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Article

Pan-African and Early Paleozoic Orogenic Events in Southern Tibet: Evidence from Geochronology and Geochemistry of the Kangbuzhenri Gneissic Granite in the Zhegu Area

by
Ming Cheng
1,
Xuming Hu
1,
Yao Tang
1,
Zhao Deng
1,
Yingzi Min
1,
Shiyi Chen
2,3,
Saijun Sun
2,3,* and
Huanzhan Zhou
1
1
Changsha General Survey of Natural Resources Center, China Geological Survey, Changsha 410600, China
2
Laboratory for Marine Geology, Qingdao Marine Science and Technology Center, Qingdao 266237, China
3
Key Laboratory of Ocean Observation and Forecasting, Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(8), 845; https://doi.org/10.3390/min14080845
Submission received: 1 July 2024 / Revised: 26 July 2024 / Accepted: 19 August 2024 / Published: 22 August 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
The Zhegu area in southern Tibet is situated in the central and eastern part of the Tethys Himalayan tectonic belt, with the Kangbuzhenri area being abundant in gneissic granites. This study examines the petrology, chronology, and geochemistry of the Kangbuzhenri gneissic granite, providing insights into its Pan-African and Early Paleozoic geological evolution. The zircon U-Pb chronology indicates an upper intercept age of ~539 Ma, reflecting Pan-African orogenic events in the eastern part of the Tethys Himalayan tectonic belt, and a lower intercept age of ~144 Ma, representing a late tectonic–thermal event. Geochemically, the gneissic granites are calc-alkaline peraluminous rocks with high SiO2 and Al2O3 contents and low TiO2, P2O5, MgO, and FeOT contents. The gneissic granites are enriched in LREE and LILEs (Rb, Pb, Th, U, etc.), but relatively depleted in HREE and HFSEs (Nb, Ti, P, etc.). Most of them show a weak negative δEu anomaly, except for two samples which show a significant negative δEu anomaly due to the crystallization of plagioclase. Based on the above study, most of the gneissic granites exhibited the characteristics of an I-type granite, while two of the samples were a highly differentiated I-type granite with S-type affinities. All the above characteristics indicate that the gneissic granite likely originated from the partial melting of crustal materials and sediments with a minor involvement of mantle-derived materials. Combined with the previous chronological studies, the Kangbuzhenri gneissic granites were formed in an extensional tectonic environment during post-collision orogeny and then they were influenced by the Kerguelen mantle plume tectonic–thermal event around ~144 Ma and the subsequent Southern Tibet Detachment System (STDS).

1. Introduction

Kennedy (1964), based on a large number of reliable Rb-Sr and K-Ar age data from the African continent, found that there was an apparent “thermal event” before ~500 Ma and called it the “Pan-African event”. The Pan-African event in the Himalayan massif of Tibet is generally regarded as an important orogenic event that occurred around ~550 Ma [1,2,3]. From north to south, the Himalayan tectonic belt includes the Tethys Himalayas, High Himalayas, Low Himalayas, and Sub Himalayas belts (Figure 1a). Among them, the Ordovician sedimentary cover of the “Pan-African event” has been discovered in the Tethys Himalayan region, which has further strengthened our understanding of the Pan-African event in the northern margin of Gondwana [4,5,6,7]. In particular, chronological data have great geological significance for the restoration of Pan-African Paleozoic geological evolution and have supplemented and improved our understanding of the “Pan-African movement” [6,7,8,9,10]. There are many metamorphic domes in the Tethys Himalayas belt, such as the Malashan Mountain, Laguiangri, Saga, Kangmar, Ranba, Cuonadong, and Yalashangbo metamorphic domes. Among them, the Yalashangbo dome is a typical representative of the North Himalayan gneiss dome belt (NHGD), which records the tectonic activity information from the collisional orogeny to extensional detachment, and is of great significance for revealing the geological tectonic evolution of the Tibetan Plateau [11]. Our predecessors have conducted extensive research on the Yalashangbo dome in the Zhegu area [12,13,14,15,16,17], but the Kangbuzhenri gneissic granite exposed to the southeast is rarely reported (Figure 1b).
A significant number of east–west-oriented granitic bodies are present in the Tethys Himalayan belt and the High Himalayan belt, forming two nearly parallel granite belts [20,21,22,23,24,25,26,27,28]. Within these regions, the Tethys Himalayan igneous rock belt intrudes into the surrounding sedimentary–metamorphic rocks primarily in the form of veins, lenses, and stocks, which mainly include two-mica monzogranite, porphyritic granite, and gneissic granite with ages of 529~457 Ma (Early Paleozoic) and 10~45 Ma (Cenozoic) [6,14,29,30,31,32,33]. The granitic rocks exposed in the Zhegu area of the Tethys Himalayan belt are predominantly located in and around the Yalashangbo dome, with ages of 520.4~501.3 Ma and 45.6~17 Ma (Figure 1b) [12,13,14,15,16]. However, there is a lack of reported research on the Kangbuzhenri gneissic granite situated southeast of the Yalashangbo dome. Notably, previous studies on the intermediate–basic dyke swarm and Sangxiu Formation volcanic rocks in the Zhegu area have indicated formation ages of 142~131.7 Ma [34,35], suggesting their origin in the rift environment of the early Cretaceous passive continental margin extension associated with the Comei Large Igneous Province (LIP). Additionally, the recently discovered metamorphic basic rocks in the Yalashangbo area of the Zhegu region were formed ~128 Ma in the Early Cretaceous period, likely in an extensional setting linked to the breakup of India and Australia under the active conditions of the Comei LIP [17]. As such, is the gneissic granite in the Zhegu area of the Tethys Himalayan belt the product of the Pan-African event or the Comei LIP? Has the whole Zhegu region even been affected by the Pan-African event? Based on petrographic, chronological, and geochemical analyses, this paper studies the diagenetic age, petrogenesis, and tectonic environment of the Kangbuzhenri gneissic granite in the Zhegu area, southern Tibet, which provides evidence for the tectono-magmatic evolution and geodynamic background of and a Pan-African event in the Tethys Himalayan belt.

2. Geological Background

The Zhegu area in southern Tibet is located in the eastern section of the Gangdise–Himalayan orogenic system in southern Tibet and is part of the Himalayan block. Positioned between the Yarlung Zangbo Suture Zone (YZSZ) and the Southern Tibet Detachment System (STDS) (Figure 1), the Tethyan Himalayan block contains numerous metamorphic domes. These gneissic domes, including Malashan Mountain, Laguigangri, Sakya, Kangma, Ramba, Cuonadong, and Yalashangbo, are primarily aligned from west to east. Following the influence of the STDS, the pre-Sinian metamorphic basement rocks of these domes are predominantly in fault contact with the upper overburden. The structural orientation of the Zhegu area is determined by regional faults and folds, generally trending in the NW-SE direction. The geological formations exposed in the region encompass Precambrian, Paleozoic, Triassic, Jurassic and Quaternary strata, with a prevalence of Triassic and Jurassic strata (Figure 1b). The Precambrian Yaduizhara Group (An∈Y) comprises metamorphic rocks, predominantly schist and gneiss, with minor occurrences of granulite, quartzite, marble, and amphibolite. The Paleozoic Qudegong Formation (Pzq) consists mainly of metamorphic rocks such as metamorphic sandstone, phyllite, and schist, with a small amount of schist. The Neru Formation (T3n) of the Upper Triassic is grayish black thin–middle silty slate with gray middle feldspar quartz sandstone and fine sandstone. The Lower Jurassic Ridang Formation (J1r) is characterized by gray, dark gray, gray-black argillaceous siltstone, silty mudstone, siltstone, and shale, with siliceous bands and lenticular limestone. The Lower Middle Jurassic Luje Formation (J1–2l) is a lithologic assemblage of gray argillaceous siltstone, mudstone, limestone, and marl. The Middle Jurassic Zara Formation (J2z) is predominantly composed of argillaceous siltstone and silty mudstone, with a primary occurrence of basic volcanic rocks.
The tectono-metamorphic belt in the Zhegu area is bounded by the upper detachment fault zone (F1) and the Lugela-Gudui fault zone (F2) (Figure 1b). Geographically, it is segmented from north to south into the Yalashangbo metamorphic core complex (Yalashangbo metamorphic subzone), Langlage-Zhaari thrust belt (Kazhu-Maru metamorphic subzone), and Changcaiga-Xuemeijiang fold-thrust belt (Zhegu-Gudui metamorphic subzone). Within this structural belt, numerous E-E-, NW-SE-, N-S-trending faults/folds are present, exhibiting a significant level of metamorphism, predominantly ranging from greenschist facies to amphibolite facies in the Zhegu area. The Zhegu area experiences intense magmatic activity, with volcanic occurrences primarily concentrated in the Middle–Late Jurassic to Early Cretaceous periods. In the western region of the area, marine volcanic rocks comprising almond basalt and andesite basalt are exposed, while in the southwest, a substantial number of dacites can be observed. Intrusive rocks in the Zhegu area exhibit a wide distribution, encompassing a variety of rock types from ultramafic to felsic, notably including Mesozoic intermediate–acid magmatic rocks, predominantly situated in the western and northeastern sectors of the area. Spatially, these rock formations are predominantly visible in the Kangbuzhenri, Dara, Yalashangbo, and other locations, with the Kangbuzhenri rocks positioned in the northeast of the Zhegu area and the southeastern periphery of the Yalashangbo dome. These rocks cover an exposed area of 8.3 km2 and intrude into the metamorphic rocks of the Yaduzala Group, primarily characterized by gneissic granite.
Gneissic granite primarily consists of plagioclase, K-feldspar, quartz, and oriented muscovite/biotite 0.2~2.5 mm in size. The rock shows blastogranitic and lepidoblastic textures, where micas are oriented or distributed around plagioclase. Plagioclase (27%~40%) appears as subhedral lenticular, angular, and columnar shapes. Creepquartz is observed at the plagioclase-K-feldspar and quartz contacts, with some occurrences possibly attributed to stress. K-feldspar (14%~25%) is anhedral granular. Quartz (30%~35%) is anhedral granular, forming conjunctive aggregates with undulatory extinction, and many grains exhibit subgrains. Biotite (6%~10%) and muscovite (6%~7%) are arranged in a lamellar orientation (Figure 2).

3. Analytical Methods

The Kangbuzhenri gneissic granites (TW2692, KB-1, KB-2, KB-3, KB-4, KB-5, KB-6) in the Zhegu area, southern Tibet, were collected for dating and whole-rock major and trace element analysis.
LA-ICP-MS zircon U-Pb dating was completed in the laboratory of Tianjin Geological Survey Center of China Geological Survey, following the analytical procedures given in references [36,37]. The instruments used were a RESOlution LR laser ablation system (Applied Spectra, Inc., West Sacramento, CA, USA) and an Agilent 7900 inductively coupled plasma mass spectrometer (Agilent Scientific Instruments, Santa Clara, CA, USA). The laser ablation spot diameter was 29 μm, the frequency was 7 Hz, the energy density was 3 J/cm2, and the carrier gas was He (800 mL/min). Zircon age calculation uses standard zircon 91500 as the external standard to correct instrument quality discrimination and element fractionation. Standard zircon Plesovice was used as a blind sample to check the quality of U-Pb dating data. The element content was calibrated with NIST SRM610 as the external standard and Si as the internal standard [36,37]. The ICPMSDataCal software (version 11.5) was used to perform the drift correction, offline selection, integration of back-ground and analysis signals [38].
The compositions of major and trace elements in the whole rock were analyzed at the Hunan Institute of Mineral testing and Utilization. Major elements were analyzed by an atomic fluorescence spectrometer (AFS-830A) (Jitian Instruments, Beijing, China) and atomic absorption spectrometer (Z-2300) (Hitachi Limited, Tokyo, Japan). Trace elements were analyzed by ICAP6300 (Thermo Fisher Scientific, Waltham, MA, USA) and rare earth elements were analyzed by ICP-MS (Thermo X2, Thermo Fisher Scientific, Waltham, MA, USA) [39,40]. The analytical precision for major elements was better than 1%, and that of the trace elements was better than 5%.

4. Results

4.1. Zircon U-Pb Geochronology

A zircon U-Pb concordia diagram of the Kangbuzhenri gneissic granite is shown in Figure 3, and the results for the U–Pb age data are supplied in Table S1. The cathodoluminescence (CL) images of zircons show that they are mainly grayish black or grayish white euhedral-hypidiomorphic crystals with a length of 50~100 μm (Figure 3). Th and U contents of zircons vary greatly (31~1541 ppm and 366~4004 ppm, respectively). The Th/U ratio is 0.03~0.77, most of which are more than 0.1, similar to magmatic zircons. However, most of the zircons are affected by the late metamorphic recrystallization, and the magmatic zoning structure becomes blurred or disappears, showing the characteristics of metamorphic recrystallized zircons. The zircon U-Pb dating results show that all the data points fit well to the discordia line (Figure 3), giving an upper intercept age of 538.9 ± 8.3 Ma and a lower intercept age of 144 ± 17 Ma, respectively [41].

4.2. Characteristics of Whole-Rock Major Elements

Data for the major elements in the Kangbuzhenri gneissic granites in the Zhegu area are shown in Table S2. The gneissic granites exhibit high SiO2 (71.4~76.8 wt.%) and Al2O3 (13.4~15.8 wt.%) contents, and low TiO2 (0.03~0.26 wt.%), P2O5 (0.04~0.12 wt.%), FeOT (0.73~1.69 wt.%), and MgO (0.07~0.67 wt.%) contents. The Rittman index (σ) is 0.77~2.29, and the total alkali content (Na2O + K2O) is 5.13~8.26 wt.%. The solidification index (SI) is 0.82~7.50, and the differentiation index (DI) is 80.4~93.4. In the SiO2-K2O diagram, most of the gneissic granite samples show calc-alkaline series characteristics (Figure 4). They are peraluminous (A/CNK = 1.06–1.75) and corundum appeared when we calculated the contents of CIPW standard minerals (Figure 4). They also have characteristics of calc-alkaline and peraluminous granites (A/CNK = 1.06~1.75) (Figure 4). The contents of Al2O3, CaO, MgO, and TiO2 systematically decrease with increasing SiO2, suggestive of the magmatic differentiation of gneissic granites (Figure 5). This trend is similar to that of Yalashangbo granitic gneiss, but more information is recorded in this study.

4.3. Characteristics of Whole-Rock Trace Elements

Trace element data for the Kangbuzhenri gneissic granites in the Zhegu area are listed in Table S2. The gneissic granites have low total rare earth elements (ΣREE = 35.4~143 ppm). In the chondrite-normalized REE diagram, the granites are shown to be rich in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE) ((La/Yb)N = 1.01~47.3) (Figure 6a). Most of them do not have δEu anomalies (δEu = 0.82~0.99), except for two samples which show δEu anomalies (0.15, 0.36) due to the plagioclase crystallization. In the primitive mantle-normalized spider diagrams, the granites show coherent patterns, with high contents of large ion lithophile elements (LILEs) such as Rb, Th, and U and low amounts of high field strength elements (HFSEs) such as Nb, Ta, Ti (Figure 6b). The distribution characteristics of the REEs and trace elements in the Kangbuzhenri gneissic granites are similar to those of Yalashangbo granite gneiss, both of which have the characteristics of island arc magmatic rocks.

5. Discussion

5.1. Geochronology

It is generally believed that the ages of granite formations are of great significance to revealing the history of the geological evolution that occurred during the Pan-African–early Paleozoic period. The LA-ICP-MS dating results of the Kangbuzhenri gneissic granites fit well with the discordia line, yielding an upper intercept age of 538.9 ± 8.3 Ma and a lower intercept age of 144 ± 17 Ma. Here, from the perspective of chronology, the magmatic–tectonic events related to the Kangbuzhenri granites are discussed.

5.1.1. Protolith Age

The Kangbuzhenri gneissic granites exhibit similar characteristics (See Section 5.2) to the Yalashangbo metamorphic core complex in terms of their lithologic and geochemical compositions (enrichment of LREE and LILE, and relative depletion of HREE and HFSE) [15]. Our predecessors have extensively investigated the chronological data of the Yalashangbo metamorphic core complex. For example, Gao et al. (2011) [12] identified ~518 Ma Yalashangbo dome gneiss, Wu et al. (2014) [14] discovered ~520 Ma Yalashangbo mylonitic gneisses, and Dong et al. (2018) [15] reported ~514 Ma and ~501 Ma granitic gneisses in the core of Yalashangbo dome. These ages closely align with the upper intercept age of 538.9 ± 8.3 Ma determined in this study, falling within the range of error. Furthermore, they are consistent with the ancient nuclear ages of zircons (546~520 Ma) in this study (Figure 3), indicating a protolith age of 538.9 ± 8.3 Ma. Moreover, these findings are indicative of the Pan-African tectonic magmatic event, suggesting that the northern margin of the Indian continent experienced intense early Paleozoic magmatism.

5.1.2. Record of Pan-African-Early Paleozoic Events

Pan-African orogeny is commonly attributed to a series of orogenic events [45,46] that occurred between East Gondwana and West Gondwana, with a time span of 750~510 Ma [47,48]. Previous studies indicate that the Himalayas and Gangdise belts, situated on the northern margin of Gondwana, represent the continental margins with the lengthiest duration and the latest end of the global Pan-African movement. Both the southern and northern regions of Tibet exhibit a unified Pan-African basement of ~500 Ma [3,5,49]. The geological records of Pan-African orogenic events previously reported for the Tethys Himalayan tectonic belt include Yalashangbo granitic gneiss, Laguigangri granitic gneiss, and Kangma gneissic monzogranite [15,50,51]. This study reveals that the protolith age of the Kangbuzhenri gneissic granite is 538.9 ± 8.3 Ma, indicating its association with a distinct geological record of the Pan-African tectonic thermal event in the Zhegu area of southern Tibet. It suggests that the Zhegu area was involved in the Pan-African orogenic event during the early Paleozoic era. Meanwhile, numerous chronological data (Table S3) from the Tethys Himalayan and High Himalayan tectonic units demonstrate the participation of the metamorphic basement rocks of the Himalayan terrane in both Pan-African events and early Paleozoic events after ~500 Ma. Combined with geotectonic location and age data, it appears that the Zhegu area may also have been involved in early Paleozoic events.

5.1.3. Mesozoic Magmatic Tectono-Thermal Event

Basalts, diabase dykes/beds, gabbros, and a small amount of layered ultramafic and felsic rocks are widely exposed in the Comei area of the Tethys Himalayan tectonic belt in southern Tibet (Figure 1). Their ages are mainly concentrated in 134~130 Ma, which are considered to be associated with the magmatic activity of the Comei LIP formed by the Kerguelen mantle plume [52,53,54,55,56,57]. Previous investigations into the mafic dikes, metamorphic basic, and volcanic rocks exposed in the Zhegu area and its neighboring regions (Gudui and Xiawa) exhibit similar geochemical characteristics to the igneous rocks of the Comei LIP, with their ages primarily falling within the range of 128~146 Ma (Figure 1b) [17,34,49,58,59]. Consequently, it is postulated that these rocks originated from a tectonic setting characterized by a strong extension of the passive continental margin and a thinning of the lithosphere, and were products of the interaction between the Kerguelen mantle plume and the lithospheric mantle [60,61,62,63]. In this study, zircons from the Kangbuzhenri gneissic granite were influenced by the Kerguelen plume and contained preserved evidence of a lead loss event (~144 Ma), which aligns closely with the previously reported ages in the Zhegu area (128~146 Ma) within the margin of error, and is proximate to the peak age (~132 Ma) in the Comei LIP. Considering the regional context, it is inferred that the Zhegu area might have experienced the thermal effects of the Kerguelen mantle plume event, suggesting that the scope of ~132 Ma magmatic activity in the Comei LIP is broader than previously reported [52].

5.2. Petrogenesis

The Kangbuzhenri gneissic granite minerals exhibit euhedral–subhedral characteristics with distinct mineral boundaries, lacking evident residual mineral metamorphism structures. At the same time, no apparent mineral alteration is observed under microscopic examination, and the ignition loss is minimal (0.94~1.77 wt.%). A linear evolution correlation is identified among mobile elements (Rb, Sr, K, Ca), transition elements (Mg, Fe, V, Cr), and immobile elements (Ti, Th, Y, Co) with Zr (Figure S1), suggesting that their compositions have remained relatively unchanged and are indicative of the protolith composition.
Low Ti and P, high Al contents are generally characteristic of S-type granites. The high P-T experimental results indicate that the solubility of apatite increases with rising SiO2 contents during magmatic differentiation in peraluminous melts (S-type) [64], while in metaluminous to slightly peraluminous melts (I-type), there is an inverse correlation [39]. In this study, excluding samples KB-4 and KB-6, the Kangbuzhenri gneissic granites show a decrease in P2O5 content as the SiO2 content increases (Figure 7b), aligning with the characteristics of I-type granites. Moreover, the absence of typical minerals (garnet and cordierite) under microscopic examination indicates that the Kangbuzhenri gneissic granites are likely to be calc-alkaline I-type gneissic granites. This observation is further supported by the positioning of almost all Kangbuzhenri gneissic granites within the I-type granite region in the K2O-Na2O diagram (Figure 7a).
High-K I-type granites may be generated by several processes, including (a) formed by partial melting of calc-alkaline to high-K calc-alkaline, mafic to intermediate rocks (e.g., Sisson et al., 2005 [65]); (b) a mixture of mantle-derived materials with crust-derived materials (e.g., Brasilino et al., 2011 [66]); and (c) formed by the advanced assimilation-fraction crystallization (AFC) of mantle-derived basaltic parental magmas (e.g., Njanko et al., 2006 [67]).
Minerals 14 00845 g007
Figure 7. (a) Na2O versus K2O diagram, (b) P2O5 versus SiO2 diagram [68], (c) Fe*O/MgO versus (Zr + Nb + Ce + Y) diagram [69], (d) (La/Yb)N versus δEu diagram for the Kangbuzhenri gneissic granites. Reference data from Yalashangbo granite gneiss [15].
Figure 7. (a) Na2O versus K2O diagram, (b) P2O5 versus SiO2 diagram [68], (c) Fe*O/MgO versus (Zr + Nb + Ce + Y) diagram [69], (d) (La/Yb)N versus δEu diagram for the Kangbuzhenri gneissic granites. Reference data from Yalashangbo granite gneiss [15].
Minerals 14 00845 g007
In view of the fact that the mafic rocks of the Pan-African period (~500 Ma) in the Zhegu area have not been reported at present, the Kangbuzhenri gneissic granites tend to originate from a hybrid melt, which can be verified by the following geochemical characteristics. In this study, the composition of the gneissic granites in terms of SiO2, Al2O3/TiO2, and CaO/Na2O are similar to those of granites formed through the partial melting of crustal sedimentary rocks (SiO2 < 74 wt.%, Al2O3/TiO2 < 100, CaO/Na2O > 0.3), indicating a potential involvement of crustal sedimentary rocks [70]. The majority of Rb/Sr ratios in the gneissic granite exceed those of the upper continental crust and continental crust (0.32, 0.24), implying a significant contribution from the crust in the source materials [50,71,72]. Moreover, most Mg# values of the gneissic granites fall below 40, and are accompanied by low Cr and Ni contents, as well as Nb/La and Nb/Th ratios, suggesting a crustal contribution with a minor presence of mantle-derived materials. This observation aligns with the result of the crust–mantle granite area in the (La/Yb)N-δEu diagram (Figure 7d).
Subduction fluids generally carry fluid-mobile trace elements (e.g., Pb, U, and LILEs) into the mantle wedge [37,73,74]. In contrast, the addition of sediments could elevate Th/Yb and La/Sm ratios [37,74,75]. The majority of samples yield low Ba/La and Ba/Th ratios, as well as high (La/Sm)N and Th/Yb ratios, which are consistent with the predominant influence of subducted sediments (Figure 8a,b), indicating that their source underwent weak metasomatism by a fluid component, with a significant contribution from sediment melts. As mentioned above, the Kangbuzhenri gneissic granites primarily originated from the input of crustal and sedimentary materials, with the minor involvement of mantle-derived materials.
However, two samples (KB-4 and KB-6) exhibit characteristics more similar to the Yalashangbo granitic gneiss with S-type affinities. For example, they range from slightly to moderately peraluminous, and their P2O5 contents increase with rising SiO2 contents (Figure 7a). However, the absence of typical minerals (garnet and cordierite) under the microscope and their high differentiation indices (89.2 and 93.4) indicate that they are most likely highly differentiated I-type gneissic granites (Figure 7c). This is also consistent with their more negative anomalous features of trace elements (Ba, Eu, Sr, Ti) due to their significant fractional crystallization (Figure 6). Crustal granites typically have a relatively low Sr content (<300 ppm), while granites originating from sources containing mantle components tend to have high Ba and Sr contents [77]. Our two samples have low Ba and Sr values, which are consistent with the crustal origin (Figure 7d). Water distribution during the melting process also plays an important role in the generation of granites [78,79]. When water is present, plagioclase easily dissolves in the melt, resulting in higher Na and Ca contents as well as positive Eu anomalies [80]. Our two samples exhibit a relatively low CaO content and negative Eu anomalies, suggesting the presence of residual plagioclase. Furthermore, negative correlation between Rb/S ratios and Sr, along with the positive correlation with Ba, supports the hypothesis of fluid-absent biotite dehydration melting (Figure 8c,d). Therefore, our two samples (KB-4 and KB-6) were probably formed through biotite-dehydration melting and significant crystallization.

5.3. Geodynamic Implication

It is generally believed that the peraluminous granite is the product of partial melting of crustal thickening in the early stages of continental collision [78,81,82]. However, it is also noted that peraluminous granite can also be produced against the background of lithospheric extension following a collision [78,82]. High-K I-type magmas usually occur in the two possible tectonic scenarios: (1) a continental arc setting similar to that of the Andes; (2) a post-collisional setting, where melting of the source rocks is caused by decompression following crustal thickening [83]. Previous research has indicated a significant magmatic activity around 500 Ma within the Tethys Himalayan belt, particularly concentrated in the North Himalayan gneiss dome belt [6,12,14]. For example, Gu et al. (2013) [50] proposed that the granitic gneiss (~514 Ma) at the core of the Laguigangri metamorphic core complex originated in an extensional tectonic environment of post-collision orogeny. Wang et al. (2016b) [10] believed that the Kangma dome gneissic monzogranite (~528.8 Ma) was formed in the extensional tectonic environment after collisional orogeny. Dong et al. (2018) [15] considered that the Yalashangbo dome granitic gneiss (~501.3 Ma, ~514.8 Ma) was produced in the extensional tectonic environment of post-collision orogeny after the end of the main collision orogenic stage.
In this study, it is found that the protolith age of the Kangbuzhenri gneissic granites in the Zhegu area is 538.9 ± 7.3 Ma, which shows that it belongs to the same tectono-magmatic event highlighted in previous studies within the error range. Furthermore, they mainly have crustal origins with an enrichment of LILE and LREE and a depletion of HFSE, with negative anomalies of Nb and Ta, which are consistent with those of post-collision magmatic rocks formed in a post-collisional environment [84]. Accordingly, in the (Y + Nb)-Rb diagram (Figure 9), they fall within the post-collision granite field, which is consistent with the Yalashangbo dome. At the same time, in the Rb-Hf-Ta triangle diagram (Figure 9), most of them are also plotted within the post-collisional setting. Considering both regional and chronological data, it is suggested that the Kangbuzhenri gneissic granites in the Zhegu area originated mainly from the extensional tectonic environment following the end of the collisional orogenic stage. Subsequently, they might have been affected by the Kerguelen mantle plume tectono-magmatic event around ~144 Ma and the subsequent STDS.

6. Conclusions

The zircon crystallization age of the Kangbuzhenri gneissic granite is 538.9 ± 8.3 Ma, representing a geological record of the Pan-African orogenic event within the eastern Tethys Himalayan tectonic belt.
The Kangbuzhenri gneissic granites are rich in LREE and LILEs, and relatively depleted in HREE and HFSEs, belonging to the calc-alkaline peraluminous I-type rocks, which come from the partial melting of crustal materials and sediments, accompanied by minor participation of mantle-derived materials.
The Kangbuzhenri gneissic granites originated within an extensional tectonic environment during the post-collisional orogeny phase following the conclusion of the primary collisional orogenic period. Subsequently, they underwent the influence of the Kerguelen mantle magmatic thermal event around ~144 Ma, followed by the subsequent STDS.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14080845/s1, Figure S1: Zr versus mobile elements (Rb, Sr, K, Ca), transition elements (Mg, Fe, V, Cr) and immobile elements (Ti, Th, Y, Co) diagrams; Table S1: Zircon U-Pb dating of the Kangbuzhenri gneissic granites; Table S2: Major (wt%) and trace elements (ppm) of the Kangbuzhenri gneissic granites; Table S3: Statistical table of age records of Pan-African-Early Palaeozoic events in the Himalayan terrane.

Author Contributions

Conceptualization, M.C. and S.S.; methodology, X.H.; software, Y.T.; validation, Z.D., Y.M. and M.C.; formal analysis, S.C.; investigation, H.Z.; resources, M.C.; data curation, X.H.; writ-ing—original draft preparation, M.C.; writing—review and editing, M.C. and S.S.; visualization, X.H.; supervision, M.C.; project administration, M.C. and S.S.; funding acquisition, M.C. and S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (42373049), the Taishan Scholar Program of Shandong (tspd20230609), Laoshan Laboratory (LSKJ202204100), the China Geological Survey Project (1212011220659, DD20230527 and ZD20220309).

Data Availability Statement

The data presented in this study are available in this paper.

Acknowledgments

We acknowledge anonymous reviewers of their constructive reviews and valuable suggestions that led to great improvement in the presentation of the paper, and we extend our thanks to Xin Jin and Long Zhang for their discussions.

Conflicts of Interest

The authors declare that they have no competing interests.

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Figure 1. (a) Geotectonic map of southern Tibet (revised by [18,19]); (b) Geological map of the Zhegu area. 1—Indian plate; 2—Northern Lhasa Massif; 3—Gangdisê island arc; 4—Tethys Himalayas; 5—High Himalayas; 6—Low Himalayas; 7—Sub-Himalayan; 8—Yarlung Tsangpo river ophiolite belt; 9—thrust fault; 10—slip fault; YLZBS—Yalung Zangbo suture; STD—South Tibet detachment; MCT—Main central thrust; MBT—Main boundary thrust; MFT—Main front thrust.
Figure 1. (a) Geotectonic map of southern Tibet (revised by [18,19]); (b) Geological map of the Zhegu area. 1—Indian plate; 2—Northern Lhasa Massif; 3—Gangdisê island arc; 4—Tethys Himalayas; 5—High Himalayas; 6—Low Himalayas; 7—Sub-Himalayan; 8—Yarlung Tsangpo river ophiolite belt; 9—thrust fault; 10—slip fault; YLZBS—Yalung Zangbo suture; STD—South Tibet detachment; MCT—Main central thrust; MBT—Main boundary thrust; MFT—Main front thrust.
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Figure 2. (a,b) Field contacts of gneissic granites from the Zhegu area in southern Tibet; (c,d) Photomicrographs of gneissic granites from the Zhegu area. Qzt—quartz; Pl—plagioclase; Kfs—K-feldspar; Ms—muscovite.
Figure 2. (a,b) Field contacts of gneissic granites from the Zhegu area in southern Tibet; (c,d) Photomicrographs of gneissic granites from the Zhegu area. Qzt—quartz; Pl—plagioclase; Kfs—K-feldspar; Ms—muscovite.
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Figure 3. Zircon U-Pb concordia diagrams of the Kangbuzhenri gneissic granitoids; Zircon cathodoluminescence image of the Kangbuzhenri gneissic granitoids.
Figure 3. Zircon U-Pb concordia diagrams of the Kangbuzhenri gneissic granitoids; Zircon cathodoluminescence image of the Kangbuzhenri gneissic granitoids.
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Figure 4. (a) SiO2 versus K2O + Na2O diagram for the Kangbuzhenri gneissic granitoids; (b) SiO2 versus K2O diagram for the Kangbuzhenri gneissic granitoids [42]. (c) A/NK versus A/CNK diagram for the Kangbuzhenri gneissic granitoids [43]. Reference data from Yalashangbo granite gneiss [15].
Figure 4. (a) SiO2 versus K2O + Na2O diagram for the Kangbuzhenri gneissic granitoids; (b) SiO2 versus K2O diagram for the Kangbuzhenri gneissic granitoids [42]. (c) A/NK versus A/CNK diagram for the Kangbuzhenri gneissic granitoids [43]. Reference data from Yalashangbo granite gneiss [15].
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Figure 5. Harker diagrams for the Kangbuzhenri gneissic granitoids. Reference data from Yalashangbo granite gneiss [15].
Figure 5. Harker diagrams for the Kangbuzhenri gneissic granitoids. Reference data from Yalashangbo granite gneiss [15].
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Figure 6. (a) Chondrite-normalized REE patterns of the Kangbuzhenri gneissic granitoids; (b) Primitive mantle-normalized trace elements spider diagrams of the Kangbuzhenri gneissic granitoids. Normalizing values are from [44]. Reference data from Yalashangbo granite gneiss [15].
Figure 6. (a) Chondrite-normalized REE patterns of the Kangbuzhenri gneissic granitoids; (b) Primitive mantle-normalized trace elements spider diagrams of the Kangbuzhenri gneissic granitoids. Normalizing values are from [44]. Reference data from Yalashangbo granite gneiss [15].
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Figure 8. (a) Ba/Th versus (La/Sm)N diagram (b) Th/Yb versus Ba/La diagram for the Kangbuzhenri gneissic granitoids. (c,d) Plots of Rb/Sr ratios versus Sr (ppm) and Ba (ppm), respectively (after Inger and Harris, 1993 [76]). Reference data from Yalashangbo granite gneiss [15].
Figure 8. (a) Ba/Th versus (La/Sm)N diagram (b) Th/Yb versus Ba/La diagram for the Kangbuzhenri gneissic granitoids. (c,d) Plots of Rb/Sr ratios versus Sr (ppm) and Ba (ppm), respectively (after Inger and Harris, 1993 [76]). Reference data from Yalashangbo granite gneiss [15].
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Figure 9. (Y + Nb) versus Rb diagram for the Kangbuzhenri gneissic granitoids [85]. Rb-Hf-Ta triangular plot for the Kangbuzhenri gneissic granitoids [86]. Reference data from Yalashangbo granite gneiss [15]. ORG-Ocean Ridge granites; WPG-Within Plate Granites; VAG-Volcanic Arc Granites; Syn/Post-COLG (Syn- and Post-collision Granites).
Figure 9. (Y + Nb) versus Rb diagram for the Kangbuzhenri gneissic granitoids [85]. Rb-Hf-Ta triangular plot for the Kangbuzhenri gneissic granitoids [86]. Reference data from Yalashangbo granite gneiss [15]. ORG-Ocean Ridge granites; WPG-Within Plate Granites; VAG-Volcanic Arc Granites; Syn/Post-COLG (Syn- and Post-collision Granites).
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MDPI and ACS Style

Cheng, M.; Hu, X.; Tang, Y.; Deng, Z.; Min, Y.; Chen, S.; Sun, S.; Zhou, H. Pan-African and Early Paleozoic Orogenic Events in Southern Tibet: Evidence from Geochronology and Geochemistry of the Kangbuzhenri Gneissic Granite in the Zhegu Area. Minerals 2024, 14, 845. https://doi.org/10.3390/min14080845

AMA Style

Cheng M, Hu X, Tang Y, Deng Z, Min Y, Chen S, Sun S, Zhou H. Pan-African and Early Paleozoic Orogenic Events in Southern Tibet: Evidence from Geochronology and Geochemistry of the Kangbuzhenri Gneissic Granite in the Zhegu Area. Minerals. 2024; 14(8):845. https://doi.org/10.3390/min14080845

Chicago/Turabian Style

Cheng, Ming, Xuming Hu, Yao Tang, Zhao Deng, Yingzi Min, Shiyi Chen, Saijun Sun, and Huanzhan Zhou. 2024. "Pan-African and Early Paleozoic Orogenic Events in Southern Tibet: Evidence from Geochronology and Geochemistry of the Kangbuzhenri Gneissic Granite in the Zhegu Area" Minerals 14, no. 8: 845. https://doi.org/10.3390/min14080845

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