Petrology and Geochemistry of Igneous Rocks in Zarinkamar Area , NE of Shahrood , Iran

Zarinkamar area is located in northeast of Shahrood, northeast of Iran, between latitudes of 36 ̊37’ 36 ̊42’ and longitudes of 55 ̊07’ 55 ̊12’. Different types of igneous rocks have an outcrop on the region. Intrusive igneous rocks include a type of syenite with aegirine pyroxene and the other includes quartz from 5 to 20 percent. Moreover, aegirine syenite, ordinary syenite, and quartz syenite have been injected into Karaj Formation with an age of middle to early Eocene. Also, some parts of trachyte magma have given rise to produce trachyte outcrops after passing the crust and current flows. Studying major and rare elements shows that a magma separation has been caused by separation smelting or a crystal separation. These magmas are alkaline and shoshonites and they have been originated in a continental rift. The magma of Zarinkamar region has been combined with acidic continent crust as it rose to the surface. High amount of Th, and Rb and low amount of Ti, P, Sr, and Ba can prove this hypothesis.


Introduction
Study area is located 28 km northeast of Shahrood in Semnan province in Iran.As far as geological classification of Iran is concerned, the area belongs to eastern Alborz zone.This zone is a subzone of northern part of Alp-Himalia orogeny belt in Western Asia.It is also collided with Caspian block and central Iran block toward north and south, respectively.Based on the geochemical databases and related geochemical figures of major, rare and cation figures, the composition of the magma is classified as Alkaline and Shoshonite magma.As far as petrological issue is concerned, the rocks include intrusive rocks, alkali feldspar aegirine syenite, alkali feldspar quartz syenite and volcanic rocks (e.g.tracite and crystalline silica) which specifies that the tectonic regime of these rocks are continental alkaline rifts with a depletion of Eu (negative anomaly).Based on geochemical studies and related geochemical figures, Eu shows a negative anomaly which is related to plagioclase separation.Also, the magma shows a concentration of LREE/HREE.A concentration of LREE is one of the specifications of OIB showing a presence of garnet in a magma resource.La/Yb is about 6.16 to 8.8 and La/Sm is approximately 2.98 to 3.49; also a notable negative anomaly of Eu is considered (Eu/Eu* = 0.05 − 0.69).Based on the geochemical database and related figures of major and rare elements, the magma of Zarinkamar region is classified as an alkaline and Shoshonite magma which consists of syenite, syenite alkali feldspar with quartz, and syenite quartz.The pattern of related normalized to diagonal values of the rare elements demonstrate the concentration of lithophile elements with high ion radius (LILE) of Th, K, and Rb; while, the elements with high ionic strength (HFSE) show depletion.A negative anomaly of Ti, P, Sr, and Ba shows a fractional crystallization and lack of depletion of Nb-Ta illustrates Shoshonite masses which have been originated within continent environments.Also, their tectonic source of these rocks is continental alkaline rifts.

Alkali Feldspar Quartz Syenite
Linear alkali feldspar with long and stretched shapes which has been getting gray and dusty color has been identified in thin sections, they are almost similar and in the same size.Also, some of them have kept their primitive shape and shown a simple macle.Their frequencies are about 90 to 95 percent which have crossed each other.Pyroxenes (acmite and aegirine) have been identified among them with a dark and olive green color to brownish green color with a zonal condition and semi shaped frame which all of them show a zonal condition vividly.Quartz has been placed in residual spaces among minerals as micro crystalline with a frequency of 5 percent and it shows a hypidiomorphic intergranular (Figure 1).

Alkali Feldspar Aegirine Quartz Syenite
Elongated minerals of feldspars are often gray and dusty and show a simple macle with a regular shape and different size.The macles are usually crossed each other and they normally have been altered and become kaolinitized.Their

Geochemistry
After plotting geochemical databases of the samples on QAP    was high in the beginning of the separation and it decreases as separation process develops and it dropped to its minimum content in the final stages of the separation and it locates finally in the crystalline structure.Total iron content increases as separation phase expands and it reaches to its maximum concentration at the final stages of the separation process and it contributes in ferrous mineral compositions (Figure 8).
K, Th, Rb, Nb, Hf, Zr, Sm, Y are concentrated in the rocks of Zarinkamar region and Ba, Sr, P, Ti are depleted in these samples.High content of Rb is related to Shoshonite properties and it also may relate to contamination with crust.Solubility of titanium minerals decreases as pressure increases in aqueous fluids.
These minerals can be remained as residual phases during partial melting processes in the depth of over 30 km and it causes a negative anomaly in the melt [9].Based on a theory [10], LREE concentration in the rocks shows an intervention of continental crust on forming the magma (Figure 9).
In the normalized multi elements of spider diagram related to chondrite [11], Ba, Sr, P, and Ti elements show depletion and Rb, Th, La, Ce, and Nd show concentration.The high amount of Rb, and Th and low amount of Ba, Sr, P, and Ti are consistent with crust properties which shows that crust materials have made some impurities during magma evaluation.Sr also enters to feldspar structures and Rb is placed in the potassium feldspar structure.Based on the theory raised by [12], negative anomalies of Ba, Sr, P, and Ti shows fractional crystallization and non-depletion of Nb-Ta shows a source of Shoshonite masses formed mainly within inter continent environment (Figure 10).
In the normalized diagrams to the REE chondrite    (Eu/Eu* = 0.05 -0.69), Figure 12, are notable.Figure 13 shows that all the samples have been placed within a negative anomaly of Eu.Eu anomalies are often controlled by feldspars since Eu is stable in its bivalency in calcium feldspars; while, other trivalent REE are inconsistent in calcium feldspars.Therefore, separating calcium feldspar from the melting phase can lead to a negative anomaly of Eu in the magma either the separation has been caused by crystalline separation or partial separation (which feldspar is remained in the magma).The Eu    depletion is probably because of plagioclase removal from the source or later discarding by fractional crystallization in the magma chamber prior to emplacement [14].Also, the absence of plagioclase in the macroscopic samples proves the negative anomaly of Eu in samples [15].

Tectonomagmatic Environments
In the diagram of Yb versus Ta [17], rock samples have been placed in the interplate tectonic region (Figure 13).Also, in the triplex diagram of Y-Nb-Ce [18]
frequencies are approximately 80 to 90 percent.Quartz crystalline appears in a shaped and hypidiomorphic formation among vacant spaces among minerals and their frequency is about 10 to 15 percent.Pyroxenes are in light to dark green colors in a stockwork texture with small to big crystalline which the bigger the pyroxenes are, the better zoning and vertical cleavage properties are identified.Apart from pyroxene, amphiboles are appeared with inclined cleavage with a green to brown and dark green color which all of them are macro crystalline with angled and amorphous shapes.Ferrous minerals are in dark red which are amorphous and mostly in stockwork texture with angled shapes.According to the thin-section interpretation, this sample has hypidiomorphic intergranular texture (Figure2).

Figure 8 .
Figure 8. Geochemical database of Zarinkamar samples on the major elements versus separation coefficient[6].