LATE MESOZOIC TO LATE CENOZOIC MAGMATIC EVOLUTION OF THE EASTERN PONTIDES, NE TURKEY

Anatolia is formed by mainly four major tectonic blocks, separated by three main high-pressure (HP) belts. Eastern Pontides (EP) is a subset of Sakarya zone, which is one of these tectonic blocks. The EP extends for 450 km along the north eastern coast of Turkey. Late Cretaceous to late Miocene magmatic activity yielded several volcanic and plutonic rocks of different age and compositions.

Late Cretaceous is represented by (1) high-K calc-alkaline and calc-alkaline I-type (~75-91 Ma, Ar-Ar on hornblend, U-Pb on zircon and Pb-Th on monazite) and shoshonitic A-type (~75-81 Ma, U-Pb on zircon) plutonic rocks, and (2) calc-alkaline (Turonian-Campanian) and shoshonitic volcanic rocks (80 Ma, Ar-Ar on hornblende). All the rocks are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display I_Sr = 0.70676 to 0.70736, ε_Nd(79 Ma) = -4.4 to -3.3, with T_DM= 0.94 to 1.36 Ga. The lead isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.79-18.87, (²⁰⁷Pb/²⁰⁴Pb) = 15.59-15.61 and (²⁰⁸Pb/²⁰⁴Pb) = 38.71-38.83. All isotopic compositions suggest that the magma has a dominantly mafic lower crustal source, though a minor mantle contribution is possible.

Late Paleocene-Early Eocene are characterized by high-K calc-alkaline adakite-like volcanic and adakite-like granitoid porphyre rocks (~ 50-54 Ma, Ar-Ar on hornblende) having a relative high I_Sr (0.70474-0.70640) and low ε_Nd (50 Ma) values (-2.3 to 0.8), which is consistent with an origin as partial melts of a mafic lower crust, and spessartite type lamprophyres (~ 48 Ma, Ar-Ar on hornblende). Their relative high I_Sr = 0.70429 to 0.71291, ε_Nd(48 Ma) = -1.4 to 3.6, with T_DM= 0.48 to 2.22 Ga and lead isotopic ratios [(²⁰⁶Pb/²⁰⁴Pb) = 18.35-18.63, (²⁰⁷Pb/²⁰⁴Pb) = 15.51-15.60 and (²⁰⁸Pb/²⁰⁴Pb) = 38.29-38.87] accord with partial melting of isotopically and chemically enriched subcontinental lithospheric mantle (SCLM).

Middle Eocene aged rocks are typical of high-K calc-alkaline I-type granitoids (40-45 Ma, K-Ar on biotite, U-Pb on zircon, Pb-Th on monazite and Pb-Th on uraninite) and high-K calc-alkaline and calc-alkaline volcanics. Initial Nd-Sr isotopic compositions for the rocks are ε_Nd(43 Ma) = -0.6 to 0.8 and mostly I_Sr = 0.70482-0.70548. The Nd model ages (T_DM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.60-18.65, (²⁰⁷Pb/²⁰⁴Pb) = 15.61-15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.69-38.85. These isotopic characters indicate that the magma has a dominantly SCLM, which is chemically enriched, but isotopically depleted in composition, though a minor mafic lower crustal melt contributes to the generation.

Neogene alkaline volcanics (5-13 Ma, K-Ar on biotite and Rb-Sr) are last magmatic products of the EP. The rocks are slightly depleted and homogeneous in isotopic composition, with respect to ⁸⁷Sr/⁸⁶Sr (ranging 0.705018 to 0.705643) and ¹⁴³Nd/¹⁴⁴Nd (ranging 0.512662 to 0.512714) that indicates young Nd model ages (0.51-059 Ga). This may imply that the parent melts tapped a homogeneous and young lithospheric mantle source, metasomatized by subduction-derived sediments during the Late Mesozoic. Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb= 18.85-18.95; ²⁰⁷Pb/²⁰⁴Pb = 15.60-15.74; ²⁰⁸Pb/²⁰⁴Pb= 38.82-39.25) may also be consistent with a model for an enriched SCLM source.

Geochemical and age data, combined with regional studies, show that the late Cretaceous magmatism formed in a subduction setting resulting from northward subduction of northern branch of Neo-Tethyan oceanic crust beneath the Eurasian plate and that the back-arc extensional period started by least ~79 Ma in the EP. A magmatic gap, coinciding with collision period, is present from late Cretaceous to late Paleocene in the region. After the collision period, first magmatic records, which are adakitic characters in composition, occurred at ~ 50 Ma. The rocks represent initial stage of crustal thinning caused by crustal extensional events in the EP. The spessartite type lamprophyres, having similar ages with the adakitic products, are evidence of initial stage of post-collisional extensional events in the region. From middle Eocene to late Miocene, lithospheric thinning and resultant upwelling of asthenosphere induced by lithospheric extension is responsible to magma generation in the EP. In this regard, the Eastern Pontides recorded a complex history of subduction-, collision- and extension related magmatic episodes from late Cretaceous to late Miocene.