Cretaceous thermal evolution of the closing Neo-Tethyan realm revealed by multi-method petrochronology

A Cretaceous paleo-accretionary wedge, the Ashin Complex, now exposed along the Zagros suture zone in southern Iran, exhibits mafic, metasedimentary and ultramafic lithologies. Field, geochemical and petrological observations point to an anomalous high-temperature event that gave rise to the formation of peritectic (trondhjemitic) melts associated with restitic garnet-bearing amphibolites. Lu-Hf isotopic dating of centimetresized garnet in amphibolite-facies metasediments yielded a crystallization age of 113.10 & PLUSMN; 0.36 Ma, possibly representing the age of prograde to near-peak metamorphic conditions. SHRIMP U-Th-Pb zircon dating from trondhjemitic leucosomes yields crystallization ages of 104 +/- 1 Ma, interpreted as the age of the temperature peak, which occurred in the upper amphibolite-facies (c. 650-680 C at 1.1-1.3 GPa), according to thermody-namic modelling and Ti-in-zircon thermometry. Rutile crystals from two leucosomes yield Zr-in-rutile temper-atures in the range of 580-640 C and a LA-ICP-MS U-Pb age range from 85 to 112 Ma, interpreted as a consequence of partial re-equilibration during incipient cooling. A late static recrystallization event is indicated by the presence of sodic-calcic clinopyroxene, sodic amphibole, Si-rich phengite, titanite overgrowths after rutile and lawsonite within former leucosomes and late fractures. This mineral assemblage is a typical blueschist-facies (high pressure-low temperature) paragenesis and is interpreted as reflecting long-term isobaric cooling that occurred until the end of the Cretaceous as a consequence of increasing slab thermal age. This first report of a melting event in the Zagros paleo-accretionary wedge reveals the presence of a transient, abnormally high thermal gradient of c. 18 C/km that occurred at c. 105-113 Ma. We speculate that this could be explained by the subduction of a thermal anomaly such as a seamount chain, a transform fault system or, more likely, a spreading ridge under the southern Iranian margin. Indeed, paleogeographic reconstructions of the Tethyan realm suggest the entrance of the Northern Tethyan basin ridge into the subduction zone shortly after 120 Ma.

Automated summary

The Ashin Complex is a Cretaceous paleo-accretionary wedge exposed in southern Iran's Zagros suture zone, consisting of mafic, metasedimentary, and ultramafic lithologies. A high-temperature event is believed to have created peritectic melts associated with restitic garnet-bearing amphibolites. Lu-Hf isotopic dating suggests a crystallization age of 113.10 +/- 0.36 Ma for garnet in amphibolite-facies metasediments, while SHRIMP U-Th-Pb zircon dating indicates a temperature peak age of 104 +/- 1 Ma. The presence of sodic-calcic clinopyroxene, sodic amphibole, Si-rich phengite, titanite overgrowths after rutile, and lawsonite suggests a late static recrystallization event. The melting event indicates a transient, abnormally high thermal gradient and could be attributed to the subduction of a thermal anomaly.