سنگ شناسی، سن سنجی، ژئوشیمی و تعیین منشأ توده های گرانیتوئیدی منطقه بجستان، شمال فردوس، استان خراسان رضوی

Introduction The investigated area is situated in the south west of the Khorasan Razavi Province along the North West of the Lut Block. Different types of metal ore bodies along with non-metal deposits have already been documented in the Lut Block (Karimpour et al., 2008). Most of the study area is covered with granitoid rocks. Metamorphic rocks with unknown age are present in the north of the area. Skarns are observed in contact with fault zones and intrusive bodies. Eocene volcanic rocks with andesite and andesibasalt composition are located in the east and north east of the area (Ahmadirouhani et al., 2015). The study area that is a part of the Lut Block has a high potentials for Cu, Fe, Au, and Barite mineralization along the observed alteration zones. In the present study, the petrography, petrogenesis, Sr–Nd isotopes, and U–Pb zircon age of acidic granitoids in the east of Bajestan were investigated. Materials and methods In the current study, 400 rock samples were collected from the field and 170 thin sections were prepared for petrography studies. Thirty samples of volcanic rocks, intrusions, and dykes were analyzed using XRF at the Geological Survey of Iran. Twenty-five samples were selected for the elemental analysis using ICP-MS by the Acme Lab Company (Canada), 16 samples of them were related to acidic intrusive bodies and dykes. In addition, zircon crystals from four samples of the granitoids bodies were collected for U–Pb dating. Approximately 50 zircon grains (i.e. euhedral, clear, uncracked crystals with no visible heritage cores and no inclusions) were hand-picked from each sample. Through cathodoluminescence imaging, the internal structure and the origin of zircon grains were examined at the Geological Survey of Vienna, Austria. Moreover, zircons were dated using the (LA)-ICP-MS method at the Laboratory of Geochronology, the University of Vienna, Austria using the methodology outlined in Klotzli et al., (2009). Sr and Nd isotopic compositions were also determined for the same samples (i.e. U-Pb samples) using the whole-rock method. The samples were analyzed in the Laboratorio de Geologia Isotopica da Universidade de Aveiro, Portugal. Results Granitoids in the study area have mostly monzogranite (biotite monzogranite, hornblende biotite monzogranite and pyroxene hornblende biotite monzogranite), granite, and syenogranite composition. Granular, micro-granular, and porphyritic textures are common textures in these rocks. Common mafic minerals in these rocks include biotite, hornblende and pyroxene. Based on mineralogy, low values of magnetic susceptibility, high aluminum saturation index, and high initial 87Sr/86Sr ratios (> 0.710) of the study of granitoid rocks belong to the ilmenite-series of the reduced S-type granitoids. These magmas originated from the upper continental crust at a syncollosion zone. Furthermore, the rocks normalizing spider diagrams showed characteristics of a crustal environment. The age of the granitoids based on zircon U–Pb age dating was determined, including granite porphyry (79±1 Ma), syenogranite (76±1 Ma), biotite monzogranite (76±1 Ma), all of which belong to the Upper Cretaceous (Campanian), except pyroxene hornblende biotite monzogranite with 30.7±1 Ma, Oligocene age (Rupelian) has a different age. The ranges of their initial 87Sr/86Sr and 143Nd/144Nd ratios for Upper Cretaceous granitoids are 0.710897–0.717908 and 0.511995–0.512186, respectively while they are 0.713292 and 0.512186 for Oligocene intrusion. The initial єNd isotope values for the syenogranite, biotite monzogranite, and granite porphyry are -10.65, -7.38 and -9.51, respectively. The initial єNd isotope value for pyroxene hornblende biotite monzogranite is -8.06. The values of the igneous rocks could be considered as representative of continental crust derived from magma, and melt derived from psammite rocks is considered to have been the source of the granitoids. Discussion Based on the U-Pb dating results, there are two magmatism phases (Upper Cretaceous and Oligocene) in the area which have not reported in the north of Lut Block yet. During the Upper Cretaceous, three localities of granitoids are reported, excluding Bajestan: Bazman (initial 87Sr/86Sr =0.7056) is located in the southern part of the Lut Block, Gazu (initial 87Sr/86Sr =0.7045) is located near the Nayband fault in the Tabas Block and Kaje is located in Ferdows (initial 87Sr/86Sr =0.7061-0.7080). All of these granitoids were formed due to the subduction zone and their magma (I type) originated from mantle. However, granitoids in Bajestan with the initial 87Sr/86Sr =0.711-0.718 were formed during the continental collision while their magma was originated from the continental crust. In addition, the Middle Jurassic granitoids in the Lut Block (Shah Kuh, KlatehAhani and SurkhKuh) with the origin of continental crustal magma have an initial 87Sr/86Sr = 0.7068-0.7081. That is, the continental crust from which Bajestan granitoid magma is originated, is different from the other parts of the Lut Block due to very high (87Sr/86Sr). This indicates that Bajestan perhaps joined the Lut Block after the Upper Cretaceous collision. In addition to Bajestan, the Oligocene granitoids in the Lut block are reported in the Chah-Shaljami, Dehsalm, Mahoor and Khunik areas. Except Bajestan, all of these granitoids were formed in the subduction zone and their magma is I type. Mineralization in Chah-shaljami, Dehsalm, and Mahoor is related to the porphyric system, whereas no mineralization in Khunik and Bajestan Granitoids has been reported yet. References Ahmadirouhani, R., Karimpour, M.H., Rahimi, B. and Malekzadeh Shafaroudi, A., 2015. Enhance of alteration zones and lineation in the east of Bajestan using SPOT, ASTER, ETM+ and Geophysics data. Scientific Quaternary Journal Geosciences, 24: 253-262. Karimpour, M.H., Malekzadeh-Shafaroudi. A., Stern. C.R. and Hidarian, M.R., 2008. Using ETM+ and airborne geophysics data to locating porphyry copper and epithermal gold deposits in Eastern Iran. Journal of Applied Science, 8: 4004–4016. Klotzli, U., Klotzli, E., Gunes, Z. and Kosler, J., 2009. Accuracy of laser ablation U–Pb zircon dating: results from test using five different reference zircons. Geostandards and Geoanalytical Research, 33: 5–15.