جستجوی مقالات مرتبط با کلیدواژه « فیروزکوه » در نشریات گروه « زمین شناسی »

Green tuffs of middle Eocene age in Hesarbon area, south west of Firoozkuh (East of Central Alborz) consist of a thick sequence of lithic-, crystal-,vitric-and calcareous-tuffs. Microscopic and x-ray diffraction studies show plagioclase (albite and oligoclase), alkali feldspar (sanidine), quartz, cristobalite, biotite and hornblende are the major minerals in the rocks studied. Secondary minerals such as analcime, chlorite, prehnite and clay minerals are mainly present in the groundmass of the rocks. Extensive tectonic activities have created a variety of structural features including numerous folds and faults and therefore, have caused the green tuffs to be crushed and converted to breccia tuffs in many parts. Veins and cavities are filled by considerable amounts of zeolitic minerals including heulandite group, clinoptilolite and natrolite along with calcite and secondary quartz. Based on geochemical data, they lie on the dacite and rhyodacite field showing a calc-alkaline nature in the corresponding diagrams. According to the chondrite and primitive mantle normalized diagrams of trace elements, negative anomalies of Eu, Nb, Ti, P and depletion of HFSE together with their position in the petrogenesis discrimination diagrams, it is most likely that these rocks are formed in the active continental margin of a subduction zone. The existence of analcime and prehnite in the groundmass demonstrate that these rocks have undergone some degrees of low-grade metamorphism due to the overburden of the layers in the temperature range 200-300 °C. The present study shows that zeolite minerals filling the fractures and cavities of tuffs are precipitated by hydrothermal fluids with a neutral pH to acidic

In this study¡ a modified “probabilistic seismic hazard assessment” (PSHA) method is used to estimate the level of the potential seismic ground motion in Firoozkouh. A problem that may be encountered in probabilistic studies of seismic hazard for a specific site¡ for engineering purposes¡ is the selection of design earthquakes corresponding to a given hazard value. In order to derive a seismic scenario consistent with the results of PSHA for a site and determine the relative contribution of events to the overall seismic hazard¡ the concept of disaggregation was introduced. The disaggregation of seismic hazard is an effective way to identify the scenario events that contribute to a selected seismic-hazard level. In other words¡ the disaggregation process separates the contributions to the mean annual rate of exceedance (MRE) of a specific ground motion value at a site due to scenarios of given magnitude M¡ distance R¡ and the ground motion error term¡ ε. Disaggregation results could change with the spectral ordinate and return period¡ thus more than one single event may dominate the hazard especially if multiple sources affect the hazard at the site. These results can provide useful information for better defining the design scenario and selecting corresponding time histories for seismic design. In most cases¡ as the probability decreases¡ the hazard sources closer to the site dominate. Larger¡ more distant earthquakes contribute more significantly to hazard for longer periods than shorter periods. In this study¡ the seismic hazard disaggregation process is performed to identify dominant scenarios in “peak ground acceleration” (PGA) and 5% damped 0.2 and 2.0 s spectral accelerations corresponding to mean return periods (MRPs) of 50 yr¡ and 475 yr (hazard levels of 63% and 10% probability of exceedance in 50 yr¡ respectively) in Firoozkouh city. In this regard¡ potential seismic sources and their seismicity parameters have been estimated based on the concept of spatial distribution function in 34º–37ºN latitudes and 52º–55º E longitudes in grid intervals of 0.1º. For each point using proper attenuation relationships¡ PGA¡ 0.2 and 2.0 s spectral acceleration values with 63% and 10% probabilities of exceedance in 50 yr have been calculated using the EZ-FRISK (version 7.43) code. The hazard can be simultaneously disaggregated in different types of bin. The result of seismic hazard disaggregation are presented in terms of 1-D M¡ R and ε bins and 2-D M-R bins. Bins of width 0.4 in magnitude¡ 10 km in distance¡ and 0.2 in ε are selected. The disaggregation results in terms of probability density function (PDF) are reported¡ which is obtained by dividing the probability mass function (PMF) contribution of each bin by the bin’s size¡ thus the PDF representation is independent of the bin’s amplitude. The results identify the distribution of the earthquake scenarios that contribute to exceedance of PGA and 5% damped 0.2 and 2. s spectral accelerations for 50 yr and 475 yr MRPs¡ in terms of magnitude and distance (M-R). Dominant scenarios are identified for interest hazard levels in Firoozkouh city.

Firouzkuh prospect area is located about 35 km northeast of Torbat-e- Jam, Khorasan Razavi Province. Geology of the area includes metamorphosed rocks of Miankouhi Formation, which has intruded by monzogranitic to dioritic intrusions. The magnetic susceptibility of the intrusive rocks varies from 0 to 43 ´ 10-5 SI and they belong to reduced granitoids of ilmenite series. These rocks are affected by sericitic, silicification, and propylitic alterations in some places. Geochemistry of intrusive rocks indicates that they vary from metaluminous to peraluminous and belong to medium-K to high-K calc-alkalin and shoshonite series. Tectonic setting of intrusions is pre-plate collision to post-collision uplift. Mineralization has controlled by fault zone and formed in the contact between intrusive rocks and metasandstones or in metamorphosed units. Primary minerals consist of gold, arsenopyrite, pyrite, chalcopyrite, and pyrrhotite and secondary minerals include covellite, hematite, and goethite. Geochemical exploration, using chip composite method, shows anomalies of Au (up to 8942ppb), As (up to 74500 ppm), Cu (up to 357 ppm), Pb (up to 45 ppm), and Zn (up to 97 ppm) in the surface samples of the area, which are related to vein mineralization. Core drilling geochemistry indicates that maximum contents of Au (30732 ppb) and Cu (3200 ppm) exist in OBH-7 and high anomaly of As (98670 ppm) and W (133 ppm) are in OBH-2. Au often shows a positive correlation with As in most samples. It mainly exists in arsenopyrite and less as native gold or pyrrhotite and pyrite minerals. Based on presence of reduced intrusive rocks of ilmenite series, reduced mineral assemblages (arsenopryrite and pyrrhotite), type and development of alteration, mineralization form and high concentrations of Au, Cu, As, and W, the area can be part of a reduced intrusion-related gold system, which requires further investigations.

The Strike-slip faults are not linear and straight features visible on the map view and nature, as they become bent in their path. Bending causes compressional or tensional stress becomes predominant in the region. Astaneh and Firuzkuh faults are the two active strike-slip faults with ENE-WSW trend. Between these two active left-lateral faults, an imbricate fault zone with thrust component occurs. Furthermore, this zone cuts and offsets an anticline. The region is uplifted in this regime and forms a compressional horst between Chashm and Bashm faults; moreover, an additional horst is forming in the south east of the zone.

This study focused on the upper parts of Karaj Formation in Damavand-Firuzkoh area. Field study indicated that the green tuffites of Karaj Formation in Kilan, Hesarbone, and Zarindasht irregularly altered and changed to zeolite and bentonite. Microscopic studies of zeolitic tuffites have shown that the main components of these tuffites are altered glass shards. The shards changed to zeolites from their margins. In addition to zeolite, clay minerals also changed to glass shards. Scanning Electron Microscopic studies confirmed the change of glass shard to zeolite and clay mineral from their margins. The presence of clinoptilolite and carbonate impurity in vitric altered tuffite are determined by thermal curves tests (TG & DTG). Also, X-ray showed clinoptilolite, crystobalite as major mineral and muscovite, montmorillonite as minor mineral in these tuffites. On the base of XRF and ICP, chemical composition of these tuffites are in the range of acid to intermediate rocks compositions. Chemical composition of altered and unaltered tuffites is unique. Base on the petrological studies, the componential magma of the tuffites is calc alkaline. Spider diagrams indicated the enrichment of K, Ba, Th, Rb, which is characteristic of arc magmatism.