Sediment-derived melt-related metasomatized mantle wedge as a source of post-subduction Quaternary adakitic porphyries associated with absarokite-shoshonite from the Karadag stratovolcano (Karaman, Central Anatolia, Turkey)


ÇOBAN H., KARSLI O., CARAN Ş. , YILMAZ K.

JOURNAL OF ASIAN EARTH SCIENCES, cilt.196, 2020 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

The Karadag stratovolcano (KS), located at the southern end of the Cappadocian Volcanic Province in Central Anatolia, Turkey, is a key volcanic vent for understanding the spatiotemporal association between adakitic and mafic potassic (absarokitic-shoshonitic-banakitic) magmas in a post-subduction intracontinental setting. Here, we present a comprehensive geochemical dataset consisting of mineral chemistry, major and trace elements, Sr-Nd-Pb isotopes and conventional K-Ar age data for the adakitic andesite-dacite series of the KS. KS was built up by the eruption of adakitic andesites (825 +/- 17 ka), absarokite-shoshonites (393 +/- 153 ka), adakitic dacites (45 +/- 6 ka) and effusive deposits (< 45 ka). The metaluminous and calc-alkaline to high-K calc-alkaline Karadag adakitic porphyries demonstrate high Sr (662 to 1053 ppm), Sr/Y ratios (39-66) and Mg# (46 to 63) along with low Y (11 to 17 ppm) and heavy rare earth element (HREE) concentrations, consistent with a slab melting origin in modern subduction settings. The samples are characterized by relative enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), with Nb-Ta depletion but an insignificant negative Eu anomalies. However, their high K2O/Na2O ratios (0.6-0.9) and Mg# and their evolved Sr-Nd-Pb isotopic compositions (Sr-87/Sr-86(t) = 0.7045 to 0.7078, Nd-143/Nd-144 = 0.512454 to 0.512612 and Pb-206/Pb-204 = 18.93 to 18.94) mark an origin from an enriched mantle source. Karadag volcanic rocks also have low Nb/La (< 0.5) ratios that show a positive correlation with SiO2 and high Nb/Ta (average 15.7) ratios close to that of primitive mantle (PM) (17.5), all of which implies a heterogeneous mantle source and an insignificant role of crustal assimilation. Based on the powerful volcanic markers of geochemical ratios (e.g., low Ba/Th, Sr/Th and high La/Sm, Th/Nb, Th/La, Hf/sm), variable Sr-Nd, and high Pb-206/Pb-204 isotopic compositions, we contend that the Karadag adakitic porphyries, similar to the absarokites-shoshonites-banakites, were formed through partial melting of the metasomatized mantle. The mantle wedge was metasomatized mainly by overlying sediments of subducted crust, as major metasomatic agents, and to a lesser degree by melts/fluids from the dehydrated oceanic slab. It is suggested that absarokitic magmas were derived from the lowermost lithospheric mantle source and adakitic magmas from the uppermost mantle lithosphere. The thermal energy required for partial melting of the metasomatized mantle lithosphere was generated by upwelling of the hot asthenosphere through deep strike-slip faults bordering a pull-apart basin in a post-subduction extensional intracontinental setting. These interpretations argue against the presence of a Quaternary arc in Central Anatolia, and adakitic and associated mafic potassic magmas do not require contemporaneous subduction of an oceanic slab or delamination of lower continental crust in a post-subduction setting. We conclude that a post-collisional transtensional stage after rapid multi-phased uplift was triggered by asthenospheric mantle flows (collision-induced, collision-related slab break-off-induced, or a combined effect of both), transferring the metasomatized mantle-derived magmas into the crust system along with the whole Cappadocian Volcanic Province.