GeolCarp_Vol58_No2_133

GEOLOGICA CARPATHICA, APRIL 2007, 58, 2, 133—144

www.geologicacarpathica.sk

Ar/

Ar dating of detrital white mica of Upper Paleozoic

sandstones in the Carnic Alps (Austria): implications for

provenance and tectonic setting

DIETER MADER*, FRANZ NEUBAUER and ROBERT HANDLER

Fachbereich Geographie und Geologie, Paris Lodron Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria;

franz.neubauer@sbg.ac.at

*Present address: Department für Lithosphärenforschung, Universität Wien, Althanstraße 14, 1090 Wien, Austria; dieter.mader@univie.ac.at

(Manuscript received April 28, 2006; accepted in revised form October 5, 2006)

Abstract: New information on the geodynamic development and the provenance for Carboniferous to Permian succes-
sions exposed within the Carnic Alps is supplied by an integrated approach of microprobe analysis and

Ar/

Ar dating

of detrital white mica.

Ar/

Ar analyses of detrital white mica concentrates (4—10 grains) from syn- and post-orogenic

Mississippian to Late Permian successions display an apparently uniform population and time lag of the isotopic ages
with respect to the stratigraphic age of their host rock: early Variscan ages (373—396 Ma) are reported within syn-
orogenic Visean-Namurian turbiditic sandstones, Variscan ages (333—309 Ma) in post-orogenic Pennsylvanian and
Permian terrestrial and shallow marine sandstones. Detrital white micas from Mississippian syn-orogenic sandstones
indicate an intermediate time interval between the cooling of the source rock and deposition, typical for compressional
accretionary wedge settings. Furthermore, these ages argue for a Middle Devonian tectonothermal event in the hinter-
land. Detrital white micas from Pennsylvanian sandstones indicate a very narrow time lag between post-Variscan cooling
of the source region and the depositional age. This points to a rapid exhumation of rocks in the source region from mid-
crustal levels prior to the deposition of these sediments.

Key words: Late Paleozoic, Southern Alps, Ar-Ar-dating, provenance, sandstones, detrital mica.

Introduction

The provenance and geodynamic development of sand-
stone successions can be classified by a variety of meth-
ods including petrographic analysis, whole rock and
mineral chemistry and K-Ar and

Ar/

Ar age dating (e.g.

Hodges et al. 2005 and references therein).

Detrital white mica within clastic sediments originates

from metamorphic or magmatic source rocks which were
formed in middle levels of the continental crust. The K-Ar
ages of these micas monitor the cooling of the crust of the
hinterland below temperatures of about 350—410 ºC, the
approximate closure temperature of the argon isotopic sys-
tem within white mica (Robbins 1972; Purdy & Jäger
1976; Hames & Bowring 1994; Kirschner et al. 1996), al-
though the system is complicated by other factors (Villa
1998; McDougall & Harrison 1999 and references there-
in). The argon isotopic system of detrital white mica has
been shown to be very resistant against mechanical and
chemical weathering and sedimentary transport and is,
therefore, very suitable for K-Ar and

Ar/

Ar dating (e.g.

Clauer 1981; Mitchell & Taka 1984).

Isotopic analyses may support stratigraphic and petro-

graphic analyses for the identification of source areas of
sediments and for the reconstruction of the paleogeo-
graphic and geodynamic evolution of sedimentary basins.
K-Ar and

Ar/

Ar isotopic analyses on detrital white

mica for provenance and paleogeography of sedimentary
basins include studies by Krylov & Silin (1959), Vistelius

(1959), Fitch et al. (1966), Kelley & Bluck (1989, 1992),
Dallmeyer (1987), Dallmeyer & Nance (1990), Renne et al.
(1990), Welzel (1991), Dallmeyer & Takasu (1992), Aron-
son & Lewis (1994), Handler et al. (1997), and Sherlock et
al. (2000). However, little attention has been paid to white
micas from sedimentary basins of specific geodynamic set-
tings. With the help of isotopic ages of detrital white mica,
the exhumation rates of the continental crust can be calcu-
lated and indicators for the geodynamic situation are thus
available (Copeland & Harrison 1990; Hodges et al.
2005). If the source rock reaches temperatures above the
argon retention temperature of white mica, the K-Ar ages
will become rejuvenated due to loss of radiogenic argon.
Thus, the crustal rejuvenation rate indicates the extent of a
young metamorphic overprint on continental crust (e.g.
Neubauer & Handler 1997). The time interval between
cooling of the source rock through the argon retention
temperature and the time of deposition is a supporting
method to classify various types of sedimentary basins.

The Carnic Alps have become a classical area for strati-

graphic investigations of Paleozoic formations, due to the
abundance of fossils, the structurally relatively undis-
turbed, although faulted, Upper Ordovician to Triassic se-
quences, and the very low-grade Variscan and Alpine
metamorphic overprints (Schönlaub 1985a). In almost all
lithostratigraphic sections of the Carnic Alps clastic sedi-
ments can be found at the base, and on the basis of ca.
640 Ma

Ar/

Ar ages of detrital white mica, a Cadomian

hinterland for Ordovician clastic sequences of the Carnic

134

MADER, NEUBAUER and HANDLER

Alps has been proposed (Dallmeyer & Neubauer 1994).
Detrital white micas from other Paleozoic siliciclastic se-
quences of the Carnic Alps were not dated up to now.

We applied

Ar/

Ar multigrain dating of detrital white

mica from Carboniferous-Permian sequences to obtain ra-
diometric ages and to evaluate current models for the geo-
dynamic evolution of the Carnic Alps in conjunction with
petrographic and geochemical data (Mader & Neubauer
2004). Contrasting

Ar/

Ar ages of detrital white mica

may indicate different provenances of detrital white mica
within different types of Carboniferous-Permian sedimen-
tary basins, such as a Mississippian syn-orogenic flysch
trough and post-orogenic Pennsylvanian molasse basin of
the Paleozoic of the Carnic Alps, as well as from the
South-Alpine Permian passive continental margin. A high
proportion of rejuvenated mica from continental hinter-
land crust, which was formed during the Variscan orogeny,
may be detected by investigations of detrital white mica
on Carboniferous syn-orogenic flysch and post-orogenic
molasse sediments.

In addition, more information about the geodynamic de-

velopment of the Paleozoic of the Carnic Alps should be
obtained by

Ar/

Ar dating of detrital white mica, be-

cause it would supply valuable information in conjunc-
tion with petrographic and geochemical data (Mader &
Neubauer 2004).

Geological setting

The study area is located in the central Carnic Alps

(Carinthia, Austria) on the Italian border (Figs. 1, 2). The
Carnic Alps comprises the sedimentary basement of the
Southern Alps, exposed in their north-eastern part, adja-
cent to and separated from the Austroalpine units by the
Periadriatic fault (Fig. 1). Reviews on early research of
this area can be found, for example in Schönlaub (1985a),
Tollmann (1985) and Schönlaub & Histon (2000).

Together with the Graywacke Zone, and the southern

Karawanken Alps, which both show some lithologic simi-
larities and a similar general trend of sedimentary evolu-
tion to the Carnic Alps, and the Gurktal and Graz
Paleozoic successions they are considered to be the south-
ern external zone of the European Variscides, similar to
the Rhenohercynic and Saxothuringic Zones, which rep-
resent the northern external zone (e.g. Schönlaub & Hein-
isch 1993).

The Carnic Alps are characterized by a continuous fos-

siliferous non- to low-grade metamorphosed sedimentary
sequence reaching from the late Middle Ordovician to the
early Pennsylvanian (with the Westphalian climax of the
Variscan orogeny). They underwent deformation during
both the Variscan and the Alpine orogeny. Evidence for
Alpine tectonometamorphic activity is indicated by Cre-

Fig. 1. Schematic geological map of Eastern and Southern Alps.

136

MADER, NEUBAUER and HANDLER

deposits with intercalations of olistoliths indicate the on-
set of the Variscan orogeny. From late Namurian to early
Westphalian (Bashkirian to Moscovian) the Carnic Alps
were uplifted and eroded for a period not longer than ca.
5 Myr (Flügel 1975; Spaletta et al. 1980).

Above a well-defined angular unconformity (Fenninger

et al. 1976), Pennsylvanian molasse deposits (Auernig
Group) occur. They grade into Lower Permian marine car-
bonates and Middle Permian siliciclastic red-beds and
overlying Upper Permian shallow-marine carbonates of
the South-Alpine Permian to Triassic succession (e.g.
Krainer 1993 and references therein).

Samples were taken from Carboniferous to Permian

sandstone-bearing formations in the central Carnic Alps
(Fig. 2). On the basis of currently available geodynamic
evidence (e.g. Schönlaub & Heinisch 1993; Neubauer &
Sassi 1993; Krainer 1993; Schönlaub & Histon 2000;
Läufer et al. 2001), previous (Dallmeyer & Neubauer
1994; Neubauer et al. 2001) and new samples, these were
grouped into (1) Ordovician to Devonian extensional
geodynamic environments (Himmelberg Sandstone, Bi-
schofalm Quartzite, Zollner Formation), (2) Visean to Na-
murian (Mississippian to lowermost Pennsylvanian)
contractional environments, as represented by syn-oro-
genic flysch formations (Hochwipfel Formation), (3)
Pennsylvanian molasse (Waidegg/Malinifer Formation,
Auernig Group), and (4) Permian extensional environ-
ments due to ongoing Alpine rifting (Grenzland Forma-
tion, Gröden Formation).

Analytical methods

White mica concentrates were prepared from sieve frac-

tion 0.125—0.200 mm by flotation in water columns. Con-
centrates were cleaned in an ultrasonic bath with acetone,
alcohol, and distilled water for 10 minutes each. Further
purification was done by subsequent dry-sieving and sep-
aration by a Frantz isodynamic magnetic separator. Final-
ly, mica grains were hand-selected under a binocular
microscope.

The mineral concentrates for the

Ar/

Ar dating were

packed into high purity Al-sample holders and irradiated
in the central position of the ASTRA reactor at the Austri-
an Research Centre in Seibersdorf, Austria for 1 to 3 hours.
The flux within the reactor is 1.1 1014 n/cm

s. Correc-

tion factors for production of interfering isotopes have
been reported by Frank et al. (1996) and are:

Ar/

Ar(Ca)=0.0003,

Ar/

Ar(Ca)=0.00065, and

Ar/

Ar(K)=0.03. Variations in the flux of neutrons were

monitored with B4M white mica standard (Flisch 1982)
for which a

Ar/

Ar plateau age of 18.6±0.4 Ma has been

reported (Burghele 1987).

Ar/

Ar analyses were carried

out in the ARGONAUT Laboratory at the Institute of Ge-
ology and Paleontology at the University of Salzburg us-
ing a UHV Ar-extraction line equipped with a combined
MERCHANTEK

UV/IR laser ablation facility and a VG-

ISOTECH

NG3600 Mass Spectrometer following proce-

dures described in Handler et al. (2004). Stepwise heating

analyses of samples are performed using a 25 W CO

-IR

laser operating in Tem

mode at wavelengths between

10.57 and 10.63 µm. The laser is defocused to a spot size
of ca. 1.0 mm. The laser is controlled from a PC, and the
position of the laser beam on the sample is monitored
through a double-vacuum window on the sample chamber
via a video camera in the optical axis of the laser beam on
a computer screen. Gas clean-up is performed using one
hot and one cold Zr-Al SAES getter. Measurement is per-
formed on an axial electron multiplier in static mode,
peak-jumping of the magnet is controlled by a Hall-probe.
For each gas increment the intensities of

Ar,

Ar and

Ar are measured, the baseline readings on mass

36.5 are subtracted. Intensities of the peaks are extrapolat-
ed over 16 measured intensities to the time of gas admit-
tance into the mass spectrometer either by a straight line
or a curved fit. Intensities are corrected for system blanks,
background, post-irradiation decay of

Ar, and interfering

isotopes. Ages and errors are calculated following sugges-
tions by McDougall & Harrison (1999) and decay factors
reported by Steiger & Jäger (1977).

Results

Composition of detrital mica

The results of electron microprobe analysis (reported in

Mader & Neubauer 2004) were plotted in ternary diagrams
(Fig. 4) with the compositional mol-percentage of musco-
vite, paragonite and aluminoceladonite (phengite of older
literature) according to the recent nomenclature proposed
by Rieder et al. (1998). We measured data both from thin
sections and from magnetically separated concentrates. In-
terestingly, the magnetically separated grains show in
general a lower percentage of aluminoceladonitic/phen-
gitic micas. The reason is unknown.

The chemical composition of mica may reflect the na-

ture of magmatic and metamorphic source rock (Speer
1984; Spear 1993). According to this the provenance from
granites and regionally metamorphosed rocks may be in-
dicated by mica compositions close to the muscovite cor-
ner. The more phengitic populations point to
high-pressure metamorphic source rocks from deeper oro-
genic levels exposed by exhumation; high paragonite
compositions (according to high sodium values) are inter-
preted as derivatives from low- to medium-grade metamor-
phic source rocks (Guidotti 1984 and references therein).

Late Ordovician sandstones display moderately phen-

gitic micas in a thin section and more muscovite-rich mi-
cas in separate. Together, these data show some diversity
of mica populations from this time slice.

The Visean-Namurian syn-orogenic samples may indi-

cate two different sources (Fig. 4). Micas from specimen
DM-9 plots near the muscovite-paragonite axis, whereas
sample DM-14 demonstrates a more scattered pattern with
a main spread along the muscovite-paragonite axis
(Fig. 4). It should be noted, however, that, because of their
scarcity, micas from the latter sample were not separated

137

Ar/

Ar DATING OF DETRITAL WHITE MICA OF UPPER PALEOZOIC SANDSTONES (AUSTRIA)

Fig. 4. Ternary plots showing the chemical variation of detrital white mica de-
termined by electron microprobe. We apply the nomenclature proposed by
Rieder et al. (1998). Results of magnetically separated micas of the grain size
fraction 125—200 µm are shown. They illustrate the chemical variability of
white mica within a sample. Because all micas analysed are primarily musco-
vite, the paragonite and aluminoceladonite corners were plotted at 50 %, where-
by the latter indicate the composition of phengite.

The micas of the Early Permian sample

(DM-22) show the highest paragonitic compo-
sition, whereas the phengitic component is
generally not more than 20 % (Fig. 4). Low
chemical diversity and a composition closely
clustered to the muscovite corner are displayed
by the Late Permian concentrate (DM-2).

Ar/

Ar data

The small grain size did not allow good sin-

gle grain analyses. So, 4 to 10 grains per sam-
ple were selected to get a first order
approximation of age populations. Fortunate-
ly, this approach yielded reasonably good re-
sults and revealed a first order distinction of
age populations from time level to time level.
Analytical results are listed in Table 1 and are
portrayed as age spectra in Fig. 5. The small
increments throughout the release spectra with
highly variable ages are not interpreted as re-
flections of disturbed argon isotopic systems
of the white micas analysed. However, possi-
ble mixtures of muscovite, aluminocelado-
nite/phengite and paragonite in the
concentrate analysed (4—10 white micas) can-
not be excluded and may be reflected by the
disturbed age spectra. Muscovite has a lower
closure temperature compared to aluminocela-
donite/phengite and paragonite (e.g. von
Blanckenburg et al. 1989; McDougall & Har-
rison 1999), which will result in unstable age
spectra. Furthermore, weak, very low-grade
Carboniferous and Cretaceous/Oligocene
metamorphic overprints have been reported
from Carboniferous rocks of the working area
adjacent to the Periadriatic fault (Läufer et al.
1997, 2001). A tectonometamorphic event,
with K-Ar ages of illite of 300—320 Ma, is re-
ported for the Hochwipfel Nappe in Läufer et
al. (2001), which may also have slightly af-
fected the K/Ar isotopic system. This resulted
in some minor Ar loss in low-temperature in-
crements of some samples (see below).

Sample DM-2 (Gröden Formation; Late Per-

mian) displays a discordant age pattern with
an apparent age of 319.7±3.4 Ma for the larg-
est increment (53.5 % of total

Ar released)

which is similar within the error to the total
gas age of 319.2±4.7 Ma. We suggest there-
fore, that this age is geologically significant,

with the highest magnetic susceptibility compared to the
other samples. This may be the reason for their scattered
plotting.

The multi-grain concentrate of the post-orogenic Penn-

sylvanian sample (DM-4) generally shows a narrower
cluster of lower aluminoceladonitic (phengitic) but signif-
icantly higher paragonitic contents (Fig. 4). Two distinct
mica populations may indicate two different sources.

and may be interpreted as the average age of cooling
through the appropriate argon retention temperature after
peak conditions of metamorphic/plutonic temperature
conditions at ca. the Mississippian to Pennsylvanian
boundary.

Both Early Permian samples display rather concordant

patterns. Sample DM-20 (Grenzland Formation) yielded
an apparent age of 334.4±2.6 Ma for the largest increment

138

MADER, NEUBAUER and HANDLER

(43.1 % of total

Ar released) and total gas age of

332.9±2.4 Ma. In sample DM-22 (Grenzland Formation)
an apparent age of 316.5±2.8 Ma is indicated by the larg-
est increment (39.2 % of total

Ar released, a total gas age

320.3±3.2 Ma and an integrated age of 323.5±2.9 Ma
(Fig. 5).

Similar results are indicated by the Pennsylvanian sam-

ples of the Auernig Group and Waidegg Formation with to-
tal gas ages ranging between 309.2±2.8 to 328.9±3.6 Ma
(Table 1). The largest increment of sample DM-15 (64.8 %
of total

Ar released) and sample DM-5 (68.8 % of total

Ar released) yielded apparent ages of 324.0±3.0 Ma and

322.9±3.0 Ma respectively. Sample DM-21e yielded an ap-
parent age of 320.9±2.8 Ma. An apparent age of
309.8±3.0 Ma is indicated by three internally concordant
steps, together comprising 69.2 % of total

Ar released,

which is interpreted as being geologically significant. The
first and last steps show an older age component 396.7±6.2

and 381.9±10 Ma which suggests the presence of an older
age group and, therefore, an inhomogeneous age composi-
tion of the sample. A component with a similar age of
404±12 Ma is indicated by step 7 of sample DM-4. The
largest increment of sample DM-11 from the Pennsylvanian
Waidegg Formation yielded an apparent age of
318.6±2.9 Ma (40.0 % of total

Ar released) which is again

relatively close to the total gas age of 325.3±3.2 Ma.

Detrital white micas of the Mississippian Hochwipfel

Formation yield total gas ages between 372.1±3.3 and
375.0±3.3 Ma (Table 1). Sample DM-9 displays a staircase
increase of ages from minimum 219.4±22.5 Ma to the larg-
est increment with an apparent age of 396.3±3.5 Ma
(66.6 % of total

Ar released). Similarly, samples 13 and

17 show a lower age in the first increments with ages of
320±11 and 222±36 Ma, respectively. An apparent age of
379.5±8.2 Ma is recorded by the increment 2—5 with
92.3 %

Ar released in sample DM-17. Detrital white mi-

Fig. 5. Apparent

Ar/

Ar age spectra of detrital white mica. Experimental laser energy output increases from left to right. Width of the

bar corresponds to the analytical 2 error.

141

Ar/

Ar DATING OF DETRITAL WHITE MICA OF UPPER PALEOZOIC SANDSTONES (AUSTRIA)

Together, the ages show clearly two age

groups although the ages are variable within
these groups: (1) an age of 373.4±3.8 to
396.3±3.5 Ma in the Hochwipfel Formation
(Visean to Namurian), and (2) an age group
ranging from 309.8±3.0 to 331.9±3.3 Ma for
post-Variscan Waidegg to Gröden Formations
(Pennsylvanian to early Late Permian).

Discussion

Our new

Ar/

Ar data from Upper Paleozoic

sandstones of the Carnic Alps have significance
both for the Paleozoic evolution of the Carnic
Alps and the basement evolution of the Alps as
a whole. We discuss first the chemical variation
of investigated micas and, then age data with
the main goal of putting all data in a sequence
of geodynamic models to explain our new data
in the context of the available literature. These
models are shown in Fig. 6.

The low chemical diversity of the micas with-

in individual samples indicates rather homoge-
neous source areas for each lithostratigraphic
unit except the Carboniferous micas display a
more diverse chemical composition, both with-
in each sample and between the individual
samples (Mader & Neubauer 2004). Hence, dis-
tinct source regions can be assumed for these
samples. The relatively homogeneous chemical
compositions indicate a rather limited possibil-
ity for mixture of different source rocks within
each sample. This is an important fact regard-
ing the multi-grain radiometric age determina-
tion of detrital white mica.

A trend from fairly aluminoceladonitic/phen-

gitic composition (Carboniferous) to muscovit-
ic-paragonitic micas (Permian) is indicated.
The composition of the Visean—Namurian
Hochwipfel Formation is explained as repre-
senting magmatic and high-grade metamorphic
sources of the just uplifted, exhumed and de-
nuded Variscan orogen. The rather large spread
of these data may be interpreted as an indica-
tion of diversity in source regions. In the Penn-
sylvanian, post-orogenic sediment material was
likely derived from high-pressure dominated
metamorphic rocks as seen in the high alumi-
noceladonitic/phengitic components, but with-
out significant magmatic supply.

Undisturbed integrated ages and ages repre-

sented by the large gas volumes are interpreted
as cooling ages of the detrital white micas in
the respective source areas. The interval be-
tween apparent cooling age of white mica (closure temper-
ature ca. 350—410 ºC, e.g. von Blanckenburg et al. 1989)
and depositional biostratigraphic age of post-orogenic
rocks can be used as a measure for the exhumation rate in

Fig. 6. Late Ordovician to Late Permian tectonic evolution of the Carnic Alps.

the source region (Copeland & Harrison 1990; Hodges et
al. 2005 and references therein). This represents the exhu-
mation of the continental crust from the depth of closing
temperature, which is assumed as middle-upper crust (ca.

142

MADER, NEUBAUER and HANDLER

10—15 km depth, at a normal geothermal gradient), to the
erodable surface (e.g. Neubauer & Handler 1997).

Previously reported

Ar/

Ar white mica and U-Pb zir-

con ages argue for a Cadomian source of clastic material
in the Late Ordovician (Dallmeyer & Neubauer 1994;
Neubauer et al. 2001), tentatively back-arc extensional
setting of the Carnic Alps (Fig. 6).

The white micas from Visean—Namurian samples may

have undergone an early Variscan tectonothermal event.
In general, only a very weak tectonothermal overprint dur-
ing Late Paleozoic Variscan orogenic events is recognized
in the Carnic Alps. Low-temperature increments of micas
from the Hochwipfel Formation display apparent ages of
286±11 Ma, 320±11 Ma, and 222±36 Ma, which can be
interpreted as reflecting minor rejuvenation during Alpine
orogenic events. The similar Middle Devonian ages for
Visean—Namurian samples may be attributed to a fairly
homogeneous source region, which is also indicated by
geochemical analysis (Mader & Neubauer 2004). The De-
vonian metamorphosed Austroalpine crystalline complex
was postulated as a possible source for these flysch depos-
its (Neubauer 1988) (Fig. 6). Similar Middle Devonian

Ar/

Ar ages have been recently reported from two Aus-

troalpine units, the Kaintaleck Metamorphic Complex of
the Eastern Graywacke Zone (Handler et al. 1999) and the
Wechsel Gneiss Complex (Müller et al. 1999). Similar De-
vonian ages by Pb-Pb zircon evaporation were reported
from the Hochwipfel Formation of the South Karawanken
(Sonntag et al. 1997) exposed towards the east of the Car-
nic Alps, which supports the significance of this age
group. Their Pb-Pb ages are explained as records of the
magmatic rocks in the source region. Interestingly,
Vozárová et al. (2005) recently reported similar early
Variscan multi-grain

Ar/

Ar ages of white mica from

Westphalian sandstones and conglomerates from the
Rudňany Formation of Western Carpathians.

The post-Variscan detrital micas of the Auernig sand-

stones record relatively rapid exhumation according to ap-
parent cooling ages between ca. 309.8±3.0 Ma and
331.9±3.3 Ma (Fig. 5). Hardly any record is present of old-
er sources. This suggests that nearly the entire upper crust,
above the Ar retention temperature of ca. 350—410 ºC was
removed prior to deposition of post-Variscan molasse.
This is entirely different from the Alpine molasses where
only a low percentage of young micas is recorded (e.g.
von Eynatten et al. 1999) and also dissimilar to the Hima-
laya orogen with a fairly low but still recognizable per-
centage of old micas (e.g. Najman et al. 2001). Also for
similarly aged clastic successions, Vozárová et al. (2005)
recently reported the same Variscan multi-grain

Ar/

ages of white mica from the Western Carpathians.

The relationships between cooling and depositional

ages suggest that the exhumation of these rocks occurred
in about 2—25 Myr or max. 39 Myr (using time intervals of
311 to 290 Ma, after Gradstein et al. 2004, for the deposi-
tional range of the post-Variscan sediments). The apparent
ages of the post-Variscan overstep sequences are interpret-
ed as post-metamorphic cooling ages of the deeply buried
source rocks recording approximately the minimum age of

the Variscan orogenic phase. The uncertainty of stratigra-
phy of the Auernig Formation does not allow a more de-
tailed correlation between cooling in the source region
and sediment deposition. Detrital white micas from the
Permian Gröden- and Grenzland Formations demonstrate
ages similar to the earliest post-orogenic sandstones of the
Waidegg Formation and Auernig Group. This suggests
that erosion affected similar tectonic horizons as during
Pennsylvanian times (Fig. 6).

It should be noted that nearly no Alpine tectonothermal

overprint is reflected in the apparent ages of both the syn-
and post-orogenic Upper Paleozoic samples. This indi-
cates weak Alpine metamorphic conditions in the realm of
the Carnic Alps, not exceeding 250—300 ºC in accordance
with previous observations (Rantitsch 1997; Läufer et al.
1997, 2001).

According to Neubauer & Handler (1997), variations in

residence time intervals between the closure temperature
and depositional age may bear on the geodynamic setting
of sedimentary basins. The new radiometric data suggest
the following sequence of events, using time calibration
data after Gradstein et al. (2004): The white micas of the
Middle Permian Gröden Formation display a lag time/resi-
dence interval of about 60—70 Myr, reflecting an exten-
sional setting with deeply eroded source regions.
Post-orogenic Pennsylvanian sandstones show very low
residence intervals (ca. 2—39 Myr), reflecting the geody-
namic setting of a peripheral molasse/intramontane basin
adjacent to a rapidly rising orogen. A residence time inter-
val of ca. 30—70 Myr is demonstrated by the detrital white
micas of the Hochwipfel Formation. Since no rejuvenated
mica occur because of insufficient Variscan tectonother-
mal overprint a higher residence time interval is shown
than expected for a syn-collisional flysch basin, which
should contain material both from older basement and
from younger sedimentary cover sequences of the overrid-
den plate.

Conclusions

Microprobe data of detrital white mica not only supply

information regarding their variable chemical composi-
tion, important for the interpretation of their distinct clos-
ing temperatures, but can also supply arguments
concerning the provenance of the detrital micas.

Ar/

data of detrital white mica from Visean—Namurian sand-
stones (Hochwipfel Formation) of the Carnic Alps record
early Variscan ages (373.4±3.8 to 396.3±3.5 Ma) in the
white mica demonstrating an undetectable to low thermal
effect of both the Variscan and the Alpine orogenies onto
these sequences. The post-Variscan sandstones record
Variscan cooling ages from (309.8±3.0 to 331.9±3.3 Ma),
likewise indicating no Alpine thermal overprint. Conse-
quently, the older source regions seem to have been nearly
entirely eliminated during Variscan tectonic processes.

Acknowledgments:

We acknowledge detailed reviews by

Peter Árkai (Budapest) and Fritz Ebner (Leoben), which

143

Ar/

Ar DATING OF DETRITAL WHITE MICA OF UPPER PALEOZOIC SANDSTONES (AUSTRIA)

helped to clarify presentation. We also acknowledge gen-
erous support by the Austrian Research Foundation FWF
(Grant No. P10506-GEO) to FN. We thank Wolfgang
Frisch (Tübingen), Konrad Hammerschmidt (Berlin),
Christoph Heubeck (Berlin), and Igor Villa (Bern) for re-
marks on an initial version of the manuscript.

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