New evidence for Jurassic continental rifting in the northern Sanandaj Sirjan Zone, western Iran: the Ghalaylan seamount, southwest Ghorveh

ABSTRACT

We address the growing controversy about the tectonic setting in which Jurassic magmatism of Iran occurred: arc or continental rift. In the Ghorveh area of the northern Sanandaj Sirjan zone (SaSZ), the Ghalayan metabasites are interlayered with marble and schist and locally cut by acidic dikes. Zircon U-Pb dating of the metabasitic rocks shows that these crystallized at ca. 145–144 Ma ago in the Late Jurassic (Tithonian). This complex was metamorphosed in the lower greenschist facies, however, some protolithic structures such as pillow lava and primary minerals are preserved. The metabasites are tholeiites with low SiO₂ (45.6–50.5 wt.%), moderate Al₂O₃ (11.3–17.0 wt.%), and high TiO₂ (0.7–2.9 wt.%) and Fe₂O₃ (9.4–14.1 wt.%). The Ghalayan metabasites are enriched in Light rare earth elements (LREEs) without significant Nb, Ta, Pb, Sr and Ba anomalies, similar to modern continental intra-plate tholeiitic basalts such as Afar and East African rifts. The Ghalaylan metabasites show wide ranges for ⁸⁷Sr/⁸⁶Sr_(i) (0.7039–0.7077) and positive ε_Nd(t) values (+0.1 to +4.6). These isotopic compositions are similar to those expected for slightly depleted subcontinental lithospheric mantle sources. Independently built discrimination diagrams indicate an intra-continental rifting regime for the source of Jurassic metabasites in the northern SaSZ. Geochemical and tectonic evidence suggests that rifting or a mantle plume was responsible for volcanic activity in the Upper Jurassic SaSZ. Considering the variation of ages of basaltic volcanism along the SaSZ, we suggest that Ghalayan basaltic magmatism reflected a submarine volcano that formed as part of the late stage continental rift, similar to Afar in the East African Rift system. Our results indicate that an extensional tectonic regime dominated SaSZ tectonics in the Middle to Late Jurassic.

Acknowledgments

We thank P. Cipollari for discussion and advice. This research was supported by the University of Kurdistan for fieldwork and Nagoya University and JSPS KAKENHI Grant (Nos. 25303029, 17H01671) and join research program of the Institute for Space – Earth Environmental Research (ISEE), Nagoya University. H. Azizi would like to thank both University of Kurdistan and Nagoya University for their support during his sabbatical periods in 2017-18. This is UTD Geosciences contribution number 13XX. This version much benefits from comments by the associate editor, Hadi Shafaii Moghadm, anonymous reviewers and Reza Maghdour-Mashhour.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Japan Society for the Promotion of Sciences (JSPS) KAKENHI Grant [Nos. 25303029, 17H01671] and join research program of the Institute for Space – Earth Environmental Research (ISEE), Nagoya University.