This paper provides new constraints on the crystallisation age of the
gabbroic rocks and associated migmatites that are exposed now along the
boundary of the Penninic and Austroalpine nappes in the Eastern Central
Alps (Val Malenco, N Italy). The gabbros intruded at the
crust-to-mantle boundary and caused granulite facies metamorphism in
the country rocks. Zircons were extracted from the gabbros and
metagabbros as well as from an associated leucogranite and single
crystals were dated by the U-Pb method. In addition the lead isotopic
composition was determined on plagioclase separated from the same
samples.
One Fe-gabbro with preserved primary mineral assemblage yielded
sufficient zircons suited for U/Pb dating. Additionally a Zr-rich
Ti-Fe-gabbro which is overprinted by Alpine metamorphism and a
leucogranite representing local anatexis of country rocks during the
gabbro intrusion were selected for dating. 15 U-Pb data points of the
U-poor (30-150 ppm) zircons from both gabbros yielded an upper
concordia intersection age of 281 +/- 19 Ma, whereas 10 U-rich
(600-4000 ppm) zircons from the leucogranite define a very similar
upper concordia intersection at 278 -2.5/+2.6 Ma corresponding to an
Early Permian age. On a regional scale, the Early Permian age of the
gabbros is consistent with a major phase of continental crustal growth
during the late Paleozoic.
Plagioclase of all but one sample of the tholeiitic gabbro suite is
characterised by a Pb isotopic composition typical for average crust.
The relatively homogeneous Pb isotopic composition of these rocks,
which is independent of the degree of differentiation, suggests a
contaminated mantle source of their parental magma rather than crustal
contamination during the emplacement at the crust-to-mantle boundary.
One Ti-Fe-gabbro dike showed elevated mu and omega values (mu = 9.98;
omega = 40.3) very similar to those of the anatectic leucogranite.
Contamination of this rock most likely took place during its intrusion
into metasedimentary country rocks. U/Pb results of some zircons of an
other Ti-Fe-gabbro suggest the presence of inherited crustal
components. Abundant zircon and a high Zr content (603 ppm) indicate
that the magma of this sample was saturated with zircon and thus
xenocrystic zircon may have survived. However, the presence of cores or
xenocrysts has not been confirmed by optical means yet. Observed cores
and the presence of inherited lead in zircons of the leucogranite are
in agreement with an anatectic origin of this rock.