New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

• Published in: Lithos Vol. 302-303; pp. 278 - 297
• Main Authors: Will, T.M., Schmädicke, E., Ling, X.-X., Li, X.-H., Li, Q.-L.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2018
• Subjects: Lu-Hf isotope data; Odenwald-Spessart basement; Paired metamorphic belt; U-Pb geochronology; Variscan orogeny; U-Pb geochronology; Variscan orogeny; Paired metamorphic belt; Odenwald-Spessart basement; Lu-Hf isotope data
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2018.01.008

New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro (1961). Thus, the Odenwald-Spessart basement represents the first recognised paired metamorphic belt in the European Variscides. [Display omitted] •Metamorphism in the Odenwald-Spessart basement occurred between 334 and 321 Ma.•Magmatic events occurred in the Spessart basement at 460, 445 and 329 Ma.•Eclogite- and granulite-facies metamorphism took place contemporaneously at ca. 330 Ma.•The Odenwald-Spessart basement is interpreted as a Variscan paired metamorphic belt.