Search

Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time. [ABSTRACT FROM AUTHOR]

It is important to understand the Jurassic tectonic setting of SW Eurasia, and the igneous rocks of the Sanandaj-Sirjan zone in SW Iran provide critical evidence. Bimodal volcanic and hypabyssal rocks exposed around Songhor in the central part of the Sanandaj-Sirjan zone have zircon U-Pb crystallization ages of 156 Ma for felsic and 153 to 146 Ma for mafic rocks. Felsic volcanic and subvolcanic rocks with affinity to A-type granite have high SiO 2 (62.6 to 73.9 wt%) and total alkalis (K 2 O + Na 2 O = 5.1–9.8 wt%), low MgO (0.1–3.1 wt%) and slightly positive εNd(t) (+0.6 to +3.8) and moderate 87Sr/86Sr(i) ratios (0.7041 to 0.7061). The mafic rocks (basalt and micro gabbro/dolerite) with lower contents of SiO 2 (47.0 to 54.6 wt%), high MgO (4.1 to 10.7 wt%) with variable Na 2 O (2.0–5.9 wt%) and K 2 O (0.1–2.6 wt%) can be divided into two groups based on TiO 2 contents: 1) low-Ti and 2) high-Ti. The low-Ti group has 0.14 to 0.62 wt% TiO 2 with εNd(t) = +3.2 to +5.8 and 87Sr/86Sr(i) = 0.7032–0.7059. The high-Ti basalts have 1.4–1.6 wt% TiO 2 with εNd(t) = +1.2 to +4.2 and 87Sr/86Sr(i) =0.7039 to 0.7057. The low-Ti suite is less fractionated, with Mg# between 55 and 74 compared to 43 to 63 for the high-Ti suite. Felsic rocks may have formed by remelting of underplated mafic rocks which were derived from subcontinental lithospheric mantle along with minor assimilation of continental crust in the early stage of extension. Thinning of continental lithosphere continued with increasing partial melting to produce high-Ti mafic melts early and low-Ti mafic melts in the late stage. Similar Sr-Nd isotope ratios for the mafic and felsic rocks with an 8–10 Ma interval between these two groups hint at gradual thinning of the continental lithosphere. This is consistent with formation of transitional crust of a passive continental margin between Neo-Tethys and Iran continental crust in Jurassic time. [Display omitted] • Bimodal magmatism occurred from 156 to 144 Ma in west Iran (Songhor). • Felsic rocks were generated from the mantle with minor involvement of older crust. • Mafic rocks are divided into low-Ti (tholeiitic) and high-Ti (alkaline) groups. • Magmatic activity suggest that SW Iran was a Jurassic volcanic rifted margin. [ABSTRACT FROM AUTHOR]

Intra-plate basalts of ~ 35–0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature ( T p ), while the Shandong basalts have the lowest T p . We infer that a deep high- T p plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low- T p plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~ 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation. [ABSTRACT FROM AUTHOR]