Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

Abstract

<jats:title>Abstract</jats:title> <jats:p>Significant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ~42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ~19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.</jats:p>

Adakites, High-Nb Basalts and Copper–Gold Deposits in Magmatic Arcs and Collisional Orogens: An Overview

“…This is an extremely fertile metallogenic environment, which includes various mineralization styles formed in diverse tectonic settings [263]. Adakites are found at different age levels in the Tethyan belt and are frequently (Iran, Tibet) associated with high-Nb basalts [65,115,137,[264][265][266] and Cu-Au deposits [110,[267][268][269][270][271][272][273][274][275]. Petrologic models for Tethyan adakites and high-Nb basalts include ridge subduction and slab melting followed by mantle hybridization and formation of HFSE-enriched sources, as well as the melting of thickened mafic lower crust (especially in Tibet), crustal delamination and subduction modification (basification) of continental crust.…”

“…Adakites are found in northwestern Iran, in the central part of the Zagros Ranges, in the eastern Iran region (around Bibi-Maryam) and in the southeastern portion of the Urumieh-Dokhtar magmatic arc [269,[283][284][285]. Adakites in NW Iran are frequently associated with contemporaneous high-Nb basalts [137,264,265] and Cu-Au porphyry and epithermal mineral deposits [269,270,283]. The Miocene Sungun Cu-Mo porphyry systems on the NW terminus of the Urumieh-Dokhtar magmatic arc (Figure 21) are centered on high-K granodiorite stocks [286] with adakite geo chemical characteristics (K 2 O = 2.4-3.5 wt.%; Sr = 363-1984 ppm; Y = 8-17 ppm; Figure 22).…”