Journal of Economic Geology (Dec 2024)
Petrogenetic evidences in geodynamics and placement of Nordoz intrusive masses in Alborz-Azerbaijan structural zone
Abstract
The Oligocene Nordoz intrusive complex is located in the eastern region of Siehroud, within the East Azerbaijan province, and within the Alborz-Azerbaijan structural zone. The results of field studies have identified the presence of gabbro diorite, monzodiorite, monzonite, tonalite, granodiorite, and granite within the region. The geochemical characteristics of these rocks indicate that they belong to the metaluminous type I granitoid classification. The rocks display calc-alkaline to high-potassium calc-alkaline and Shoshonite series characteristics, exhibiting an enrichment of LREE elements relative to HREE, a positive Pb anomaly, and a depletion of Nb and Ti elements. The presence of disequilibrium textures, such as sieve texture and zoning in plagioclase, suggests the existence of multiple magma chambers and mixing between them. The isotopic geochemistry indicates that a closed system process and crystal differentiation are the primary processes of formation, with a combination of DMM and EMII identified as the principal sources for magmas in the region. This behavior can be explained by contamination of magma derived from a depleted asthenospheric mantle with lithospheric sediments during oceanic crust subduction and upper mantle wedge metasomatism. In summary, the magmatic plateau of Azerbaijan has been affected by tectonic processes and lithospheric thinning during the Eocene-Oligocene period. Introduction The Azerbaijan region is located in northern Iran, in close proximity to the borders with Turkey, Azerbaijan, and Armenia. This area constitutes part of the Iran-Turkey zone, which encompasses Central Eastern Anatolia and Northwestern Iran (Dilek et al., 2010; Shafaii Moghadam and Shahbazi Shiran, 2011). As the central portion of the Alpine-Himalayan orogenic belt, this region has experienced a number of geologic events as a consequence of the closure of the branches of the Neotethys Ocean (Stampfli et al., 1991). Cenozoic igneous rocks are pervasive in the Iran-Turkey region (Dilek et al., 2010). The magmatic complex in question is distributed throughout northwestern Iran, Armenia, and eastern Turkey in the northern and eastern parts around the Arabian Plate. Materials and methods In order to facilitate petrologic studies, 11 samples exhibiting the least degree of alteration were selected and sent to the Zarazma company for chemical analysis of rare elements by ICP-MS and major oxides by XRF method. Furthermore, three samples displaying the lowest percentage of loss on ignition (LOI) were selected for isotopic analysis at the laboratory of the Faculty of Earth Sciences at the University of British Columbia in Canada. The results of the whole-rock chemical and isotopic analyses are presented in Tables 1, 2, 3, 4 and 5, respectively. Discussion Petrography Petrographic studies have revealed that the granitoid bodies of the Nordoz region are composed of a range of basic to acid igneous rocks, including gabbrodiorite, monzodiorite, monzonite, tonalite, granodiorite, and granite. The predominant texture is granular, although sieve and perithetic textures are also present. The principal minerals of these rocks are plagioclase, alkali feldspar, quartz, amphibole, clinopyroxene, and olivine (in base samples). Additionally, opaque minerals and secondary minerals of calcite, sericite, chlorite, and epidote are observed in this rock group. geochemistry The results of whole rock chemical analyses of 16 samples extracted from the intrusive mass in the Nordoz area are presented in Tables 1 and 2. The classification of the rocks was conducted using the Delaroche et al. table (De La Roche et al., 1980). As illustrated in Figure 3A, the samples under examination are classified as gabbrodiorite, monzodiorite, monzonite, tonalite, granodiorite, and granite, thereby corroborating the findings of the petrographic investigations. To ascertain the magmatic nature of the intrusive rocks, a K₂O vs. SiO₂ plot (Peccerillo and Taylor, 1976) was employed. As illustrated in Figure 3B, the samples under examination predominantly occupy the calc-alkaline range with elevated potassium concentrations, as well as the Shoshonite range. Na₂O versus K₂O diagrams adapted from Chappell and White (2001) (Fig. 4A) indicate that all samples fall within the range of Type I granites. Additionally, to differentiate between the granitoids of the region and the A granitoids (non-orogenic granitoids) depicted in the diagram (Kleeman and Twist, 1989), which is plotted based on the Nb versus SiO2 variation (Fig. 4B), all samples fall within the range of Type I granites. In order to ascertain the provenance of the primary magma and the subsequent processes, including contamination and mixing of the magma, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic analyses were conducted on the samples. The results of the isotopic analysis of the samples are presented in Table 5 for reference. The combined spectrum of the 87Sr/86Sr isotopic ratio of the investigated samples exhibits a range of values between 0.704412 and 0.705081. Similarly, the 143Nd/144Nd ratios of these intrusive rocks range from 0.512769 to 0.5128255. The 143Nd/144Nd versus 87Sr/86Sr isotope correlation plots, as presented by Zindler and Hart (1986), were employed to ascertain the provenance of the intrusive rocks in the Nordoz region. All samples are plotted on the conventional isotope diagram in the mantle region near BSE (Bulk Silicate Earth) and indicate that the main source for the magmas of the study area is a combination of depleted mantle (DM) and enriched mantle (EMII) (Fig. 7). This behavior can be attributed to the contamination of large volumes of depleted asthenospheric mantle (DM) magmas with lithospheric fluids enriched in LILE and radiogenic Sr elements, which primarily result from the dehydration of detrital sediments during subduction. The data presented in Figure 8 illustrates that the 208Pb/204Pb ratios of all samples from the region are above the NHRL line and within the EMI and EMII ranges (Zindler and Hart, 1986), with the majority of samples falling close to the EMII range. It is important to note that EMI is associated with an enriched mantle with moderate amounts of 87Sr/86Sr, low amounts of 143Nd/144Nd, and low ratios of 206Pb/204Pb. tectonic setting As illustrated in the Al2O3 vs. TiO2 variation diagram from Muller and Groves (1997), all intrusive samples from the Nordoz area are situated within the range of magmatic arcs (Fig. 6, 7, 8, 9 and 10A, B, C). In accordance with the diagram of Rb vs. (Y+Nb) (Pearce et al., 1984), which is utilized to differentiate the tectonic environments of granitoids associated with disparate tectonomagmatic environments, the acidic samples are situated within the volcanic arc range, predominantly at the juncture of this range with the analogous range. Additionally, acidic samples exhibit parallels with post-collisional environments in the tectonomagmatic diagram (Maniar and Piccoli, 1989) (Fig. 10). In the diagram presented by (Meschede, 1986), which is drawn based on the values of Nb*2-Zr/4-Y. The basaltic samples of the region are included in the range of volcanic arc basalts. Conclusion and discussion The results of the field observations indicate the presence of fractionated mafic magma injected into acidic magma (Fig. 2). It can thus be posited that the intrusive masses observed in the Nordoz region are the result of the differentiation of basic magma in the region. It appears that the basic magma is situated within a vast magma chamber beneath the region, exhibiting elevated temperatures. During the subsequent compressive phases, it propagated to higher levels, specifically into the lower crust, where it caused an increase in temperature. The fluid phase, in conjunction with the temperature increase resulting from the intrusion of the basic magma into this region of the crust, has led to the melting of a portion of the underlying crust and the formation of magma with a granodiorite and calc-alkaline composition. The evidence supporting the retreat of the subduction slab and extensional tectonics in Alborz, as presented in previous studies, can be attributed to a decrease in the rate of subduction in the Eocene. This decrease was caused by a reduction in the opening of the Indian Sea, which led to the retreat of the Neotethys subduction slab and subsequent tectonic extension in central Iran (Hassanzadeh et al., 2004). The presented data, in conjunction with the findings of geochemical and isotopic studies, as well as the positioning of the Nordoz region samples within tectonic environment diagrams, permit the formulation of a model for the genesis of these rocks. This model proposes that the mantle portion of the subcontinental lithosphere is the most probable origin for these rocks. The magmatic plateau of Azerbaijan has been subjected to a number of processes, including the uplift of the asthenosphere source and its thermal effect in the region. These processes have occurred as a result of events such as slab failure or lithosphere layer separation during the Eocene-Oligocene period, which have resulted in some degrees of partial melting. Consequently, it is possible that the resulting magma is located in a tensional environment behind the arc affected by the orogenic events of the collision of the Arabian plate with Eurasia.
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