Zircon U–Pb dating, geochemistry, and geology of Shotorsang hypabyssal granitoids, southern Quchan (northeast of Iran)

IntroductionAt the northeastern end of the Sabzevar ophiolitic zone and the southern edge of the Binalud zone and south of Quchan, a 200 km long young magmatic arc consisting mostly of calc-alkaline to adakitic volcanic-intrusive rocks are exposed which extended to Esfarayen. In the present study, the Shotorsang hypabyssal granitoids in the northeast of Sabzevar and the Binalud structural zone are investigated. No dating data and comprehensive geochemical study of these units have been published, and the origin of hypabyssal units is unclear. Accordingly, new petrographic and geochemical data and U–Pb zircon ages of intermediate-acidic intrusives of Shotorsang in the northeast of Iran are reported.GeologyThe hypabyssal Shotorsang granitoids, as a part of the magmatic arc in the north of the Sabzevar ophiolitic belt, are located 82 km northeast of Sabzevar and in the Binalud structural zone. Based on the field studies and 1:5000 geological map, the rock units of the Shotorsang area include early Cretaceous limestone, Eocene sedimentary and volcanic rocks, the Miocene hypabyssal stocks and dykes, and new Plio-Quaternary deposits. Intrusive units including dikes and hypabyssal stocks composed of dacite, granodiorite, and quartz monzonite. The dikes with acidic composition crosscut the Eocene sedimentary and volcanic succession.Materials and MethodsDuring the field investigations in the Shotorsang area, 80 samples were taken from most of the geological units, and 40 thin sections were studied by a polarizing microscope for petrographic studies. Six representative and least altered samples from the Shotorsang hypabyssal granitoids were analyzed for major oxides and trace elements using XRF and ICP-MS in the Acme Laboratories (Canada). Two samples from dacite dike and granodiorite stock (samples Ab8 and Ab59) were selected for dating studies according to the suitable size and abundance of zircon crystals.U-Pb dating was conducted by laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) in the Laboratory of Isotope Geology at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China.PetrographyBased on petrographic studies, the Shotorsang hypabyssal units can be divided into three groups including granodiorite and quartz monzonite stocks as well as dacite dikes. The granodiorite is composed mainly of quartz, plagioclase, orthoclase, hornblende, and biotite with accessory zircon, sphene, and apatite. This unit dominantly displays a porphyritic to glomeroporphyric texture.Dacite dikes have mineralogical and textural similarities with those of the granite stocks. Mineralogically, these units include quartz, plagioclase, orthoclase, biotite, and hornblende with a felsitic porphyry texture.Magmatic fluids released during the intrusion of quartz monzonite bodies are the main factor for skarnization in the area.GeochemistryOn the total alkali-silica diagram (Middlemost, 1985), the intrusive bodies and dikes are plotted in the fields of quartz monzonite, granodiorite, and granite, which is consistent with their petrographic observations. On the SiO2 versus K2O discrimination diagram (Peccerillo and Taylor, 1976), the Shotorsang hypabyssal granitoids fall mainly in the medium- to high-K calc-alkaline domains. On chondrite and primitive mantle-normalized diagrams, the investigated samples display enrichment in LILE and LREE and depletion in HFSE and HREE.On Rb versus Nb+Y tectonomagmatic discrimination diagram (Pearce et al., 1984), the Shotorsang hypabyssal granitoids are plotted in volcanic arc granite field.U-Pb datingThe two samples from the dacitic dike and granodioritic stock yielded zircon U-Pb ages of 22.52±0.38 and 22.56±0.35 Ma, respectively. The Th/U ratio for zircons from the dacitic dike and granodioritic stock is more than 0.1, which is different from metamorphic zircons and compatible with magmatic zircons.DiscussionThe remarkable geochemical criteria of hypabyssal Shotorsang granitoids are Sr (382.5-607 ppm), high Sr/Y ratios (26.4-62.5), LaN/YbN (9.5-17.1). The low values of Y (9.7-16.3) and HREE are similar to those of adakites. The low values of Yb and Y, and high ratios of Sr/Y, as well as La/Yb, could be attributed to the presence of residual garnet and hornblende or as fractionated minerals.The LILE (large-ion lithophile elements) enrichment and HFSE (high-field strength elements) depletion are typical features of calc-alkaline magmas related to subduction zones, which originated from partial melting of a subducted oceanic slab or a supra-subduction mantle wedge.The lack of Eu anomaly in adakites, as well as the Shotorsang hypabyssal granitoids with abundant plagioclases, is related to magma ƒO2. Under oxidizing conditions, Eu occurs dominantly as Eu3+, leaving lesser Eu2+ to be incorporated into plagioclase.There are evident changes in Sr-Nd isotopic ratios of the magmatic rocks of the northern Sabzevar ophiolitic belt, and it seems that the (87Sr/86Sr)i ratios increase, however, the εNdi values decrease from the old to the young rocks, which likely indicates the effect of the crustal materials on the source magma.It can be inferred that the Cretaceous-Paleocene magmatism is subduction-related, and from Eocene onwards, the igneous rocks of this zone, including the Miocene Shotorsang adakites with an age of 22.5 million years, have a post-collisional nature.ConclusionsThe Shotorsang hypabyssal granitoids, as a part of the magmatic arc of the northern Sabzevar ophiolitic belt, formed as dike and stock with dominantly porphyritic textures and intruded the Cretaceous-Eocene volcano-sedimentary succession. The two samples from the dacitic dike and granodioritic stock yielded zircon U-Pb ages of 22.52±0.38 Ma and 22.56±0.35 Ma, respectively, indicating the importance of Miocene time in iron mineralization in this zone. The geochemical features of the Shotorsang hypabyssal granitoids correspond to the field of post-collisional volcanic arc rocks (VAG) and possess similarities with those of the adakitic rocks, originating from the subducting slab.These rocks probably have an eclogite or amphibolite garnet origin resulting from the metamorphism of the Sabzevar Neotethyan oceanic lithosphere, which was subducted under the Alborz zone.

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