نوع مقاله : مقاله پژوهشی

نویسنده

گروه زمین شناسی، دانشکده علوم طبیعی، واحد شبستر، دانشگاه آزاد اسلامی، شبستر، ایران

چکیده

آتشفشان سهند در کمربند ولکانو‌ـ پلوتونیک ارومیه‌ـ دختر واقع شده و ساختمان آن شامل تناوبی از گدازه‌ها و مواد آذرآواری مختلف است. این آتشفشان از نظر سن نسبتاً جوان (میوسن تا پلئیستوسن) است؛ طوری که بر اساس مطالعات رخساره‌ای می‏توان ساختمان آتشفشان سهند را در چهار رخساره تفکیک کرد. رخسارۀ مرکزی در محل کالدرا واقع شده و واحدهای سنگی خاصی، چون توده نفوذی نیمه‌ژرف دگرسان‌شده و برش‌های گدازه‌ای و دایک‌های حلقوی و شعاعی، دارد. رخسارۀ نزدیک آتشفشان به صورت دیواره‌های پرشیب و مرتفع در اطراف کالدرا گسترش پیدا کرده و ساختار چینه‌ای را به‌خوبی نشان می‌دهد که در آن توالی‌های ستبر واحدهای آذرآواری ریزشی، خیزابی، جریانی، و گدازه به صورت متناوب قرار گرفته‌اند. رخسارۀ میانی، با بیشترین گسترش، به طور عمده از مواد آذرآواری خیزابی، جریانی، گدازه، و لایه‌های متعدد لاهار تشکیل شده است. رخسارۀ دور آتشفشان سهند ستبرای کمی دارد و شامل رسوبات رودخانه‌ای درشت‌دانه‌ای است که در اثر تحرک دوباره محصولات اولیۀ آتشفشان و حمل آن‌ها توسط رودخانه‌ها به وجود آمده‌اند. مطالعات رخساره‌ای نشان می‌دهد ساختمان آتشفشان سهند در اثر چندین فاز فورانی انفجاری ایجاد شده که برخی از آن‌ها با دخالت آب همراه بوده‌اند و فاصلۀ زمانی زیادی میان فوران‌ها وجود نداشته است. ویژگی‌هایی چون جوش‌خوردگی شدید و رنگ نهشته‌های آذرآواری نشان می‌دهد این واحدها در زمان تشکیل دمای بالایی داشته و در خشکی تشکیل شده‌اند.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Facies Studies of Sahand Stratovolcano (East-Azarbijan Province) Farhad Pirmohammadi Alishah

نویسنده [English]

  • Farhad Pirmohammadi Alisha

Department of Geology, Faculty of Natural Sciences, Shabest branch, Islamic Azad University, Shabest, Iran

چکیده [English]

1-Introduction
Sahand Volcano, with an area of ​​about 7200 km2, is a stratosphere located in East Azarbaijan Province, and since it has retained some of its original structure and volcanic facies due to its young age, its facies studies can be well studied. Understanding the mechanism of volcanic activity in this region, examining the possibility of various deposits and finding their exact location and recognizing related geological processes and separating urgent phases, shows the importance and necessity of epistemological studies in this volcano. The purpose of this study is to identify the facies of Sahand volcano and to achieve this goal, we have tried to identify and study the facies by using extensive field studies and introducing and describing the facies and volcanic products.
 
2-Materials and methods
In order to properly understand the geology of Sahand volcano and to determine the relationship between different rock units and to study the epistemology by conducting field visits, geological features from the central part of Caldera to the walls and also from the outer parts to Caldera, by conducting several surveys. Various studies and sections were examined. Due to the fact that the domes of the last volcanic eruptions of Sahand volcano are mostly in the central and eastern part of the volcano, the sections worked in the central and eastern parts of the region are located. In the next step, by examining the facets of volcanic products and studying the sedimentary tissues and structures present in pyroclastic and opioclastic deposits of different sections, it was tried to use these tools to some extent to characterize the sedimentation environment and how Make the faces clear.
 
3-Results and discussion
In the evolution of sedimentary basins close to volcanic centers, there is a complex relationship between processes such as subsidence of the basin, processes related to the evolution of volcanic arcs, and climate phenomena (Catonino 2004; Smis et al., 2002). Climatic phenomena have led to the re-emergence of volcanic deposits during eruptions and intermittent eruptions, and sedimentation has preserved evidence of how these processes have changed. This evidence is better preserved in areas with rapid subsidence (Catonino 2004; Leader 2000, Gaussorpe, and Incele 2000). One of the features of Sahand volcanic rocks is the very thick thickness of the products. It seems that volcanic eruptions in the Sahand region initially led to the formation of large volumes of emergency materials. Then, when volcanic activity has stopped, pyroclastic deposits and lavas are eroded and transported by river networks, and epic-plastic sequences are produced, and these deposits are generally processed by processes. Called epiclastic, they are transported and re-deposited (Cass and Wright 1987). Recent studies on sedimentation in sedimentary environments affected by explosive volcanic activity have shown that the alternation of sedimentation conditions with eruption and interstitial sedimentary conditions is the most significant factor influencing the formation of deposits. They are in these environments (Martina et al. 2006; Manasro et al. 2000; Katuka and Nakaju 2002; Nakayama and Yoshikawa 1997). Powerful erosion periods in volcanic areas are likely to be related to climate change (declining water levels, different types of plants along the range) or volcanic-structural changes in the region (Calcutta et al. 2007). Exploratory studies of volcanic products are very useful tools for understanding the nature of sedimentary environments at the time of eruption. Because volcanic rocks are formed in both continental and marine environments, describing the features of their epistemological form can provide valuable information about the characteristics of the sedimentation environment. And provide researchers with the type of environment and even the type and intensity of volcanic eruptions.
 
4- Conclusion, Keywords
Sahand Volcano is a stratovolcano that erupts its material from numerous scattered craters, and its volcanic activity has alternated with explosive and lava flows that have led to the formation of pyroclastic and lava deposits. The eruptions were mostly explosive and not far from each other, but during periods of eruption between volcanic eruptions, volcanic material was eroded by rivers and deposited on the riverbed or sedimentary basins of the low-lying continent. They were left deep. Chinological studies have shown that Sahand operated for a relatively long period of time, with its first volcanic activity beginning in the middle of the Miocene and continuing intermittently until the end of the Pleistocene. The first eruption in Sahand was very explosive and involved water interference. The reaction of water and acidic magma beneath the Earth's surface caused huge ferromagmatic explosions, and pieces of host rocks, along with volcanic ash and Pamis, were thrown into the air, making Sahand the first product. The detection of this immediate phase is possible only by studying the medial facies, because the Azarvari deposits, which were the result of this eruption, have the best outcrops in this facies. Then the eruption stopped and erosive agents created holes in the surface of the flow deposits. These side effects are visible on the middle face. Subsequent immediate events, which have created a large number of Azarvari deposits, have been associated with the frequent rise of Andesite and Dacite magmas, successive eruptions, the formation of lava domes and the collapse of domes, and sometimes the release of lava. The effects of these events can be seen in near and medium facies. After this, the Sahand caldera collapses, and the next phase involves simultaneous eruptions after the formation of the caldera, accompanied by the creation of the last Azeri deposits and lavas. The effects of this phase can be seen in the central, near, and middle facies. Sahand's central facade is located on the site of Caldera, and after the cessation of Magmayi's activities, it has undergone extensive heat transfer and severe erosion. Thermal alteration has had the greatest effect on the semi-volcanic infiltration mass, causing it to undergo completely alterations and a set of secondary minerals.

کلیدواژه‌ها [English]

  • Sahand
  • Stratovolcano
  • Volcanic Facies
  • Pyroclastic deposits
  • Stratification