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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Europe
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Central Europe
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Bohemian Massif (2)
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Czech Republic
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Barrandian Basin (1)
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Czech Erzgebirge (1)
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Czech Sudeten Mountains (1)
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Erzgebirge
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Czech Erzgebirge (1)
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Fichtelgebirge (1)
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Sudeten Mountains
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Czech Sudeten Mountains (1)
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elements, isotopes
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chemical ratios (1)
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isotope ratios (2)
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isotopes
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stable isotopes
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Nd-144/Nd-143 (2)
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metals
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rare earths
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neodymium
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Nd-144/Nd-143 (2)
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samarium (1)
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geologic age
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Paleozoic
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Cambrian
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Upper Cambrian (1)
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Precambrian
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upper Precambrian
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Proterozoic
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Neoproterozoic (1)
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Saxothuringian (1)
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igneous rocks
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igneous rocks
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plutonic rocks
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diorites
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plagiogranite (1)
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granites
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A-type granites (1)
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volcanic rocks
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basalts
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alkali basalts
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spilite (1)
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trachytes (1)
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metamorphic rocks
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metamorphic rocks
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gneisses
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orthogneiss (1)
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metaigneous rocks
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metabasalt (1)
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Primary terms
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crust (2)
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Europe
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Central Europe
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Bohemian Massif (2)
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Czech Republic
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Barrandian Basin (1)
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Czech Erzgebirge (1)
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Czech Sudeten Mountains (1)
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Erzgebirge
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Czech Erzgebirge (1)
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Fichtelgebirge (1)
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Sudeten Mountains
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Czech Sudeten Mountains (1)
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-
-
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geochemistry (2)
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igneous rocks
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plutonic rocks
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diorites
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plagiogranite (1)
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granites
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A-type granites (1)
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-
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volcanic rocks
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basalts
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alkali basalts
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spilite (1)
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-
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trachytes (1)
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-
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isotopes
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stable isotopes
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Nd-144/Nd-143 (2)
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-
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mantle (2)
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metals
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rare earths
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neodymium
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Nd-144/Nd-143 (2)
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samarium (1)
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-
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metamorphic rocks
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gneisses
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orthogneiss (1)
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metaigneous rocks
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metabasalt (1)
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paleogeography (1)
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Paleozoic
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Cambrian
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Upper Cambrian (1)
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petrology (1)
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plate tectonics (1)
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Precambrian
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upper Precambrian
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Proterozoic
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Neoproterozoic (1)
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Ca. 500 Ma orthogneisses and bimodal suites are widespread along the northern part of the Bohemian Massif (central European Variscides) and are interpreted to document intense magmatism during a continental break-up episode along the northern periphery of Gondwana. Based on geological setting, and geochemical and isotopic evidence, these felsic igneous rocks record the generation of: (1) magmas of pure or predominantly crustal derivation, represented by minor extrusives and much more voluminous orthogneisses similar to S-type granitoids; (2) subordinate magmas of exclusively mantle origin (ranging from within-plate alkali trachytes to oceanic plagiogranites) corresponding to felsic derivatives of associated basalts; and (3) magmas of hybrid origin, produced either as a result of large degrees of contamination of mantle-derived magmas ascending through the crust, or alternatively, generated by partial melting of mixed sources, such as interlayered sediments and mafic rocks or graywackes containing a juvenile component. The high-temperature dehydration melting process responsible for the generation of the most abundant rock-types necessitated the advection of mantle heat, in a context of continental lithosphere extension, as documented by broadly coeval basaltic magmatism at the scale of the igneous province. The large volumes of felsic magmas generated during the 500-Ma anorogenic event are interpreted to result from the combination of a hot extensional tectonic regime with the widespread availability in the lower crust of fertile lithologies, such as metagraywackes. This in turn reflects the largely undifferentiated nature of the crustal segment accreted some 50–100 m.y. earlier during the Cadomian orogeny.
On the basis of immobile trace elements and Nd isotope signatures, the Barrandian meta-basalts may be ascribed to two major groups, extracted from contrasting mantle sources: A depleted group, with strong light rare earth element depletion, elevated Zr/Nb ratios (>30), and highly radiogenic Nd isotopes (ϵNd 600 from +7.8 to + 9.3). Multi-element patterns normalized to normal mid-ocean ridge basalt all show negative anomalies of Nb, and to a lesser degree, Zr and Ti. Eight samples may define a 605 ± 39-Ma whole-rock isochron with ϵNd i of +8.8 ± 0.2. An enriched group, comprising both mildly enriched (Zr/Nb 12–18) and strongly enriched (Zr/Nb 4–7) samples, with ϵNd 600 ranging from +8.2 to +3.8. The depleted group is interpreted to reflect generation from depleted mantle sources fluxed by subduction-related components, probably in an intraoceanic back-arc basin. In contrast, the younger enriched group is typical of the within-plate style of mantle enrichment and documents the extinction of the subduction-related component. The switch from suprasubduction zone to within-plate magmatism suggests that new mantle material flowed into the former arc and back-arc system sources. This flow might have occurred simply as a result of ocean-ward migration of the subduction zone. Alternatively, the subduction fluxing might have stopped as a result of impingement of a spreading ridge with the intraoceanic trench, leading to mutual annihilation, a switch to a transform plate boundary, and opening of a slab window that allowed the inflow of new mantle and the generation of late-stage, within-plate enriched basalts. In terms of modern analogues, the Neoproterozoic of the Barrandian and other Cadomian regions of western Europe resemble arc and back-arc systems from the western Pacific region, where large intraoceanic subduction systems fringe major continental masses with a complex mosaic of microplates and magmatic arcs, including intervening basins floored either by oceanic crust or attenuated continental crust.