Petrography, Geochemistry and Tectonics of Harris Granite, East Azerbaijan Province: Evidence for Intracontinental Rift Magmatism

Introduction Granites are interesting because of their abundance in the continental crust and the presentation of valuable information from the depths of the earth and their close dependence on tectonic and geodynamic processes (Bonin, 2007). The Mishu granites are exposed over an area around 50 km2 in the northwestern Iran near the city of Tabriz (Figures 1 and Figures 2). The Mishu granites have been injected into the Neoproterozoic shales, carbonates, sandstones, and tuffs of the Kahar Formation (Asadian et al. 1994). Mafic and ultramafic rocks (gabbro, basalt, and dunite) occur at the north and northeast of the Mishu granites and seem to be the host of granites. Field observations show a magmatic injection of the Mishu granites into the mafic-ultramafic rocks. There are several outcrops of granite rocks in the northwest Iran, including Takab–Zanjan, Khoy, Soursat, and Mishu. Among these outcrops, there are no systematic geochemical and geochronological studies on the Mishu rocks. In this paper, we investigate the genetic relationship between different parts of the mass, origin of the constructive magma and the tectonic position of this intrusion with the help of the results of field studies governing different parts of the Harris intrusion mass, petrography and geochemical analysis of the main and rare elements. Material and methods A total of 150 samples were collected from Mishu granites. Polished thin sections were prepared from all the collected samples. Based on petrographic observations, 20 samples with minimal effects of hydrothermal alteration were selected for whole-rock geochemical analysis (Table 1). These selected samples were analyzed for major and trace elements at the ACME Laboratory (in the ACME Analytical Laboratories of Vancouver, Canada). Analytical errors for major elements are assessed as <1% of the determined concentrations. Results are reported in Supplementary Table 1. Major element oxide analysis was performed by Lithium Borate Fusion and Inductively Paired Plasma Emission Spectrometer (ICP-ES). In this method, the number of oxides of the main elements is measured based on weight percentage. The measurement accuracy for the main elements in this method was 0.01 Wt.%. Also, in this method, the number of volatiles in the form of L.O.I. was measured with an accuracy of 0.01%. The induced coupled plasma mass spectrometer (ICP-MS) method was used to measure the amount of trace and rare elements. The detection threshold of these elements, depending on the element, varied from close to 0.01 ppm to 10 ppm. Results Harris granite rocks are in the northwestern Iran and about 20 km west of Shabestar city. This mass is composed of alkaline feldspar granite. The most abundant texture seen in these rocks is micro-pertite and myrmicite and based on lithographic and geochemical properties, they belong to A2-type granites. The samples are meta-aluminous to per-aluminous is based on the saturation index of alumina. In general, the studied granites have higher amounts of Na2O + K2O, Fe / Mg, Ga / Al, HFSEs and lower amounts of CaO, Sr and Eu. Also, the content of REEs of the samples in the normalized graph concerning chondrite shows a negative Eu anomaly. In other words, it is likely that A-type alkaline granites after collision have been created in this area following collision events and during their placement the tensile structure is predominant. Normalized multi-element diagrams as well as high Rb indicate that the continental crust has played a significant role in the formation of the Harris granite producing magma, possibly due to the melting of the lower crust by a tonalitic-granodioritic combination. Discussion All lithographic and geochemical data show that Harris granite rocks are of A-type nature. Negative anomalies of Ba, Nb, Ti, Sr and Eu and enrichment in LILEs, especially Rb and Th, indicate the crustal origin of these rocks separation of feldspar during crystallization or the presence of feldspar as a residual phase in the origin and the anomaly of P and Ti to iron-titanium and apatite oxides. Enrichment in LILE and HFSE elements with negative anomalies of Nb and Ti is a characteristic of subduction-dependent. The negative anomaly of Eu in the trace element pattern can be attributed to granites, usually attributed to the mantle origin, previously due to the metamorphic activity of fluids from sediments deposited by LILE and HFSE elements (Pearce et al., 1984), or may be the nature of magmas rooted from a subcontinent meteorite mantle formed during early subduction. In addition, enrichment at Th, Rb, and depletion at Sr, Eu, Ba, Nb, and Ti indicate that the granites are rooted in crustal lavas (Zhao and Zhou., 2007).

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