Unravelling metamorphic ages of suture zone rocks from the Sabzevar and Makran areas (Iran): Robust age constraints for the larger Arabia–Eurasian collision zone

• Published in: Journal of metamorphic geology 2021-12, Vol.39 (9), p.1099-1129
• Main Authors: Bröcker, Michael, Omrani, Hadi, Berndt, Jasper, Moslempour, Mohammad E., White, Richard
• Format: Article
• Language: English
• Subjects: Age; Amphibolite facies; Amphibolites; blueschists; Contamination; Cretaceous; Eocene; Geochronology; Geochronometry; Lead; Lithosphere; Makran; Metamorphism; Metamorphism (geology); Mountains; Outcrops; Palaeocene; Paleocene; Plates; Potassium; Rock; Rocks; Sabzevar; Sedimentary facies; Subduction; Subduction (geology); Subduction zones; Tectonics; Uranium; Zircon
• ISSN: 0263-4929; 1525-1314
• DOI: 10.1111/jmg.12603

In this study we establish a time frame for metamorphic processes affecting Neotethyan suture zones at the northern (Sabzevar) and southern (Makran) margin of the Central Iranian Microcontinent. For this purpose, we have applied Rb–Sr and U–Pb geochronology to blueschist‐ and amphibolite facies rocks from these occurrences. So far, no isotopic ages are available for high‐P/low‐T rocks from the Sabzevar area which are exposed in the Govain and Chili mountains near Soltanabad. Three newly dated blueschists from the Govain mountain yielded Rb–Sr ages of 56.52 ± 0.32, 57.20 ± 0.32, and 55.12 ± 0.33 Ma respectively. These ages provide the first unambiguous evidence for late Palaeocene to early Eocene high‐P metamorphism in the Sabzevar zone and confirm the accuracy of earlier age assignments that were derived indirectly. The clearly confirmed age difference between late Palaeocene–early Eocene blueschists of the Govain mountain and kilometre‐scale dismembered HP granulites with Early Cretaceous (Albian) metamorphic ages from outcrops c. 60 km to the west of Soltanabad further corroborates interpretations that these rocks are not related to the same subduction zone complex. The blueschists record closure of an Albian to Late Cretaceous back‐arc basin, whereas the granulites formed due to subduction of older oceanic lithosphere. Uranium–Pb zircon dating of an amphibolite from the Iranshahr region in the Makran Zone yielded an intercept age of 87.8 ± 1.8 Ma and weighted mean age of 88.1 ± 1.7 Ma. Rb–Sr mineral isochrons for seven blueschist‐ and amphibolite facies samples from the same area generated very consistent ages of c. 84 Ma (± 85 Ma) have not yet been confirmed but can be reconciled with the larger geological frame implying that northward subduction of the Arabian plate beneath the Ba