Emplacement depths and radiometric ages of Paleozoic plutons of ...

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ranging from 890 to 940 jC for Opx–Prg–Spl
assemblages (Vsˇeruby and Neukirchen olivine gabbro;
Fig. 6D) and 810 to 920 jC for Opx–Cpx–Spl
coronas (Hoher–Bogen olivine metagabbro; Fig. 6F).
For these rocks, a pressure estimate is possible based
on the reaction: Ol + Pl F H2O ! Opx + Am or Cpx +
Spl. Such coronitic microfabrics are typical features in
olivine gabbros that underwent magmatic cooling or
high-grade metamorphism at P =6.5–8 kbar (e.g.,
Grantham et al., 1993; Jan and Karim, 1995). According
to Bucher and Frey (1994), such microfabrics are
diagnostic for pressures >6 kbar. In Fig. 6D and F, the
spinel-in reaction in the system CaO–MgO–Al2O3–
SiO2 is illustrated according to Gasparik (1984). In
natural systems containing iron (XMg = ca. 0.9), spinel
may occur at some lower pressures (Bucher and Frey,
1994).
6. Radiometric dating
For carrying out K–Ar age dating, biotite and
hornblende have been separated from samples of the
Smrzˇovice and Teufelsberg intrusions. Light brown to
greenish magnesio-hornblende sampled from finegrained
hornblende–biotite diorite of the large Smrzˇovice
quarry (sample SK 4, Location 2 in Fig. 1) shows
weak alteration along the cleavage planes. Magnetic
and unmagnetic fractions yield 547 F 7 and 549 F 7
Ma, respectively (Table 2). Dark brown to reddish
brown biotite separated from biotite–hornblende
quartz diorite of the Smrzˇovice quarry (sample 113)
shows few inclusions of plagioclase, ilmenite
F hornblende. The degree of biotite alteration is
very weak. In a few places, biotite is moderately
altered to clinozoisite (along the cleavage planes)
and/or chlorite at the margins. K–Ar dating of this
biotite yields 495 F 6 Ma (Table 2).
K–Ar dating of biotite of the Teufelsberg pyroxene
diorite (Location 7 in Fig. 1) yields 342 F 4 Ma.
Similar to the Smrzˇovice samples, the Teufelsberg
biotite is relatively fresh. Rare alteration in form of
weak chloritization is restricted to the marginal parts.
The separated zircons of the Teufelsberg diorite
(Location 7 in Fig. 1) are colourless and yellow
belonging to the subtypes S20 to S25 and J1 to J5
according to the classification of Pupin (1980). The
internal structure of the zircons is perceptible in
C. Bues et al. / Tectonophysics 352 (2002) 225–243
cathodoluminescence (CL) images oriented parallel
to the zircon c-axis. The different light–dark contrasts
and bright color display different amounts of trace
element content. The bright color represents more or
less pure zircon phase, whereas the different dark
colors reflect variable amount of U and Y contents.
The internal structure of all zircons is complex (e.g.,
Fig. 7). A rounded heterogeneous core is surrounded
by a thin discordant bright zone (A in Fig. 7), the
latter suggesting a first stage of dissolution. A narrowspaced
oscillatory zoning follows combined with
compositional sector zoning between dark prisms
and light–dark 101-pyramides until a second stage
of dissolution is indicated by a thin discordant bright
zone (B in Fig. 7). Towards the margin, a similar
combination follows including small light–dark 211pyramids
ceasing with a third stage of dissolution
indicated by a thick discordant bright zone (C in Fig.
7). The margin of the zircon consists of dark prisms
and pyramids.
The single zircons have a weight of 37–41 Ag and
thus, a total lead from 513 to 998 pg, resulting in high
beam intensities and allowing static-mode measurements
in all cases. The concordant single zircon 1 and
four discordant single zircons 2 to 5 define a discordia
with an upper intercept at 361 F 4 Ma and a lower
intercept at 16 F 12 Ma (Fig. 8F, Table 3). These data,
together with the age of a concordant zircon at
359 F 2 Ma, are interpreted to reflect the age of the
zircon crystallization and thus, the magmatic emplacement
age of the Teufelsberg diorite. Although the CL
images show a complex core pattern in the zircons, no
inherited lead could be detected (Table 3).
7. Discussion and conclusions
The Vsˇepadly granodiorite and the Smrzˇovice
diorite of the NE part of the NKM intruded at shallow
crustal levels (< 7 km depth) as is indicated by Al-in-
Hbl barometry. Similar emplacement depths have
been determined for the Těsovice granite of the
adjacent Stod pluton (Zulauf et al., 1997). A supracrustal
emplacement level is also indicated by the
small differences between U–Pb zircon ages (that
reflect the time of pluton emplacement) and K–Ar
and Ar–Ar ages of hornblende and mica (that reflect
cooling of the pluton through particular isotherms).