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Jiri K. Novák, Institute of Geology (Czech Republic)
Jaromir Ulrych, Institute of Geology ASCR (Czech Republic)
Lukas Ackerman, Institute of Geology ASCR (Czech Republic)
Emil Jelinek, Faculty of Science, Charles University (Czech Republic)
Martin Chadima, Institute of Geology ASCR (Czech Republic)
Jiri Mizera, Nuclear Physics Institute ASCR (Czech Republic)
Zdenek Randa, Nuclear Physics Institute ASCR (Czech Republic)
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A specific high-titanium basaltic rock association (with 3.5-5.2 wt% TiO2), represented by olivine-poor melanephelinite to tephrite occurs in the uplifted shoulder of the Ohre (Eger) Rift. It is noteworthy that these rocks were generated in an intracontinental rift setting. Their bodies are either synchronous with the 37-28 Ma old Loucna - Oberwiesenthal Volcanic Centre on the Bohemia/Saxony border or represent younger satellite bodies (24-18 Ma in age). The elevated Ti/Y (1158-924 vs. 491), Ti/Nb (253-174 vs. 94) and Zr/Nb (3.5-2.8 vs. 2.5) ratios discriminate the sample set into the high-Ti and moderately-Ti series. The low Mg# (39-44), low contents of Cr, Ni, Co and enrichment in LILE, Zr, Hf, Nb, Ta, U, Th and LREE contrast with the chemical composition of primary mantle-derived melts with high Mg# and incompatible element contents. They often entrain alkali pyroxenite to ijolite xenoliths with 87Sr/86Sr (0.70361-0.70365) and 143Nd/144Nd (0.51271-0.51276) ratios, being related to a hidden intra-crustal alkali complex. The initial ratios are consistent with mantle source of HIMU-affinity, with possible presence of some other lithospheric units.
Based on opaque mineralogy and magnetic experiments, the anomalously high magnetic properties (magnetic susceptibility =77,000-155,000x10-6 SI units) are explained as a result of deuteric titanomagnetite oxidization (thermochemical RM). Magnetic susceptibility (MS) is dependent upon geomagnetic field and increases with its amplitude; the MS increase is ∼9% for moderate-Ti tephrite (2.41 wt% TiO2) and ∼22.5% for high-Ti melanephelinite (4.97 wt% TiO2), due to the exsolution of secondary phases after thermal treatment. The oxy-exsolution of titanomagnetite is also sensitively reflected in the Curie temperature (Tc). Two Tc were identified for moderate-Ti tephrite: ∼310 °C and ∼430 °C. The Tc for high-Ti melanephelinite sample is more complex showing the first Tc at ∼200°C, the steeper MS increase in the range of 350-420°C, and the second Tc at ∼420 °C. Bulk and trace element geochemistry of rocks, mineral chemistry and magnetic petrological analyses were employed to characterize some differences in the development of high-Ti, moderately-Ti and low-Ti basaltic rocks from the western Ohre (Eger) Rift.
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