Silurian A-type metaquartz-syenite to -granite in the Eastern Anatolia: Implications for Late Ordovician-Silurian rifting at the northern margin of Gondwana

The Anatolide-Tauride block in the Tethyan belt represented passive northern continental margin of Gondwana throughout Paleozoic time. In this paper, we report on a relatively large amphibolite-facies metaquartz-syenite to -granite complex (~ 60 km²) in the Eastern Anatolia, the easternmost part of the Anatolide-Tauride block. Igneous textures in the metaintrusive complex are variably preserved. The preserved igneous minerals include mesoperthite + quartz ± aegirine-augite ± sodic amphibole ± calcic amphibole ± biotite and ± allanite. Mesoperthite is the sole feldspar in well-preserved igneous textures, suggesting hypersolvus nature of the original quartz syenite to granite. In the well-recrystallized samples, mesoperthite is partially or totally replaced by discrete grains of plagioclase and K-feldspar. U–Pb dating on igneous zircons from three samples yielded Silurian igneous crystallization ages (430–440 Ma). All rock types have typical geochemical characteristics of A-type syenite to granite with relatively high abundances of FeO*, Na₂O, K₂O, Nb, Ga, Zr, Ce, and Zn, and low abundances of MgO, CaO and Sr, resulting in high ratios of Fe/Mg and Ga/Al. Geochemical features such as (i) the absence of negative Nb–Ta anomalies on multielement variation diagrams, (ii) the enrichment of Nb and Ce relative to Y, and (iii) Y/Nb, Yb/Ta and Ce/Nb ratios similar to those in ocean island basalts point to A₁-type syenite and granite which are thought to be differentiates of ocean island basaltic magmas. Estimated magma temperatures based on zirconium saturation range from 850 to 990 °C, indicating derivation of the original quartz syenite and granite from high-temperature juvenile magmas. Initial εHf values of the igneous zircons from two samples are 2.59 ± 1.14 and 2.30 ± 1.29, while δ¹⁸O values of the zircons range from 5.41 to 7.32‰ with a median of 6.50 ± 0.48‰. All these isotopic characteristics suggest that the quartz syenite to granite were derived from high-temperature mantle magmas with insignificant crustal assimilation. The Late Ordovician-Silurian A-type igneous rocks in the northern part of the Anatolide-Tauride block were probably related to a rifting event at the northern margin of Gondwana, leading to the opening of the Paleo-Tethys. This interpretation is also supported from (i) regional geological features of the Early Paleozoic sedimentary successions suggestive of extensional setting during the Late Ordovician-Silurian, (ii) the presence of Silurian anorogenic magmatism in continental blocks detached from Gondwana during Early Paleozoic time, and (iii) the presence of Silurian deep sea sedimentary blocks in Paleo-Tethyan accretionary complexes.