ارزيابي اثر تركيب كاني‎شناسي واحدهاي سنگي توده نفوذي الوند برمقاومت رخنمون‎ها در برابر هوازدگي و فرسايش

An Assessment about the Effect of Mineralogical Composition of Alvand Pluton Rock Units on Outcrops Resistance against Weathering and Erosion

توده نفوذي الوند، يكي از بزرگترين توده‎هاي نفوذي در كمربند دگرگوني سنندج ـ سيرجان است. اين مقاله به ارزيابي اثر تركيب كاني‎شناسي واحدهاي سنگي بر مقاومت آنها در برابر هوازدگي و فرسايش پرداخته است. اين كار با ارايه يك روش پيشنهادي، يعني استفاده از نمودار QAPF انجام گرفته است. در اين روش با توجه به قرارگيري كاني‎هاي كوارتز و فلدسپاتوييدها در دو قطب مخالف نمودار و نقش تعيين‎كننده آنها در مقاومت رخنمون‎ها، ابتدا سطح نمودار QAPF به ده محدوده تقسيم و ارزش‎گذاري شد كه ارزش هر محدوده به‎صورت نسبي، معرف تاثير تركيب كاني‎شناسي بر مقاومت سنگ‎هاي داخل آن در برابر هوازدگي و فرسايش است. سپس درجه مقاومت رخنمون واحدهاي سنگي توده نفوذي الوند تعيين و ضمن طبقه‎بندي در چهار گروه، نقشه پهنه‎بندي مربوطه تهيه شده است. با تعيين درجه مقاومت كاني‎شناسي رخنمون واحد‎هاي سنگي، مي‎توان نقش تركيب كاني‎شناسي را به‎صورت يك متغير كمّي در تحليل‎هاي ژيومورفولوژيكي براي تبيين فرم و فرآيند‎هاي حاكم بر ناهمواري‎ها مورد توجه قرار داد.

Introduction In general, strength of rocky outcrops is associated with two factors, feature lithology that includes mineralogical composition, texture and rock Structure and environmental factors that is the area stones are located in (Hafezy moghadas, 2011, 229 after Ulusay, 1994). The mineralogical composition determines sensitivity of rocks against physical, chemical and biochemical attacks (Mahmoodi, 2010). Rocks because of containing different minerals show various stability against the degradation factors (Nikoodel, 2011). This is the first time research to use classification system of in geology for naming rocks to determine the degree of resistance of the stones against weathering and erosion. Methodology The Alvand Pluton is one of the largest intrusive masses in Sanandaj- Sirjan metamorphic zone (Sepahi, 2008).The exposed area of this mass are approximately 362.92 square kilometers (excluding Quaternary deposits). Alvand Plutonism was started from middle Cretaceous and continued till early Tertiary (Paleocene) (Sepahi & Moeen vaziry, 2000). Alvand pulotonic rocks have been marked on the geological map (Tuyserkan and Hamadan1:100,000 scale) in 100 limited areas with 8 different symbols. That usually has light grey to white color and fine to coarse grains (2-5mm in diameter). About 9 square kilometers of cordierite andalousites and cordierite hornfelses (metamorphic rocks) in the 23 areas with different extent are over the surface of porphyroid granite which mostly matches top heights (especially Heights Gavboreh). This shows diapirism in placement of the mass. In this research, library and field observation have been used as the methods of data collection, (sampling and observation) and the research method has been descriptive and analytical. For evaluating the effect of mineralogical composition of the Alvand pluton rock on outcrop resistance some steps have been taken. In next step, the classification was proposed by the International Union of Geological Sciences (IUGS), and diagram (QAPF) associated with it, was selectedwith range diagram, according to the mineralogical composition. Numerical values are determined for the degree of outcrop resistance of each area against weathering and erosion. In order to determine the degree of Alvand pulotonic outcrops resistance we have specified surface of QAPF diagram. The resistance specified as numerical value is ranged from 1 to 10 classes started from foidolites with least degree of resistance and with ratio of decreasing F and P and increasing A and Q. It is the most sensitive igneous rocks against chemical weathering considering mineralogical composition. Thus, quartz-rich granitic-rocks are in 9 areas and finally quartzolit (silexite) which is the most resistant intrusive igneous rock against weathering. To test the proposed method, 10 types of Alvand pulotonic rocks which gathered by Zarian et al., 1972 with Modal analysis method was studied and selected by using diagram QAPF for resistance range. In addition, finding equivalent name in classification system of IUGS, the degree of the resistance against weathering and destruction in terms of mineralogical composition is given in. Results and Discussion Naming Alvand plutonic rock units on geological maps is based on IUGS classification. Thus, based on provided method, relative strength of the rock outcrop in terms of the effects of mineralogical composition on the resistance against weathering and erosion have been identified and presented in Table 1. Specific degree of resistance for Alvand plutonic rock units are presented on Table 4. Rock units of this mass can be classified in four groups, the least of resistance is related to Olivine gabbro outcrops with 2 degree which cover 14.8% of the mass area.The most resistant rock outcrops are the unit pegmatitic granite, pegmatic- aplite granite, tourmalin granite and granite bearing garnet with a resistance degree of 8. Among other units this later covers just 3.15% of surface area. Degree resistance was determined for different rock units in Alvand pluton and it shows 6 degree resistance differences between them.This difference can be affected by weathering and differential erosion between different units. Average weighted degree rock units outcrop resistance of Alvand in terms of mineralogical composition is obtained for total mass about 6.59. It shows resistence of the mineralogical composition of the outcrops and the results can be found in relation to high percentage of quartz and alkali feldaspar, and the approximate homogeneity in mineralogical composition of the rock mass. In order to determine the applicability of the results obtained from the proposed method in geomorphological and hydrological analyses, the Digital Elevation Model (DEM) of Alvand pluton was provided. Conclusion In this paper, the effect of mineralogical composition of Alvand plutonic outcrops against the weathering and erosion with the numeric range 1-10 and using QAPF diagrams have been determined. In this method, Surface of QAPF diagram between Q and F has been divided into ten areas with in numerical range of 1-10. The range of values represents the effect of the mineralogical composition on degree of resistance of outcrops of rock units. According to degree resistance designated for Alvand pluton rock units in terms of the effects, this mass can be classified in four groups.In this classification the least resistance is related to Olivine gabbro outcrops with degree resistance of 2 and the most resistant rock outcrops with resistance degree of 8 are the units including pegmatitic granite pegmatitic– aplite granite, tourmaline granite, and granite bearing granite. According to average weighted degree granitic outcrop resistance of Alvand in terms of mineralogical composition, the degree is obtained 6.59for total mass.. Determining resistence of Alvand rock units in quantitative mineralogical composition and the properties of texture and structure characteristics can be used for geomorphological analysis and explanation of predominant form over Alvand mass.