Transient, synobduction exhumation of Zagros blueschists inferred from P‐T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics

Abstract

[1] We present the first P-T, deformation time, and kinematic constraints on the only known blueschist facies rocks (BS) present in the Zagros (Hajiabad area). The BS were underplated below the Sanandaj-Sirjan zone and crop out as kilometer-scale bodies within extensive colored melange units marking discontinuously the Neotethyan suture zone. P-T estimates point to high-pressure/low-temperature (HP-LT) conditions around 11 kbar and 520-530°C for the majority of BS, along a $15°C km À1 gradient. Some exotic blocks in matrix serpentinite reached 17-18 kbar at $500°C. In situ laser probe 40 Ar-39 Ar radiometric age constraints on phengite cluster between 85 and 95 Ma and suggest that (1) synconvergence exhumation of Zagros BS from 35-50 km to depths <15-20 km was accomplished before 80 Ma, (2) BS exhumation corresponded to a transient process with respect to the long-lived subduction beneath Iran ($150-35 Ma), and (3) age constraints for Zagros BS are 5-10 Myr older than for the nearby Oman HP-LT rocks and broadly coincide with obduction processes in the region (circa 95-70 Ma). We propose that the mechanical coupling across the Neotethyan subduction zone (NSZ) beneath Iran was modified by the large-scale plate rearrangement accompanying obduction, allowing for a short-lived exhumation of Zagros BS. Exhumation ceased at the end of obduction, when subduction of the Arabian continental margin stopped. Kinematic calculations suggest that convergence velocities across the NSZ likely doubled (to $5-6 cm yr À1 ) during the period 118-85 Ma, so that BS exhumation may have been promoted by a combination of obduction movements and increased convergence velocities.

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

“…The presence north of Shahr-e-Kord (the southern part of the SSZ) of metabasites, amphibolites and eclogites with mid-ocean ridge (MORB)-like compositions suggests remnants of Neo-Tethyan oceanic lithosphere (Davoudian et al 2006(Davoudian et al , 2007, thought to be exhumed subduction zone materials. Blueschists are an important component of SSZ rocks in the Haji-Abad area, where Agard et al (2006) documented metamorphic conditions around 11 kbar and 520-530 • C, and laser-ablation 40 Ar/ 39 Ar ages on phengite clustering at 85-95 Ma; similar rocks are found in the SSZ near Neyriz (Davoudian et al 2006(Davoudian et al , 2007. These blueschist occurrences show that some SSZ rocks were subducted and later exhumed.…”

“…In some of these models the SSZ is a continental fragment rifted from Arabia and was accreted to Eurasia during Cretaceous time (e.g. Glennie, 2000;Golonka, 2004;Agard et al 2005Agard et al , 2006. However, Robertson (2007) observed that the rocks of the SSZ cannot be correlated with Arabian rocks.…”

“…These blueschist occurrences show that some SSZ rocks were subducted and later exhumed. Agard et al (2006) concluded that exhumation of Haji-Abad blueschist was completed by 80 Ma. The Early Eocene uplift and therefore hiatus in overlying sediments of all Zagros ophiolites during PaleoceneEarly Eocene time may be a further expression of the exhumation of the Sanandaj-Sirjan accretionary complex.…”

“…In many interpretations the Nain-Baft ophiolite belt represents a suture on the site of a small Mesozoic ocean basin (e.g. Stampfli & Borel, 2002;Agard et al 2006;Shafaii Moghadam et al 2009) with a number of microcontinents. However, there is no agreement as to how such microcontinents and ocean basins should be reconstructed in space and time, or in the rift processes (Robertson, 2007).…”

“…Some geologists prefer a model whereby the IB marks the position of a Campanian back-arc basin (e.g. Stampfli & Borel, 2002;Agard et al 2006;Shafaii Moghadam et al 2009). This back-arc basin represented a narrow seaway with discontinuous oceanic crust (Shafaii Moghadam et al 2009).…”