Transition from arc- to post-collision extensional setting revealed by K-Ar dating and petrology: an example from the granitoids of the Eastern Pontide Igneous Terrane, Araklı-Trabzon, NE Turkey

Abstract

The Eastern Pontide Igneous Terrane (EPIT) includes several Cretaceous to Neogene intrusive rocks, ranging in composition from low-K tholeiitic gabbros through calc-alkaline and high-K calc-alkaline metaluminous granitoids or peraluminous leucogranites to alkaline syenites. Such high diversity in age and composition is also accompanied by a broad spectrum in terms of geodynamics—i.e. from arc through syn-collisional thickening to post-collisional extensional regimes. Shallow-seated porphyritic acidic to intermediate rocks are from oldest to youngest, on the basis of field relations, the Gundogdu altered microgranite, the Bogali K-feldspar-megacrystic monzogranite and the Uzuntarla porphyritic granodiorite. These rocks, exposed in the southern part of the Arakli region, east of Trabzon, Turkey, were studied in terms of their mineralogy and petrography, whole-rock geochemistry and hornblende K–Ar dating. The mineralogical and geochemical data reveal an apparent diversity in incompatible-element enrichment and depletion, for the Bogali unit and Uzuntarla unit, respectively. The Bogali and Uzuntarla units yield hornblende K–Ar ages ranging from 75.7 ± 1.55 to 61.4 ± 1.47 Ma and from 42.4 ± 0.87 to 41.2 ± 0.89 Ma, respectively. The diversity in both mineralogy–geochemistry and hornblende K–Ar ages suggests that the Bogali and Uzuntarla units are parts of the Cretaceous arc and Eocene post-collision extensional-related igneous activity, respectively, in the EPIT of northern Turkey. Copyright © 2005 John Wiley & Sons, Ltd.