Geochemistry, petrogenesis, and tectonic setting of Nepheline syenite intrusive rocks in Razgah Sarab, East of Azerbaijan Province

Razgah, Kalibar, and Bozghosh intrusive rocks, located in East Azarbaijan province, are among the most important alkaline nepheline syenite rocks known in Iran. Several theories reported regarding the geochemical characteristics as well as the geotectonic environment of the Tertiary igneous rocks exposed in the Ahar-Arasbaran belt (Jamali et al., 2012). Some workers have considered the magmatism of this belt as an important extensional phase of the late Cretaceous, while, some others believe it is related to subduction zones. Although the mineralogy, petrology, geochemistry, and petrogenesis of these rocks have been studied by several people (i.e., Esmaeili, 1997; Ameri, 2004; Ashrafi, 2009), in none of them the origin and geotectonic setting of the area has been discussed in detail. Thus, for the present study, mineralogical, petrological, and geochemical studies are applied to understand the origin, the petrogenesis, and the geotectonic setting of the intrusive rocks from the nepheline syenite exploration areas, particularly the Razgah intrusive rocks.

 Regional Geology:

The Razgah intrusive. a part of the Western Alborz-Azarbaijan zone lies on the southern edge of the Ahar-Arasbaran volcanic belt. Geologically, the Eocene volcanic rocks cover the Jurassic-Cretaceous flysch is intruded by the Oligo-Miocene plutonic rocks (Mahdavi and Fazli, 2008). The Razgah nepheline syenite intrusive intruded the sediments of evaporate basins including gypsum marl, limestone marl, sandstone, and the Miocene conglomerates. Their contact with previous rocks has vanished due to weathering and erosion of itself and the Quaternary sediments surrounding it. Copper mineralization (malachite and azurite) can be identified in the western part of Razgah intrusive. In addition, a number of dykes are distributed especially in the western and northwestern parts. Also, numerous silica veins were observed in different parts, mainly along, in parallel, and in contact with dykes, more widespread in the western part.

Following the sampling (150 rock pieces) and preparation of 70 thin sections for petrographic and mineralogical investigations, trace and REE were determined on 93 samples using the ICP-MS method.

Petrography:

The Razgah intrusive rocks are dominated by olivine-bearing nepheline gabbro, nepheline monzosyenite, pseudolucite syenite, and K-feldspar nepheline syenite. Also, basic dykes (lamprophyre), similar to their host (altered syenite nepheline), syenite and microsyenite dykes and finally altered dykes with siliceous and mineralized veins are present. Nepheline, pseudolucite, potassium feldspar, plagioclase, clinopyroxene, and olivine are among the most important major minerals. Apatite, zircon, and iron-oxide are the minor while chlorite, calcite, adularia, and hematite along with sericite, epidote, kaolinite, and biotite present as secondary minerals. Granular and porphyroid textures, as well as poikilitics, graphics, and replacements (causing pseudomorphous forms), are the common textures of the rocks under study. Lamprophyre dykes with porphyry texture and olivine, pyroxene, and Kaersutite (brown amphibole) are present. The other dykes, to some extent, have similar mineralogical characteristics as nepheline syenites, in which clay minerals occur.

The Co and Th contents, the amount of Ce/Yb, Ta/Yb, and Th/Yb ratios along with low TiO2 content point to the nature of the parent magma as a high potassium calc-alkaline type (shoshonite). The pattern of REE, LILE, and HFSE, the negative Nb, Ti, and HFSE anomalies as well as Pb and LILE positive anomalies, indicate the mantle metasomatism process or continental crust contamination occurrence (Rollinson, 1993; Soesoo, 2000). In addition, the decrease in P solubility with the high level of K and REE gave rise to the separation of apatite from the parent melt (Green and Adam, 2002). The similarity of REE patterns indicates the same magmatic processes involved in the generation of the rocks under study. The mineralogical and geochemical data on Razgah intrusive rocks and the associated lamprophyres point to 1% partial melting of a spinel lherzolite source. Based on the presented tectonic model, following the Arabian plate subduction beneath the Iranian plate, several processes including the slab breakoff, extensile regime, magma formation from the enriched mantle, and subducted slab melting took place which followed by crystal fractionation and contamination at the base of the crust and finally differentiated melts intruded the high levels. As the gases release, the lamprophyre dykes injected into the fractured roof and the other dykes crosscut the Razgah intrusive rocks.