One of the pre-Jurassic metamorphic basements of the Western Sakarya Zone (NW Turkey) is the Havran Unit which contains metasedimentary (i.e. the Kalabak formation) and metaintrusive rocks of Devonian age. An LA-ICP-MS U-Pb zircon dating study was conducted on 125 zircon grains obtained from a metasedimentary rock to better understand the origin and evolution of the Kalabak formation. Around 107 of 175 spot analyses are concordant with rates 90–110% and the zircon ages vary between 426 ± 5.2 and 3406.9 ± 195.7 Ma. Zircon populations cluster in ranges: 426 ± 5.2–535 ± 13Ma (8.4%, Palaeozoic), 549 ± 14–999 ± 11Ma (54.21%, Neoproterozoic), 1012 ± 200–1543 ± 50Ma (6.54%, Mesoproterozoic), 1736 ± 123 –2414 ± 58Ma (18.7%, Palaeoproterozoic), and 2512 ± 30–3406.9 ± 195.7 Ma (12.15%, Archean). Dominating Neoproterozoic ages notate Cadomian-Avalonian terrains and Mesoproterozoic ages should mainly be related to Avalon terrains. The Karacabey metagranite consists of coarse-grained, equigranular, hypidiomorphic, and rarely foliated monzogranite and granodiorite. It is located to the north of the town of Karacabey in the eastern part of the Biga Peninsula. Main mineral assemblage is quartz, K-feldspar, plagioclase, and biotite. Zircon, apatite, monazite, magnetite, and sphene occur as accessory phases. Aluminum Saturation Index (ASI) values vary between 0.91 and 1.63, indicating the pluton as a metaluminous-peraluminous, I-type granite. The metagranite samples are plotted in the high-K-calcalkaline field on the SiO2 vs. K2O diagram. N-MORB-normalised spidergrams display a profound enrichment in the large-ion lithophile elements (LILE), light rare earth elements (LREE), and depletion in high field strength elements, suggesting that its genesis relates to hydrous melting of a mantle wedge in a subduction zone and/or melting of subduction zone-influenced source areas. Chondrite-normalised rare earth element spidergrams are indicative of the importance of plagioclase and amphibole fractionation. In tectonic discrimination diagrams, all of the metagranite samples fall into the volcanic arc granite and late post-collisional granite fields. U/Pb LA-ICP-MS zircon dating of the metagranite yielded an age of 400.3 ± 1.4 Ma (Early Devonian), interpreted as the age of igneous crystallisation. These age data show that the Karacabey metagranite is a member of the lower to mid Devonian granites, recently described from the Biga Peninsula.
In light of these findings, we argue that the Havran Unit was an exotic terrain which possibly was once a part of far-travelled Avalon terrains and was attached to Sakarya Zone.
Keywords: Western Sakarya Zone; Avalon terrain; detrital zircon; Devonian magmatism; NW Turkey