Geochemistry and geochronology of granitoid rocks of the Taatsiin Gol pluton of the Khangai Complex, Central Mongolia

The Taatsiin Gol pluton is one of the major constitute the intrusive body of the Khangai Complex, and is composed the first phase of diorite, the second phase of porphyritic granite, biotite-hornblende granite, and granodiorite, and the third phase of biotite granite and alkali granite. This paper presents new geochemical and U-Pb zircon age data from intrusive rocks of the Taatsiin Gol pluton. Geochemical analyses show that the granitoid rocks of the pluton are high-K calc-alkaline, and metaluminous to weakly peraluminous I-type granites, depleted in HFSE such as Nb, Ta, Ti and Y and enriched in LILE such as Rb, Cs, Th, K and LREE, where some variations from early to later phases rock. Zircon U-Pb dating on the biotite granite of the third phase yielded weighted mean ages of 241.4±1.2 Ma and 236.7±1.4 Ma. Based on the new and previous researchers’ age results, the age of the Taatsiin Gol pluton of the Khangai Complex is 256-230 Ma consistent with the late Permian to mid-Triassic time. Although showing variated geochemical features, the rocks of the three phases are all suggested to form at an active continental margin setting, probably related to the southwestward subduction of the Mongol-Okhotsk Ocean plate during the late Permian to mid-Triassic period.


INTRODUCTION
Mongolian territory is situated in the central part of the Central Asian Orogenic Belt (CAOB), which is bounded by the Siberian and the East European cratons in the north and the North China and the Tarim cratons in the south (Xiao et al., 2015;Badarch et al., 2002;Windley et al., 2007). The formation time of the Khangai Complex has long been an issue of debate. In the past, the Khangai Complex was considered to be Paleozoic or older in age for a long time. Taking the Taatsiin Gol pluton, one major constitute the intrusive body of the Khangai Complex, as an example, it was mapped as the Tarvaragtai Complex and was designated a Carboniferous age in the 1:200,000 L-47-72 map Sheet (Tumurchudur et al., 1990), but was regarded as the Proterozoic in the L-48-61 Sheet (Togtokh et al., 1984), as the granitic rock of the Khangai Complex in the L-47-84 Sheet (Zabotkin et al., 1988), neighboring map sheets. Recently, accompanying accumulation of dating data, the Khangai Complex has been considered to be Original Article Mongolian Geoscientist mainly formed during the late Permian to Triassic period ( Fig. 1; Kozlovskiy, 2013, Yarmolyuk et al., 2008;Orolmaa et al, 2008, Orolmaa andDolzodmaa, 2019;Dolzodmaa et al., 2020). The tectonic setting of the Khangai Complex is another major topic for discussion. At present, three main points of view in this regard have been invoked, including mantle plume (Yarmolyuk et al., 2016;Yarmolyuk et al., 2013b;Kovalenko, 2003a, 2003b;Kuzmin et al., 2010), subduction of the Mongol-Okhotsk Ocean plate (e.g., Donskaya et al., 2012;Yarmolyuk et al., 2016;Ganbat et al., 2021), and post-collision (Dolzodmaa et al., 2020). In order to verify the formation age and determine the tectonic setting of the Khangai Complex, we selected the Taatsiin Gol pluton, which contains rocks of three phases and thus is representative of the Khangai Complex, to do detailed field investigation, and geochronological and geochemical analyses. This paper presents the results, which have important implications for the regional tectonic evolution.

GEOLOGICAL SETTINGS
The CAOB hosts a number of large batholiths formed during the Paleozoic and Mesozoic which are formed in the various geological environment, the most interesting products of this magmatism are granitoid batholiths of the Late Paleozoic-Early Mesozoic age because of their unique size and role during the late evolution of CAOB (Yarmolyuk et al., 2016). (2) granitoid massifs of the Khangai batholith; (3) rift zones (GARZ is the Gobi Altai rift zone and NMRZ is the northern Mongolian rift zone) and exposed bimodal volcanic associations and alkaline granites; (4) Late Triassic volcanic fields and granitoid massifs; (5) faults; (6) boundaries among the continental blocks or terranes (S-Songino, D-Zavkhan, T-Tarvagatai, KH-Khangai); (7) sampling sites for isotopic dating. The origin of the batholiths was closely related to alkaline and subalkaline complexes associated with Late Paleozoic-Early Mesozoic rift systems (Yarmolyuk and Kovalenko, 2003b). The Permian-Early Triassic magmatic province generally comprises diverse magmatic complexes, which occur over an area of >4 million km 2 and one of its distinguishing features is zonal magmatic areas: Khangai, Barguzin, and Mongolia-Transbaikalia which consist of batholiths surrounded by coeval rift zones (Yarmolyuk et al., 2016;Yarmolyuk et al., 2013b). The Khangai zonal magmatic area ( Fig. 1) covers approximately 250000 km 2 and includes the Khangai batholith and the Gobi Altai and northern Mongolian rift zones that surround the batholith in the south and north (Yarmolyuk et al., 2016). The Khangai batholith occurs within a half of the magmatic area and is composed of a number of rock associations, which are dominated by granitoids of normal alkalinity grouped into the Khangai Complex and the Fig. 2. Geological map of the study area second most abundant rock type is subalkaline leucocratic granites of the Sharus Gol Complex (Fedorova, 1977;Dergunov et al., 2001). The granitoids are classified according to their composition into two magmatic associations: granite-granodiorite, which is made up mostly by rocks of the Khangai Complex, and graniteleucogranite, which corresponds to the Sharus Gol Complex (Yarmolyuk et al., 2016). Intrusive rocks of the Khangai Complex intrude the Carboniferous Tarvaragtai Complex in the western and northwestern parts and are intruded by Triassic leucogranites of the Sharus Gol Complex. In the study area, the intrusive rocks of the Khangai Complex intruded the Silurian to Lower Devonian Tsoroidog Formation, the Lower-Middle Devonian volcanogenic-silicicsedimentary rocks of the Erdenetsogt Formation, upper Devonian-Mississippian, the Mississippian terrigenous-sedimentary rocks of the Dulaankhairkhan Formation, and the Cisuralian volcanogenic rocks of the Khoshigt Formation. They are crosscut by the Triassic Sharus Gol Complex and late Jurassic to early Cretaceous subvolcanic dykes and covered by the early-mid Jurassic Saikhan-Ovoo Formation and Quaternary sediments. In the contact zones with the Erdenetsogt Formation, mafic volcanics are metamorphosed into amphibolite and amphibolic schist. The contact metamorphic zones are normally 5-10 m in width, usually with lesser content of dark minerals and finergrained textures compared with the pluton (Fig.  2). The Khangai Complex is comprised of several plutons, such as Taatsiin Gol (5000 km 2 ), Baits, Khanui, Mandal, Upper Orkhon (each 1000-2000 km 2 ) in Khangai area. The Taatsiin Gol pluton is the biggest (5000 km 2 ) and is representative of the Khangai Complex and is composed of three phases: the first phase of fine -medium grained, dark grey diorite, the second phase of coarse-grained, pinkish grey, porphyritic granodiorite and granite, and the third phase of fine-grained, light grey, biotite granite, and leucogranite (Orolmaa and Dolzodmaa, 2019).

METHODS
Field observation and study were conducted on the intrusive rocks of the Khangai Complex around the Uyanga area, with 10 samples selected for petrographic study, 32 samples for geochemistry, and 2 samples for zircon U-Pb dating. Thin-section observations were conducted at the Geological Investigation Center, State owned Enterprise of Mongolia. Major and trace element analyses were performed by XRF and ICP-MS (for 56 elements), respectively, at the "SGS Mongolia" laboratory in accordance with the standardized methodology. Zircons used for U-Pb dating were contracted from fresh rock samples of ca. 2-3 kg by conventional heavy liquid method and the U-Pb isotope analysis was carried out by using the LA-ICPMS at the laboratory of the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The internal structure of zircons was examined by using an optical microscopy at both the transmitting and reflective modes, and by cathode-luminescence (CL) imaging using a microprobe. The size of the target analytical spots is 32 μm in diameter and the concentrations of U, Th, and Pb were calibrated by using the 29Si internal standard.

Geochemistry
In order to determine the geochemical characteristics of granitoid rocks of the Taatsiin Gol pluton a totally of 7 samples of first phase diorite, 20 samples of second phase granodiorite, and porphyritic biotite-hornblende granite and, 12 samples of third phase biotite granite were analyzed for major trace and rare earth elements. Representative samples of each phase are shown in Table 1.
In the K 2 O-Si 2 O diagram (Rickwood, 1989), all the samples plot in the high-K calc-alkaline field (Fig. 12). According to the A/NKC vs C/ NKC diagram, all the granitoids of the Taatsiin Gol pluton are metaluminous to weakly peraluminous, except for two samples of the third phase granite which plot in the strongly peraluminous field (Fig. 13). In the AFM diagram, the granitoids of the pluton show an obvious evolutionary trend of the calc-alkaline series (Fig. 14). Primitive mantle-normalized trace element diagram shows that the granitoid rocks are depleted in high field strength elements (HFSEs) of Nb, Ta, Ti and Y and enriched in Cs, Rb, Th, and K (Fig. 15). The third phase granites of the Taatsiin Gol pluton, Khangai Complex display negative anomalies in Ba and Sr compared with the neighboring elements and are more richment in light rare earth elements (LREEs) and more intense negative Eu anomaly compared to the first and second phase rocks (Fig.16). In the diagram of the petrotectonic classification (Pearce et al., 1984), the Taatsiin Gol pluton samples of the first and the second phase rocks plot into the volcanic arc granite filed and the third phase granites into the syncollision granite field (Fig.17). According to the FeO(T)-ASI classification diagram (Pearce et al., 1984) all the samples plot in the I-type granite field, except for 2 samples of the second and third phases falling in the S-type granite field (Fig. 18).  Complex (Pearce et al., 1984) are elongate subhedral to prismatic and are 100-250 µm long. These zircons mostly show a coreovergrowth texture, with the cores displaying weak or no oscillatory zoning in CL images but the overgrowths having well-developed oscillatory zoning (Fig. 19A), which indicates an igneous origin for the zircon overgrowths, as well as the zircons. A total of 29 analyses were made on 20 zircon grains for this sample and all the analyses gave coherent results (Table 2), which yielded a weighted mean 206 Pb/ 238 U age of 241.4±1.2 Ma (MSWD = 0.99; Fig.19B). Zircon grains from the biotite granite (sample Si -391), are essentially similar in internal texture, except for their relatively stubby morphology (Fig. 19C). Totally, 12 analyses were done on 12 zircon grains (Table 3). These analyses yielded a weighted mean 206 Pb/ 238 U age of 236.3±1.4 Ma (MSWD = 0.0016; Fig.19D).    (Takahashi et al., 2000). Some researchers explain that the Khangai Batholith is formed within the time span of 269-241 Ma (Yarmolyuk et al, 2016;Yarmolyuk et al, 2013aYarmolyuk et al, , 2013b

Tectonic settings
As aforementioned, granitoid rocks of the Taatsiin Gol pluton can be divided into three phases according to the field relations and petrographical characteristics. The first phase is mainly characterized by fine-medium-grained diorite, the second phase is medium to coarsegrained, porphyritic biotite-hornblende granite and granodiorite, and the third phase is represented by biotite granite. In terms of geochemistry, all the rocks of the three phases belong to the high-K calc-alkaline series and Itype granites (Fig. 14 and Fig.18) although they display a gradual change in geochemical signature .
The rocks of all the three phases contain relatively more contents of water-bearing minerals (e.g., hornblende and mica), implying formation in a subduction zone environment. If the high-K feature of these rocks is taken into consideration, we can further suggest that the subduction zone environment was likely confined to a continental arc or an active continental margin setting because the continental crust is enriched in K. Geochemically, the rocks of the former two phases display major and trace element features similar to those of typical arc igneous rocks. For example, the rocks are depleted in HFSE such as Nb, Ta, Ti and Y and enriched in LILE such as Rb, Cs, Th, K and LREE. As a result, these features demonstrate that the first two phases of rocks were formed in an arc (or subduction zone) setting. The third phase rocks have similar trace element and REE patterns to the first two phase rocks, except for that they reveal more evident HFSE depletion and stronger negative anomalies of Ba, Sr, and Eu than the former two -phase rocks, suggesting that the latter rocks were generated from a source where plagioclase is a major residual phase, or the magmas experienced stronger plagioclase crystal fractionation. From the Harker diagrams (Fig.  13), it is certain that the third phase granites were not formed by crystal fractionation from the first two phase granites, as evidenced by the nonlinear relations of Na 2 O and Al 2 O 3 with SiO 2 . Therefore, we believe that the highly negative Eu and Sr anomalies of the third phase granites resulted from the residence plagioclase in the source. In the case of plagioclase as a major residual phase, the magma source would be located at a relatively shallow crustal level.
So, we propose that the third phase granites of the Taatsiin Gol pluton were also formed in a subduction setting. Alternatively, granitoids forming in a postcollision environment can also show geochemical features similar to those of the third phase granites. Considering the very narrow age gap of the three phase intrusions of the Taatsiin Gol pluton (256-230 Ma), which seems to be too short for an orogenic belt to change from an oceanic plate subduction regime to a postcollision one, we prefer an active continental margin setting (subduction) for the third phase granites of the Taatsiin Gol pluton. In a word, the Taatsiin Gol pluton, as well as the whole Khangai complex, were most likely formed in an active continental margin setting.

Implications for the Mongol-Okhotsk Belt evolution
The Khangai area is located at the conjunction of two orogenic systems, the CAOB to the northeast and the Mongol-Okhotsk Belt to the south and west. Our new results and published data demonstrate that the intrusive rocks of the Khangai Complex were formed during the latest Permian and mid-Triassic period, and therefore the possibility that its formation was directly related to the subduction of the Paleo Asian Ocean plate can be precluded because the Khangai area belongs to the Early Paleozoic Domain of Mongolia the concurrent subduction zone of the during that time the Paleo Asian Ocean was located far south on the Mongolia-China border area (Sengor and Natal'in, 1996;Xiao et al., 2003). Regarding the origin of the Khangai Complex, it has been intensely discussed in the literatures. Some researchers believe that it formed by convergence processes during the closure of the Mongol-Okhotsk Ocean (e.g., Donskaya et al., 2012;Yarmolyuk et al., 2016); some others explain that it was formed due to mantle plume (Yarmolyuk et al., 2016;Yarmolyuk et al., 2013b;Kovalenko, 2003a, 2003b;Kuzmin et al., 2010), while there is a suggestion about a gradual change in the geodynamic environment from collision to intraplate setting (Litvinovsky et al., 2011). Recently, Dolzodmaa et al. (2020) concluded that ~237 Ma granitoid magmatism in the Khangai area is related to the post-collisional environment in Mongol-Okhotsk Belt whereas Ganbat et al. (2021) proposed that late early Mesozoic granitoids (~230 Ma) in the Khangai-Khentii basin, Central Mongolia, were probably formed during the southward subduction of the Mongol-Okhotsk Ocean lithosphere. As mentioned above, the third phase rocks of the Taasiin Gol pluton show geochemical features of post-or syn-collisional granites, and this geochemical characteristic is mainly caused by its shallow source region and thus does not change the explanation of the subduction geodynamic setting for the Taatsiin Gol pluton. As a consequence, we urge that the whole Khangai Complex were likely formed in a subduction zone environment during the latest Permian to the mid-Triassic period (256-230 Ma).

SUMMARY
The Taatsiin Gol pluton of the Khangai Complex contains intrusive rocks of three phases. The first phase is composed mainly of diorite, the second phase consists of fine-to medium-grained granodiorite and granite, and the third phase comprises fine to mediumgrained biotite granite. Although showing variated geochemical features, the rocks of the three phases are all suggested to form at an active continental margin setting, probably related to the southwestward subduction of the Mongol-Okhotsk Ocean plate during the late Permian to mid-Triassic period.