MINERALOGY, GEOCHEMISTRY AND GEODYNAMIC SETTING OF THE GRANITOIDS FROM NW IRAN

The Mahneshan granitoids intruded the Neoproterozoic-Lower Cambrian regional metamorphic rocks in northwestern Iran. These granitoids consisting mainly of K-feldspar, plagioclase, quartz, muscovite, garnet and biotite display a number of subtypes in terms of structure, texture and mineralogy. Geochemically, they are peraluminous and (or) slightly peraluminous with variable normative corundum contents (0.28– 4.50%), and medium to high potassic with calc-alkaline affinity. Chondrite-normalized REE patterns indicate that these granitoids can be divided into three distinct groups, supported by petrographic data. The REE patterns of the first group are shallow-sloping in LREE relative to HREE ([La/Yb]n=1.37–2.48), exhibiting pronounced negative Eu (Eu/Eu_¼0.23–0.35). The second group granitoids are characterized by strong LREE-enrichment relative to HREE ([La/Yb]n=3–6.20), with positive Eu anomalies (Eu/Eu*=1.15–1.47). The third group of granitoids is depleted in the middle REE relative to other LREE and HREE. These REE patterns suggest the role of plagioclase and hornblende in their source of granitoids for group 1 and groups 2 and 3, respectively. The trends of Eu/Eu_ ratio versus silica contents suggest mixing of mafic material with components formed by crustal melting with a plagioclase-rich residue. Furthermore, thermobarometric estimations indicate that these rocks may have been formed at depths of 15–18 km at relatively low temperatures. The Mahneshan granitoids are S-type and may have been emplaced in a syn- to post-collisional tectonic setting.

1. GEOCHEMISTRY

1.1. Major and trace elements

These granitoids have SiO2 contents between 69-76.8 wt %, low MgO and very low abundance of high-field strength elements (Nb, Ta, Zr and Hf). For example, Nb is generally lower than the average value of felsic I-type (21 ppm) granites from Lachlan Belt of southeastern Australia (Chappell and White 1992). The chemical compositions of the rocks are plotted on the total alkali versus silica diagram of Middlemost (1994). The samples plot in the granite field of this diagram. Using AFM diagram of Rickwood (1989) and TAS diagrams of Irvine and Baragar (1971) and Cox et al. (1979), all samples present a subalkaline composition and are plotted on the calc alkaline field . The SiO2 versus K2O plot yield a medium to high-potassic calc-alkaline characteristics. Based on the aluminium saturation index (ASI) of (Shand 1947); the Mahneshan granitoids are peraluminous to slightly peraluminous and have S and I-type tendencies.

1.2. Rare earth elements

Based on chondrite-normalized REE patterns (normalization values from Sun and McDonough 1989), Mahneshan granitoids can be subdivided into three distinct groups. The REE patterns of the first group (are shallow-sloping in light rare earth elements (LREE) relative to heavy rare earth elements (HREE). The samples exhibit moderately fractionated REE patterns ([La/Yb)n=1.37-2.48), flat heavy REE patterns, and a large negative europium anomalies (Eu/Eu*=0.23-0.35; the depletion of Eu/Eu* ratios reflects melting with residual plagioclase and/or plagioclase as a major fractionating phase (Mark 1999). Therefore, the chondrite-normalized REE patterns and the Eu/Eu* ratio versus silica contents, display the role of hornblend and plagioclase minerals in their source, which probably suggests that the Mahneshan granitoids may be originated by partial melting of crustal material with different compositions.

1.3. Tectonic and geodynamic setting

Compared with the typical Lachlan S- and I-type granites (White and Chappell, 1983), the Mahneshan Granitoid rocks have higher Na₂O but similar K2O content. The I-type affinities shown in a few of the Mahneshan granitoid samples may result from hydrothermal alteration or even may be due to weathering, which are reflected in the variation of K₂O/Na₂O ratios and Na2O>K2O. Secondary minerals, such as sericite, some muscovite and cloudy feldspar are resulted from late hydrothermal activities.

Discrimination diagrams R2 [6Ca+2Mg+Al] vs. R1 [4Si-11 (Na+K)-2(Fe+Ti)] indicates that the Mahneshan granitoids represent syn- to post- collisional granites. Furthermore, these granitoids using the geotectonic classification of Pearce et al. (1984) and Pearce (1996) are classified as syn- to post- collisional granites similar to those of recent syn- to post- collision granites (high-Rb, low-Zr, Hf and Sr contents). The (Hf, Rb/10, Ta*3) diagram after Harris et al. (1986), in Figure 2k clearly shows a collisional tectonic setting for the studied rocks.

2. CONCLUSION

-The geochemical characteristics, mineralogy and petrography of these granitoids are comparable with typical S-type granites and were emplaced in syn- to post- collisional tectonic setting.

-Magmatic origin from water-saturated, heterogeneities source rocks (i.e. crustal material having different compositions) under low pressure conditions are suggested to the Mahneshan granitoids origin.