Early-Palaeozoic magmatic evolution of the Austroalpine basement

Delia Rösel

Institute for Mineralogy, TU Bergakademie Freiberg, Germany

Abstract. Meta-igneous rocks within the Austroalpine basement to the south of

the Tauern Window preserve information about the Early-Palaeozoic evolution

from an active to a passive continental margin along the north Gondwanan

periphery. Several magmatic suites can be distinguished according to their whole-

rock geochemistry. Protolith single zircon Pb-Pb-geochronology and whole rock

Ta/Yb-Th/Yb data allow to define two principal magmatic evolution lines. The

older evolution with 590 Ma N-MORB-type and 550-430 Ma volcanic arc basalt-

type suites indicates magmatism related to an active margin, which finished at

470-450 Ma by the formation of acid magmatites. The younger magmatic

evolution line is composed of passive margin related within-plate basalt-type

rocks (430 Ma) and metabasalts which are transitional to MORB.

Introduction

The Austroalpine crystalline basement was part of the northern Gondwanan margin. Since the Neoproterozoic microcontinents and terranes split off from the north Gondwana margin and were moved northward until they collided with the southern part of Laurussia. The Austroalpine basement was part of the Hun terrane, which split up from Gondwana during the opening of the Palaeo-Tethys. The Hun terrane encloses the Celtic terrane as a former pre-Early-Ordovician active margin in the north and the Noric Composite terrane as a passive margin in the south (Frisch and Neubauer 1989; Schulz et al. 2004).

In the Austroalpine basement to the south of the Tauern Window distinct suites of meta-magmatic rocks exist. Isotopic and geochemical analysis of these suites give information about the pre-Mesozoic tectonic evolution of the northern margin of Gondwana (Schulz and Bombach 2003; Schulz et al. 2004, 2006).

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Geological Setting

The investigated area of the Austroalpine basement is located between the Southern Alps and the central Tauern Window (Fig.1). It is limited to the south by the Periadriatic Lineament. To the north, the Austroalpine crystalline basement overlies the alpine metamorphosed penninic units of the Matrei Zone and the upper “Schieferhülle” (Schulz et al. 2001).

The Austroalpine basement to the south of the Tauern Window is divided by a sinistral W-E striking shear zone called Defereggen-Antholz-Vals line (DAV) into a northern and a southern block (Borsi et al. 1978; Steenken and Siegesmund 2000).

Lithological units south of the Tauern Window

Four principal units (Fig.1) can be distinguished according to their lithological associations (Schulz and Bombach 2003; Schulz et al. 2004, 2006). The Northern-Defereggen-Petzeck Group (NDPG) to the north and to the east of the DAV is composed of meta-psammopelitic rocks with interlayered marble and meta-quartzite. Different types of metabasites with distinctive geochemical characteristics occur in this group. The Prijakt Subgroup (PSg) is composed of N-MORB-type eclogitic amphibolites and volcanic arc basalt (VAB) type hornblende gneisses. MORB-type amphibolites and volcanic arc basalt type hornblende-plagioclase gneisses appear in the Rotenkogel Subgroup (RSg). The Torkogel Subgroup (TSg) consists of MORB- and within-plate basalt-type (WPB) amphibolites. Similar amphibolites can be found in the Michelbach Subgroup (MSg). The Croda Nera Subgroup (CNSg) contains MORB- and WPB-type metabasites.

The monotonous garnet-muscovite-schists and garnet-muscovite-quartzitic gneisses of the Durreck Muscoviteschist Group (DMG) form a unit within the NDPG. In comparison to the NDPG, only few amphibolite lenses exist which show within-plate basalt-type signatures.

To the south of the DAV the Thurntaler Phyllite Group (TPG) overlies the meta-psammopelitic Defereggen Group (DG). The latter is composed of banded quartzitic gneisses, paragneisses and mica schists which contain thin layers of amphibolite, marble, graphitic gneisses and quartz-feldspar gneisses. Only one serpentinite lens exists. MORB-type and tholeiitic to alkaline WPB-type rocks are typical metabasites from the TPG. The TPG is mainly composed of phyllitic sequences with interlayered amphibolites, epidote-amphibolites, chlorite schists, quartzites and marbles.

Acid pre-Variscan magmatites can be found in every unit. The Thurntaler Phyllite Group contains meta-porphyroids which are supposed to be metatuffites. Different groups of granitoid orthogneisses occur in the Nothern-Defereggen-Petzeck Group, Durreck Muscoviteschist Group and the Defereggen Group.

Early-Palaeozoic magmatic evolution of the Austroalpine basement 3

Fig.1. Lithological units in the Austroalpine basement to the south of the central Tauern Window, Eastern Alps. The investigated area is marked (modified after Schulz et al. 2004). A Antholz muscovite orthogneiss; BQ Quartzphyllites of Brixen; C Casies biotite orthogneiss; CA Carnic Alps; CNSg Croda Nera Subgroup (metabasites); CT Campo Tures orthogneiss; DAV Defereggen-Antholz-Vals line; DG Defereggen Group; DMG Durreck Muscoviteschist Group; EW Engadine Window; H Hochgrabe biotite orthogneiss; IQ Quartzphyllite of Innsbruck; K Kristeinertal biotite orthogneiss; KV Kalkstein-Vallarga line; MSg Michelbach Subgroup (amphibolites); MZ Matreier Zone (Penninic); NDF Northern Drauzug fault; NDPG Northern-Defereggen-Petzeck Group; NCA Northern Calcareous Alps; P Penninic Upper Schieferhülle; PG Palaeozoic of Graz; PGN Palaeozoic of Gurktal Nappes; PGZ Palaeozoic of Greywacke Zone; PL Periadriatic Lineament; PSg Prijakt Subgroup (eclogitic amphibolites and hornblende-plagioclase-gneisses); R Rieserferner tonalite; RSg Rotenkogel Subgroup (hornblende-gneisses); SA Southern Alps; SAM Southern limit of Alpine metamorphism; T Permo-Trias and Trias; TPG Thurntaler Phyllite Group; TSg Torkogel Subgroup (amphibolites); Z Zinsnock tonalite.

The lithological units underwent a pre-Alpine amphibolite-facies metamorphism and deformation (Schulz et al. 2004). To the north of the DAV, Early- and Late-Alpine ductile deformation occur. Ductile Variscan deformation and an Alpine brittle overprinting prevail to the south of the DAV (Schulz et al. 2001).

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Geochemistry of meta-magmatites

It is possible to distinguish and discriminate magmatic rocks produced in different tectonic settings on the basis of their chemistry. In this concept, it is important that only element signatures and ratios determined by igneous petrogenesis and differentiation and not by post-magmatic mobilization are used as indicator elements (Schulz et al. 2004).

Magmatites can be discriminated in several ways. By normalizing the concentration values of the relative element abundances in geochemical variation diagrams characteristic depletion or enrichment occur, which can be referred to distinct tectonic settings. The Zr-Ti discrimination diagram can be used to distinguish basalts having a within-plate, ocean floor or volcanic arc basalt character. The Th/Yb versus Ta/Yb plot shows the difference between subduction related basalts and basalts derived from depleted sources (MORB) as well as enriched sources (OIB). Acid magmatites can be classified according to their alkaline/aluminium ratio in continental arc granite (CAG) and continental collision granite (CCG) in a discrimination diagram after Maniar and Piccoli 1989.

Meta-magmatic suites

According to Schulz and Bombach 2003, Schulz et al. 2004 and 2006 several meta-magmatitic suites can be distinguished (Fig.2). This division is based on whole-rock main and trace element distribution (Schulz and Bombach 2003; Schulz et al. 2004, 2006). Whole rock oxygen isotope values as well as strontium, neodymium and samarium concentrations and isotopic compositions are described in Schulz et al. 2004. Protolith ages were dated by the Pb-Pb single zircon evaporation method (Schulz and Bombach 2003).

Variations of elements within one suite are caused by magmatic fractionation. No significant change of the element abundance due to metamorphic processes could be determined (Schulz et al. 2006).

Fig.2. Zr-Ti plot of meta-magmatic suites in the Austro-alpine basement to the south of the Tauern Window. Samples which were dated by Pb-Pb single zircon evaporation method are marked (modified after Schulz et al. 2004).

Early-Palaeozoic magmatic evolution of the Austroalpine basement 5

N-MORB-type suite

N-MORB-type geochemical characteristics can be found in eclogitic amphibolites of the Prijakt Subgroup (Fig.3a). The Zr-Ti abundances plot in the MORB field (Fig.2), although the large ion lithophile (LIL) elements and Ce are slightly enriched compared to MORB (Fig.3b). The eclogitic amphibolites show a tholeiitic composition with 46- 49 wt % SiO2. Light rare earth elements (LREE) are slightly enriched with chondrite normalized (La/Yb)N values between 0,68 and 1,85 (Fig.3b). A (Tb/Ta)N ratio of 0,93-3,31 and (Th/Ta)N ratio between 1,38 and 2,50 indicates a back arc basin magmatism. δ18O values range from 6,9 to 8,2 ‰ and are therefore slightly elevated compared to a mantle melt. This indicates crustal contribution. The formation of the N-MORB-type basalts is the oldest magmatic event (590 ± 4 Ma) found in the Austroalpine basement.

Fig.3. MORB normalized variation diagram (Fig.3a) and chondrite normalized REE pattern (Fig.3b) of eclogitic amphibolites found in the Prijakt Subgroup (modified after Schulz and Bombach 2003).

Volcanic arc basalt type suite

Volcanic arc basalt (VAB) type magmatites occur as hornblende-plagioclase-gneisses in the Rotenkogel and Prijakt Subgroup (Fig.4). The plutonic protoliths were clinopyroxene-gabbros with a calc-alkaline fractionation trend. Multi-element plots show an enrichment of LILE (Sr, K2O, Rb, Th, Ce) and a depletion of HFSE (Nb, Zr, Hf, Sm, Ti, Y, Yb, Cr) relative to MORB (Fig.4a). This is characteristic for melts formed in island arcs or active continental margin setting. Negative anomalies from Ta, Nb and TiO2 (Fig.4a) and whole-rock δ18O values of 6,9-8,2 ‰ points to a contribution of upper crustal material during magma formation. The chondrite normalized (La/Yb)N values ranging from 2,6 to 8,1 show a strong enrichment of LREE compared with HREE (Fig.4b). Protolith ages are between 533 ± 3,8 Ma, 550 ± 5,9 Ma and 553 ±2,3 Ma.

(a) (b)

6 Delia Rösel

Fig.4. MORB normalized variation diagram (Fig.4a) and chondrite normalized REE pattern (Fig.4b) of hornblende-plagioclase-gneisses found in the Rotenkogel Subgroup (modified after Schulz and Bombach 2003).

Within-plate and MORB-type suite

MORB-type signatures occur in numerous amphibolites within the NDPG. The LIL elements are 2-10 times enriched compared with MORB-composition (Fig.5a). Rare earth elements show a significant negative Eu-anomaly and LREE pattern are depleted to flat at (La/Yb)N = 0,5 - 1,1 (Fig.5b).

Within-plate basalt (WPB) can be found in amphibolites and in the metabasic subgroups within the NDPG with protolith ages about 430 ± 1,6 Ma. These rocks are differentiated by low Ti and Zr-content (Fig.2) and have tholeiitic, transitional and alkaline compositions. The variation diagrams (Fig.5a, 5b) indicate a significant LILE, HFSE and LREE enrichment with (La/Yb)N = 14,6 - 19,6. Whole-rock δ18O values are between 9,3 and 11,9 ‰.

The metabasites of the Thurntaler Phyllite Group plot in the Zr-Ti diagram in a transitional field between MORB- an WPB-type and have a tholeiitic to alkaline composition. The transitional MORB-type rocks show a slight enrichment of LILE and LREE according to chondrite normalized (La/Yb)N values between 1,5 and 2.3. The WPB-type rocks have a clear enrichment of LREE with a chondrite normalized (La/Yb)N ratio of 7.0. Whole-rock δ18O values are 11,2 - 11,9 ‰.

Fig.5. MORB normalized variation diagram (Fig.5a) and chondrite normalized REE pattern (Fig.5b) of amphibolites found in the Northern-Defereggen-Petzeck Group (modified after Schulz and Bombach 2003).

(a) (b)

(a) (b)

Early-Palaeozoic magmatic evolution of the Austroalpine basement 7

Pre-Variscan acid magmatites

Acid magmatites, such as orthogneisses and meta-porphyroids, are differentiated from the other meta-magmatic rocks due to their negative Eu-anomaly (Fig.6b).

The orthogneisses occur in the NDPG, DMG, DG and can be subdivided into the following groups: tonalitic to granodioritic, rarely dioritic biotite bearing orthogneisses of Gsies, fine grained and strongly foliated biotite orthogneisses in the eastern parts of DG (Blankenstein, Hochgabe, Sauspitze, Kristeinertal) and leucocratic muscovite bearing orthogneisses of Antholz.

These orthogneisses have a predominant peraluminous character with SiO2 contents which range from 63 to 75 wt. %. The whole-rock δ18O depends on the SiO2 value and ranges between 10,3 and 9,2‰. The orthogneisses of Gsies and the biotite orthogneisses plot in the CAG field (Fig.6a), whereas the muscovite orthogneisses have a CCG s-type affinity and higher HREE fractionation (Fig.6b). The protolith ages of the CAG are measured by 478 ± 15 Ma, 468 ± 5 Ma, 463 ± 8 Ma and 461 ± 4 Ma and the slightly younger CCG-type orthogneisses have ages about 457 ± 5 Ma and 447 ± 6 Ma.

The meta-porphyroids occur in the Thurntaler Phyllite Group and their δ18O value ranges from 12,7 to 13,3‰ at a SiO2 content between 67 and 79 wt. %. They cover only the CCG field in the diagram of Maniar and Piccoli 1989 (Fig.6a). The protolith ages of the meta-porphyroids range between 468 ± 5,1 and 473 ± 6,7.

Fig.6. Geochemistry of acid Magmatites in the Austroalpine basement. Fig.6a shows the discrimination diagram after Maniar and Piccoli (1989). The arrow marks a chronological and chemical evolution trend in the orthogneisses. In the chondrite normalised REE pattern (Fig.6b) a significant Eu-anomaly occur (modified after Schulz and Bombach 2003).

Magmatic evolution lines in the Austroalpine basement

There are two major magmatic evolution lines in the Austroalpine basement (Fig.7) according to Schulz et al. 2004. The older evolution line I is composed of N-MORB (590 Ma), volcanic-arc basalts (550 - 530 Ma) and acid meta-magmatites (480 - 450 Ma). In the Ta/Yb -Th/Yb evolution diagram the meta-magmatites follow firstly the subduction-type or crustal contamination vector and then a fractional crystallization trend. The younger evolution line II contains

8 Delia Rösel

MORB and WPB-type rocks and follows an intraplate metasomatism or within-plate enrichment vector.

The 87Sr/86Sr versus time evolution lines (not shown) of the N-MORB-type sample and volcanic-arc basalt-type rocks are subparallel to a depleted mantle growth curve (Schulz et al. 2004). The WPB-type rocks follow Sr- evolution lines parallel and in most cases above the enriched mantle growth curve. The acid magmatite suites have highly variable SrINIT (Schulz et al. 2004). This is an indication of inconsistent contribution of mantel material to the continental crust source and suggests that the sources of the suites had different geochemistry even if the trace element signatures are similar (Schulz et al. 2004).

Fig.7. Evolution lines in Ta/Yb versus Th/Yb coordinates. Line I: subduction related evolution including N-MORB and VAB type metabasites as well as orthogneisses and meta-porphyroids. Line II: MORB and alkaline WPB-type metabasites. Vectors indicate the influence of subduction (S), crustal contamination (C), within plate enrichment (W) and fractional crystallization (F). Dashed lines separate the boundaries of the tholeiitic (TH), calc-alkaline (CA) and shoshonitic (S) field. ACM stands for active continental margin and OIA for oceanic island arc.(modified after Schulz et al. 2004).

Conclusions

According to geochemical data and the evolution lines of the magmatic suites a plate tectonic model can be proposed, which is illustrated as cross sections in Fig.8.

The oldest magmatic event in the Austroalpine basement is the formation of N-MORB-type basalts at 590 ± 4 Ma. These magmatites can be found as eclogitic amphibolites in the Prijakt Subgroup. Trace element signatures indicate a subduction related basic magmatism with depleted mantle sources. In the geodynamic framework, this type of magmatites could have been formed in a back arc basin of an active margin situated in the north-Gondwanan periphery.

Early-Palaeozoic magmatic evolution of the Austroalpine basement 9

The 550 to 530 Ma old meta-gabbros from the Rotenkogel and Prijakt subgroup provide the main evidence of an active continental margin in the Early-Palaeozoic. This evolution continued during the Ordovician indicated by acid continental arc magmatites (Gsies-type granitoids) and anatectic continent-collision-type granitoids from Antholz.

The 430 ± 2 Ma old within-plate basalts were possibly formed in intracontinental rifts, which are related to the Palaeo-Tethys opening event and the end of the active margin evolution.

Fig.8. Cross sections, which illustrate the possible plate tectonic evolution of the Austroalpine basement at the northern margin of Gondwana during the Late-Proterozoic and Early-Palaeozoic. Fig.8a and Fig.8b show the Early-Palaeozoic active margin evolution. The Ordovician acid magmatism is demonstrated in Fig.8c. The passive margin evolution and the opening of the Palaeo-Tethys is illustrated in Fig.8d. DG Defereggen Group; DMG Durreck Muscoviteschist Group; NDPG Northern-Defereggen-Petzeck Group; TPG Thurntaler Phyllite (modified from Schulz et al. 2004).

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