Petrogenesis of Middle Eocene Granitoids and Their Mafic Microgranular Enclaves in Central Urmia-Dokhtar Magmatic Arc (Iran): Evidence for Interaction Between Felsic and Mafic Magmas.

Abstract

Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0 ± 0.7 Ma for the granodiorite phase. Mafic microgranular enclaves (MMEs) are common in these plutons, and have relatively low SiO₂ contents (53.04–57.08 wt.%) and high Mg# (42.6–60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous (A/CNK = 0.69–1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO₂ contents (59.7–66.77 wt.%) and lower Mg# (38.6–52.2); they are considered a product of partial melting of the mafic lower crust. Chondrite-normalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies (Eu/Eu* = 0.60–0.93). The host granodiorite samples yield (⁸⁷Sr/⁸⁶Sr)_i ratios ranging from 0.70498 to 0.70591, positive ε_Nd(t) values varying from +0.21 to +2.3, and T_DM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves (⁸⁷Sr/⁸⁶Sr)_i = 0.705111–0.705113, ε_Nd(t) = +2.14 to +2.16, T_DM2 = 697–785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantle-derived mafic magmas in an active continental margin arc environment.