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Variable origin of clinopyroxene megacrysts carried by Cenozoic volcanic rocks from the eastern limb of Central European Volcanic Province (SE Germany and SW Poland)

Abstract : Clinopyroxene megacrysts occurring in Cenozoic mafic alkaline volcanic rocks from the northern margin of the Bohemian Massif (SW Poland, SE Germany) could be subdivided by colour and Mg# (Mg/(Mg + Fetot)) in three groups. Megacryst with the highest Mg# = 90.0-91.5 ("high Mg#", HMg#) is transparent and strongly Light Rare Earth Element (LREE) depleted and contains abundant orthopyroxene lamellae. The clinopyroxene megacrysts with "medium Mg#" (MMg#) values (76.8-83.4) are transparent to light grey and are all LREE-enriched. The group with the lowest Mg# (LMg#; 62.2-74.6) is intensively coloured (from grey to green) and may enclose apatite, Ti-magnetite or pseudomorphs after amphibole. The "low Mg#" (LMg#) clinopyroxenes are LREE-enriched, and some display positive Zr-Hf anomalies.

The single HMg# megacryst records pressures ~1GPa and temperature of 1280 °C, pointing to its mantle origin. It is the first megacryst described in European lavas and possibly worldwide, which shows affinity to Depleted MORB Mantle (DMM). The MMg# megacrysts formed from fractionating mafic magmas at variable pressures and temperatures - from those corresponding to mantle depths (>1 GPa, 1230-1350 °C) to lower/middle crustal values (0.5-0.9 GPa, 1120-1150 °C). The parental melts for this group are isotopically related to the Cenozic volcanic rocks from the study area. The LMg# megacrysts crystallized from strongly alkaline melts, mostly at lower- to middle-crustal pressures (0.4-1.0 GPa). Their parental melts were also related to Cenozoic volcanism, but their strongly evolved nature resulted in local, but significant enrichment in Zr and Hf. The LMg# megacrysts from three localities in Poland are the first evidence of strongly alkaline magmatism in the north-easternmost part of Cenozoic European Volcanic Province.

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Submitted on : Saturday, May 7, 2022 - 8:09:08 AM
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Magdalena Matusiak-Małek, Jacek Puziewicz, Theodoros Ntaflos, Alan Woodland, Laura Uenver-Thiele, et al.. Variable origin of clinopyroxene megacrysts carried by Cenozoic volcanic rocks from the eastern limb of Central European Volcanic Province (SE Germany and SW Poland). Lithos, 2021, 382, ⟨10.1016/j.lithos.2020.105936⟩. ⟨insu-03661476⟩

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