Mantle-derived high-K magmatic fluxes in northeast Iran arc: Constraints from zircon U-Pb-O-Hf and bulk rock major-trace elements and Sr-Nd-Pb isotopes

Abstract

Most continental arcs are built up over a long time (≥100 myr), and while subduction may be ongoing throughout this interval, magmatism appears to be highly episodic. This episodic behaviour is characterized by high-flux magmatic events but an overall low rate of magmatism. The causes of high-flux magmatic events (“flare-ups”) are enigmatic in many continental arcs. Bulk-rock Sr, Nd, and Pb isotopes, as well as zircon O and Hf isotopes, imply that the mantle and the continental crust can be involved in magmatic flare-ups. However, the relative contributions of mantle vs. crust with changes in eruption rates can differ from arc to arc. The Cenozoic magmatic arcs of Iran, built on mature continental crust, are an excellent candidate for studying the geochemical-isotopic feedback of magmatic pulses to understand the triggers for a flare-up. Our new data constrain the timing of the flare-up in NE Iran to the Early to Middle Eocene (51–43 Ma). This flare-up is characterized by the outpouring of high-K calc-alkalic to shoshonitic magmas at ∼110 ± 8 km3/myr - km. Geochemical modelling using the “Arc Basalt Simulator version 3″ shows that the high-K trachybasalts, moderately to extremely depleted in high-field strength elements, can be derived from the shallower (3.0 GPa; 870 °C) to deeper parts (5.0–5.4 GPa; 965–980 °C) of a subducting slab with ∼1.0 to 5.5 % slab melt flux. Mixing modelling using Sr, Nd, and Pb isotope data indicates that the Torud mafic-intermediate magmatic rocks can be generated by adding ∼ 1% to <6% of slab components (50% AOC: 50% sediment) to an Indian MORB-like mantle. Our results suggest that the high magmatic fluxes in NE Iran were instigated mainly by Eocene slab steepening after Paleocene flat-slab subduction, resulting in enhanced upwelling and melting of a volatile-enriched asthenospheric mantle.