Mineral chemistry, whole-rock geochemistry and petrology of eocene ı-type shoshonitic plutons in the Golkoy area (Ordu, NE Turkey)

Abstract

The Eocene intermediate to felsic plutons are widespread in varying sizes and compositions throughout the Eastern Pontides Orogenic Belt in NE Turkey. Of these, two monzonitic bodies (namely the Eriko Tepe and Gol Tepe Plutons) in the Golkoy (Ordu) area, extend nearly in the orientation of NW-SE and E-W and were emplaced into the Upper Cretaceous and/or Eocene volcanic and sedimentary rocks. Petrographically, the studied monzonitic plutons are compositionally fine to medium grained monzonite, monzodiorite and subordinate quartz-monzonite. They consist of plagioclase (An(35-67)), K-feldspar (Or(61-96)), quartz, clinopyroxene (Wo(28-49)En(35-51)Fs(10-25)), biotite (Mg#: 0.53-0.73) +/- hornblende (Mg#: 0.65-0.82), Fe-Ti oxide with monzonitic, poikilitic, perthitic, rare antirapakivi and graphic textures. Mineral thermobarometer estimations imply that the plutons were crystallized in P-T conditions of mid to shallow crustal levels. Petrochemically, these monzonitic plutons show post-collisional, I-type, metaluminous (A/CNK=0.76-0.93) and shoshonitic features. The whole-rock major oxide and trace element variations suggest that fractionational crystallization played a significant role in the evolution of these monzonitic plutons. The primitive mantle-normalized trace element patterns of the studied plutons are similar to each other with enrichment in large ion lithophile elements, Th, Ce and negative Nb and Ti anomalies. Moreover, the chondrite-normalized rare earth element plots of the plutons show moderately enriched concave-shaped patterns (La-N/Lu-N = 9.3-12.6) with negative Eu anomalies (Eu-N/Eu*=0.69-0.84), all of which imply plagioclase and clinopyroxene +/- hornblende fractionations during their evolution. The geochemical data suggest that the monzonitic plutons have evolved from parental magmas derived from the melts of enriched lithospheric mantle, in a post-collisional setting.