Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria

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 small Stomanovo monzonite porphyry intrusion in Central Rhodope Massif, Bulgaria. The dated at 30.50 ± 0.46 Ma is intruded into voluminous Oligocene (31.63 0.40 Ma) Bratsigovo–Dospat ignimbrite. hosts both normally and reversely zoned clinopyroxene phenocrysts. characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas has green Fe-rich cores (Mg 71–55) mantled 87–74). Neither core nor equilibrium with host monzonite. This shows more radiogenic Sr isotopes (( 87 Sr/ 86 Sr) i = 0.71066) ϵNd(t) −7.8 to −8.0 that distinct from ignimbrites ( 0.70917–0.70927 −4.6 −6.5. Sr–Nd isotopic data presence copious zircon xenocrysts underlying metamorphic basement suggest extensive assimilation. Our observations indicate lower or middle fractional crystallization an evolved magma producing cumulates. process was followed hybridization primitive mantle-derived subsequent continuous contamination. We instead inheriting their high K 2 O large-ion lithophile element enrichments slab-derived/metasomatic fluids, may owe some its unusually alkalinity assimilation potassium-rich phases Massif rocks.