فهرست مطالب چیولی لی

At 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.

Geology:

The 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.

During 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.

Petrography:

Based 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.

Geochemistry:

On 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.

The 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.

The 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.

The Maher abad and Khopik porphyry Cu deposits occurred in the Upper Eocene (39-37 Ma) in Lut block. All of them associated with intermediate (mostly monzonite) rocks. Porphyry deposits are closely associated with oxidized magmas. Oxygen fugacity (fO2) is a key factor that controls the formation of porphyry Cu deposits. The composition of the major and trace elements of zircon grains related to several ore-bearing monzonite were measured in Maher abad and Khopik porphyry copper indices. Zircon grains show moderate to low Ce4+/Ce3+ with a range of 19 to 610 and an average of 155. The average of oxygen fugacity (logfO2) values of Meher abad and Khopik ore-bearing magmas, range ∆FMQ -3.2 to MFMQ -1.3 with mean ∆FMQ -2.2, indicate formation under moderate oxidation conditions (between Ni-NiO (NNO) and Faylite magnetite-quartz (FMQ) buffers, but magnetite-hematite (HM) buffer, which ), which is not ideal for the formation of porphyry deposits. This is supported by whole-rock and Sr-isotopic data, and absence of high oxidation minerals such as hematite, and the poor adakitic charactristic of rocks in both deposits, which are due to factors involved in magma origin such as rock type and partial melting rate (possibly peridotite with low participation of slab).

Oxygen fugacity (fO2) and zircon crystallization temperatures (Ti-in-zircon) of ore-bearing quartz monzonite intrusive rocks of ChahShaljami and Dehsalam Cu-Mo porphyry deposits were measured by using calculations of zircon trace element values and oxidation conditions of parental magma were investigated. The Ce4+/Ce3+ values for ChahShaljami and Dehsalam are in a range of 8 to 295 with an average of 125 and 3 to 171 with an average of 35, respectively, therefore they were not in the favorable range for porphyry mineralization. The estimated fO2 values for ChahShaljami and Dehsalam are in the range of ∆FMQ -1.8 to MFMQ +0.6 with an average of ∆FMQ -0.6 and ∆FMQ -1.3 to ∆FMQ -0.4 with an average of ∆FMQ -0.8 which is at least ∆FMQ +1 less than the values of porphyry copper mineralization in the world.. Based on the zircon log fO2 values of ore-bearing intrusive rock, the parental magma originated from partial melting of supra-subduction mantle peridotite zone within oceanic slab. Melting under unfavorable conditions (under magnetite-hematite (HM) buffer and in faylite-magnetite-quartz (FMQ) buffer condition, and lack of sufficient Cu and sulfur at the source), which is supported by evidences in deposit such as lack of magnetite-hematite intergrowths, was not ideal for porphyry Cu porphyry mineralization in the two regions. The results are supported also by previous whole-rock major and trace element geochemistry and Sr-Nd isotopic values.

Dioritic plutons and pinkish alkali-feldspar granitic dikes of Kalateh area (northwest of Khur city) intruded in Neoproterozoic basemental of Jandagh- Arousan complex. Field observations, petrography and geochemical studies indicate that these intrusive rocks have I-type nature and belong to medium-high K calc-alkaline granitiods and also have genetically relationship with each other. Their parental magmas produced by partial melting of metasomatized mantle wedge which was located above the subducted Neotethyan oceanic slab. The mentioned magmas generated in an intracontinental back arc extensional setting in relation to the subduction of oceanic Neotethys slab. U-Pb dating results on separated zircon from dioritic and alkali-feldspar granitic rocks indicated 213.9 ± 1.6 to 221.16 ± 1.6 Ma interval age (corresponding to Late Triassic). These igneous rocks are indications of magmatic manifestations of Early Cimmerian Orogeny.

Ayrakan granitoid pluton with 12 km2 area cropped out in the northeast of Mohammad Abad Kooreh Gaz village from Khur o Biabank County and in the farthest NE end of Jandagh-Arousan Metamorphic-Igneous belt. The main part of this granitoid pluton consists of porphyroidic biotite granites. A few leucocrate aplitic granite dikes and apophyse as a late magmatic phase cut porphyroidic biotite granites. Both type of the rocks, suffered intensively plastic (mylonitisation) and brittle (catclastic or fragmentation) deformations. Porphyroidic biotite granites are supposed to show more intensive mylonitization. Based on geochemical characteristics, isotopic ratios of 87Sr/86Sri, 143Nd/144Ndi and εNd, Ayrakan mylonitized porphyroidic biotite granites and aplitic granites are belonging to peralumious S-type granites. , 87Sr/86Sri (0.712714 and 0.712490), 143Nd/144Ndi (0.5115227 and 0.511581), associated with negative εNd (-8.19 and -7.13) for porphyroidic biotite granites and aplitic granites, respectively, indicate that they are originated from melting of metapelitic lower crust. Based on, Zircon U-Pb dating, porphyroidic biotite granites and aplitic granites have 545±11 and 537±3.8 Ma age, respectively. These values confirm the earlier dating results (549±15 Ma age). Therefore, Ayrakan granitoids certainly are part of the basement and Iranian Gondwanan terrains with the Late Neoproterozoic-Early Cambrian age