Petrogenesis of the Late Cretaceous Demirkoy Igneous Complex in the NW Turkey: Implications for magma genesis in the Strandja Zone

摘要

The Srednogorie Zone in Bulgaria and its eastern continuation in NW Turkey, the Strandja Zone, consist of a Late Cretaceous magmatic belt comprised of plutonic, volcanic and associated volcano-sedimentary rocks. The Demirkoy Igneous Complex is the biggest Cretaceous pluton within the Strandja Zone, and its age is between 71 and 84 Ma. The Demirkoy pluton and additional smaller plutons are intruded into Triassic and older metamorphic basement rocks. A contact metamorphic aureole with hornblende-hornfels facies conditions was developed around the Demirkoy pluton.The WNW-trending Demirkoy pluton has an elliptical shape and a concentrically zoned internal structure. In this structure, gabbro/diorites and quartz diorites are only seen in the SW corner of the pluton, while granodiorites form the main body of the pluton surrounded by a zone of granite. The granodiorites contain dioritic mafic microgranular enclaves. Small quartz monzonite lenses and/or bodies and mafic dykes are also observed in the Demirkoy Igneous Complex.Geochemically, the Demirkoy Igneous Complex has calc-alkaline, metaluminous and medium-K characteristics. There is a significant correlation between the LREE/HREE ratios and SiO2 values. Chondrite-normalized REE patterns are relatively flat (La_N/Lu_N = 2) in gabbros. This ratio ranges from 3 to 11 for the diorite/ granodiorites, and from 14 to 35 for the granites. Dioritic enclaves have REE patterns similar to those of the main body and have La_N/Lu_N values between 3 and 6. Relatively enriched LILE (Sr, K, Rb, Ba and Th) and relatively depleted HFSE (Ta, Nb and Ti) values indicate the classic subduction-related origin of the complex. The initial isotopic signatures range from ε_(Nd(i)) = -0.45 to -2.57 and ~(87)Sr/~(86)Sr(i) = 0.7042-0.7064 for gabbros and ε_(Nd(i)) = - 0.43 to - 2.67 and ~(87)Sr/~(86)Sr(i) = 0.7048-0.7059 for granodiorites and granites. δ~(18)O values range from +6.18 to +7.80‰ (VSMOW) for gabbros and from +6.86 to +8.89‰ (VSMOW) for granodiorites and granites.Based on geochemical and isotopic characteristics this magmatic complex can be subdivided into 4 groups: a) gabbro, b) diorite to alkali granite (main body), c) syenite/monzonite and d) mafic dykes. Geochemical data and modelling results favour a typical subduction-related enriched lithospheric mantle and lower crustal source for the Demirkoy lgneous Complex. Slab-derived fluids and crustal contamination also played a key role in the petrogenesis of this magmatic suite.