فهرست مطالب سمیه عباس زاده

Abstract The Early Cretaceous succession in the southern part of Yazd Block begins with terrestrial sediments of the Sangestan Formation. Little attention has been paid to this formation and most studies have focused on its lithostratigraphy. In this study, some geochemical studies have been performed on this formation. Twenty seven siliciclastic samples were taken from the Sangestan Formation in two sections of Bidakhvid and Alavi Mountain in order to identify the type of siliciclastic sediments, the nature of the sediments, and palaeoclimate conditions. Mineralogical studies and modal analysis showed that the siliciclastic sediments have been classified as sandstone (arkose) and siltstone. The results of factor and cluster analysis depicted that the chemical composition of siliciclastic sediments is different in the study areas. Based on multivariate analysis, siltstones that have high positive values of Zr, V, U, Sc, Sr, REE, MgO, Fe2O3, TiO2, and CaO elements are related to cement, while sandstones showed high positive values for K2O, Na2O, SiO2, Al2O3 elements. The results showed that siltstone sediments have more calcite and iron cement than the sandstone samples. By studying the variations of these elements versus the profile of the Sangestan Formation, it was observed that the elements related to rock cement are concentrated in siltstones due to the increasing depth of the sedimentary basin or more extension related to rift event, and the entrance of new elements into the basin. Also, the results of weathering indices depicted that both rocks have been formed in the same climate conditions which is consistent with the palaeogeography of Yazd Block during the Early Cretaceous.Keywords: Sangestan Formation, Yazd Block, Geochemistry, multivariate analysis, sedimentary environment, weathering index  IntroductionThe Sangestan and Taft formations dating back to the Early Cretaceous were deposited in many locations of Yazd Block which is located in the western part of the Central–East Iranian Microcontinent.In general, most studies have focused on the Taft Formation because it hosts lead and zinc deposits such as Mahdiabad, Farahabad, Mansourabad, etc. (Mojtahedzadeh 2002; Maghfouri and Hosseinzadeh 2018; Maghfouri et al. 2019; Maghfouri et al. 2020). On the other hand, a few studies have been performed on the Sangestan Formation focusing on its stratigraphy and paleontology. Until now, the geochemistry of Sangestan sediments and their sedimentary environment have not been paid attention to.The use of major, trace, and rare earth elements of siliciclastic sediments is very common in sedimentary geochemistry studies used by researchers to identify various purposes, including the tectonic setting, palaeoweathering conditions of the source area, and the nature of the source rock (Shadan and Hosseini-Barzi 2013; Wang et al. 2013; Nowrouzi et al. 2014; Salehi et al. 2014; Zaid et al. 2015; Moallemi et al. 2017; Okon et al. 2017; Periasamy and Venkateshwarlu 2017; Somasekhar et al. 2018; Abubakar et al. 2019; Etesampour et al. 2019; Sci et al. 2019; Xu et al. 2019).Multivariate statistical methods have been utilized to determine the relationship between several elements and the nature of sediments; something impossible by univariate statistical analysis. Despite the advantage of multivariate statistical methods, they have not been paid considered in siliciclastic geochemistry and they have been used only in a few studies.The main purpose of this study is to evaluate the composition and petrography of siliciclastic rocks of the Sangestan Formation, the relationship between the composition of the sediments and the deepening of the sedimentary basin, the relation between the nature of the sediments and the major and trace elements by multivariate analysis, and the climate conditions in Yazd Block during the Early Cretaceous. Material & MethodsAfter field observation, sampling was performed from the Sangestan Formation at the Bidakhavid and Alavi Mountain sections. Seventeen samples (nine siltstone and eight sandstone samples) from Alavi Mountain and 10 samples (three siltstone and seven sandstone samples) were collected from Bidakhavid.Sixteen thin sections from siliciclastic samples were studied petrographically by the Olympus model polarizing microscope.  The point counting of more than 500 points per thin section was performed on twelve sandstone samples by the Gazzi-Dickinson method.Twenty-seven samples of siliciclastic rocks of the Sangestan Formation were selected for geochemical objectives. The preparation steps of the samples which include the cutting of the samples in order to obtain an un-weathered surface, crushing, and grinding to achieve the particle sizes under 200 mesh were performed at Yazd University. Then, the representative samples were analyzed by Zarazma Mineral Studies Company (Iranian company). To determine the major oxide elements, the samples were fused by lithium metaborate and dissolved in dilute nitric acid and, finally, determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). An ICP-OES 735 instrument in Australia was used to analyze the samples. Moreover, the rare earth elements (REE) and other trace elements were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The instrument used was an ICP-MS Agilent series HP 4500 in the United States.SPSS and GCDkit software were used to carry out multivariate statistical studies and to plot ternary diagrams, respectively. In addition, Rockwork 14 software was used to draw the stratigraphic column. Discussion of Results & ConclusionsThe preliminary study of thin sections showed that the siliciclastic samples of the Sangestan Formation include sandstone and siltstone in the two studied sections. Sandstone samples are often medium-grained, with a small percentage of coarse-grained, moderately rounded and immature. Modal analysis was performed on sandstone samples for more detailed studies. The main constituents are quartz grains (56% and rock fragments ( less than 1%), as well as feldspar (25%).Siltstone samples are similarly composed of quartz, feldspar, and rock fragments. They have about 52% quartz, most of which is monocrystalline, and only less than 1% is polycrystalline quartz. The amount of feldspar is less than sandstone samples and they constitute about 16% of the rock volume. Similar to the sandstone samples, a few rock fragments were observed in siltstone samples. Furthermore, on the basis of the classification of Folk (1980), sandstone samples of the Sangestan Formation showed arkosic composition.  This result was validated by geochemical data confirmed by the geochemical classification of Herron (1988).In factor analysis, the positive and large factor loadings demonstrate a significant correlation between the variable and corresponding factor in the matrix. The results of factor analysis identified two first factors which discriminated the elements associated with the main constituents of siliciclastic sediments and those  related to the cement of siliciclastic sediments.For example, factor 1 showed positive factor loadings with Zr, V, U, Sc, Sr, REE MgO, Fe2O3, TiO2, CaO and negative factor loading with Cr, Na2O, SiO2, and Al2O3.  This factor depicted that the cement of rocks was formed from carbonate, iron oxide, and so on whereas the main constituents of siliciclastic samples were formed from elements such as Na2O, SiO2, and Al2O3. Furthermore, the results of the hierarchical cluster analysis shown in the dendrogram confirmed those of factor analysis.In order to evaluate the changes of these groups of elements, they were plotted along the sedimentary log. The results showed that the elements which are associated with the framework constituents of siliciclastic samples are more in sandstone rocks than siltstone rocks. Conversely, the elements related to the cement of siliciclastic sediments are more in siltstones than sandstone rocks.  Thus, it can be interpreted that the time of siltstone deposition was probably accompanied by an increase in the depth of the basin and the presence of more cement in the basin. Finally, the results of (A-CN-K) Al2O3-(CaO*+Na2O)-K2O ternary diagram and diagram which was proposed by Suttner and Dutta (1986) depicted that these sediments have been deposited in arid to semi-arid climatic conditions.

In geochemical exploration, there are various techniques such as univariate and multivariate statistical methods available for recognition of anomalous areas. Univariate techniques are usually utilized to estimate the threshold value, which is the smallest quantity among the values representing the anomalous areas. In this work, a combination of the Sequential Gaussian Simulation (SGS) and Gap Statistics (GS) methods was utilized as a new technique to estimate the threshold and to visualize the anomalous regions in the Hararan area, which is located in SE Iran, and consists of copper mineralization that seems to be connected to a porphyry Cu-Mo system. Furthermore, the most important advantage of this method is the reliable assessment of the anomalous areas. In other words, the anomalous areas were discriminated in terms of their probability values. The regions with high probability values were reliable and appropriate to locate the drilling points for a detailed exploration. It not only decreases the risk, cost, and time of exploration but also increases the drilling point reliability and precision of reserve estimation after drilling. In this research work, the results of analysis of 607 lithogeochemical samples for the element Cu were used. The SGS method was performed on the transformed data and 50 realizations were obtained. In the next step, the back-transformed realizations were utilized to obtain an E-type map, which was the average of 50 realizations. Moreover, the results of the GS method showed that the Cu threshold value was 228 ppm in the area. Therefore, using the E-type map, areas with values greater than 228 ppm were introduced as the anomalous areas. Finally, the probability map of the exceeding threshold values was acquired, and the anomalous districts located in the southern part of the studied area were considered as more reliable regions for future detailed exploration and drilling.

One of the silver resources is Cu-Mo porphyry deposits, producing silver as by product. For this research, the results of analysis of 607 lithogeochemical samples for W, Zn, Ag, As, Ba, Co, Cu, Mo, Pb, Sb, Sn, Sr, Bi elements were used to investigate geochemical behavior of silver element in Hararan area which is located in the southeast of Iran and consists of copper mineralization seems to be connected to a porphyry Cu-Mo system. In this study, gap statistics and multivariate analysis techniques for identification anomalous areas of silver element and its associated elements were used. Anomalous areas maps achieved using gap statistics method showed that anomalous areas of Ag, Cu, Pb and zinc elements covered approximately each other. Dendrogram, which was earn from hierarchical cluster analysis, depict that Ag and Cu elements have associated with each other and formed one cluster. Although these elements are linked to Pb and Zn elements with lower correlation. As well, the results of factor analysis showed that the third factor include Cu and Ag elements with the highest eigenvalues. Consequently, geochemical bimodal behavior of Ag element i.e. accompaniment with Cu element by locating in Cu –bearing minerals such as chalcopyrite and bornite from one side and accompaniment with Pb and Zn elements by locating in mineral from other side was validated by using gap statistics and multivariate analysis techniques.

In geological sciences, especially geochemistry, multivariate statistical methods have many applications. So that, they were employed as a new application for recognition main alterations in Cu-Mo porphyry system such as argillic, potassic, and propylitic alterations. For determination of alteration areas, results of 607 lithogeochemical samples were utilized in Hararan area, which appears to possess the potentiality for porphyry mineralization and locates in Baft sheet (1:100,000 series) in the southeast of Iran. 607 rock samples were analyzed by ICP-MS in Amdel laboratory for 45 elements. However, it used just the results of Ca, Na, Al, Fe, S, K, Rb, Mg elements for this paper. Results of hierarchical cluster and k- means Cluster methods showed two different clusters: K, Rb, S elements and Mg, Ca, Na, Al, Fe elements have been in cluster 1 and cluster 2, respectively. Cluster1 and cluster2 introduced as potassic and argillic alterations and propylitic alteration, respectively. The results of factor analysis not only confirmed the above results but also the first factor map depicted small areas of argillic and potassic alterations in the southern parts of the region, so that these areas did not identify by k-means cluster method.

Ghale–Askar area is located in the east north of Baft city in which Cu-porphyry mineralization has occurred. 63 samples were analyzed for 24 elements such as U، Th، Ba، W،As، Mo، S، Bi، Re، Sn، Cr، Ni، V، Co، Sr، Zn، Mn، Cd، Pb، Sb، Au، Cu، Ag and Hg with ICP-Ms method. One and multivariate statistical techniques were applied on drainage geochemical data. To draw the map of elements’ anomaly، the threshold was calculated with gap statistics and methods; the results of the mentioned methods specified that the threshold value obtained from approach was greater than that of gap statistics and its map represented smaller region. The results of hierarchical cluster method showed six separated clusters that the fourth cluster including (Bi، Cu، S) and the fifth cluster including (Ni، U، Mo، S، Th، Cr) were identified as the center of mineralization. Moreover، the results of K-mean cluster method confirmed the results of hierarchical cluster method and represented the center of mineralization in red and green colors in that region. Besides، factor analysis was carried out and studying eigenvalues of the sixth factor described that it was the center of mineralization. Finally، the results were integrated and identified the best regions for the future studies.

Nostalgia in terminology has regret and sorrowful sense for brilliant past and mislaid things that don’t exist now. Social and political and economic conditions are effective completely in evident of grief and sad for past conditions and circumstances. Accepts of this sentiment is seen in Persian contemporary poem clearly. Contemporary poets have explained variously the sentiments and feelings in this connection. In this paper we try to research the nostalgia terminology and definition of this word in psychologist's view. We are struggle in this writing to precede the effective factors in establishment of nostalgia and various reflections in four Persian contemporary poets.