Petrology and Geochemistry of Dikes in the North of Saveh in Iran

Dikes in the north of Saveh are located in a region with an area of approximately 200 square kilo-meters, in a distance of approximately 100 kilometers south of Tehran. Dikes mentioned in terms of petrological composition, are divided into two categories: alkaline and intermediate to acidic. Alka-line dikes include: andesitic basalt and andesite and intermediate to acidic dikes include: trachyte and trachyandesite. In terms of geochemical, dikes in the north of Saveh have a dual nature of alkaline and calc-alkaline. Both groups are derived from more enrichment source than primitive mantle. Despite similarity of pattern of both groups, varieties of alkaline having less silica, in the elements Sr, Ti, Nb and Ta show more enrichment and in the elements Hf, Rb, Th, K show less enrichment than varieties of calc-alkaline.


Introduction
The studied area with an area of approximately 200 square kilometers, in the north of Saveh city, is located between 55˚15' to 55˚50' East and 35˚00' to 35˚15' North.In this area, more than 250 dikes with an approximate thickness of 0.4 to 12 m and a length of approximately 50 to 3000 meters have been intruded in an area of approximately 200 square kilometers and with a combination of andesitic basalt, andesite and trachyte in Eocene volcanic rocks (Figure 1).Studied swarm dikes, are alike type IV swarm dikes of Ernst [1] which are due to creation of the regional stress fields [4].According to the studies of Hou [2] and [3], dikes in this area can be considered similar to dikes parallel to the linear pattern which have created in a relatively wide area and influenced by regional tensions caused by a phenomenon such as subduction [4].
Figure 1.Dikes in the north of Saveh on the geological map of the studied area [4].
Previous studies in this area have been carried out by different people and Geological Survey of Iran, in the form of preparing geological mapping at a scale of 1:100,000 and 1:250,000.Based on the studies, the mentioned dikes are acidic and alkaline dikes [5] and have the feeder role of Eocene volcanic rocks of the region [4].In this article it is tried to introduce a variety of petrology, geochemical characteristics and tectonomagmatic environment of dikes in the north of Saveh.In this regard, after doing field studies and detailed petrography, 15 samples of dikes have been subjected to chemical analysis by XRF ICP_MAS and methods in laboratories of the University of Tarbiat Modarres and Zarazma Company for major, minor and trace elements (Table 1), then on two selected samples, 44 point microprobe analyses were carried out by electron microprobe machine type GEOL8200 in the laboratory of the University of Lisbon in Portugal to determine the mineral types (Table 2 and Table 3).

Geology of the Area
In the division of geological units, the studied area is a part of the volcanic zone of Orumieh-Dokhtar.The most ancient rocks in this area is dependent on the Middle Eocene.In general, structures in this area have a trend of the West-North West, East-South East.The most important structural elements in this area are original fractures and folding.Due to governing movement pattern of the area (shear-compressional) in most cases, regional faults zone have both horizontal and vertical displacement components [6].Swarm dikes investigated in this study, are cut Tertiary volcanic rocks (Figure 2).

Petrography
Dikes in the studied area are generally divided into two categories: alkaline and intermediate to acidic.

Alkaline Dikes
These dikes in terms of petrology include andesitic basalt, basaltic andesite and andesite.Their texture is Porphyritic, Hyalo porphyritic with microlitic pulp and sometimes glomeroporphyritic.Plagioclase Phenocrysts with a combination of Andesine (An% = 42.59) to labradorite (An% = 50.73)semi-shaped to self-shaped make their coarsely crystals (Figure 3(a) and Figure 3(b)).Some of them are quite normal and have zoning (An% = 47.77-50.5) (Figure 3(c)) with inclusions of biotite.Plagioclase strongly altered and changed to sericite.
Pyroxenes are in the form of semi-shaped Phenocrysts of augite type with chemical changes of En43.45 Fs15.37Wo41.43 to En40.06Fs23.51Wo36.43 (Figure 4) which partly are altered to chlorite and amphibole.In glomeroporphyritic sectors, pyroxene phenocrysts are often as integrated crystals with each other and completely altered so that only their shape remains and their empty space is filled with calcite and quartz and opaque minerals.Just in a few spots small parts of them remain.
Another mafic mineral in these rocks, is olivine in very low quantity which is completely altered and their vacancies are filled by epidote and calcite and remain only spherical forms of them in size about one millimeter.These minerals are often in a basis of composed plagioclase microlites and fine opaque minerals and scattered and a little glass with an eligible spherical cavities in the size of mm and are filled by radial calcite and iron oxides.

Intermediate to Acidic Dikes
Intermediate to acidic dikes are included Trachyte and trachyandesite.Trachytic dikes of the area show trachytic, microlitic and glomeroporphyritic texture.Phenocrysts in some trachytic cases are very rare.These phenocrysts are types of self-shaped sanidine and have 500 microns dimension.Other phenocrysts samples are pyroxene and plagioclase (Figure 5).Almost all phenocrysts completely altered and only original form of these phenocrysts is remaining.Background of these rocks entirely consists of oriented microlites of feldspar which along with them secondary fine crystal biotites are seen.These biotites are result of recrystallization of primary biotites.Finegrained opaque and amorphous minerals are scattered in the total amount of rocks and their quantity is less than 5%.Sericites are scattered and can be seen in the context as fine crystals.Calcite and hematite are scattered in the whole context of stone and in pores sized in mm also calcite and hematite is formed with opaque minerals.

Geochemistry
In the diagram Na 2 O + K 2 O-SiO 2 [7] the composition of dikes samples are in two sub-alkaline and alkaline   By examining variation diagrams of main elements' oxides versus SiO 2 (Figure 7), at first, a discontinuity in the value of silica of 46 to 55 is observed.This dual role does not prevent the regular changes of various oxides.So that from variation diagrams of FeO and MgO versus SiO 2 , generally show an obvious reduction trend with development process of subtraction.Reducing these elements suggests the idea of their participation in the structure of ferromagnesian minerals such as olivine, hornblende and biotite and titanium oxide during magma differentiation.
However, distribution of points in the CaO diagram, which probably due to propylitic alteration in some samples, a decreasing trend can be seen in the collections of basic and intermediate-acidic, this decline may be due to their participation in the structure of plagioclase and pyroxene.Al 2 O 3 content of the samples is relatively fixed with the progress of differentiation in two types of stones, but shows an overall downward trend for it.Reducing this oxide indicates its consumption to make a variety of silicates.
In the changing Na 2 O graph there is no specific process.This element in the structure of minerals that are firstly crystallized is not used and with differentiation progress, its increased amount in the melt has been   increased and finally inters into the feldspar structure.Rising trend of K 2 O showed that with progress of differentiation its value in the melt increases and fractional and eventually enter the structure of feldspar.In general, in most variation diagrams, dispersions are observed that may be a sign of digestive processes that affect the differentiation process.Irregular distribution of sodium, potassium and calcium plots and decreasing amounts of iron, phosphorus, calcium and aluminum may be a sign of digestion or react with the rocks in the path that have had high content of elements above.
Spider diagrams: examining normalized plot [9], the average of both dikes, compared to primitive mantle, following results are obtained (Figure 8): • Both groups compared to primitive mantle are originated from more enriched source.This enrichment in LILE elements such as Rb, Th, U and K is higher.• Despite pattern similarities between two groups, alkaline types which have less silica, in elements Sr, Ti, Nb and Ta have more enrichment and in elements Hf, Rb, Th, K have less enrichment than the calc-alkaline.• Abundant elements in oceanic island basalts than the crust such as Re and La in alkaline types are more abundant.

Discussion
In geochemical investigation of samples of dikes in the north of Saveh it is clear that they have a dual nature of alkaline and calc-alkaline.From the view point of Pearce [10] evaluating the ratio of highly incompatible elements, per less incompatible, such as Zr and Th (shell maker) and Nb and Ta (mantle maker) to Y and Yb (i.e.mantle arrangement diagram) is a very useful method to understand the way of metasomatism and other processes role in the magma genesis and different enrichments.According to [11] and [12], abundance of elements with high stability such as Ta and Nb than light earth elements in the lithospheric mantle is low, so ratios more than one in Nb/La is a sign of asthenosphere mantle source like OIB and less than 0.5 is a sign of lithospheric mantle source.In diagram of (Figure 9(a)) used by [13], most samples are in lithosphere sector.
Pearce [10] indicates that the most useful charts to explore the role and involvement of mantle or subduction components, is chart Th/Yb-Ta/Yb, because Th is a sensitive index for subduction constructive interference and Ta is related to asthenospheric mantle constructive (in-plane) [10].Incompatible element Yb (which is non-moving in subduction), only changes during partial melting process in the presence of remaining Garnett.Being rich or empty of magma source, is determined by high or low proportion of the Ta/Yb and adding subduction melt with high ratio of Th/Yb [14] and [15].The position of the studied samples shows enriched origin influenced by the subducted material with fractionation process (Figure 9(b)).According to El-Bialy [16] placement of the samples in parallel arrangement of the mantle, is a sign of mantle enriched by the rim, AFC process or both.
On the other hand, consecutive exposure of a variety of calc-alkaline enriched by potassium and alkaline adjacent to each other, have characteristics of post-collision areas [17]- [19].These features, along with enrichment LREE and FREE with LIEL and depletion of HREE and HFSE, can be a sign of magmas after the collision, and   [13].OIB average from Fitton [22], the lower crust from Chen [23], OIB and HIMU limits from Weaver [24].Limit separator lines from Smith [12].a result of low degree of partial melting of depleted peridotite origin in sub-continental lithospheric mantle influenced by subduction metasomatism (SCLM) [20] and [21].
Ghasemi Barghi [25] believes that in the north-west of Meshkin Shahr, the diversity of petrology, the number of eruptive phases and various forms of Eocene volcanic rocks are associated with volcanism.Due to high relative density of dikes (1.2%), he believes that they are swarm dikes that are lava Eocene feeder and a sign of an extensional tectonic media.According to being after collision of these lavas and related dikes, he represents that, collision of irregular margins of continental convergent plates, causes collision crevasses [26].Also he believes that, geometry and intensity of the collision was not so that the crevasse forms, but could have created multiple fractures perpendicular to the collision or reactivate old faults.Fractures which are deep are fed from deep sources of magma and alkaline magmas, and those which were associated with shallow nests, have reached evolved magmas to the surface.Also [27] in the site of continental collision in the north of Zagros-Belitis suture zone (Iran, Azerbaijan and Turkey) introduce two magmatic periods of Eocene and Miocene-Quaternary with composition of calc-alkaline-shoshonite and alkaline which have magmatism characteristics of the post-collision.They believe that increasing the amount of alkaline elements of the recent volcanic is caused by entering the asthenosphere magma during the final stages of post-collisional magmatism which due to partial melting and crustal contamination, they are associated with more differentiated types with calc-alkaline nature.

Conclusion
Dikes in the north of Saveh are swarm dikes with the nature of calc-alkaline and alkaline that a variety of calcalkaline types have more silica and lower relative age than alkaline types.Discontinuity of changes in main elements oxide in Harker diagrams in both dikes, although is not associated with increasing specific trends in the moving elements such as sodium and potassium, but genetic affinities that can be seen in the less-animated elements and dual nature with simultaneous presence of alkaline and calc-alkaline magmas, can be in an environment after collision and impact of AFC processes on penetrated primary magma in the crust lithosphere.

Figure 3 .
Figure 3. (a) Plagioclase phenocrysts in basic dikes; (b) The mineralogical composition of andesitic basalt feldspars (sample M 34 ) based on microprobe analysis; (c) Changes in mineralogical composition zoned feldspar structure M 45-6-1 to M 45-6-4 .sectors.Dikes with high silica have calc-alkaline nature and more basic samples have alkaline nature (Figure 6(b)).Also this duality is well observed in the diagram of Cox [8] (Figure 6(a)).Basic samples have petrology composition of alkaline basalt and hawaiite, and dikes more enriched by silica have trachyandesite and rhyolite composition.By examining variation diagrams of main elements' oxides versus SiO 2 (Figure7), at first, a discontinuity in the value of silica of 46 to 55 is observed.This dual role does not prevent the regular changes of various oxides.So that from variation diagrams of FeO and MgO versus SiO 2 , generally show an obvious reduction trend with development process of subtraction.Reducing these elements suggests the idea of their participation in the

Figure 6 .
Figure 6.(a) and (b) The geochemical nature of dikes in the north of Saveh.

Figure 7 .
Figure 7. Variation diagrams of the main elements oxides versus SiO 2 .

Figure 8 .
Figure 8. Patterns of normalized rare elements to primitive mantle in two groups of dikes in Saveh.