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(40)A/Ar-39 step-heating of mica and amphibole megacrysts from hauyne-bearing olivine melilitite scoria/tephra from the Zelezna hurka yielded a 435 +/- 108 ka isotope correlation age for phlogopite and a more imprecise 1.55 Ma total gas age of the kaersutite megacryst. The amphibole megacrysts may constitute the first, and the younger phlogopite megacrysts the later phase of mafic, hydrous melilitic magma crystallization. It cannot be ruled out that the amphibole megacrysts are petrogenetically unrelated to tephra and phlogopite megacrysts and were derived from mantle xenoliths or disaggregated older, deep crustal pegmatites. This is in line both with the rarity of amphibole at Zelezna hurka and with the observed signs of magmatic resorption at the edges of amphibole crystals.
Controversy 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-Pgranulite below from low-Pmetasedimentary 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-Pamphibolite 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 publishedP-T-tdata for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of similar to 8 mm/year and similar to 80 degrees C/Ma, with a drop in exhumation rate from similar to 20 to similar to 2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag ofc. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90 degrees 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 ofc. 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.
Early Carboniferous to Permian magmatism associated with rifting within the northern foreland of the Variscan Orogen was widespread across Europe. During the long period of magmatic activity the regional tectonic setting changed across the region from early Carboniferous extension and basin formation to a rifting-wrenching style of deformation in the late Carboniferous (Stephanian) to early Permian. Wrenching and faulting were accompanied by widespread, voluminous and episodic magmatic extrusion, intrusion and underplating. This was followed by thermal relaxation and the development of the Northern and Southern Permian Basins in later Permian times. Thermal relaxation was punctuated by a Permo- Triassic phase of extension and graben formation. Ar-40/Ar-39 Ar step-heating dating for mineral separates and whole- rock samples of magmatic rocks from southern Scandinavia (Oslo Graben and south Sweden) and Rugen (north Germany) provides further radiometric evidence for three of the proposed periods of magmatic activity in the region. Latest Carboniferous to earliest Permian ages (c. 300-310 Ma) were obtained for volcanic rocks in the Oslo Graben and dolerite sills and dykes in south Sweden and north Germany. This phase can be time-correlated with magmatic activity that occurred throughout Europe during large-scale dextral wrenching that followed the Variscan Orogeny. A second phase of alkaline intrusions is confined to the Oslo Graben and related to caldera collapse around c. 275 Ma. The third, Permo- Triassic phase (c. 250 Ma) is considered to be related to a new tectonic cycle involving extension that triggered minor melting of enriched, fertile mantle.
Permian basins
(2005)