U-Pb zircon geochronology of a tholeiitic intrusion and associated migmatites at a continental crust-mantle transition, Val Malenco, Italy

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 μ and ω values (μ = 9.98;ω = 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.