Reappraisal of the Karamustafa mafic rocks (Eastern Pontides, NE Türkiye): A Late Cambrian orthoamphibolite roof pendant recording incipient Rheic back-arc extension

Eyüboğlu Y.

LITHOS AN INTERNATIONAL JOURNAL OF MINERALOGY, PETROLOGY, AND GEOCHEMISTRY, cilt.544, sa.545, ss.1-7, 2026 (SCI-Expanded, Scopus)

Özet

The Karamustafa mafic rocks in the Eastern Pontides Orogenic Belt (EPOB, NE Türkiye) were previously interpreted as Late Triassic lamprophyric intrusions based principally on amphibole 40Ar/39Ar geochronology. Here I present zircon Usingle bondPb geochronology, zircon Hf isotope and trace-element data, whole-rock geochemistry, and Sr–Nd–Pb isotope data showing that these rocks are instead Late Cambrian orthoamphibolites preserved as a roof-pendant–like basement fragment within the Late Carboniferous Gümüşhane Pluton. Zircon Concordia ages of 498.8 ± 3.7 Ma for Group 1 and 492.0 ± 2.3 Ma for Group 2 contrast with ages of 323.3 ± 2.1 Ma and 317.3 ± 1.2 Ma from crosscutting felsic intrusions, establishing a ∼ 180 m.y. age gap. The orthoamphibolites exhibit supra-subduction geochemical signatures, including negative Nbsingle bondTa and positive Pb anomalies, weakly enriched to nearly flat REE patterns, and positive εNd(t) and zircon εHf(t) values. The ∼7 m.y. transition from the more arc-influenced Group 1 to the more depleted-mantle-dominated Group 2 records increasing juvenile mantle input during progressive extension. The Karamustafa orthoamphibolites constitute the first directly dated Late Cambrian basement assemblage recognized within the magmatic core of the EPOB and provide the first regional record of incipient Rheic back-arc development. Together with previously documented Devonian–Carboniferous oceanic and supra-subduction assemblages, this discovery extends the preserved Rheic record of the Eastern Pontides back to its initial opening stage and indicates that the belt records both the opening and closure of the Rheic Ocean system.