J. Cent. South Univ. (2019) 26: 3420-3435
DOI: https://doi.org/10.1007/s11771-019-4264-7
Geochronology, petrogenesis and tectonic significance of Dahongliutan pluton in Western Kunlun orogenic belt, NW China
DING Kun(丁坤)1, LIANG Ting(梁婷)1, 2, YANG Xiu-qing(杨秀清)1, 2, ZHOU Yi(周义)1,
FENG Yong-gang(凤永刚)1, 2, LI Kan(李侃)3, TENG Jia-xin(滕家欣)3, WANG Rui-ting(王瑞廷)4
1. College of Earth Sciences and Resources, Chang’an University, Xi’an 710054, China;
2. Mineralization and Dynamics Laboratory of Chang’an University, Xi’an 710054, China;
3. Xi’an Geological Survey Center, China Geological Survey, Xi’an 710054, China;
4. Northwest Nonferrous Metals Geological Mining Group Limited Company, Xi’an 710054, China
Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
Abstract:
The Dahongliutan granitic pluton, in the eastern part of the West Kunlun orogenic belt, provides significant insights for studying the tectonic evolution of West Kunlun. This paper presents a systematic study of LA-ICP-MS zircon U–Pb age, major and trace elements, Sr-Nd-Hf isotopes, and the first detailed Li isotope analysis of the Dahongliutan pluton. LA-ICP-MS zircon U–Pb dating shows that the Dahongliutan granites were emplaced in the Late Triassic ((213±2.1) Ma). Geochemical data show relatively high SiO2 contents (68.45 wt%-73.62 wt%) and aluminum saturation index (A/CNK=1.11-1.21) indicates peraluminous high-K calc-alkaline granite. The Dahongliutan granites are relatively high in light rare earth elements (LREE) and large ion lithophile elements (LILEs) (e.g., Rb, K, Th), and relatively depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, Ti). The εNd(t) values range from -8.71 to -4.73, and (87Sr/86Sr)i=0.7087-0.71574. Zircons from the pluton yield 176Hf/177Hf values of 0.2826181 to 0.2827683, and εHf(t) values are around 0; the two-stage Hf model ages range from 0.974 to 1.307 Ga. The δ7Li values are 0.76‰-3.25‰, with an average of 2.53‰. Isotopic compositions of the pluton suggest a mixed trend between the partial melting of the Middle Proterozoic ancient crustal material and a juvenile mantle-derived material. This study infers that the Dahongliutan rock mass is formed in the post-collisional extension environment, when the collision between South Kunlun and the Tianshuihai terranes results in the closure of the Palaeo-Tethys. The mantle-derived magma results in partial melting of the lower crust.
Key words:
Cite this article as:
DING Kun, LIANG Ting, YANG Xiu-qing, ZHOU Yi, FENG Yong-gang, LI Kan, TENG Jia-xin, WANG Rui-ting. Geochronology, petrogenesis and tectonic significance of Dahongliutan pluton in Western Kunlun orogenic belt, NW China [J]. Journal of Central South University, 2019, 26(12): 3420-3435.
DOI:https://dx.doi.org/https://doi.org/10.1007/s11771-019-4264-71 Introduction
The West Kunlun Orogenic Belt (WKOB) in the southwestern part of Xinjiang is an important part of the Central Orogenic Belt of China, as it occupies a key tectonic junction between the southward growth of the Paleo-Asian continent and the evolution of the Tethyan tectonic domains from the early Paleozoic to Mesozoic [1, 2]. The WKOB is of importance to research, with tectonic movement, frequent magmatic activity, wide distribution of granitoid rocks, and a large range of ages. Many researches have been conducted on the precise time of closure of the Paleo-Tethyan ocean. It is generally agreed that the WKOB has undergone the collisional orogenic process in the Early Paleozoic (Calientian period); however, the formation and evolution of the Paleo-Tethyan ocean still remain controversial, and various processes have been proposed, principally that the Paleo- Tethyan ocean is subducting to the north or south [2-7], or that there is no the Paleo-Tethyan ocean in the area [8].
In order to explore the extinction process of the Paleo-Tethyan ocean from the eastern part of the WKOB, we selected the Dahongliutan rock mass, situated in the eastern part of the WKOB, to conduct systematic geological, geochemical and chronological work. Previous studies on the Dahongliutan rare metal pegmatite deposit were focused on the deposit’s geochemical characteristics, diagenesis and mineralization age, and the genesis of its deposit [9-14]. It has the guiding significance for understanding the regional tectonic evolution of the West Kunlun belt.
The diagenetic age of the granites, which is closely related to mineralization is still debated, and there has been a lack of systematic geochemical research, which restricts the understanding of the formation age, genesis and tectonic evolution of the granites in this area. This paper presents detailed field and microscopic geological research, as well as the first application of the Li isotopic analysis method, LA-ICP-MS zircon U–Pb ages, whole-rock geochemistry, and Sr-Nd-Hf isotopic data on the Dahongliutan pluton. These systematic results will finally provide significant insights into exploring the age and origin of these granites, and their relationship to the tectonic evolution of the WKOB.
2 Geological setting and petrographic characteristics
The WKOB, on the northwestern margin of the Tibetan Plateau, occupies a key geotectonic location, at the junction of the Indian plate and the Eurasian plate and is a composite orogenic belt formed by multiple arc-continent collisions (Figure 1). The magmatism in the WKOB, especially of the Caledonian and Indosinian-Yanshanian granites, is widespread from the early Proterozoic to the Mesozoic-Cenozoic. Geotectonic studies show that the evolution of the WKOB is complex, and that the belt is generally distributed in a giant inverted S-shape with a NW-SE trend. In recent studies, the orogen has been tectonically subdivided into four major units, from north to south: the West Kunlun North terrane (NKT in Figure 1), the West Kunlun South terrane (SKT in Figure 1), the Tianshuihai terrane (TST in Figure 1), and the Karakorum terrane (KKT in Figure 1) [19]. The Dahongliutan tectonic location is located in the Tianshuihai terrane (previous studies show that the Dahongliutan tectonic location is in the Late Paleozoic-Mesozoic Bayankala belt [20]). The present study focuses on the Tianshuihai terrane. A brief description of the geological setting of the terrane is given below.
Figure 1 Schematic diagram of distribution of Western Kunlun granite (after ROBINSON [15], ZHANG et al [16], LIU et al [17], WANG et al [18])
The geographically remote Tianshuihai terrane is the western extension of the Songpan-Ganzi block and covers widespread glacy. The accretionary complex of the Tianshuihai terrane consists of the neritic facies of a volcanic-clastic- carbonate rock association of a Precambrian basement (the Kangxiwa group and middle Meoproterozoic Tianshuihai group).
The Dahongliutan rock mass is located between the Tianshuihai terrane in the South China plate (also known as the Tibetan plate) to the north, and the Mazha-Kangxiwa suture to the south, and adjacent the Dahongliutan fault (Figure 2). The secondary faults and joint structures are mainly developed by SN and EW directions. The characteristics of the fracture zone are very obvious, and its strike matches well with the regional structure.
The Dahongliutan pluton is dominated by Indosinian granites, and the lithology mainly consists of biotite monzonitic granite, biotite soda granite and two-mica granite (Figure 3). The biotite monzonitic granite is located in the northwest of the Dahongliutan rock mass, and the two-mica granite is located in the southeast of the Dahongliutan granite. It is discovered through field observation that the biotite monzonitic granite is intruded by the two-mica granite. The abundant mafic microgranular enclaves are contained in the Dahongliutan pluton (Figure 3(b)). The enclaves consist of plagioclase + K-feldspar + amphibole + biotite + quartz, are notably darker-colored and finer-grained than the host rocks, and generally ellipsoidal with diameters up to a few tens of centimeters and sharp contacts with the host granite, which implies microgranular enclaves may have formed through deep-level hybridization of enriched mantle magmas with crustal melts.
3 Sampling and analytical methods
3.1 Sampling
In this study, seven samples from the Dahongliutan pluton were collected for whole-rock chemistry and whole-rock Li-Sr-Nd-Hf isotope analyses. Sample D002 was selected for zircon U–Pb dating. All samples were powdered into 75 μm using an agate mill for whole-rock geochemical analysis. The petrography of the samples is described below.
All the granitic rocks were slightly altered with feldspars showing localized sericitization and mafic minerals showing localized chloritization. Sample D002 is located along the south of Zhanbei Bridge on 219 National Highway. The biotite monzonites are gray-white in color and show a medium texture. The mineral assemblage is comprised of quartz (30%-35%), plagioclase (25%-30%), K-feldspar (25%-30%) and biotite (10%-15%), with the accessory minerals of zircon, apatite, epidote, and sphene (<5%); it has massive structure and medium to coarse grained texture (Figures 3(a) and (e)). The K-feldspar is commonly subhedral and shows evidence of partial kaolinization. The plagioclase is generally subhedral, and is partly altered, resulting in sericite and shows irregular stripes on the surface of K-feldspar.
Figure 2 Geological map of Kangxiwa-Dahongliutan area
Figure 3 Field characteristics and photomicrographs of granitic rocks from Dahongliutan:(Grt-Garnet; Tur-Tourmaline; Ab-Albite; Mc-Microcline; Chl-Chlorite; Bt-biotite)
Sample D006 is located on the 219 National Highway about 3 km from Shenxianwan. The two-mica granites are gray-white in color and granitic texture and massive structure. They are mainly composed of quartz (30%-40%), plagioclase (20%-25%), biotite (15%-20%), muscovite (15%-20%) and K-feldspar (10%-15%). Accessory minerals are tourmaline, garnet and zircon (Figure 3(d)). The biotite is dark-gray in color and shows an orientated flake assemblage, containing quartz, apatite, zircon and tourmaline inclusions, and partly occurs as chlorite.
3.2 Analytical methods
3.2.1 Major and trace element analyses
Major and trace elements were obtained at the Mineralization and Dynamics Laboratory of Ministry of Land and Resources, Chang’an University, Xi’an, China. Major elements were analyzed by XRF (Rikagu RIX 2100) with analytical uncertainties of 5%. Trace and rare earth elements were analyzed by ICP-MS (Agilent 7500a) with analytical uncertainties of approximately 5%.
3.2.2 LA-ICP-MS U–Pb and trace element analyses
Zircons for LA-ICP-MS U–Pb dating were handpicked under a binocular microscope, then zircon grains were mounted in epoxy. Zircon cathodoluminescence (CL) image and zircon U-Pb isotopic dating were completed at the Mineralization and Dynamics Laboratory of the Ministry of Land and Resources, Chang’an University, Xi’an, China. The spot size used was 30 μm in diameter. Detailed operating conditions and analytical procedures were described by LIU et al [21]. Calculations of zircon isotope ratios were performed by ICP-MS DataCal [21], and age calculations were performed using Isoplot 3.7 software [22]. The errors for the mean ages are quoted at 90% confidence levels.
3.2.3 Zircon in situ Hf isotopic analyses
In situ zircon Lu-Hf isotopic analysis was performed using a Nu PlasmaHR MC-ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. The spot size was used for the analysis with a beam diameter of 43 μm, the laser repetition rate was 5 Hz, and the energy density applied was 6 J/cm2. The GJ-1 standard solution was used for monitoring the condition of the analytical instrumentation during Hf isotope measurement. Details of instrumental conditions and data acquisition methods were as described by YUAN et al [23].
3.2.4 Sr-Nd isotopic analyses
The Rb-Sr and Sm-Nd isotopic analyses were conducted at the Laboratory of Beijing Kehui Testing Technology Limited, using a Thermo Finnigan Triton Thermal Ionization Mass Spectrometer (TIMS). During analysis, the NBS- 987 Sr standard and JNdi Nd standard were used as the reference standard, with average values for the 87Sr/86Sr ratio of 0.710269±19 (n=9), and for the 143Nd/144Nd ratio of 0.512185±16 (n=28) respectively. Chemical separation and analytical details were the same as described by ZHANG et al [24].
3.2.5 Li isotopic analyses
Lithium isotopic ratios after column separation were analyzed at the Laboratory of Beijing Kehui Testing Technology Limited, China using a Nu Plasma MC-ICP-MS. The NIST L-SVEC standard solution was measured to optimize the isotopic compositions for the instrument parameters. The analytical errors for Li concentration measurement were <±10%, as estimated from the reproducibility of standard rocks [25, 26].
4 Results
4.1 Whole-rock geochemistry
The pluton contains relatively high SiO2 contents (68.45 wt%-73.62 wt%, average of 71.38%), low K2O (3.70 wt%-5.05 wt%, average of 4.45%) and P2O5 (0.07 wt%-0.20 wt%, average of 0.13%) contents, which is close to the granites with high S content (P2O5=0.14%), and distinctly different from the A-type granite [27]. These rocks are all peraluminous with A/CNK (1.11-1.21)>1.0 (Table 1 and Figure 4(a)). The total alkali content of the rock (K2O+Na2O) is (7.11%-7.96%), and the alkali aluminum index (AKI) is 0.60-0.66. As shown in the K2O vs SiO2 diagram (Figure 4(b)), all samples fall in the high-K calc-alkaline series [28, 29].
In summary, the Dahongliutan granites are generally high in SiO2 and K2O, and the A/CNK values are greater than 1.1; they contain aluminum minerals such as muscovite and garnet, which are peraluminous high-K calc-alkaline granite. The overall characteristics are consistent with S-type granite [30].
The samples are characterized by relatively low ΣREE (38.52×10-6-196.76×10-6) and enriched in light rare earth elements (LREE) and large ion lithophile elements (LILEs) (e.g., Rb, K, Th), depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, Ti) (Figure 5(a)) with the flat heavy rare earth elements (HREE). The biotite monzonitic granite (D002) shows a more marked negative Eu anomaly than D006 (Figure 5(b)), which suggests that plagioclase fractionation is also required after partial melting. This difference may be related to the difference in formation depth between the two plutons; the biotite monzonitic granite is shallower in formation depth.
4.2 LA-ICP-MS U–Pb dating of zircons
Zircons from the biotite monzonitic granite were analyzed for U–Pb dating. The results are summarized in Table 2, and illustrated in Figure 5.
Table1 Major and trace element and Li isotopic compositions for Dahongliutan granites (%)
Continued
Figure 4 A/NK vs A/CNK (a) (according to MANIAR et al [28]), and w(K2O) vs w(SiO2) (b) from Dahongliutan pluton (according to PECCERILLO et al [29])
The lengths of the zircons are between 80 and 240 μm with aspect ratios of 1:2-1:3. All zircons show regular magmatic oscillatory zoning, and have wide ranges of U (315.4×10-6-2853.1×10-6) and Th (92.7×10-6-429.4×10-6) contents, and moderate Th/U ratios (0.095-0.5085) (Table 2). Twenty-eight zircon cores from sample D002 plot in a group on the Concordia curve, yielding a weighted mean 206Pb/238U age of (213±2.1) Ma (N of 28, mean square of weighted deviates of 1.3). CL images of representative zircon grains are shown in Figure 6(a).
Figure 5 Diagrams of chondrite-normalized REE patterns (a) and primitive mantle-normalized multi-element plots (b) from Dahongliutan pluton (Normalizing values are from Ref. [31])
Table 2 LA-ICP-MS U–Pb isotopic data of zircon for Dahongliutan biotite adamellite
4.3 Hf isotopes of zircons
The results of in situ Hf isotope measurements for the zircon grains are listed in Table 3, and illustrated in Figure 6(b). All samples have low 176Lu/177Hf ratios, from 0.00062 to 0.001494 (all <0.002), revealing little radioactive Hf accumulation after the formation of the rock mass. The zircon 176Hf/177Hf ratio can, therefore, be used to explore the genetic information on rock generation [32]. Fourteen representative magmatic zircons from the biotite monzonitic granite sample were analyzed. The initial 176Hf/177Hf ratios vary from 0.2826181 to 0.2827683, and εHf(t) values range from -0.91 to 4.3 (average of 1.91). The single-stage Hf model ages (TDM1) range from 0.695 to 0.897 Ga (average of 0.789 Ga), and the two-stage Hf model ages range from 0.974 to 1.307 Ga (average of 1.128 Ga).
4.4 Whole-rock Li-Sr-Nd isotopes
Whole-rock Rb-Sr and Sm-Nd isotopic compositions for the Dahongliutan granites are listed in Table 4. The initial 87Sr/86Sr value and εNd(t) value were calculated back to the LA-ICP-MS U–Pb zircon age (213 Ma). The granites had initial 87Sr/86Sr ratios of 0.7087 to 0.71574 and εNd(t) values of -8.71 to -4.73. Lithium isotopic compositions for the Dahongliutan granites are reported in Table 1. The δ7Li content of 7 granite samples from Dahongliutan was 0.76‰-3.25‰ (average of 2.53‰).
Figure 6 CL images and U–Pb ages of zircons in biotite adamellite, the smaller solid circles represent locations for U–Pb analyses, and the bigger dotted circles locations for LA-MC-ICP-MS in situ Hf isotopic analyses (a) and εHf(t)-t diagram for zircon in biotite adamellite (b)
Table 3 LA-MC-ICP-MS Lu-Hf isotopic data of zircon for Dahongliutan biotite adamellite
Table 4 Sr-Nd isotopic data for Dahongliutan granites
5 Discussion
5.1 Diagenetic age of Dahongliutan pluton
As described above, previous researchers have carried out extensive isotopic dating on the Dahongliutan rock mass. Lots of chronological results were found in published literatures. The Second Geological Brigade of Xinjiang Geological and Mineral Bureau (1985) utilized the biotite K-Ar dating method to determine the biotite monzonite age of 163.44 Ma. QIAO et al [9] obtained the age of (220±2.2)-(217.4±2.2) Ma by SHRIMP zircon U–Pb dating (monzonitic granite); WEI et al [13] reported the age of (209.6±1.5) Ma by LA-ICP-MS zircon U–Pb dating (diamica granite), ZHANG et al [14] measured the age of granites by SIMS zircon U–Pb dating as (217.5±2.8) Ma.
Although some previous geochronology work has been carried out for some granites on the Dahongliutan pluton, the ages reported have varied widely, and are still a matter of debate. This new zircon LA-ICP-MS dating on the biotite monzonitic granite samples (D002) yielded crystallization ages of (213±2.1) Ma. The results are relatively close to those of QIAO et al [9], WEI et al [13], and ZHANG et al [14], which is consistent with the geological fact that there is no obvious contact boundary between the biotite monzonitic granite and the granites in the Dahongliutan deposit, indicating that the biotite monzonite is a part of Dahongliutan granite.
The results of this study show the high-precision diagenetic ages of the granites in the Dahongliutan pluton are all (220±2.2)-209.6 Ma. This indicates that they are the result of magmatic activity, and formed in the Late Triassic magmatism.
5.2 Magma source areas
Variation of Rb-Sr-Ba compositions in peraluminous granite is related to pelite-derived melts, and to the remelting mechanism of psammite-derived melts. Compared to clay-rich sources, the plot of Rb/Sr vs Rb/Ba ratios probably reflect the lower Rb/Ba and Rb/Sr ratios of clay-poor ones [34]. In the Rb/Ba-Rb/Sr diagram (Figure 7(a)), the granite samples in this study lie closely to the clay-poor source trend.
Experimental results suggest that the CaO/Na2O ratios of peraluminous granites (pelite-rich, plagioclase-poor) are less than 0.3; in particular, the ratios of the granites (pelite-poor, plagioclase-rich) are >0.3 [34]. The CaO/Na2O ratios of the Dahongliutan granite are between 0.50 and 0.88, and the total is greater than 0.3, which shows the typical characteristics of melts derived from a psammite source. In comparison with five typical peraluminous granite fields in the world (the Alps, Himalaya, and Hercynian, Caledonian, and Lacklun belts), post collisional granites in different orogenic belts were divided into pelite-derived melts and psammite-derived melts by their inherit different Al2O3/TiO2 and CaO/Na2O ratios. As shown in Figure 7(b), the Al2O3/TiO2 and CaO/Na2O ratios of the Dahongliutan rock mass samples support their derivation mainly from clay-poor psammite, consistent with the results on Rb-Sr-Ba content variation.
The Dahongliutan granites show more similar Mg# contents to pure crustal partial melts than mantle melts (Figure 8), suggesting that they might also be produced by partial melting of crustal rocks and that a small amount of mantle-derived magmas are mixed into the crustal melts. This view is reinforced by their Sr-Nd-Hf isotopic compositions. Li-Sr-Nd-Hf isotope geochemical analyses can be used to investigate the source region of granites. The Dahongliutan pluton displays higher initial 87Sr/86Sr ratios, 0.7087-0.71574; (143Nd/144Nd)i 0.511918-0.512122; a wide range of δ7Li values from 0.76‰ to 3.25‰; and low negative to low positive εHf(t) values, -0.91-4.3. It is generally believed that the initial 87Sr/86Sr ratios of chondrite are 0.699, the initial 87Sr/86Sr ratios of MORB are 0.7020-0.7060, and the initial 87Sr/86Sr ratios of continental crust are usually greater than 0.719. XIAO et al [34] proposed that the initial 87Sr/86Sr ratios of granite are between 0.706 and 0.719, the magma source is a primary crust source with juvenile mantle-derived materials. The (87Sr/86Sr)i of the Dahongliutan rock mass shows that the magma is derived from a crust source, but it is still difficult to determine the exact source area of the pluton. Values of εNd(t) (-8.71 to -4.73) are the characteristic of the crust, except for one sample that was less than 0, and this interpretation is consistent with the information from the strontium isotopes. The Dahongliutan biotite adamellite has εNd(t) values higher than that of the two-mica granite, revealing the involvement of juvenile mantle materials in the diagenetic process of the granitic plutons. The value of εHf(t) varies from -0.91 to 4.3, but is mainly close to 0, and there is a certain gap away from the contemporary depleted mantle, indicating that the primitive magma has undergone a long period of crustal retention after separating from the depleted mantle.
Figure 7 Diagrams of Al2O3/TiO2 versus CaO/Na2O (a) and Rb/Sr versus Rb/Ba (b) after SYLVESTER [34]
Figure 8 Diagram of SiO2 versus Mg#
The range of δ7Li values is mainly concentrated between 2.25‰-3.25‰, which is between the values expected for mantle (3‰-5‰ [35]) and crust (the average value of δ7Li is 1‰ [25]), indicating that both a crustal source and lithospheric mantle components are involved in the magmatic source. NISHIO et al [36] conducted lithium isotope determination of mantle-derived mites in Sikhote Alin (eastern Russia) and Bullenmerri (eastern Australia) by MC-ICP-MS. They suggested that the δ7Li value of the Sikhotte Alin samples from enriched mantle I is extremely low, ranging from -17.1‰ to -3.1‰, and the Delta δ7Li values of the Bullenmerri samples from enriched mantle II are relatively high, ranging from 5‰ to 6‰. However, it is difficult for granitic magma to be directly melted from mantle peridotite [37], so it is unlikely that the Dahongliutan pluton was directly derived from partial melting of enriched mantle. This is also consistent with the magma mixing characteristics in a large number of mafic microgranular enclaves of the Dahongliutan pluton.
From a regional perspective, existing research has confirmed that the Muztagata pluton, Arkarz pluton, Alamas pluton, Ayilixi pluton and Datong pluton from the West Kunlun are all of mixed crust-mantle magma origin [38-42], the Arkarz pluton was formed by the intrusion of mantle-derived magma under the Late Triassic thickened continental crust [43]. In summary, it is considered that the Dahongliutan pluton is formed in the thickened continental crust as a result of the underplating of mantle-derived magma.
5.3 Tectonic settings
The Dahongliutan muscovite-bearing granite with low content of heavy rare earth elements (HREE), and presence of the aluminum-rich minerals muscovite and garnet as proposed by BARBARIN [44], is a partially melted product of thickened continental crust. This suggests that the muscovite granites are derived from continental crust thickening or a continental collision environment, which accords with the view that the K-rich calc-alkaline granites revealed by DUCHESNE et al [45], are most likely formed in a transitional setting, from an extruded setting to an extensional one. In addition, in the Rb vs (Y+Nb) and Rb vs (Ta+Yb) diagrams [30], the Dahongliutan granites share the characteristics of the volcanic arc granite and post-collisional granite area (Figure 9).
However, there are multiple solutions to the structural discriminant diagram, and the study of the tectonic setting of the rock mass should also be combined with that of the tectonic evolution of the whole region. The WKOB is a multi-stage orogenic belt with frequent magmatic activities and extensive distribution of granitoid rocks, spanning from Early Proterozoic to Mesozoic-Cenozoic. The Late Hercynian-Indosinian-Yanshanian period was the peak period of its the magmatism of the WKOB. The Indosinian granites were mainly distributed between the South Kunlun terrane and the Tianshuihai terrane (on the north and south sides of the Mazha-Kangxiwa suture zone) [4]. At this time, the area entered the evolution stage of the Paleo-Tethys ocean, and formed the Indosinian orogenic belt under the transformation from collisional compression to post-collisional extension [2]. After the Tianshuihai terrane collided with the South Kunlun terrane, the collision resulted in the end of oceanic crust subduction and the closure of the Paleo-Tethys Ocean, and the entire West Kunlun area was transferred to the intracontinental stage [46]. Previous study has confirmed that the Indosinian granite in the WKOB is closely related to the closure of the Paleo-Tethyan Ocean [9, 47]. However, the exact closure process of the Paleo-Tethys Ocean between the South Kunlun and the Tianshuihai terranes remains controversial and uncertain. The timing of the closure has been variously ascribed to the Late Permian [4], Middle and Late Triassic [48] and Late Jurassic [1]. The Mesozoic Indosinian granitic rock belt in the WKOB is the largest tectono-magmatic belt [6, 40] and is considered to be an important area for grasping the tectonic evolution of the WKOB. A lot of published geochronological data have been reported the Indosinian magmatic rocks in the West Kunlun in recent years (Table 5). The zircon U–Pb dating result presented in this study, and those presented in recent years, enable us to place a systematic and powerful constraint on the timing of the Indosinian tectono-thermal events in the WKOB.
Figure 9 Illustration of Rb vs Yb+Ta (a) and Nb+Y vs Rb (b) in Dahongliutan granite (Modified after PEARCE et al [30], Muztagata pluton data are from KANG et al [39], and Mazha pluton data are from LIU et al [52].)
Table 5 Isotopic age information for Indosinian intrusive rocks in West Kunlun
However, there is a lack of isotopic dating evidence of magmatic activity from 240-230 Ma (Middle-Late Triassic), and of the Late Permian to Middle Triassic sedimentary records in the western Kunlun area. The marine sediments were mainly deposited before the Late Permian, and continental deposition began in the Middle or Late Triassic, indicating that the transition period from the main collisional orogeny to the post-collisional orogeny should be between the Middle Triassic and the Late Triassic Isochron age [39]. ZHANG et al [55] investigated the structural characteristics of garnet- containing gneissic granite in the middle of the West Kunlun and the regional metamorphic events occurring at 240-230 Ma [47], proposing that the Paleo-Tethys Ocean subsided northward and finally closed at 240 Ma. The regional tectonic environment at this time was extruded.
Combined with the regional tectonic setting of the Yuqikapa rock mass [47], the Arkarz pluton [16], and the Mazha rock mass [17, etc., it is demonstrated that the Middle-Late Triassic granitoids in the West Kunlun are related to the Paleo-Tethys subduction and subsequent continental collision orogeny [17, 47, 56]. In the Rb vs (Y+Nb) and Rb vs (Ta+Yb) diagrams (Figure 9), the Dahongliutan granites, together with the coeval WKOB granitoids, plot near the domains of VAG + POG. Several typical Late Triassic granitoids in the WKOB which show similar ages to the Dahongliutan intrusions are also interpreted to have formed in a post-collisional setting, including granitoids in the Mazha area [57] and Muztagata area [39], suggesting they were probably generated in the same tectonic setting.
In summary, the continental collision between the South Kunlun and Tianshuihai terrane during the Middle-Late Triassic resulted in the northward reduction of the Paleo-Tethys ocean, as a consequence of progressive thickening of the continental crust and the formation of the Mazha- Kangxiwa suture, followed finally by the closure of the Paleo-Tethyan Ocean at 243 Ma. The Dahongliutan pluton was formed in the post-collisional extension environment. The observations are compatible with those described in YAN et al [10] and ZHANG et al [14].
6 Conclusions
1) The Dahongliutan pluton is mainly composed of biotite monzonitic granite, biotite soda granite and two-mica granite. Geochemical analysis shows that the Dahongliutan rock mass is peraluminous high-K calc-alkaline granite with the same characteristics as S-type granite and clay-poor psammite melts.
2) LA-ICP-MS U–Pb dating of zircons shows that the biotite monzonitic granite is emplaced at (213±2.1) Ma, related to the closure of the Triassic Paleo-Tethyan Ocean. Isotopic characteristics of zircon Hf and whole rock Li-Sr-Nd indicate that the rock mass is formed by the mixing of crust-mantle material.
3) Geochronological and regional geological evidence indicate that the Dahongliutan pluton is formed in the post-collisional environment during the collision orogenesis between the South Kunlun and the Tianshuihai terranes. It is produced by the partial melting of the primary crustal material with input of a juvenile mantle source.
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(Edited by FANG Jing-hua)
中文导读
西昆仑造山带大红柳滩岩体年代学、成因及其构造意义
摘要：西昆仑造山带东部大红柳滩花岗岩体对于理解西昆仑造山带大陆演化具有重要的指示意义。本文对该花岗质岩体主、微量元素、Li-Sr-Nd-Hf同位素和锆石的U-Pb年龄进行了系统研究。LA-ICP-MS锆石U-Pb年龄数据显示，大红柳滩花岗岩体的侵位时代为(213±2.1) Ma，说明大红柳滩岩体形成于晚三叠世。地球化学数据显示该岩石SiO2含量为68.45%~73.62%，铝饱和指数(A/CNK)变化区间为1.11~1.21，A/NK为在1.52~1.67，属于富钾钙碱性过铝质花岗岩。岩石富集Rb、Th、K等大离子亲石元素，亏损Nb、Ta、P、Ti等高场强元素；大红柳滩花岗岩锶同位素初始比值为(87Sr/86Sr)i=0.7087～0.71574，εNd(t)=-8.71~-4.73；锆石176Hf/177Hf比值为0.2826181~0.2827683，εHf(t)值在0附近，与同时期的亏损地幔的εHf(t)偏离较远，二阶段模式年龄(tDM2)为974~1307 Ma；花岗岩样品的δ7Li含量为0.76‰~3.25‰，平均2.53‰。推测岩体为中元古代古老地壳物质部分熔融形成，同时可能有少量幔源物质的加入。综合前人研究成果，本文认为大红柳滩岩体形成于南昆仑地体和甜水海地体发生陆-陆碰撞造山过程的后碰撞伸展环境下，幔源岩浆底侵作用引起下地壳部分熔融的结果。
关键词：LA-ICP-MS锆石U-Pb年龄；岩石地球化学；Li-Sr-Nd-Hf同位素组成；大红柳滩岩体；西昆仑
Foundation item: Project(2017YFC0602701) supported by the National Key Research and Development Plan, China; Project(DD20160004-8-3) supported by the Geological Survey of China
Received date: 2018-12-18; Accepted date: 2019-04-16
Corresponding author: LIANG Ting, PhD, Professor; Tel: +86-29-82339100; E-mail: liangt@chd.edu.cn; ORCID: 0000-0001-9286- 4276
Abstract: The Dahongliutan granitic pluton, in the eastern part of the West Kunlun orogenic belt, provides significant insights for studying the tectonic evolution of West Kunlun. This paper presents a systematic study of LA-ICP-MS zircon U–Pb age, major and trace elements, Sr-Nd-Hf isotopes, and the first detailed Li isotope analysis of the Dahongliutan pluton. LA-ICP-MS zircon U–Pb dating shows that the Dahongliutan granites were emplaced in the Late Triassic ((213±2.1) Ma). Geochemical data show relatively high SiO2 contents (68.45 wt%-73.62 wt%) and aluminum saturation index (A/CNK=1.11-1.21) indicates peraluminous high-K calc-alkaline granite. The Dahongliutan granites are relatively high in light rare earth elements (LREE) and large ion lithophile elements (LILEs) (e.g., Rb, K, Th), and relatively depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, Ti). The εNd(t) values range from -8.71 to -4.73, and (87Sr/86Sr)i=0.7087-0.71574. Zircons from the pluton yield 176Hf/177Hf values of 0.2826181 to 0.2827683, and εHf(t) values are around 0; the two-stage Hf model ages range from 0.974 to 1.307 Ga. The δ7Li values are 0.76‰-3.25‰, with an average of 2.53‰. Isotopic compositions of the pluton suggest a mixed trend between the partial melting of the Middle Proterozoic ancient crustal material and a juvenile mantle-derived material. This study infers that the Dahongliutan rock mass is formed in the post-collisional extension environment, when the collision between South Kunlun and the Tianshuihai terranes results in the closure of the Palaeo-Tethys. The mantle-derived magma results in partial melting of the lower crust.
