Geochemistry and petrogenesis of Mashhad granitoids: An insight into the geodynamic history of the Paleo-Tethys in northeast of Iran

Abstract

Mashhad granitoids in northeast Iran are part of the so-called Silk Road arc that extended for 8300km along the entire southern margin of Eurasia from North China to Europe and formed as the result of a north-dipping subduction of the Paleo-Tethys. The exact timing of the final coalescence of the Iran and Turan plates in the Silk Road arc is poorly constrained and thus the study of the Mashhad granitoids provides valuable information on the geodynamic history of the Paleo-Tethys. Three distinct granitoid suites are developed in space and time (ca. 217–200Ma) during evolution of the Paleo-Tethys in the Mashhad area. They are: 1) the quartz diorite–tonalite–granodiorite, 2) the granodiorite, and 3) the monzogranite. Quartz diorite–tonalite–granodiorite stock from Dehnow–Vakilabad (217±4–215±4Ma) intruded the pre-Late Triassic metamorphosed rocks. Large granodiorite and monzogranite intrusions, comprising the Mashhad batholith, were emplaced at 212±5.2Ma and 199.8±3.7Ma, respectively. The high initial 87Sr/86Sr ratios (0.708042–0.708368), low initial 143Nd/144Nd ratios (0.512044–0.51078) and low εNd(t) values (−5.5 to −6.1) of quartz diorite–tonalite–granodiorite stock along with its metaluminous to mildly peraluminous character (Al2O3/(CaO+Na2O+K2O) Mol.=0.94–1.15) is consistent with geochemical features of I-type granitoid magma. This magma was derived from a mafic mantle source that was enriched by subducted slab materials. The granodiorite suite has low contents of Y (≤18ppm) and heavy REE (HREE) (Yb<1.53ppm) and high contents of Sr (>594ppm) and high ratio of Sr/Y (>35) that resemble geochemical characteristics of adakite intrusions. The metaluminous to mildly peraluminous nature of granodiorite from Mashhad batholiths as well as its initial 87Sr/86Sr ratios (0.705469–0.706356), initial 143Nd/144Nd ratios (0.512204–0.512225) and εNd(t) values (−2.7 to −3.2) are typical of adakitic magmas generated by partial melting of a subducted slab. These magmas were then hybridized in the mantle wedge with peridotite melt. The quartz diorite–tonalite–granodiorite stock and granodiorite batholith could be considered as arc-related granitoid intrusions, which were emplaced during the northward subduction of Paleo-Tethys Ocean crust beneath the Turan micro-continent. The monzogranite is strongly peraluminous (Al2O3/(CaO+Na2O+K2O) Mol.=1.07–1.17), alkali-rich with normative corundum ranging between 1.19% and 2.37%, has high initial 87Sr/86Sr ratios (0.707457–0.709710) and low initial 143Nd/144Nd ratios (0.512042–0.512111) and εNd(t) values (−5.3 to −6.6) that substantiate with geochemical attributes of S-type granites formed by dehydration-melting of heterogeneous metasedimentary assemblages in thickened lower continental crust. The monzogranite was emplaced as a consequence of high-temperature metamorphism during the final integration of Turan and Iran plates. The ages found in the Mashhad granites show that the subduction of Paleo-Tethys under the Turan plate that led to the generation of arc-related Mashhad granites in late-Triassic, finally ceased due to the collision of Iran and Turan micro-plates in early Jurassic.