Abstract
AbstractControversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.
Highlights
Recent attempts to assimilate accumulated geological and geophysical evidence into a plate tectonic model of the late Palaeozoic Variscan orogen in Europe and North-West Africa have renewed the controversy over the involvement of microplates in the Variscan plate convergence
The Cambro-Ordovician protolith ages of the Saxon granulites overlap with U–Pb zircon ages in the range 500–470 mean age (Ma) obtained throughout the Variscan belt for a late stage of rift magmatism in the northern Gondwana margin (e.g. Díez Fernández, Pereira, & Foster, 2015; Höhn et al, 2018; Kryza & Fanning, 2007; Kusbach et al, 2015; Tichomirowa, Sergeev, Berger, & Leonhardt, 2012)
The Saxon granulites derive from crustal sources in the rifted northern Gondwana margin, where oceanic opening produced the protoliths of the Metagabbro/Serpentinite Unit overlying the granulites
Summary
Recent attempts to assimilate accumulated geological and geophysical evidence into a plate tectonic model of the late Palaeozoic Variscan orogen in Europe and North-West Africa have renewed the controversy over the involvement of microplates in the Variscan plate convergence. The Cambro-Ordovician protolith ages of the Saxon granulites overlap with U–Pb zircon ages in the range 500–470 Ma obtained throughout the Variscan belt for a late stage of rift magmatism in the northern Gondwana margin (e.g. Díez Fernández, Pereira, & Foster, 2015; Höhn et al, 2018; Kryza & Fanning, 2007; Kusbach et al, 2015; Tichomirowa, Sergeev, Berger, & Leonhardt, 2012). Another age population of the Saxon granulites dated by Sagawe et al (2016) to c. The late-kinematic granite intrusions in the granulite body and its boundary shear zone have yielded zircon ages in the range 335–333 Ma based on pooled data from spot and single-grain measurements, and an upper intercept age of c. 338 Ma for discordant multigrain zircon fractions (references in the caption to Figure 2)
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