Origin of the Early-Middle Devonian magmatism in the Sakarya Zone, NW Turkey: Geochronology, geochemistry and isotope systematics

Abstract

A unique example of isotopically-dated Devonian metagranitoid (the Çamlık metagranite; Okay et al., 1996) crops out in the Biga Peninsula, NW Turkey, although its contact relationships with the country rocks and geodynamic setting have remained to be enigmatic so far. Our field work, however, has shown that a number of metagranitoid bodies similar to the Çamlık metagranite intruded the country rocks and developed contact metamorphic zones, consisting of andalusite and calcsilicate hornfelses, garnet–epidote and diopsite–wollastonite skarns. The country rocks of these metagranitoids are made up of regionally metamorphosed metaclastic successions with subordinate metacarbonate–metachert–metabasites (the Kalabak formation), intercalated with tectonic slices of meta-serpentinites. The metagranitoids and the Kalabak formation are collectively termed here the Havran Unit which forms a NE–SW trending, 20 km wide and 80 km long belt in the Biga Peninsula. The Havran Unit is unconformably overlain by the Late Triassic shallow marine sediments and is in tectonic contact with the Permo-Triassic Karakaya Complex, interpreted as the Palaeotethyan subduction–accretion complex. U–Pb SHRIMP-II and LA–ICP–MS dating of the zircons from four individual plutons yielded crystallization ages ranging from 389.1 ± 2.6 to 401.5 ± 4.8 Ma (i.e. Early to Middle Devonian). Based on their geochemical characteristics, the metagranitoids are divided into two groups named the Çamlık and Yolindi metagranitoids. Both granitoids display distinct subduction signature and plot in the volcanic arc granite and post-collisional granite fields on tectonic discrimination diagrams. 87Sr/ 86Sr ( T ) values of these intrusions vary between 0.707367 and 0.715588 while their 143Nd/ 144Nd ( T ) values range from 0.51166 to 0.51187. Very low values of εNd ( T ) (from −5.3 to −9.1) imply that the Lower to Mid-Devonian metagranitoids in the Havran Unit might have been generated by partial melting of lower continental crust, probably driven by a major magma underplating event. Delamination and/or slab-breakoff models are considered to be the most plausible mechanisms for the genesis of these metagranitoids. The data obtained in this study has shown for the first time the widespread existence of Devonian and older basement in NW Turkey. Regional geological correlations imply that this basement may be an exotic terrane in the Sakarya Zone, emplaced during the Late Triassic, prior to its amalgamation with the Palaeotethyan Karakaya Complex.