Thorium–lead (Th-Pb) crystallization
ages of hydrothermal monazites
from the western, central and eastern Tauern Window provide new insights
into Cenozoic tectonic evolution of the Tauern metamorphic dome. Growth
domain crystallization ages range from 21.7 ± 0.4 to 10.0 ± 0.2 Ma.
Three major periods of monazite growth are recorded between
∼ 22–20 (peak at 21 Ma), 19–15 (major peak at 17 Ma) and
14–10 Ma (major peak around 12 Ma), respectively, interpreted to be
related to prevailing N–S shortening, in association with E–W extension,
beginning strike-slip movements and reactivation of strike-slip faulting.
Fissure monazite ages largely overlap with zircon and apatite fission track
data. Besides tracking the thermal evolution of the Tauern dome, monazite
dates reflect episodic tectonic movement along major shear zones that took
place during the formation of the dome. Geochronological and structural data
from the Pfitschtal area in the western Tauern Window show the existence of
two cleft generations separated in time by 4 Ma and related to strike-slip
to oblique-slip faulting. Moreover, these two phases overprint earlier
phases of fissure formation.
Highlights.
In situ dating of hydrothermal monazite-(Ce).
New constraints on the exhumation of the Tauern metamorphic dome.
Distinct tectonic pulses recorded from east to west.
Introduction
In situ thorium–lead (Th-Pb) dating of hydrothermal fissure monazite-(Ce) (in the following
simply monazite) has recently been demonstrated to be a reliable method for
dating tectonic activity under retrograde metamorphic conditions
(Bergemann
et al., 2017, 2018, 2019, 2020; Berger et al., 2013; Fitz-Diaz et al., 2019;
Gasquet et al., 2010; Gnos et al., 2015; Grand'Homme et al., 2016a; Janots
et al., 2012; Ricchi et al., 2019). These studies conducted through the
entire Alpine orogenic belt allowed constraining tectonic activity in
relation with exhumation and fault activity under retrograde lower
greenschist to sub-greenschist facies metamorphic conditions.
Hydrothermal fissure monazite, concentrating light rare earth elements (LREE), Th and U, generally
crystallizes in Ca-poor lithologies, outside the stability field of titanite
or epidote / allanite. However, once formed, hydrothermal processes may cause
dissolution–reprecipitation events leading to resetting of the monazite
Th-Pb decay system in parts of the crystal. Chemically and isotopically
homogeneous crystals indicate a single, rapid growth episode (e.g.
Grand'Homme et al., 2016a). However, crystals showing different
growth domains indicative of successive growth episodes are more common. In
other cases, parts of, or entire, grains display a patchy zoning due to
dissolution–reprecipitation processes (e.g.
Ayers
et al., 1999; Grand'Homme et al., 2016b). These processes involve element
fractionation resulting in crystal zones with often distinct Th/U values
(Seydoux-Guillaume
et al., 2012).
The advantage of using hydrothermal monazite for dating tectonic activity is
related to the high closure temperature of monazite of > 800 ∘C,
implying that diffusion in monazite is negligible
(Cherniak
et al., 2004; Gardés et al., 2006, 2007) under P–T conditions at or
below 450–500 ∘C and 0.3–0.4 GPa (e.g.
Mullis et al., 1994; Mullis,
1996; Sharp et al., 2005) where hydrothermal
fissures form. Fissure monazites date crystallization following chemical
disequilibrium within a fissure. This causes a dissolution–precipitation
cycle that may include dissolution or partial dissolution of existing
fissure monazite. This has the consequence that late
dissolution–precipitation steps may be well recorded, whereas earlier growth
domains may be completely destroyed. Thus, monazite crystallization due to
chemical disequilibrium is interpreted as being related to tectonic activity
(e.g. volume change, fissure propagation, exposure of fresh host rock,
delamination of fissure wall, seismic activity, fluid loss or gain).
Recent studies have shown that fissure monazite typically forms between
generally lower to higher 200–400 ∘C
(Gnos et al., 2015; Janots et al.,
2019). For this reason, fissure monazite ages are generally interpreted as
dating crystallization or re-crystallization. Monazite geochronology can
thus be utilized to constrain shear and damage zone activity under
greenschist and very low-grade metamorphic conditions at least down to
200 ∘C (e.g.
Bergemann
et al., 2017, 2018; Gnos et al., 2015).
Fissures and clefts develop close to the brittle–ductile transition
(< 450 ∘C; Mullis, 1996) and are
usually oriented perpendicular to the foliation and lineation of the
host rock (Gnos et al., 2015). Fissures are
generally straight when they form and either became enlarged by subsequent
tectonic activity to form fluid-filled decimetre- to metre-sized clefts, displaying a
more open shape with rounded surfaces (e.g. Ricchi et al., 2019) when the
stress field retains the same orientation, or become completely filled to
form mineral veins. However, they may show a complex shape when the stress
field direction changes during deformation. Fluid inclusion studies (e.g.
Mullis, 1996) show that clefts generally suffered several deformation episodes.
Interaction of the fluid that fills the fissures with the surrounding rock
leads to dissolution of minerals in the wall rock and mineral precipitation
in the fissure. As long as deformation continues, fluid-filled clefts will
react to deformation via dissolution–precipitation cycles due to
disequilibrium between fluid, rock wall and mineral assemblage within the
cleft (e.g. Putnis, 2009). Thus, hydrothermal minerals
like monazite do not only grow following the initial fissure formation but
form, continue to grow or dissolve during subsequent deformation stages.
The timing of these growth or alteration stages may not always be resolvable
with the precision of currently available geochronological methods, but
different growth stages may be distinguishable through differences in the
chemical composition (Grand'Homme et al., 2018). In
contrast to the surrounding country rock, the fissures and clefts remain
highly reactive at low temperature due to the presence of fluids. For this
reasons, deformation steps during brittle deformation may be registered
through mineral growth or recrystallization in clefts (e.g. Berger et al.,
2013) down to conditions where clay minerals form in fault gauges.
The Tauern Window (TW) is a thermal and structural dome of the eastern Alps
(Fig. 1) exhumed over a period of about 30 Ma starting from the Early
Oligocene (e.g. Rosenberg
et al., 2018; Schmid et al., 2013). Previous monazite crystallization ages
obtained in the eastern subdome of the TW record tectonic activity between
19.0 ± 0.5 and 15.0 ± 0.5 Ma
(Gnos et al., 2015). In the current study,
monazite geochronology is extended to the entire TW in order to investigate
its Cenozoic deformation history. We particularly aim to establish a
chronological record for the younger exhumation history recorded by fissure
monazite crystallization, to be compared with known deformation phases.
Tectonic map of the TW dome modified after Bertrand et al. (2017),
Scharf et al. (2013), Schmid et al. (2013) and Schneider et al. (2013).
Yellow stars on the map represent sample locations, and numbers inside the
stars refer to samples listed in Table 1. Range of weighted mean growth
domain ages are indicated for each grain from this study and Gnos et al. (2015),
labelled in black and green, respectively, on the map (see Table 4 for an exhaustive
summary of all the ages). Only the spot date range is indicated for grains 1,
4 and 6. Locations of AA', BB' and CC' cross sections are indicated by black
lines, and individual cross sections are presented in Fig. 6 together with
monazite crystallization ages. Two normal faults delimit the western and
eastern borders of the TW, the Brenner normal fault (BNF) and the Katschberg
normal fault (KNF), respectively. Note that the KNF prolongation results in
dextral and sinistral strike slips in the north and south, respectively
(KSZS: Katschberg shear zone system). Several sinistral strike-slip faults
(AhSZ: Ahrntal shear zone; ASZ: Ahorn shear zone; DAV: Defereggen–Antholz–Vals
fault; GSZ: Greiner shear zone; InF: Inntal fault; MüF: Mur–Mürz fault;
NF: Niedere Tauern southern fault; OSZ: Olperer shear zone; SEMP:
Salzach–Ennstal–Mariazell–Puchberg fault; SpSZ: Speikboden shear zone;
TSZ: Tuxer shear zones; ZWD: Zwischenbergen–Wöllatratten and Drautal faults),
dextral shear zones (HoF: Hochstuhl fault; IsF: Iseltal fault; KLT: Königsee–Lammertal–Traunsee
fault; Mölltal fault (MöF); PF: Pustertal fault) and a reverse fault
(MM: Meran–Mauls fault) are also visible in red on the map.
A total of 23 monazite grains together with provenance data, and in some
cases host-rock information, were dated (Table 1). Seven grains come from the
western limb of the TW (INNB1 ZEI1, SCHR1, MAYR4, PFIT1, BURG2 and PLAN1;
samples 1–7; Fig. 1), another seven grains come from the eastern border
of the western subdome (central TW; SCHEI1, HOPF2, GART1, NOWA3, GART3,
STEI2 and KNOR1; samples 8–14; Fig. 1), and nine grains were collected in
the eastern subdome (KAIS6, SALZ18, LOHN4, ORT1, EUKL2, HOAR1, MOKR1, SAND1
and REIS1; samples 15–25; Fig. 1, Table 1). In order to capture at best
the tectonic activity during the exhumation of the TW, the investigated
samples were selected in a way that gives priority to sample localities in
regions affected by major fault zones and at lithological boundaries. In the
following, we will discuss the ages in terms of sample ID numbers (1–25)
provided in Table 1.
Summary of monazite samples investigated in this study and by Gnos et
al. (2015). Samples name, number, location, host-rock lithology, metamorphic
degree and fissure mineral association are provided. Samples with approximate
finding location are marked with “approx.”.
Ab – albite; Adl – adularia;
Ank – ankerite; Ant – anatase; Ap – fluorapatite; Asc – aeschynite; Brk – brookite;
Cc – calcite; Chl – chlorite; Clc – clinochlore; Hm – hematite; Ilm – ilmenite;
Lm – limonite; Pyr – pyrite; Qtz – quartz; Rt – rutile; Sd – siderite; Snt –
senaite; Str – strontianite; Syn – synchisite; Trm – tourmaline; Xnt – xenotime;
Alpine metamorphism: AM (amphibolite facies), GAT (greenschist–amphibolite transition),
UGS (upper greenschist facies). a Bloc in glacial moraine. b Bloc in rock slide.
Geologic settings
In the largest tectonic window of the Austroalpine nappe stack, the TW,
Penninic (Glockner nappe system and Matrei zone; Fig. 1) and Subpenninic
nappes (mainly the Venediger Duplex) are exposed (e.g.
Schmid et al., 2013; Fig. 1). The TW metamorphic and
structural dome consists of two subdomes, with E–W-striking upright folds
in the internal parts and bordered by two major normal faults, the
Katschberg normal fault (KNF) in the east and the Brenner normal fault (BNF)
in the west (Fig. 1). The western subdome is dissected by numerous
sinistral shear zones (Ahorn shear zone (ASZ), Olperer shear zone (OSZ),
Tuxer shear zones (TSZ), Greiner shear zone (GSZ) and Ahrntal shear zone
(AhSZ)) and is bordered by the Salzach–Ennstal–Mariazell–Puchberg fault
(SEMP) in the north (Fig. 1). The eastern subdome is bordered to the east by
the Katschberg normal fault (KNF), continuing to the north into the dextral
Katschberg shear zone system (KSZS) and to the south into an unnamed
sinistral shear zone and oriented parallel to the Mölltal fault
(MöF). The deformation history of these fault complexes will be
discussed later.
Summary of deformation phases in the Tauern metamorphic dome.
Age (Ma)PhaseFaultDomainCharacteristicsRef.RemarksEstimated peaks of deformation ∼65D1Penninic nappesAccretion and subduction ofPiemonte–Liguria OceanE∼41D2Penninic and SubpenninicnappesSubduction of Valais Ocean and parts of the distal European marginE∼35D3Central TWExhumation of high-pressure unitsEFolding of D2 thrust, decompression∼29D4Subpenninic nappesEuropean slab break-off, VenedigerDuplex formation and “Tauernkristallisation”EContemporaneous intrusion ofPeriadriatic plutons and incipientNE-wards subduction of theAdriatic slab∼23–21East of the Giudicarie beltIncipient indentation of the southern alpine units in the Eastern AlpsD, E∼17D5TWIndentation, doming andlateral extrusionEFaults' motion 33–15ASZWestern TWSinistral ductile shearF, GDuctile continuation of the SEMP fault24–12TSZWestern TWSinistral ductile shearB, F20–7GSZWestern TWSinistral ductile shearF21–10BNFWestern TWNormal faultC22–13KNFEastern TWNormal faultC
A: Bertrand et al. (2017, 2015); B:
Blanckenburg et al. (1989); C: Favaro et al. (2017); D: Scharf et al. (2013); E:
Schmid et al. (2013); F: Schneider et al. (2013); G: Rosenberg and Schneider (2008). ASZ: Ahorn shear zone, BNF: Brenner normal fault, GSZ:
Greiner shear zone, KNF: Katschberg normal fault, SEMP: Salzach–Ennstal–Mariazell–Puchberg fault, TSZ: Tuxer shear zones.
The Alpine evolution of the TW started in the Early Paleocene with the
accretion and subduction of the Piemonte–Liguria Ocean (Matrei zone; Fig. 1)
under the Apulian margin (Austroalpine nappe stack; e.g.
Schmid et al., 2004, 2013; D1 deformation of
Schmid et al., 2013; Fig. 1, Table 2). In the Middle Eocene, the Valais Ocean
and parts of the distal European margin (Glockner nappe system, Eclogite
zone and parts of the Modereck nappe system; Fig. 1) were equally subducted
below the Austroalpine nappe stack and the Matrei zone accreted during D1
deformation (D2 deformation of Schmid et al., 2013; Table 2). In the Late
Eocene, exhumation was achieved by extrusion of the high-pressure units that
went together with major folding of the D2 thrust formed between the
subducted Glockner nappe system and Modereck nappe system (D3 deformation of
Schmid et al., 2013; Table 2). In the Early Oligocene, nearly contemporaneous
break-off of the subducting European slab and formation of the Venediger
Duplex (crustal-scale duplex structure) occurred, followed by the
“Tauernkristallisation” (reheating of the whole nappe stack to
amphibolite facies conditions) (D4 deformation of Schmid et al., 2013; Table 2).
This was followed by an inversion of subduction polarity at
∼ 23 to 21 Ma (e.g.
Rosenberg et al., 2018; Scharf et al., 2013; Schmid et al., 2013; Table 2).
The following exhumation of the TW started in the Early to Middle Miocene by
Alpine N–S collisional shortening and E–W orogen-parallel extension leading
to folding, erosion and lateral extrusion through shear zone development
(e.g.
Luth
and Willingshofer, 2008; Rosenberg and Berger, 2009; Rosenberg and Garcia,
2011; Schmid et al., 2004, 2013; Selverstone, 1988; D5 deformation of Schmid
et al., 2013). Previous shear zone age dating in the TW was achieved using
different geochronometers: Rb-Sr whole-rock–phengite dating (20 Ma;
Blanckenburg
et al., 1989), Rb-Sr whole-rock–white mica dating (39–16 Ma;
Glodny et al., 2008), Sm-Nd dating on
garnet (27.5–20 Ma;
Pollington
and Baxter, 2010, 2011) and 40Ar /39Ar dating on mica (35–28 Ma;
Urbanek et al., 2002). A recent detailed study by
Schneider et al. (2013) using
texturally controlled in situ 40Ar /39Ar dating of syn-kinematic
phengite and K-feldspar returned ages of 33–15, 24–12 and 20–7 Ma.
They were interpreted as recording deformation along three major shear
zones (ASZ, TSZ and GSZ, respectively) of the western subdome.
Sample location
Fissure monazite is rare and difficult to find, meaning that this study
could not have been conducted without the help of crystal searchers who
provided samples. Fissure monazites were selected to cover all parts of the
TW areas with known shear zones within it. It was, however, unfortunately
not possible to obtain exact coordinates for all of the samples (Table 1).
This is due to the rarity of fissure monazite, so that some samples were
obtained from old finds or collections. In other cases, the crystal searcher
could not anymore precisely identify the fissure in which the monazite was
found. These samples are marked with “approx.” in Table 1. We could
therefore only revisit some of the sample locations in order to add
structural information. Experience from other parts of the Alps (e.g.
Bergemann et al., 2017, 2019; Ricchi et al., 2019) shows that fissure
monazite sampled within the damage or central zones of a shear zone
generally records shear zone activity well. Information on the source
localities, host rocks, degree of alpine metamorphism and mineral
associations is in Table 1.
Methods
The crystals were polished individually on a lapidary disc and embedded in
epoxy together with the monazite standard “44069” (425 Ma,
Aleinikoff et al., 2006),
following the same procedure as that in
Bergemann
et al. (2017). Backscatter electron (BSE) images were acquired in order to
investigate the internal textural features of each grain (e.g. zoning,
evidence of alteration) using an energy dispersive spectrometer (EDS)-equipped JEOL JSM7001F and a
Zeiss DSM940A electron microscope at the University of Geneva with a beam
current of 3.5 nA and acceleration voltage of 15 kV. BSE images helped in the
selection of secondary-ion mass spectrometry (SIMS) spot analysis points,
carefully placed in chemically distinct domains.
Ion probe U-Th-Pb analyses of 15 monazite crystals were conducted at the
SwissSIMS ion microprobe facility, University of Lausanne, Switzerland, and
analyses of another eight crystals were performed at the NordSIMS facility,
Swedish Museum of Natural History, Stockholm (Tables 1 and 3). Both
laboratories are equipped with a Cameca IMS 1280-HR instrument. The
instruments were run following the procedure of Janots et
al. (2012), applying a -13 kV O2- primary beam, an intensity of
∼ 3 and 6 nA focused on the sample (SwissSIMS and NordSIMS,
respectively) to produce a spot of 15–20 µm in diameter. A mass
resolution of 4300–5000 (M/ΔM, 208Pb /232Th at 10 %
peak height) and an energy window at 40 eV were applied, with data collection
in peak hopping mode using an ion-counting electron multiplier. All the
unknowns were standardized to 44069 (425 Ma;
Aleinikoff et al., 2006)
monazite, and the uncertainty on the standard 208Pb /232Th-ThO / Th
calibration in each session was 1.7 % on average.
Th-U-Pb analyses of monazite by ion microprobe (SwissSIMS and NordSIMS).
Analyses resulting in unreliable dates (e.g. presence of cracks, affected by Pbc
causing high uncertainty) were not considered and are written in italic.
A 207Pb and 204Pb common lead (Pbc) correction calculated at time
zero was applied to the data acquired at the SwissSIMS and NordSIMS (Table 3) using the
terrestrial Pb evolution model of Stacey and
Kramers (1975). Cameca customizable ion probe software (CIPS) was used
for data reduction. 204Pb- and 207Pb-corrected ages agree within
uncertainty (Table 3), but we preferred to discuss 207Pb-corrected ages
because they are more robust and consistent (better statistics and less
scatter in the data). Calculation of weighted mean ages, based on
207Pb correction, and plotting was carried out using the
IsoPlot Ex 4.1 programme (Ludwig, 2003). Single and weighted mean
ages (or average ages) are quoted at the 1σ and at the 95 % confidence
level in the text, respectively.
Weighted mean 208Pb /232Th ages were calculated for each growth
domain following the approach of Bergemann et al. (2017, 2018, 2019,
2020)
and Ricchi et al. (2019). Distinct chemical and textural domains were
carefully defined in each grain based on Th concentrations as function of U
concentrations and BSE image information. Since fissure monazite is
dissolved and re-precipitated under changing chemical conditions (e.g.
Grand'Homme et al., 2018), spot analyses affected by
Pbc (resulting in older dates directly related to higher Pbc, i.e. positive
age-f208 correlation), inclusions or those with high uncertainty (1σ > 1 abs.)
were removed from the dataset and marked in italic in
Table 3. However, spot dates located on dissolution trails, generally
providing younger dates, were considered in the age ranges because they
likely record a later phase of monazite crystallization.
ResultsField observations
An example of deformed fissures and different stages of fissure formation is
well exposed in outcrops along the road leading to Pfitscherjoch (in
proximity to the PFIT1 sample locality 5, western TW; Table 1), where two
fissure generations are present (Fig. 2a and b). In this outcrop, an earlier
fissure generation (C2, green ellipses) is partly deformed during
subsequent deformation, and a younger generation of fissures (C3, blue
ellipses) is also present. Subhorizontal fissures (C3) seem linked to
a strongly inclined lineation (L3, blue arrows), whereas older
subvertical fissures (C2) seem related to a weakly inclined
strike-slip lineation (L2, green arrows). The older fissures are wider
and sigmoidal in shape and contain muscovite which is not found in the
younger fissures. In some cases, younger fissures crosscut older ones (Fig. 2b).
Moreover, the orientation of the foliation (S2,3; Fig. 2c) of
these two fissure generations (C2 and C3) is different from the
foliation (S1; Fig. 2c) of early fissure formation mainly observed in
the eastern part of the TW (C1, Fig. 2c, discussed below). This
suggests that, in the Pfitscherjoch area, early fissures C1 were
overprinted by younger tectonic movements.
(a) Two generations of late fissures visible in a road
outcrop located between monazite locality (46∘59.436′ N, 011∘39.240′ E)
and Pfitscherjoch. Steeply oriented fissures (C2: ∼090/65) are older and deformed
(green ellipses), and seem related to a flatter lineation (L2: ∼250/30, green arrows)
visible on some of the foliation planes. Younger and flatter oriented fissures (C3: ∼085/30)
are straight (blue ellipses) and seem related to a steeper lineation (L3: 270/70, blue arrows).
These observations indicate that a fissure can be deformed during its existence. The length of the
hammer handle is 60 cm. (b) Enlargement of panel (a). (c) Schematic illustration
of the three fissure generations observed in this study (C1,C2 and C3), together with
respective orientation, foliation (S1,S2 and S3) and lineation (L1,L2 and
L3). The first fissure generation (C1) is related to E–W extension, the second
fissure generation (C2) is linked to strike-slip movements and the third fissure generation
(C3) is related to the oblique-slip movements.
The large majority of the fissures present in all the investigated
localities are oriented subvertically (C1 and C2 type in Fig. 2c),
roughly striking NE–SW. For C2, this would indicate a similar
direction of extension for the development of this fissure type, which is in
line with palaeostress orientations provided by
Bertrand et al. (2015). However, even if all
subvertical fissures are subparallel, at least two generations exist. (i) Early
subvertical fissures (C1, Fig. 2c) are related to flat foliation
(S1) and E–W-stretching lineation (L1); these are oriented
perpendicular to the main fold axes (and lineation) of the TW and are
associated with E–W extension
(e.g. Gnos et al., 2015; Rosenberg et al., 2018; Schneider et al., 2013). (ii) Younger
subvertical fissures (C2, Fig. 2c) are associated with
subvertical E–W-oriented foliation (S2) and flat to inclined lineation
(L2), and are oriented perpendicular to strike-slip faults (mainly in
the western part of the TW; Fig. 2c). At Pfitscherjoch, the shape of C2
fissures, indicating overprinting by sinistral sense of shear, is in
agreement with the larger-scale sinistral shearing of the GSZ shear zone.
(iii) A third generation of fissures (C3, Fig. 2c) is locally observed,
for example, in the Pfitscherjoch locality (Fig. 2a and b) and is at a high
angle with C1 and C2 fissures. This third fissure generation
observed in the Pfitscherjoch locality is associated with a subvertical
E–W-striking foliation (S3). Stretching lineation related to the BNF
activity is subparallel to C3 lineation; however, its foliation is
striking N–S (Fig. 2c). We suggest that C3 fissures are related to
oblique-slip movements, in line with the observed E–W-striking foliation and
not the BNF activity.
Summary of weighted mean ages of monazite growth domains and spot age ranges of each grain from the TW.
The monazite grains selected for in situ Th-Pb dating are millimetre sized and, when
BSE zoning is visible, they show two distinct textures: regular and patchy
(Figs. 3, 4 and 5; Table 4). The term “regular” refers to crystals showing
growth zonation, whereas a patchy texture is interpreted as a replacement by
secondary dissolution–reprecipitation processes
(e.g.
Ayers et al., 1999; Bergemann et al., 2018, 2019, 2020; Gnos et al., 2015).
Thorium and uranium (U) contents of the dated fissure monazites display a large
variability, ranging from ∼ 200 to 63 000 ppm Th and
∼ 2 to 2000 ppm U, with variations in Th/U ratio from 1 up to
∼ 7000 (Figs. 3, 4 and 5; Table 3). 232Th-208Pb ages
presented on the right-hand panel of Figs. 3, 4 and 5 are arranged
according to the order established in Table 3. A detailed description of
each monazite grain is provided in the Supplement. Average
ages are reported for groups of dates on texturally and/or chemically similar
domains. In order to ensure that a group of dates from a domain is
internally consistent, rare outliers have been excluded to bring the mean square weighted deviation (MSWD) within acceptable values (MSWD < 3; Spencer et
al., 2016). In a few cases, the dates for specific monazite domains have a
scatter above analytical uncertainty (e.g. grains 6, 9 and 24), which probably
reflects the complex formation process of fissure monazite.
The investigated grains from the western part of the western TW subdome
come from the Venediger Duplex, with the exception of sample 6, which comes
from the Glockner nappe system (Fig. 1; Table 1). Samples 2, 4 and 6
were collected near the major Brenner normal fault, which delimits the TW
to the west, and samples 1, 3, 5 and 7 were collected in the vicinity of
sinistral strike-slip faults (Fig. 1). Average growth domain ages range from
20.9 ± 0.6 to 10.0 ± 0.2 Ma (samples 3 and 2), with the youngest
ages recorded in the western TW (Figs. 1, 3 and 6a; Tables 3 and 4).
Chemical, textural and geochronological information of monazite
grains from the western TW. On BSE images, colour-filled circles correspond
to ion probe spot locations. Note that the square-shape shading in grain 4 is
due to an artefact of composing BSE images with diverse contrast.
The central part of the TW displays growth domain ages between 18.3 ± 1.1
and 10.4 ± 0.2 (samples 8 and 14; Figs. 1 and 4; Tables 3 and 4),
but the majority of the dated crystals in this area record ages around 17 Ma
(Fig. 6b). Samples 8, 9 and 10 were collected between the eastern and
western termination of the ASZ and the SEMP fault (Fig. 1). Another three
samples (11, 12 and 13; Table 1) were collected in the northern prolongation
of the AhSZ, and a seventh monazite (grain 14) was sampled near the southern
border of the eastern part of the western subdome (Fig. 1).
The oldest ages are principally recorded in the eastern part of the TW at
around 21 Ma (Fig. 6c). Average ages of growth domains range from 21.7 ± 0.4
to 13.6 ± 0.6 Ma (samples 19a and 25; Figs. 1, 4 and 6c;
Tables 3 and 4). The samples were mainly collected at the western border of
the eastern subdome, in the Venediger Duplex or near the boundary with the
Glockner–Modereck nappe systems (Fig. 1). Sample 25 was taken at some
distance from the other samples, near the southeastern border of the
eastern subdome (Fig. 1).
Chemical, textural and geochronological information of monazite grains
from the central TW. On BSE images, colour-filled circles correspond to ion
probe spot locations. Note that the square-shape shading in grains 10 and 11
is due to an artefact of composing BSE images with diverse contrast.
Chemical, textural and geochronological information of monazite grains
from the eastern TW. On BSE images, colour-filled circles correspond to ion
probe spot locations. Note that the square-shape shading in grains 15, 17,
18 and 20 is due to an artefact of composing BSE images with diverse contrast.
Discussion Fissure monazite ages
The oldest monazite ages of 21.7 ± 0.4 to 19.9 ± 0.3 Ma (found
in samples 19a and 20; Figs. 1, 6c and 7a and d) are common in the eastern
TW but can also be found in the western area (Fig. 7a, c and d, red
symbols). This in line with regional fault activity recorded at
∼ 21 Ma based on Pleuger et al. (2012) (Fig. 8a) which
corresponds to the main indentation phase
(Favaro et al., 2017). We interpret these as a
first monazite crystallization event during E–W extension in association
with the dome formation (N–S shortening) when the existing clefts reached
P–T conditions at which fissure monazite starts to grow (phase 1, red
symbols in Fig. 7). When comparing an assumed fissure formation temperature
of 450 ∘C (typically obtained in quartz fluid inclusion studies on
early alpine fissures (e.g. Mullis, 1996) with
thermochronological data of the eastern TW (compiled in
Wölfler et al., 2012), the onset of
fissure formation, predating primary monazite crystallization, is estimated
at around 25 Ma. Based on a comparison with thermochronological data,
monazite crystallization recorded between 19 and 15 Ma was estimated to have
occurred at ∼ 200–300 ∘C in the eastern TW
(Gnos et al., 2015). New monazite ages
from this study in the eastern TW are up to ∼ 22 Ma (sample 19
in Fig. 1), suggesting that early monazite crystallization in the area may
have occurred at higher temperatures of 350–400 ∘C.
Cross sections of (a) the western part of the western subdome, (b) the central
part of the western subdome and (c) the western end of the eastern
subdome, modified after (Schmid et al., 2013). See Fig. 1 for locations and
legend. Sample locations are indicated by yellow stars and identified by sample
numbers listed in Table 1. Monazite crystallization ages are present in the
lower part of the figure and are linked to each sample by light-grey dashed
lines. Weighted mean ages from this study and from Gnos et al. (2015) are
presented by yellow diamonds and yellow circles, respectively, and blue
bars correspond to the range of single spot dates.
(a) Map
of the TW from Fig. 1 with sample locations coloured as function of deformation
episodes (coloured stars). See Fig. 1 for legend. (b) DD' NE–SW cross
section across the BNF, (c) AA' NW–SE cross section perpendicular to the
axial plane of the western subdome and (d) EE' longitudinal cross section
parallel to the main axial plane of the TW metamorphic dome, modified after
Bertrand et al. (2017). In the upper part, coloured and numbered stars correspond
to sample locations and are linked to corresponding Th-Pb monazite ages by
dashed vertical lines. Sample numbers refer to Table 1. In the lower part,
monazite weighted mean ages from this study and from Gnos et al. (2015) are
labelled by coloured diamond and circles, respectively, and the range of
single spot dates is depicted by blue bars. The colour code used for
diamonds and circles follows deformation episodes explained in the
discussion. Note that most error bars are smaller than the size of
the diamonds and circles. Zircon and apatite fission track ages are
from the Bertrand et al. (2017) compilation; light- and dark-grey dots
with error bars are displayed for comparison. Square brackets shown
to the right delimit the main periods of monazite growth discussed in the
text: (1) early record of N–S shortening and associated E–W extension,
(2) contemporaneous N–S shortening and strike slip, (3) reactivation
of strike slip to oblique slip.
While early fissure formation is related to E–W extension (leading to flat
foliation and E–W mineral lineations; Fig. 2c), we suggest that monazite
formation also occurred along the sinistral strike-slip to oblique-slip
movements (vertical foliation and flat to inclined lineation; Fig. 2),
particularly developed in the central and western parts of the TW (e.g.
Rosenberg
et al., 2018; Schneider et al., 2013). These shear zones developed as a
result of bending of the E–W-oriented upright folds around a vertical axis
(leading edge of the Dolomite indenter) (Fig. 1). This occurred when N–S
shortening could no longer be accommodated by folding and doming within the
TW. Associated with these movements is the formation of a younger generation
of fissures (see Pfitscherjoch example above), the peak activity of which is
recorded at ∼ 17 Ma (phase 2, green symbols in Fig. 7).
This fissure generation is associated with a steep foliation and a flat
lineation (Fig. 2) but subparallel in orientation to the earlier fissure
formation. The monazite ages at ∼ 17 Ma found in the western
and central TW (Figs. 1, 6 and 7; samples 5, 8, 13; Table 4) are
associated with sinistral fault zones, as in the Pfitscherjoch region or
near the eastern termination of the ASZ and AhSZ faults (see above).
Unfortunately, we do not have structural information on the westernmost and
easternmost analysed samples (6 and 25; Figs. 1, 6 and 7), but they can be
speculated to also have formed in association with a strike-slip shear zone
or the BNF and KNF in the case of samples 6 and 25, respectively. At larger
scale, these movements seem to have been associated with the development of
the sinistral Giudicarie fault (GF, located at the southwestern corner of
the TW), offsetting the Periadriatic fault (PF; Fig. 8b, e.g. Pleuger et
al., 2012). Ages of ∼ 17 Ma are also recorded in the eastern
part of the TW, likely linked to the KNF and Mölltal fault (MöF)
activity (samples 16, 21, 24 and 25; Fig. 7a and d; e.g. Favaro et al.,
2017). In grains located in the western part of the eastern subdome, in the
prolongation of the MöF (e.g. Kurz
and Neubauer, 1996) (Fig. 1), numerous monazite growth domains yield ages
between 15.6 ± 0.7 and 15.0 ± 0.5 Ma (bracketed by samples 22
and 21 from Gnos et al., 2015; Figs. 1, 6c, 7a and d; green circles in
Fig. 7d; Table 4). These ages date the latest known activity of this shear zone
to ∼ 15 Ma. Whereas younger ages, associated with reactivation
of fault zones are widespread in the central and western TW, tectonic
movements seem to cease in the eastern TW after this time (Fig. 8c).
The youngest monazite growth domain ages, principally recorded in the
western subdome, range from 13.2 ± 0.3 to 10.0 ± 0.2 Ma
(samples 5 and 2; Table 4), indicating steps of reactivation of the different
sinistral strike-slip to oblique-slip movements along different faults
(phase 3 and blue symbols in Fig. 7). Based on our monazite
crystallization data, the oldest activities of this younger phase are
recorded near the GSZ (sample 5) and in the prolongation of the AhSZ
(samples 7 and 9). The youngest activities are recorded in association with
the ASZ, OSZ and TSZ in Fig. 7a–c (samples 1, 2 and 4), and in the central
TW in an area located south of the main fault systems (sample 14; Fig. 7a).
In addition to faults activity at ∼ 12 Ma (Fig. 8),
coeval strike-slip activity has also been documented in many areas of the
central and western Alps (e.g.
Bergemann
et al., 2017, 2019; Berger et al., 2013; Gasquet et al., 2010; Grand'Homme
et al., 2016a; Pleuger et al., 2012; Ricchi et al., 2019).
In summary, in the western TW, monazite ages (Fig. 1) constrain the
activities of the ASZ (18–12 Ma, samples 8 and 9), AhSZ (17–12 Ma,
samples 13 and 7), TSZ/OSZ (11.5–10 Ma, samples 1 and 2; older ages of
sample 3 are probably related to extensional unroofing) and GSZ (17–13 Ma).
In the eastern part, the MöF is active between 19 and 15 Ma.
Tectonic map of the Alps based on Pleuger et al. (2012) showing
active Cenozoic faults at ∼21 (in red), 17 (in green) and 12 Ma
(in blue), respectively. Note that after 17 Ma the Giudicarie fault (GF)
becomes active and hence the Periadriatic fault (PF) and the Mölltal
fault (MöF, dextral fault at the southeastern corner of the TW) become
inactive. Sinistral strike-slip faulting starts at ∼19 Ma and is
affecting the western and central parts of the TW until at least 7 Ma.
Future active faults are depicted in grey and inactive faults in black.
Th as function of U content obtained for all the monazite grains
analysed in this study. Samples indicated by an asterisk are from Gnos et
al. (2015). Fissure monazite grains associated with hematite (oxidizing
conditions) are labelled in red, whereas grains hosted in graphite-bearing rocks
(reducing conditions) are labelled in blue. Samples with intermediate composition
and/or for which we have no information on the presence of hematite or graphite
in the fissure environment are labelled in grey.
Comparison with shear zone dating
A number of attempts to date shear zone activity in the TW using Ar-Ar,
Rb-Sr and Sm-Nd techniques have been made in the past, which were, however,
based on mineral separation techniques without a clear structural control on
the dated grains (e.g.
Blanckenburg
et al., 1989; Glodny et al., 2008; Pollington and Baxter, 2010, 2011;
Urbanek et al., 2002). An exception to this is the 40Ar /39Ar study
of Schneider et al. (2013) on syn-kinematic phengite and K-feldspar which
will be used in the following as a comparison
(Table 2). Fissure monazite ages
largely corroborate this work, similarly showing the longevity of different
shear zones in the TW. The ages confirm that even though most of the dated
monazite samples are only located in the damage zone in the vicinity of the
core of the shear zones, fluid-filled fissures provide a sensitive system
where tectonic activity triggers fluid-enhanced dissolution–precipitation
reactions at lower greenschist to sub-greenschist facies conditions.
While Schneider et al. (2013)
obtained crystallization age ranges of 33–15 Ma for the ASZ, 24–12 Ma
for the TSZ and 20–7 Ma for the GSZ (Table 2), our data confirm fluid
activity, and thus possible tectonic activity, at 18–12, 11.5–10 and
17–13 Ma, respectively (Fig. 7). However, the oldest dates from Schneider
et al. (2013) might also be interpreted as older grains that have been
aligned in the new foliation (Fig. 8). The data presented here indicate that
all of the shear zones where potentially active at least until
∼ 13–12 Ma, and the Tuxer and/or Olperer shear zones even
until ∼ 7 Ma, as suggested by younger dates observed in grain 2
(Figs. 3b, 6a and 7, Tables 3 and 4). However, the fissure monazite data
do not date the initiation of the GSZ (Selverstone et
al., 1991) nor the earliest activity of the TSZ (greenschist to amphibolite
facies; Selverstone et al., 1984,
1991) or the ASZ (greenschist facies; Cole et al.,
2007), since their formation already started at amphibolite facies
conditions. As Alpine fissures only form under greenschist facies
conditions, the oldest monazite crystallization ages are younger than the
data obtained by Schneider et al. (2013). This indicates that shear zone
activity started earlier than the fissure monazite record. As the monazite
age range of the younger fault activity is comparable to the data of
Schneider et al. (2013) but is not the same for individual shear zones, it
seems likely that all shear zones of the western TW were active as recently
as 8–7 Ma.
Comparison with fission track data
There is a wealth of zircon fission track (ZFT) data that can assist in
describing the exhumation and low-grade tectonic activity in the TW
(Bertrand,
2014; Bertrand et al., 2017; Dunkl et al., 2003; Fügenschuh et al.,
1997; Mancktelow et al., 2001; Most, 2003; Pomella et al., 2011; Steenken et
al., 2002; Stöckhert et al., 1999; Viola et al., 2001; Wölfler et
al., 2008) and apatite fission track (AFT) data
(Bertrand,
2014; Bertrand et al., 2017; Coyle, 1994; Di Fiore, 2013; Foeken et al.,
2007; Fügenschuh et al., 1997; Grundmann and Morteani, 1985; Hejl, 1997;
Mancktelow et al., 2001; Most, 2003; Pomella et al., 2011; Staufenberg,
1987; Steenken et al., 2002; Viola et al., 2001; Wölfler et al., 2008,
2012).
Three cross sections, DD' (perpendicular to the BNF), AA' (perpendicular to
the western limb of the western subdome) and EE' (parallel to the main
axial plane of the TW), are presented in Fig. 7, redrawn after Bertrand et
al. (2017) and Schmid et al. (2013). Zircon and apatite fission track data
compiled by Bertrand et al. (2017) are displayed in the lower part of Fig. 7b–d
and compared to fissure monazite ages. As described in Bertrand et al. (2017)
(first model), fission track data along the AA' cross section (Fig. 7c)
nicely display a dome-like shape, with younger ages recorded near the
subdome axial plane, where cooling was slower. By contrast, along the EE'
longitudinal cross section (Fig. 7d), ZFT and AFT are younger on the western
and eastern borders of the TW where the two major extensional faults, the BNF
and KNF, are respectively located. Perpendicular to the BNF (DD' cross
section, Fig. 7b), the fission tracks record cooling ages younging from the
footwall toward the plane of the normal fault (from 10 to 4 Ma for AFT;
second model of Bertrand et al., 2017). Along the EE' cross section, the youngest
monazite ages (15–10 Ma) lie between zircon and apatite fission track
data (grey and blue symbols), whereas the older ages (> 17 Ma) do
not follow the cooling trend and are equal to or older than the ZFT data.
This means that at least the fissure monazites recording older ages
crystallized somewhere above ZFT closure temperatures of ∼ 240–280 ∘C (Bernet,
2009; Bernet and Garver, 2005; Reiners, 2005; Yamada et al., 1995)
(Fig. 7d).
Monazite Th/U as monitor of oxidizing and reducing conditions
Extreme low and high Th/U ratios described in fissure monazite by Gnos et
al. (2015) (T1, T2 and T3 samples in Fig. 9) are also observed in some grains
from this study (red and blue labels in Fig. 9). Hydrothermal monazite
from the TW associated with hematite in fissure typically displays very high
Th/U ratios of around 1200 (Fig. 9, red labels; Table 1), whereas grains
obtained from graphite-bearing host rocks show very low Th/U ratios around 8
(Fig. 9, blue labels; Table 1). This attests for oxidizing and reducing
fluid conditions in the fissure environment, respectively.
The Th/U in monazite grains PFIT1 and MOKR1 would instead record a dynamic
oxidation environment due to variable fluid conditions. In PFIT1 monazite,
the Th/U decreases from core to rim, whereas within MOKR1 the opposite
evolution is observed (Fig. 9). Thus, in the first case, the fissure
environment evolves toward reducing conditions, whereas in the second case
there is an evolution towards more oxidizing conditions. Many of the other
grains indicate intermediate oxidizing conditions and they could not be
assigned to one of the two categories defined above, as the presence of
either hematite or graphite is uncertain (Fig. 9; grey labels).
Conclusions
Th-Pb ages of fissure monazite provide an extended record of exhumation of
the TW during the Miocene. The investigated monazites crystallized at
temperatures < 400 ∘C in the presence of hydrothermal
fluids that circulated in open fissures formed through tectonic movements.
The Th-Pb ages recorded by fissure monazites are in general agreement with
previously published geochronological data and range between 21.7 ± 0.4
and 10.0 ± 0.2 Ma. Spot dates suggests that monazite
crystallization in the metamorphic and structural TW dome occurred over a
period of ∼ 16 Myr. The combination of structural and
geochronological information allows relating monazite growth to tectonic
movements that affected the TW. The three major growth episodes identified
in this study, by dating monazite growth domains, are interpreted to be
associated with N–S shortening associated with the E–W extension (22–20 Ma),
contemporaneous N–S shortening and sinistral strike-slip movements (19–15 Ma)
and reactivation of strike-slip/normal faulting (14–10 Ma). Overall,
fissure monazite age recording indicates that in the TW Cenozoic faults show
increased activity at ∼ 21, ∼ 17 and
∼ 12 Ma, probably due to reorganization of plate movements
occurring at those times. Comparison of Th-Pb fissure monazite
crystallization ages with existing crystallization and cooling ages (e.g.
AFT, ZFT, white mica from fault zones) shows that the latest stages of
monazite crystallization occurred at temperatures between apatite and zircon
fission track “closure” temperatures. This enlarged dataset also supports
previous observations on fissure monazite chemistry displaying extremely
high Th/U ratios (∼ 1200) under oxidizing conditions in
association with hematite.
Data availability
The data used in this study are available in Tables 3 and 4.
The supplement related to this article is available online at: https://doi.org/10.5194/se-11-437-2020-supplement.
Author contributions
Fissure monazite samples were organized by EG and FW. Monazite samples for dating
were selected by ER, CAB, EG and AB according to tectonic settings and fault
activity of the study area. ER prepared the manuscript during her PhD
project under the supervision of EG, with contributions from all co-authors.
Sample preparation and BSE imaging were performed by ER and CAB. Data
acquisition and reduction at the SwissSIMS and NordSIMS facility was,
respectively, carried out by ER and CAB under the supervision of DR and MJW.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Sepp Brugger, Kurt Novak, Franz Gartner, Peter Hellweger, Adolf Meyer, Sebastian Planken-Steiner, Johann Rappold, Josef Rathgeb, Alexandre Salzmann, Maria Schaffhauser, Andreas Steiner, and Ermin Welzl for providing samples for this study. Urs Klötzli and Jan Pleuger are thanked for their helpful comments.
Financial support
This research has been supported by the Swiss National Science Foundation (grant no. 200020-165513). The NordSIMS facility received funding from the Swedish Research Council (infrastructure grant no. 2014-06375) and the Swedish Museum of Natural History; this is NordSIMS publication no. 636.
Review statement
This paper was edited by Bernhard Grasemann and reviewed by Jan Pleuger and Urs Klötzli.
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