150
formed by Variscan anatexis and/or metamorphic segregation. On minor hornblendites and coarse-grained amphibolites as well as
the contrary, Loth and Holl )1994) inferred two dierent magmatic various leucocratic gneisses. The basaltic protoliths of the ®ne-
rock types of pre-Variscan age and rejected the idea of Variscan grained amphibolites were intruded by pyroxenitic and gabbroic
anatexis based on petrographical and zircon morphological studies. melts as well as small patches of normal `I-type' granitoids and
Loth et al. )1997) further argued that the two rock types may have later by altered, dierentiated granitic melts )Eichhorn et al. 1999;
formed either coeval in a layered intrusion or ophiolite complex, or Holl and Eichhorn 2000). Slices of subducted fore-arc oceanic
noncoeval where the crustal dioritic precursor of the hornblende- crust, which have been accreted to the main volcanic arc )`sedi-
plagioclase gneiss has been intruded by gabbroic sills in the wake of mentary formation'), comprise an up to 700-m-thick sequence of
initial )back-arc?) rifting. Sample SG-1 has been chosen to resolve dark phyllites as well as minor micaschists and quartzites, locally
this age problem by comparing our radiometric data with existing intercalated with metavolcanics and ultrama®cs. The ultrama®cs
gabbro )now metamorphosed coarse-grained amphibolites and represent remnants of oceanic crust which dier geochemically
garnet amphibolites) formation ages.
from Stubach Group equivalents )Puhl et al. 1998). The ®nds of
The muscovite gneisses )sample SG-5) are characterized by a various well-preserved microfossils in the dark phyllites describe a
modal composition of about 70% quartz, 10% muscovite and 10% sedimentation period of Late Riphean to Early Vendian )i.e., 1.0±
albite )`white schists'). The high content of quartz is interpreted as 0.6 Ga; Reitz and Holl 1988; Reitz et al. 1989; Gilg et al. 1989).
resulting from the breakdown of alkali feldspar during metamor-
Hornblendites and coarse-grained amphibolites )HG-6) form
phism and deformation. Muscovite gneisses have been found only numerous, parallel to subparallel lenses, up to some meters thick.
near an occurrence of poophyroid gneisses )sample SG-8) in the Gradual transitions as well as distinct contacts between the two
Sturmmannseck, and have thus at ®rst been interpreted as tecton- cogenetic rock types have been described by Holl )1975). The
ized equivalents )Hammer 1937). Field geological, petrographical amphiboles comprising 75±98 vol% of these dark-green hornb-
and zircon morphological studies subsequently revealed that the lendites are mainly composed of brownish cores of Ti-rich tscher-
muscovite gneisses are alkaline granitoid intrusives of mainly makitic to Mg-hornblende within actinolitic hornblende. Accessory
mantle origin, whereas the porphyric texture of the latter with minerals are scheelite, biotite, plagioclase, carbonate, epidote-
many rock fragments and magmatic mineral relics, its wealth of group minerals, magnetite, hematite, ilmenite, Cr spinel, and some
xenocrysts and inherited zircon cores, and its nondeformed ap- sul®des )Holl and Eichhorn 2000). The coarse-grained amphibolites
pearance arms its presumably Variscan volcanic origin )Loth and contain less amphibole but up to 50 vol% plagioclase, together
Holl 1994). The high deformational degree of the muscovite with the same minor to trace minerals. We dated a scheelite-poor
gneisses may be explained by the fact that they occur near the coarse-grained amphibolite to serve as a crosscheck for the `banded
nappe sole thrust of a tectonic klippe of the Tux nappe )Fig. 2b). amphibolite' data of the Stubach Group rocks and as a reference
The two samples SG-5 and SG-8 were studied in order to get some for published ages of the hornblendites.
information of the later evolution of the convergent plate margin
and of the provenance age)s) of the source rocks, respectively.
Lensoid occurrences of meta-conglomerates within the `sedi-
mentary formation' reveal elongated, seldom well-rounded clasts
embedded within ®ne-grained biotite-plagioclase gneisses or pla-
Methods
gioclase-biotite schists. The clasts consist of amphibolites, various
The samples comprised 5±30 kg of fresh material. Mineral fractions
leucocratic schists and gneisses, marbles and quartzites. Sample SG-
for isotopic analyses were processed through conventional mineral
4 is a huge 40-cm leucocratic orthogneiss clast which derived from a
separation techniques, including a Wil¯ey table, heavy liquids and
1.4-m thick, unsorted meta-breccia of gneisses and amphibolites.
the Frantz isodynamic separator. Final mineral separates consisted
The deposition mechanism of the breccias and conglomerates is still
of handpicked, top-quality zircon grains, homogeneous in terms of
a matter of debate. Gilg et al. )1989) considered a deposition by
transparency, color and ¯uorescence.
debris and mud ¯ows triggered by local submarine earthquakes and/
Zircon grains from the sample and a zircon standard are
or a deposition by drifting icebergs. The former interpretation is
mounted and polished in a 24-mm diameter epoxy disc and gold
corroborated by the broad variety of clasts, the latter by the ®nds of
coated. The zircons are then photographed, examined by ca-
some exotic, large single clasts which were interpreted as dropstones
thodoluminescence )CL) imaging and analyzed using the SHRIMP
from melting icebergs, embedded in ®ne-grained laminated sedi-
II ion microprobe. During data reduction, correction for common
ments )now metamorphosed biotite-plagioclase gneisses). We dated
Pb in the zircon uses the methods of Compston et al. )1984). The
the orthogneiss clast to gain some information about the age of the
calculation of 206Pb/238U ages is based upon the assumption that
orthogneiss protolith and, in consequence, to get an upper estimate
the bias of the measured 206Pb+/238U+ ratio relative to the true
for the deposition time of the meta-breccia.
ratio can be described by the same power law relationship )Claoue-
Additionally, we dated a migmatitic leucosome )SG-3) from the
Long et al. 1995) between 206Pb+ 238U+ and UO+/U+ for both
the CZ3 standard zircon )Pidgeon 1997) and the unknown zircon
/
Serie der Alten Gneise to elucidate the time of migmatization.
Variscan nappe-stacking )Ri nappes; Fig. 2b) produced a heter-
ogeneous association of pre-Variscan Stubach Group rocks in form
of high-grade gneisses, migmatites and )eclogitic) amphibolites to-
gether with Variscan, 340-Ma-old `in-situ' anatexites. Eichhorn et al.
)1999) argued that the formation time of the anatexites may be
identical to the time of migmatization of the Stubach Group rocks.
This interpretation is further corroborated by the fact that the
Variscan amphibolite-facies metamorphism is regarded as compa-
rable to, or even more intense than Alpine lower amphibolite-facies
conditions )Eichhorn et al. 1995), and that the thermal peak of the
Variscan metamorphism occurred between 324 and 282 Ma )Holl
and Eichhorn 2000). On the other hand, ages between 415 and
422 Ma from )eclogitic) amphibolites have been interpreted to in-
dicate a high-pressure metamorphism which converted these gabb-
ros into eclogites during a Silurian subduction )Quadt et al. 1997).
analysis. The uncertainty of the 206Pb/238U age of the unknown
zircon includes the uncertainty of the 206Pb+ 238U+ ratio of the
/
standard. For this reason, approximately twice the number of
`standard' analyses are interspersed between `unknowns' analyses
in any analytical session. Reproducibility for the `standard' Pb/U
ratio ranged between 1.05 and 2.22% for the ®ve SHRIMP
sessions required to analyze the samples. The concentrations of U,
Th and Pb are calculated using a similar approach to that used for
the calculation of U/Pb ratios, with the unknown referenced to the
standard with known U, Th and Pb abundances )Compston et al.
1984; Claoue-Long et al. 1995; Williams et al. 1996).
The zircons were examined with a Zeiss DSM 960A scanning
electron microscope )SEM) at the Institut fur Allgemeine und
Angewandte Geologie, University of Munich, and with a JEOL
6400 at the Electron Microscopy Centre, University of Western
Australia, Perth, in cathodoluminescence )CL) mode on polished
and gold-or carbonc-oated grain mounts for internal structures.
The CL information was obtained prior to the SHRIMP analyses,
Habach Group
The volcanic arc rocks of the Habach Group, referred to as in order to select appropriate zircons for analysis, to place the ion
beam without straddling dierent zones within the same grain, and
`magmatic formation', consist mainly of ®ne-grained amphibolites,