Geochemical and Isotopic Characteristics of the Protolith of Eclogites from the North Shahrekord Metamorphic Complex: Evidence of Late Neoproterozoic Back-arc Basin development in Sanandaj-Sirjan Zone

All of the major tectonic zones of Iran, except for the Kopet-Dagh, contain Pan-African crystalline basement. Subduction of the Proto-Tethys at about 630 Ma caused Cadomian arc– and backarc magmatism in the northern margin of Gondwana (Hassanzadeh and Wernicke 2016). Sanandaj-Sirjan Zone with NW-SE trend is the most active zone in Iran that extends to the southeast of Turkey and then to Bitlis Massif. There are many similarities between the Precambrian basement in the zone from Iran with Bitlis and Menderes massifs from Turkey. The study area is a part of a large-scale ductile shear zone, containing a wide range of metamorphic rocks with sedimentary and magmatic origins. Metasedimentary rocks comprise of paragneiss, various schists and meta-carbonates that have cropped out through the shear zone which extends along the Zayandeh-Rood River. Metabasic rocks of the North Shahrekord Metamorphic Complex (NSMC) are composed of amphibolites, garnet amphibolites and eclogites as lenses in metagranitoid bodies and other metamorphic rocks. The work is focused on the origin of the eclogites based on field geology, petrography, geochemistry and Sr-Nd isotopic ratios. The new petrological and geochemical analyses are presented to show a geodynamic model for the protolith of the eclogites and their relations to Proto-Tethys subduction during Late Neoproterozoic.

After microscopic studies, nine fresh samples were selected for whole-rock geochemical analysis (XRF and ICP-MS analysis) for determining the major trace elements, and REE contents. Six samples were analyzed at Nagoya University (Japan) and three samples were analyzed for XRF at Salzburg University (Austria) and ICP-MS, in ACME lab (Vancouver, Canada).

Chondrite normalized REE diagrams display minor enrichment of LREE in comparison with HREE. (La/Yb)cn ratio varies between 1.7 to 2.7 without Eu anomaly. Primary mantle normalized diagram of trace elements show negative anomaly in P, Ti, Nb and Zr. Initial magma had been a basalt to andesite-basalt composition. Tholeiitic magma are revealed by relatively flat REE patterns and geochemical diagrams for their protolith. The geochemical data of the NSMC eclogites shows compositional characteristics of E-MORB which is composed of a mix of lithospheric and asthenospheric mantle, and final melt segregation has occurred at depths between 10 to 30 km. Spinel was aluminum bearing phase in the mantle. Tectonic discrimination diagrams display that magma is formed in a back-arc basin environment. Studying Sr-Nd isotopes specifies a range of 0.707 to 0.711 for 87Sr/86Sr and 0.5129 as an average of 143Nd/144Nd. ɛNdt varies between 3 to 7. Moreover, the initial 87Sr/86Sr ratios of the samples vary from 0.705–0.709 (Malek-Mahmoudi et al., 2017). The isotopic evidences indicate that the initial magmas are formed from the mixture of EMII and MORB reservoir with a 0.4 to 0.8 influence of the subduction component.

Iranian basement rocks that were affected by the Cadomian orogeny are reported from different zones of Iran with an age range between 630 to 514 Ma (Hassanzadeh and Wernicke 2016). Neoproterozoic rocks of the Gondwana supercontinent, which were formed in a back-arc basin setting, have been reported from some zones of Iran, such as Alborz, Central Iran and Zagros Hormoz complex (e.g. Etemad Saeed et al., 2015; Faramarzi et al., 2015; Hosseini et al., 2015( and also from Turkey (e.g. Gürsu and Göncüoglu, 2005). Ages of the back-arc successions range from 570 to 530 Ma (e.g., Abbo et al., 2015; Shafaii Moghadam et al., 2016). Our geochemical evidences indicate the formation of a continental back-arc basin in Sanandaj-Sirjan Zone during Late Neoproterozoic. The association of eclogites with meta-sedimentary rocks including paragneiss, schist, quartzite, metadolomite and metasandstone, display a shallow marine sedimentary environment. The combination of field observations and chemical composition of the eclogites shows that the protolith of the rocks are formed at a rifted back-arc basin at the Gondwana during late Neoproterozoic to early Cambrian. Then, high-pressure metamorphic phase was affected on the rocks during Early Jurassic (Davoudian et al., 2016).