The Initial Subduction Time of the Proto-Tethys Ocean in the Eastern Section of the East Kunlun Orogen: The Constraints from the Zircon U-Pb Ages and the Geochemistry of the Kekesha Intrusion

Abstract

The Cambrian period marks a crucial phase in the initial subduction of the Proto-Tethys Ocean beneath the East Kunlun Orogen. Studying the I-type granites and mafic–ultramafic rocks formed during this period can provide valuable insights into the early Paleozoic tectonic evolution of the region. This paper incorporates petrology, LA-ICP-MS zircon U-Pb geochronology, and whole-rock major and trace element data obtained from the Kekesha intrusion in the eastern section of the East Kunlun Orogen. The formation age, petrogenesis, and magmatic source region of the intrusion are revealed, and the early tectonic evolution process of the subduction of the Proto-Tethys Ocean is discussed. The Kekesha intrusion includes four main rock types: gabbro, gabbro diorite, quartz diorite, and granodiorite. The zircon U-Pb ages are 515.7 ± 7.4 Ma for gabbro, 508.9 ± 9.8 Ma for gabbro diorite, 499.6 ± 4.0 Ma for quartz diorite, and 502.3 ± 9.3 Ma and 501.6 ± 6.2 Ma for granodiorite, respectively, indicating that they were formed in the Middle Cambrian. The geochemical results indicate that the gabbro belongs to the high-Al calc-alkaline basalt series, the gabbro diorite belongs to the medium-high-K calc-alkaline basalt series, the quartz diorite belongs to the quasi-aluminous medium-high-K calc-alkaline I-type granite series, and the granodiorite belongs to the weakly peraluminous calc-alkaline I-type granite series, all of which belong to the medium-high-K calc-alkaline series that have undergone varying degrees of differentiation and contamination. Gabbro and gabbro diorite exhibit significant enrichment in light rare earth elements (LREEs), depletion in heavy rare earth elements (HREEs), and an enhanced negative anomaly in Eu (Europium). Compared to gabbro and gabbro diorite, quartz diorite and granodiorite exhibit more pronounced enrichment in LREEs, more significant depletion in HREEs, and an enhanced negative anomaly in Eu. All four rock types are enriched in large-ion lithophile elements (LILEs) such as Cs, Rb, Th, Ba, and U, and are depleted in high-field-strength elements (HFSEs) such as Nb, Ta, and Ti. This indicates that these rocks originated from the same or similar mixed mantle source regions, and that they are formed in the island-arc tectonic environment. This paper suggests that the gabbro and gabbro diorite are mainly derived from the basic magma formed by partial melting of the lithospheric mantle metasomatized by subducted slab melt in the oceanic crust subduction zone and mixed with a small amount of asthenosphere mantle material. Quartz diorite results from the crystal fractionation of basic magma and experiences crustal contamination during magmatic evolution. Granodiorite forms through the crystal fractionation of basic magma, mixed with partial melting products from quartz diorite. While the lithology of the intrusions differs, their geochemical characteristics suggest they share the same tectonic environment. Together, they record the geological processes associated with island-arc formation in the East Kunlun region, driven by the northward subduction of the Proto-Tethys Ocean during the Early Paleozoic. Based on regional tectonic evolution, it is proposed that the Proto-Tethys Ocean began subducting northward beneath the East Kunlun block from the Middle Cambrian. The Kekesha intrusion formed between 516 and 500 Ma, marking the early stages of Proto-Tethys Ocean crust subduction.

1. Introduction

The Kunlun Orogenic Belt is an important component of the Central China Orogenic Belt (CCOB) [1,2,3,4,5,6,7,8,9,10]. The Kunlun Orogenic Belt is bounded to the west by the Pamir tectonic node and to the east by the Qinling Orogenic Belt. The Kunlun Orogenic Belt is divided into two tectonic units by the NE–SW Altyn Tagh strike-slip fault zone: the East Kunlun Orogenic Belt and the West Kunlun Orogenic Belt (Figure 1a). The East Kunlun Orogenic Belt is located in the transition zone between the Proto-Tethys Qilian Orogenic Belt on the northern margin of the Qinghai–Tibet Plateau and the Paleo-Tethys Orogenic Belt inland of the Qinghai–Tibet Plateau. It records at least two significant tectonic events, including the tectonic evolution of the Early Paleozoic Proto-Tethys Ocean and the Late Paleozoic to Early Mesozoic Paleo-Tethys Ocean [4,5,7]. The subduction related to the opening of the Proto-Tethys Ocean made the whole of East Kunlun enter the evolution process of the active continental margin of the trench-island arc-back-arc basin system, forming the Muzitag–Buqingshan–Animaqen ophiolitic mélange belt, the magmatic arc composed of intermediate-acid magmatic rocks in the south of East Kunlun, and the back-arc basin-type ophiolitic mélange belt in the middle of East Kunlun. Therefore, investigating the tectonic framework and evolutionary history of the Eastern Kunlun Orogen is of great significance not only for the research of the subduction/accretion orogenic process in the Early Paleozoic, but also for understanding the tectonic evolution and geodynamic process of the Central China Orogenic Belt (CCOB).

Previous studies, based on investigations of sedimentary strata, igneous rocks, and regional metamorphic events, have suggested that the Eastern Kunlun Orogen exhibits a geological structure characterized by a “multi-block and multi-island arc configuration” and a dynamic history of “multiple subductions and collisional orogeny” [2,4,5,7,11,12,13,14,15,16,17]. These studies have also identified three ophiolitic mélange zones that are parallel to the East Kunlun Orogenic Belt (EKO). From north to south, they are the Qimantagh–Xiangride ophiolitic mélange zone (QXM), the Central East Kunlun ophiolitic mélange zone (CKM), and the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM) (Figure 1b) [4,5]. However, the petrogenesis, geological background, and tectonic correlation of these ophiolitic mélanges zones are still controversial, such as the northward subduction of the Proto-Tethys Ocean [17], the southward subduction [13], the bidirectional subduction [18], and the slab retreat after the northward subduction [3].

The investigated region is located in the southern East Kunlun terrane between CKM and MBAM. The tectonic evolution of the Proto-Tethys in the East Kunlun Orogen is quite complex: various types of plutons are exposed, and formed under different tectonic environments in the south of East Kunlun, such as the ophiolites or mafic–ultramafic complexes in the subduction environment of the ocean in the Qingshuiquan–Kekesha–Kekekete area [18,19,20,21,22] and the monzonite in the back-arc extensional environment in the Dundeshaerguole area [7,23]; the intermediate-acid intrusions in the island-arc environment of the Kekesha and Zhiyu areas in the Bulhanbuda Mountains [13,24]; and the Manite–Berichet–Yikehalaer magmatic arc [25,26,27,28,29,30,31,32]. However, the spatiotemporal distribution and evolutionary processes of these ophiolites, mafic–ultramafic complexes, and intermediate-acid magmatic rocks remain poorly understood, resulting in uncertainty regarding the duration and nature of the Early Paleozoic magmatic arc in the East Kunlun terrane. This limits our understanding of the tectonic evolution of the Eastern Kunlun Orogenic Belt and the Proto-Tethys Ocean. The intrusions within the southern East Kunlun terrane are closely linked to the formation and development of its associated subduction orogenic belt. The intrusions in the East Kunlun terrane are closely associated with the evolution of the East Kunlun Orogenic Belt and the Proto-Tethys Ocean. In this study, we conducted a detailed field geological survey, petrological analysis, LA-ICP-MS zircon U-Pb geochronology, and rock geochemistry of the Early Paleozoic intrusions in the Kekesha region, located in the eastern segment of the East Kunlun. Through comprehensive analysis, the study aimed to uncover the geological structure and duration of the Proto-Tethys tectonic domain within the Eastern Kunlun Orogenic Belt, providing valuable insights into its role in the tectonic evolution of the region.

2. Geological Background

The East Kunlun Orogenic Belt is located on the northern margin of the Qinghai–Tibet Plateau. To the north, the Eastern Kunlun Orogenic Belt is bounded by the Golmud concealed fault, adjacent to the Qaidam Block. To the south, it is bordered by the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM), which is adjacent to the Bayanhar terrane. Based on three ophiolitic mélange zones from north to south—the Qimantagh–Xiangride ophiolitic mélange zone (QXM), the Central Kunlun ophiolitic mélange zone (CKM), and the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM)—the East Kunlun Orogenic Belt is divided into three segments: the North Qimantagh terrane, North Kunlun terrane (NKT), and South Kunlun terrane (SKT) (Figure 1b) [1,4,5,7,33]. According to previous research, the East Kunlun Orogenic Belt is a continental composite orogenic belt with two stages of tectonic evolution, which records a long and complex evolution history of oceanic crust subduction, collision, tectonic deformation, and magmatic activity. Magmatic activity of varying scales occurred during the Early Paleozoic and Late Paleozoic to Early Mesozoic [4,7]. Throughout the tectonic evolution of the Early Paleozoic, the Eastern Kunlun Orogenic Belt accumulated a diverse range of rock types, including ophiolitic suites, magmatic rocks, and island-arc volcanic rocks, all of which record the region’s complex tectonic history.

2.1. North Qimantagh Terrane (NQT)

The Qimantagh belt is located along the southern margin of the Qaidam Block [34], where it is underlain by a Paleoproterozoic metamorphic basement and overlain by a discontinuous sedimentary cover, comprising Ordovician–Silurian, Devonian, Carboniferous–Permian, Middle-Upper Triassic, and Jurassic sequences (Figure 1c). The Paleoproterozoic high-grade metamorphic basement is primarily composed of gneiss and amphibolite. The Ordovician–Silurian Qimantagh Group consists mainly of basalt, andesite, and rhyolite, with a small proportion of clastic and carbonate rocks. The Devonian Maoniushan Formation is characterized by typical synorogenic conglomerates and coarse-grained sandstones. Carboniferous–Lower Permian clastic rocks, carbonate rocks, Middle-Upper Triassic volcanic-sedimentary rocks, and Jurassic continental clastic rocks are discontinuously developed in the North Qimantagh belt. These stratigraphic units are separated by angular unconformities.

2.2. North Kunlun Terrane (NKT)

The geological characteristics of the NKT are mainly manifested in the large-area distribution of Proterozoic metamorphic basement, Early Paleozoic metamorphic rock series, and Paleozoic to Early Mesozoic granites, and in the development of Neoproterozoic granitic gneiss. The Pre-Cambrian basement of the NKT is primarily composed of the Paleoproterozoic Baishahui Formation (Pt₁b) (also known as the Jinshuikou Group) [30], which consists of para-amphibolite, migmatite, schist, amphibolite, and marble. The Mesoproterozoic Xiaomiao Formation (Pt₂x) and Langyashan Formation (Pt₂l) are also present, with the former being composed of quartzite and gneiss metamorphosed under low-amphibolite facies, and the latter predominantly consisting of lightly metamorphosed carbonates. Early Paleozoic metamorphic rocks are represented by the Naij Tal Group (Pz₁N), which includes volcanic-sedimentary rocks metamorphosed under green schist to low-amphibolite facies [15]. The Late Silurian to Early Devonian Kalachuka Formation (D₁k) and Bulakbash Formation (D₂b) are mainly composed of limestone, sandstone, and siltstone. The Yaoniushan Formation (D₁m), found locally, consists of conglomerate, sandstone, and a small amount of bimodal basalt-rhyolite suites. The NKT is extensively exposed and features Middle to Neoproterozoic (1006–840 Ma) S-type granitoids [35,36], Paleozoic (474–383 Ma) gabbro-granite suites [4,7,37,38], and Permian-Triassic (277–204 Ma) magmatic rocks predominantly consisting of granitic intrusions [9,39,40,41,42,43,44].

2.3. Central Kunlun Ophiolitic Mélange Zone (CKM) and Its Early Paleozoic Back-Arc Basins

The CKM is located between NKT and SKT. The belt extends west to Bokaliketagh, bounded by the Altyn Tagh sinistral strike-slip fault, and extends eastward through the Dagangou and Qingshuiquan area, and is cut off by the NW-trending Elashan fault. Numerous ophiolitic suites have been identified within this belt, and it is hypothesized that the ophiolitic mélange in this region represents a complex product of multi-stage rifting and tectonic reassembly. Among them, from east to west are the Changshishan ophiolite in an extensional environment (537 Ma) [45], the Qushiang ophiolite in a subduction environment (505 Ma) [46], the Tatuo ophiolite in an extensional environment (521 Ma) [46], the Qingshuiquan ophiolite in a subduction environment (518 Ma, 522 Ma, 481 Ma, 452 Ma) [10,22,47,48], and the Kekekete mafic–ultramafic complex in a subduction environment (509 Ma, 501 Ma) [19,49]. The Qingquangou ophiolite (510 Ma) [23] in the subduction environment shows that CKM is a back-arc basin environment with extension and subduction in the Early Paleozoic.

2.4. South Kunlun Terrane (SKT) and Its Early Paleozoic Island Arc

The SKT region is characterized by a complex stratigraphy, including Pre-Cambrian metamorphic basement rocks, Early Paleozoic shallow-metamorphic sedimentary-volcanic sequences (e.g., Naij Tal Group and Saishiteng Formation), and Late Paleozoic to Mesozoic sedimentary strata, along with Early Paleozoic and Triassic granites [15,35,50] (Figure 1c and Figure 2). The Pre-Cambrian sequence comprises the Baishahe Formation (Pt₁b) (amphibolite, marble, and quartzite), Xiaomiao Formation (Pt₂x) (quartz–feldspathic–quartzite, and gneiss), Kuhai Group (Pt₂K) (gneiss and amphibolite), and Wanbaogou Group (clastic rocks, volcanic rocks, and carbonates). The Cambrian–Ordovician Naij Tal Group (Pz₁N) is dominated by island-arc volcanic rocks and the associated turbidites. The Saishiteng Formation (Ss) includes conglomerates, sandstones, and siltstones. The southern SKT margin exposes Carboniferous marine carbonates and clastics, Late Permian coarse clastics, and Triassic–Jurassic marine, transitional, and continental deposits. Notable units include the Halaguole Formation (C₁hl), Haoteluowa Formation (C₂P₁ht), Maerzheng Formation (P_1-2m), Gequ Formation (P₃g), Hongshuichuan Formation (T₁h), Naocangjiangou Formation (T₂n), Babaoshan Formation (T₃b), and Yangqu Formation (J₁y) [10,14,33,51]. A series of Early Paleozoic intermediate-acid intrusive rocks and mafic–ultramafic complexes are exposed in the middle of the SKT. From west to east, the Bulhanbuda Mountains feature island-arc quartz diorite (510–515 Ma) in the Kekesha area, island-arc metaluminous diorite (474 Ma) in the Gouli area, and island-arc intermediate-acid intrusions (483 Ma, 449 Ma) in the Zhiyu area [52]. Additionally, amphibolite (447–450 Ma) and granodiorite (466 Ma) from the appinitic pluton in the Xiadawu area further support the presence of an island-arc environment in the Early Paleozoic of the East Kunlun [9]. These findings collectively confirm the presence of an island-arc environment in the region during the Early Paleozoic.

Figure 1. (a) Geotectonic sketch map of China [53]; (b) tectonic map showing main tectonic units in the East Kunlun Orogen [53]; and (c) distribution of the Kekesha intrusion in the East Kunlun Orogen [7]. Q—Quaternary; E—Paleogene; J₁y—Lower Jurassic Yangqu Formation; T₃e—Upper Triassic Elashan Formation; T₃b—Upper Triassic Babaoshan Formation; T₂x—Middle Triassic Xilikete Formation; T₂n—Middle Triassic Naocangjiangou Formation; T₁h—Lower Triassic Hongshuichuan Formation; TB—Triassic Bayan Har Group; P₃g—Upper Permian Gequ Formation; P_1-2m—Lower-Middle Permian Maerzheng Formation; C₂P₁sh—Upper Carboniferous–Lower Permian Shuweimenke Formation; C₂ht—Upper Carboniferous Haoteluowa Formation; Dm—Devonian Maoniushan Formation; Pz₁N—Lower Paleozoic Naij Tal Group; Pt₃W—Neo-Proterozoic Wanbaogou Group; Pt₂K—Meso-Proterozoic Kuhai Group; Pt₂l—Meso-Proterozoic Langyashan Formation; Pt₂x—Meso-Proterozoic Xiaomiao Formation; and Pt₁b—Paleo-Proterozoic Baishahe Formation.

Figure 1. (a) Geotectonic sketch map of China [53]; (b) tectonic map showing main tectonic units in the East Kunlun Orogen [53]; and (c) distribution of the Kekesha intrusion in the East Kunlun Orogen [7]. Q—Quaternary; E—Paleogene; J₁y—Lower Jurassic Yangqu Formation; T₃e—Upper Triassic Elashan Formation; T₃b—Upper Triassic Babaoshan Formation; T₂x—Middle Triassic Xilikete Formation; T₂n—Middle Triassic Naocangjiangou Formation; T₁h—Lower Triassic Hongshuichuan Formation; TB—Triassic Bayan Har Group; P₃g—Upper Permian Gequ Formation; P_1-2m—Lower-Middle Permian Maerzheng Formation; C₂P₁sh—Upper Carboniferous–Lower Permian Shuweimenke Formation; C₂ht—Upper Carboniferous Haoteluowa Formation; Dm—Devonian Maoniushan Formation; Pz₁N—Lower Paleozoic Naij Tal Group; Pt₃W—Neo-Proterozoic Wanbaogou Group; Pt₂K—Meso-Proterozoic Kuhai Group; Pt₂l—Meso-Proterozoic Langyashan Formation; Pt₂x—Meso-Proterozoic Xiaomiao Formation; and Pt₁b—Paleo-Proterozoic Baishahe Formation.

Figure 2. Simplified geological map of the Kekesha area in the East Kunlun Orogen (according to Zhang et al.) [51]. 1—Quaternary; 2—Tuotuohe Formation; 3—Naocangjiangou Formation; 4—Hongshuichuan Formation; 5—Naij Tal Group; 6—Xiaomiao Formation; 7—Baishahe Formation; 8—Halagatu granodiorite; 9—K-feldspar granite; 10—porphyritic monzonitic granite; 11—monzonitic granite; 12—granodiorite; 13—quartz diorite; 14—gabbro diorite; 15—gabbro; 16—pyroxenite; 17—serpentine; 18—basalt; 19—Dundeshaerguole hornblende monzonite; 20—geological boundary; 21—unconformity boundary; 22—reverse fault; 23—translational fault; 24—ductile shear structure interface; and 25—sampling location.

Figure 2. Simplified geological map of the Kekesha area in the East Kunlun Orogen (according to Zhang et al.) [51]. 1—Quaternary; 2—Tuotuohe Formation; 3—Naocangjiangou Formation; 4—Hongshuichuan Formation; 5—Naij Tal Group; 6—Xiaomiao Formation; 7—Baishahe Formation; 8—Halagatu granodiorite; 9—K-feldspar granite; 10—porphyritic monzonitic granite; 11—monzonitic granite; 12—granodiorite; 13—quartz diorite; 14—gabbro diorite; 15—gabbro; 16—pyroxenite; 17—serpentine; 18—basalt; 19—Dundeshaerguole hornblende monzonite; 20—geological boundary; 21—unconformity boundary; 22—reverse fault; 23—translational fault; 24—ductile shear structure interface; and 25—sampling location.

2.5. Muztagh–Buqingshan–Anemaqen Ophiolitic Mélange Zone (MBAM) and Its Early Paleozoic Main Ocean

The MBAM is the boundary between the South Kunlun terrane and the Bayan Har terrane, and it is pinched out in the Naij Tal area in a narrow wedge shape in the near east-west direction. The matrix of the mélange zone is mainly composed of flysch deposits of the Early–Middle Permian Maerzheng Formation (P_1-2m). Among them, there are at least two stages of Early Paleozoic and Late Paleozoic ophiolites, as well as the Mesoproterozoic Kuhai Group (Pt₂K) basement, the Silurian island-arc granodiorite, and the dacite porphyry–rhyolite porphyry blocks, Carboniferous oceanic island/seamount basalt/limestone blocks, and other complex mélange systems [4,5,7,53]. In addition, in the Buqingshan area, it can be seen that the large-scale external nappes of the platform facies carbonate rocks of the Shuweimenke Formation thrust on different tectonic positions of the tectonic mélange zone. The tectonic belt is the product of long-term tectonic evolution from the Paleozoic to the Early Mesozoic. It experienced Late Neoproterozoic–Early Paleozoic ocean expansion and subduction–collision; Late Paleozoic ocean expansion and subduction orogeny; Late Paleozoic–Early Mesozoic comprehensive collision orogeny; the long and complex evolution process of Mesozoic–Cenozoic intracontinental orogeny. MBAM has developed a large number of the Early Paleozoic oceanic crust-type and island-arc-type rock combinations, such as the Early Paleozoic Delistan ophiolite in the Buqingshan area and the Kuhai oceanic island-type gabbro in the Anemaqen area [29,30,32], indicated that MBAM existed in the main oceanic basin of the Proto-Tethys Ocean in the Early Paleozoic [7]. Formations such as the Bairiqiete granodiorite (439 Ma) and the dacite porphyry–rhyolite porphyry (441 Ma) [30]; the Yikehalaer island-arc granodiorite (436 Ma) [23]; and the Manite granodiorite (495.6 Ma) [28] all indicate that the MBAM underwent island-arc magmatism caused by oceanic crust subduction in the Early Paleozoic.

3. Petrological Characteristics of the Kekesha Intrusion

The Early Paleozoic Kekesha intrusion is located in the south part of Xiangjia Township, Dulan County, Qinghai Province. It belongs to the SKT (Figure 1c and Figure 2). The exposed area is about 62 km². The main lithology is quartz diorite, which is mostly irregular rock stock intruded in the Paleoproterozoic Baishahe Formation. Other basic intrusions or acidic intrusions are small in scale. They occur in the form of small rock stocks or rock branches, or in the form of restite around the main intrusion. The northwestern part is in intrusive tectonic contact with the lower-grade metamorphic clastic rocks of the Early Paleozoic Naij Tal Group and the quartzite of the Mesoproterozoic Xiaomiao Formation. The southeastern part is in fault contact or unconformity contact with the glutenite and marl strata of the Early Triassic Hongshuichuan Formation. After the intrusion, it was affected by the magmatic emplacement and tectonic destruction during the Late Paleozoic–Early Mesozoic orogenic movement. The residual intrusion has an obvious segmentation dislocation phenomenon, but the whole is irregular lenticular, which is obviously different from the direction of the regional tectonic line in the NW–SE direction, but it generally extends from SW to NE. The intrusion generally develops a weak gneissose structure, which is basically consistent with the schistosity or gneissic texture occurrence in the metamorphic surrounding rock, and the intrusion develops weak deformation and alteration phenomena, strong chloritization, and epidotization. The intrusion transits from basic gabbro to intermediate-basic gabbro diorite and from intermediate-acid quartz diorite to acidic granodiorite. Most of the lithology shows abrupt contact relationships, but some shows gradual contact. Among them, the gray quartz diorite is the main lithology of the Early Paleozoic Kekesha intrusion, accounting for about 80~85%, which is concentrated in distribution in the Kekesha area. The dark gray gabbro and gabbro diorite are relatively less distributed, with most consisting of large blocks or basal–medium basal intrusions within and near the main quartz diorite. The gray-white granodiorite, which occupies a small proportion, is mainly exposed on the edge of the main intrusion, and the shape is very irregular. A gradual transition in the structure of the quartz diorite from coarse- to medium-fine-grained can be clearly observed.

The gabbro is dark gray, with a medium-fine-grained subhedral gabbro structure and massive structure (Figure 3a). The main mineral compositions are plagioclase (about 50%), hornblende (35%~45%), pyroxene (about 5%~8%), K-feldspar (about 5%), and a small amount of biotite (<5%). The plagioclase is an euhedral–subhedral plate, and sericitization is strong. Polysynthetic twin and Carlsbad-albite compound twin can be observed. The texture is fine and dense, the grain size is different, and the grain size is 1~2.2 mm. Some have bending and breaking phenomena, and the arrangement has obvious directionality and is generally distributed in parallel. The hornblende is short columnar or fibrous, with a grain size of 1~2 mm. Under the crossed polarized light, it can be seen that it has been replaced by chlorite and epidote (Figure 3b,c), leaving only obvious mineral cleavage. The pyroxene is subhedral columnar and fibrous, with a grain size of 0.01~2.0 mm. Most of the pyroxenes have been altered into hornblende and chlorite, which only retain their short columnar crystal appearance. Accessory minerals include zircon, apatite, sphene, and magnetite.

The gabbro diorite is dark gray, with medium-fine-grained subhedral granular structure and massive structure (Figure 3d). The main mineral composition is plagioclase (65%~70%), hornblende (about 25%~20%), K-feldspar (about 5% ±), a small amount of biotite (less than 5%), and quartz (less than 5%). The plagioclase is an euhedral plate, and the grain size is generally (0.6~1) × (1.5~2.5)mm. The surface of the crystal is turbid and unclear, with sericitization and epidotization, but a clear schistose twin can be observed. The hornblende occurs as euhedral granular and irregular crystals, exhibiting a slightly preferred orientation. The alteration of larger hornblende grains is relatively moderate, while the smaller hornblende crystals within the plagioclase fractures are almost entirely transformed into chlorite and biotite (Figure 3e,f). Quartz is distributed in anhedral form between the plagioclase and hornblende, sometimes with concentrated distribution, and wavelike extinction is obvious. K-feldspar is euhedral–anhedral granular, and some grains are large, up to 2.5~2.8 mm, with lattice twin crystal development. Most of the pyroxenes have been altered into hornblende and chlorite. The accessory minerals are zircon, apatite, and magnetite.

The quartz diorite is gray–light gray, with medium-coarse-grained subhedral granular structure and massive structure (Figure 3g). The main minerals are plagioclase (50%~60%), hornblende (about 10%~15%), quartz (10%~15%), K-feldspar (10% ±), and biotite (5% ±). The plagioclase is generally more euhedral plate or short column, sericitization is obvious, and the grain size is generally (0.4~0.8) × (0.8~1.7)mm. The plagioclase shows schistose twin development; see the ring structure. The hornblende is distributed between the plagioclase in a short columnar and needle-like shape. It is, basically, all chloritization or biotiteization, and a few parts still maintain some incomplete cleavage. The quartz is distributed in its shape between feldspar particles, with wavy extinction. The K-feldspar and plagioclase have a monzonitic texture, which is euhedral to anhedral, and display micro-plagioclase with lattice twin characteristics and a small amount of card-type twin orthoclase (Figure 3h,i). The accessory minerals include zircon, apatite, magnetite, and sphene.

The granodiorite is light gray–gray-white, with medium-grained granite structure and massive structure (Figure 3j). The main minerals are plagioclase (55%~60%), quartz (20% ±), K-feldspar (10%~15%), hornblende (5%~10%), and biotite (less than 5%). The plagioclase is generally more euhedral tabular and the grain size is (0.4~0.8) × (0.8~1.7)mm, with ring structure and schistose twin development; it is fine and dense, and occasionally the schistose twin is bent. The K-feldspar is less euhedral than the plagioclase, which is euhedral to anhedral, and includes orthoclase, perthite, and microcline. The quartz is distributed in the intergranular spaces of the hornblende, biotite, K-feldspar, and plagioclase (Figure 3k,l). The hornblende is euhedral columnar, and about 2~3 mm long. The biotite is brown, and is Fe-biotite. The accessory minerals are zircon, apatite, and magnetite, with sometimes a small amount of hematite, ilmenite, and sphene.

4. Analytical Methods

4.1. Whole-Rock Geochemical Analyses

The analysis of major elements and trace elements was carried out in the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences. The major elements are tested by X-ray fluorescence spectrometer (XRF-1500) with an accuracy of better than 2%~3%. The samples of trace elements and rare earth elements were prepared by acid dissolution method and analyzed by ICP-MS (Element II) test with an accuracy of better than 10%. The chemical analysis procedure, as referenced by Chen et al. (2000 and 2002), is outlined herein [54,55].

4.2. LA-ICP-MS Zircon U-Pb Dating

The zircon U-Pb isotope age test was carried out on the LA-ICP-MS instrument of the State Key Laboratory of Continental Dynamics, Northwest University. The analytical instruments were Elan6100DRC quadrupole mass spectrometer and Geolas200M laser ablation system. The laser was 193 nm ARF excimer laser. The laser ablation spot diameter is 30 μm and the laser ablation depth is 20~40 μm. The zircon age was calculated using the internationally calibrated zircon 91,500 as the external standard, and the standard sample for isotope ratio monitoring is as follows: the element content was corrected using the synthetic silicate glass NIST610 of the National Institutes of Standards and Materials as the external standard and ²⁹Si was used as the inner table element for correction. The experimental data are processed by ICP-MS DataCal 10.9 program and Isopiot 4.15 program. The detailed experimental principles, test procedures, and instruments are shown in the relevant literature [56].

5. Results

5.1. Zircon U-Pb Age

The zircon U-Pb isotopic geochronology of gabbro (3998-1), gabbro diorite (11006-1), quartz diorite (11009-1), and granodiorite (11016-1, 11030-1) in the Kekesha intrusions were studied. The test results are shown in Table S1.

Of the 25 zircon grains analyzed from the gabbro sample (3998-1), 23 exhibited concordance greater than 90%, while two grains were highly discordant (Figure 4a,b). These zircon crystals are clear and complete, with most being transparent to semi-transparent, long columnar, semi-conical, and plate-like columnar shapes. The zircon grains are euhedral to subhedral, with lengths ranging from 100 to 200 µm and aspect ratios from 1:1 to 3:1. Most zircons exhibit typical oscillatory zoning. The ²³²Th content ranges from 191 to 4269 ppm, and the 238U content ranges from 460 to 2279 ppm. The Th/U ratios vary from 0.32 to 1.87 (with the majority of samples > 0.4), which are characteristic of magmatic zircon. The weighted average ²⁰⁶Pb/²³⁸U age is 515.7 ± 7.4 Ma (MSWD = 2.0, N = 23) (Figure 4b), indicating that the crystallization age of the gabbro is Middle Cambrian.

Zircon ages from the gabbro diorite sample (11006-1) were classified into two groups, after excluding two highly discordant grains (Nos. 04 and 17). The first group includes 14 zircon grains (Nos. 01, 05–12, 15, 18, 20, 22, and 25). These zircons are clear, intact, and exhibit an aspect ratio of approximately 2:1, with euhedral, elongated columnar shapes. The grain sizes range from 200 to 350 μm. The zircons display rhythmic zoning and a core-mantle structure, and have a predominantly gray color. The ²³²Th content ranges from 98 to 780 ppm, and the 238U content ranges from 238 to 3026 ppm. The Th/U ratios range from 0.13 to 0.52 (with the majority > 0.4), characteristic of typical magmatic zircon. These zircons belong to the category of magmatic crystallization products (Figure 4c) [57]. They show virtually no Pb loss and are located on or near the U-Pb concordia lines. The weighted mean ²⁰⁶Pb/²³⁸U age is 508 ± 9.8 Ma (MSWD = 2.2, N = 14), indicating that the crystallization age of the gabbro diorite is Middle Cambrian. The second group consists of nine zircons, with ²⁰⁶Pb/²³⁸U ages ranging from 608 to 786 Ma. These zircons may represent early xenocrysts that were lead-adjusted due to late metamorphic-thermal events, leading to their deviation from the U-Pb concordia lines (Figure 4d). This suggests that the formation age of the gabbro diorite is Middle Cambrian, which is consistent with the age of the previously analyzed gabbro.

The zircon crystals from the quartz diorite sample (11009-1) are light yellow to colorless. Most of the zircons are large, euhedral, and columnar, with a few smaller grains exhibiting short columnar shapes. The grain size is primarily between 200 ìm and 350 ìm. The CL images of the zircons reveal distinct oscillatory zoning, characteristic of magmatic growth. The 232Th content ranges from 86 to 426 ppm, while the 238U content varies from 322 to 822 ppm. The Th/U ratios range from 0.27 to 0.52, with the majority of samples showing ratios greater than 0.4, indicating a magmatic origin for these zircons (Figure 4e) [57]. A total of 25 zircons were selected for LA-ICP-MS analysis. All zircons plot on the U-Pb concordia lines. The weighted average ²⁰⁶Pb/²³⁸U age is 499.6 ± 4.0 Ma (MSWD = 0.65, N = 25) (Figure 4f), and this suggests that the formation age of the quartz diorite is Middle Cambrian.

The zircon crystals from the granodiorite sample (11016-1) are light yellow to colorless, with some showing darker hues. The zircon grains are generally larger, with most exhibiting euhedral short columnar to nearly circular shapes, while a few are long columnar. The grain size is between 200 ìm and 350 ìm, and the zircons show well-defined rhythmic zoning (Figure 4g). The ²³²Th content ranges from 35 to 314 ppm, and the 238U content varies from 74 to 1669 ppm. The Th/U ratios range from 0.05 to 1.82, with the majority of samples showing ratios greater than 0.4, suggesting a magmatic origin for these zircons [57]. After excluding four highly discordant zircon grains (Nos. 1, 10, 21, and 24), the zircon ages can be grouped into two main categories. The first group consists of 12 zircons (Nos. 04–08, 12, 15–18, 23, and 25), with a weighted average ²⁰⁶Pb/²³⁸U age of 502.3 ± 9.3 Ma (MSWD = 1.4, N = 12) (Figure 4h), representing the crystallization age of the granodiorite. The remaining zircons, which are considered inherited or older xenocrystic zircons, display complex compositions and significant age variations, likely corresponding to earlier geological events. These zircons have 206Pb/238U ages ranging from 540 to 846 Ma.

The zircon crystals from the granodiorite sample (11030-1) are transparent, ranging from colorless to light yellow. The zircon grains are predominantly subhedral, with shapes including long columnar, short columnar, bipyramidal, semi-cut cone, and irregular forms. These zircons exhibit noticeable erosion, and their edges and corners are often unclear. The crystal lengths range from 150 to 350 ìm, with widths between 50 and 200 ìm, resulting in a length-to-width ratio of 2:1 to 3:1. A small number of zircons (e.g., Nos. 05, 16, and 27) display distinct residual cores or irregular dark areas, which may indicate inherited or xenocrystic cores (Figure 4i). The ²³²Th content ranges from 62 to 1545 ppm, and the 238U content varies from 126 to 2321 ppm. Th/U ratios range from 0.05 to 1.82, with the majority of samples showing ratios greater than 0.4, suggesting a magmatic origin for the zircons [57]. The zircon crystals from the granodiorite sample (11030-1) are transparent, ranging from colorless to light yellow. A total of 29 zircon grains were analyzed. Excluding two highly discordant zircon grains (Nos. 05 and 28), the zircons can be divided into two main groups. The first group consists of 22 zircon grains, with a weighted average ²⁰⁶Pb/²³⁸U age of 501.6 ± 6.2 Ma (MSWD = 1.4, N = 22) (Figure 4j), representing the crystallization age of the granodiorite in the Kekesha intrusion. The second group, comprising five zircons (Nos. 07, 08, 16, 22, and 27), shows Pb re-adjustment due to late metamorphic-thermal events, causing them to deviate from the U-Pb concordia lines. The ²⁰⁶Pb/²³⁸U ages of these grains range from 520 to 855 Ma, indicating they may represent inherited zircon ages. By combining the results from samples 11016-1 and 11030-1, the formation age of the granodiorite is determined to be Middle Cambrian, consistent with the age of the quartz diorite.

5.2. Major Elements

The Kekesha intrusion contains a variety of rock types. Through field geological survey and thin section observation under a microscope, the rock name was first determined by a QAP diagram. In view of the wide variation range of major elements (for example, SiO₂ content is 48.92%~74.76%, total alkali (Na₂O + K₂O) content is 1.86%~9.53%, and Na₂O/K₂O ratio is 0.49~6.40), the major element analysis data for the Kekesha intrusion are shown in Table S2. The Kekesha intrusion has undergone alteration, and the total alkali (Na₂O + K₂O) content may change. Therefore, in addition to the QAP diagram (Figure 5a) and the total alkali–silicon diagram (TAS) (Figure 5b), the relatively stable element SiO₂-Zr/TiO₂*0.0001 diagram (Figure 5c) is supplemented. Most of the samples in the diagram fall within the normal diorite area (with the distribution of gabbro, gabbro diorite, quartz diorite, and granite diorite), which is consistent with the naming of rocks in the field.

The gabbro exhibits a high SiO₂ content, ranging from 48.92 wt.% to 51.94 wt.%. The TiO₂ content varies from 0.54 wt.% to 1.29 wt.%, which is comparable to that of island-arc basaltic volcanic rocks (0.58 wt.% to 0.85 wt.%) [61]. The content of Al₂O₃ changes greatly, ranging from 10.90 wt.% to 19.75 wt.%, which is characteristic of high-aluminum basalts typically found in subduction zones [62]. The Na₂O content ranges from 1.13 wt.% to 3.53 wt.%, while the K₂O content varies between 0.48 wt.% and 1.67 wt.%, with total alkali content (Na₂O + K₂O) fluctuating between 1.61 wt.% and 5.2 wt.%, although Na₂O generally exceeds K₂O. The P₂O₅ content ranges from 0.09 wt.% to 0.37 wt.%. The MgO content spans from 3.82 wt.% to 13.99 wt.% and the Mg^# (molar Mg/(Mg + Fe)) ranges from 48.92 to 79.17. In the total alkali–silica (TAS) diagram (Figure 5b), the gabbro plots within the sub-alkaline field. In the (Na₂O + K₂O)-FeOT-MgO (AFM) diagram (Figure 6a), it falls into the calc-alkaline series, indicating that the gabbro belongs to the calc-alkaline basalt group. Therefore, the gabbro is classified as part of the calc-alkaline series.

The SiO₂ content of gabbro diorite ranges from 52.51 wt.% to 56.79 wt.%, with TiO₂ content ranging from 0.53 wt.% to 0.90 wt.%. The Al₂O₃ content is slightly higher and more stable, ranging from 14.63 wt.% to 17.87 wt.%. The Na₂O content varies between 2.19 wt.% and 3.97 wt., while the K₂O content shows significant variation, from 0.62 wt.% to 3.98 wt.%. The total alkali content (Na₂O + K₂O) fluctuates between 3.77 wt.% and 5.33 wt.%. The P₂O₅ content is low, ranging from 0.1 wt.% to 0.2 wt.%. The MgO content spans from 2.92 wt.% to 6.36 wt.%, and the CaO content ranges from 6.10 wt.% to 9.52 wt.%. The average Mg^# value is 51.38, with a range from 47.47 to 70.79. In the (Na₂O + K₂O)-FeOT-MgO (AFM) diagram (Figure 6a), the samples plot within the calc-alkaline field. On the SiO₂-K₂O diagram (Figure 6b), most samples are positioned within the calc-alkaline and high-K calc-alkaline fields, suggesting that they belong to the medium-high-K calc-alkaline series.

For quartz diorite, the SiO₂ content ranges from 57.20% to 61.30%, with the TiO₂ content varying from 0.67% to 0.80%. The Al₂O₃ content is stable and slightly higher, ranging from 16.71% to 17.27%. The Na₂O content ranges from 2.23% to 4.09%, and the K₂O content varies significantly from 0.72% to 3.98%. The total alkali content (Na₂O + K₂O) spans from 3.77% to 5.33%. The MgO content ranges from 2.72% to 3.49%, and the CaO content varies from 5.30% to 7.17%. The average Mg^# value is 51.54, with a range from 47.18 to 60.14. In the SiO₂-K₂O diagram (Figure 6b), the samples are dispersed, with most falling within the calc-alkaline and high-K calc-alkaline fields. The A/CNK ratio ranges from 0.82 to 0.97, with an average of 0.91. In the A/NK-A/CNK diagram (Figure 6c), quartz diorite falls within the metaluminous field, indicating it belongs to the metaluminous medium-high-K calc-alkaline series.

Granodiorite shows a high SiO₂ content, ranging from 67.17% to 72.59%, and a low TiO₂ content (0.28% to 0.43%). The Al₂O₃ content ranges from 14.75% to 16.54%. The Na₂O content is relatively high, ranging from 4.28% to 4.61%, and the K₂O content is moderate, ranging from 1.73% to 3.97%. The total alkali content (Na₂O + K₂O) is high, ranging from 6.19% to 7.02%. The MgO content ranges from 0.80% to 1.30%, and the CaO content ranges from 3.21% to 3.80%. The Mg^# value is relatively low, ranging from 33.62 to 46.48. In the SiO₂-K₂O diagram (Figure 6b), most samples fall within the calc-alkaline field. The A/CNK ratio ranges from 0.99 to 1.14. In the A/NK-A/CNK diagram (Figure 6c), the granodiorite samples fall within the peraluminous field, indicating that the granodiorite belongs to the weakly peraluminous calc-alkaline series.

The Harker diagrams for the major and trace elements in gabbro and gabbro diorite from the Kekesha intrusion reveal negative correlations between FeOT, TiO₂, and P₂O₅ with Mg^# (Figure 7a–c), while CaO, Ni, Cr, CaO/Al₂O₃, and Sc/Y exhibit positive correlations with Mg^# (Figure 7d–h). The Harker diagrams for the major elements in quartz diorite and granodiorite show significant negative correlations between Al₂O₃, P₂O₅, Fe₂O₃T, TiO₂, CaO, and MgO with SiO₂ (Figure 8a–f).

5.3. Trace Elements

The geochemical analysis data of trace elements in the Kekesha intrusion are shown in Table S2.

The content level of REEs in the gabbro of the Kekesha intrusion is high, ranging from 66 ppm to 197 ppm, which is significantly greater than that in the original mantle (ΣREE = 7.430 × 10⁻⁶). Overall, the REE distribution exhibits a right-leaning pattern, with relative enrichment in light REEs (LREEs) (55 ppm to 186 ppm) and relative depletion in heavy REEs (HREEs) (10 ppm to 20 ppm) (Figure 9a). The fractionation between the LREEs and HREEs is pronounced, with ΣLREE/ΣHREE ranging from 4.97 to 9.52, except for sample 11028/4, which has a ratio of 17.06. The (La/Yb)_N ratio ranges from 4.75 to 11.77, except for sample 11028/4, which is 28.64. The degree of LREE fractionation is high, with (La/Sm)_n values between 2.80 and 7.68, while the fractionation of the HREEs is relatively weak, with (Gd/Yb)_N values ranging from 1.32 to 2.14. The δEu values range from 0.64 to 1.30, with some samples showing negative Eu anomalies. The primitive mantle-normalized trace element spider diagram (Figure 9b) reveals relative enrichment in large-ion lithophile elements (LILEs), such as Rb, Ba, Pb, and Sr, and depletion in high-field-strength elements (HFSEs), including Nb, Ta, Ti, Zr, and Hf. These characteristics suggest that the gabbro may originate from an enriched mantle source related to subduction, resembling the signature of island-arc basaltic magma (IAB).

The total amount of REEs in the gabbro of the Kekesha intrusion varies greatly (Figure 9a), ranging from 77 × 10⁻⁶ to 313 × 10⁻⁶. The LREEs are relatively enriched (69 ppm to 178 ppm), while the HREEs are relatively depleted (10 ppm to 46 ppm). The fractionation between LREEs and HREEs is pronounced, with ΣLREE/ΣHREE ratios ranging from 4.97 to 7.25, and (La/Yb)_N values between 4.50 and 7.89. The fractionation of LREEs is more pronounced, with (La/Sm)_N values ranging from 2.16 to 4.34, while the fractionation of HREEs is relatively weak, with (Gd/Yb)_N values between 1.26 and 2.15. The Eu anomaly (δEu) ranges from 0.31 to 0.91, with most samples exhibiting strong negative Eu anomalies. In the primitive mantle-normalized trace element spider diagram (Figure 9b), all samples display a similar pattern to that of the gabbro in the Kekesha intrusion, with enrichment in large-ion lithophile elements (LILEs) such as Rb, Ba, Th, and K, as well as LREEs. At the same time, there is varying depletion in high-field-strength elements (HFSEs), including Nb, Ta, and Ti, suggesting a subduction zone-related island-arc basalt (IAB) characteristic [65].

The total REE content in the quartz diorite of the Kekesha intrusion exhibits significant variation, ranging from 86 ppm to 265 ppm (Figure 9c). The LREEs are relatively enriched, with concentrations ranging from 44 ppm to 267 ppm, while the HREEs are relatively depleted, ranging from 6 ppm to 34 ppm. A clear fractionation between LREEs and HREEs is observed, with ΣLREE/ΣHREE ratios ranging from 4.29 to 9.45 and (La/Yb)_N values between 3.79 and 10.07. The LREEs show a more pronounced fractionation, with (La/Sm)_N values between 3.09 and 5.28, whereas the fractionation of HREEs is less pronounced, as indicated by (Gd/Yb)_N values ranging from 1.37 to 2.28. The Eu anomaly (δEu) varies from 0.03 to 1.04, with some samples exhibiting negative Eu anomalies. In the primitive mantle-normalized trace element spider diagram (Figure 9d), the samples display enrichment in large-ion lithophile elements (LILEs) such as Rb, Ba, Th, and K, while showing varying degrees of depletion in high-field-strength elements (HFSEs) like Nb, Ta, and Ti. This distribution pattern reflects the subduction zone-related island-arc basalt (IAB) characteristics [65].

The total REE content in the granodiorite of the Kekesha intrusion is relatively low, ranging from 95 ppm to 102 ppm. Within this range, the LREEs are relatively enriched (62 ppm to 64 ppm), while the HREEs are relatively depleted (6.9 ppm to 7.6 ppm). A clear fractionation between LREEs and HREEs is observed, with ÓLREE/ÓHREE ratios ranging from 7.7 to 8.0 and (La/Yb)_N values between 7.6 and 8.9. The LREEs exhibit a high degree of fractionation, as evidenced by (La/Sm)_N values ranging from 6.03 to 7.34, with a flat LREE pattern. The HREEs show a moderate fractionation, with (Gd/Yb)_N values ranging from 1.40 to 2.06. In the chondrite-normalized REE diagram, the granodiorite exhibits a distinct right-leaning pattern (Figure 9c). The Eu anomaly (äEu) ranges from 0.04 to 1.39, with some samples showing a weak positive Eu anomaly, while others exhibit a strong negative Eu anomaly. The primitive mantle-normalized trace element spider diagram (Figure 9d) reveals enrichment in large-ion lithophile elements (LILEs), such as Rb, Ba, and Th, and a relative depletion in high-field-strength elements (HFSEs), particularly in a pronounced “Ta-Nb-Ti” negative anomaly.

6. Discussion

6.1. Petrogenesis

6.1.1. Petrogenesis of Gabbro

Geochemically, the gabbro from the Kekesha intrusion has high SiO₂ (48.92%~51.94%), MgO (3.82%~13.99%, average 7.39%), and Fe₂O₃^T content levels, which are much higher than the average continental crust [67], indicating that the initial magma should have been derived from the mantle [70]. The FeO^T content (7.19%~9.24%) is slightly higher than that of the average continental crust (FeO^T = 6.71%) [67]. The gabbro from the Kekesha intrusion has relatively low Nb/La and Nb/Ce ratios (0.22~0.47 and 0.11~0.24, respectively), and these are lower than those of the primitive mantle, average crust, and average lower crust (respectively: 1. 02, 0. 4; 0.69, 0.33; 0.83, and 0.39). The Th/Nb (0.08~1.27, mean 0.66) ratio is significantly higher than that of the primitive mantle (Th/Nb = 0.12) and continental crust (Th/Nb = 0.44), suggesting that the mafic magma may be a mixture of crustal and mantle materials [71,72,73].

The study indicates that contamination of crustal materials during magma evolution results in a significant increase in the La/Sm ratio of rock trace elements (>5). The gabbro from the Kekesha intrusion exhibits a notable variation in the La/Sm ratio (4.3–11.7), suggesting that certain portions of the magma underwent extensive crustal contamination during its formation, while other portions were relatively unaffected by such contamination [74]. In addition, a high degree of crustal contamination often leads to Zr-Hf positive anomalies in rocks [75]. The Zr-Hf content of most gabbro samples is distributed between IAB and MORB on the primitive mantle-normalized trace element spider diagram, and a small number of gabbro samples have slight Zr-Hf positive anomalies (Figure 9b), which also proves that there are two different degrees of crustal contamination during magmatic evolution. The gabbro samples from the Kekesha intrusion show a lower MgO content than the mantle magma, indicating that the Mg-rich minerals such as olivine have undergone a certain degree of crystal fractionation during the formation process. In addition, the gabbro of the Kekesha intrusion has significantly lower Cr, Ni, and Mg^# values (Cr = 19.6 × 10⁻⁶~667 × 10⁻⁶, Ni = 6.6 × 10⁻⁶~264 × 10⁻⁶, and Mg^# = 48.92~79.17) than the original basaltic magma (Cr > 1000 × 10⁻⁶, Ni > 400 × 10⁻⁶, and Mg^# > 73) [76]. The Hacker diagram constructed with Mg^# as the abscissa shows that FeO^T, TiO₂, and P₂O₅ have a significant negative correlation with Mg^#, which may indicate that the magma has undergone crystal fractionation of apatite and Fe-Ti oxide; CaO, Cr, Ni, CaO/Al₂O₃, Sc/Y, and Mg^# show a significant positive correlation (Figure 7d–h), indicating that the magma occurred during the formation of clinopyroxene and olivine crystal fractionation [54,77,78]. The δEu shows a weak negative anomaly, indicating a slight crystal fractionation of the plagioclase (Figure 7a–c). In summary, this study suggests that the crystal fractionation of gabbro in the Kekesha intrusion occurred during magmatic evolution, and that two types of crustal materials with varying degrees of contamination were involved.

6.1.2. Petrogenesis of Gabbro Diorite

The gabbro diorite samples from the Kekesha intrusion have less dark mineral content than the gabbro samples, and have higher SiO₂ (52.51%~56.79%), Na₂O (2.19%~3.97%), K₂O (0.62%~3.98%), and LILE (77 × 10⁻⁶~313 × 10⁻⁶) content, and lower P₂O₅ (0.1%~0.2%) and TiO₂ (0.1%~0.2%) content than gabbro samples, suggesting that the crustal contamination of gabbro diorite is increased [75,79]. The research results have shown that in the process of magma evolution and emplacement, crustal contamination and crystal fractionation often occur simultaneously (the AFC process) [79,80,81]. The gabbro diorite from the Kekesha intrusion has relatively low Nb/La and Nb/Ce ratios (0.230.70 and 0.12~0.35, respectively), and is located between the Nb/La and Nb/Ce ratios of the average crust and the average lower crust [71]. The Nb/Ta and Zr/Hf values of most gabbro diorite samples (7.87~27.65 and 29.88~79.27, respectively) are between MORB (17.7 and 36.1) [66] and continental crust (11 and 33) [71], indicating that the mafic magma may have been formed by a mixture of more crustal and mantle materials. The gabbro diorite from the Kekesha has lower Cr, Ni, and Mg^# values (Cr = 20.5 × 10⁻⁶~150 × 10⁻⁶, Ni = 3.06 × 10⁻⁶~47.9 × 10⁻⁶, and Mg^# = 47.47~70.79), and it shows the same correlation between main and trace elements as the gabbro. The δEu, meanwhile, shows a strong negative anomaly (Figure 9a), indicating that the gabbro diorite has a stronger crystal fractionation of clinopyroxene, olivine, apatite, iron-titanium oxide, and plagioclase. In summary, this paper suggests that the gabbro diorite of the Kekesha intrusion has undergone stronger crystal fractionation and crustal contamination in the process of magma evolution.

6.1.3. Petrogenesis of Quartz Diorite

There are few dark minerals in the quartz diorite samples from the Kekesha intrusion, and hornblende is observed both in hand specimens and under the microscope. It is preliminarily identified as I-type granite from the perspective of mineralogy. The major elements show that the intrusion has high SiO₂ content (57.20%~61.30%) and Na₂O content (2.23%~4.09%), and low A/CNK (0.82~0.97), indicating it belongs to the mateluminous medium-high-K calc-alkaline series. In the Hacker diagram, Al₂O₃, P₂O₅, Fe₂O₃^T, and TiO₂ are negatively correlated with the SiO₂ content (Figure 8a–d) and, with the evolution of granitic magma to acid, it has experienced the separation and crystallization of apatite and Fe-Ti oxides. There is a negative correlation between CaO, MgO, and SiO₂ (Figure 8e–f), and Eu shows a negative anomaly (Figure 9c), indicating that there may have been crystal fractionation of the hornblende and plagioclase during the magmatic evolution. The quartz diorite is characterized by the enrichment of LREEs, significant depletion of Nb, Ta, and Ti, weak negative Eu anomaly, and low ¹⁰⁴Ga/Al ratio (2.12~2.36, with an average of 2.21) (Figure 10a), showing it has the characteristics of I-type granites [82]. The FeO^T/MgO-(Zr + Y + Ce + Nb) diagram shows that the sample is a differentiated I-type granite (Figure 10b). The FeO^T/MgO-(Zr + Y + Ce + Nb) diagram shows that the sample is an unfractionated I-type granite (Figure 10b). In addition, there is a good linear evolutionary trend between SiO₂ and the major oxides, and this falls on the magma mixing line in the MgO-FeO^T related diagram, reflecting the magma evolution (Figure 10c) and indicating that there is certainly magma mixing in the intrusion. The Zr-Zr/Nb diagram shows that crystal fractionation and partial melting existed at the same time (Figure 10d). Therefore, this paper suggests that the quartz diorite from the Kekesha intrusion is I-type granite partially melted by magmatic rock, and that there is a weak degree of crystal fractionation and magma contamination.

6.1.4. Petrogenesis of Granodiorite

A small amount of hornblende is also observed in the granodiorite of the Kekesha intrusion, both in hand specimens and under the microscope. This hornblende exhibits the mineralogical characteristics typical of I-type granites. The Hacker diagram shows that MgO, Na₂O, TiO₂, CaO, Fe₂O₃^T, and P₂O₅ decrease with the increase in SiO₂ content, and P and Ti are obviously negative anomalies in the primitive mantle-normalized trace element spider diagrams, indicating that the crystal fractionation of ilmenite oxide, apatite, and hornblende is likely to exist in the magmatic evolution [79,80,81]. The content of P₂O₅ and SiO₂ is negatively correlated (Figure 7c), showing it has the characteristics of I-type granites [82]. In the Nb-¹⁰⁴Ga/Al and FeO^T/MgO-(Zr + Y + Ce + Nb) diagrams, the sample points are basically located in the I- and S-type granites region (Figure 10a) and on the boundary between FG and OGT, which are fractioned I-type granites (Figure 10b). In the MgO-FeO^T diagram, it falls below the magma mixing line (Figure 10c). In the MgO-FeO^T diagram, it falls below the magma mixing line (Figure 10c), indicating that there is stronger magma mixing in the intrusion. The Zr-Zr/Nb diagram (Figure 10d) shows that crystal fractionation and partial melting occurred at the same time, and the degree of partial melting was higher. Therefore, the granodiorite from the Kekesha intrusion is an I-type granite with partial melting of magmatic rocks and has stronger crystal fractionation and magmatic contamination.

6.2. Magma Source and Magmatic Processes

6.2.1. Magma Source of Gabbro and Gabbro Diorite

The gabbro and gabbro diorite samples from the Kekesha intrusion exhibit characteristics consistent with calc-alkaline basalt, including low TiO₂, high Al₂O₃, and relative enrichment in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs). These rocks are characterized by a depletion in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs) (Figure 9a,b), which closely resembles melts derived from the partial melting of an enriched lithospheric mantle [62,75,86,87], showing typical island-arc basalt (IAB)characteristics. In addition, in the La/Yb-Nb/La diagram, most of the gabbro and gabbro diorite samples in the Kekesha intrusion fall into the lithospheric mantle source region, but a few fall into the mixed mantle source region, and gather around the boundary between the lithospheric mantle and the mixed mantle (Figure 11a), indicating that they originate from the partial melting of the enriched mantle source region and are partially affected by the asthenosphere mantle material [67,88]. The research results have shown that there are significant differences in the activity of trace elements in plate subduction, in which the fluid in the subduction material and the magma caused by melt metasomatism will show different trace element content characteristics [89]. The Th/Yb, Ba/La, Th/Nb, and U/Th ratios indicate a relatively stable Th/Yb ratio and a broad Ba/La ratio (Figure 11b), as well as a broad Th/Nb and a narrow U/Th ratio (Figure 11c), providing evidence for fluid-induced metasomatism during subduction [90,91] (Figure 11c).

In addition, gabbro and gabbro diorite samples have obvious HREE fractionation characteristics (Figure 9a), indicating that the residual garnet is unstable in its source region. In the La/Yb-Dy/Yb diagram, the samples are projected on the garnet lherzolite melting line, indicating that the melting degree of the mantle source region is about 15%~20% (the conversion area between the spinel lherzolite stable area and the garnet lherzolite stable area is 75~80 km) [92] (Figure 11d). It also shows that the primitive magma of these gabbro and gabbro diorite samples was involved in the mantle material at 75~80 km. Previous research on the ophiolite in the Qingshuiquan–Kekesha–Kekekete area in the SKT [48,78] involved conducting a comprehensive analysis of the petrology and geochemistry and found that it has the geochemical characteristics of island-arc basalt (IAB), similar to the gabbro and gabbro diorite in the Kekesha intrusion, which is formed in the subduction tectonic environment of an island arc. In summary, the source characteristics of the gabbro and gabbro diorite in the Kekesha intrusion are complex and are influenced by fluid metasomatism in the subduction zone and the incorporation of crustal materials. This research project proposes that the primary magma resulted from partial melting of a lithospheric mantle metasomatized by subducted slab melt in an oceanic crust subduction zone [93].

6.2.2. Magma Source of Quartz Diorite

The results indicate that the quartz diorite samples from the Kekesha intrusion were primarily formed through crystal fractionation of mantle-derived magma. The Nd/Th ratio (2.11~12.27) is distributed between the crust-derived rock value (Nd/Th ≈ 3) [94] and the mantle-derived value (>15) [95], suggesting that the magma originated from partial melting of mantle materials and was mixed with crustal materials. In addition, the low Sr/Y ratio (<20) of the quartz diorite samples from the Kekesha intrusion indicates that it is an arc granite rather than an adakitic rock, and the Y-Sr/Y diagram shows that all samples are located in the basic rock partial melting curve of the normal arc magmatic rock area (Figure 12a). The experimental research on the melting of different source rocks in the lower crust of the continent shows that the partial melting of the basic rocks in the lower crust of the continent forms a relatively basic diorite magma. In the discrimination diagram of (Al₂O₃ + FeO^T + MgO + TiO₂)-Al₂O₃/(FeO^T + MgO + TiO₂) source region (Figure 12b), the quartz diorite sample falls into the partial melting area of the basic rock. In the Zr-Zr/Nb diagram, which reflects magma evolution (Figure 10d), the samples are positioned between the partial melting and magma mixing lines, suggesting that partial melting was the primary process in rock formation, with a smaller contribution from magma mixing. The primitive mantle-normalized trace element spider diagram shows similarities to the gabbro from the Kekesha intrusion, being enriched in LILEs and LREEs, while relatively depleted in HREEs and HFSEs. Notably, it exhibits typical island-arc signatures, including depletion in Nb, Ta, and Ti, which are indicative of a subduction zone setting (Figure 9b,d). Therefore, it is inferred that the quartz diorite of the Kekesha rock mass originated from the partial melting of the lithospherically enriched mantle rock, metasomatized by slab fluids and mixed with crustal material introduced by slab subduction.

6.2.3. Magma Source of Granodiorite

The Nb/Ta ratios of the granodiorite from the Kekesha intrusion range from 13.69 to 27.75, with an average of 18.8, which is higher than the continental crust average of 11. The Zr/Hf ratios range from 36.98 to 47.98, approaching the mantle value of 37, indicating a mantle-derived source for the granodiorite. The rock exhibits high SiO₂ (67.17–72.59%) content, elevated Al₂O₃ (14.75–16.54%), and significant K₂O + Na₂O (6.19–8.58%) content, along with high Mg^# values (33.63–46.48). The content levels of Sr, Y, (La/Yb)_n, and Yb are 261–560 ppm, 9.89–12.4 ppm, 13.07–17.43, and 1.07–1.40 × 10⁻⁶, respectively. Except for samples 11006-1 and 11030-1, most of the samples do not conform to the typical characteristics of adakitic rocks (SiO₂ > 56%, Al₂O₃ > 15%, Sr > 400 × 10⁻⁶, Y < 19, very low HREEs, and Yb < 1.9 ppm). In the Y-Sr/Y diagram (Figure 12a), the samples plot within the overlapping zone of normal island-arc and adakitic rocks. In the (Al₂O₃ + FeO^T + MgO + TiO₂)-Al₂O₃/(FeO^T + MgO + TiO₂) diagram (Figure 12b), the majority of the granodiorite samples fall into the partial melting region of basic rocks. Previous studies suggest that adakites are formed through partial melting of young, hot subducting plates or thickened lower crust [96,97]. In addition, in a mature continental arc environment, granites can undergo partial melting, resulting in granitic compositions with high Sr/Y ratios that resemble the properties of adakites [98]. The granodiorite in the Kekesha intrusion is enriched in Rb, Th, and Zr, while being depleted in Ba, Nb, Ta, P, and Ti, and exhibits characteristics typical of crust-derived materials [24,99]. This suggests that samples 11006-1 and 11030-1 may be mixtures of crustal materials with adakitic signatures. On the Zr-Zr/Nb and MgO-FeO^T diagrams (Figure 10c,d), the granodiorite samples from the Kekesha intrusion deviate from the expected trend of magma evolution and instead align with partial melting and magma mixing lines. Based on these observations, this paper hypothesizes that granodiorite forms through the crystal fractionation of basic magma, mixed with partial melting products from quartz diorite.

6.2.4. Magmatic Processes

In summary, the Kekesha intrusion completely covers all rock types of basic rock, intermediate rock, and acidic rock. With the evolution of rock to acidity, each lithology has a roughly gradual relationship in terms of mineral composition and main trace element content. The content level of Mg^# gradually decreases, and the intrusion differentiation index (DI) (gabbro 13.79~35.56; gabbro diorite 32.48~47.06; quartz diorite 43.17~52.14; and granodiorite 70.73~90.81) increases gradually. The chondrite-normalized rare earth element distribution diagram and the primitive mantle-normalized trace element spider diagram show roughly the same positive and negative anomaly trends (Figure 9), and the Harker diagram has a good linear relationship (Figure 7 and Figure 8), indicating that these four types of rocks come from the same or an interrelated magma source region. Combined with the zircon U-Pb age determination results for the Kekesha intrusion in this paper, they are gabbro (515.7 ± 7.4 Ma, 3998-1), gabbro diorite (508.9 ± 9.8 Ma, 11006-1), quartz diorite (499.6 ± 4.0 Ma, 11009-1), and granodiorite (502.3 ± 9.3 Ma, 11009-1 and 501.6 ± 6.4 Ma, 11030-1), which shows that the age of the rock formation becomes gradually younger during the evolution from basic to acidic. This paper proposes that the formation of the Kekesha intrusion occurred during the subduction of the oceanic crust. The heat generated by the subducting slab, along with the fluids released during slab dehydration (which are rich in H₂O and large-ion lithophile elements such as K, Rb, Sr, U, Th, and rare earth elements), induced partial melting of the lithospheric mantle material. At depths of 75–80 km, a small amount of asthenospheric mantle material mixed with the lithospheric mantle, resulting in the formation of basic magma (515.7–508.9 Ma). Due to the difference in relative density, the basic magma migrates upward and resides at the bottom of the thick and light continental crust. These hot basic magmas provide heat for the partial melting of basic rocks in the lower crust and form new magmas, which are then mixed with basic magmas, and assimilated and homogenized (MASH) to form basic or intermediate diorite magmas (508.9–499.6 Ma). During the continuous subduction, basic or intermediate mixed magmas are continuously generated and accumulate at the bottom of the lower crust, and the temperature and pressure of the magma gradually increase, which leads to the crystal fractionation of the magma. The basic minerals such as pyroxene, hornblende (most of which forms hornblende crystals due to rich water-bearing fluids), and plagioclase first begin to crystallize and form crystal mush. The pressure between crystals leads to the retention of more basic crystals, while more intermediate melts migrate upward and stay in the middle crust due to pressure and density differences [100] and continue to crystal fractionation to generate intermediate-acid magma. With the decrease in temperature and pressure, the crystal fractionation continues, and the residual melt forms anhedral minerals (such as quartz and potassium feldspar) between the crystals. Finally, it is mixed with a small amount of crustal material to form an intermediate or acidic granitic magma (502.3–499.6 Ma). These different melting source regions and degrees of crystal fractionation led to intrusions in different lithologies, forming a complete set of cross-stratigraphic island-arc magmatic systems with a time transition relationship (Figure 13) [100,101,102].

6.3. Tectonic Environment

6.3.1. The Tectonic Environment of Gabbro and Gabbro Diorite

Two primary tectonic environments have been proposed for the formation of Early Paleozoic mafic rocks in the SKT: (1) the opening of the “Buqingshan Paleo-Ocean Basin” (previously part of the Proto-Tethys Ocean) along the southern margin of the East Kunlun, followed by subsequent northward subduction into an island-arc environment [29,30,32] and (2) a back-arc basin environment in the Qingshuiquan–Kekesha–Kekekete region of the CKM and SKT [7,12,22,39,45,46,48,78,103]. This study examines the gabbro and gabbro diorite samples from the Kekesha intrusion. Based on the formation age, the gabbro (515.7 ± 7.4 Ma) in the Kekesha intrusion was formed during the early stage of northward subduction of the “Buqingshan ancient ocean”. In terms of major elements, all sample points are located in the continental margin arc domain (comprising island-arc calc-alkaline basalt, ICA, and island-arc basalt, IAB) (Figure 14). Therefore, it is inferred that the gabbro and gabbro diorite in the Kekesha intrusion were formed within an island-arc tectonic environment due to subduction.

6.3.2. The Tectonic Environment of Quartz Diorite and Granodiorite

The tectonic environment of the Early Paleozoic diorite and granite in the SKT is relatively uniform. The “Buqingshan Paleo-Oceanic Basin”, located on the southern margin of the East Kunlun terrane, expanded and continued to subduct northward. During the Middle Cambrian, the East Kunlun terrane transitioned into an active continental margin, characterized by a trench-island arc-back-arc basin system. In this context, the Manite–Berechet–Yikehara magmatic arc, nearly parallel to the Early Paleozoic ophiolite zone on the southern side, was formed in the SKT [9,25,27,28,29,30,31,32,61]. The quartz diorite (499.6 Ma) and granodiorite (502.3 Ma, 501.6 Ma) in the Kekesha intrusion are slightly later than the gabbro and gabbro diorite samples (515.7 Ma, 508.9 Ma) in terms of formation age. In terms of geochemical characteristics, they have the characteristics of the medium-high-K calc-alkaline series (Figure 6b) and a right-leaning rare earth element distribution curve (Figure 9a,c) similar to that of the gabbro and gabbro diorite samples. The negative anomaly of Eu is more obvious, which shows it has roughly the same source region, but the stronger crystal fractionation is carried out in the process of magmatic evolution. The primitive mantle-normalized trace element spider diagram shows that it is enriched in large-ion lithophile elements (LILEs) (Cs, Rb, Ba, and K) and depleted in high-field-strength elements (HFSEs) (Nb, Ta, and Ti), showing it has the characteristics of island-arc magmatic rocks related to the subduction zone (Figure 9b,d) [106,107]. Furthermore, tectonic background diagrams (Rb-Y + Ta, Nb-Y, Rb-Y + Nb, and Ta-Yb diagrams, Figure 15a–d) show that the quartz diorite samples plot within the volcanic arc granite field area. Additionally, there are several reports of Early Paleozoic adakites in the SKT, such as Manite arc granite (479~487 Ma) in the Buqingshan area and adakitic granodiorite (438 Ma) formed by the melting of oceanic slab in the subduction zone of the Yikehalaer area [27]. Quartz diorite–tonalite (498~486 Ma) and hornblende gabbro (492 Ma) with TTG granite properties have the obvious geochemical characteristics of the magmatic arc granite in Maerzheng area [108]. Therefore, it is inferred that the quartz diorite and granodiorite of the Kekesha intrusion were also formed in the island-arc tectonic environment under subduction, and that the crustal material was mixed in the later stage of magmatic evolution. It is suggested that part of the crustal material is likely to be the surrounding adakitic rock.

6.3.3. Tectonic Setting

Combined with a series of subduction-related magmatic intrusions and volcanic rock stratigraphic records in the CKM and NKT, the subduction of oceanic crust may have begun in the late Early Cambrian–Middle Cambrian, and with the northward subduction of the oceanic basin, a series of subduction-related island arcs, back-arc basins, and magmatic and metamorphic events related to collisional orogeny occurred in the SKT.

Based on a comprehensive analysis of regional magmatic activity, it is clear that the island-arc magmatic rocks of the SKT are significantly older than the magmatic rocks produced in the back-arc extension environment of the NKT. This supports the conclusion that the study region was part of an active continental margin where the Proto-Tethys Ocean was subducting northward, with the MBAM zone serving as the main oceanic basin. In the Kekesha intrusion, the older gabbro and gabbro diorite are primarily distributed in the northeast, while the younger quartz diorite and granodiorite are mainly found in the southwest. This distribution aligns with the regional magmatic proliferation during the Early Paleozoic. The formation age of the Kekesha intrusion is earlier than that of the Dundeshaerguole hornblende monzonite (544.8 Ma) [7,23], while the basic magmatic rocks in the Kekesha intrusion are slightly older than the ophiolite or mafic–ultramafic mélange (509–452 Ma) found in the Qingshuiquan–Kekesha–Kekekete area [22,48,68]. These observations suggest that, at 544.8 Ma, the SKT was experiencing an extensional environment, with no evidence of subduction or island-arc magmatism (Figure 16a). As the Buqingshan paleo-ocean expanded, the Proto-Tethys Ocean began to subduct northward, leading to the development of an active continental margin characterized by a trench-island arc-back-arc basin system. This subduction process resulted in the formation of island-arc-type basic magmatic rocks in the Kekesha area (516–509 Ma) (Figure 16b). The ongoing extension and expansion of the back-arc basin gave rise to a small back-arc ocean basin along the Qingshuiquan–Kekesha–Kekekete region (509–452 Ma) [22,48,68]. Furthermore, the intermediate to acidic magmatic rocks (502–500 Ma) of the Kekesha intrusion were formed after the onset of the small back-arc ocean basin in the Qingshuiquan–Kekesha–Kekekete area (Figure 16c), but prior to the formation of the Manite–Berichet–Yikeharaer magmatic arc on the southern margin of the SKT (493–438 Ma) [9,25,27,29,30,31,32,62,68]. This temporal relationship suggests that the Kekesha intrusion represents island-arc magmatism occurring in the early stages of subduction. With the continued expansion of the back-arc ocean basin in the Qingshuiquan–Kekesha–Kekekete area, island-arc magmatism migrated southward from the Kekesha area to the Yikehalaer region (Figure 16d). In summary, the Kekesha intrusion is a record of island-arc magmatic activity associated with the initial phase of the northward subduction of the Proto-Tethys Ocean.

7. Conclusions

(1) The Kekesha intrusion is an island-arc magmatic rock with quartz diorite as the main body. It consists mainly of gabbro, gabbroic diorite, quartz diorite, and granodiorite. The zircon U-Pb ages for these rocks are as follows: gabbro (515.7 ± 7.4 Ma), gabbroic diorite (508.9 ± 9.8 Ma), quartz diorite (499.6 ± 4.7 Ma), and granodiorite (502.3 ± 9.3 Ma and 501.6 ± 6.4 Ma). These data indicate that the intrusion formed during the Early Paleozoic, in the Middle Cambrian.

(2) The petrographic and geochemical characteristics indicate that the Kekesha intrusion is a typical island-arc intrusive complex. It is mainly composed of gabbro from the high-alumina calc-alkaline basalt series, gabbro diorite from the medium-high potassium calc-alkaline basalt series, quartz diorite from the quasi-aluminous medium-high-potassium calc-alkaline I-type granite series, and granodiorite from the weakly peraluminous calc-alkaline I-type granite series. The presence of these rock types reflects the complex geochemical evolution during the formation of the rock mass, highlighting its close relationship with magmatic activities in an island-arc environment.

(3) Under the influence of oceanic crust subduction, high temperature, and slab-derived fluids, partial melting occurred in the lithospheric mantle, depositing material in the lower crust and forming the primary magma for the Kekesha intrusion (515.7–508.9 Ma). Due to differences in relative density and elevated temperatures, the basic magma accumulated in the lower crust, partially melting the lower crust and promoting magma mixing. The magma then ascended, collected in the middle crust, and underwent crystal fractionation, resulting in the dioritic magma that constitutes the main body of the Kekesha intrusion. Subsequently, the magma continued to fractionate and mixed with crustal material, forming a small amount of granitic magma. This process established a time-transgressive, cross-stratified island-arc magmatic system.

(4) Based on the synthesis of this study and regional geological data, it is inferred that the Proto-Tethys Oceanic crust began subducting northward during the Middle Cambrian (516–500 Ma). The Kekesha intrusion represents an island-arc magmatic complex formed by the subduction of oceanic crust in the eastern part of the East Kunlun Orogenic Belt. It provides direct evidence and material records of the initiation of the Proto-Tethys Ocean’s subduction.