The southwestern Tauern Window in the Eastern Alps is a classic area for investigating the interplay between tectonics and metamorphism of the Alps. This easily accessible area with excellent outcrops is also popular for field trips that capitalize on the perfectly exposed important features of the Eastern Alps (e.g., Morteani, 1974; Lammerer and Morteani, 1990; Lammerer et al., 2011). This tectonic window exposes part of the Subpenninic and Penninic units, which are otherwise hidden by the overthrusted Austroalpine superunit (Fig. 1).

The Pfitscher Joch area has been the focus of a particularly large amount of research aimed at reconstructing the pressure–temperature–time path of different tectonic units in the Tauern Window (Selverstone et al., 1984; Selverstone and Spear, 1985; Selverstone, 1988; Schulz et al., 1995; Schulz, 1996), their structural evolution (Selverstone, 1985, 1993), and age data using different isotope systems and minerals (e.g., Glodny et al., 2008; Schmid et al., 2013; Rosenberg et al., 2018; Ricchi et al., 2020, and references therein). This work has generated a coherent picture of regional metamorphic tectonic Alpine history. The peak of orogenic metamorphism reached temperatures of up to 500–550 ∘C and pressures of up to 1.0–1.1 GPa (Selverstone et al., 1984). Nearly isothermal decompression was rapid (Selverstone et al., 1984) and occurred during orogen-parallel exhumation (e.g., Ratschbacher et al., 1991) and large-scale E–W extension (Behrmann, 1988; Selverstone, 1988).

The Subpenninic and Penninic units exposed by the Tauern Window have been interpreted as the pre-Alpine distal European margin and remnants of the Tethys Ocean, respectively (e.g., Schmid et al., 2013, and references therein). The post-Variscan metasedimentary units described herein (Fig. 1) belong to the Subpenninic nappes, exposed in the core of the Tauern Window. These strata are the stratigraphically lowermost tectonic units and primarily consist of (pre-)Variscan basement rocks intruded by late- and post-Variscan granitoids and subvolcanic rocks (e.g., Cesare et al., 2002; Veselá et al., 2011). The latter units are traditionally referred to as the Zentralgneis (“gneiss cores”, e.g., Morteani, 1974; “gneiss horsts”, Veselá and Lammerer, 2008). In the western Tauern Window, the Zentralgneis is divided into the Ahorn, Tux, and Zillertal nappes, stacked to form the Venediger Duplex (e.g., Frisch, 1980; Lammerer and Weger, 1998; Lammerer et al., 2008; Schmid et al., 2013). The Tux Zentralgneis is characterized as a strongly fractionated, calc-alkaline, I-type granitoid (Finger et al., 1993); hosts the molybdenite deposit of the Alpeinerscharte; and shows features of intermediate argillic alteration (Mostler et al., 1982; Melcher et al., 1995), typical of porphyry granitoids.

The metasedimentary cover overlies the Tux and Zillertal units and is folded between these two rigid bodies into an elongated basin, the Pfitsch–Mörchner basin, together with other post-Variscan sedimentary basins. Veselá et al. (2008) and Veselá and Lammerer (2008) proposed post-Variscan intramontane basins as the depositional environment for the Carboniferous to Mesozoic sediments. Such basins would have formed part of a network of basins in the paleogeographical central European and Alpine region of Pangea, which Elter et al. (2020) described as comprising pull-apart basins between the landmasses of Laurussia and Gondwana.

Today, the metasedimentary rocks of the Pfitsch–Mörchner basin form a large, ENE–WSW-striking, isoclinally folded syncline that runs parallel to the Greiner shear zone, which is situated between the Tux and the Zillertal Zentralgneis units (e.g., Behrmann, 1988; Behrmann and Frisch, 1990; Selverstone et al., 1991), together with the Ahorn, Olperer, and Tux shear zones (e.g., Rosenberg and Schneider, 2008; Ricci et al., 2020). The original bedding is apparent in lithologic contacts, except near to the fold hinge where shearing has obliterated the original sedimentary structure. The Carboniferous to Jurassic age (Veselá et al., 2011) of the major basins of the western Tauern Window limits these post-Variscan cover units to having experienced only Alpine greenschist to amphibolite facies metamorphism. Any indications of Permian low-temperature hydrothermal metamorphism found in other parts of the Alps (review in Schuster and Stüwe, 2008) have probably been erased by the stronger Alpine metamorphism.

The sample set is subdivided into the SE limb and the NW limb of the fold (Fig. 2). It comprises basement rocks and the overlying metasedimentary rocks, including what Veselá and Lammerer (2008) called the Pfitsch Formation and the Windtal Formation, up to an overlying marble unit, the Aigerbach Formation. The geological map of the Brenner Basistunnel 1 : 50 000 (Brandner et al., 2011) classifies the sequence as the Triassic Aigerbach Formation overlying Permian(?) metaconglomerates. In the compilation of Schmidt et al. (2013) the sequence corresponds to their units 1 and 2 (basement), 4 (conglomerates), 5 (Permo-Scythian quartzites), and 6 (limestone).

Beneath the SE limb of the fold, the basement consists of thin (< 100 m) orthogneiss lamella of the Zillertal Zentralgneis, amphibolite (variety hornblende garbenschist), and small bodies of serpentinite (Fig. 2). Further south lies a thick (pre-)Variscan series of graphitic mica schists called Furtschagl schist (Christa, 1931; Selverstone and Muñoz, 1987). Northward, the uppermost unit of the SE limb is a quartzite, separated from the metasedimentary sequence by a 1–2 m thin layer of hornblende garbenschist (see Supplement 4.3.4 in Franz et al., 2021). It marks the top of the basement, characterized by a conspicuous reddish weathering of pyrite from which the local name of the Rotbachl (“red creek”) is derived, best exposed in a profile at the crest of the Rotbachlspitze (Fig. 3a). This pyrite quartzite has a well-developed schistosity with some white mica. It is feldspar(–albite)-bearing in several layers near the base, where it is interspersed with a biotite–chlorite–plagioclase gneiss. It also has a zone rich in quartz and kyanite and a smaller lens rich in staurolite, in addition to several quartz segregations. Near the top, magnetite-rich lenses appear. These lenses extend up to 30 m along strike and a few meters perpendicularly to strike; the assemblage contains magnetite–chloritoid–staurolite–chlorite–ilmenite–white mica–quartz, locally with albite, in varying proportions (Fig. 3c; Barrientos and Selverstone, 1987), abbreviated below as MCSC lenses.

The metasedimentary succession of the Pfitsch–Mörchner basin starts with a strongly sheared metaconglomerate (Fig. 3d). Up-section the metaconglomerate grades into a thin layer of mica schist, followed by ∼ 30 m of a tourmaline-rich, poorly foliated feldspathic rock called tourmaline gneiss (Berryman et al., 2017). The high tourmaline content, which was formed during metamorphism, is a characteristic feature of these rocks, and tourmaline also occurs in segregations. The gneiss contains lenticular quartz- and feldspar-rich, mica- and chlorite-poor bodies, up to 5 cm long and up to 1.5 cm thick, interpreted as relict pebble structures (Fig. 3e; for more images see Supplement 4.3.8 in Franz et al., 2021). In some areas, ∼ 20 cm thick lenses of fine-grained, muscovite-bearing quartzite appear between the tourmaline gneiss and layers of mica schist (with chlorite and biotite). In sharp contact with the latter is a fine-grained, chlorite–biotite schist grading upwards into biotite schist, grouped here together and subsequently referred to as mica schist. The Pfitsch–Mörchner basin succession is topped by an approximately 250 m thick unit of carbonate–mica schist distinguished in the field by ankerite–biotite aggregates (sites for the reaction ankerite + muscovite = calcite + biotite). The same unit is only ∼ 50 m thick in the NW limb of the basin.

In the NW limb, the basement (Fig. 2) is formed by the large Tux Zentralgneis body that consists of several intrusions of granitoids with aplite dikes. In the investigated area, we distinguish two types: one with the local name “Augenflasergneis” (describing the augen texture and irregular foliation) and a leucocratic type with a poorly developed foliation that is referred to here as “Schrammacher Gneis” (interpreted as a highly differentiated granite with local beryl–phenakite mineralization; Franz et al., 1986). The granitoids are cut by post-Variscan mafic dikes, which were transformed into biotite schist during Alpine metamorphism (Fig. 3b). The host rocks of the granitoids, known as “Altes Dach” (old roof pendant), are predominantly amphibolites with minor amounts of garnet-bearing (para)gneiss, hornblende garbenschist, and a series of serpentinite bodies with blackwall zoning. A pyritic quartzite unit with MCSC lenses corresponding to that identified at the top of the basement units in the SE limb could not be identified. The succession of the Pfitsch Formation starts with a poorly sorted metaconglomerate and in places a meta-breccia, well exposed at a small lake (Langsee; De Vecchi and Baggio, 1982; Lammerer, 1986). The large pebbles (Fig. 3d) are up to 30 cm in size; angular to sub-rounded; and made of macroscopically distinguishable gneisses (meta-aplite, biotite-bearing metagranite), calc-silicates (rich in epidote minerals), amphibolite, marble, biotite schist, mica schist with Cr-bearing muscovite, and quartz in a fine-grained matrix of mica–chlorite–plagioclase–epidote–quartz. The mica schist with Cr-bearing muscovite (Lammerer, 1986; Schön and Lammerer, 1993) is a characteristic pebble type and guides identification of the metaconglomerate unit in areas of high strain, where the shape of the pebbles has been obliterated by Alpine shearing and flattening. This is especially the case in the SE limb, where these rocks show a well-developed schistosity defined by white mica and small-scale layering. In the NW limb, the metaconglomerate crops out over approximately 200 m, and in the SE limb it crops out over approximately 100 m. Lammerer (1986) estimated the original thickness of the unit as nearly 500 m.

The contact between the Pfitsch Formation and the next unit (Windtal Formation), a muscovite-bearing lazulite quartzite with a well-developed schistosity, is gradual over a distance of approximately 2 m. This contact zone has thin layers (centimeters to tens of centimeters) of carbonate–mica schist alternating with muscovite-bearing quartzite. The quartzite contains conspicuous blue lazulite (Fig. 3f), together with kyanite, staurolite, magnetite, hematite, chlorite, tourmaline, and a number of different Al–P minerals (lazulite, svanbergite–goyazite, bearthite, apatite), described in detail in Morteani and Ackermand (1996). Staurolite occurs only in the SE limb and is not observed in samples with lazulite. Heavy minerals are concentrated in layers, suggesting the original bedding (Morteani and Ackermand, 1996). The high content of quartz indicates a compositionally mature sandstone protolith. Magnetite (± chlorite) dominates in the NW limb. Flattened quartz nodules, up to approximately 10 cm in length, record dominantly plane strain and are sites of major volume loss by pressure solution (Selverstone and Hyatt, 2003).

Quartz veins and tourmalinite veins, ∼ 1–2 cm thick, consisting of quartz and very fine grained tourmaline, cut the lazulite quartzite (Henry et al., 2002; Selverstone and Hyatt, 2003). Tourmaline-rich and quartz-rich domains comprise the tourmalinite veins (Fig. 3g, h). Tourmaline-rich bands contain up to 75 vol % tourmaline, and quartz-rich bands contain < 10 vol % tourmaline (Fig. 3i; Henry et al., 2002). In places where they cut the original bedding, the veins are isoclinally folded (Fig. 3g); in other areas, the veins are (sub)parallel to the foliation (Fig. 3h). In some occurrences, the veins are dismembered by shearing and resemble centimeter-sized pebbles (Lammerer, 1986); for more images see Supplement 4.3.6 in Franz et al. (2021) and Henry et al. (2002). These relicts of the tourmalinite veins were also found in the metaconglomerate and in the tourmaline gneiss.

The uppermost metasedimentary unit in this area is a dolomite marble (Aigerbach Formation; Veselá and Lammerer, 2008), which forms the central part of the fold.

Overall, the metasedimentary rocks have a wide range of Al2O3 and SiO2 contents (Fig. 4a), whereas the orthogneisses (Zentralgneis) show a restricted SiO2 content between ∼ 70 and ∼ 78 wt %. Data from the serpentinite bodies include metasomatically altered border zones (blackwall zones of talc, actinolite, chlorite, biotite; similar to rocks described by Trumbull et al., 2008, from the Tauern Window east of the study area), which are high in Al2O3 and K2O. Most data from the MCSC lenses have > 20 wt % Al2O3, but there is a significant overlap with data from the pyrite quartzite. The lazulite quartzite has SiO2 contents up to 90 wt %; samples with high Al2O3 contents (> 25 wt %) are mica-rich selvages around quartz nodules, indicative of silica loss during Alpine deformation (Selverstone and Hyatt, 2003).

Mica schist, carbonate–mica schist, and tourmaline gneiss follow a common positively correlated trend in MgO and Fe2O3(tot) contents (Fig. 4b). The lazulite quartzite and MCSC lenses, corresponding to the top and bottom of the Pfitsch Formation, respectively, both have high Fe2O3(tot) and relatively low MgO. In contrast, the pyrite quartzite that surrounds and underlies the MCSC lenses has a low Fe2O3(tot) content and a relatively high MgO content. Alkali elements Na2O and K2O (Fig. 4c) of metaconglomerate, tourmaline gneiss, and mica schists show a negative correlation, the lazulite quartzite and the pyrite quartzite being very poor in Na2O, except the feldspar-bearing variety of the pyrite quartzite. In terms of CaO and Na2O (not shown), the mica schist, carbonate–mica schist, and tourmaline gneiss are Ca-dominant, whereas many samples of the metaconglomerate, MCSC lenses, and pyrite quartzite are Na-dominant. The Ca contents are variable and reach 10 wt % CaO in the tourmaline gneiss and 17 wt % CaO in one metaconglomerate sample. The carbonate–mica schist (3.75 wt % CaO) and mica schist (3.05 wt % CaO) do not differ significantly.

The K2O and Rb contents are well correlated (Fig. 4d), forming two trends: one for the tourmaline gneiss, carbonate–mica schist, and mica schist and the other for the lazulite quartzite, MCSC lenses, and the pyrite quartzite. Values for the heterogeneous metaconglomerate scatter strongly and are not included in these trend lines. The first trend corresponds to an Rb / K ratio in the range of ∼ 0.003 to 0.008, centering around a value of 0.005 (R2=0.79). This trend is consistent with the average K2O and Rb values for clastic sediments from upper continental crust (sandstone and shale; Turekian and Wedepohl, 1961). The orthogneisses (variety Augenflasergneis and gneiss from the Zillertal unit) show a comparable trend; data for the Schrammacher Gneis similarly display a large amount of scatter (R2=0.65). Lazulite quartzite, MCSC lenses, and pyrite quartzite trend closely near an average of 0.002 Rb / K, suggesting fractionation of Rb from K (R2=0.84).

Strontium and Ca contents (Fig. 4e) are extremely variable and poorly correlated. The lazulite quartzite, despite having a very low Ca content, is strongly enriched in Sr, up to 20 000 µg g-1, and contains accessory celestite (see Morteani and Ackermand, 1996). Serpentinite rocks with blackwall zoning also show high Sr values of up to 700 µg g-1 in several samples. A similarly strong heterogeneity is observed for Ba (not shown), which can reach > 20 000 µg g-1 in samples of the metaconglomerate, indicating local enrichment of a Ba mineral, likely baryte. In the case of the MCSC lenses and the metasedimentary rock, no correlation exists between Sr and Ba. Whereas the behavior of Rb was observed to be largely controlled by the behavior of K, Sr is only poorly correlated with Ca in both the metasedimentary rocks (trend line with R2=0.46) and the orthogneiss (R2=0.55).

The high-field-strength elements (HFSEs) Ti, Nb, and Zr, assumed to be less fluid-mobile and hosted in the sediments predominantly by heavy minerals (e.g., rutile-anatase-brookite, ilmenite, and zircon), are poorly correlated with Fe2O3(tot) (Fig. 4f, g; Nb not shown). Contents of Zr and Ti reflect the amounts of detrital zircon and ilmenite, which are scattered and also reflect maturity of the precursor sediments. However, TiO2 is broadly correlated with Fe2O3(tot) in all samples, with the exception of the lazulite quartzite and the pyrite quartzite. The ∼ 1 : 10 TiO2 : Fe2O3(tot) trend likely reflects the relative abundance of primary ilmenite/Ti magnetite inherited from the sediment source (Fig. 4g). Good correlation in the orthogneiss records the preservation of a magmatic fractionation trend, since the low fluid mobility of these elements implies limited movement during metamorphism.

Elevated concentrations of the trace elements Cr, V, (Fig. 4h) and Ni (not shown) in sediments can be considered a signature of an ultramafic-mafic source rock, such as the serpentinites. Chromium (and Ni) reaches its highest values of up to 300 µg g-1 in the metaconglomerate and in the lazulite quartzite. In the tourmaline gneiss, carbonate–mica schist, and mica schist, these elements are generally positively correlated, with some samples showing local enrichment in either V or Cr. Within the siliciclastic rocks of the Pfitsch Formation, the tourmaline gneiss has low Cr (15 to 150 µg g-1), V (15 to 130 µg g-1), and Ni (up to 40 µg g-1) contents, but these are still much higher than in the orthogneiss, which has values near or below the detection limit of ∼ 0.10 µg g-1 in most samples. Most samples of the MCSC lenses are enriched in V.

Average REE(+Y) patterns of the different rock units are shown normalized to post-Archean Australian shale (PAAS; Fig. 5). The individual patterns for the metasedimentary rocks differ mainly in terms of absolute REE contents, due to the fact that quartz (with negligible REE content) varies greatly in abundance in many samples. To account for this, we corrected all average REE values by a factor f= wt % SiO2sample / wt % SiO2PAAS. No REEs were determined for the highly heterogeneous metaconglomerates. The data for the mica schist exclude a quartz-rich sample (no. 28-14) with strong enrichment of the middle to heavy REEs and a pronounced negative Eu anomaly. The reason for this unusual pattern in this particular sample is its atypical enrichment of xenotime, determined by X-ray mapping of the thin section (see Supplement 4.3.5 in Franz et al., 2021). In some cases, individual patterns have different ratios of light to heavy REEs, because the REE budget of the precursor sediments is controlled mainly by the amount and type of heavy mineral, such as monazite (light REE-bearing), xenotime (heavy REE–Y-bearing), and zircon.

There are no Ce anomalies in any of the analyzed rocks, suggesting a common oxidation state in the sediments. The mica schist and tourmaline gneiss patterns are flat, having abundance levels of ca. 1 for the light REEs, including Eu, which has a slightly negative Eu anomaly (Eu - (Sm + Gd) / 2) of -0.2. The middle to heavy REEs are slightly enriched. The orthogneiss, representing the inferred proximal source area, has a flat pattern, with total abundances slightly below that of PAAS, except for Sm and Eu that are both close to 1. The lazulite quartzite shows a similar pattern to that of the Zentralgneis, with total abundances below those of PAAS and with a slightly positive Eu anomaly of 0.2. The MCSC lenses and pyrite quartzite both display steep REE patterns, depleted in light REEs with REE contents increasing towards Lu. The pyrite quartzite REE contents are consistently lower than the MCSC lenses, except for Lu. In chondrite-normalized patterns (see Supplement 4.3.9 in Franz et al., 2021) all units show a negative Eu anomaly, in line with the fact that the Zentralgneis is one of the dominant sources, most prominent in the REE similarity to the lazulite quartzite and the Zentralgneis.

The B contents and whole-rock δ11B values are shown in Fig. 6 (for data see Franz et al., 2021). Average B contents in the Pfitsch Formation (combined data for the tourmaline gneiss, mica schist, and carbonate–mica schist) are 215 µg g-1; they are near 30 µg g-1 in the lazulite quartzite (Windtal Formation) and 4 µg g-1 in marble from the Aigerbach Formation. In contrast, the basement rocks (Zentralgneis, amphibolite, and serpentinite from the roof pendant Altes Dach and Furtschagl schist) all contain ≤ 14 µg g-1 of B (average 5.0 µg g-1). The B-isotope compositions vary widely, ranging from ca. -33 to -3 ‰ δ11B, with the siliciclastic metasedimentary rocks of the Pfitsch Formation having values below ca. -14 ‰ δ11B. Values for the lazulite quartzite and its associated tourmalinite veins are restricted to -9 to -12 ‰ δ11B, similar to those of the overlying marble. In most of the basement rocks (including a tourmaline-bearing quartz–feldspar segregation), δ11B tends towards more positive values with the lowest value of -17 ‰ δ11B measured in the Furtschagl schist.

The protolith of metamorphic rocks can be constrained by the geochemical fingerprint imparted by the sedimentary, metamorphic, or igneous precursor. To be able to infer the sedimentary protoliths of the Pfitsch Formation, our initial assumption is that the present metasedimentary rocks retain some signature of the original sedimentary compositions. The composition of the sedimentary precursor itself was altered from that of the hinterland rocks through several processes, which began during chemical weathering, continued during sediment mixing and transport along rivers and sorting during sediment deposition, and ended with porewater interaction and diagenesis.

During regional metamorphism, deviation from isochemical deformation results from element transport by metamorphic fluids and metamorphic differentiation. In our study area, metasomatism and element mobility on a small scale are evident in certain rock samples, such as the selvages surrounding quartz nodules in the lazulite quartzite (Selverstone and Hyatt, 2003), or on the scale of tens of centimeters from rock to segregation as shown in the tourmaline gneiss (Berryman et al., 2017). Blackwall zones around serpentinite bodies (Trumbull et al., 2009) are another example of strong metasomatism in the area. Large-scale fluid movement that significantly changes the bulk composition is typically expressed in channels (e.g., Ague, 2011, and references therein). For instance, in the SW Tauern Window, the Greiner shear zone channelized fluids into a narrow zone 2–10 m in width (Selverstone et al., 1991), distinguished in the field by a macroscopically visible change in mineralogy and fabric. Although the Greiner shear zone extends into the Pfitscher Joch area (Fig. 1), channelized fluid flow is not observed except in the hornblende garbenschist (Selverstone et al., 1984; Steffen et al., 2001; Steffen and Selverstone, 2006). Within these basement rocks, deformation was locally responsible for fluid access and chemical alteration but did not pervasively change the chemical composition of the rock unit. Moreover, even in the case of the high-pressure rocks of the Tauern Window, fluid heterogeneities between rock layers on the millimeter-to centimeter scale have been observed to persist during metamorphism (Selverstone et al., 1992). Taken together, these observations support our assumption that during regional, near-peak, and post-peak Alpine metamorphism – except for the transition from diagenesis to low-grade metamorphism – pervasive fluid movement and associated element mobility were limited, leaving the whole-rock composition of the metasedimentary rocks largely intact. In addition, the use of a large dataset from each rock series in our investigation allows the average element values to cancel out the chemical signature of small-scale element mobility.

The chemical signatures of the metasedimentary rocks of the Pfitsch–Mörchner basin allow us to reconstruct the following pre-metamorphic history. At the time of sediment erosion and transport, the crystalline basement, consisting of granitoids and related roof pendants with amphibolites, serpentinites, and gneisses, was exposed to an arid to semi-arid climate, likely characterized by strong physical weathering (Fig. 9). This is in line with the low latitudes for the position of this part of Eurasian continental crust at the Permian–Triassic (Scotese and Schettino, 2017). Portions of the granitoids might have experienced strong hydrothermal alteration and weathering, with such rocks then being transformed during Alpine metamorphism into the characteristic pyrite quartzite. According to chemical classifications of the metasedimentary rocks, the hinterland was a continental volcanic arc, in line with subduction-related magmatism producing the Zentralgneis granitoids (e.g., Finger et al., 1993; Cesare et al., 2001) and the associated, porphyry-related molybdenite deposit (Melcher et al., 1996). The presence of coarse conglomerates and breccias on the base of this major erosional nonconformity supports a proximal position of the sediment source area. A closed intramontane Permian basin (Veselá and Lammerer, 2008, and references therein) is completely in accordance with the data as are the high B contents and very low δ11B values, typical of lacustrine, saline basins. Sedimentation was dominated by fluvial–lacustrine sediments, with indications of organic-rich sediments being absent. The interpretation here of the tourmaline-bearing gneiss as a meta-arkose, in contrast to a meta-rhyolite protolith by Veselá and Lammerer (2008), is important. Within our metasedimentary interpretation, the zircons from this unit dated by Veselá et al. (2011) are proposed as detrital. The presence of zircon indicates volcanism in the source area, defining a maximum age of sedimentation as 280 ± 3 Ma.

On top of the Pfitsch Formation, a sedimentary facies change is represented by the Windtal lazulite quartzite with distal transport producing the well-sorted sediment, possibly already existing near the seashore, as hypothesized by Morteani and Ackermand (1996) in a saline lagoon, or sabkha, environment (Fig. 9). Further east, in the Slovenian Karawanken mountains, marine shallow-water carbonates at the Permian–Triassic boundary are exposed in the Brsnina section (Williams et al., 2021) and are also explained as deposited at hypersaline conditions in a sabkha. It remains open if the transition from the Pfitsch Formation to the Windtal Formation is a sequence boundary or a disconformity (with a period of no deposition), but phosphatic sandstones, metamorphosed into Al–P quartzite (Morteani et al., 2007), are often associated with unconformities. From a similar continental setting in the Cañizar and Eslida formations of the south Iberian range (Spain), Galán-Abellán et al. (2013) described Early Triassic Al–P sandstones and the development of Al–P minerals during early diagenetic interaction with acidic meteoric waters. There are numerous other reports of Al–P sandstones related to unconformities, e.g., Gaboreau et al. (2005, 2007) from Proterozoic basins in Canada and Australia and Pe-Piper and Dolansky (2005) from the terrestrial Cretaceous Chaswood Formation in Nova Scotia, Canada. Hall et al. (1997) identified Al–P minerals on a former karstic erosional surface, derived from the leaching and dissolution of pre-depositional carbonates (Pennsylvanian Cheltenham Formation, Missouri, USA), and Mordberg et al. (2000) described Al–P minerals in a Devonian bauxitic weathering profile on Precambrian alkaline rocks in the Timan Range, northern European Russia. In the Pfitsch–Mörchner basin, lazulite quartzite marks the transition from a closed intramontane basin to an open basin, possibly connected with a disconformity. The depositional age of this and the next unit above, the Aigerbach Formation marble, is not restricted by radiometric age data, but comparison with the sedimentary record on the northern, Eurasian part of the basement allows for the hypothesis in which they are Early and Middle Triassic (Lammerer, 1986), corresponding to the classic “Germanic trilogy Buntsandstein–Muschelkalk–Keuper”. Radiogenic 87Sr / 86Sr for the marble and the Pfitsch Formation (Supplement 4.3.3 in Franz et al., 2021), which were determined to check for a stratigraphic correlation, are in line with this hypothesis but due to the likely influence of Rb in the marble do not allow a more detailed interpretation.

South of the Pfitscher Joch area in the South Alpine realm, other unmetamorphosed or only slightly metamorphosed Permo-Mesozoic sedimentary basins are known, such as the Collio and Tione basins. In the Tione basin near the city of Trento, a sedimentary sequence of similar age was described by Cortesogno et al. (1998). Its basement also consists of late Hercynian granitoids (“Dos del Sabion”) with diverse metamorphic country rocks (cf. Zentralgneis with Altes Dach). The cover sequence begins with coarse fluvial–lacustrine deposits of the Permian Collio Formation (cf. metaconglomerates of the Pfitsch Formation), separated by an unconformity from the overlying medium- to coarse-grained Val Daone conglomerates (cf. mica schist unit and tourmaline gneiss). This sequence leads into the detrital Verrucano Lombardo–Val Gardena deposits, which are covered by Lower Triassic sandstones and limestone of the Servino Formation (cf. lazulite quartzite and Aigerbach Formation marble). However, there are also significant differences among these sequences, because the Tione basin contains a large amount of dacitic to andesitic volcanic rocks, which are absent in the Pfitscher Joch area.

The Permo-Triassic was a time of major extensional stress, which affected Eurasia in the western European domain (Nikishin et al., 2002), producing several terrestrial basins in the Southern Alps. Cassinis et al. (2008) documented mafic magmatism throughout these basins and their crystalline basements, occurring as mafic dike swarms/sills having a Late Triassic age of 217 ± 3 Ma (Ar–Ar dating) in the Tione basin. In the Collio basin to the west, Feijth (2002) determined the Ar–Ar age of detrital white mica and interpreted ages of 270 to 255 Ma and 250 to 180 Ma as records of hydrothermal events. In line with these observations, the metamorphosed dikes in the Zentralgneis of the Pfitscher Joch area would represent analogous igneous activity of the 217 Ma mafic dike swarms, with the tourmalinite veins resulting from this early Mesozoic hydrothermal activity. A similar but marine setting has been described in the Austroalpine nappes north of the Tauern Window, in the evaporitic Haselgebirge Formation of the northern calcareous Alps (Schorn et al., 2013). These authors reported upper Permian (to lowermost Triassic) crystallization ages of ca. 270 to 248 Ma for dolerite dikes, interpreted as indications of the incipient rifting and opening of the ancient Meliata Ocean after the Variscan orogeny. A metagabbro of oceanic affinity in the Tauern Window yielded an intrusion age of 157 ± 2 Ma (Gleißner et al., 2021), consistent with other ages of 167 to 156 Ma along the Alpine chain (Schaltegger et al., 2002, and references therein) testifying to an already-open ocean during the Jurassic. Initial (earlier) basaltic magmatism of this spreading event might have also left traces on the passive continental margin during the opening of the Valais Ocean on the European basement, in our field area represented by the Zentralgneis.

In summary, our study shows that the geochemical signature of metasedimentary rocks allows determining the protoliths, diagenetic processes in the sediments, and some conclusions about the source region. We emphasize that (1) the chemical signature of the sedimentary protoliths was largely preserved during regional metamorphism and (2) individual samples may differ greatly in whole-rock geochemistry, but using averages of a sufficiently large sample set we can show that the metasedimentary rocks represent a mixture of the different rock types of the source area, as demonstrated by Na2O / (Na2O + K2O) and MgO / (MgO + Fe2O3) ratios. The trace elements Cr, V, and Ni can also identify components derived from mafic–ultramafic source rocks. REE patterns of individual metamorphic rock samples are difficult to interpret (due to heterogeneous distribution of REE-rich minerals in sediment), but if averages of a unit are used, these can clearly distinguish metasedimentary rocks with an upper continental crustal signature from hydrothermally altered rocks by their steep pattern with rising contents from La to Lu.

Diagenetic processes in the sediment can be reconstructed via chemical signatures; the behavior of fluid-mobile elements K, Rb, and Sr is especially helpful in identifying pre-metamorphic processes. This insight is due to the crystal chemical coupling of Rb to K and the preferential fractionation of Rb into mica/feldspar over hydrous fluid. Any decoupling of K and Rb requires unusual fluid compositions such as Cl-rich saline brines or hydrothermal fluids rich in S. This is in contrast to the behavior of Sr, which always preferentially partitions into a hydrous fluid. Although Sr is generally coupled to Ca, the presence of Sr minerals such as strontianite or celestite can cause a strong decoupling.

K metasomatism of the sediments is preserved in the metamorphic product and is responsible for the large variation and locally strong enrichment in K2O. Potassium and Sr are fixed in mica and feldspar, and although generally considered fluid-mobile during metamorphism, the variation caused by metamorphism is less important than that produced by diagenesis. This K metasomatism is also considered responsible for B enrichment in the precursor sediments of the study area, resulting in tourmaline-rich metamorphic rocks and explaining petrographic features such as the high tourmaline content. The B-isotope ratios and whole-rock B contents give important hints about the depositional environment in the Pfitsch–Mörchner basin, for a continental, intramontane setting with high evaporation rates.

Al- and P-bearing quartzites can be an important indication of a disconformity (however, they should not be considered a proof of a dis- or unconformity). In the Pfitsch–Mörchner basin, such rocks mark the depositional transition from a closed-basin setting to an open basin. In their protoliths of phosphatic sandstones, P minerals are difficult to identify, but during metamorphism and the formation of macroscopically identifiable blue lazulite, easily found during fieldwork, the presence of these quartzites can hint at a possible disconformity.

Basinal brines, enriched in B, can lead to the formation of tourmalinite, with crosscutting field relations. Characteristic for the origin of these tourmalinites is fluid immiscibility of B-rich and Si-rich gel-like fluids. In the case of the Pfitsch–Mörchner basin, the heat source of this hydrothermal event is unknown, but we speculate that it was associated with Permo-Triassic magmatism that took place following incipient rifting after the Variscan orogeny and before the onset of the Alpine orogeny.