زمین‌ شناسی، سنگ‌ شناسی، ژئوشیمی و سن‌ سنجی ایزوتوپی توده‌ های نفوذی منطقه اکتشافی تاریک‌ دره

منطقه تاریک دره به‌دلیل بررسی اکتشافی طلاتنگستن در شمال‌شرق ایران و ناحیه تربت‌جام شناخته‌شده است. این منطقه در زون ساختمانی کپه‌داغ قرار دارد. سنگ‌های نفوذی با ترکیب حدواسط و طیف سنگ شناسی گابرو–دیوریت تا کوارتز مونزونیت در منطقه رخنمون‌‌داشته‌ و در این میان، دیوریت ها ترکیب غالب هستند. اغلب توده های نفوذی یادشده ماهیت متاآلومین، کالک‌آلکالن با غنی‌شدگی پتاسیم نشان می دهند و پذیرفتاری مغناطیسی این توده‌ها بسیار پایین ( lt، 3×103 si) است. روند تغییرات عناصر اصلی و کمیاب در توده های نفوذی منطقه تاریک‌ دره، نشان‌دهنده یک ارتباط زایشی و بیانگر مراحلی از تبلور تفریقی کانی های سازنده این سنگ‌ها از ماگمایی مشترک است. الگوی پراکندگی ree تمامی نمونه ها تمایل به غنی شدگی در بخش lree دارد. در مجموع جدایش محسوسی از lree به سمت hree و با شیب نزولی مشاهده می شود و تمامی نمونه ها با تهی شدگی جزئی در eu همراه هستند. ویژگی‌های ژئوشیمیایی و نسبت پایین n(la/yb) تمامی سنگ‌های منطقه (بین 6/6 تا 9/12)، دخیل‌بودن ماگمایی با منشا جبه ای و خارج از حوزه پایداری گارنت و یا پوسته زیرین را برای تولید آنها محتمل می‌کند. جایگاه زمین‌ساختی سنگ‌های منطقه شباهت‌هایی با محیط کمان های آتشفشانی داشته و همچنین ویژگی محیط‌های درون صفحه‌ای را نشان می دهند. سن‌سنجی‌های upb بر روی دانه‌های زیرکن نشان‌داده است که قدیمی‌ترین سنگ‌های نفوذی با ترکیب گابرویی در محدوده زمانی 9/0±5/215 میلیون (تریاس بالاییاشکوب نورین) جای‌گیری شده‌اند. همچنین بر این اساس، سن سنگ‌های میزبان این توده ها که تاکنون به نورین منتسب‌شده بود نیز قدیمی تر از تریاس پیشین پیشنهاد می شود.

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

IntroductionThe investigated area in northeastern Iran that is known as the Tarik Darreh arsenopyriteAuW 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 AuW quartz lode mineralization carried out by Taghizadeh (1965). Since then, the Geological Survey of Iran (e.g., Alavi Naini and MossaviKhorzughi, 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. Materials and methodsExpansive 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 KT10 and GMS2 Fugro instrument (precision of 0.0001 SI units).Sixteen samples of intrusive bodies and dykes were analyzed for major and trace elements using 4EReserch package (INAA, FUS ICPOMS, ICPMS) at the Acme Lab (Canada). Gabbroic units were selected for UPb geochronological study. In the heavy liquid process, zircon crystals were separated, handpicked under microscope. Zircon grains for ThUPb dating were examined by an Agilent 7700 quadrupole ICPMS at the University of Tasmania (Australia). Discussion and ResultsPlutonic 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 gabbroicdiorite 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 Kcalcalkaline 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 3×103 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 UPb 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 UPb 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 ( lt،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 garnetfree 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. AcknowledgementsThe authors wish to thank Prof. Yuanming Pan (University of Saskatchewan) for supporting this study and Eng. Amin Haghdoost for his help during the field geology work. ReferencesAlavi Naini, M. and MossaviKhorzughi, E., 2006. Geological Map of TorbateJam, scale1:100,000. Geological Survey of Iran.Behroozi, A., Eftekharnezhad, J. and Alavi Naini, M., 1993. Geological Map of TorbateJam, scale 1:250,000. Geological Survey of Iran, Tehran.Ghavi, J., Karimpour, M.‌H., Mazaheri, S.‌A., Ghaderi, M. and Rahimi, B., 2013. Identifying secondary dispersion halos and geochemical element distribution in soils on the Tarik Darreh AuW prospect area, north of TorbateJam, NE Iran. Iranian Journal of Crystallography and Mineralogy, 21(1): 107–120. (in Persian)Shafi Niya, H., 2002. Geochemical and geological studies of Tarik Darreh gold prospect, M.Sc. Thesis, Shahid Beheshti University, Tehran, Iran, 157 pp. (in Persian)Taghizadeh, N., 1965. Prospecting at Tarik DarehCheshmegool area. Geological Survey of Iran, Tehran, Report 1, 8 pp.