Eocene to Late Oligocene extension dominated mafic magmatism from South Kaleybar, Iran

Abstract

Vast and geochemically diverse volcanic rocks from the western Alborz Magmatic Assemblage (AMA) represent the back-arc of the central Iran Neotethyan arc (Urumieh-Dokhtar Magmatic Assemblage; UDMA). Volcanic rocks of the west AMA record valuable information on the timing, source region(s) and geodynamic setting of magmatism. Over 30 days of field study and sampling, investigation of 170 thin sections, 30 whole-rock geochemical analyses, 13 whole-rock Sr-Nd isotopic ratios and U-Pb age dating of zircon separates from 7 samples furnished the present study data. Eocene (38.5 Ma) OIB-type volcanic rocks from South Kaleybar indicate an anorogenic (extensional) setting. This lithospheric-scale extensional event induced influx of asthenospheric mantle into the sub-arc wedge, of which the partial melts differentiated to produce OIB-type melts. The OIB-type melts incorporated some inherited zircons in their ascent through the Cadomian crust. A continued extensional regime led to asthenospheric upwelling and produced mafic melts that produced 27.5-Ma-old subalkaline series volcanics. The LILE-depleted signature of the South Kaleybar subalkaline volcanic rocks implies that their mantle source region experienced previous partial melting event(s), probably during OIB-type magmatism in the Eocene. Alkaline volcanism (24.4 Ma) and concurrent high-silica adakitic volcanism (24.3 Ma-23.4 Ma) followed subalkaline magmatism. The alkaline rock signature in the study area range from 'Nb-Ta depleted' to 'plume-type'. This is consistent with lithosphere-asthenosphere interaction in an arc-related setting. Simultaneous partial melts of delaminated lower crustal rocks reacted with the asthenosphere and produced adakitic melts. Asthenospheric, lithospheric and crustal contribution to the magmatism in South Kaleybar express the back-arc signature of magmatism in Eocene to Late Oligocene times.