A slab detachment and delamination model for the generation of Carboniferous high-potassium I-type magmatism in the Eastern Pontides, NE Turkey: The Köse composite pluton

The Hercynian Köse composite pluton (KCP) is located in the Eastern Pontides, Turkey, and consists of two units of high-K calc-alkaline, primarily peraluminous granites: (i) the internal body, and (ii) the external body. The internal body, which was emplaced at 322–318 Ma (40Ar/39Ar ages on biotite and hornblende, respectively), displays a wide compositional range (49–71 wt.% SiO2) and contains several lithologies: hybrid equigranular rocks, microgranular magmatic enclaves, mafic dikes, porphyry dikes and mylonites. The external body, which was emplaced at 306.7 Ma (40Ar/39Ar age on K-feldspar), consists exclusively of monzogranite (> 71 wt.% SiO2). Field relationships, mineralogy, major- and trace element geochemistry, and initial Sr–Nd isotope values (ISr = 0.70821 to 0.71002, eNd(t) = −6.6 to −8.0) show that the internal body was differentiated and evolved by crystal fractionation and magma mixing processes. The end-members of the mixing process were a mafic rock and a felsic rock. Mafic magma was derived from a relatively deep-seated (25–30 km) crustal storage reservoir, not directly from the mantle, and underwent significant differentiation by fractional crystallization and crustal contamination before mixing. In addition, these magma storages probably supplied the additional heat necessary to initiate crustal melting. Some of the additional heat may have also been released by the radiogenic decay of heat producing elements. Eventually, the existing felsic magma from the melting of K-bearing meta-greywackes was raised to its emplacement level at a depth of ~ 10–16 km. After partial crystallization, it was sporadically intruded by modified mafic magma from the deeper crustal reservoir to generate hybrid rocks. The hybrid rocks were then elevated to a shallower depth by normal faults during the collapse of the orogen and erosion. Mylonites that were later overprinted by pseudotachylites are typically constrained to the internal body and are regarded as markers of this event. The external body is characterized by a significantly less radiogenic and limited range of Sr–Nd isotope values (ISr = 0.70639 to 0. 70792, eNd(t) = −4.4 to −6.5) than those of the internal body and a lack of rocks documenting the open system differentiation processes. Fractional crystallization is the exclusive process responsible for the elemental range within the body. The rocks also contain less biotite relative to those of the internal body. All these involve less K-bearing mid-crustal rocks (orthogneisses) in their source, which was probably located at depths near the lower crust. The absence of purely lower crustal-derived melts can be explained by the removal of this type of material during the formation of the parental melt. This melt later ascended to its emplacement level at a depth of around ~ 5–10 km and cut the hybrid rocks of the internal body and regional metamorphic rocks that had been raised previously due to ongoing erosion. The melt that injected into the cracks of the internal body crystallized into porphyries because there was not enough time for the entire crystallization of magma. The data presented here indicate that late Early Carboniferous and Late Carboniferous magmatism occurred in a collisional setting. Slab detachment and subsequent delamination seem to be the most plausible mechanisms for the generation of the Hercynian high-K calc-alkaline magmatism in the Eastern Pontides, Turkey.

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► Mixing between mantle-derived mafic and upper crust-derived felsic magmas accounts for the large lithological and geochemical variations within the internal body of the Köse composite pluton (KCP) in the Eastern Pontides, Turkey.
► Partial melting of mid-crustal rocks resulted in the formation of the external body.
► The melt that injected into the cracks of the internal body crystallized into porphyries.
► Slab detachment and subsequent delamination provide the most plausible mechanisms for the Carboniferous magmatic and tectonic evolution of the Eastern Pontides.