Causes, melt sources and petrological evolution of adakitic magmatism in NW Turkey

We document the occurrence of adakite like porphyries in NW Anatolia and report a comprehensive data set (petrography, major-trace element geochemistry, Sr-Nd isotope compositions and 40Ar/39Ar radiometric ages) from them in order to evaluate the causes, melt sources and petrological evolution of Adakitic magmatism in NW Turkey. A number of Eocene plutons and porphyries emplaced into the Izmir-Ankara suture zone (IASZ), which represents the collision zone between Anatolide-Tauride platform and Sakarya continent. Adakite-like porphyries form stocks, sills and dikes that are spatially and temporally associated with Eocene plutons emplaced into ophiolitic and blueschist rocks of Tavşanli{dotless} zone. Petrographically, they can be classified as porphyritic microgranite, and microgranodiorite. They display holocrystalline, microgranular porphyritic textures, and composed mainly of plagioclase, K-Feldspar, quartz, hornblend and biotite. Geochemically, adakitic rocks have high SiO₂ (69.43-63.80), Al₂O₃ (15.44-16.64) contents, high Sr/Y (113.39-214.92) and La/Yb(n) (35.33-80.18) ratios and low Y(6.2- 11.5), Yb (1.07-0.54) contents, and display enrichments of LILE and LREE, depletion of HFSE and lack of Eu anomaly. Our 40Ar/39Ar ages obtained from adakite-like porphyries (53.7 to 54.0 Ma) indicate that adakitic magmatism occured during the Early Eocene in NW Anatolia, as documented in Eastern Pontides (NE Turkey). They have moderately enriched ⁸⁷Sr/⁸⁶Sr (54) (0.70620-0.70660) and low εNd (54) -2.73 to 0.59). Geochemistry and isotope compositions of adakitic porphyries indicate that they most likely to be produced by partial melting of garnet-bearing lower crustal lithologies and its interaction with mantle melt. Assimilation combined with fractional crystallization (AFC) was played an important role during the evolution of these melts at shallower crustal levels. Geochronology, geochemistry and geology of Adakitic porphyries and geology of the NW Anatolia indicate that adakitic magmatism was not formed above an actively dehydrating subducted slab. Instead, our observations are consistent with a magmatism that is more typical of syn-convergent extensional tectonic settings. Formation of adakitic melts require increased heat flow at about 54 Ma that might have occurred by slab break-off.