Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Granitoids are the main rock units in the continental crust. Study of granitoids reveals significant information on tectonic mantle and upper crust. Many researchers have investigated petrogenesis and origin of granitoids (e.g., Chappell and White, 2001; Barbarin, 1999; Frost et al., 2001). For example, Chappell and White (1992), Pitcher (1993) and Chappell et al., (1998) have divided granites into two major groups of: (1) I-type granites (high-temperature or Cordellerian granitoids, including low-K granitoid to high-Ca tonalite, without inherited zircons) formed by partial melting of mafic rocks at >1000 ℃ in mantle or subduction zones of continental margins, and (2) S-type (low-temperature or Caledonian granitoids with inherited zircons) granites formed by partial melting of felsic crust at ~700-800 ℃. Northeast of Iran is a key location for studying the Cimmerian Orogeny, which is related to the Late Triassic collision between it and Eurasia, and the closure of the Paleo-Tethys (Samadi et al., 2014). Mesozoic Mashhad granitoids have cropped out along with the Paleo-Tethys suture zone. Distinct granitoid suites, i.e., monzogranite, granodiorite, tonalite, and diorite occur in Mount Khalaj located in the south of Mashhad. It comprises of monzogranite and granodiorite. However, monzogranite is the most abundant. To study the plutonic events during the Turan and Central Iran collision, the origin and tectonic setting of monzogranite of Mount Khalaj are investigated in this study based on whole rock geochemical data.

This research study is based on field studies and petrography. Fresh thin sections samples were selected for geochemical analysis. Whole rock composition was measured on pressed powder tablets by X-ray fluorescence (XRF) using a Philips PW 1480 wavelength dispersive spectrometer with a Rh-anode X-ray tube and a 3 MeV electron beam Van de Graaff Accelerator, at the center for Geological Survey of Iran. The trace element data of a sample was measured at the Activation Laboratories, Ontario, Canada (ActLabs). Samples were digested by lithium metaborate/tetraborate fusion and analyzed with a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP/MS. GCDkit 4.1 and CorelDraw software packages were used for plotting diagrams and calculation of saturation temperatures.

The Khalaj granitoid is mineralogically composed of quartz, potassic feldspar, plagioclase, mica, and accessory minerals of zircon and apatite. Geochemically, it is an unaltered acidic intrusion with ~72-73 wt.% SiO2. It is a granitoid (monzogranite) based on various classification diagrams (e.g., Cox et al., 1979; etc.). It shows the peraluminous nature (A/CNK~ 1.08-1.24) with negative Eu anomaly of ~0.62-0.73 (Eu/Eu*<1), low HREE and high LREE and LILE contents.

Geochemically, the low HREE and high LREE and LILE content in the Mount Khalaj monzogranite indicate a more differentiated melt for it. Monzogranite samples from the Khalaj-Khajeh Morad regions are similar to ferroan alkali-calcic, felsic peraluminous S-type granitoids based on discrimination diagrams by various researchers (e.g., Chappell and White, 2001; Villaseca et al., 1998). In fact, the Mount Khalaj monzogranite is a collisional granite (based on diagrams by: Batchelor and Bowden, 1985; Sahin et al., 2004), produced by anatexis and partial melting of felsic upper crust pelitic sediments (based on diagrams by: Almeida et al., 2007; Patiño Douce, 1999). It is classified as a low-temperature S-type granite formed at 730-800 ℃ (based on the diagram of Rapp and Watson, 1995), with TZr of ~732-745 ℃ (by using GCDKit software). Therefore, S-type syn- to post-collisional Mount Khalaj monzogranite is a consequence of partial melting (anatexis) of hydrous sedimentary rocks of upper crust after Paleo-Tethys subduction under Turan plate and continental collision and compressional tectonism.