GEOLOGICA CARPATHICA, 48, 6, BRATISLAVA, DECEMBER 1997
347–352
MIDDLE-LATE TRIASSIC
40
Ar/
39
Ar HORNBLENDE AGES
FOR EARLY INTRUSIONS WITHIN THE DITRAU ALKALINE
MASSIF, RUMANIA: IMPLICATIONS FOR ALPINE RIFTING
IN THE CARPATHIAN OROGEN
DAVID R. DALLMEYER
1
,
HANS-GEORG KRÄUTNER
2
and FRANZ NEUBAUER
3
1
Department of Geology, University of Georgia, Athens, GA 30602, U.S.A.
2
Institut für Allgemeine und Angewandte Geologie, Maximilians-Universität, Luisenstrasse 37, D-80333 München, Germany
3
Institut für Geologie und Paläontologie, Paris-Lodron-Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
(Manuscript received March 10, 1997; accepted June 24, 1997)
Abstract:
Multigrain hornblende concentrates from two samples of massive gabbro and diorite collected within
„early“ intrusive phases of the Ditrau Alkaline Complex (Rumania) record well-defined
36
Ar/
40
Ar vs.
39
Ar/
40
Ar pla-
teau isotope correlation ages of 231.5 ± 0.1 Ma and 227.1 ± 0.1 Ma; 2
σ
intralaboratory error). These are interpreted as
dating relatively rapid post-magmatic cooling at high crustal levels following pluton emplacement in the Middle-Late
Triassic. The magmatic activity predated Early Jurassic rifting in the Eastern Carpathian orogen.
Key words:
Eastern Carpathians, Triassic, alkaline magmatism, rifting.
Introduction
The Alpine-Carpathian orogen displays only local and limit-
ed evidence for magmatic activity during tectonic rifting.
Rift-related mantle-plume activity is apparently entirely lack-
ing throughout the Alpine-Carpathian orogen. Most recent
models have postulated two distinct phases of Alpine rifting
which led to the development of two different oceanic tracts
between Laurasia and Gondwana during Mesozoic times
(Dal Piaz et al. 1995; Neubauer 1994; Sandulescu 1994;
Stampfli et al. 1991; Trümpy 1988). Permian to Middle Trias-
sic rifting resulted in formation of the Middle Triassic Melia-
ta-Vardar and North Dobrogea extended rift and ocean do-
mains. A second rift resulted in the separation of the
continental Austroalpine/Southalpine domains from extra-Al-
pine Europe. Evidence for the latter rift and subsequent pas-
sive continental margin formation is widely distributed
throughout the Austroalpine, Southalpine, and Dinaric do-
mains although magmatic activity appears to have been mi-
nor during the rift phase.
Middle Triassic magmatic activity in the Southalpine-Di-
naric units included (Fig. 1): (1) Triassic pegmatite intrusions
in westernmost sectors of the Southalpine unit (Hanson et al.
1966) due to crustal extension (e.g. Bertotti et al. 1993); (2)
Middle Triassic intrusion of Predazzo-Monzoni suites in the
central Southern Alps; and (3) Middle Triassic volcanic activ-
ity in the South Tyrolian Dolomites. The variably altered,
green tuffs associated with this partly subaerial volcanism
(„pietra verde“) are widely distributed in Southalpine and
Austroalpine units, and the Dinarides. The latter has been in-
terpreted to have related to subduction of oceanic crust rather
than to rifting processes (e.g. Bebien et al. 1978; Pe-Piper 1982).
This contribution presents geochronological evidence for
a Middle-Late Triassic age of early gabbro and diorite intru-
sions within the Ditrau Alkaline Massif that occurred close to
the opposite margins of the Alpine-Carpathian belt. These
data suggest that intrusion of the Ditrau Alkaline Massif
was associated with mantle-plume activity which predated
Jurassic rifting within the Eastern Carpathian orogen.
Geological setting
The Ditrau Alkaline Massif is located within westernmost
exposure of basement within the Eastern Carpathians or Ru-
mania (Fig. 1). Detailed description of the intrusion complex
and general geological relationships may be found in Ianovi-
ci (1938), Streckeisen (1952, 1954, 1960), Codarcea et al.
(1958), Streckeisen & Hunziker (1974), Anastasiu & Con-
stantinescu (1984), Sandulescu (1984), Anastasiu et al.
(1994) and Kräutner & Bindea (1995). The Ditrau Massif is
considered to represent an intrusion body with a internal zon-
al structure, which was emplaced into pre-Alpine metamor-
phic basement complexes of the Bucovinian Nappe Com-
plex. The center of the Ditrau Massif was formed by
nepheline syenite, which is surrounded by syenite and
monzonite. Northwestern and norteastern marginal sectors
are composed of hornblende gabbro/hornblendite, diorite,
monzonite and alkali granite. Hornblende gabbro/hornblen-
dite and diorite represent the earliest intrusive phase, and are
embedded within younger syenite and granite. All these rocks
are cut by late-stage dikes with a large variety of composi-
tions including tinguaite, microsyenite, and aplite, and later
lamprophyre (Streckeisen 1952, 1954; Codarcea et al. 1958;
Streckeisen & Hunziker 1974; Anastasiu & Constantinescu
1984; Anastasiu et al. 1994). Lithologies and variations of
petrographic compositions suggest that the Ditrau pluton rep-
resents an alkaline massif with mantle-plume related origin.
348 DALLMEYER, KRÄUTNER and NEUBAUER
A contact aureole is well-developed mainly within the oth-
erwise low grade, host metamorphic Tulghes Group metased-
iments and metavolcanic rocks. The contact aureole displays
statically grown andalusite, biotite, and cordierite (Streckeisen
1952; Streckeisen & Hunziker 1974). Neogene volcanic and
sedimentary rocks discordantly overlie the complex.
Previous interpretations of the age of intrusion are exclu-
sively based on K-Ar geochronology. K-Ar biotite ages rang-
ing from Late Triassic to Cretaceous have been reported from
the intrusive complex (218–103 Ma: Bagdasarian 1972;
Streckeisen & Hunziker 1974; Minzauti et al. 1981, unpubl.
report: data listed in Kräutner & Bindea 1995; Molnár &
Avra-Sós 1995). Recently, Molnár & Avra-Sós (1995) report-
ed K-Ar amphibole ages ranging from 237 to 177 Ma, and K-
Ar feldspar ages between 255 and 113. The Ditrau Alkaline
Massif was recently interpreted as representing an Early Ju-
rassic incipient intrusion within the Bucovinian intra-conti-
nental rift zone of the Eastern Carpathians (Kräutner & Bin-
dea 1995).
Analytical methods
The techniques used during
40
Ar/
39
Ar analysis of the Di-
trau hornblende concentrates generally followed those de-
scribed by Dallmeyer & Gil-Ibarguchi (1990). Optically pure
(>99 %) amphibole concentrates were wrapped in alumini-
um-foil packets, encapsulated in sealed quartz vials, and ir-
radiated for 80 hr in the central thimble position of the
TRIGA Reactor in the U.S. Geological Survey, Denver.
Variation in the flux of neutrons along the length of the irra-
diation assembly was monitored with several mineral stan-
dards, including MMhb-1 (Sampson & Alexander 1987).
The samples were incrementally heated until fusion in a
double-vacuum, resistance-heated furnace. Temperatures
were monitored with a direct-contact thermocouple and are
controlled to ± 1
o
C
between increments and are accurate to
± 5
o
.
Measured isotopic ratios were corrected for total
blanks and the effects of mass discrimination. Interfering
isotopes produced during irradiation were corrected using
factors reported by Dalrymple et al. (1981). Apparent
40
Ar/
39
Ar ages were calculated from corrected isotopic ratios us-
ing the decay constants and isotopic ratios listed by Steiger
& Jäger (1977).
Intralaboratory uncertainties reported here have been cal-
culated by statistical propagation of uncertainties associated
with measurement of each isotopic ratio through the age
equation. Interlaboratory uncertainties are ca. ±1.25–1.5 %
of the quoted age. Total-gas ages have been computed for
each sample by appropriate weighting of the age and percent
39
Ar released within each temperature increment. A „pla-
teau“ is considered to be defined if the ages recorded by two
or more continuous gas fractions each representing >4 %,
constituting together >50 % of the total
39
Ar evolved are mu-
tually similar within a ±1 % intralaboratory uncertainty.
Analyses of the MMhb-1 monitor indicate that apparent K/
Ca ratios may be calculated through the relationship 0.518
(±0.0005)
×
(
39
Ar/
37
Ar)
corrected
.
Plateau portions of the hornblende analyses have been
plotted on
36
Ar/
40
Ar vs.
39
Ar/
40
Ar isotope correlation dia-
Fig. 1.
Generalized tectonic map of the Alpine-Carpathian orogen showing Triassic magmatic suites and location of the Ditrau Alkaline
Massif in the Eastern Carpathians, Rumania.
MIDDLE-LATE TRIASSIC
40
Ar/
39
Ar HORNBLENDE AGES FOR EARLY INTRUSIONS 349
grams. Regression techniques followed methods described
by York (1969). A mean square of the weighted deviation
(MSWD) has been used to evaluate the isotope correlations.
Results
Multigrain hornblende concentrates were prepared from
two samples of massive gabbro and diorite collected within
northern sectors of an „early“ gabbroic phase of the Ditrau
Complex. Sample locations are indicated in Fig. 2. Coordi-
nates of sample locations and petrographic descriptions of
the dated samples are provided in the Appendix. The
40
Ar/
39
Ar analytical data are provided in Table 1 and portrayed as
apparent age spectra in Fig. 3.
The two hornblende concentrates display variably discordant
apparent age spectra (Fig. 3). The relatively small volume low-
temperature gas fractions record considerable variation in appar-
ent ages. These are matched by fluctuations in apparent K/Ca
ratios which suggest that experimental evolution of argon oc-
curred from compositionally distinct, relatively nonretentive
phases. These could have been represented by: 1) very minor,
optically undetectable mineral contaminants in the hornblende
concentrates; 2) petrographically unresolvable exsolution or
compositional zonation within constituent hornblende grains;
Fig. 2.
Simplified geological map of the Ditrau Alkaline Massif
showing
40
Ar/
39
Ar sample locations (strongly simplified after
Streckeisen & Hunziker 1974).
Release
temp. (
o
C)
(
40
Ar/
39
Ar)* (
36
Ar/
39
Ar)* (
37
Ar/
39
Ar)
c
39
Ar % of
total
%
40
Ar non-
atmospheric
+
36
Ar
Ca
%
Apparent age (Ma) and
analytical error (Ma) **
Sample 1: J = 0.009792
600
55.10
0.10548
1.475
0.77
43.64
0.38
381.7
±
0.3
700
19.27
0.02223
0.957
0.41
66.28
1.17
212.7
±
0.3
800
14.12
0.00364
2.567
9.10
93.79
19.16
220.2
±
0.1
850
13.85
0.00154
2.752
25.59
98.26
48.53
226.0
±
0.1
875
13.80
0.00135
2.798
32.64
98.70
56.36
226.1
±
0.1
900
13.79
0.00129
2.892
22.20
98.88
60.93
226.4
±
0.1
925
13.81
0.00202
3.319
7.37
97.55
44.52
223.9
±
0.1
950
14.32
0.00281
6.965
1.47
98.08
67.52
233.3
±
0.1
Fusion
15.66
0.01041
14.453
0.44
87.74
37.77
229.6
±
0.4
Total
14.19
0.00259
2.920
100.00
97.48
50.54
226.7
±
0.1
Total without 600-700
o
C,
950
o
C -fusion
96.91
225.4
±
0.1
Sample 1: J = 0.009862
600
30.22
0.05297
1.309
0.81
48.54
0.67
239.8
±
0.3
700
12.64
0.00849
0.366
3.16
80.34
1.17
169.1
±
0.1
740
12.63
0.00843
0.694
2.18
80.66
2.24
169.7
±
0.1
770
12.72
0.00733
1.516
1.87
83.88
5.62
177.5
±
0.2
800
13.32
0.00379
2.316
3.82
92.94
16.62
204.5
±
0.1
825
13.65
0.00298
2.466
12.29
94.95
22.47
213.5
±
0.1
850
14.30
0.00301
2.707
13.26
95.26
24.46
223.9
±
0.1
875
14.34
0.00214
2.891
18.29
97.18
36.82
228.7
±
0.1
900
14.17
0.00163
3.044
20.14
98.29
50.93
228.6
±
0.2
930
14.18
0.00179
3.245
16.70
98.07
49.35
228.2
±
0.1
960
14.23
0.00156
4.421
3.98
99.23
77.29
231.8
±
0.2
Fusion
15.16
0.00330
7.116
3.49
97.31
58.74
241.9
±
0.1
Total
14.18
0.00312
2.924
100.00
95.41
37.20
221.7
±
0.1
Total without 600-825
o
C,
960
o
C -fusion
68.39
227.6
±
0.1
* measured
c corrected for post-irradiation decay of
37
Ar (35.1 day ½-life)
+
[
40
Ar
tot.
- (
36
Ar
atmos.
) (295.5)
]
/
40
Ar
tot.
** calculated using correction factors of Dalrymple et al. (1981); two sigma, intralaboratory errors.
Table 1:
40
Ar/
39
Ar analytical data for incremental heating experiments on hornblende concentrates from gabbros of the Ditrau Massif,
Rumania.
350 DALLMEYER, KRÄUTNER and NEUBAUER
3) minor chloritic replacement of hornblende; and/or intracrys-
talline inclusions. Most intermediate- and high-temperatures
gas fractions display little intrasample variation in apparent K/
Ca ratios, suggesting that experimental evolution of gas oc-
curred from compositionally uniform sites. The intermediate-
and high-temperature gas fractions experimentally evolved
from sample 1 record generally similar apparent
40
Ar/
39
Ar ages
which define a plateau age of 225.4±0.1 Ma.
36
Ar/
40
Ar vs.
39
Ar/
40
Ar isotope-correlation of the plateau data is well-defined
(MSWD = 1.38), and defines an inverse ordinate intercept
(
40
Ar/
36
Ar ratio) of 293.6. This is similar to that of the present-
day atmosphere, and suggests no significant intracrystalline
contamination with extraneous („excess“) argon components.
Using the inverse abscissa intercept (
40
Ar/
39
Ar ratio) in the
age equation yields a plateau isotope-correlation age of
227.1±0.1 Ma. Because calculation of isotope correlation ages
does not require assumption of a present-day
40
Ar/
36
Ar ratio,
they are considered more significant than those directly calcu-
lated from the analytical data. The 227 Ma isotope-correlation
age recorded by the hornblende concentrate from sample 1 is
considered geologically significant, and is interpreted to date
post-magmatic cooling temperatures required for intracrystal-
line retention of argon in constituent hornblende grains. Harri-
son (1981) suggested ca. 500±25
o
C are appropriate for argon
retention within most hornblende compositions in the range of
cooling rates likely to characterize most geological settings. In
view of the high-level intrusive character of the Ditrau Com-
plex, it is likely that post-magmatic cooling was relatively rap-
id. Therefore the ca. 227 Ma isotope-correlation age is interpret-
ed as probably closely dating initial pluton emplacement.
The hornblende concentrate prepared from sample 2, a
hornblende diorite, displays more extensive internal spectra
discordance. However, the 850–930
o
C increments record
similar apparent ages which define a plateau of
227.6±0.1 Ma. Isotope-correlation of the plateau data is well-
defined (MSWD = 1.88) with an inverse ordinate intercept of
294.7. A plateau isotope-correlation age of 231.5±0.1 Ma is
defined. This is also interpreted as dating a relatively rapid
post-magmatic cooling following pluton emplacement.
Considered together the plateau isotope-correlation ages
defined by the two hornblende concentrates suggest em-
placement of early magmatic suites of the Ditrau Complex
occurred at ca. 232–228 Ma. These correspond to the Mid-
dle-Late Triassic boundary following the time-scale calibra-
tion of Gradstein et al. (1994).
Discussion
The continental portion of the Eastern Carpathian orogen,
represented by the Bucovino-Getic microcontinent, has been
separated from stable Europe by Early Jurassic rifting (e.g.
Sandulescu 1984, 1994; Debelmas & Sandulescu 1987;
Trümpy 1988) which is expressed by general subsidence asso-
ciated with extension of stable continental lithosphere. The
Ditrau Alkaline Massif is interpreted, therefore, as having de-
veloped as a result of mantle-plume activity within continental
crust (South European margin; now Bucovinian Nappe Com-
plex) predating the onset of Jurassic rifting (Fig. 4a). On this
continental crust a sedimentary sequence records subsequent
rift and passive continental margin formation (Fig. 4b). Deep
water and oceanic sequences structurally occur both beneath
Fig. 3.
40
Ar/
39
Ar apparent ages and apparent K/Ca spectrum of
multigrain hornblende concentrates from the Ditrau Alkaline Mas-
sif. Analytical uncertainties (two sigma, intralaboratory) repre-
sented by vertical width of bars. Experimental temperatures in-
crease from left to right. Note that plateau ages are listed. For
discussion of ages, see text.
MIDDLE-LATE TRIASSIC
40
Ar/
39
Ar HORNBLENDE AGES FOR EARLY INTRUSIONS 351
the present Bucovinian Nappe Complex (e.g. deep water sedi-
ments and basalts within the Black Flysch and Ceahlau nappes
within the sedimentary Civcin rift that widened towards the
south within the Southern Carpathians) and above it, within
the oceanic elements that are exposed in the Transylvanian
nappes and in the Bucovinian wildflysch sequence of the
Haghimas, Rarau and Persani mountains (Sandulescu 1984;
1994; Burchfiel 1976, 1980). The latter correlate with the Ju-
rassic Mures ophiolite sequence that is exposed in the south-
ern Apuseni mountains. Therefore, the Bucovino-Getic mi-
croplate is now structurally interleaved with Jurassic oceanic
sequences. We interpret the Bucovino-Getic microcontinent
as representing an extensional allochthon which was only de-
tached from the European continent during Jurassic rifting.
Acknowledgements:
We acknowledge grants from the Tec-
tonics Program of the U.S. National Science Foundation to
RDD (EARTH-9316042) and from the Austrian Research
Foundation to FN (P8652-GEO). We thank to Albert Streck-
eisen, Tudor Berza and Urs Schaltegger for thougthful remarks
on an earlier version of the manuscript, and Hans-Peter Steyrer
for providing figures. Also, we would like to take this opportu-
nity to thank Mrs. Patti P. Gary for her outstanding and expe-
dite skills applied to the magnificent typing of this paper.
Appendix 1: Sample descriptions
Sample 1
. Coordinates of location: 46
o
50´ 02´´ N, 25
o
34´ 19´´ E; ca. 7 km
east of Ditrau town on the road to Tulghes. Medium-grained (average
grain size between 1 and 2 mm) massive gabbro. Main constituents are
brown amphiboles and plagioclase. The amphibole is mostly free of in-
clusions, some grains include large magnetite, carbonate, and biotite
crystals. In a few amphibole grains crystallographically oriented exsolu-
tion of sphene occur. In the center rare clinopyroxene inclusions occur.
Plagioclase is often optically zoned, and includes some fine inclusions
in the center as the product of post-magmatic alteration. Fine-grained
phyllosilicates may occur along plagioclase grain boundaries. Further
constituents are biotite, nepheline, clinopyroxene, rare alkali feldspar,
and zircon.
Sample 2.
Coordinates of location: 46
o
52´15´´ N; 25
o
30´ 01´´ E; Sarmas-
Jolotca mine in the Sarmas Valley NE of Ditrau. Sligthly deformed and
altered, medium-grained massive hornblende diorite. The main constit-
uents are predominant plagioclase and subordinate amphibole, biotite,
clinopyroxene, minor constituents alkali feldspar, opaque minerals,
sphene, calcite and epidote. Plagioclase occurs in irregularly zoned,
sometimes cataclastically deformed grains. These are sometimes altered
into a fine aggregate of phyllosilicates. Biotite is kinked and contains
amphibole and clinopyroxene inclusions, and sagenite exsolutions. Am-
phibole occurs in three textural types: 1) large greenbrown crystals with
some narrow rims; 2) medium-grained, isometric crystals with straigth
grain boundaries; these crystals are free of inclusions, and form aggre-
gates; and 3) some minor actinolite.
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