Subduction-related volcanic rocks are widespread in the Central Pontides of Turkey, and represented by the Hamsaros volcanic succession in the Sinop area to the north. The volcanic rocks display high-K calc-alkaline, shoshonitic and ultra-K affinities. 40 Ar/ 39 Ar age data indicate that the rocks occurred during the Late Cretaceous (ca 82 Ma), and the volcanic suites were coeval. Primitive mantle-normalized trace element patterns of all the lavas are characterized by strong enrichments in large ion lithophile elements (LILE) (Rb, Ba, K, and Sr), Th, U, Pb, and light rare earth elements (LREE; La, Ce) and prominent negative Nb, Ta, and Ti anomalies, all typical of subduction-related lavas. There is a systematic increase in the enrichment of incompatible trace elements from the high-K calc-alkaline lavas through the shoshonitic to the ultra-K lavas. In addition, the shoshonitic and ultra-K lavas have significantly higher 87 Sr/ 86 Sr (0.70666-0.70834) and lower 143 Nd/ 144 Nd (0.51227-0.51236) initial ratios than coexisting high-K calc-alkaline lavas ( 87 Sr/ 86 Sr 0.70576-0.70613, 143 Nd/ 144 Nd 0.51245-0.51253). Geochemical and isotopic data show that the shoshonitic and ultra-K rocks cannot be derived from the high-K calc-alkaline suite by any shallow level differentiation process, and point to a derivation from distinct mantle sources. The shoshonitic and ultra-K rocks were derived from metasomatic veins related to melting of recycled subducted sediments, but the high-K calc-alkaline rocks from a lithospheric source metasomatized by fluids from subduction zone.
Early Miocene (ca. 21–18 Ma) volcanism in the Karacadaǧ area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly‐alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post‐collisional tectonic setting. The calcalkaline suite rocks with medium‐K in composition resemble those of subduction‐related lavas, whereas the mildly‐alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within‐plate lavas. Incompatible element and Sr‐Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacadaǧ area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction‐related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post‐collision volcanism.
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