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

Document Type : Full length article


1 Ph.D. Candidate in Geomorphology, Dep. of Geography Science and Research Branch, Islamic Azad University

2 Prof. of Geomorphology, Faculty of Geography, University of Tehran

3 Associate Prof. of Geomorphology, Faculty of Earth Sciences, University of Shahid Beheshti


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.
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
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2 Physical Geography Research Quarterly, 46 (1), Spring 2014
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)1 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
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
1. Q = Quartz, A= Alkali Feldspar, F= Feld spathoids, P= Plagioclase.
Physical Geography Research Quarterly, 46 (1), Spring 2014 3
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.
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.