Marchev, P, Raicheva, R, Georgiev, S et al. (2 more authors) (2022) Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria. Geological Magazine, 159 (1). pp. 81-96. ISSN 0016-7568
Abstract
Generally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i = 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i = 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © The Author(s), 2021. Published by Cambridge University Press. This article has been published in a revised form in Geological Magazine https://doi.org/10.1017/S0016756821000868. This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. |
Keywords: | crustal contamination; crystal-melt mixing; green-core clinopyroxene; Rhodope Massif; ultrapotassic monzonite |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Inst of Geophysics and Tectonics (IGT) (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 05 Jan 2022 15:41 |
Last Modified: | 18 Apr 2022 00:38 |
Status: | Published |
Publisher: | Cambridge University Press |
Identification Number: | 10.1017/S0016756821000868 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:181875 |