Hochreichart Plutonic Suite (MM3, MM69, MM51) and represent highly fractionated melts (Figs. 7,8,10). The pegmatitic dikes (MM5 and SP61) found in the paragneiss of the Glaneck Metamorphic Suite show generally higher TiO2, Al2O3, MgO,

CaO, Sr and lower K2O and Rb values than pegmatitic (MM51), leucogranitic (MM3) and granitic dikes (MM69) penetrating the granodiorite gneiss of the Hochreichart Plutonic Suite. U/Pb zircon ages of pegmatitic dikes display a concordia age of 361 ± 13 Ma (MM3) and 365 ± 11 Ma (MM5) (Mandl et al., 2018) which is slightly older compared to the group 2 dikes. The different types of the group 1 dikes represent most likely residual melts from the main stage of the intrusive activity of the Hintertal Suite. Dikes belonging to group 2 (MM14, MM15, MM54) represent evolved granitic melts similar to the Griessstein Pluton that penetrate mainly the paragneiss of the Glaneck Metamorphic Suite. They yield a slightly younger U-Pb zircon age of 348 Ma ± 14 Ma, 331 ± 10 Ma and 349 ± 8 Ma, respectively (Mandl et al., 2018) and are interpreted as late fractionation products from the same source region of the Pletzen Pluton, as can be seen by different variation plots e.g., Al2O3, MgO, P2O5 vs. SiO2 (Fig. 7) as well as e.g., Sr vs. SiO2 and V vs.

SiO2 (Fig. 8).

6.2 Paleogeographic setting of the Seckau Complex Early Variscan (365 ± 8 Ma to 343 ± 12 Ma; or mid Variscan after Raumer and Stampfli, 2008) plutonites of the Seckau Complex represent calc-alkaline magmatism with meta- to peraluminous (Pletzen Pluton) and peraluminous character (Griessstein Pluton), respectively and can be interpreted as volcanic arc granites. These granitoids correspond to the early Variscan “Cordillerian” I-type granitoids that mainly occur in e.g., the Saxothuringian (Odenwald), Central Bohemia (Central Bohemian Batholith; Zák et al., 2011) and the intra-Alpine realm (e.g., eastern part of the Hohe Tauern Batholith, Schladming Batholith). Granitoids with similar geochemical characteristics (“Cetic granitoids”) also occur, as exotic boulders, along the southern margin of the Bohemian Massif, close to the northern front of the Eastern Alps. The so-called “Cetic granitoids” are embedded in sediments of the Ultrahelveticum and the Rheno-Danubian Flysch Belt and are

81 considered to be part of the Cetic Massif, a hypothetical crystalline basement block which adjoins the Moldanubian Unit to the south below the Eastern Alps (Finger et al., 1997). The general paleogeographic position of the Seckau Complex during the Variscan orogenic phase is considered to be south to southeast of the Bohemian Massif, proximal to the eastern Hohe Tauern, the Schladminger Tauern and the Western Carpathians (Schermaier et al., 1997) before the onset of Jurassic rifting and separation of the Adriatic Plate. Therefore, the Seckau Complex is interpreted as being part of an early Variscan magmatic arc along the southern margin of a continental sliver, now constituting Bohemia. This continental terrane containing Armorica by ca. 350 Ma was termed Galatia (von Raumer et al., 2013). Granitoid emplacement can be related to the closure of the Galicia-Moldanubian Ocean, alternatively termed Paleotethys (Fig. 11). At Late Devonian to early Carboniferous time this arc is represented by the Hintertal Plutonic Suite (in particular the Pletzen Pluton). The ca. 500 Ma old Hochreichart Pluton was already part of the consolidated crust now comprising Bohemia and its extension to the Alps.

Fig. 11. Plate tectonic situation of the central European Variscides during Middle Devonian to Middle Carboniferous (modified after Franke et al., 2017; Stampfli et al., 2013).

82 7 Conclusions (1) Late Cambrian to Early Ordovician metagranitoids of the Hochreichart Plutonic Suite show a peraluminous character (A/CNK= 1.08-1.51). They are characterized by negative fractionation trends of most major elements and e.g., Rb and Ba against 87 86 SiO2. They have a significant negative Eu anomaly and initial Sr/ Sr ratios of 0.7056 to 0.7061.

(2) Late Devonian to early Carboniferous metagranitoids of the Pletzen Pluton (Hintertal Plutonic Suite) are meta- to weakly peraluminous in composition (A/CNK= 0.88 to 1.10). These rocks are characterized by negative fractionation trends of most major

elements and e.g., Sr against SiO2. K2O and Rb represent a positive fractionation 87 86 trend against SiO2. A significant Eu anomaly is absent and initial Sr/ Sr ratios vary between 0.7051 and 0.7061.

(3) Late Devonian to early Carboniferous metagranitoids of the Griessstein Pluton (Hintertal Plutonic Suite) are considered to be the product of fractionation processes from the Pletzen Pluton. They indicate a peraluminous character (A/CNK= 1.10 to 1.23) and show similar fractionation trends for most main and trace elements against 87 86 SiO2 to their parental melt. A slight Eu anomaly has developed and initial Sr/ Sr values range from 0.7054 to 0.7063.

(4) The early Variscan granitoids of the Seckau Complex are interpreted as being part of the southern margin of Bohemia. Early Variscan magmatism was related to the subduction of the Paleotethys prior to the final collison of Gondwana and Laurussia.

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Chapter 3 |

U-Pb zircon geochronology and sedimentary provenance of detrital zircons from sedimentary rocks of the Rannach Formation, Eastern Alps

89 Abstract The Rannach Formation within the Seckau Nappe (Eastern Alps) represents one of the widespread sedimentary cover units within the Austroalpine Unit. Although the Rannach Formation comprises more than 1000 meter thick sediment sequences there is only limited information about the age distribution and the ancestry of detrital zircons in literature. This study determines U-Pb ages from 284 zircons of eight samples to identify possible internal and external source domains by matching detrital zircon ages with crystallization ages of potential source regions. Analyzed zircons from sedimentary rocks of the Rannach Formation indicate a wide range of ages and span over considerable time ranging from late Archean to Permian times. The two most prominent peaks of the detrital age spectrum at 555 ± 7 Ma and at 508 ± 7 Ma reflect most likely derivation from a final stage of the Cadomian orogeny and a subsequent re- attachment event of the Cadomian Belt to Gondwana during Cambrina times. A significant peak at 457 ± 7 Ma indicates Middle/Upper Ordovician rifting due to the opening of the Rheic Ocean whereas a further dominant peak at 358 ± 5 Ma demonstrates Meso-Variscan collision tectonics. The youngest peak at 293 ± 6 Ma reflects magmatic input from Permian postorogenic rifting and lithospheric thinning. This detrital zircon age spectrum displays predominantly the same U-Pb ages as observed from different crystalline basement units of the Seckau Complex e.g., the Glaneck Metamorphic Suite, the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. Consequently, a primary local provenance is suggested for most of the clastic sediments forming the Rannach Formation. Only Ordovician and Permian zircon ages are not documented in the aforementioned suites of the Seckau Complex and suggest more regional sources e.g., domains to the south of the Tauern Window or close to the Southern Alps.

1 Introduction The study of detrital zircons within different nappes of the Austroalpine Unit provides information on the spatial distribution of pre-Alpine basement units and ancestry of Austroalpine units. Each peak in the detrital zircon age spectrum displays information about a magmatic or metamorphic event the source area had experienced during its history. Zircon is often used to identify potential source areas because it is a very common mineral in

90 magmatic, metamorphic and sedimentary rocks and resistant to mechanical abrasion and chemical alteration. Permomesozoic metasedimentary units are widespread in the Eastern Alps. These sedimentary units are characterized by coarse-grained clastic sediments at its base and are thought to be Permian in age, overlain by thick Triassic carbonate layers (Schnabel, 1980a, b). Today the Permo-Mesozoics are dissociated from each other due to Eo-Alpine tectonics (Schmid et al., 2004). The Rannach Formation, east of the Tauern Window, represents one of these Permomesozoic units and is of special interest because of its extraordinary thick Permian sedimentary sequences and the almost complete absence of carbonate beds. As part of this study U-Pb detrital zircon geochronology from different levels of the Rannach Formation is performed to identify less revealed provenances. This makes a substantial contribution to decipher the complex pre-Alpine history of the Alps.

2 Geological setting Permian to early Mesozoic metasedimentary sequences are widely dispersed within the Austroalpine Nappe System (Fig. 1). These Permo-Mesozoic metasediments, except those of the Northern Calcareous Alps and the sediments of the Drau-Gurktal basement nappes, are generally known as Central Austroalpine Mesozoic (CAM) that represents the cover of most high-grade Austroalpine basement nappes (e.g., Tollmann, 1977; Schmid et al., 2004). The CAM comprises e.g., the Brenner and the Stangalm Mesozoic that covers parts of the Ötztal and the Bundschuh basement nappes, the Thörl and the Rannach Permo-Mesozoic that cover the Troiseck-Floning-Zug as part of the Gleinalpe Unit and the crystalline basement of the Seckau Nappe. The Permian to Lower Triassic sequences of the Brenner Unit, the Stangalm Unit and the Thörl Unit have a minor thickness of about 5 to 10 meters, 25 to 30 meters and up to 200 meters, respectively, whereas the metasedimentary successions of the Rannach Formation have a thickness of hundreds of meters and reach a maximum of more than 1000 meters (Pfingstl, 20013, Master Thesis; Schnabel 1980a, b). The Koralpe-Wölz Nappe System is the only Upper Austroalpine Unit that lacks Permomesozoic cover series – those were completely stripped off during an early phase of the Eo-Alpine orogenic event (Schuster et al., 2004). Close to the western margin of the Austroalpine nappes Permo-Mesozoics occur as un- metamorphed Landwasser-Ducan sediments covering parts of the Silvretta basement nappes

91 and along the eastern margin Permo-Mesozoic metasediments, summarized as Semmering Quarzite that overlies the Lower Austroalpine nappes of the Semmering Wechsel Nappe System (e.g., Pfingstl et al., 2015; Schmid et al., 2004; Schuster, 2004; Schuster et al., 2004).

Fig. 1. Simplified geological map of the Eastern Alps showing the distribution of Permo- Mesozoic cover units (yellow colored) south of the Northern Calcareous Alps (based on Schmid et al., 2004). The study area is marked with a black rectangle. Abbreviations: BN: Bundschuh Nappe; DZ: Drauzug Nappe; Gl: Gleinalpe; GN: Gurktal Nappe; GP: Graz Paleozoic; GWZ: Greywacke Zone; Kor: Koralpe; NCA: Northern Calcareous Alps; Ötz: Ötztal Nappe; SC: Schladming Nappe; Sil: Silvretta Nappe; SN: Seckau Nappe; TW: Tauern Window; WZ: Wölz Nappe.

2.1 Rannach Formation The Rannach Formation, situated east of the Tauern Window, represents the metasedimentary cover of the Seckau Nappe and overlies the northern part of the Seckau Complex and the northwestern part of the Bösenstein area. Both tectonic units, the crystalline basement and the sedimentary cover of the Seckau Nappe, experienced a low grade Eo-Alpine metamorphic overprint with upper greenschist facies conditions (520°C at 0.9 Ga) (Faryad and Hoinkes, 2001). 92 The deepest unit of the Rannach Formation consists of “Alpiner Verrucano” (Schnabel 1980b), defined by Tollmann (1972) as Permian, coarse- to fine- grained, non or poorly stratified, continental detrital sedimentary rocks. These basal conglomerates are overlain by early Triassic rocks e.g., sericite- quartz schists, sericite quartzites, sericite schists and phyllites (Scharbert, 1980; Schnabel, 1980b). Several profiles across the Rannach Formation show a fining upward sequence with coarse-grained conglomerates at the base, followed by sandstones and fine-grained clastics on the top. A profile from the Antonikreuz to the Mount Bremstein comprises actually two fining upwards sequences, incipient by an approximately 10 m thick marble layer at its base, followed by ca. 110 m thick quartz-rich metaconglomerates (medium-grained) and phyllites, with a thickness of about 60 m. The second fining upwards sequence comprises coarse- to fine-grained metaconglomerates with a total thickness of approximately 750 m (Pfingstl, 2013, Master Thesis).

3 Methods Whole rock compositions for the metasediments were determined by a Bruker Pioneer S4 X- ray fluorescence (XRF) spectrometer under standard conditions at the Institute of Earth Sciences, NAWI Graz Geocenter, Austria. U-Pb detrital zircon analysis were performed by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) techniques (Košler, 2008) using a NewWave 193 nm ArF excimer laser coupled to a Nu Plasma II mass spectrometer at the NAWI Graz Central Lab for Water, Minerals and Rocks, NAWI Graz Geocenter, Austria. Zircon separation, sample preparation, measurement conditions, data evaluation and age calculations follow the procedures of Mandl et al. (2018). All single zircon ages and plotted Concordia ages are quoted at the 2σ level.

4 Sample location/sample description For this study different (mainly fine-grained) metasedimentary rocks of the Rannach Formation were sampled. Most of the rocks, with exception of sample MM11, were taken in the southern part of the Rannach Formation, close to the contact to the crystalline basement of the Seckau Complex (Fig. 2). An overview of the sample locations is provided in Figure 2 and Table 1.

Fig. 2. Simplified geological map of the Seckau Complex and the overlying Rannachformation indicating the sample locations of the metasedimentary rocks taken for U-Pb zircon age dating (based on Mandl et al. 2018, Metz 1967; Moser, 2015). Abbreviations: Ghpt: Geierhaupt; Gr.Grst: Grosser Griessstein; Plz: Pletzen; Rk: Rosenkogel; SZ: Seckauer Zinken; Zwflkpf: Zwölferköpfl.

Table 1 Samples analyzed for U-Pb geochronology.

Sample Lithology Locality Latitute Longitude MM11 Metaconglomerate Roßbachgraben, Gaal 47°17'03.2" N 14°33'50.7" E MM24 Metaconglomerate Schwarzlacken, north of Mount Bremstein 47°22'38.6" N 14°47'03.4" E MM25 Metaconglomerate Mount Bremstein 47°22'25.2" N 14°46'47.4" E MM26 Metaconglomerate Mount Bremstein 47°22'20.9" N 14°46'44.5" E MM29 Metaconglomerate Mount Zwölferköpfl/Hühnerkar 47°23'55.2" N 14°37'47.6" E MM30 Metaconglomerate Mount Zwölferköpfl/Hühnerkar 47°23'48.7" N 14°37'43.3" E MM31 Metaconglomerate Mount Zwölferköpfl (ascent to the summit) 47°23'46.1" N 14°37'54.3" E MM34 Metaconglomerate Mount Lattenberg 47°24'05.3" N 14°36'05.0" E

The Rannach Formation predominantly comprises slightly to strongly deformed, fine- to coarse-grained metasedimentary rocks. Representative outcrop photos and photomicrographs of characteristic rocks are shown in Figures 3a-d and 4a-f.

Fig. 3. Metasedimentary rocks of the Rannach Formation. Outcrop of sample (a) MM11, (b) MM26, (c) MM31 and (d) MM34.

Fine-grained metaconglomerate (MM11) situated in the Roßbachgraben, in the southwestern part of the Seckau Complex, represents a small outcrop of a few tens of meters (Fig. 3a), enclosed by paragneiss of the Glaneck Metamorphic Suite (Mandl et al., 2018). The well- foliated metaconglomerate generally strikes northeast to southwest with a dip angle of 70° towards northeast. This rock predominantly comprises quartz (clasts) characterized by undulatory extinction and muscovite shear bands. The quartz clasts are typically aligned parallel to the dominant mineral lineation (L=130/20) and indicate bulging recrystallization microstructures (Fig. 4a). In the southeastern part of the Rannach Formation three metaconglomerate samples, two located in horizontal layers close to the top of Mount Bremstein (MM25 and MM26; Fig. 3b) and one situated at lower altitudes close to the Schwarzlacken, north of Mount Bremstein (MM24), show different fabrics. Sample MM24 has an equigranular texture dominated by fine-grained recrystallized quartz and only little coarser-grained feldspar and muscovite (Fig 4b). In contrast, samples MM25 and MM26 are composed of inequigranular textures. The

95 fine-grained matrix (predominantly quartz) comprises large quartz and feldspar clasts of more than 1 mm in size. These clasts are partly surrounded by muscovite bands (Fig. 4c) and quartz mainly indicates undulatory extinction. In the southwestern part of the Rannach Formation samples MM29, MM30 and MM31 are taken on a path on the ascend to the top of Mount Zwölferköpfl (Fig. 3c), whereas sample MM34 represents the metaconglomerate on the top of Mount Lattenberg (Fig. 3d) further west. These metasedimentary rocks also indicate different textures und mineral compositions similar to the rocks in the southeastern part just mentioned before. The fine-grained metaconglomerate, sample MM29, is dominated by an equigranular texture of recrystallized quartz and fine-grained muscovite bands as well as minor feldspar (Fig. 4d). The westernmost sampled metaconglomerate (MM34) indicated an inequigranular texture comprising larger quartz and feldspar clasts within a fine-grained matrix of predominantly quartz. From special interest is the metaconglomerate sample named MM30 that contains, beside quartz, feldspar and muscovite, also calcite with a size of up to 1 mm (4e).

Fig. 4. Representative photomicrograph (crossed polarizers) of metasedimentary rocks of the Rannach Formation. Abbreviations: cal: calcite, fsp: feldspar, ms: muscovite, qz: quartz.

5 Geochemistry Major and selected trace element compositions for eight metasedimentary rocks of the Rannach Formation, sampled for U-Pb zircon age dating, are presented in Table 2.

These rocks typically show high SiO2 values in the range between 75.52 and 84.41 wt%.

Their TiO2 values vary between 0.204 and 0.741 wt%, the Al2O3 content ranges from 8.49 to nearly 12.00 wt% and Fe2O3 (total Fe) is in the range between 1.49 and 4.03 wt%. MnO values are, with exception of the calcite-rich sample MM30 (0.090 wt%), constant with an average of 0.007 wt%. Sample MM30 also displays, not surprisingly, the upper limit of MgO

97 and CaO content varying from 0.24 to 1.58 wt% and from 0.04 to 2.35 wt%, respectively.

Na2O is in the range between 0.89 and 2.68 wt%, K2O values vary from 1.32 to 2.54 wt% and

P2O5 is nearly constant with an average of 0.064 wt%. Barium content ranges between 216 and 453 ppm, rubidium varies from 54 to 76 ppm, strontium content is between 22 and 56 ppm and vanadium displays values from 27 to 60 ppm. Zirconium varies from 53 to 281 ppm and other selected trace elements e.g., Ce and Cr often show values under the detection limit (u.d.l.) of < 20 ppm.

Table 2 Samples analyzed for U-Pb zircon age dating. Abbreviations: u.d.l.: under detection limit Sample MM11 MM24 MM25 MM26 MM29 MM30 MM31 MM34

SiO2 80.30 79.83 81.50 81.18 75.52 75.90 81.22 84.41

TiO2 0.23 0.48 0.20 0.34 0.74 0.37 0.53 0.29

Al2O3 11.06 9.95 9.35 8.85 11.95 9.34 9.89 8.49

Fe2O3 1.83 2.06 2.48 2.26 4.03 2.17 1.59 1.49 MnO 0.01 0.01 0.01 0.01 0.01 0.09 0.01 0.00 MgO 0.41 1.18 0.47 0.49 0.86 1.58 0.59 0.24 CaO 0.13 0.14 0.10 0.13 0.12 2.35 0.20 0.04

Na2O 1.34 2.68 2.53 2.19 1.77 1.07 1.96 0.89

K2O 2.54 1.33 1.32 1.33 2.55 1.94 1.35 2.10

P2O5 0.06 0.07 0.05 0.07 0.09 0.06 0.08 0.05 LOI 1.37 1.23 0.98 1.06 1.57 4.55 1.24 1.10 Sum (wt.%) 99.28 98.95 98.99 97.91 99.21 99.41 98.64 99.11

Ba 291.20 221.90 262.00 216.00 330.10 453.30 304.00 358.70 Ce 38.20 u.d.l. u.d.l. 53.50 38.90 55.60 u.d.l. 46.90 Cr u.d.l. 22.10 u.d.l. u.d.l. 41.70 20.70 u.d.l. 35.90 Rb 74.60 64.40 53.80 53.70 96.70 75.80 53.70 74.10 Sr 22.20 34.60 29.70 28.70 36.40 53.00 55.50 22.80 V 26.60 28.10 44.20 46.70 60.30 43.00 37.00 40.30 Zr 78.12 147.60 65.76 87.00 281.28 103.80 160.92 52.80

98 6 Detrital zircons Zircon separation from metasedimentary rocks of the Rannach Formation was often difficult because of the small size of many zircon grains. The small-sized zircons have an average grain size of < 100 µm and are typically well rounded. Beside these small rounded zircon grains also rare larger-sized, rounded to euhedral long prismatic zircons with an aspect ratio of mainly 1:2, 1:3 or 1:4 were picked (Fig. 5). The euhedral long prismatic zircons are usually broken in the middle and look very similar to zircon grains separated from the crystalline basement rocks of the Seckau Complex (Mandl et al., 2018). By inspection with a binocular microscope these well-shaped zircon grains appear predominantly colorless and indicate only few cracks and inclusions, in contrast to the smaller well-rounded zircon grains.

6.1 U-Pb zircon ages Detrital zircons from eight metasedimentary rocks of the Rannach Formation yield various U-Pb ages and span a considerable time period from late Archean to Permian times. For each sample U-Pb zircon age data were plotted in a 207Pb/235U vs. 206Pb/238U diagram and a Kernel density estimation plot (KDE) is given for each sample to estimate the relative likelihood of the different ages in the population (Fig. 6a-h) (Vermeesch, 2012). A metasedimentary rock sampled in a small outcrop in the Roßbachgraben (MM11), embedded within basement rocks of the Seckau Complex, provides one dominant Cambrian Concordia age at 507 ± 15 Ma (n=12) and three subordinate concordant ages at 2085 ± 70 Ma (n= 3), 543 ± 26 Ma (n= 4) and 366 ± 20 Ma (n=5) (Fig. 6a). Sample MM24, collected to the north of Mount Bremstein, shows its most two prominent KDE peaks at 291 ± 10 Ma (n=10) and 344 ± 11 Ma (n= 7), respectively. Subordinate peaks comprise beside Neoarchean to Mesoproterozoic ages, zircon grains with a concordant Ordovician age of 455 ± 20 Ma (n= 4) (Fig. 6b). Two fine-grained metasedimentary rocks, close to the top of Mount Bremstein (MM25 and MM26), indicate predominantly late Neoproterozoic (557 ± 14 Ma (n=11; MM25) and Cambrian (518 ± 11 Ma (n=31; MM26) U-Pb Concordia ages as well as early Variscan Concordia ages of 366 ± 15 Ma (n= 5; MM25) and 348 ± 19 Ma (n= 9; MM26), respectively. Beside Neoarchean to Paleoproterozoic zircon ages between 2055 and 2714 Ma (without analytical error) these samples also contain Ordovician-aged zircons (e.g., 457 ± 32 Ma; n= 6; MM26). In both samples of Mount Bremstein Permian zircons are absent (Fig. 6c, d).

Fig. 5. Cathodoluminescense images of zircons from metasedimentary rocks of the Rannach Formation including corresponding 206Pb/238U ages (± 2 σ). White colored tracks indicate the approximated laser track used for U-Pb age dating.

100 The four remaining samples were taken further west, three on the path from Gasthof Jansenberger to Mount Zwölferköpfl (MM29, MM30 and MM31) and the westernmost sample (MM34) derived from the top of Mount Lattenberg. The metasedimentary rock sampled in the area of Mount Zwölferkopfel (MM29) comprises the largest number of concordant age cluster on the 207Pb/235U vs. 206Pb/238U diagram of all studied metasedimentary rocks. Two most dominant KDE peaks are at 548 ± 19 Ma (n= 11) and 297 ± 12 Ma (n= 9). Neoarchean and Paleoproterozoic Concordia ages cluster at 2661 ± 93 Ma (n= 3) and 2070 ± 69 Ma (n= 5) and Cambrian, Ordovician and early Variscan ages at 507 ± 25 Ma (n= 5), 462 ± 32 Ma (n= 3) and 347 ± 19 Ma (n= 5), respectively (Fig. 6e). The calcite-rich sample MM30 shows its maximum KDE peak at 558 ± 13 Ma (n= 15). Interestingly, late Cambrian zircon ages, as observed in the aforementioned samples, are absent. Only four zircon grains indicate early Cambrian ages (at approximately 530 Ma) but have been added to the well-fitting late Neoproterozoic data set. Subordinate Ordovician and early Varican peaks occur at 455 ± 15 Ma (n=10) and 367 ± 10 Ma (n= 4). The youngest Concordia age clusters at 297 ± 12 Ma (n= 5) (Fig. 6f). From sample MM31 only few zircons (n=14) were analyzed, but all calculated zircon ages fit within the age clusters just mentioned before. The maximum KDE peak of sample MM31 displays a concordant age of 554 ± 27 (n= 6) (Fig. 6g). Most zircons of sample MM34, located in metasedimentary rocks at the top of Mount Lattenberg, indicate a late Neoproterozoic Concordia age of 552 ± 17 Ma (n= 8). But there are also Neoarchean to Paleoproterozioc, late Cambrian (as observed in sample MM30), Ordovician, early Variscan and Permian concordant age cluster at 2545 ± 74 Ma (n= 4), 2049 ± 67 Ma (n= 3), 529 ± 22 Ma (n= 6), 462 ± 10 Ma (n= 5), 363 ± 17 Ma (n= 7) and 283 ± 17 Ma (n= 6), respectively (Fig. 6h). Nearly all discussed metasedimentary samples comprise beside well-defined concordant ages, also few single Concordia ages (mainly Archean to Proterozoic). Together, these ages reflect concordant ages at 2831 ± 70 Ma (n= 4), 2693 ± 41 Ma (n=7), 2513 ± 50 Ma (n= 8), 2362 ± 56 (n= 4) as well as at 919 ± 46 Ma (n= 4), 725 ± 36 Ma (n= 4) and 641 ± 24 Ma (n= 5). Only very few analyzed age data (< 2%) had to be excluded.

101

Fig. 6. U-Pb diagrams including Kernel density estimation (KDE) plots for metasedimentary rocks of the Rannach Formation. Error ellipses are given at the 2 σ level. 102 Considering all individual detrital zircons from different metasedimentary rocks of the Rannach Formation throughout Archean to Permian times, six main concordant zircon ages can be observed (Fig. 7, Fig. 8a-f): (1) Paleoproterozoic zircons with a Concordia age of 2063 ± 30 Ma (n= 16; Fig. 8f), (2) late Neoproterozoic zircons yielding a Concordia age of 555 ± 7 Ma (n= 59; Fig. 8e), (3) Cambrian zircon grains indicating a concordant age of 508 ± 7 Ma (n= 64; Fig. 8d), (4) Middle to Upper Ordovician zircons with a concordant age of 457 ± 7 Ma (n=28; Fig. 8c), (5) Late Devonian/early Carboniferous zircons displaying a Concordia age of 358 ± 5 Ma (n= 44; Fig. 8b) and (6) Permian zircon grains with a Concordia age of 293 ± 6 Ma (n= 31; Fig. 8a). Most detrital zircons indicate, with nearly 45 %, predominantly Cambrian (23 %) and late Neoproterozoic (21 %) U-Pb zircon ages for the metasedimentary rocks of the Rannach Formation. About 16 %, of in total 284 dated zircons, yield Late Devonian/early Carboniferous (early Variscan) ages, approximately 14 % comprise Archean to Proterozoic ages, around 11 % yield Permian ages and approximately 10 % indicate Ordovician ages. The remaining 5 % comprise poorly defined Neoproterozoic ages scattering between ca. 625 and 963 Ma.

Fig. 7. Kernel density estimation (KDE) plot for detrital zircons of the Rannach Formation analyzed by U-Pb zircon age dating. A Concordia plot for each dominant peak is given in Figure 8.

103

Fig. 8. (a-f) U-Pb Concordia diagrams indicating Concordia ages for six dominant KDE peaks in Figure 7. Error ellipses are given at the 2 σ level.

104 7 Discussion 7.1 Potential source areas Various U-Pb zircon ages from different sedimentary rocks of the Rannach Formation, interpreted as crystallization age of potential source domains, suggest that the sedimentary deposit originates most likely from multiple source domains formed by different tectonic and magmatic events through late Archean to Permian. Dominant peaks in the detrital zircon age spectrum (Fig. 7) suggest (1) a predominant local provenance for late Neoproterozoic, Cambrian and Late Devonian/early Carboniferous zircons and (2) external source areas for Ordovician and Permian zircons. Ordovician and Permian ages have not been found in the basement underneath (Mandl et al., 2018) and hence the sources are defined as external sources. A subordinate Paleoproterozoic peak at around 2063 ± 30 Ma also indicates the presence of old cratonic material derived from Gondwana fragments (e.g., von Raumer et al., 2002).

7.1.1 “Internal Source” Most determined U-Pb zircon ages from sedimentary rocks of the Rannach Formation are in good agreement with analyzed zircons from crystalline basement rocks of the Seckau Complex. Prominent age peaks in the detrital zircon age spectrum let assume that different lithological units of the Seckau Complex e.g., the Glaneck Metamorphic Suite, the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite have provided “internal source material” to form parts of the Rannach Formation. More detailed, late Neoproterozoic zircons (555 ± 7 Ma) represent most likely eroded material from paragneiss of the Glaneck Metamorphic Suite which comprises beside Archean to Paleoproterozoic inherited zircon cores also late Neoproterozoic zircons with Concordia ages between 572 ± 7 Ma and 559 Ma ± 11 Ma. Cambrian (508 ± 7 Ma) and Late Devonian/early Carboniferous (358 ± 5 Ma) zircons derived predominantly from metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite that gave U-Pb zircon ages between 508 ± 9 Ma and 486 ± 9 Ma and in the range from 365 ± 11 Ma to 343 ± 12 Ma, respectively (Mandl et al., 2018).

7.1.2 “External Source” Middle to Upper Ordovician zircon ages of 457 ± 7 Ma and Permian zircon ages of 293 ± 6 Ma are, up to now, not documented in the different crystalline basement units of the Seckau Complex. Therefore, these zircons could represent material from adjacent or from

105 basement rocks further away. Evidence for Middle to Upper Ordovician magmatism can be found in almost all low-grade Paleozoic units in the Eastern Alps: e.g., the Greywacke Zone (e.g., “Blasseneck Porphyroid”) or the Gurktal Nappe (e.g., Haas et al., 2020; Heinisch, 1981; Löschke, 1989; Söllner et al., 1991; Tropper et al., 2016). Within high-grade Austroalpine basement units Ordovician magmatism occurs e.g., to the south of the Tauern Window (e.g., Schulz and Bombach, 2003), in the Ötzal-Stubai Complex (Hoinkes et al., 1997; Thöni and Miller, 2004; Tropper et al., 2016), in the Silvretta Nappe (Poller, 1997; Müller et al., 1996, 1995) as well as in the Carpathians (high-grade and low-grade) (e.g., Gaab et al., 2005; Haas et al., 2020). Permian zircon ages at 293 ± 6 Ma are also not documented in the crystalline basement units of the Seckau Complex, as mentioned before. They might be derived either from late mafic dikes– (not dated – “internal source”), crosscutting several lithological units that were covered by the Rannach Formation (Pfingstl 2013, Master Thesis) or from more regional sources of widely distributed Permian magmatic rocks. Domains hosting Permian magmatic rocks predominantly occur in the southern part of the Austroalpine Unit (e.g., Drauzug Goldeck-Kreuzeck) as well as in Southalpine units or the Carpathians realm (Marocci et al., 2008; Schuster et al., 2001). Based on geochronological and paleogeographic reasons the external source was placed to the south of the todays Tauern Window, close to the Southern Alps. In that region both Ordovician and Permian magmatics are widespread.

7.2 Tectono-metamorphic events The different source areas for sediments of the Rannach Formation had experienced various tectono-metamorphic events during its history and are documented by dominant zircon age peaks in the detrital zircon age spectrum (Fig. 7). A short overview of possible events is given below. (1) The late Neoproterozoic peak around 555 ± 7 Ma documents a late stage of the Cadomian orogenic event that affects the northern margin of Gondwana between 650 Ma and 550 Ma. During the Cadomian orogeny peri-Gondwana fragments started rifting off and the Cadomian magmatic arc (Cordilleran-type) developed by the subduction of oceanic crust below Gondwana. The final stage of the Neoproterozoic event is characterized by extensional tectonics and the formation of back-arc basins in the back of the Cadomian Arc, where in early to middle Cambrian small and short-lived oceans opened and separated fragments from Gondwana (Canadan et al., 2006; Haas et al., 2020; Linnemann et al., 2007; von Raumer and

106 Stampfli, 2008). (2) The Cambrian event around 508 ± 7 Ma indicates probably the closure of back-arc basins, developed in late Neoproterozoic, caused by the collision of the Cadomian Arc with its cratonic hinterland. This event is not very common in the Eastern Alps and reflects most likely the re-attachment of the Cadomian Arc to Gondwana (Haas et al., 2020). (3) The Middle/Upper Ordovician event around 457 ± 7 Ma is characterized by intensive acidic magmatism that is very common in the Eastern Alps, but not documented in the basement rocks of the Seckau Complex. This phase indicates the rift-off of Gondwana fragments due to the opening of the Rheic Ocean that separates Laurussia from Gondwana. From Silurian onwards (ca. 420 Ma) microcontinents e.g., Galatia drifted off Gondwana and the Paleotethys opened (Kroner and Romer, 2013; Franke et al., 2017). (4) The Late Devonian/early Carboniferous event around 358 ± 5 Ma (Meso-Variscan event) represents the amalgamation of Galatia with Laurussia-derived terranes through the closure of the Rheic Ocean just before the major collision between Gondwana and Laurussia forms the supercontinent Pangea around ca. 340 Ma (e.g., Franke et al., 2017; von Raumer et al., 2003). (5) The Permian event (293 ± 6 Ma) affects the Austroalpine Unit by postorogenic extensional tectonic and lithospheric thinning accompanied by magmatic underplating and high temperature/low pressure metamorphism (e.g., Haas et al., 2020; Plant et al., 2003; Schuster and Stüwe, 2008; Stampfli et al., 2002; von Raumer et al., 2015). Extensive Permian magmatism is evident from widespread Permian pegmatites and volvaniecs (e.g., Bozen porphyry). All of those concentrate in southern domains of the Alps (southern Austroalpine and Southalpine). Before the Meliata Ocean opened during the middle Triassic graben structures were formed to host the sediments of the future Rannach Formation. Detrital zircons with ages of about 290 Ma provide a maximum age for onset of sedimentation within the Rannach Formation.

8 Conclusions (1) Detrital zircon age spectrum of sedimentary rocks from the Rannach Formation comprises: Neoarchean to Paleoproterozoic (e.g., 2063 ± 30 Ma), late Neoproterozoic (555 ± 7 Ma), Cambrian (508 ± 7 Ma), Ordovician (457 ± 7 Ma), Late Devonian/early Carboniferous (358 ± 5 Ma) and Permian (293 ± 6 Ma) zircon ages.

107 (2) The main source domain for sediments forming the Rannach Formation is predominantly the Seckau Complex on which the Rannach Formation was deposited (“internal source”).

(3) Middle/Upper Ordovician zircons as well as Permian zircons are absent in the basement rocks of the Seckau Complex and derived most likely from more external sources e.g., from domains south to the Tauern Window or in areas close to the Southern Alps.

(4) Prior to their Alpine overprint most detrital zircons of the Rannach Formation indicate the distinct geodynamic settings e.g., the Cadomian collision event, the re-attachment of the Cadomian Arc, a rifting period due to the opening of the Rheic Ocean, the Meso-Variscan collision event and finally a Permian rift setting.

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112

Discussion and general conclusions

Based on lithological field criteria, geochemical characteristics and U-Pb zircon ages, a new subdivision of the Seckau Complex into three lithological units (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite is proposed.

(1) The Glaneck Metamorphic Suite predominantly composed of paragneisses (and mica schists) comprises the oldest analyzed zircons of the Seckau Complex with ages between 2.7 and 2.0 Ga as well as in the range between 572 ± 7 Ma and 559 ± 11 Ma, respectively. These pre-Cambrian rocks represent the wallrocks for younger (late Cambrian to Early Ordovicain and Late Devonian to early Carboniferous) intrusive bodies.

(2) The Hochreichart Plutonic Suite, predominantly found in the central part of the Seckau Complex, contains weak to strong deformed granites and granodiorites indicating highly fractionated melts with mainly S-type affinity. These intrusive rocks display a peraluminous character (A/CNK= 1.08-1.51), a negative fractionation trend

of most major elements against SiO2 and Rb/Sr ratios > 45. They show a significant negative Eu anomaly, initial 87Sr/86Sr ratios of 0.7056 to 0.7061 and U-Pb zircon ages clustering between 508 ± 9 Ma and 486 ± 9 Ma. These late Cambrian to Early Ordovician intrusive bodies can be related to the re-attachment of the Cadomian Arc to Gondwana.

(3) The Hintertal Plutonic Suite, located in the north-eastern and south-western part of the Seckau Complex, is mainly composed of slightly to strongly deformed intermediate to acidic intrusion bodies. These plutons typically indicate a magmatic fractionation trend with Rb/Sr ratios in the range between 0.09 and 0.45 and U-Pb zircon ages varying between 365 ± 11 Ma and 343 ± 12 Ma. These Late Devonian to early Carboniferous intrusives can be related to the subduction of the Paleotethys prior to the final collison of Gondwana and Laurussia.

(a) The Pletzen Pluton comprises predominantly intermediate to acidic intrusion bodies that are mainly meta- to weakly peraluminous in composition (A/CNK= 0.88

113 to 1.10). They are characterized by I-type affiliation, the absence of a significant Eu anomaly and initial 87Sr/86Sr ratios vary between 0.7051 and 0.7061. A typical feature for this sub-unit is the green colored biotite and large titanite crystals. (b) The Griessstein Pluton is mainly composed of granitic rocks indicating a peraluminous character (A/CNK= 1.10 to 1.23) with S-type features. These granites show a weakly developed Eu anomaly and initial 87Sr/86Sr ratios between 0.7054 and 0.7063.

(4) The Late Devonian to early Carboniferous rocks of the Griessstein Pluton are considered to be the product of a fractionation process from the more primitive Pletzen melt. Twenty to thirty percent of the Pletzen melt needs to crystallize in order to produce a composition close to the Griessstein Pluton. A contamination of the Pletzen Pluton by the rocks of the Hochreichart Plutonic Suite is considered to be unlikely to produce the composition of the Griessstein Pluton.

(5) The general paleogeographic position of the Seckau Complex during the Variscan orogenic event is considered to be south to southeast of the Bohemian Massif. The Seckau Complex is assumed to be part of an early Variscan magmatic arc along the southern margin of a continental sliver now constituting Bohemia. The Late Devonian to early Carboniferous granitoid emplacement of the Hintertal Plutonic Suite (Pletzen Pluton) can be related to the closure of the Paleotethys. The approximatively 500 Ma old Hochreichart Pluton, excluded as source for the plutons of the Hintertal Plutonic Suite, was already part of the consolidated crust now comprising Bohemia and its extension to the Alps.

(6) Sedimentary rocks of the Rannach Formation comprise detrital zircons derived from Neoarchean to Paleoproterozoic (e.g., 2063 ± 30 Ma), late Neoproterozoic (555 ± 7 Ma), Cambrian (508 ± 7 Ma), Ordovician (457 ± 7 Ma), Late Devonian/early Carboniferous (358 ± 5 Ma) and Permian (293 ± 6 Ma) crustal material. The basement rocks of the Seckau Complex, containing most of the determined U-Pb zircon ages, and domains in the southern part of the Austroalpine, comprising Ordovician and Permian zircon ages (that have not been found in the basement rocks of the Seckau Complex), are suggested to represent the main source regions for the sediments of the Rannach Formation.

114 Appendix

Appendix A p. 116-134

Mandl, M., Kurz, W., Hauzenberger, C., Fritz, H., Klötzli, U., Schuster, R. (2018). Pre-Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constraints from in- situ LA-ICP-MS U-Pb zircon geochronology. Lithos 296-299, 412-430, doi.org/10.1016/j.lithos.2017.11.022.

Digital Appendix (DVD)

(01) U-Pb data determined by LA-MC-ICP-MS

(02) Zircon images used for U-Pb zircon age dating

115