Unravelling metamorphic ages of suture zone rocks from the Makran and Sabzevar areas (Iran): new constraints for the temporal evolution of the Arabia-Eurasian collision zone
Final Report Abstract
Many aspects of the geological, tectono-metamorphic and magmatic evolution of suture zones surrounding the Central Iranian Microcontinent (CIM) have been deciphered in recent years, but knowledge about the timing of regional metamorphic processes remained fragmentary. The aim of our studies in two spatially widely separated areas was to better constrain the time frame for subduction processes in the periphery of the CIM. We have dated metamorphic rocks from mélanges of the Makran and Sabzevar regions to determine age constraints for different P–T stages in their metamorphic histories, either operating at the same time or successively, and to help resolving spatial and temporal patterns in the geodynamic evolution on a broader regional scale. The new data provide essential and precise age constraints for further refinement of geodynamic concepts, and can be summarized as follows: 1. The newly dated blueschists from the Sabzevar zone yielded Rb–Sr ages of 56.52 ± 0.32, 57.20 ± 0.32 and 55.15 ± 0.33 Ma that are interpreted to closely approximate the time of mineral crystallization/recrystallization at HP/LT peak conditions. 2. These ages provide the first unambiguous evidence for Late Paleocene-Early Eocene high-pressure metamorphism in this area. The existence of pre-58 Ma blueschists is currently not corroborated by direct isotopic dating of such rocks but could possibly be deduced indirectly from field relationships indicating that 58 ± 2 Ma granitoids intruded blueschist-bearing mélanges. 3. An Eocene age for HP/LT metamorphism was only determined in the Sabzevar zone at the northern margin of the CIM; similar rocks from other suture zones around the CIM only yielded Late Cretaceous ages. 4. The new geochronological data for the northern part of the Makran zone further substantiate the geological relevance of a Cretaceous HP/LT episode. Rb–Sr internal mineral isochrons for samples from the coloured mélange of the Iranshahr area yielded a remarkable homogeneous age group, regardless of metamorphic grade. All blueschist- and amphibolite-facies samples gave ages of ca. 81–84 Ma that overlap within analytical uncertainty, suggesting a combination of tectonic mixing during exhumation and very rapid exhumation. U–Pb zircon dating of an amphibolite indicate a metamorphic age of ca. 88 Ma. 5. The new Rb–Sr and U–Pb ages are similar to many of the previously reported K–Ar dates from the Iranshahr and Fannuj area, but much more precise, and suggest that most of the existing data had not significantly been compromised by contamination with extraneous Ar. 6. Indications for poorly constrained older blueschist- and/or amphibolite-facies metamorphism that might be recorded by some K–Ar dates could not yet be verified but can be reconciled with the larger geological frame implying that northward subduction of the Arabian plate beneath the Bajgan-Durkan continental block started shortly after ca. 120 Ma.
Publications
- (2021). Unravelling metamorphic ages of suture zone rocks from the Sabzevar and Makran areas (Iran): robust age constraints for the larger Arabia-Eurasian collision zone. Journal of Metamorphic Geology
Bröcker, M., Omrani, H., Berndt, J., Moslempour, M.E.
(See online at https://doi.org/10.1111/jmg.12603)
