Original paper
Quaternary low-SiO2 adakites from the Alborz-Azerbaijan magmatic belt (NW Iran) and their geodynamic implications
Faridazad, Morovvat; Ashrafi, Nasser
Neues Jahrbuch für Mineralogie - Abhandlungen Band 198 Heft 2 (2023), p. 149 - 169
102 references
published: Jun 6, 2023
published online: May 16, 2023
manuscript accepted: May 7, 2023
manuscript revision received: Apr 24, 2023
manuscript revision requested: May 2, 2022
manuscript received: Oct 25, 2021
ArtNo. ESP154019802002, Price: 29.00 €
Abstract
Adakitic rocks in the Alpine-Himalayan belt are indicative for the geodynamic evolution of collision zones and often associated with mineralization. This study reports the presence of Quaternary low-SiO2 andesitic to trachyandesitic (Mg# 51–56) adakites of medium-K calc-alkaline composition that are coeval with normal calc-alkaline rocks of northwestern Iran. The adakitic rocks are characterized by high Sr (998–1250 ppm) and low Y (6.5–8.4 ppm), whereas the normal calc-alkaline rocks are low in Sr (296–530 ppm) and high in Y (14.3–22.6 ppm). The adakitic samples have an average of ~56 wt.% SiO2, ~3 wt.% MgO, and ~12 wt.% CaO+Na2O, indicating that they fall into the low-SiO2 adakite group. The steep rare earth elements patterns with chondrite-normalized (La/Yb)n of 35–46 and absent to minor positive Eu anomalies, low contents of Y, Nb, and Ti, as well as high ratios of Sr/Y indicate the presence of garnet ± amphibole ± Fe-Ti oxides and the absence of plagioclase in the melt residue. Although these rocks belong to a post-collisional environment, they have inherited geochemical signatures of a subduction zone. The plausible scenario for the simultaneous occurrence of adakitic and non-adakitic magmas in northwestern Iran is partial melting at different depths within the mantle wedge that was metasomatized by slab-derived melts. The important drivers of post-collisional magmatism were probably slab break-off and/or sub-lithospheric convection. Semi-parallel active strike-slip faults most likely played a key role in the Plio-Quaternary magmatism by facilitating crustal magma ascent. Despite known Cu-Mo-Au mineralizations in the form of porphyry and epithermal deposits in connection with Oligocene-Miocene sub-volcanic stocks and dykes that share the adakite-like and high-K calc-alkaline compositions with the Plio-Quaternary rocks in the study area, no related mineralization has been observed.
Keywords
Trachyandesite • adakite • Varzaghan area • Arasbaran Quaternary Magmatism • post-collisional setting