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Article

Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean

by
Peipei Dong
1,2,
Yingjie Li
1,2,*,
Yan Xie
3,
Jinfang Wang
1 and
Hongyang Li
1
1
College of Earth Sciences, Heibei GEO University, Shijiazhuang 050031, China
2
Hebei Key Laboratory of Strategic Critical Mineral Resources, Heibei GEO University, Shijiazhuang 050031, China
3
Inner Mongolia Mining Exploitation Co., Ltd., Hohhot 010020, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(10), 1212; https://doi.org/10.3390/min12101212
Submission received: 6 July 2022 / Revised: 31 August 2022 / Accepted: 21 September 2022 / Published: 26 September 2022
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve the understanding of plate tectonic processes and the history of continental growth. Here, we report on newly recognized trondhjemite in the north of Diyanmiao ophiolite belt in North China. The trondhjemite was found along the Erenhot-Hegenshan suture zone. U-Pb zircon dating revealed that the trondhjemite crystallized at 309 ± 2.1 Ma. The trondhjemite had a high amount of SiO2 (68.94–76.45 wt %), Al2O3 (13.37–15.90 wt %), and Sr (232–601 ppm); and a low amount of K2O (1.57–2.70 wt %), Y (6.91–9.39 ppm), Ni (1.10–4.19 ppm), and Cr (1.55–13.50 ppm). The Na2O/K2O ratios were 1.90–4.37. There was a lack of negative Eu anomalies. It was relatively enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr; was depleted in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P; and had low total rare-earth element (REE) contents (27.73–49.63 ppm) with distinct REE fractionation (chondrite-normalized (La/Yb)N of 5.76–10.52), which was similar to adakitic rocks formed by partial melting of subducted oceanic crust. The trondhjemite, together with Diyanmiao ophiolite (335.6 Ma), may have formed during the stages of intra-oceanic subduction, suggesting that in the Early Carboniferous–Late Carboniferous, the southern Paleo-Asian Ocean was in its subduction stage.

1. Introduction

Tonalite–trondhjemite–granodiorite (TTG) suites have always been one of the important subjects in the field of geology [1,2]. As an important component of the crust, Tonalite–trondhjemite–granodiorite (TTG) rocks are not only widely distributed in the Archean Paleoproterozoic granite greenstone belt, but also an important part of the Phanerozoic accretionary orogenic belt (the oceanic subduction belt). Geochemical and experimental petrological studies have confirmed that TTG suites were formed by partial melting of water-bearing basaltic rocks in garnet amphibolite facies or eclogite facies [2,3]. It is generally believed that the Phanerozoic TTG suites are a typical igneous tectonic assemblage formed in the island arc environment of the oceanic subduction zone and were the product of dehydration and melting of oceanic subduction basalt slabs [1,4,5]. This is an important rock record of crustal accretion and tectonic evolution in orogenic belts. The study showed that the Phanerozoic TTG magmatism was an important geological process for the growth of the Cenozoic continental crust. A tonalite–trondhjemite–granodiorite (TTG) assemblage represents a growth event of the Cenozoic continental crust or accretion of the continental crust, which is of great significance in the formation of the continental prototype and is also the key to characterizing the accretion of the Phanerozoic accretionary orogenic belt and the ocean continent transition [6,7]. Therefore, a study on the petrogenesis, tectonic environment, and formation age of TTGs can provide important petrological, chronological, and geochemical bases for the formation, accretionary mechanism, and tectonic evolution of an new continental crust.
The Central Asian Orogenic Belt (CAOB) is located between the Siberian plate and the Tarim and North China plates (Figure 1a). The CAOB is one of the Earth’s largest accretionary orogens and records a complex history of subduction and collision episodes pertaining to the Neoproterozoic–Paleozoic closure of the Paleo-Asian Ocean (PAO) [8,9]. Its prolonged, episodic, diachronous, and mostly Paleozoic tectonic evolution is characterized by accretion of a diverse array of terranes including continental-margin fragments, seamounts, fore-arc and back-arc basins, and oceanic arcs [10,11,12,13,14,15,16,17,18]. The archipelago-type southwestern Pacific oceanic is often regarded as a modern tectonic analog [10,14,19]. It is considered to be a typical accretionary orogenic belt (Figure 1b) [19,20,21]. However, the closing time and location of the Paleozoic Paleo-Asian Ocean in the eastern part of the CAOB is still obviously controversial. Xiao et al. [22] and Jian et al. [23] considered that the Paleo-Asian Ocean was in the ocean continent transition stage in the Carboniferous Permian and closed along the Solonker-Xilamulun suture in the Late Permian-Early Triassic. Another view is that the Paleo-Asian Ocean closed in the middle-late Devonian and the late Paleozoic igneous rocks were formed in an intracontinental rifting setting [24]. The main reason for the inconsistency is that there is no unified understanding of the reconstruction of the paleo-ocean basin, the late Paleozoic tectonic magmatic process, and its geotectonic background. There is a lack of systematic identification and research from the ophiolite to the initial subduction intra-oceanic arc [20]. Recent studies on the Izu–Bonin–Mariana (IBM) forearc revealed that the SSZ type ophiolite and forearc rock assemblages were first formed during the initial subduction [25]. Therefore, identifying and studying the spatiotemporal distribution and internal genetic relationship between island arc magmatic rocks and SSZ ophiolites can deepen the understanding of the subduction and extinction process of the Paleo-Asian Ocean.
The southeastern CAOB contains several sub-parallel ophiolite belts with variable ages [26] (Figure 1b). The Hegenshan ophiolite–arc–accretionary belt is located in the northern of the southeastern CAOB (Figure 1b). The Hegenshan suture zone is widely distributed with Paleozoic ophiolites (belts) and subducted island-arc magmatic rocks, especially the Early Carboniferous–Late Carboniferous ophiolites and Early Carboniferous–Early Permian island-arc magmatic rocks. Most previous studies considered that the Hegenshan ophiolites were formed in a mid-ocean ridge environment [27,28], and some scholars also believed that the Hegenshan ophiolites have the characteristics of a subduction zone, such as an island arc-marginal basin system or back arc extensional ocean basin [29], but lacking systematic identification of an intra-ocean arc [21].
Recent studies have shown that some ophiolites in the Hegenshan ophiolite–arc–accretionary belt were formed in an intra-oceanic forearc setting, such as Diyanmiao ophiolite [30] and Meilaotewula ophiolite [31]. Therefore, the recognition of intra-oceanic arcs in SSZ-type ophiolites is an effective method to understand the process of intra-oceanic subduction and subsequent orogenesis [32].
Our regional geological mapping revealed the occurrence of Late Carboniferous trondhjemite in the north of the Diyanmiao SSZ-type ophiolite zone (Figure 2), providing an opportunity for a systematic study of intra-oceanic subduction of the southeastern part of the Paleo-Asian Ocean (PAO). Fore-arc basalt, boninitic rocks, and adakites indicated that the Diyanmiao ophiolite formed in the incipient forearc of suprasubduction zone [21,32]. The newly discovered Gaolihan trondhjemite provides new information for the systematic study of intra-oceanic subduction in the southern PAO. Here, we present the field relations, petrography, major and trace-element geochemistry, and zircon U-Pb geochronology of this trondhjemite. We provide clear evidence for a progressively evolving of an intra-oceanic arc and intra-oceanic subduction in the Late Carboniferous.

2. Geological Setting

The southern CAOB extends from southern Mongolia via Inner Mongolia to the Great Xing’an Range [21,33]. This region has been subdivided into three tectonic zones, the southern subduction–accretionary orogen, the Solonker suture zone, and the northern subduction–accretionary orogen. [22,34] (Figure 1b).
The Hegenshan ophiolite-arc-accretionary complex extends ∼500 km from Erenhot to the Hegenshan Mountains in the northeast [30]. This complex is sandwiched between the Chagan Obo–Arongqi and Erenhot faults and contains the Erenhot ophiolitic complex, Hegenshan ophiolite, and Meilaotewula and Diyanmiao ophiolites [31,32], The Erenhot–Hegenshan fault zone contains abundant Early Carboniferous to Late Carboniferous SSZ-type ophiolites and Late Carboniferous to Early Permian island arc magmatic rocks. In these island arc magmatic rocks, geologists [35,36] have continuously identified TTG rocks. These island arc magmatic rocks, particularly the arc crust or continental crust formed by the tectonic combination of intra-oceanic arc or intra-oceanic arc igneous rocks, record the information of ocean land-transformation processes such as subduction of the Paleo-Asian ocean crust, TTG magmatism, and growth of the new continental crust. Therefore, this paper attempted to provide constraints for the subduction and extinction process of the Paleo-Asian Ocean and the growth of the Cenozoic continental crust tectonic magmatic events through a detailed study of the petrology, geochemistry, and zircon U-Pb chronology of the Gaolihan trondhjemite, which is closely related to the Diyanmiao intra-oceanic arc.

3. Field Relationship and Petrography

The newly recognized Gaolihan trondhjemite is located in the north of Diyanmiao area (Figure 2) (118°21′, 45°01′). The trondhjemite is discordantly overlain by Jurassic intermediate volcanic rocks and are intruded by Late Carboniferous tonalite (Figure 2). The Diyanmiao SSZ ophiolite is in faulted contact with Early Permian marine flysch deposits [21] (Figure 2), which comprise mainly, from bottom to top, harzburgite, layered-massive gabbro, diabase veins, fore-arc basalts (FAB), boninite, chert, and siliceous mudstone [21], a lithological assemblage that is similar to the SSZ-ophiolite-type sequence of the IBM system [37]. Most units of the ophiolite and its related rocks were subjected to mylonitization (Figure 2).
The Gaolihan trondhjemite is greyish-white to grey in color and medium- to coarse-grained (Figure 3a–d). It contains mainly plagioclase (60–65%), quartz (25–30%), and biotite (5–10%). The oligoclase occurs as euhedral to subhedral grains. The composition of the plagioclase is sodic oligoclase with An = 10–15 based on extinction angle measurements. The plagioclase is typically partially sericitized. The quartz is xenomorphic granular. The biotite is commonly altered to chlorite.

4. Analytical Methods

The whole-rock geochemical and zircon U-Pb isotopic analyses were undertaken at the Wuhan Sample Solution Analytical Technology Co., Ltd, Wuhan, China.
A Zsx Primus II wavelength dispersive X-ray fluorescence spectrometer (XRF) produced by RIGAKU, Japan, was used for the analysis of major elements in the whole rock; the X-ray tube was a 4.0 Kw end window Rh target. All major element analysis lines were kα and the standard curve used the national standard material: the rock standard sample GBW07101-14. The relative standard deviation (RSD) was less than 2%.
Trace-element analyses of whole rock were conducted on an Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The detailed sample-digesting procedure was as follows: (1) Sample powder (200 mesh) was placed in an oven at 105 °C for drying for 12 h; (2) 50 mg of sample powder was accurately weighed and placed in a Teflon bomb; (3) 1 mL of HNO3 and 1 mL of HF were slowly added into the Teflon bomb; (4) the Teflon bomb was enclosed in a stainless steel pressure jacket and heated to 190 °C in an oven for >24 h; (5) after cooling, the Teflon bomb was opened and placed on a hotplate at 140 °C and evaporated to incipient dryness, and then 1 mL HNO3 was added and evaporated to dryness again; (6) 1 mL of HNO3, 1 mL of MQ water, and 1 mL of internal standard solution at 1 ppm In were added, and the Teflon bomb was resealed and placed in the oven at 190 °C for >12 h; (7) the final solution was transferred to a polyethylene bottle and diluted to 100 g via the addition of 2% HNO3.
Zircons were separated using standard density and magnetic techniques and then handpicked under a binocular microscope. Grains were then imaged using cathodoluminescence (CL) to characterize their internal morphologies. Zircon 91,500 and glass NIST610 were used as external standards for U-Pb dating and trace-element calibration, respectively. The Excel-based software ICPMSDataCal was used to perform off-line selection and integration of the background and analyzed signals, time-drift correction, and quantitative calibration for the trace-element analysis and U-Pb dating [38]. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Berkeley CA, USA, Kenneth R. Ludwig, 2003) [39].

5. Analytical Results

5.1. Major and Trace-Element Analyses

The loss-on-ignition (LOI) contents of the Gaolihan trondhjemite was 1.06–1.69% (Table 1 [6,35,36,40,41]), which was consistent with petrographic evidence for partial alteration. Furthermore, elements such as Li, Rb, Cs, K, U, and P may be mobile in rocks during hydrothermal alteration [42]. Therefore, in the following discussions, the major element contents were normalized to 100 wt % on an anhydrous basis and we considered some relatively immobile elements (such as high field strength elements: Th, Zr, Hf, Nb, Ta, Ti, and rare-earth elements) for interpretation of the trace-element chemistry.
The Gaolihan trondhjemite samples had high SiO2 (68.94–72.58 wt %) and Al2O3 (14.85–15.90 wt %) contents and low MgO (0.76–1.33 wt %), TiO2 (0.27–0.52 wt %), and P2O5 (0.09–0.14 wt %) contents. The rocks had relatively low concentrations of K2O (1.37–2.70 wt %) but were enriched in Na2O (4.80–5.99 wt %); the Na2O/K2O ratios were 1.88–4.38 (Table 1). In an An-Ab-Or diagram (Figure 4a), all of the five trondhjemite samples fell into the field of trondhjemite. In a K-Na-Ca diagram (Figure 4b), the trondhjemite samples fell into the field of Archaean TTGs or at the boundary. In a SiO2 vs. Na2O + K2O (TAS) diagram, three trondhjemite samples fell into the field of granodiorite, with two plotting at the quartz monzonite field (Figure 5a). On a K2O vs. SiO2 diagram (Figure 5b), the five trondhjemite samples plotted within the calc–alkaline field.
The samples had low total rare-earth element (REE) contents (30.58–77.80 ppm) and showed light rare-earth element (LREE) enrichment and heavy rare-earth element (HREE) depletion in the chondrite-normalized REE diagrams ([La/Yb]N = 5.76–10.52) with no pronounced Eu anomaly (Eu/Eu* = 0.89–1.00; average = 0.96) (Figure 6a). The striking chemical characteristics were the high contents of Sr (286–601 ppm; average = 465 ppm) and low contents of Y (7.12–11.10 ppm; average = 8.66 ppm) and Yb (0.81–1.16 ppm; average = 0.91 ppm); the Sr/Y ratios were 36.48–64.00. The samples had relatively low abundances of compatible elements such as Cr (1.55–5.42 ppm) and Ni (1.10–5.74 ppm). In addition, the samples displayed a relative enrichment in large-ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr; and depletion in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P (Figure 6b), showing the trace-element composition characteristics of the island arc magmatic rocks in the oceanic subduction zone [36].
Figure 6 [6,40,41,47,48] shows that the trace-element patterns of the Gaolihan trondhjemite were consistent with those of Phanerozoic TTG, adakites within the Diyanmiao SSZ-type ophiolite, and Late Carboniferous tonalite, displaying enrichment in LILEs (K, Rb, and Sr) and depletion in HFSEs (Nb, Ta, and Ti), which was consistent with the island arc and oceanic subduction background [49,50].

5.2. U-Pb Zircon Ages

The results of the zircon LA–ICP–MS U-Pb dating are summarized in Table 2. Zircon grains separated from this sample (P16TW08) showed an intact crystal shape and subhedral to euhedral morphologies. All analyzed zircon grains were transparent and had oscillatory zoning and broad magmatic zoning in cathodoluminescence (CL) images (Figure 7a). The zircon grains showed relatively narrow, clear, and dense oscillatory zoning with no evidence of resorption or inherited cores; all zircon grains had high Th/U ratios (Table 2), which was characteristic of crystallization from acidic magma [51]. A total of 16 analyses of zircons from sample P16TW08 yielded concordant results with 206Pb/238U ages ranging from 301 ± 4 to 316 ± 5 Ma. These data yielded a weighted mean age of 309.2 ± 2.1 Ma (MSWD = 0.77) (Figure 7b,c), reflecting the crystallization age of the Gaolihan trondhjemite.

6. Discussion

6.1. Petrogenesis of the Trondhjemite

The Gaolihan trondhjemite was characterized by high concentrations of SiO2, Al2O3, Na2O, and Sr, with high Sr/Y ratios (average = 53.44 > 40) but low MgO, Y, and Yb. They were also enriched in LREEs but had low total HREE contents and no clear Eu anomalies (Figure 6a; Table 1), all of which were similar to classic adakite and TTG rocks (Figure 6) and showed a trondhjemitic differentiation trend (Figure 4b). The Gaolihan trondhjemite was high in LREEs and low in HREEs without negative Eu anomalies (Eu/Eu* = 0.91–1.03) that were similar to the REE patterns of Phanerozoic TTG and TTG rock data published in the CAOB [52] (Figure 6a). All samples were enriched in large-ion lithophile elements (LILEs) such as Ba, K, Rb, and Sr; were depleted in high-field-strength elements (HFSEs) such as Nb, Ta, P, and Ti; and were similar to the primitive mantle (PM)-normalized spider-diagram patterns of Phanerozoic TTG [6] (Figure 6b). Furthermore, the Gaolihan trondhjemite showed a trondhjemitic differentiation trend [7,44] as the evolution of Na-rich magmas (Figure 5b). Therefore, through the comparison of petrological characteristics and geochemical characteristics with Phanerozoic TTG and TTG rock data published in the CAOB (Table 2); and by referring to the An-Ab-Or diagram (Figure 4a), K-Na-Ca diagram (Figure 4b), rare-earth-element patterns (Figure 6a), and trace-element diagram (Figure 6b) of the Late Carboniferous tonalite, the Gaolihan trondhjemite may belong to TTG rocks [5].
The basalt melting experiment inferred that TTG-like trondhjemite melts can only be produced when there is free water in the source, while dehydration melting generates granitic liquids [53]. However, there are different opinions on the genetic mode or tectonic setting of TTGs. Two main but different views have been proposed: (1) Partial melting genesis of mafic material underplated beneath thickened crust [54,55,56]; (2) partial melting of the subduction oceanic crust [6] (for example, the subduction of young oceanic crust, the subduction initiation, and ridge subduction [57]). The Phanerozoic TTGs were similar to adakites known to be formed in subduction environments and were in association with fore-arc basalts (FABs), boninites (high-Mg andesite), and Nb-enriched basalts/Nb-enriched gabbros [7,48].
It is generally believed that Phanerozoic TTG rocks are sodic rocks formed in the island arc environment of the ocean subduction zone [1,2,4,7,36]. In the Rb-(Y + Nb) and Rb-(Yb + TA) diagrams (Figure 8a,b [6,48,58,59,60]), the Gaolihan trondhjemite fell within the field of volcanic arc granites, indicating an island arc environment [61,62]. In the Rb-(Y + Nb) and Rb-(Yb + TA) diagrams (Figure 8c), the samples plotted mainly in the TTG area and all five fell within the field of adakite and TTG, reflecting an island arc environment of the ocean subduction zone. In the (Th/Yb)-Nb/Yb) diagram (Figure 8d), the samples fell in the overlapping area of an oceanic island arc and continental arc. All these diagrams indicated that the Gaolihan trondhjemite formed in the island arc environment of the oceanic subduction zone.
The experimental results showed that TTG-like felsic melts can be produced only when free water is available in the source, “however free water is only available in the case of subducted slab melting, thus leading to the genesis of trondhjemites” [53,63]. All these arguments suggest that TTGs are generated in a subduction-related geodynamic setting.
The Gaolihan trondhjemite was characterized by typical geochemical characteristics formed in an island arc of the suprasubduction zone; were consistent with the fore-arc basalts, boninites, and adakites in the Early Carboniferous Diyanmiao ophiolite; and composed a relatively complete (intra-oceanic) arc igneous rock structural assemblage [21] (Figure 1 and Figure 2), indicating the trondhjemite represented a partial melting of a subducted oceanic slab.
The characteristics of subducted oceanic basalts are Na-rich, K-poor, and high Na2O/K2O values. In addition, the Gaolihan trondhjemite had a higher content of Si and Mg# (Table 1). It is generally believed that the Mg# of the partial melting of basalts is less than 45, while the TTG melt formed by the melting of oceanic subduction plates needs to pass through mantle wedge peridotite in the process of rising to the surface and must interact with the mantle wedge peridotite. This interaction will increase the Mg# of the residual melt [4]. Therefore, the Gaolihan trondhjemite may be the TTG melt formed by the partial melting of the subduction oceanic crust, which interacted with the mantle wedge peridotite in the process of rising to the surface through the subduction zone.

6.2. Tectonic Implications of the Trondhjemite: Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean

TTG occurs throughout geologic time and has contributed to the growth of continental crust [56]. It is crucial to understand the source and origin of TTGs to better understand the formation and tectonic evolution of continental crust [56]. Many scientists believe that intra-oceanic arcs are important sites of juvenile continental crust formation and that the subduction-related TTG-type granitoid magmatism is a main contributor to continental growth [7]. As one of the typical igneous rock assemblages formed in the island arc environment, Phanerozoic TTGs are combined with island arc magmatic rocks such as forc-arc basalt, high magnesium andesite (boninite)/high magnesium diorite, magnesium andesite, adakite, and Nb-enriched gabbro basalt/gabbro, and are important petrological records of oceanic subduction and Cenozoic arc crustal growth events [1,7,64].
The Hegenshan suture zone, which is located in the southeastern CAOB (Figure 1b), carries numerous ophiolite slices and island arc magmatic rocks. Safonova [7] reported on intra-oceanic arcs and TTGs in the CAOB and considered that TTG magmatism in intra-oceanic and continental margin arcs is an important geological process for the growth of new continental crust. Therefore, the recognition of the Diyanmiao primary arc igneous rock association may reveal that the Paleo-Asian Ocean was still in the stage of (intra-oceanic) oceanic crust subduction, intra oceanic arc TTG magmatism, and neocontinental crust growth in the Late Carboniferous. Şengör et al. [64] proposed that the CAOB was mainly formed as a subduction-related accretionary complex and magmatic arc and considered that the continental crust was accretionary mainly through tectonic accretion along the oceanic subduction zone (forming a subduction accretionary complex) and emplacement of Cenozoic magmatic rocks (forming a magmatic arc). The authors of [64] pointed out that the Paleo-Asian Ocean finally closed along the Solonker suture zone in the Triassic. The island-arc-type TTG in the Late Carboniferous oceanic subduction zone reported in this paper formed a relatively complete structural combination of primary arc igneous rocks with the Early Carboniferous Diyanmiao SSZ ophiolite (harzburgite and layered-massive gabbro), forearc basalt (FAB), boninite, and adakite (granodiorite) [21,30], creating the Early Carboniferous–Late Carboniferous Diyanmiao ophiolite-TTG rock belt. The identification of island arc TTGs in the oceanic subduction zone in the Late Carboniferous (309.2 ± 2.1 Ma) indicated that the Paleo-Asian Ocean was in the stage of oceanic subduction and juvenile continental crust formation in the Late Carboniferous.
The zircon U-Pb age of 309.2 ± 2.1 Ma obtained for the Gaolihan trondhjemite coincided with the formation age of Late Carboniferous island arc magmatic rocks in central Inner Mongolia combined with the Early Carboniferous Diyanmiao ophiolite (~340 Ma) [30], forc-arc basalt (335.6 Ma), boninite (~328 Ma) [21], and Late Carboniferous adakite (315 Ma) [47] identified in central Inner Mongolia in recent years, which further supported the interpretation that the southern PAO had not closed by the Late Carboniferous but was undergoing intra-oceanic subduction. In other words, the Late Carboniferous recorded the initial stages of subduction of the Diyanmiao intra-oceanic arc within the southern PAO, with subsequent subduction of oceanic crust. Combined with the studies on the initial subduction in the ocean, the characteristic lithological assemblage of an intra-oceanic forearc of the initial subduction in the ocean in the eastern part of the Paleo-Asian Ocean from the Early Carboniferous to Late Carboniferous can be preliminarily established: SSZ-type ophiolite (serpentinized harzburgite and layered-massive gabbro), fore-arc basalt (FAB), boninite, and adakite TTGs. Based on our field observations, a geochemical data analysis of the Diyanmiao ophiolite and TTGs, and a comparison of the IBM initial subduction system [37], we inferred a subduction initiation model in the Diyanmiao intra-oceanic arc during the Early Carboniferous to the Late Carboniferous. The Early Carboniferous intra-oceanic subduction initiation in the southeast of the Paleo-Asian Ocean began along a transform fault or fault zone [32] (Figure 9a). After the initial sinking of the slab, asthenospheric upwelling and decompression melting [65] led to forearc spreading and the formation of Diyanmiao FABs (335.6 Ma) (Figure 9b) [21,32]. The Diyanmiao FABs represented the initial subduction stage. Asthenospheric upwelling and decompression melting [66] following the initial sinking of the slab resulted in forearc spreading and formation of the Diyanmiao FABs (335.6 Ma) (Figure 9b). As the subduction initiation process continued, leading to further decompression melting, boninites (328 Ma) were generated at this stage (Figure 9c). The newly discovered trondhjemite (309 Ma) may represent the partial melting of the subducted oceanic crust at a greater depth (Figure 9d). The above information revealed that the initial subduction of the initial intra-oceanic may have occurred in the eastern part of the Paleo-Asian Ocean from the Early Carboniferous to the Late Carboniferous.

7. Conclusions

(1)
The petrological and geochemical characteristics indicated that the Gaolihan trondhjemite belonged to the tonalite–trondhjemite–granodiorite (TTG) series and formed via melting of the subducted oceanic crust.
(2)
The Gaolihan trondhjemite crystallized at 309.2 ± 2.1 Ma, reflecting the subduction-related TTG-type granitoid magmatism and indicating that the southern Paleo-Asian Ocean was in the process of subduction, TTG magmatism, and juvenile continental crust formation in the Late Carboniferous.

Author Contributions

Conceptualization, P.D. and Y.L.; methodology, P.D., Y.L., H.L., and J.W.; software, P.D. and Y.L.; validation, P.D. and Y.L.; formal analysis, P.D.; investigation, P.D., Y.L., and J.W.; resources, P.D. and Y.L.; data curation, P.D. and Y.L.; writing—original draft preparation, P.D. and Y.L.; writing—review and editing, P.D., Y.L., Y.X., H.L., and J.W.; visualization, P.D.; supervision, P.D.; project administration, Y.L.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant/award number: 41972061), the China Geological Survey (grant/award number: 1212011120711), and the Science and Technology Innovation Team Program of Hebei GEO University (KJCXTD-2021-07).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Tectonic framework of the North China–Mongolia segment of the Central Asian Orogenic Belt [16]. (b) Geological sketch map of the northern China–Mongolia tract [17,18].
Figure 1. (a) Tectonic framework of the North China–Mongolia segment of the Central Asian Orogenic Belt [16]. (b) Geological sketch map of the northern China–Mongolia tract [17,18].
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Figure 2. Geological map of the Gaolihan trondhjemite.
Figure 2. Geological map of the Gaolihan trondhjemite.
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Figure 3. Field occurrences (a,c) and photomicrographs (b,d) of the Gaolihan trondhjemite. (a) Field occurrence of the fine-medium-grained trondhjemite. (b) Characteristic fine-medium-grained subhedral granular texture of the trondhjemite. (c) Field occurrence of the coarse-grained trondhjemite. (d) Photomicrograph of the trondhjemite. Olg—oligoclase; Qtz—quartz; Bt—biotite.
Figure 3. Field occurrences (a,c) and photomicrographs (b,d) of the Gaolihan trondhjemite. (a) Field occurrence of the fine-medium-grained trondhjemite. (b) Characteristic fine-medium-grained subhedral granular texture of the trondhjemite. (c) Field occurrence of the coarse-grained trondhjemite. (d) Photomicrograph of the trondhjemite. Olg—oligoclase; Qtz—quartz; Bt—biotite.
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Figure 4. (a) Normative An-Ab-Or triangle [43]; (b) K–Na–Ca triangle [44]. Late Carboniferous tonalite data from Wang et al., 2021 [41]. To = tonalite; Gd = granodiorite; Tr = trondhjemite; Gr = granite; Td = trondhjemitic differentiation trend; CA = classical calc-alkaline trend characterized by K-enrichment during differentiation.
Figure 4. (a) Normative An-Ab-Or triangle [43]; (b) K–Na–Ca triangle [44]. Late Carboniferous tonalite data from Wang et al., 2021 [41]. To = tonalite; Gd = granodiorite; Tr = trondhjemite; Gr = granite; Td = trondhjemitic differentiation trend; CA = classical calc-alkaline trend characterized by K-enrichment during differentiation.
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Figure 5. (a) SiO2 vs. (K2O + Na2O) for trondhjemite [45]; (b) SiO2 vs. K2O for the Gaolihan trondhjemite [46]. Phanerozoic TTG data are from Condie [6].
Figure 5. (a) SiO2 vs. (K2O + Na2O) for trondhjemite [45]; (b) SiO2 vs. K2O for the Gaolihan trondhjemite [46]. Phanerozoic TTG data are from Condie [6].
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Figure 6. (a) Chondrite-normalized REE distribution patterns and (b) primitive mantle-normalized trace-element diagram for the Gaolihan trondhjemite. The normalizing values are from Sun and McDonough [47]. Phanerozoic TTG data from Condie [6]; adakite (granodiorite) data from Wang et al., 2020 [48]; Daqihundi TTG data from Fan 2020 [40]; Late Carboniferous tonalite data from Wang et al., 2021 [41].
Figure 6. (a) Chondrite-normalized REE distribution patterns and (b) primitive mantle-normalized trace-element diagram for the Gaolihan trondhjemite. The normalizing values are from Sun and McDonough [47]. Phanerozoic TTG data from Condie [6]; adakite (granodiorite) data from Wang et al., 2020 [48]; Daqihundi TTG data from Fan 2020 [40]; Late Carboniferous tonalite data from Wang et al., 2021 [41].
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Figure 7. (a) Representative zircon CL images, (b) Concordia curves, and (c) weighted mean age of LA-ICP-MS U-Pb data for the Gaolihan trondhjemite.
Figure 7. (a) Representative zircon CL images, (b) Concordia curves, and (c) weighted mean age of LA-ICP-MS U-Pb data for the Gaolihan trondhjemite.
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Figure 8. Rb vs. (Yb + Ta) diagram [58] (a), Rb vs. (Y + Nb) diagram [58] (b), (La/Yb)N vs. YbN diagram [48,59] (c), and (Th/Yb) vs. (La/Yb) vs. Th diagram [60] (d) of the Gaolihan trondhjemite. Phanerozoic TTG data from Condie, 2005 [6].
Figure 8. Rb vs. (Yb + Ta) diagram [58] (a), Rb vs. (Y + Nb) diagram [58] (b), (La/Yb)N vs. YbN diagram [48,59] (c), and (Th/Yb) vs. (La/Yb) vs. Th diagram [60] (d) of the Gaolihan trondhjemite. Phanerozoic TTG data from Condie, 2005 [6].
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Figure 9. Evolution of the early Carboniferous to late Carboniferous Diyanmiao intra-oceanic forearc system in central Inner Mongolia as based on the subduction infancy model [21,32,65,67,68]. (a) Oceanic crust boundary preceding subduction initiation. (b) Early Carboniferous subduction initiation, leading to forearc spreading and formation of FABs. (c) As the s6ubduction initiation process continued, boninites were formed. (d) As the subduction process continued, the oceanic crust partially melts at a deeper depth, trondhjemite were form. DMM is depleted MORB mantle. FAB—forearc basalt. PAO—Paleo-Asian Ocean.
Figure 9. Evolution of the early Carboniferous to late Carboniferous Diyanmiao intra-oceanic forearc system in central Inner Mongolia as based on the subduction infancy model [21,32,65,67,68]. (a) Oceanic crust boundary preceding subduction initiation. (b) Early Carboniferous subduction initiation, leading to forearc spreading and formation of FABs. (c) As the s6ubduction initiation process continued, boninites were formed. (d) As the subduction process continued, the oceanic crust partially melts at a deeper depth, trondhjemite were form. DMM is depleted MORB mantle. FAB—forearc basalt. PAO—Paleo-Asian Ocean.
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Table
Table 1. Major and trace-element data for the Gaolihan trondhjemite.
Table 1. Major and trace-element data for the Gaolihan trondhjemite.
SampleP21XT02-1P21XT05-1P21XT05-2P16XT06-1P16XT06-2Phanerozoic TTGTTG Rock in the Central Asian Orogenic Belt Late Carboniferous Tonalite
Major element (wt %) a
SiO272.58 68.94 69.24 71.81 71.30 65.90 69.0569.2064.4677.1771.1066.5866.45
TiO20.12 0.440.52 0.25 0.27 0.47 0.300.310.650.230.450.500.49
Al2O315.55 15.90 15.83 14.85 15.23 16.50 13.8713.1915.7211.2313.8515.5716.14
TFe2O31.24 3.17 3.38 2.01 2.05 1.563.41
MnO0.06 0.050.050.060.050.09 0.090.080.110.140.040.070.07
MgO0.36 1.24 1.33 0.76 0.77 1.67 1.111.252.200.561.171.641.94
CaO1.11 0.78 0.56 1.54 1.32 4.36 3.722.374.822.562.583.392.90
Na2O5.735.915.994.805.13 4.00 4.084.113.653.843.784.254.45
K2O2.081.57 1.37 2.55 2.70 2.14 1.652.781.720.891.441.911.81
P2O50.050.140.14 0.090.09 0.12 0.090.080.210.040.150.150.16
Total b99.94 99.83 99.94 99.80 100.05 97.998.3599.67100.0999.9799.8399.88
LOI1.06 1.69 1.55 1.08 1.14 2.111.95
Mg #40.3447.6247.7346.9246.71 41.79 4651
Trace element (ppm)
Rb38.5032.6028.1048.6053.1063.0030.0250.9459.506.8357.4239.8025.70
Cr1.554.945.423.933.8332.003.483.909.882.853.0111.5012.30
Co1.266.407.153.583.6440.704.516.0013.51.626.6510.609.30
Ni1.104.195.742.822.3312.002.652.035.143.074.006.205.90
Sc1.675.415.853.614.0034.907.167.16 7.886.12
V11.4062.0062.5034.7036.70243.0075.9479.10 13.56 90.5067.50
Zr68.1087.9092.4084.1082.00122.00118.6096.84156.0045.86168.37113.2099.40
Hf2.042.682.962.472.563.403.613.224.281.374.187.007.52
Ta0.190.180.230.180.180.750.190.190.440.030.170.200.23
Sr286.00601.00586.00439.00412.00493.00207.60143.74336.0071.82293.56458.60421.20
Ba426.00527.00452.00427.00432.00716.00354.80444.20359.0059.77394.60528.20413.20
Nb2.172.893.511.912.196.702.212.226.270.333.162.782.93
Cs0.792.382.041.090.96 0.440.813.76
Ga17.1017.8017.5018.3018.20 14.3715.94
Th0.913.904.623.003.077.602.973.264.780.595.094.293.73
U0.310.830.850.800.911.900.733.471.040.162.091.040.95
La6.4814.3016.208.1010.8017.005.697.1917.807.4417.4810.8912.67
Ce13.9030.7036.8018.7023.1034.0013.3114.4236.2021.1335.1621.6227.35
Pr1.733.594.322.412.78 1.752.044.702.503.222.733.13
Nd6.6814.0016.309.2510.7016.007.908.7419.2010.0211.5410.9013.58
Sm1.422.623.231.832.093.102.182.244.062.611.922.162.35
Eu0.400.790.890.560.590.840.710.681.100.470.620.740.83
Gd1.302.102.421.501.562.802.602.544.442.561.941.831.99
Tb0.240.310.370.210.230.400.440.410.62 0.250.300.32
Dy1.251.621.871.141.39 3.282.943.863.091.371.711.82
Ho0.250.310.390.220.25 0.660.580.800.660.270.310.35
Er0.750.861.040.720.74 2.272.002.261.970.780.891.12
Tm0.120.140.160.110.11 0.340.300.350.310.100.140.18
Yb0.810.981.160.790.821.162.642.402.542.190.700.911.05
Lu0.120.160.190.140.150.180.400.360.360.350.120.130.18
Y7.849.3911.17.127.9014.5017.0414.6920.1020.547.667.868.84
Total REE35.4372.5185.4145.7455.33 44.1746.8398.2955.7375.4755.2666.89
LREE30.5866.0477.8040.9150.08
HREE4.856.477.614.835.25
LREE/HREE6.3110.2010.238.479.54
(La/Yb) N5.76 10.52 9.97 7.34 9.49
δEu0.911.030.981.031.00 0.910.87 0.99
a All oxide contents in the samples were recalculated to 100% on a volatile-free basis; b total before normalization. TFe2O3 as total Fe2O3. (La/Yb)N is ratio of La and Yb normalized by chondrite. Mg# = 100× [Molar Mg/(Mg+Fe2+)]. Phanerozoic TTG data from Condie [6]. TTG rock data in the Central Asian Orogenic Belt from [35,36,40]; Late Carboniferous tonalite data from Wang et al., 2021 [41].
Table
Table 2. LA-ICP-MS zircon U-Pb isotopic analysis of the Gaolihan trondhjemite.
Table 2. LA-ICP-MS zircon U-Pb isotopic analysis of the Gaolihan trondhjemite.
Spot
No.		Element (ppm)Th/UIsotopic RatiosApparent Age (Ma)
ThU 206Pb/238U1σ207Pb/235U1σ207Pb/206Pb1σ206Pb/238U1σ207Pb/235U1σ207Pb/206Pb1σ
P16TW08 Trondhjemite
0177125 0.620.0479 0.0007 0.3585 0.0206 0.0540 0.0030 301 4311 15369 131
0247 94 0.500.0497 0.0008 0.3580 0.0224 0.0525 0.0035 313 5311 17309 152
0398 146 0.670.0488 0.0006 0.3396 0.0164 0.0505 0.0025 307 4297 12217 115
04104 161 0.640.0493 0.0007 0.3465 0.0174 0.0514 0.0027 310 4302 13257 86
0583 127 0.650.0483 0.0007 0.3509 0.0191 0.0535 0.0029 304 4305 14350 131
0648 97 0.500.0498 0.0009 0.3682 0.0228 0.0536 0.0034 313 5318 17367 144
0749 99 0.490.0492 0.0008 0.3192 0.0190 0.0480 0.0030 310 5281 1598.2 141
0871 121 0.590.0499 0.0007 0.3395 0.0192 0.0494 0.0028 314 4297 15165 131
0950 98 0.510.0492 0.0008 0.3740 0.0216 0.0561 0.0035 309 5323 16457 134
1092 131 0.700.0486 0.0007 0.3323 0.0168 0.0502 0.0028 306 4291 13206 127
11100 138 0.720.0496 0.0007 0.3461 0.0170 0.0512 0.0027 312 4302 13250 122
12339 241 1.410.0488 0.0005 0.3305 0.0136 0.0489 0.0020 307 3290 10143 94
1338 790.480.0488 0.0009 0.3311 0.0220 0.0496 0.0039 307 5290 17176 174
1468 108 0.630.0496 0.0007 0.3878 0.0203 0.0571 0.0031 312 5333 15494 122
1553 106 0.500.0503 0.0008 0.3486 0.0217 0.0501 0.0033 316 5304 16198 156
1667 118 0.560.0494 0.0007 0.3640 0.0200 0.0536 0.0032 311 4315 15354 133
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Dong, P.; Li, Y.; Xie, Y.; Wang, J.; Li, H. Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean. Minerals 2022, 12, 1212. https://doi.org/10.3390/min12101212

AMA Style

Dong P, Li Y, Xie Y, Wang J, Li H. Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean. Minerals. 2022; 12(10):1212. https://doi.org/10.3390/min12101212

Chicago/Turabian Style

Dong, Peipei, Yingjie Li, Yan Xie, Jinfang Wang, and Hongyang Li. 2022. "Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean" Minerals 12, no. 10: 1212. https://doi.org/10.3390/min12101212

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