Petrography, geochemistry, petrology and isotope sistematics of gabbroic rocks in the Kovanlık (Bulancak-Giresun) area

Late Cretaceous aged felsic intrusions are common in varying ages, sizes and compositions but mafic ones are very limited in the Eastern Pontides. One of the mafic ones, namely the Kovanlık Pluton, cut Late Cretaceous-aged volcanic and volcaniclastic rocks. Petrographically, the Kovanlık Pluton is predominantly pyroxene gabbro and lesser hornblende gabbro in composition with phaneritic, ophitic and rarely porphyritic textures. The pyroxene gabbros comprise of plagioclase (An54-92), clinopyroxene (Wo31-49En41-60Fs1-5), ortho-pyroxene (Wo1-4En66-74Fs24-31) and Fe-Ti oxides, whereas the hornblende gabbros contain plagioclase (An57-87), hornblende, and lesser biotite and Fe-Ti oxides. Based on whole-rock geochemistry, the studied gabbroic rocks show tholeiitic-calc-alkaline transition characteristics. Differences in the major and trace element variations can be explained by the fractionation of cpx ± opx ± plg ± Fe-Ti oxide in pyroxene gabbros and that of hbl ± plg ± Fe-Ti oxide in hornblende gabbros. Trace and REE distrubitions of these rocks indicate derivation of similar origin. Primitive mantle-normalized multi-element variations exhibit slight enrichment in large ion lithophile elements, as well as depletion of Th, Zr, Nb, La, and Ce, thus revealing that gabbroic samples evolved from a parental magma derived from an enriched mantle source. The pyroxene gabbros have 87Sr/86Sr ratios 0.70471-0.70652, 143Nd/144Nd ratios 0.512870-0.512884 and εNd values (+4.53)-(+4.80), whereas the hornblende gabbros have 87Sr/86Sr ratio 0.70493, 143Nd/144Nd ratio 0.512871 and εNd value (+4.55). All these data suggest that the parental magma(s) of the Kovanlık Pluton have derived from lithospheric mantle enriched by subduction fluids in the extensional arc setting.