Geochemical and Nd-Sr Isotopic Compositions of Hypabyssal Adakites in the Torud-Ahmad Abad Magmatic Belt, Northern Central Iran Zone: Analysis of Petrogenesis and Geodynamic Implications

Abstract

Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt (TAMB), in the northern part of the Central Iran zone, are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahrood. These igneous rocks include hypabyssal dacite, trachyte, andesite, trachy-andesite, and basaltic andesite; they are mainly composed of phenocrysts and microcrystalline groundmass of pyroxene, amphibole, and plagioclase, with minor biotite and titanomagnetite; they form domal structures (plugs and stocks), dikes, and sills that intruded into Neoproterozoic to cogenetic Eocene volcano-sedimentary sequences. Based on isotopic analysis of these intermediate to acidic rocks, initial ratios of 143 Nd/ 144 Nd range from 0.512 775 to 0.512 893 and initial ratios of 87 Sr/ 86 Sr range from 0.703 746 to 0.705 314, with quite positive ɛ Nd(i) values of +3.69 to +6.00. They are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high-field strength elements, the SiO 2 content is (52-62) wt.%, and Na 2 O content >3 wt.%, Al 2 O 3 content >16 wt.%, Yb <1.8 ppm, and Y <18 ppm. These geological, geochemical, and Sr and Nd isotopic data are consistent with adakitic signatures originating by partial melting of the subducted Neo-Tethys oceanic slab (Sabzevar branch) and lithospheric suprasubduction zone mantle. The mantle signatures typifying the rapidly emplaced adakitic rocks (slab (high-silica adakite) and suprasubduction zone (low-silica adakite) melts) together with their locally voluminous extent are evidences that support a locally extensional geodynamic setting; and the evidence is consistent with an evolution to local transpression in the Late Eocene in this convergent margin arc environment to rifting (basalts to adakites) towards submarine conditions in the Neogene. KEY WORDS: isotope geochemistry, adakite rocks, Central Iran zone, Shahrood, Iran.

Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt: New Constraints from U‐Pb Geochronology and Sr‐Nd‐Hf Isotopes

“…Various models have been proposed to explain the origins of adakitic felsic magmas (Moyen, 2009), including: (1) partial melting of subducted oceanic crust, followed by interaction with the overlying mantle wedge (Defant and Drummond, 1990;Martin, 1999;Yousefi et al, 2021); (2) assimilation and fractional crystallization (AFC) or high-pressure differentiation (HPFC) involving a mantle-derived basaltic magma (Castillo et al, 1999;Macpherson et al, 2006); (3) partial melting of a high Sr/ Y (and La/Yb) source such as delaminated mafic lower arc crust (Rapp et al, 2002(Rapp et al, , 2003Xiao and Clemens, 2006;Wang X S et al, 2018); and (4) partial melting of mafic lower continental crust (Jiang et al, 2007).…”