ALBERT

Description: 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 centimetre-sized garnet in amphibolite-facies metasediments yielded a crystallization age of 113.10 ± 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 thermodynamic modelling and Ti-in-zircon thermometry. Rutile crystals from two leucosomes yield Zr-in-rutile temperatures 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.

Description: We investigate the late Cretaceous blueschist-facies (480 °C-1.8 GPa) segment of the Zagros suture zone, a well-preserved block-in-matrix paleo-subduction channel. We aim to determine the relative chronology, conditions of deformation, and potential fluid sources and processes associated with the widespread occurrence of lawsonite + clinopyroxene + glaucophane veins and aragonite-bearing hydraulic breccias. We use a multi-scale approach methodology to provide new insights into deep fluid flow mechanisms as well as to constrain possible sinks of CO2-bearing fluids in the subducting slab. Petrological analyses suggest that silicate-rich vein systems began precipitating during early burial and evolved with ongoing burial and shearing-related deformation in the blueschist-facies, while most carbonate-rich veins and hydrofractures formed at near-peak P-T conditions. In situ LA-ICP-MS trace element analyses reveal that: (i) individual silicate host-vein pairs have similar REE signatures, reflecting local-scale fluid-mediated element redistribution, (ii) carbonate-bearing veins and metasediments also have similar trace element signatures and (iii) lawsonite in blueschist-hosted veins exhibit REE enrichments along their rims, suggesting an increasing contribution of metasedimentary-derived fluids upon approaching peak P-T. Carbonate Osingle bondC isotope compositions of the veins and metasedimentary rocks range from +13.6 to +17.9‰ (δ18OVSMOW) and − 1.0 to +3.1‰ (δ13CVPDB), demonstrating metasedimentary-derived fluid sources related to large-scale H2O homogenization with far-traveled mafic- ultramafic-derived fluids. Srsingle bondNd isotopic ratios in carbonate veins and the adjacent host resemble their host composition indicating that host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix, possibly during episodic porous flow. Thermodynamic modeling predicts that decarbonation via fluid-assisted reactions is inefficient at blueschist-facies and that carbon release likely occurs deeper along the subduction interface (i.e., at eclogite-facies). We propose that deeply produced H2O-rich fluids interacted with the carbonate-bearing lithologies along the subduction interface facilitating fluid-mediated decarbonation and further fluid transport as hydraulic pulses (e.g., porosity waves) that traveled at the kilometer-scale parallel to the subduction interface, (i) contributing to the isotopic homogenization herein observed and (ii) triggering episodic hydrofracturing in the lawsonite-blueschist-facies (≈50-60 km depth). Veinsets in exhumed subducted rocks hence provide a unique opportunity to understand fluid-rock interaction processes in the region at which episodic tremor and slow slip events phenomena occur.