نشریه علوم زمین خوارزمی
سال پنجم شماره 2 (پاییز و زمستان 1398)

In the present research biostratigraphy of the filyschoid deposits of the Silak section located at the eastern margin of Lut Block based on calcareous nannofossils was investigated. These deposits are about 420 m thick, and mainly contain shale, marl, calcareous sandstone and sandstone. From this section 118 samples were taken and prepared with smear slide method and then samples studied based on calcareous nannofossils. On the basis of the studied samples, 34 nannofossil species belong to 17 genera with medium preservation were identified and photographed. Based on the identified calcareous nannofossils, CC21 - CC26 biozones of Sissingh [24] in the section were determined. Determined biozones represent the late Campanian to the end of late Maastrichtian age for the investigated deposits in the Silak section. Study and comparison of the number of identified nannofossil species and determined biozones in the three sections: Silak, Sarayan and Shushud in the Lut Block show relatively same diversity of the species. However, species have better preservation in Sarayan section. In addition, difference in the biozones thickness in the studied sections may indicate a different sedimentation rate in the time interval of each biozones in different parts of the east of the Lut Block sedimentary basin.

In this research, paleoecology and depositional environment of Surgah Formation at the Maleh Kuh section have been investigated. The thickness of Surgah Formation is 82 meters and consists of argillaceous limestone, calcareous shales with intercalated limestones. Based on the planktonic foraminifera, the age of Surgah Formation at the study area is late middle Turonian to Early Santonian. Statistical studies of the morphotype groups of Epicantinental Sea Forms (ESF), Shallow Water Forms (SWF) and Deep Water Forms (DWF) show that these foraminifera are related to DWF, which indicate specific condition of oligotrophic. Also high planktonic/benthic foraminifera ratio confirms the semi deep to deep marine condition of sedimentary environment. The salinity as an ecological parameter in the section was changed to very low amount at the Late Turonian-Early Santonian based on abundance of Muricohedbergella and Whiteinella. Finally, four mirofacies have been identified based on stratigraphic and biostratigraphy position of planktonic foraminifera which formed in the semi deep to deep open marine. This microfacies belongs to the outer ramp of a carbonate platform.

The Gurpi Formation with a thickness of 190 m at the Kouh-e sefid anticline is investigated with regard to the calcareous nannofossils. In this section, Gurpi Formation is mainly consists of shale, marls and argillaceous limestones. As a result of this study, 21 genera and 41 species of calcareous nannofossil have been recognized. Based on distribution of index species calcareous nannofossils bio zones of have been recognized, including CC16-CC25 biozones, that corresponding to UC12-UC20aTP. According to the identified biozones, the age of the Gurpi Formation in this section is middle/late Santonian to early late Maastrichtian. The study in the Kouh-e sefid anticline, show that the sedimentation of Gurpi Formation has been started at the middle/late Santonian and to continue to the early late Maastrichtian.

The Lake Siah magnetite ± apatite deposit is situated in the northeastern of Bafq and Central Iran tectonic zone. The host rock of deposit is composed from lower Cambrian volcano-sedimentary sequence that has exposed as caldera complex. The iron mineralization is as massive ore and includes magnetite and hematite which form with apatite, quartz and calcite gangue minerals. Based on fluid inclusions petrography in quartz and apatite, at least five types of fluid inclusions are recognized including, multi-phase solid bearing (L+V+S1+S2), three phase solid bearing (L+V+S), liquid-rich two phase (L+V), vapor-rich two phase (V+L) and monophase vapor (V(. The three phase fluid inclusions are the most abundant of fluid inclusions type and indicate homogenization temperature between 123 to 530°C (avg. 344°C) and salinity content 30 to 58 wt% NaCl equiv. (avg. 40.60 wt% NaCl equiv.). Three phase fluid inclusions in the apatite are characterized by homogenization temperature between 175 to 420°C (avg. 385°C) and salinity between 34 to 53 wt% NaCl equiv. (avg. 41.58 wt% NaCl equiv.). Calculated pressure on the basis of microthermometric measurements on fluid inclusions in the H2O-NaCl system is calculated between 10 to 500 bar which is corresponding with depth between 0.5 to 1.5 km. The values δ18O of fluid in equilibrium with quartz is 7.5 to 11 ‰ which indicated magmatic waters origin. According to wide range of fluid inclusion homogenization temperature and salinity accompanied with positive correlation between salinity and homogenization temperature of fluid inclusions, mixing of hot and high salinity fluid with cold and low salinity water (similar to meteoric waters) is main factor for occurrence of the Lake Siah deposit.

We used 2D seismic profiles, well data and field observation to describe the sub- surface structural styles and geological history of two basins in the northern part of Central Iran Basin (the Howz Soltan and Zavieh basins) from Late Oligocene to Pliocene. Two basin- bounding faults with normal- slip component, the South Moreh Kuh Fault (SMF) and the Howz Soltan Fault (HSF), controlled the thickness of Qom Formation and lower part of Upper Red Formation (URF) in the Howz Soltan basin, particularly during the Early Miocene to early- Middle Miocene. In the Moreh Kuh area, there is also a facies change within the Early Miocene succession from the hanging wall to the footwall of a south- dipping inverted normal fault, the North Moreh Kuh Fault (NMF). The change from carbonated deposits of the Qom Formation (in the hanging wall) to clastic dominated URF (in the foot wall) indicates that the NMF had been a normal fault which was subsequently subjected to inversion. In the Zavieh basin, the Oligocene- Early Miocene succession thins onto the basin margins. The lower part of this interval shows an onlap from the north towards the basin margin. This could be an indication that the basin wall had been slightly inclined prior to the marine transgression. Since the Middle Miocene and/or Late Miocene, the initiation of a regional compressional phase resulted in folding and reverse faulting in the Zavieh and Howz Soltan basins. The deposition of upper URF and Pliocene units was simultaneous with this stage.

Gabbroic-dioritic rocks of Hurjand area northeast of Kerman within the Posht-e-Badam block of the Central Iran microcontinent have intruded into the Precambrian-Cambrian sedimentary units of Desu Series. Amphibole, pyroxenes and plagioclase are the main minerals of these rocks. Also, granular and poikilitic are dominant textures of these rocks. Mineral chemistry studies show that clinopyroxene composition is augite and diopside. The plagioclase has a range of chemical composition from bytownite (An75.2Ab23.8Or1.1) to andesine (An40.2Ab55.9Or3.9). Amphibole crystals present in these rocks are calcic and magmatic types with chemical composition of magnesio-hastingsite. Thermometry by utilizing of Ti content in amphibole composition shows range of crystallization temperatures from 769 to 889°C. Also, based on coexisting amphibole-plagioclase pair geothermometry, shows the temperature ranges from 793 to 896°C for crystallization temperatures these two minerals. Results of the geobarometry show that amphibole crystals formed at pressure of about 6.3 to 8.8 kbar, equivalent to the depth of 23 to 30 km. Mineral chemistry studies indicate the formation of these rocks from magma of alkaline to sub-alkaline nature related to intra-plate tectonic setting.

The Eocene volcanic rocks are exposed in Dastgerdo and Molla Ahmad areas (East of Isfahan province); the middle part of Urumieh - Dokhtar magmatic arc (UDMA). In this area, the UDMA width is about 13 kilometers which is one of its narrowest parts. These Eocene rocks consist of lava (trachyte, dacite, andesite, basaltic andesite and basalt) and pyroclastic rocks (littic tuff, tuff breccia and ignimbrite) which cross cut by Oligocene granodiorites. Andesites are the predominant rock unit with good outcrops. The main rock- forming minerals of andesites are clinopyroxene (augite), orthopyroxene (enstatite), plagioclase (andesine to bytonite), amphibole (magnesian hornblande), biotite, quartz, magnetite, ilmenite and sphene. Secondary minerals which are produced by alteration are chloritized olivine, malachite, chlorite, calcite, epidote, actinolite, albite and montmorillonite. The geochemistry of volcanic rocks show that the volcanic rocks of Dastgerdo to Molla Ahmad areas have a wide range of SiO2 (50.4 to 65.5 wt%) and belong to the calc-alkaline magmatic series. These rocks are more enriched in light rare earth elements (LREEs) than the heavy rare earth elements (HREEs) and represent evident negative anomaly of Eu. Whole rock chemistry and horizontal pattern of HREEs reveal that the spinel lherzolite is the source rock. Petrography and geochemistry of minerals and whole rock samples indicate that the studied andesites are formed by the same petrogenetic processes and are affected by magma mixing and contamination during crystallization. The field, petrography and geochemical data show that Dastgerdo to Molla Ahmad volcanic rocks belong to volcanic arc tectonic setting and are formed by subduction of Neothetys oceanic crust beneath the Central Iran.

Late Neoproterozoic Majerad basementic igneous - metamorphic complex with NE-SW trend located in the 150 Km southeast Shahrood and north edge of the central Iran structural zone. Same as the other basementic complex of southeast of Shahroud, in the north of Majerad pass, this complex crosscut by several small scale gabrrodioritic intrusions and numerous diabasic dikes. In these gabrodiorite intrusions observed field evidences of fractionation from gabbro to tonalite. These gabrrodiorits show granular, ophitic, subophitic and poikilitc textures and composed of augitic clinopyroxene, green hornblende, plagioclase and biotite (as an essential minerals). Magma forming of these rocks have calc-alkaline nature, and enriched in large ion lithophile elements (LILEs) and also light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). For the first time carried out U-Pb age dating on zircons extracted from the mentioned gabbrodiorites, represented an average age around 166 million years  ago (equivalent to middle Jurassic) for their formation. These magmas originated from the partial melting of the metasomatized subcontinental lithosoheric mantle source with spinel- lherzolithic primary nature in an extensional intracontinental back arc basin tectonic setting located on the subduction zone of the Zagros neothetyan basin on the northern edge of central Iran structural zone during the middle Jurassic period.

The northwest-southeast Rag sefid thrust is a seriesof the tensile faults formed during the opening of the Neo-Tethys Ocean in the Permian Triassic. After the collisions of Iranian and Arabic sheets, the final phase of the inversion-related folding of this fault in the Pliocene, has led to the rise of the Rag sefid anticline. The Tectonic inversion rate calculated on the seismic reflection sections of the Rag sefid fault (Ri = 0.75) shows the high inversion values along this fault. Considering the high rate of inversion, high angle Rag sefid inverted fault dip, apparent geometry and evolutionary history in the Zagros foreland, the small thrust fault with a lower separation depth in the southern limb of the Zahge anticline can be considered as a shortcut thrust and also a branch of the Rag sefid thrust that has developed for the deformation transfer in the footwall of the Rag sefid fault.