Zircon U-Pb Dating and Geological Significance of Late Paleozoic Intrusive Rocks in the Khanbogd-Erdene Area, Southern Mongolia

Abstract

The Khanbogd-Erdene region in southern Mongolia is a globally important copper–polymetallic metallogenic province, hosting large to super-large deposits, such as Oyu Tolgoi and Tsagaan Suvarga. The area experiences frequent tectonic–magmatic activity, particularly Late Paleozoic subduction-related magmatism, which controls the occurrence of large-scale copper–polymetallic mineralization. This study focuses on the Late Paleozoic granitic intrusive rocks in the Khanbogd-Erdene region of southern Mongolia. Using LA-ICP-MS and SHRIMP dating techniques, precise zircon U–Pb ages were obtained for 10 samples. A total of 209 zircon grains from these 10 intrusive rocks were analyzed, with most cathodoluminescence (CL) images of zircon grains showing distinct oscillatory zoning. Th/U ratios ranging from 0.11 to 2.92 indicate they are magmatic. The younger group of granitic rocks yielded ages between 260.2 ± 1.2 Ma and 286.6 ± 0.9 Ma, indicating an Early Permian geological age. The other seven samples yielded older ages between 315.9 ± 1.8 Ma and 340.9 ± 0.9 Ma, indicating a Carboniferous geological age. These large-scale Carboniferous to Early Permian intrusive rocks in the Khanbogd-Erdene region are products of tectonic–magmatic activity during specific stages of crustal evolution. The findings provide reliable chronological data for regional tectonic–magmatic activity and offer new evidence for constraining the timing of the Variscan orogeny in southern Mongolia.

1. Introduction

The Khanbogd-Erdene region in southern Mongolia, adjacent to China (Figure 1), is a globally significant copper–polymetallic metallogenic province and one of the world’s three major porphyry copper metallogenic belts [1,2,3,4,5,6,7,8]. The area has experienced frequent tectonic events that induced intensive magmatism with ages ranging from the Neoarchean to the Late Mesozoic, predominantly composed of granitic rocks that mostly occur as plutonic batholiths or small- to medium-sized stocks [9,10,11,12]. Among those, Late Paleozoic magmatism represented the most intensive magmatic activity in the Khanbogd-Erdene district, constituting the main body of the magmatic rock belt in the Khanbogd-Erdene region. More importantly, the Late Paleozoic magmatism shows a close spatial relationship and has been considered to have governed the formation of abundant critical metal deposits, including copper, gold, molybdenum, tungsten, tin, and silver–lead–zinc [13,14,15,16,17], such as the world-class Oyu Tolgoi porphyry copper–gold deposit [18,19,20,21] and the giant Tsagaan Suvarga porphyry copper–molybdenum deposit [22,23,24,25] (Figure 1c). Based on Sino-Mongolian international geoscientific cooperation, this study focuses on the Late Paleozoic granites in the Khanbogd-Erdene region of southern Mongolia. This study obtained new robust ages for ten intrusive massifs employing high-precision zircon U–Pb dating techniques (LA–ICP–MS and SHRIMP). These results establish a refined geochronological framework for Late Paleozoic magmatism, thereby providing critical constraints on the Late Paleozoic tectonic–magmatic evolution of the region and providing new insights into the timing of the Variscan orogeny in southern Mongolia. Furthermore, the precise dating of these intrusions allows us to investigate potential relationships between specific magmatic pulses and mineralization events.

Figure 1. Simplified tectonic map of Mongolia (a), simplified tectonic map of southern Mongolia (b), and distribution map of intrusive rocks in the Khanbogd-Erdene region (c).

2. Geological Background

The Khanbogd-Erdene region in southern Mongolia is situated at the intersection of the Baruun Urt-Hutag Uul-Dong Ujimqin-Arxan arc-basin belt and the Sulinheer-Mandula-Holingol arc-basin belt [9]. Within Mongolia, this area corresponds to the Hutag Uul and Baruun Urt terranes as defined by Professor Tomurtogoo [26]. The Baruun Urt–Hutag Uul–Dong Ujimqin–Arxan arc-basin belt is mainly composed of Early Paleozoic island arcs and back-arc basins, Late Paleozoic island arcs and back-arc basins, and Neoproterozoic island arcs [27]. The Early Paleozoic island arcs were developed in an active continental margin setting and consist of Early–Middle Silurian volcanic–sedimentary sequences. The Early Paleozoic back-arc basins, also formed in an active continental margin environment, comprise Early Cambrian to Ordovician–Late Silurian carbonate rocks and continental marginal clastic deposits [27,28]. The Late Paleozoic island arcs originated in a Late Paleozoic active continental margin setting and are composed of Early Devonian–Early Carboniferous intermediate volcanic rocks and carbonate formations [9]. The Late Paleozoic back-arc basins, likewise formed in an active continental margin environment, are represented by Early Devonian–Late Carboniferous carbonate rocks and terrigenous clastic sequences. The Neoproterozoic island arcs, developed in an active continental margin setting, are characterized by dismembered ophiolite suites and Neoproterozoic basalt–andesite formations [27,28]. The Sulinheer–Mandula–Holingol arc-basin belt primarily consists of Early Paleozoic island arcs and foreland basins, Late Paleozoic island arcs, back-arc basins, and foreland basins, as well as Early Paleozoic accretionary wedges and Paleoproterozoic uplifts. Within Mongolian territory, this belt includes the Sulinheer–Mandula–Holingol Late Paleozoic island arc, the Sulinheer–Mandula–Holingol Late Paleozoic back-arc basin, and the Duulgant–Ganqimaodu Early Paleozoic accretionary wedge. The Sulinheer–Mandula–Holingol Late Paleozoic island arc formed in an active continental margin setting as a thrust nappe complex, consisting of ophiolite fragments devoid of dyke intrusions and carbonate–volcanic mélanges that preserve Tethyan-type Early Paleozoic fossil remnants [29]. The Sulinheer–Mandula–Holingol Late Paleozoic back-arc basin, situated in the southwestern part of the Hutag Uul terrane of Mongolia, is represented by Late Carboniferous–Middle Permian continental marginal clastic formations. The Duulgant–Ganqimaodu Early Paleozoic accretionary wedge extends in an approximately E–W trend along the southern border region of Mongolia, where it connects with China’s Ganqimaodu area and consists of Ordovician greenschist formations.

In recent years, significant exploration breakthroughs have been made in copper–gold deposits in the Khanbogd-Erdene region, with newly discovered copper–gold deposits primarily associated with plutonic intrusive rocks. Large areas of intermediate–acidic intrusive rocks are developed along both sides of the fault zone, including lithologies such as granite, granodiorite, monzogranite, syenogranite, and minor basic–ultrabasic rocks. This subduction-related magmatism persisted from the Devonian and Carboniferous through to the Permian to Early Triassic [30,31,32], playing a crucial role in the formation of copper–polymetallic deposits. This study selects 10 intrusive rock bodies in the Khanbogd-Erdene region as research subjects (Figure 1): Sample (M7-9-3) is medium-coarse-grained granite, coordinates 111°05′15″ E, 44°13′54″ N; Sample (M7-9-4) is medium-fine-grained granite, coordinates 111°06′34″ E, 44°11′10″ N; Sample (M7-9-6) is medium-coarse-grained granite, coordinates 111°02′09″ E, 44°04′28″ N; Sample (M7-9-9) is medium-coarse-grained granite, coordinates 111°19′45″ E, 43°43′28″ N; Sample (M7-10) is fine-grained K-feldspar granite, coordinates 110°56′40″ E, 43°41′59″ N; Sample (M7-13-4) is medium-fine-grained granite, coordinates 108°47′27″ E, 42°32′40″ N; Sample (M7-14) is fine-grained granite, coordinates 109°40′17″ E, 42°34′23″ N; Sample (M9-3) is coarse-grained granite, coordinates 106°40′53″ E, 43°01′26″ N; Sample (M9-4) is medium-fine-grained monzogranite, coordinates 106°43′17″ E, 42°55′10″ N; and Sample (M9-5) is fine-grained syenogranite, coordinates 106°47′26″ E, 42°49′17″ N.

3. Analytical Methods

Zircon separation was conducted at the Langfang Regional Geological Survey Institute in Hebei Province using flotation and electromagnetic separation methods. Zircon grains with well-developed crystal forms and good transparency were selected under a binocular microscope, mounted on epoxy resin, and polished to create grain mounts. These mounts were then subjected to transmitted light, reflected light, and cathodoluminescence (CL) microscopic observation and imaging. For the majority of our samples, the zircon grains were of sufficient size to be effectively analyzed using LA-ICP-MS. However, for sample M7-9-4, some zircon grains were too small to use LA-ICP-MS to guarantee data quality. As such, we use the SHRIMP technique, which possesses superior spatial resolution, for M7-9-4 in this context.

The LA−ICP−MS zircon U−Pb testing was completed at the Isotope Laboratory of the Tianjin Center of Geological Survey. The experimental instrumentation utilized a Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICP-MS), specifically comprising a Neptune multi-collector ICP-MS from Thermo Fisher Scientific and a NEW WAVE 193 nm FX ArF excimer laser from ESI. During the experimental procedure, a 193 nm laser was used to ablate the zircon samples and conduct in situ U−Pb isotope determination. The laser ablation spot size was set to 35 μm. Zircon standard 91,500, which has a ²⁰⁶Pb/²³⁸U age of 1065 ± 0.6 Ma, was used to calibrate the mass discrimination and isotope fractionation. Zircon standard GJ-1 was used as an unknown for verification of data quality. Common lead was corrected based on ²⁰⁸Pb, and the NIST SRM 610 glass standard was used as an external standard to calculate the U and Pb concentrations in the zircon samples. Data and diagram processing were performed using the ICP−MS DataCal program [33] and the Isoplot program [34].

The SHRIMP zircon U-Pb dating was completed at the Beijing SHRIMP Center. The SHRIMP standard used was TEMORAl zircon, with a ²⁰⁶Pb/²³⁸U ratio of 0.0668, corresponding to an age of 417 Ma. The standard index was 2.00, and the standard ²⁰⁷Pb/²⁰⁶Pb ratio was 0.551. The standard was analyzed once after every 2–4 measurements of unknown-age zircon grains. Data analysis and processing methods followed those described by Claesson et al. [35] and Compston et al. [36]. Age calculations employed the decay constants recommended by Steiger et al. [37].

4. Results

4.1. Zircon Cathodoluminescence (CL) Characteristics

Cathodoluminescence (CL) imaging (Figure 2 and Figure 3) reveals that the zircon grains predominantly exhibit elongated prismatic, short prismatic, and irregular subangular morphologies, displaying euhedral to subhedral crystal forms with particle sizes ranging from 90 to 200 μm. Most zircon grains show distinct oscillatory zoning, indicative of primary magmatic origin. Some grains exhibit core-rim structures, with a few displaying weakened zoning at the rims and signs of hydrothermal alteration. A total of 209 zircon grains from 10 samples were analyzed. For sample M7-9-4, the Th/U ratios range from 0.31 to 1.41, while those from the other nine samples vary between 0.11 and 2.92. These values exceed the critical thresholds of 0.1 or 0.4 [38,39,40], and the hydrothermal activity did not compromise the closed-system behavior of the U-Pb isotopic system in zircon, demonstrating typical magmatic zircon characteristics.

Figure 2. Cathodoluminescence (CL) images of zircons from intrusive rocks in the Khanbogd-Erdene region, southern Mongolia (samples M7-9-3, M7-9-4, M7-9-6, M7-9-9, and M7-10). The white scale bar is 100 μm.

Figure 3. Cathodoluminescence (CL) images of zircons from intrusive rocks in the Khanbogd-Erdene region, southern Mongolia (samples M7-13-4, M7-14, M9-3, M9-4, and M9-5). The white scale bar is 100 μm.

4.2. Zircon U−Pb Dating Results

Based on preliminary selection using zircon cathodoluminescence images, measurement spots were carefully positioned to avoid internal fractures and inclusions. A total of 209 zircon grains were selected for in situ analysis (Figure 4, Appendix A Table A1, Appendix A Table A2), with most measurement points located on distinct magmatic zoning. The primary criteria for excluding analyses from the weighted age calculation are as follows: (1) High Discordance: Analyses that have significant discordance (>10) are excluded, as this may indicate Pb-loss or mixing of age domains; (2) High Analytical Uncertainty: Analyses with exceptionally large uncertainties are scrutinized and may be excluded if they are statistical outliers that disproportionately inflate the overall uncertainty; (3) Presence of Inherited Core or Metamorphic Overprint: We use Cathodoluminescence (CL) images and dating results to guide our calculation so that the different age populations (e.g., inherited core) are excluded from the calculation of the magmatic crystallization age.

Figure 4. Zircon U−Pb concordia diagram for intrusive rocks from the Khanbogd-Erdene area, southern Mongolia.

Sample (M7-9-3) yielded ²⁰⁶Pb/²³⁸U ages ranging from 317 ± 4 Ma to 359 ± 4 Ma, with a weighted mean age of 329.5 ± 1.9 Ma (after excluding grains #1, #3, #11, #12, #14, #16, #17, and #18, MSWD = 1.8, n = 12, 95% conf.). The Th/U ratios range from 0.85 to 2.94. Sample (M7-9-4) yielded ²⁰⁶Pb/²³⁸U ages ranging from 325.2 ± 3.6 Ma to 338.5 ± 2.4 Ma, with a weighted mean age of 331.6 ± 1.4 Ma (after excluding grain #8, MSWD = 0.33, n = 13, 95% conf.). The Th/U ratios range from 0.31 to 1.41. Sample (M7-9-6) yielded ²⁰⁶Pb/²³⁸U ages ranging from 326 ± 4 Ma to 342 ± 10 Ma, with a weighted mean age of 334.3 ± 1.9 Ma (MSWD = 0.86, n = 19, 95% conf.). The Th/U ratios range from 0.40 to 0.90. Sample (M7-9-9) yielded ²⁰⁶Pb/²³⁸U ages ranging from 267 ± 4 Ma to 285 ± 2 Ma, with a weighted mean age of 275.4 ± 1.7 Ma (MSWD = 2.7, n = 27, 95% conf.). The Th/U ratios range from 0.11 to 0.59. Sample (M7-10) yielded ²⁰⁶Pb/²³⁸U ages ranging from 268 ± 1 Ma to 283 ± 2 Ma, with a weighted mean age of 273.1 ± 0.6 Ma (after excluding grain #26, MSWD = 3.9, n = 25, 95% conf.). The Th/U ratios range from 0.45 to 0.66. Sample (M7-13-4) yielded ²⁰⁶Pb/²³⁸U ages ranging from 240 ± 3 Ma to 273 ± 3 Ma, with a weighted mean age of 263.2 ± 1.4 Ma (after excluding grains #9, #10, #15, #17, and #20, MSWD = 3.4, n = 15, 95% conf.). The Th/U ratios range from 0.40 to 0.85. Sample (M7-14) yielded ²⁰⁶Pb/²³⁸U ages ranging from 296 ± 3 Ma to 335 ± 4 Ma, with a weighted mean age of 316 ± 2 Ma (after excluding grains #1, #2, #3, #15, and #17, MSWD = 1.4, n = 13, 95% conf.). The Th/U ratios range from 0.25 to 1.05. Sample (M9-3) yielded ²⁰⁶Pb/²³⁸U ages ranging from 339 ± 2 Ma to 345 ± 2 Ma, with a weighted mean age of 340.9 ± 0.9 Ma (after excluding grain #21, MSWD = 0.53, n = 20, 95% conf.). The Th/U ratios range from 0.32 to 0.77. Sample (M9-4) yielded ²⁰⁶Pb/²³⁸U ages ranging from 334 ± 2 Ma to 335 ± 2 Ma, with a weighted mean age of 334.9 ± 1.3 Ma (after excluding grains #2, 4, 6, 7, 8, 10, 12, 13, 15, 16, and 19, MSWD = 0.052, n = 11, 95% conf.). The Th/U ratios range from 0.55 to 0.77. Sample (M9-5) yielded ²⁰⁶Pb/²³⁸U ages ranging from 285 ± 2 Ma to 288 ± 2 Ma, with a weighted mean age of 286.6 ± 0.9 Ma (after excluding grains #3, 4, 5, 6, 8, 10, 11, 12, 13, 16, and 22, MSWD = 0.36, n = 11, 95% conf.). The Th/U ratios range from 0.84 to 1.18.

5. Discussion

This study employs high-precision LA-ICP-MS and SHRIMP zircon U–Pb dating to determine the emplacement ages of ten intrusive massifs in the Khanbogd–Erdene region of southern Mongolia. Our results reveal two major episodes of magmatic activity: a Carboniferous phase (315.9 ± 1.8 Ma to 340.9 ± 0.9 Ma) and an Early Permian phase (260.2 ± 1.2 Ma to 286.6 ± 0.9 Ma). The findings illustrate multiple phases of tectono-magmatic activity in the Khanbogd–Erdene region, indicating that the southern Central Asian Orogenic Belt (CAOB) experienced at least two significant magmatic pulses during the Carboniferous and Early Permian, suggesting that mineralization in the area was not a single event, but rather a complex process linked to multiple tectonic–thermal events. The Carboniferous magmatism (ca. 316–341 Ma) is likely associated with the southward subduction of the Paleo-Asian Ocean plate beneath the North China Craton or other microcontinental blocks, leading to the formation of a subduction-related magmatic arc (e.g., [41,42]). In contrast, the Early Permian magmatism (ca. 260–287 Ma) likely marks a critical transition in the tectonic regime from compressional subduction-related orogenesis to a post-collisional or post-orogenic extensional setting (e.g., [12]). Such tectonic transitions often trigger crustal decompression melting and asthenospheric upwelling, creating highly favorable conditions for the development of large-scale ore-forming fluid systems and mineral deposits (e.g., [43]). The establishment of this high-precision geochronological framework significantly enhances our understanding of the tectono-magmatic processes along the southern margin of the CAOB and provides robust geochronological constraints for future mineral exploration targeting in this region.

In combination with previous geochronological and petrological studies [27,30,44], this study demonstrated that the subduction-related magmatism in the Khanbogd–Erdene region was prolonged, persisting from the Devonian to the Permian. This protracted magmatic activity corresponded to a series of tectonic regimes, including Devonian subduction, Early Carboniferous subduction–collision, and the development of the Paleozoic Andean-type magmatic island arc, and subsequent Mesozoic collisional and post-collisional events [27,30,44]. These multiphase and compositionally diverse magmatic events have directly controlled the formation of a variety of deposits, with porphyry mineralization being the dominant type [4,5,6,7,8]. Gerel et al. suggested that the Early Paleozoic granites in the Khanbogd-Erdene area are I-type granites, exhibiting subduction-related characteristics, while the Late Paleozoic was dominated by Middle–Late Devonian intermediate–acidic volcanic arc magmatism, which controlled copper–gold mineralization [25]. Nie, F.J. et al. divided the regional crustal evolution and metallogenesis in southern Mongolia during the Late Paleozoic into two stages: the syn-orogenic stage from the Devonian to Early Permian, which formed volcanic rock-hosted Cu–Zn deposits, porphyry Cu (Au), and Cu (Mo) deposits, and the Middle–Late Permian post-orogenic stage with relatively weak mineralization [4]. Li, J.J. et al. compiled a 1:1,000,000 series of geological maps for the Sino–Mongolian border region, identifying six periods of porphyry deposit mineralization and proposing that the Oyu Tolgoi–Tsagaan Suvarga metallogenic belt in southern Mongolia extends westward to connect with the East Tianshan metallogenic belt in China [8]. Jiang, S.H. et al. systematically summarized the metallogenic patterns of copper deposits in Mongolia, classifying the southern Mongolia region as a porphyry Cu–Mo–Au metallogenic belt related to the Mongol–Okhotsk Ocean tectonic system, further subdivided into a Devonian VMS-type Cu–Zn metallogenic sub-belt, a Late Devonian porphyry Cu–Au–Mo metallogenic sub-belt, and a Carboniferous porphyry Cu–polymetallic metallogenic sub-belt. They suggested a close relationship between the evolution of the Paleo-Asian Ocean branches, the tectonic nature of the Zun-Bayan Fault, and the extension of the copper metallogenic belt [8]. Zhang Weibo et al. proposed that the Late Paleozoic crustal evolution in southern Mongolia experienced a south-to-north single subduction model, where the Paleo-Asian oceanic crust subducted northward at a relatively shallow angle, forming a series of intrusive rocks and associated porphyry deposits [45].

Based on the above results, we propose that the Paleozoic crustal evolution and copper metallogenic framework in the Khanbogd-Erdene area can be summarized as follows: During the Early Paleozoic, multiple episodes of plate subduction triggered large-scale tectonic–magmatic activities, resulting in the formation of thick marine volcanic–sedimentary sequences and intrusive rocks within the Mandalovoo-Gurvansayhan terrane, thereby forming an Early Paleozoic magmatic belt within the island arc. In the Late Paleozoic, the syn-orogenic stage represented the main metallogenic epoch for porphyry copper deposits. Subduction of the Paleo-Asian Ocean plate from the Early Devonian to Early Carboniferous induced intense magmatism, leading to the extensive development of Variscan intermediate–acidic volcanic rocks and granitoid intrusions. Concurrently, hydrothermal fluids associated with intermediate–acidic magmas interacted with country rocks through water–rock interaction, forming porphyry copper deposits such as Oyu Tolgoi and Tsagaan Suvarga.

6. Conclusions

Based on high-precision LA-ICP-MS and SHRIMP zircon U–Pb dating, this study identifies two major magmatic events in the Khanbogd–Erdene region of southern Mongolia: a Carboniferous event (315.9 ± 1.8 Ma to 340.9 ± 0.9 Ma) and an Early Permian event (260.2 ± 1.2 Ma to 286.6 ± 0.9 Ma). In combination with previous studies, the Carboniferous magmatism is interpreted to be related to a continental arc formed by the southward subduction of the Paleo-Asian Ocean, while the Early Permian magmatism likely marks a key transition in the tectonic regime from subduction-related compression to post-collisional extension. The Late Paleozoic syn-orogenic stage (Early Devonian to Early Carboniferous) represents the main epoch of porphyry copper mineralization. These high-precision geochronological data provide new constraints for understanding the tectono-magmatic evolution of the southern Central Asian Orogenic Belt (CAOB)