A unique period of bimodal volcanism at 130–110 Ma in the northern Sanandaj-Sirjan Zone: Evidence for an extensional setting

Abstract

The Sanandaj-Sirjan metamorphic-magmatic Zone (SaSZ) represents the Mesozoic active continental margin of the Neotethys Ocean between the Arabian plate and the Iranian plateau. The zone evolved at the margin of ancient Ediacaran basement, as documented in this paper with an age of 557 Ma for a trondhjemite. The Mesozoic activity of the SaSZ was most intense with sedimentation and magmatism in the Jurassic between 170 and 150 Ma, but this period was then followed by a 20 million years gap with almost no magmatic activity, which has been attributed to flat slab tectonics. The present paper is concentrated on the geochemistry, and zircon CA-ID-TIMS geochronology, of a bimodal, but mainly basaltic, volcanic and subordinate plutonic succession that represents the resumption of magmatism in a deep and gradually shallowing basin in the northern SaSZ between 130 and 110 Ma. The mafic to intermediate volcanic rocks are predominantly calc-alkaline and have εNd values of +0.3 to +3.9. The felsic volcanic rocks are dominantly high-K calc-alkaline with εNd values of −0.5 to +1.9. A suite of Cretaceous trondhjemites yields εNd values of −4.6 to −4.9. Together, the geochemical, lithological and geochronological indications suggest that the thick Barremian-Aptian bimodal magmatic suite represents magmatism in a continental arc to back-arc setting, controlled by extension and probably related to foundering of the flat slab and slab roll-back. These events initiated melting of the mantle in the northern SaSZ, heralding a period of extension and the subsequent development of ophiolites all along the Neothetyan suture.