Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

Zircon formation versus zircon alteration — New insights from combined U–Pb and Lu–Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt

Principles of Isotope Geology

Abstract The mid-European segment of the Variscides is a tectonic collage consisting of (from north to south): Avalonia, a Silurian-early Devonian magmatic arc, members of the Armorican Terrane Assemblage (ATA: Franconia, Saxo-Thuringia, Bohemia) and Moldanubia (another member of the ATA or part of N Gondwana?). The evolution on the northern flank of the Variscides is complex. Narrowing of the Rheic Ocean between Avalonia and the ATA occurred during the late Ordovician through early Emsian, and was accompanied by formation of an oceanic island arc. By the early Emsian, the passive margin of Avalonia, the island arc and some northern part of the ATA were closely juxtaposed, but there is no tectonometamorphic evidence of collision. Renewed extension in late Emsian time created the narrow Rheno-Hercynian Ocean whose trace is preserved in South Cornwall and at the southern margins of the Rhenish Massif and Harz Mts. Opening of this ‘successor ocean’ to the Rheic left Armorican fragments stranded on the northern shore. These were later carried at the base of thrust sheets over the Avalonian foreland. Closure of the Rheno-Hercynian Ocean in earliest Carboniferous time was followed by deformation of the foreland sequences during the late lower Carboniferous to Westphalian. Closure of narrow oceanic realms on both sides of Bohemia occurred during the mid- and late Devonian by bilateral subduction under the Bohemian microplate. In both these belts (Saxo-Thuringian, Moldanubian), continental lithosphere was subducted to asthenospheric depths, and later partially obducted. Collisional deformation and metamorphism were active from the late Devonian to the late lower Carboniferous in a regime of dextral transpression. The orthogonal component of intra-continental shortening produced an anti-parallel pair of lithospheric mantle slabs which probably joined under the zone of structural parting and became detached. This allowed the ascent of asthenospheric material, with important thermal and rheological consequences. The strike slip displacements were probably in the order of hundreds of kilometres, since they have excised significant palaeogeographic elements.

The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution

Age and evolution of a late Archean to Paleoproterozoic upper to lower crustal section in the Wutaishan/Hengshan/Fuping terrain of northern China

The Central Zone of the Limpopo belt in southern Africa has previously been interpreted as a segment of Archaean crust which experienced its main deformation and metamorphism around 2.7 Ga ago. We report new single zircon U/Pb and Pb/Pb ages for granitoid gneisses, supracrustal rocks and anatectically derived granitic melt patches in the area around Messina, South Africa. The Sand River Gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2.6 and 3.2 Ga. The Singelele gneiss, a heterogeneous granodioritic to quartz monzonitic rock, has protolith ages between 2.55 and 2.58 Ga. Since both the Sand River and Singelele gneisses experienced polyphase high-strain ductile deformation this must have occurred later than 2.55 Ga ago. Granulite-facies pelitic gneisses of the Beit Bridge Complex contain abundant spherical, multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism. These zircons provide ages with a mean at 2026.5±6.3 Ma which is interpreted as reflecting a high P – T event (>10 kbar, 825±25°C). Granitic melt patches in the metapelites as well as in the Sand River Gneiss and anatectic granites are probably related to rapid near-isothermal decompression to below 3–5 kbar and 600–750°C). These rocks contain new magmatic zircons which yielded a mean age of 2005.6±4.4 Ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage. Our zircon data support previous suggestions for only one single granulite-facies event in the Central Zone, and we suggest that this event occurred c. 2027 Ma ago. Since most of the deformation seen in the gneisses of the Messina area must have occurred later than 2.55 Ga ago, it is likely that the ‘Limpopo Orogeny’, at least in the Central Zone, is not an Archaean event but took place in the early Proterozoic.

Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa

Conventional multigrain U-Pb systematics of zircon and single-grain evaporation data of zircons in combination with Nd model ages are presented for pyroxene-free, felsic, garnetiferous granulites and orthopyroxene-bearing granulites of felsic, mafic and intermediate composition from the Blansky Les mountains granulite complex, S Bohemia. Zircons from mafic and intermediate granulites yield Concordia intercept ages between 346 ± 5 and 351 ± 6 Ma as well as a single-grain evaporation age of 346 ± 12 Ma. These ages define the time of Variscan high-grade metamorphism in S Bohemia and are supported by an internal Sm-Nd garnet-zircon-plagioclase-whole rock isochron age of 343 ± 21 Ma. Euhedral zircons from felsic granulites of probable granitic derivation yield U-Pb ages of 366 ± 5 Ma and 373 ± 12 Ma interpreted as intrusion ages of the granitic precursor. An inherited Proterozoic crustal component is documented by poorly defined Concordia upper intercept ages around 1700 Ma and Nd-model ages between 1.3 and 2.1 Ga.

J. I. Wendt

Papers

Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa

U-Pb zircon and Sm-Nd dating of Moldanubian HP/HT granulites from South ohemia, Czech Republic