Geology, petrology, geochemistry and geochronology of Tarik Darreh plutonic rocks, northeastern Iran

The investigated area in northeastern Iran that is known as the Tarik Darreh arsenopyrite-Au-W prospecting target is situated in the Kopeh Dagh zone (Fig. 1). Most of the study area is covered with black slate rocks which most authors have referred to as Upper Triassic with Norian age (e.g., Behroozi et al., 1993). Plutonic rocks with gabbro, diorite, and quartz diorite and monzodiorite composition were introduced in the slate rocks. The previous studies in the Tarik Darreh are the preliminary report of arsenopyrite with Au-W quartz lode mineralization carried out by Taghizadeh (1965). Since then, the Geological Survey of Iran (e.g., Alavi Naini and Mossavi-Khorzughi, 2006) a few geologic companies and the universities (Shafi Niya, 2002) and Ghavi et al. (2013) have been studying in the area. In the present study, we provide more information, complete with new information about the geology of the Tarik Darreh area.

Expansive field geology surveying to draw a geology map in 1:8000 scale and sampling from plutonic rocks was performed and two main fault system were distinguished in Tarik Darreh (Figs. 2 and 3). Thin section microscopic studies were carried out following field investigations. Magnetic susceptibility was measured with a portable Model KT-10 and GMS-2 Fugro instrument (precision of 0.0001 SI units).Sixteen samples of intrusive bodies and dykes were analyzed for major and trace elements using 4E-Reserch package (INAA, FUS ICP-OMS, ICP-MS) at the Acme Lab (Canada). Gabbroic units were selected for U-Pb geochronological study. In the heavy liquid process, zircon crystals were separated, hand-picked under microscope. Zircon grains for Th-U-Pb dating were examined by an Agilent 7700 quadrupole ICPMS at the University of Tasmania (Australia).

Plutonic rocks in the Tarik Darreh area are in the form of bodies and dykes. The plutonic bodies are mostly gabbro, gabbro diorite, diorite (hornblende pyroxene diorite, biotite hornblende diorite and hornblende biotite diorite), biotite hornblende monzodiorite, quartz diorite, and quartz monzonite in composition. However, it is diorite with granular textures as a common rock (Figs. 4 and 6). Other rocks as dyke have more hornblende diorite composition with porphyritic textures. Common mafic minerals in the plutonic rocks in the Tarik Darreh include pyroxene, hornblende and biotite. The alteration assemblages of uralite, chlorite, calcite and sericite are very common gabbroic-diorite rocks (Fig. 5). The plutonic rocks have SiO2 with values in the range of 46 to 56% (Table 1). Therefore, these rocks are intermediate in composition.  Most of these rocks also have high K-calc-alkaline composition with metaluminous characteristic (Fig. 7).Enrichment in LREE relative to HREE, low La/By ratio, enrichment in LILE and negative anomaly of Eu, Ti, Zr, Y and Ba are shared in all of the studied plutonic rocks and dykes (Figs. 8 and 9). All the plutonic rocks have low values of magnetic susceptibility (less than 3×10-3 SI). Two main systems of fault have affected the area (Fig. 10). Plutonic rocks from the Tarik Darreh area are placed within the volcanic arc to within plate’s environment in tectonic setting discrimination diagrams (Fig. 11).Zircon U-Pb dating results on the gabbroic rocks are shown in Table 2. Zircon grains through cathodoluminescence imaging show euhedral, clear crystals with no visible heritage cores. However, some of them have inclusions. Zircon U-Pb dating indicates that the gabbroic rocks formed at 215.5±0.9 Ma (late Triassic) (Fig. 12). Thus, it is obvious that the country rock (called Miankuhi Formation), must be older than the late Triassic plutonic rocks rather than Norian age as previously thought.The variation of major elements with SiO2 in the plutonic rocks from Tarik Darreh (Fig. 13) indicates that the primary magma of these plutonic rocks underwent fractional crystallization from gabbro to quartz monzodiorite. Geochemical characteristics such as low ratios of (La/Yb)N (<13), high contents of MgO (7%), high Mg# (57) and high Sr (on average 572 ppm) indicate that the magma of the studied rocks have probably originated from partial melting of garnet-free source from mantle or low continental crust. However, the presence of slate as country rocks suggest possible assimilation of country rocks that indicate processes such as assimilation were more likely to be responsible for the evolution of plutonic rocks in the Tarik Darreh area.