Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Oh?e (Eger) Rift,Western Bohemia |
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| Authors: | Jaromír Ulrych Emil Jelínek Zdeněk ?anda Felicity E Lloyd Kadosa Balogh Ernst Hegner Ji?í K Novák |
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| Institution: | 1. Institute of Geology, v. v. i., Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Praha 6, Czech Republic;2. Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic;3. Nuclear Physics Institute, v. v. i., Academy of Sciences of the Czech Republic, 250 68 ?e?, Czech Republic;4. Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, United Kingdom;5. Institute of Nuclear Research, Hungarian Academy of Sciences, Bemtér 18/C, Debrecen H-4026, Hungary;6. Department für Geowissenschaften, Universität München, Theresienstraße 41, D-8033 München, Germany |
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| Abstract: | High-Ti melanephelinite (3.8–5.9 wt% TiO2), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO2), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO2) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial 87Sr/86Sr of ca. 0.7034–0.7038 and εNd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial 87Sr/86Sr of ca. 0.7035–0.7036 and εNd of 4.3–5.5. The large variation in initial 87Sr/86Sr ratios at similar εNd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial 87Sr/86Sr ratios of ca. 0.7036 and εNd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks. |
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