Eocene tonalite-granodiorite from the Havza (Samsun) area, northern Turkey: adakite-like melts of lithospheric mantle and crust generated in a post-collisional setting

摘要

Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Taslik Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (+/- 1.4) and 40.5 (+/- 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95-1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO2 (67-68 wt%) and Al2O3 (15.5-16.0 wt%) content and Ba/La (17-23), Sr/Y (40.7-61.6), and La-N/Yb-N (14.4-23.7) ratios and low Y (8.2-9.9 ppm) and Yb-N (3.1-4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (La-N/Lu-N = 14.2-21.6) with insignificant negative Eu anomalies (Eu-N/Eu* = 0.86-1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have Sr-87/Sr-86 ratios of 0.704767 to 0.704927, Nd-143/Nd-144 ratios of 0.512767-0.512774, epsilon Nd values of (+2.52) - (+2.65), and delta O-18 values of 7.9-9.7 parts per thousand. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.