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Article

Triassic Thermal Pulse of TARIM Mantle Plume: Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China

by
Jungang Sun
1,2,
Ting Liang
1,*,
Xiaohuang Liu
3,
Xiong Zhang
2,
Bei Liu
2,* and
Guorong Quan
2
1
College of Earth Science and Resourses, Chang An University, Xi’an 710054, China
2
Xi’an Center of Mineral Resources Survey, China Geological Survey, Xi’an 710100, China
3
Command Center of Natural Resources Survey, China Geological Survey, Beijing 100055, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(3), 283; https://doi.org/10.3390/min14030283
Submission received: 5 January 2024 / Revised: 13 February 2024 / Accepted: 26 February 2024 / Published: 8 March 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Owing to the paucity of research on synchronous mafic rocks in the Tarim Basin, the late Paleozoic–early Mesozoic tectonic development of this region is not well defined. The Halaqi region is situated on Tarim’s northwest edge, and numerous mafic dikes can be found cross-cutting the Permian strata. The whole-rock geochemistry, zircon U–Pb age, and Sr–Nd isotopic signature of these mafic rocks have never been reported before, and this contribution can offer geochronological and petrogenetic investigations that provide fresh insight into the geodynamic development of the area. Major oxide contents in the Halaqi mafic rocks vary, including SiO2 (45.74–50.31 wt.%), Al2O3 (13.28–14.8 wt.%), FeOT (16.48–19.19 wt.%), MgO (7.58–10.32 wt.%), CaO (7.19–12.39 wt.%), Na2O (2.97–4.50 wt.%), K2O (0.24–0.63 wt.%), TiO2 (1.11–1.29 wt.%), MnO (0.14–0.16 wt.%), and P2O5 (0.13–0.17 wt.%). The mafic rocks are enriched in high-field-strength elements (e.g., Zr and Hf) and large-ion lithophile elements (e.g., Sr, Th, and U) but depleted in Nb, Ta, and P. The total REEs in the rocks are lower (ΣREE = 72.80–86.85 ppm), and HREEs are somewhat depleted in comparison to LREEs, with positive Eu anomalies (Eu/Eu* = 1.05–1.17) but weak negative Ce anomalies (Ce/Ce* = 0.91–0.93). Zircon U–Pb ages of 201–247 Ma were obtained from a total of 18 magmatic zircon grains found in the mafic rocks that were studied. These results point to a middle-to-late Triassic emplacement. The mafic dikes exhibit somewhat enriched Nd isotopic compositions (εNd(t) = –1.6~–0.2) and an older Nd model age (TDM = 1.24–1.37 Ga). The Halaqi middle–late Triassic mafic dikes are thought to have originated from the same tectonic background as the Permian Tarim Large Igneous Province, along with similar geochemical and isotopic compositions. This suggests that they are all products of the interaction between asthenospheric and lithospheric mantles in an intraplate extensional environment. Research indicates that the Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP.

1. Introduction

Permian volcanic rocks are extensively developed in the Tianshan region and northern margin of Tarim, and many productive studies on the formation and distribution range of these rocks have been conducted. The rocks are mainly composed of large-scale basalt, gabbro, diabase, granite, and mafic–ultramafic complexes, and their formation ranges from the early to the late Permian. They are commonly thought to be the products of the Tarim Large Igneous Province (TLIP) and may be connected to mantle plume activity [1,2].
There are, however, limited research reports on the Triassic igneous rocks in this area [3,4,5,6], and different understandings over their magmatism still exist. According to some researchers, they might be the continuation of Permian magmatism, which is related to the TLIP [4,5]. Some researchers think they developed in the intraplate extensional environment of the Triassic and could be a precursor to the western Tarim Basin’s strong extension throughout the Cretaceous–Paleocene [3,6].
The study area, located in the northwestern margin of the Tarim Basin, has been dominated by passive continental margin sediments since the late Paleozoic [7,8,9]. It has been challenging to effectively constrain the mantle properties and the deep dynamic mechanism of this period in the area. Understanding the mantle properties and deep tectonic background of the late Paleozoic–early Mesozoic in this region, as well as the tectonic evolution of this period in the Tarim Basin, can be greatly enhanced by studying magmatic rocks. Furthermore, it is important to think about and investigate whether there is a relationship between the petrogenesis, magmatic evolution, source composition, and deep process of igneous rocks in the areas of Bachu, Keping, and Halaqi, which are all on the northwest edge of the Tarim Basin.
Based on previous studies, this study analyzed the petrological geochemistry, zircon chronology, and Nd isotopic characteristics of mafic dikes in Halaqi. A comparative analysis was also carried out with igneous rocks in Bachu and Keping, and this study has provided new evidence for the time limits of magmatic activity in the TLIP. In addition to the temporal and spatial distributions of mafic dikes, this study investigated the characteristics and evolving patterns of magma evolution in Tarim’s northern margin, as well as the geodynamic setting and deep geological process.

2. Regional Geological Background

With intraplate tectonic settings or geochemical affinities, large igneous provinces (LIPs) are defined as magmatic provinces with areal extents of >0.1 Mkm2, igneous volumes of >0.1 Mkm3, and maximum life spans of approximately 50 Ma. They are characterized by igneous pulses of a short duration (1–5 Ma), during which a significant portion (>75%) of the total igneous volume has been emplaced [10,11]. The Tarim Large Igneous Province is another major discovery after the Emeishan Large Igneous Province in China. After the concept of the Tarim Large Igneous Province was proposed by Jiang Changyi et al. (2004a) [12] and Yang et al. (2006) [13], it quickly attracted the attention of domestic and international scholars and became an important area for large-igneous-province research. Within the Tarim plate, the Tarim Large Igneous Province spans an area of approximately 2 × 105 km2 (Figure 1) and has a thickness ranging from tens to hundreds of meters. The TLIP is marked by voluminous occurrences of widespread igneous rocks, including flood basalts, layered mafic–ultramafic intrusions, diamondiferous kimberlites, lamprophyres, bimodal dike swarms, alkaline igneous complexes (such as syenites and A-type granites), rhyolites, and pyroclastic rocks [2,14,15,16,17]. The magmatic activity in the Tarim Large Igneous Province lasted from the Early Permian to the Late Permian, with a peak period in the Early Permian [18]. This magmatic activity can be divided into two types. One type is mainly composed of basalt with a very small amount of basic rocks widely distributed within the basin and was formed mainly between 292 and 280 Ma [19,20]. The other type mainly consists of basic and ultrabasic intrusive rocks, felsic intrusive rocks, and volcanic rocks scattered at the edge of the basin near the orogenic belts, on a small scale, with ages mostly concentrated between 282 and 264 Ma [19,20,21,22].

3. Petrography

This region’s mafic rocks extensively intrude into Carboniferous, Devonian, and Ordovician strata (Figure 2a–f). There are two distinct occurrences: one occurs in the form of a dike and is approximately perpendicular to strata (Figure 2d), and the other occurs as veins along strata (Figure 2b). Mafic dikes vary greatly in size, ranging from 0.6 to 1.2 m (maximum from 80 to 100 m) in width and extending from 8 to 100 m (a maximum of 400 m) in length. There are many types of rocks, including diabase porphyrite, olivine diabase porphyrite, and olivine pyroxene porphyrite, mainly diabase. Our study mainly focused on diabase dikes.
The fresh surface of the rocks is gray–green and gray–black, with a fine ophitic texture and a massive structure (Figure 2f), and some samples with little weathering are yellowish green, dark green, and tan. The grain size of the vein gradually increases from the rim to the core. Typically, the grain size of the edge is between 0.5 and 3.5 mm, whereas the grain size of the core is between 5 and 20 mm. The mineral composition is plagioclase (~43 vol.%), pyroxene (~37 vol.%), and magnetite (~10 vol.%), with secondary minerals, such as calcite (~4 vol.%), epidote (~3 vol.%), chlorite (~2 vol.%), and sericite (~1 vol.%) (Figure 2g). Plagioclase has better crystals, with a polysynthetic twin and a particle size from 0.8 to 1.5 mm, distributed in a tabular–stripy shape and forming a tripod structure. With a xenomorphic granular structure and a particle size range from 0.5 to 1.0 mm, pyroxenes often fill the tripod structure that is composed of plagioclase. Magnetites are xenomorphic granular in structure, with a particle size range from 0.5 mm to 2.0 mm, and are distributed between plagioclase particles, coexisting with pyroxene. Plagioclase often exhibits epidotization, sericitization, or carbonation. Pyroxenes are partially replaced by chlorites or carbonates.

4. Sampling and Analytical Methods

To maintain the samples’ freshness, we made an effort to avoid samples that had experienced more weathering and alteration during field collection. The samples’ locations are as follows: P56-19-1(77°52′00″, 40°48′50″); P56-19-2(77°53′30″, 40°49′20″); P56-19-3(77°55′30″, 40°48′00″); P56-19-4(78°12′30″, 40°45′20″); P56-19-5(78°10′00″, 40°38′20″); and P56-19-6(78°12′00″, 40°39′20″). Prior to selecting as many fresh and representative samples as possible, all the samples that were chosen for the whole-rock analysis were identified under a microscope. During the sample treatment, cross-contamination and external pollution were avoided. Manual grinding was carried out after primary crushing. A high-pressure blower was utilized to clean the crusher during the primary crushing, and alcohol cotton swabs were used to clean the mortar and pestle during the fine grinding to prevent the samples from contaminating each other.

4.1. Whole-Rock Major and Trace Element Analyses

The major and trace elements of the bulk rock were analyzed in the laboratory at the China University of Geosciences (Beijing). The samples were ground to below 200 mesh with a hammer and agate mortar. Then, the prepared samples were mixed with a lithium metaborate/lithium tetraborate reagent, and lithium nitrate was used as an oxidant. After that, the mixture was poured into a platinum mold to form a fused disk for X-ray fluorescence (XRF) analysis of the major elements. The loss on ignition (LOI) of each sample at 1000 °C was measured, and the element concentrations were calculated based on the results of both analyses. The analysis was performed using inductively coupled plasma mass spectrometry (ICP–MS) for rare earth and trace elements. The prepared samples were mixed with lithium metaborate/lithium tetraborate flux and fused in a furnace at 1025 °C. The resulting melts were cooled and dissolved in a mixture of nitric acid, hydrochloric acid, and hydrofluoric acid before the analysis. The testing instrument type was AB–104L PW2404 for the major elements, with an analytical error from 1% to 5%; for the rare earth and trace elements, the instrument was ELEMENT, with an analytical error from 5% to 10%. Wet method analysis was used to examine Fe2O3 and FeO separately.

4.2. Sr and Nd Isotopic Ratio Measurements

The whole-rock Sr and Nd isotopic ratios were measured in the laboratory at the China University of Geosciences (Beijing). A total of 5 diabase samples were analyzed by thermal ionization mass spectrometry (TIMS) according to the analysis method described by Yang et al. (2012) [24]. The sample powders were dissolved in Teflon capsules with mixed acid (HF:HClO4 = 3:1) for one week, and cationic ion-exchange resin columns were used to separate Sr and Nd in solution. The measured 87Sr/86Sr and 143Nd/144Nd ratios were corrected using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. To ensure the reproducibility of the method during the measurement, the standard sample and sample were tested simultaneously. The ratios of the measured 87Sr/86Sr and 143Nd/144Nd were 0.705020 ± 0.000012 (2σ) and 0.512635 ± 0.000005 (2σ) respectively, which were consistent with the reference values of 0.705015 [25] and 0.512637, respectively [26].

4.3. Zircon U–Pb Dating

The zircon-sorting work was completed at the Rock and Mineral Experimental Testing Center at the Langfang Geological Surveying and Mapping Institute in Hebei Province. Zircon cathode photoluminescence (CL) photography was performed using a JSM6510 scanning electron microscope at Beijing Geoanalysis Co., Ltd. (Beijing, China). The zircon U–Pb dating was performed at the MC–ICP–MS laboratory at the China University of Geosciences (Wuhan), using a Finnigan Neptune-type MC–ICP–MS and a Newwave UP213 laser ablation system. The diameter of the laser beam’s spot was 25 μm, at a frequency of 10 Hz and an energy density of 2.5 J/cm2, and He was used as the carrier gas. The zircon international standard GJ1 was used as the external standard for dating, and the U and Th mass fractions were corrected using zircon M127 as the external reference. The data were processed using the ICP–MS–DataCal 4.3 program [27], and the zircon age harmonic diagram was obtained using the Isoplot 4.15 program [28]. The detailed experimental testing process is given by Hou et al. (2009) [29].

5. Results

5.1. Zircon U–Pb Geochronology

The diabase dike was selected for LA–ICP–MS zircon U–Pb dating in this study, and the dating results are shown in Table 1. Zircon crystals are subhedral–euhedral in shape, with a size from 30 to 80 μm. Most have columnar shapes, while some have nearly equiaxed shapes. The edges of the crystals are straight and smooth. Only a minority of zircons lack an obvious core, mantle, and rim zoning; the majority exhibit oscillatory zones (Figure 3). Some zircon surfaces were re-formed by magmatism in the later period, and the boundary between the core and the zone was obvious. Therefore, in this experiment, some zircons with obvious magmatic zircon characteristics were selected for testing to obtain their formation age [30].
This test yielded eighteen effective measuring points: >323 Ma (1 point), 299–273 Ma (3 points), 273–259 Ma (4 points), 259–252 Ma (2 points), 252–247 Ma (1 point), 247–237 Ma (4 points), and 237–201 Ma (3 points). Zircon ages of 337–252 Ma are related to the Tarim igneous province, which extends from the Carboniferous to the Permian, while the young zircon ages (247–201 Ma) may represent the formation age of the Halaqi mafic dikes.

5.2. Major Element Compositions

The analysis results for the major elements of the Halaqi mafic dikes are shown in Table 2. They have high contents of Fe2O3 = 11.05–13.10% (average 12.17%), MgO = 7.58–10.32% (average 8.41%), and CaO = 7.19–12.39% (average 9.24%); moderate contents of Al2O3 = 13.28–14.80% (average 14.09%), TiO2 = 1.11–1.29% (average 1.21%), FeO = 5.75–7.40% (average 6.37%), and Na2O = 2.97–4.50% (average 3.74%); and low contents of SiO2 = 45.74–50.31% (average 47.33%), MnO = 0.14–0.16% (average 0.15%), K2O = 0.24–0.63% (average 0.43%), and P2O5 = 0.13–0.17% (average 0.15%), with a consistent ratio of CaO/Al2O3 = 0.52–0.88 (average 0.66). The contents of FeOT and Mg# were high, from 16.48% to 19.19% (average 17.32%) and from 0.68 to 0.71 (average 0.70), respectively. In the TAS rock classification diagram (Figure 4), Halaqi mafic dikes fall within the basalt region, close to the alkaline and subalkaline boundary, and can be classified as a transitional rock series of alkaline–subalkaline.

5.3. Trace Elements

The analysis results for the trace elements of the Halaqi mafic dikes are shown in Table 2. The mafic dikes have low REE contents ranging from 72.80 to 86.85 ppm (average 81.87 ppm). In contrast to HREEs (13.11–15.24 ppm), LREEs (59.69–72.01 ppm) are relatively enriched, with LREE/HREE ratios of 7.6–8.7 and LaN/YbN ratios of 6.07–6.60. In addition, the mafic dikes exhibit slightly negative Ce anomalies (Ce/Ce* = 0.91–0.93) and slightly positive Eu anomalies (Eu/Eu* = 1.05–1.17) (Figure 5). In the primitive mantle–normalized-trace-element spider diagram (Figure 6), the Halaqi mafic dikes are relatively enriched in high-field-strength elements (HFSEs; e.g., Zr and Hf) and depleted in Nb, Ta, and P, showing clear positive anomalies for large-ion lithophile elements (LILEs; e.g., Sr, Th, and U).

5.4. Sr–Nd Isotopic Compositions

The analysis results for the Sr and Nd isotopes of the Halaqi mafic dikes are shown in Table 3. The samples have variable 87Rb/86Sr and 87Sr/86Sr values of 0.0303–0.7402 and 0.706511–0.707702, respectively, with the initial 87Sr/86Sr varying from 0.70491 to 0.70748. For the Sm–Nd isotopic system, the 147Sm/144Nd values range from 0.1251 to 0.1383, while the 143Nd/144Nd values are relatively consistent (0.512432–0.512522), displaying relatively consistent εNd(t) (from –0.2 to –1.6), fSm/Nd (from –0.30 to –0.36), initial 143Nd/144Nd (from 0.512215 to 0.512286), and depleted mantle model (TDM) ages (from 1236 to 1367 Ma).
It is worth mentioning that the Sr isotope may be affected by weathering or crustal contamination due to the large fluctuation in the LOI and initial Sr value [35,36]. Therefore, we avoided the use of Sr isotopes in the investigation of petrogenesis and tectonic setting.

6. Discussion

6.1. Petrogenic Age

For the Permian magmatic activity in the northwest margin of Tarim, a series of studies has been carried out previously. Li et al. (2007) [37] obtained a crystallization age of 272 ± 6 Ma for Bachu diabase by the zircon ICP–MS dating method. According to the field occurrence of Xiaohaizi ultramafic dikes in the Bachu area and their intercalated relationship with diabase dikes, Yang et al. (2007) [22] considered that their emplacement time was early–middle Permian. Yang et al. (2014) [23] used SHRIMP U-Pb zircon dating to determine the age of the syenite intrusion of Mazhashan in the Bachu area and obtained an age of ~286 Ma. The eruption time of basalts in the Keping area is 288–290 Ma [18,38]. These results are consistent with the formation time of the TLIP (about 270–290 Ma) [39], showing that these rocks are all products of the Permian Large Igneous Province in the northwest margin of Tarim.
Triassic mafic magmatism has also been discovered in the northwest margin of Tarim. Luo et al. (2006) [3] conducted 40Ar/39Ar dating in Atushi diabase and obtained ages of 235.6 and 203.7 Ma. Zhang et al. (2009) [40] mentioned the existence of diabase with an intrusion time of 225 ± 15 Ma in Bachu (A detailed report is unavailable.). Zheng et al. (2016) [5] obtained an age of 245 ± 11 Ma for a basic complex rock mass in Keping. Yan et al. (2021) [6] found the latest zircon age of 229–233 Ma for diabase in Keping. The latest zircon age of a Halaqi mafic dike found in this study is 201–247 Ma, indicating that the dike was formed in the middle-to-late Triassic. In addition, this study also discovered that the mafic dike intruded into the limestone of the Kangkelin Formation (late Carboniferous–early Permian), suggesting that it was formed after the early Permian, which also supports the above analysis. Therefore, it may be confirmed that a mafic magmatism event existed from the middle to the late Triassic in the northwest margin of Tarim.

6.2. Petrogenesis

Crystallization differentiation played a major role in magmatic evolution. The Halaqi mafic dikes have larger changes in La (12.7–16.3 ppm; average: 14.9) but have a smaller change in La/Sm (4.10–4.62 ppm; average 4.39), indicating that the magmatic evolution was affected by fractional crystallization to a certain extent (Figure 7a–i). FeOT and Ni are positively correlated with MgO (Figure 7d,g), indicating that these rocks experienced the fractional crystallization of olivine. FeOT is positively correlated with MgO (Figure 7d), implying the fractional crystallization of iron oxide. P2O5 is positively correlated with MgO (Figure 7f), which may be related to apatite’s separation crystallization.
Previous studies have found that the content and ratios of some stable elements in magma can remain relatively stable when contaminated by the crust or lithosphere, which can be used to discriminate whether magma is contaminated by the crust [41,42,43]. Woodhead et al. (1989) [44] studied the basalts of Pitcairn Island in the Pacific Ocean and concluded that the Nb/U ratio is related to the source material but unrelated to the magmatic process (crystallization separation, assimilation/contamination, magmatic mixing, and liquid segregation). By studying oceanic basalt, Hofmann et al. (1986) [45] considered that the Nb/U ratios of MORB and OIB are usually high (47 ± 10). Through their research, Talor et al. (1995) [41] concluded that the Nb/U ratios of the continental crust are low (mostly around 12). The Nb/U ratios of the Halaqi mafic dikes, which range from 8.21 to 11.03 and are all lower than those of the continental crust, obviously show that crustal contamination is not the major cause. The Nb/La ratios of the primitive mantle and MORB are greater than 1, whereas that of the continental crust is about 0.7. The Nb/La ratios of the Halaqi mafic dikes are from 0.56 to 0.63, and all are lower than that of the continental crust, which also shows that this phenomenon is not caused by crustal contamination. The Nb/Ta value of the primitive mantle is 17.5 ± 2.0, and that of the crustal material is about 11 [46]. The Nb/Ta ratios of the Halaqi mafic dikes are from 22.55 to 24.44, which are higher than those of the primitive mantle and crustal material. This once again suggests that crustal contamination is not the dominant factor and that the metasomatism of the magma source could be the main reason.
Metasomatism often occurs in two ways in the magma source region. One is subduction-related metasomatism, which results in the enrichment of LILEs and LREEs and the depletion of HFSEs in the magma. The other is asthenospheric fluid metasomatism into the lithosphere, which results in the enrichment of LILEs, LREEs, and HFSEs in the magma [47]. The Halaqi mafic dikes are characterized by enrichment in LREEs; LILEs (Sr, Th, U, etc.); and HFSEs (Zr, Hf, etc.), which is similar to a magma source transformed by asthenospheric materials. As shown in the Th/Yb–Ta/Yb disgrams (Figure 8), the Halaqi mafic dikes plot in the ACM, implying that the Halaqi basic rocks show certain properties of ACM magmatic rock, which may be affected by asthenospheric metasomatism. Under this condition, we argue that the crust–mantle interaction process governs the magma generation of the rock.

6.3. Source Characteristics

Research shows that the mantle plume activity during the Permian led to the upwelling of the asthenospheric mantle, which interacted with the lithospheric mantle, forming the TLIP [38,49,50,51]. The TLIP exhibits two different stages of magmatism. Most of the early magmatic activity (~290 Ma) produced basalt, distributed in the basin’s interior, with a Nb–Ta negative anomaly and an enriched Nd isotopic composition (εNd = −1.8~−3.9) [12,38,52,53], which possibly derived from enriched lithospheric mantle. The late magmatic activity (~280 Ma) produced basic–ultrabasic intrusive rock, distributed in the basin’s margin, with a Nb−Ta positive anomaly and a depleted Nd isotopic composition (εNd = ~+5) [22,53,54], which probably originated from the convective mantle. In the Tianshan orogenic belt, Permian magmatism is dominated by basalt, basic dike, and ultramafic–mafic complexes, which may come from strongly depleted lithospheric mantle metasomatized by subduction-zone melting [53]. The Halaqi mafic dikes have a negative Nb−Ta anomaly and a slightly enriched Nd isotopic composition, showing characteristics of an enriched lithospheric mantle source, which is similar to early magmatism in the Permian Igneous Province.
Scholars have also found that Nd isotopic compositions are depleted in Bachu Mazhashan syenite and Wajilitag ultramafic rock [12,33], while Nd isotopic compositions of Keping basalt are enriched [55]. Meanwhile, the Halaqi mafic dikes and Bachu diabase exhibit slightly enriched or depleted Nd isotopic characteristics [33]. These indicate that the source characteristics of the TLIP vary with the evolution stage and spatial distribution. Moreover, it may also imply that the Halaqi mafic dikes derived some inheritance from the TLIP.
The Triassic magmatic rocks in the research area show great similarity, and their geochemical characteristics indicate that they are all generated in an intraplate tectonic environment. Regarding the nature of the source, the magmatic rocks in this area exhibit geochemical features of an enriched mantle source (Figure 9 and Figure 10) [3,4,5,6]. In addition, the Halaqi mafic dikes show geochemical characteristics of a slightly enriched mantle, indicating that the characteristics of the mantle source varied in different regions of the northern margin of Tarim during the Triassic period.

6.4. Tectonic Significance

Previous studies have shown that the South Tianshan Ocean, which is located between the Tarim block and Yili block, began to close in the early Carboniferous [7,58] and finally closed at the end of the early Carboniferous [59]. It entered the post-orogenic evolution stage between the late Carboniferous and early Permian. From the early Permian, the mantle plume rose, forming a huge number of basalts, ultramafic–mafic complexes, and basic dike swarms, resulting in the Tarim Permian Large Igneous Province. The Permian magmatic rock activity continued from the early Permian to the late Permian, with an activity peak in the early Permian [18]. From the Triassic to the Jurassic, the northwest margin of the Tarim block lay in an uplifted denudation area of the continental margin with limited Triassic and Jurassic strata.
The Halaqi mafic rocks obtained in this study were formed in the middle–late Triassic. The tectonothermal event they represent is a continuation of the magmatic activity of the Tarim Permian LIP, or this represents a new stage of the Triassic magmatic event, which is worthy of further discussion. As discussed above, the geochemical characteristics of the middle–late Triassic basic rocks in Halaqi provide a certain inheritance to the Tarim Permian LIP. In addition, previous studies have also found that the heat associated with mantle plumes tends to be contained in the mantle near the plume for a considerable time [60,61]. Therefore, we consider that the Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP. Although they differ in time and space from the Tarim Permian LIP, the middle–late Triassic basic rocks that formed in the Halaqi area share a similar tectonic background and source. They are all the result of the lithospheric–asthenospheric interaction in an intraplate setting (Figure 11).
During the extensive upwelling of the mantle plume, the roots of the lithosphere were eroded, resulting in thin spots in the thick lithosphere. These thin spots allowed the production of a large volume of OIB-like basaltic magmas via the decompression and partial melting of the mantle plume itself. These basaltic magmas ascended and then underplated at the base of the continental crust, where metasomatization took place between basaltic magmas and the lithosphere before erupting at the surface to produce the basalts [17,63]. Furthermore, the basaltic magmas rose up and pooled in magma chambers at shallower crustal levels, where they experienced the exchange of heat and components with the lithosphere more extensively, and mafic–ultramafic intrusions were formed by decompression and melting. The Halaqi mafic dikes formed in this process.
Furthermore, late Triassic magmatic rocks have also been found in the southwestern Tianshan orogenic belt, which is close to Tarim’s northwest margin. Hu et al. (2016) [4] discovered a late Triassic (about 224 ± 4 Ma) mafic–ultramafic layered complex in Wushi County, southwest Tianshan. Gao et al. (2009) [64] obtained a magmatic age of 233–226 Ma for eclogite from the southwestern Tianshan orogenic belt. These rocks may be the products of the same tectonothermal event as the basic dikes from Tarim’s northwest margin.

7. Conclusions

(1)
The Halaqi mafic dikes belong to the high-K calc-alkaline rock series and are characterized by an enrichment in LILEs (e.g., Sr, Th, and U) and HFSEs (e.g., Zr and Hf) and a depletion in HFSEs (e.g., Nb, Ta, and P), showing metasomatized lithospheric mantle signatures of the asthenospheric mantle in magma sources. They experienced little fractional crystallization with negligible crustal contamination. Zircon U–Pb dating suggests that the Halaqi mafic dikes were emplaced during the middle-to-late Triassic (247–201 Ma).
(2)
The Triassic mafic magmatism in northwest Tarim could be the product of the continuous thermal pulse of the Tarim mantle plume and be a part of the Tarim LIP. It has a source and a tectonic setting similar to those of the Tarim LIP, and both are products of the interaction between asthenospheric and lithospheric mantles in an intraplate environment.

Author Contributions

Writing—original draft, J.S.; writing—review and editing, J.S., T.L. and B.L.; visualization, G.Q.; investigation, J.S. and X.L.; formal analysis, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the China Geological Survey, grants number DD20221695-43 and DD20190379 (T. Liang), and the Project of the China Geological Survey, grant number DD20230378.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

We would like to thank all the members of the Halaqi Project team for their full support and help in field work. We are grateful to the laboratory staff at the China University of Geosciences (Beijing) for their assistance in testing and analysis. Thanks to the anonymous reviewers for their valuable comments and suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Tectonic sketch map of the Tarim Basin, with distribution of the Tarim LIP (modified after Yang et al., 2014 [23]; Zhu et al., 2022 [17]).
Figure 1. Tectonic sketch map of the Tarim Basin, with distribution of the Tarim LIP (modified after Yang et al., 2014 [23]; Zhu et al., 2022 [17]).
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Figure 2. Field occurrence and petrology of the mafic dikes: (a) diabase within the marlite in the Upper Carboniferous Kalazhierjia Formation; (b) diabase intruding along the Kangkelin Formation limestone in the late Carboniferous–early Permian; (c) injection of diabase into limestone in the late Carboniferous–early Permian Kangkelin Formation; (d) diabase dike invading the marlite in the Upper Carboniferous Kalazhierjia Formation; (e) emplacement of the diabase dike within Kangkelin Formation limestone; (f) ophitic texture; (g) photomicrograph of the mafic dikes. Pl—Plagioclase; Px—Pyroxene; Cc—Calcite.
Figure 2. Field occurrence and petrology of the mafic dikes: (a) diabase within the marlite in the Upper Carboniferous Kalazhierjia Formation; (b) diabase intruding along the Kangkelin Formation limestone in the late Carboniferous–early Permian; (c) injection of diabase into limestone in the late Carboniferous–early Permian Kangkelin Formation; (d) diabase dike invading the marlite in the Upper Carboniferous Kalazhierjia Formation; (e) emplacement of the diabase dike within Kangkelin Formation limestone; (f) ophitic texture; (g) photomicrograph of the mafic dikes. Pl—Plagioclase; Px—Pyroxene; Cc—Calcite.
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Figure 3. CL images of zircons from the mafic dikes in Halaqi area. Minerals 14 00283 i001—measuring point.
Figure 3. CL images of zircons from the mafic dikes in Halaqi area. Minerals 14 00283 i001—measuring point.
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Figure 4. TAS diagram of the mafic dikes (after Le Maitre et al., 2002 [31]; Keping diabase data are from Zhang et al., 2018 [32]; Keping basalt data are from Dai et al., 2017 [18]; Bachu diabase data are from Sun et al., 2007 [33]; Bachu ultramafic dyke data are from Yang et al., 2007 [22]). Ir—Alkaline and Subalkaline boundary.
Figure 4. TAS diagram of the mafic dikes (after Le Maitre et al., 2002 [31]; Keping diabase data are from Zhang et al., 2018 [32]; Keping basalt data are from Dai et al., 2017 [18]; Bachu diabase data are from Sun et al., 2007 [33]; Bachu ultramafic dyke data are from Yang et al., 2007 [22]). Ir—Alkaline and Subalkaline boundary.
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Figure 5. Chondrite–normalized REE patterns of the mafic dikes (normalization data are from Sun et al., 1989 [34]; other rock data sources are same as those in Figure 4).
Figure 5. Chondrite–normalized REE patterns of the mafic dikes (normalization data are from Sun et al., 1989 [34]; other rock data sources are same as those in Figure 4).
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Figure 6. PM–normalized-trace-element spider diagrams of the mafic dikes (normalization data are from Sun et al., 1989 [34]; other rock data sources are same as those in Figure 4).
Figure 6. PM–normalized-trace-element spider diagrams of the mafic dikes (normalization data are from Sun et al., 1989 [34]; other rock data sources are same as those in Figure 4).
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Figure 7. Correlation diagrams of major oxides (SiO2 (a), Al2O3 (b), TiO2 (c), FeOT (d), CaO (e), P2O5 (f)) and trace elements (Ni (g), Cr (h), Sr (i)) vs. MgO for the mafic dikes.
Figure 7. Correlation diagrams of major oxides (SiO2 (a), Al2O3 (b), TiO2 (c), FeOT (d), CaO (e), P2O5 (f)) and trace elements (Ni (g), Cr (h), Sr (i)) vs. MgO for the mafic dikes.
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Figure 8. Ta/Yb–Th/Yb disgrams of the mafic dikes (after Pearce, 1982 [48]; other rock data sources are same as those in Figure 4. IAT—island arc tholeiite; ICA—island calc-alkaline; SHO—shoshonite; TH—tholeiite; TR—transition ridge; ALK—alkali; ACM—active continental margin; MORB—mid ocean ridge basalt; WPB—intraplate basalt).
Figure 8. Ta/Yb–Th/Yb disgrams of the mafic dikes (after Pearce, 1982 [48]; other rock data sources are same as those in Figure 4. IAT—island arc tholeiite; ICA—island calc-alkaline; SHO—shoshonite; TH—tholeiite; TR—transition ridge; ALK—alkali; ACM—active continental margin; MORB—mid ocean ridge basalt; WPB—intraplate basalt).
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Figure 9. Zr–Nb diagrams of the mafic dikes (after Deniel, 1988 [56]; other rock data sources are same as those in Figure 4).
Figure 9. Zr–Nb diagrams of the mafic dikes (after Deniel, 1988 [56]; other rock data sources are same as those in Figure 4).
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Figure 10. La/Yb–Th/Ta diagrams of the mafic dikes (after Emst et al., 2005 [57]; other rock data sources are same as those in Figure 4).
Figure 10. La/Yb–Th/Ta diagrams of the mafic dikes (after Emst et al., 2005 [57]; other rock data sources are same as those in Figure 4).
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Figure 11. Zr–Zr/Y disgrams of the mafic dikes (after Pearce, 1979 [62]; other rock data sources are same as those in Figure 4. IAB—island arc basalt; WPB—intraplate basalt; MORB—mid ocean ridge basalt).
Figure 11. Zr–Zr/Y disgrams of the mafic dikes (after Pearce, 1979 [62]; other rock data sources are same as those in Figure 4. IAB—island arc basalt; WPB—intraplate basalt; MORB—mid ocean ridge basalt).
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Table 1. LA–ICP–MS zircon U–Pb dating results of the mafic dikes.
Table 1. LA–ICP–MS zircon U–Pb dating results of the mafic dikes.
Measuring PointsThUTh/UIsotopic RatiosIsotopic Ages (Ma)
(ppm)207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U
Ratio1σRatio1σRatio1σAge1σAge1σAge1σ
B02-5-10-021994510.440.0560.00260.28210.0120.03680.0006454104252102333
B02-5-10-063013950.760.05420.00230.2840.0120.03760.00053899425492383
B02-5-10-071433930.360.04980.0020.28030.01150.04030.00061839325192554
B02-5-10-1083.52480.340.05060.00320.25120.01530.03590.0007233144228122274
B02-5-10-111653950.420.05110.00260.26710.01380.03770.0006256120240112384
B02-5-10-167907341.080.05130.00190.27110.01040.03810.00052548724482413
B02-5-10-1747810800.440.04950.00150.28590.0090.04140.00051726925572613
B02-5-10-1849749710.05280.0020.30310.01210.04130.00053208726992613
B02-5-10-193189520.330.04930.00140.26280.00770.03840.00051616923762433
B02-5-10-122683870.690.05020.00190.37610.01490.05370.000820689324113375
B02-5-10-132693550.760.04760.00220.31110.01510.04680.000879.7107.4275122955
B02-5-10-151575750.270.04950.0020.27870.01120.04040.00051699325092563
B02-5-10-094302621.640.05240.00280.33430.01780.04620.0008302122293142915
B02-5-10-081505540.270.05680.00240.33450.01430.04280.000648394293112704
B02-5-10-052032560.790.0550.01410.31080.17880.04410.0035413488.8275142792
B02-5-10-042855730.50.05290.00210.30010.01190.0410.00063249326792594
B02-5-10-0371.61480.480.04990.00490.24710.02190.03690.0008191215224182335
B02-5-10-011494410.340.05110.00230.27660.01210.03930.000624373248102494
Table 2. The whole-rock major and trace element compositions of the mafic dikes.
Table 2. The whole-rock major and trace element compositions of the mafic dikes.
Sample NumberP56-19-1P56-19-2P56-19-3P56-19-4P56-19-5P56-19-6
SiO2 (%)45.7447.2647.1946.7346.7750.31
TiO21.211.211.241.291.211.11
Al2O314.1114.1814.4314.813.7213.28
Fe2O312.312.3412.0712.1613.111.05
FeO5.86.755.985.757.46.53
MnO0.150.150.150.150.160.14
MgO7.868.867.947.5810.327.9
CaO12.398.128.7710.267.198.71
Na2O2.973.914.53.653.853.58
K2O0.240.630.40.250.510.56
P2O50.130.140.170.150.160.14
Loss on ignition2.723.062.942.842.883.10
Total amount99.8299.8699.899.8699.8799.88
Na2O + K2O3.214.544.903.904.364.14
K2O/Na2O0.080.160.090.070.130.16
CaO/Al2O30.880.570.610.690.520.66
Mg#0.710.700.700.700.710.68
FeOT16.8717.8616.8416.6919.1916.48
La (ppm)12.714.915.813.915.716.3
Ce25.128.330.127.13030.5
Pr3.313.63.883.433.883.9
Nd14.315.216.31516.916.5
Sm3.13.293.493.233.673.53
Eu1.181.21.221.261.291.28
Gd3.123.343.583.313.753.66
Tb0.7060.7450.790.7490.840.817
Dy3.733.854.073.884.264.15
Ho0.7220.7410.7880.7490.8160.794
Er2.072.112.272.162.412.32
Tm0.3240.3330.3620.3410.3720.367
Yb2.122.172.322.192.422.37
Lu0.3210.3280.3550.3350.3690.362
Y18.419.220.219.520.919.9
Sc28.432.629.63028.431.8
V242244235247215219
Co47.759.554.949.163.254.5
Ni160202166146269194
Cu48.553.712416458.741.7
Zn56.781.565.161.998.767.8
Ga2019.820.320.619.119.6
Rb3.178.75.073.0410.16.93
Sr800360536707245458
Zr155156170157168170
Nb8.018.449.098.489.279.2
Cs0.0870.0680.0540.0490.0850.063
Ba52.614810758.813798.7
Hf3.393.553.883.593.863.9
Ta0.3550.3580.3870.3470.4110.387
Pb2.332.031.592.051.881.92
Th2.512.62.82.612.742.84
U0.8230.9320.8550.7690.8171.12
ΣREE72.8080.1185.3377.6486.6886.85
LREE59.6966.4970.7963.9271.4472.01
HREE13.1113.6214.5413.7215.2414.84
LREE/HREE4.554.884.874.664.694.85
LaN/YbN4.304.934.894.554.654.93
δEu1.151.101.051.171.051.08
δCe0.930.920.910.930.920.91
Table 3. Sr–Nd isotopic compositions of the mafic dikes.
Table 3. Sr–Nd isotopic compositions of the mafic dikes.
Sample NumberP56-19-1P56-19-2P56-19-3P56-19-5P56-19-6
Rb (ppm)24.61.841.165.851.43
Sr (ppm)96.414786.4328136
87Rb/86Sr0.74020.03620.03890.05160.0303
87Sr/86Sr0.7077020.707380.7076280.7065110.706884
(87Sr/86Sr)i0.704910.707240.707480.706320.70677
εSr(t)10.343.446.830.233.8
Sm (ppm)3.252.733.044.033.25
Nd (ppm)14.212.114.717.614.5
147Sm/144Nd0.1380.13610.12510.13830.1352
143Nd/144Nd0.5124780.5125220.5124320.5124750.512481
(143Nd/144Nd)i0.5122390.5122860.5122150.5122350.512246
εNd(t)−1.1−0.2−1.6−1.2−1.0
fSm/Nd−0.30−0.31−0.36−0.30−0.31
TDM (Ma)13551236123713671301
T2DM (Ma)11281053116511331115
These data were analyzed at the laboratory at the China University of Geosciences (Beijing).
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Sun, J.; Liang, T.; Liu, X.; Zhang, X.; Liu, B.; Quan, G. Triassic Thermal Pulse of TARIM Mantle Plume: Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China. Minerals 2024, 14, 283. https://doi.org/10.3390/min14030283

AMA Style

Sun J, Liang T, Liu X, Zhang X, Liu B, Quan G. Triassic Thermal Pulse of TARIM Mantle Plume: Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China. Minerals. 2024; 14(3):283. https://doi.org/10.3390/min14030283

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Sun, Jungang, Ting Liang, Xiaohuang Liu, Xiong Zhang, Bei Liu, and Guorong Quan. 2024. "Triassic Thermal Pulse of TARIM Mantle Plume: Evidence from Geochronology, Geochemistry, and Nd Isotopes of the Mafic Dikes from the Halaqi Area, Xinjiang, China" Minerals 14, no. 3: 283. https://doi.org/10.3390/min14030283

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