Abstract
The East Kunlun Orogenic Belt has undergone a composite orogenic process consisting of multiple orogenic cycles and involving many types of magmatic rocks spread over the whole district. However, due to bad natural geographical conditions and complex superimposed orogenic processes, most of the Caledonian orogenic traces were modified by the late tectonic uplift and denudation, so these rocks are poorly studied. Multiperiodic magmatic activity during the Late Silurian (approximately 420 Ma)–Late Devonian (approximately 380 Ma) exists in the Qimantagh area. We obtained 5 zircon U–Pb ages from the Late Silurian–Late Devonian granitoids in the Qimantagh area. Those ages are 420.6 ± 2.6 Ma (Nalingguole biotite monzogranite), 421.2 ± 1.9 Ma (Wulanwuzhuer potassium granite), 403.7 ± 2.9 Ma (Yemaquan granodiorite), 391.3 ± 3.2 Ma (Qunli granite porphyry), and 380.52 ± 0.92 Ma (Kayakedengtage granodiorite). These granitoids belong to the sub-alkaline, high-K calc-alkaline, metaluminous or weakly or strongly peraluminous series. The rocks are right oblique types, having overall relative LREE enrichment and HREE depletion, though rocks from different times may exhibit different degrees of Eu anomalies or overall moderate Eu depletion. The rocks are rich in large ion lithophile elements (LILE), such as Rb, Th, and K, and high field strength elements (HFSE), such as Zr and Hf, and are depleted in Ba, Nb, Ta, Sr, P, Eu, and Ti. The rocks have complex composition sources. The Late Silurian granitoids are mainly crust-derived. Most of the Devonian granitoids are crust-mantle mixed-source and only some parts of them are crust-derived, especially the Middle Devonian granitoids. Those mid-acidic and acidic intrusive rocks are formed in a post-collision tectonic setting, lithosphere delamination may have occurred in the Early Devonian (407 Ma), and the study area subsequently experienced an underplating of the mantle-derived magma at least until the Late Devonian (380 Ma).
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1 Introduction
The East Kunlun Orogenic Belt (EKOB) is a long (5000 km) tectonic belt across Mainland China and is a giant magmatic belt equivalent to the Gangdise belt in the Tibetan Plateau (Mo et al. 2007). The EKOB is located in the Northern Qinghai–Tibet Plateau, bounded by the Qaidam Basin to the north (Cheng 1994; Wang et al. 1990) and belonging to the western part of Mainland China’s central orogenic belt (Jiang 1992; Jiang et al. 2000; Yin et al. 1999; Liu et al. 2003) across the Paleo-Asian and Tethys tectonic domains. The belt has undergone a composite orogenic process consisting of multiple stages and cycles over several eras, such as the Early Paleozoic Caledonian cycle and the Late Paleozoic–Early Mesozoic Hercynian–Indosinian cycle. The plate subduction and collision in different periods have left their traces on this area (Jiang 1992; Pan et al. 1996; Yin and Zhang 1997, 1998; Zhu et al. 1999, 2005; Yang et al. 2009). Magmatic rocks of various types and ages are spread over the whole district, and the Caledonian and Variscan–Indosinian tectonic-magmatic activity is significant here. Previous research on the Variscan–Indosinian granites is relatively abundant, but due to the bad natural geographical conditions and the complex superimposed orogenic process, most of the Caledonian orogenic traces were modified by the late tectonic uplift and denudation, and are not well preserved, so these traces are poorly studied.
In recent years, many geologists (Bai et al. 2004; Wang et al. 2004; Zhao et al. 2008; Guo et al. 2011; Tan et al. 2011; Wang 2011) found multiple Late Silurian–Late Devonian granitoids in the Qimantagh region in the western part of the East Kunlun Mountains and researched their geochemical characteristics and tectonic environment. However, they never performed a specific and detailed discussion about the evolution process of the tectonic environment during this period. We have carried out detailed geological field studies, and with the help of systematic geochemistry and LA-ICP-MS zircon geochronological age data, confirmed that mid-acidic and acidic granitoids existed during the Late Silurian (approximately 420 Ma)–Late Devonian (approximately 380 Ma). Hence, we contribute to a beneficial discussion about the geological tectonic environment and evolution of the area providing more detailed information on the early Paleozoic orogenic cycle of the Qimantagh area, laying a foundation towards understanding the tectonic evolution of the EKOB and providing a basis for the Proto-Tethys Ocean changing into the Paleo-Tethys Ocean.
2 Geological setting
The EKOB is bounded by the Qaidam basin to the north, the Bayan Har-Songpan Ganzi block to the south, the Qinling–Dabie orogenic belt to the east and the NE-trending Altyn Tagh Fault to the west (figure 1a). The belt has the feature of north–south dissimilitude and west–east differentiation on regional geologic structures. The northern, central, and southern Kunlun faults (Huang et al. 1977, 1984; Jiang et al. 1994; Qu et al. 1994; Yang 1994; Cao et al. 1999) divide the Kunlun region into three distinct zones from north to south (Yuan et al. 2000), while Wutumeiren serves as the boundary between the western (Qimantagh area) and eastern (Dulan area) segments of the EKOB.
Geological sketch map of intrusive rocks in Qimantagh area, East Kunlun Mountains.
Qimantagh, across all of its geological tectonic units from north to south, is the widest and most exposed orogenic belt of the EKOB. According to Pan’s (2002) classification scheme, it is composed of five major tectonic units from north to south (figure 1a): the Qaidam continental block (I1), the northern early Paleozoic magmatic arc and junction belt of the Qimantagh (I2), the Kunlun block (I3), the South Kunlun subduction–collision complex rock zone (I4), and the Yulong–Bayankala marginal foreland basin (II1). There are also five important deep faults in the region corresponding to each unit: the Golmud buried fault (F5, the northern branch of the northern Kunlun fault), the Adatan fault (F4), the Nalingguole river fault (F3, the southern branch of the southern Kunlun fault), the central Kunlun fault (F2), and the southern Kunlun fault (F1).
The research of this article includes only the Qimantagh area, which is limited to the north of the central Kunlun fault (F2), the east of Yaziquan, and the west of Wutumeiren across multiple tectonic belts. Tectonic activities in the study area are well developed, and the main faults are F5, F4, F3, and F2 (figure 1a). These faults have obvious control significance to the regional tectonic-magmatic activity. The magma activity of the Qimantagh area is noticeably strong and has characteristics from multiple eras corresponding to four orogenic cycles: Precambrian (Proterozoic), Early Paleozoic (∈–D 3), Late Paleozoic–Early Mesozoic (D 3–T 3), and Late Mesozoic–Cenozoic (after Early Jurassic). The magmatic activity happened mainly during the Caledonian, Variscan, and Indosinian–Yanshan orogenies. The distribution of magmatic rocks, especially mid-acidic and acidic intrusive rocks, is very extensive, and the rock types include mainly granodiorite-monzonitic granite-moyite. The distribution of the pluton (long axis direction), controlled by regional tectonics, is to the north–west, which is consistent with regional tectonic lines (figure 1b). The strata outcropped in the study area are relatively complete and range from Archean to Cenozoic in age. The strata mainly include the following: Changcheng Period, Jinshuikou Group intermediate-deep metamorphic rocks; Jixian Period, Langyashan Group shallow metamorphic carbonates mingling with clastic rocks; upper and middle Ordovician shallow metamorphic clastic rocks mingling with a small amount of altered ultrabasic rocks; upper Devonian sandy conglomerates and volcanic rocks; Carboniferous carbonates and fine clastic rocks; lower Triassic carbonates and thinly layered siltstones; upper Triassic continental intermediate-acid volcanic series; and Cenozoic continental sandy conglomerates, siltstones and some loose accumulation.
The tectonic evolution of the EKOB underwent four orogenic cycles. A metamorphic crystalline basement formed in the Paleoproterozoic, and the tectonic environment belonged to a passive continental margin environment in the Mesoproterozoic–Neoproterozoic. The ancient Qaidam land collided with the micro-landmass of central Kunlun in the Late Neoproterozoic, while the EKOB formed under a wide range of events in the Cambrian–Ordovician, forming many ocean basins, such as the central Kunlun ocean basin and the northern side of the slightly smaller Qimantagh ocean basin. Ocean basins within the eastern segment of the EKOB formed and expanded towards the end of the Neoproterozoic and the Middle Cambrian, and subduction lasted from the Middle Cambrian to the Late Ordovician. The ocean basins had completely closed by the Late Silurian–Middle Devonian and subsequently experienced the principal collision–post-collision orogenic stages (Mo et al. 2007). The slight difference between the western and eastern segments of the EKOB is that the western segment of the central Kunlun Ocean subducted towards the north in the Late Ordovician to the Early Silurian, while the Qimantagh Ocean subducted from north to south. The ocean basin completely closed in the Early Silurian (the closing time is slightly earlier than that of the eastern segment), and the development of the continental molasse formation in the Late Devonian marks the end of the early Paleozoic orogenic cycle (at present, there is no unified theory of when the Qaidam block and the central Kunlun continent collided or when the study area turned into a post-collision environment). After the early Paleozoic orogenic cycle, the regional Paleo-Asian Ocean began converging from west to east (Xiao et al. 1990), while the Paleo-Tethys Ocean of the southern EKOB underwent extension (Fang and Liu 1996). The south Kunlun ocean basin in the EKOB formed in the Early Permian, inferring that the south Kunlun ocean basin began opening in the Early Carboniferous. The small, previously closed central Kunlun ocean basin opened once again and became the marginal sea or back-arc basin in the eastern segment of the EKOB. The EKOB was in the major subduction orogenic period in the Middle–Late Permian to Early Triassic (240–260 Ma), when the southern Kunlun ocean subducted towards the north, and entered the collision–post-collision intracontinental orogenic stage in the Late Triassic. The EKOB experienced lithosphere delamination (Gao and Jin 1997; Be’dard 2006; Chen et al. 2005) and an underplating of mantle-derived magmas at the end of the Late Triassic to the Early Jurassic, which resulted in the formation of the Qaidam basin (Mo et al. 2007). Following the Late Paleozoic–Mesozoic orogenic stage, the Tethys tectonic domain transferred into the Neo-Tethys tectonic evolution stage, and the subjected position of the Neo-Tethys also moved southward to the Bangong Nujiang and Yarlung Zangbo Suture Zones. In the Late Mesozoic–Cenozoic, East Kunlun only experienced remote effects, mainly displaying the formation and development of the thrust structure system, the shortening level of the continental crust, the formation and evolution of the Qaidam basin, and the strong uplift of the East Kunlun Mountains (Mo et al. 2007).
3 Sample descriptions
Five representative plutons were selected and 5 samples corresponding to each pluton in the Qimantagh were collected. The details are as follows.
Most of the Nalingguole pluton is covered by Quaternary sediments, so there are few exposed outcrops. The rock types of this pluton are primarily monzonitic granite; for example, sample K5 (biotite monzonitic granite) is located to the east of the Nalingguole River, and the latitude, longitude, and height of the sample are 92°51.585′E, 36°48.309′N, and 3191 m, respectively. Biotite monzonitic granite is grey to off-white in colour with a medium-fine granular texture and massive structure, consisting mainly of quartz (approximately 25%), plagioclase (approximately 30%), K-feldspar (approximately 30%), biotite (approximately 10%), hornblende (approximately 5%), and accessory minerals including titanite, magnetite, apatite and zircon. In figure 2(a and b), the following are observed: the quartz possesses a subhedral–anhedral texture and exhibits wavy extinction; the plagioclase displays multiple twins; the centre of the K-feldspar exhibits kaolinisation; the dark brown to brown biotite shows chloritisation and opaque iron materials; and the light green hornblende displays a band structure, two groups of diamond cleavage and simple twins.
Petrographic characteristics of granites in Qimantagh area. Abbreviations: Qtz: quartz; Pl: plagioclase; Kfs: potassic feldspar; Hbl: hornblende; Bi: biotite.
The Yemaquan granodiorite pluton has only sparse exposure. Sample K13-1 (GPS: 92°00.654′E, 36°58.939′N, height: 3707 m) was collected from drill core ZK10033. It is off-white in colour with a medium-fine granitic texture and massive structure. The main minerals are subhedral–anhedral quartz (approximately 30%), plagioclase (approximately 40%), and potassium feldspar (approximately 20%). The secondary minerals are biotite and hornblende (total approximately 10%), and the accessory minerals are mainly zircon, apatite, magnetite, and ilmenite. In figure 2(c and d), we can see that kaolinisation has occurred in the plagioclase and K-feldspar, so the surfaces are dirty. The biotite has partly turned into pennine and presents an indigo interference colour, and the yellow-green hornblende, which has developed a simple twin, has an interference colour of level 3 blue.
Granite porphyry (K21-2) in the Qunli mine was collected from borehole ZK1701, and it is off-white to light reddish in colour with a medium grain porphyritic structure and massive structure. In figure 2(e and f), it can be seen that the phenocrysts and the matrix mainly consist of quartz, K-feldspar and seldom biotite. Some quartz phenocrysts have euhedral crystals, while some were corroded into harbour by the matrix. The potassium feldspar phenocrysts exhibit kaolinisation, so the surface is dirty. The biotite phenocryst has developed a dark rim due to alteration.
The Kayakedengtage complex, located in the southeastern Qimantagh Mountain, is a batholith, and the exposed area covers approximately 386 km 2. The distribution direction of the complex is in the WNW–ESE direction, and the long axis direction roughly parallels the Nalingguole river fault (F3). The complex has multiple types of rocks, mainly consisting of gabbro, diorite, quartz diorite, granodiorite, monzonitic granite and syenogranite, which are products of a typical magmatic mixing process (Chen et al. 2006). The monzonitic granite and granodiorite are the main rock types, and their exposed areas are 218 and 83 km², respectively. We collected one sample (K36) of the Kayakedengtage complex in the field, and the specific location of sample K36 (granodiorite) is 91°39.075′E, 36°56.232′N with a height of 4409 m. The rock is off-white in colour with medium-fine granitic texture and massive structure. The main minerals are quartz (approximately 30%), plagioclase (approximately 40%), and potassium feldspar (approximately 20%). The secondary minerals are biotite and hornblende (total approximately 10%), and the accessory minerals are mainly zircon and apatite. In figure 2(g and h), the plagioclase contains long and short columnar crystals, multiple twins and carbonation. The K-feldspar has Carlsbad twins, while some alkali-feldspars are microcline with lattice twins. The dark brown biotite has partly turned into pennine and presents an indigo interference colour, whereas the yellow-green hornblende has developed diamond cleavage and has an interference colour of level 3 blue.
The Wulanwuzhuer pluton body has an irregular shape, but because it was affected by the later tectonic and erosion, it is even more complex. The overall distribution direction of the body is towards the northwest. The rock association of this pluton is primary granodiorite-monzogranite-potassium granite. We collected one sample (K46-2) of the Wulanwuzhuer pluton in the field, and the location of sample K46-2 (potassium granite) is 91°54.198′E, 37°14.968′N. The rock is reddish in colour with a medium-fine granular texture and massive structure. In figure 2(i and j), quartz (approximately 25%), potassium feldspar (approximately 45%), and plagioclase (approximately 20%) are the major mineral components, while biotite and hornblende (total approximately 10%) are the secondary minerals and zircon and apatite are the main accessory minerals. There are two stages of quartz: the early quartz is larger and irregularly shaped, while the later particles are smaller and are distributed around the gap between the larger quartz and feldspar. The K-feldspar has Carlsbad twins, and the minority of alkali-feldspar is microcline with lattice twins. Generally, the biotite irregularly fills the gap between the potassium feldspar and quartz.
4 Analytical methods
4.1 Zircon U–Pb geochronology
Representative samples for zircon separation are selected according to standard procedures, followed by mechanical crushing and magnetic and electromagnetic analyses. Pure zircon grains were handpicked using a binocular microscope, and together with several grains of standard zircon TEMORA, they were mounted onto an epoxy resin disc and ground down so that their interiors were exposed and polished. After preparing the mount, the zircons were first photographed by optical microscopy, and then cathodoluminescence (CL) images were obtained. Next, LA-ICP-MS in situ trace element and isotope analysis were performed. Related tests and analyses were conducted at the Tianjin Institute of Geology and Mineral Resources, according to the procedures from Li et al. (2009). ICPMS Datacal procedures developed by Dr. Liu at the China University of Geosciences and Isoplot software developed by Kenneth R Ludwig were used for data processing, and common lead correction was handled through the Andersen (2002) method.
4.2 Major and trace element analyses
As part of a detailed observation of collected samples, whether in the field or under a microscope, representative samples with no weathering were crushed in a contamination-free environment to less than 200 mesh. Major elements and trace elements were analyzed at the Southwest Metallurgical Geology Laboratory, Chengdu City, Sichuan Province, China. The major elements were analysed by XRF (X-ray fluorescence spectrometer). The test methods included the weight, X-ray fluorescence, and titration methods, and the precision was better than 5%. The trace elements were analysed by the spectrometer ICE3500 atomic absorption instrument, iCAP6300, XRF, AFS2202E atomic fluorescence spectrometer, and the NexION ICP-MS 300X, 802w spectrograph. The precision was better than 5% if the trace element content was more than 10 ×10 −6; otherwise, the precision was better than 10%.
5 Results
5.1 Zircon U–Pb geochronology
Five representative samples were selected for the LA-ICP-MS zircon U–Pb analysis. All the results are listed in table 1 and the CL images are shown in figure 3.
Cathodoluminescence (CL) images of representative zircon grains and Concordia plots of granites in Qimantage area. Solid circles indicate the locations of LA-ICP-MS U–Pb dating.
Zircon grains from sample K5 are mostly light- yellow and transparent to semi-transparent. The grains occur as short column-long columnar prismatic crystals with lengths of 100 ∼200 μm, individually 300 μm, and length/width ratios of 1.0 to 3.0. The CL image clearly shows oscillatory zoning, suggesting a magmatic origin. The analysed zircons yield Th (48–701 ppm) and U (218–2134 ppm) with Th/U (0.11–0.65), although Th/U is relatively milder, demonstrating the magmatic origins of the grains. A total of 32 spots on 29 zircon grains are used for dating, and the edge and core gave the same age. In addition, the ages of spots 3, 9, 20, and 21 (respectively, the 206Pb/ 238U ages are 353, 373, 287, and 193 Ma) are younger, while the other 28 spots’ 206Pb/ 238U ages are relatively concentrated from 405 ∼431 Ma. These data yield a weighted mean age of 420.6 ±2.6 Ma (MSWD =2.8) (figure 3a), which belongs to the Late Silurian and represents the formation age of the monzonitic granite. Moreover, because the oscillatory zoning suggests a magmatic origin, the 420.6 ± 2.6 Ma can represent the crystallisation age of the unit.
Zircon grains from sample K46-2 are mostly light-yellow and occur as short column-long columnar prismatic crystals with lengths of 100 ∼ 200 μm and an aspect ratio of 1:1 to 2:1. The CL image clearly displays oscillatory zones. Thirty-four better shaped zircon grains, which exhibit clear oscillatory zoning, were selected for dating (a total of 36 spots). Save for spots 3, 9, 20, and 21, whose ages are younger due to missing lead (respectively, the 206Pb/ 238U ages are 399 Ma, 348 Ma, and 397 Ma), the 206Pb/ 238U ages for the other 32 spots are relatively concentrated from 409 ∼435 Ma. These data yield a weighted mean age of 421.2 ± 1.9 Ma (MSWD =2.1) (figure 3b), belonging to the Late Silurian and representing the emplacement age of moyite.
Zircon grains from sample K13-1 are light-yellow and occur as short column-long columnar crystalline forms that are 100 ∼ 300 μm long. The CL image clearly shows oscillatory zones, suggesting a typical magmatic origin, and most of the zircons have dark hyperplasia edges. The analysed zircons have Th content and U content values of 10–334 and 31–710 ppm, respectively, with Th/U ratios from 0.28 to 0.63. Although the Th/U is relatively mild, the ratio as a whole is greater than 0.4, suggesting a magmatic origin for the zircon (Belousova et al. 2002; Zhong et al. 2006). A total of 32 spots on 28 zircon grains are used for dating, though the ages of spots 29 and 31 (respectively, the 206Pb/ 238U ages are 352 and 211 Ma) are younger due to missing lead. The age of spot 29 may be the age of a late stage magmatic transformation, while the age of spot 31 may be that of late stage metamorphic or hydrothermal zircon. The other 30 206Pb/ 238U ages are relatively concentrated from 381 ∼419 Ma. These data yield a weighted average age of 403.7 ± 2.9 Ma (MSWD =3.9) (figure 3c), which belongs to the Early Devonian and represents the formation age of granodiorite.
Zircon grains from sample K21-2 are mostly short column and much less than 100 μm, individually 100 ∼ 200 μm, and mostly show clear oscillatory zones in CL image 3, suggesting a magmatic origin. The concentrations of the trace elements Th and U in the zircon grains are larger (Th: 125–861 ppm; U: 127–629 ppm), and the change in Th/U (0.5– 3.27) is relatively larger, denoting magmatic zircon as the ratios are all >0.4 (Belousova et al. 2002; Zhong et al. 2006). A total of 32 spots on 31 zircon grains are used for dating, but spot 3 has no result and spots 1, 5, 6, 23, 27, and 30 (respectively, the 206Pb/ 238U ages are 326, 373, 334, 230, 159, and 375 Ma) are younger. The 206Pb/ 238U ages of the other 25 spots are relatively concentrated from 380 to 407 Ma. These data yield a weighted average age of 391.3 ±3.2 Ma (MSWD = 4.3) (figure 3d), belonging to the Middle Devonian and representing the diagenetic age of granite porphyry.
Zircon grains from sample K36 are mostly light-yellow and transparent to semi-transparent, and they occur as short column prismatic crystals, though only some are long and the crystals are mostly 100 ∼ 200 μm long. The CL image shows clearly oscillatory zones, suggesting a magmatic origin. The concentrations of the trace elements Th and U in the zircon grains are 17–134 and 30–220 ppm, respectively. The change in Th/U (0.39–0.81) is relatively milder, though almost all the ratios are >0.4, showing magmatic zircon (Belousova et al. 2002; Zhong et al. 2006). Because of the numerous zircon grains in this sample, we only selected 31 of them (a total of 32 spots) for dating. Spot 18 has no result, while the 206Pb/ 238U ages of the other 31 spots are relatively concentrated from 378 to 393 Ma. These data yield a weighted mean age of 380.52 ± 0.92 Ma with a small MSWD value (0.62) (figure 3e), belonging to Late Devonian and representing the formation age of granodiorite.
5.2 Whole rock geochemistry
The major and trace element analysis results of the Late Silurian–Late Devonian granitoids in the Qimantagh area are listed in table 2.
5.2.1 Major elements
The Late Silurian granitoid belongs to the subalkaline series (figure 4a), consisting mainly of monzonitic granite and rarely granodiorite and alkali-granite (figure 4b). Overall, the rock types plot in the high-K calc-alkaline field in the K 2O vs. SiO 2 diagram (figure 4c). They exhibit metaluminous to weakly and strongly peraluminous characteristics on a plot of A/NK vs. A/CNK (figure 4d).
The Early Devonian granitoid also belongs to the subalkaline series (figure 4a), but the lithology is relatively scattered, including quartz diorite, quartz monzodiorite, and especially granodiorite and monzogranite (figure 4b). In the K 2O vs. SiO 2 diagram (figure 4c), the rock types mainly plot in the high-K calc-alkaline field in general. All the same, they also exhibit metaluminous to weakly and strongly peraluminous characteristics on a plot of A/NK vs. A/CNK (figure 4d).
The Middle Devonian granitoid belongs to the subalkaline series, but it has a trend towards an alkaline series evolution (figure 4a). The lithology of the Middle Devonian granitoid is mainly monzonitic granite and granodiorite (figure 4b), but at the same time, this stage shows high potassium characteristics. The rock types mainly plot in the high-K calc-alkaline field, though parts of them drop into the shoshonite field (figure 4c). In the A/NK vs. A/CNK diagram, they exhibit metaluminous to weakly peraluminous characteristics (figure 4d).
The Late Devonian granitoid also belongs to the subalkaline series (figure 4a), but the lithology is relatively scattered, including quartz diorite, quartz monzodiorite, and especially granodiorite and monzonitic granite (figure 4b). The rock types mainly plot in the high-K calc-alkaline field, though parts of them distributed into the medium-K calc-alkaline field in the K 2O vs. SiO 2 diagram (figure 4c). As with the Late Silurian and Early Devonian granitoids, the Late Devonian granitoid exhibits metaluminous to weakly and strongly peraluminous characteristics on a plot of A/NK vs. A/CNK (figure 4d).
5.2.2 Trace elements
The Late Silurian granitoid has chondrite-normalised REE patterns (figure 5a) that are enriched in light rare earth elements (LREE) (LREE/ HREE =4.76–18.15, average 9.08 (La/Yb) N= 4.33–33.36, average 11.91), with moderately negative Eu anomalies (0.15–0.86, average 0.53), and are depleted in heavy rare earth elements (HREE) ((Tb/Yb) N=1.06–2.5, average 1.51).
The Early Devonian granitoid has chondrite-normalised REE patterns (figure 5b) that are also enriched in LREE (LREE/HREE =5.73–12.35, average 8.07 (La/Yb) N =4.64–19.66, average 10.06), with weakly negative Eu anomalies (0.35–1.08, average 0.72) that occasionally trend towards positive anomalies, and border on flat HREE ((Tb/Yb) N = 0.65–2.90, average 1.67) on the whole, though some may be slightly enriched in HREE.
The Middle Devonian granitoid has chondrite-normalised REE patterns (figure 5c) that are also enriched in LREE (LREE/HREE =3.57–20.67, average 9.40 (La/Yb) N=2.30–32.78, average 12.75), with somewhat strongly negative Eu anomalies (0.08–0.81, average 0.40), and are depleted in HREE ((Tb/Yb) N=0.77–2.37, average 1.58), though some may be slightly enriched in HREE.
The Late Devonian granitoid has chondrite-normalised REE patterns (figure 5d) that are greatly enriched in LREE (LREE/HREE =6.30–29.09, average 13.20 (La/Yb) N=7.23–53.96, average 19.97), with weakly negative Eu anomalies (0.47–1.18, average 0.82) that trend towards strongly negative, and as a whole have nearly flat HREE ((Tb/Yb) N=1.42–2.40, average 1.79). In addition, the Late Devonian granitoid shows similar characteristics to the Early Devonian granitoid, which may reflect the similarity of their source rock and tectonic environment.
In the primitive-mantle normalised spidergrams (figure 6), the Late Silurian–Late Devonian granitoid shows similar or roughly consistent trace element characteristics; namely, they are enriched in large ion lithophile elements (LILE), such as Rb, Th and K, and high field strength elements (HFSE), such as Zr and Hf, and are depleted in Ba, Nb, Ta, Sr, P, Eu, and Ti. The enrichment and depletion of trace elements in the Late Silurian–Late Devonian granitoid may be slightly different over time, which may be related to the main and accessory minerals in the rock and the specific environment. However, these trace element characteristics are still very similar to those of volcanic arc granites in a subduction environment.
6 Discussion
6.1 Timing of multiperiodic magmatism in the Late Silurian to Late Devonian
According to the LA-ICP-MS zircon U–Pb dating results, multiperiodic magmatic activity existed in the Qimantagh area during the Late Silurian (approximately 420 Ma)–Late Devonian (approximately 380 Ma). In addition, the majority of 206Pb/ 238U ages of the zircon grains are small values, mainly in the two periods 399–373 and 353–326 Ma (respectively, Middle–Late Devonian and Early Carboniferous), which suggests that there was significant magmatic activity that made some zircon grains recrystallise. K21-2 and K36 (391.3 and 380.52 Ma recrystallisation ages, respectively) also confirmed that Middle–Late Devonian magmatic activity existed in the study area. Early Carboniferous (353–326 Ma) magmatic activity may be associated with the opening of the south Kunlun ocean basin. In addition, there are smaller 206Pb/ 238U ages matching the Early Permian (287 Ma), Late Triassic (230 and 211 Ma), Early Jurassic (193 Ma) and Late Jurassic (159 Ma), which primarily correspond to tectonic events from the Late Paleozoic to Early Mesozoic orogenic cycle including the Early Permian–Late Triassic subduction and collision, and the Jurassic post-orogeny spreading and collapse.
In recent years, a growing number of Late Silurian to Late Devonian granitoids were gradually found, such as the Wulanwuzhuer porphyry granite (416.7 ± 3.3 Ma) (Sun et al. 2009) and peraluminous monzongranite (413 ± 5 Ma) (Tan et al. 2011), the Adatan syenogranite (412.9 ± 2.1 Ma) (Wang 2011), the Donggou biotite monzogranite (410.2 ± 1.9 Ma) (Wang et al. 2004), the Akechukesai quartz diorite (407.7 ± 7.5 Ma) (Luo et al. 2004), the Bokelike porphyritic monzongranite (408.3 ± 5.3 Ma) (Luo et al. 2004), the Bayinguole gabbro (386.9 ± 2.6 Ma, 386.4 ± 3.2 Ma) (Luo et al. 2004), and the Kayakedengtage complex pluton of gabbro and monzonitic granite (respectively, 403.3 ± 7.2 and 394 ± 13 Ma) (Chen et al. 2006).
From the Late Silurian to the Late Devonian, the outcropped granitoids have almost no time gaps among them, further confirming that multiperiodic magmatic activity existed in the Qimantagh area during this period.
6.2 Compositions of the rock
Granitoids are genetically classified as either mantle origin (e.g., Turner et al. 1992; Han et al. 1997; Volkert et al. 2000), mixed origin, with various proportions of crust- and mantle-derived components (e.g., Poli and Tommasini 1991; Barbarin and Didier 1992; Wiebe 1996; Altherr et al. 2000; Chen et al. 2002; Bonin 2004) or crustal origin (e.g., Chappell and White 1992; Chappell 1999).
Based on the major elemental systematics, the Late Silurian–Late Devonian granitoids are typical of the high-K calc-alkaline, metaluminous or weakly or strongly peraluminous rocks. The lithology is mainly granodiorite and monzogranite. But from the Early Devonian, the rocks have a trend of quartz-diorite–quartz-granodiorite–monzogranite, apparently containing intermediate-basic rocks, indicating that mantle component most likely participated in the magmatic activities. Some scholars (Luo et al. 2004; Chen et al. 2006; Mo et al. 2007) also noted that underplating of the mantle-derived magma had happened in the study area since the Early Devonian. The origin of the high-K calc-alkaline rocks has been the subject of some studies, and two main models have been proposed to interpret their petrogenesis: (1) pure crustal melts from partial melting of mafic lower crust at relatively high pressures (e.g., Roberts and Clemens 1993; Liu et al. 2002) or (2) evolution result of a mixture of crustal- and mantle-derived magmas (e.g., Barbarin 1999; Ferre’ and Bernard 2001; Chen et al. 2003; Yang et al. 2007). The Late Silurian granitoids may be generated by partial melting of mafic lower crust at relatively high pressures and belong to crustal origin. But from the Devonian, there are mixed origin granitoids, with various proportions of crust- and mantle-derived components.
From the REE distribution patterns and Eu anomaly, the REE distribution characteristics of crust-mantle mixed-source granites are light REE enrichment, inconspicuous Eu depletion and δEu >0.5 (an average of 0.8 or so), and the REE distribution patterns are a right-dipping, nearly smooth type (Luo 1986; Zhao et al. 2008). Nevertheless, the REE distribution patterns of a crust-derived granitoid are a ‘V’ type curve, showing relatively obvious Eu depletion (approximately moderate to significant depletion) and relative HREE enrichment (Luo 1986). There are both ‘V’ type curves and smooth curves for the REE distribution in the study area. The REE distribution patterns are mainly ‘V’ type curves for the Late Silurian and approximately moderate to significant Eu depletion, suggesting that the Late Silurian granitoids are of crustal origin. However, the patterns are mostly an obviously smooth curve for the Devonian and weakly negative or inconspicuous Eu anomalies, especially the Early Devonian and Late Devonian, implying that the Early Devonian and Late Devonian granitoids are mainly mixed origin, although there are still some crustal source granites at the same time, especially the Middle Devonian granitoids. Obvious Eu anomalies (average 0.40) in the Middle Devonian granitoid may be related to strong fractional crystallisation of the plagioclase during magma evolution, which is corroborated by the Middle Devonian rocks with high differentiation indices (DI) (70.56–94.68).
According to the primitive-mantle normalised spidergrams, the granitoids show similar or roughly consistent trace element characteristics of enriching in LILE (such as Rb, Th and K), HFSE (such as Zr and Hf) and depleting in Ba, Nb, Ta, Sr, P, Eu, and Ti. They also present a certain evolution trend of the orogenic belt, namely, the common enrichment of Rb, Th, Zr and Hf, significant depletion of Ti, Sr, P and Ba. As shown in the spider diagram, compared to Rb and Th, Ba is obviously depleted. The negative Ba anomalies in the granite suggest a partial melting of crustal rock (Wan 1999). The granitoids are characterized by pronounced negative Ba, Nb and Ti anomalies and enriched in LILEs and LREEs, suggesting typical crustal melts. However, these features are not always related to the crustal-derived melts and they might point to partial melting of an enriched mantle, which was metasomatized by fluids prior to melting (Hawkesworth et al. 1993; Rottura et al. 1998; Cameron et al. 2003). Compared to rocks from other times, the Middle Devonian granites have pronounced Ba, Sr, P and Ti depletion. This observation may be due to the strong fractional crystallisation of feldspar minerals such as potassium feldspar, plagioclase and apatite, while Ti depletion may be related to the fractional crystallisation of Ti-rich minerals such as ilmenite, sphene, and rutile during the evolution process. The Ti depletion also suggests that the magma was derived from the crust because Ti does not easily enter the melt and residue in the source. Thus, the strong Ti depletion and the relatively weak Nb and Ta depletion in the study area are due to the fractional crystallisation of ilmenite in the source. This observation is consistent with the rocks that experienced strong fractional crystallisation and belonged to highly differentiated granite, which mainly reflects the characteristics of the source.
In addition, some elemental ratios could indicate compositions of the rock. Bea et al. (2001) note that the Nd/Th ratio is approximately 3 for crust-derived rocks and over 15 for mantle-derived rocks. The Nd/Th ratios of the Late Silurian granitoid (0.91–3.73, average 1.97) mostly show characteristics of a crustal source. However, the Nd/Th ratios of granitoids since the Early Devonian (0.44–7.04) indicate crust and mantle mixing, which denotes an underplating of mantle magma and a change in the tectonic setting during this period. Taylor and McLennan (1985) believe that K and Rb travelled upward to the sialic layer, so they are more depleted in the mantle. Sr mainly exists in plagioclase as a substitute for Ca. As a result, high Rb/Sr ratios in the granitoids suggest a primarily upper crust source. Tischendorf (1986) also stated that the Rb/Sr ratio was an important parameter to trace the origin. The Rb/Sr ratio is <0.05 for mantle-derived magma, 0.05–0.5 for mixed crust-and-mantle magma, and over 0.5 for crust-derived magma. The Rb/Sr ratios of the Late Silurian granitoid (0.71–10.98, average 1.83) suggest a crustal reservoir as the major source, but the Rb/Sr ratios of the granitoids since the Early Devonian (0.12–22.73) indicate crust and mantle mixing in parts of the samples.
Therefore, on the whole, the Late Silurian to Late Devonian granitoids have complex composition sources. The Late Silurian granitoids are mainly crust-derived, high-K calc-alkaline peraluminous granite. However, mantle-derived compositions were involved in the magmatism starting from the Early Devonian, and the granitoids contained characteristics of a crust-mantle mixed source. Most of the Devonian granitoids are crust-mantle mixed-source, high-K calc-alkaline metaluminous and weakly peraluminous granite, and only some parts of them are crust-derived, high-K calc-alkaline peraluminous granite, especially the Middle Devonian granitoids. Due to strong crystallisation differentiation, highly fractionated granites possibly emerged in the Middle Devonian. These complex composition sources may be related to a relatively more complex tectonic environment and even its evolution process.
6.3 Tectonic setting
Some scholars (Bai et al. 2004; Wang et al. 2004, 2011; Sun et al. 2009; Tan et al. 2011) state that the research area was in a syn-collision tectonic environment during the Late Silurian (approximately 421–410 Ma). Zhang et al. (2012) firmly contend that the Qaidam continental block and the central Kunlun block were still in a state of collision in the Early Devonian (395 Ma). Other scholars (Cao et al. 2011; Wu et al. 2012) believe that the research area had been in the beginning of a syn-collision convergence environment since the Early Silurian (approximately 430 Ma), while others (Wang et al. 2004; Chen et al. 2006; Li et al. 2013) argue that the Qimantagh area was in a collision orogenic stage that continued until at least the Late Devonian. Zhao et al. (2008) contend that the Early Devonian granitoid was formed in a post-collision tectonic setting; due to the geochemical plots, parts of the granite belong to syn-collision S-type granite, so some scholars believe that it formed in an intercontinental collision orogenic process, namely, a syn-collision tectonic setting.
In recent years, research found that a continental collision phase is not conducive to the ascent of magma. Although magmatism was widely distributed during the plate period, it was very sparse. However, most of the magmatism typically occurs after, rather than before, the collision period, namely, the post-collision tectonic setting described by Liegeois (1998). In this sense, the exposed Late Silurian to Late Devonian high-K calc-alkaline granites indicate that the study area may have been in an intracontinental collision orogenic stage, during which significant amounts of post-collision high-K calc-alkaline peraluminous granite and metaluminous or weakly peraluminous granite form.
In our present study, we suggest that the regional tectonic setting during this period should belong to an intracontinental collision orogenic process, further developing post-collision granites. Moreover, additional granites emplaced in the syn-collision stage may have formed during the post-collision process, which is also confirmed in the tectonic setting discrimination diagrams (figures 7, 8). In figure 7(a), almost all the Late Silurian–Late Devonian granitoids drop into the collision setting granites and show an evolutionary tendency towards intraplate granites; in figure 7(b), parts of the granitoids drop into volcanic-arc granites (VAG), which may be related to a source that inherited the characteristics of volcanic arc; in the Rb–Y + Nb and Rb–Yb + Ta tectonic environment discrimination diagrams (figure 8), the Late Silurian–Late Devonian granitoids are also virtually all post-collision granites in the research area. Therefore, the Late Silurian–Late Devonian granitoids are post-collision granites that formed during an intracontinental collision orogeny.
(a) Rb/10-Hf-3Ta and (b) Rb/30-Hf-3Ta. VAG: volcanic arc granites, ORG: ocean ridge granites, WPG: within-plate granites, Syn-COLG: syn-collision granites and Post-COLG: post-collision granites. Symbols as in figure 4(a).
(a) Rb/10-Hf-3Ta and (b) Rb/30-Hf-3Ta. VAG: volcanic arc granites, ORG: ocean ridge granites, WPG: within-plate granites, Syn-COLG: syn-collision granites and Post-COLG: post-collision granites. Symbols as in figure 4(a).
In addition, the trace elements have consistent characteristics with volcanic-arc granites in a subduction setting. However, the ocean basin closed sometime before the Late Silurian (Chen et al, 2002; Mo et al. 2007; Lu et al. 2010; Zhang et al. 2010; Liu et al. 2012). Therefore, it is impossible that the study area was in a subduction stage in the Late Silurian–Late Devonian. The only possible explanation is that the magma source rock inherited the characteristics of a volcanic arc (Hooper et al. 1995; Li and Lu 1999); that is to say, the granites were derived from the partial melting of the original rocks, which were restored in the crust or the lithosphere during the subduction and collision periods in a certain tectonic setting (Liu 2000; Xiao et al. 2002). Li and Lui (1999) also suggested the concept of ‘retarded type calc-alkaline volcanic rocks’, which describes a phenomenon where the rocks show characteristics of volcanic-arc granite without the presence of a volcanic arc.
Just as Bonin et al. (1998) pointed out, the post-collision event started with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intracrustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic. Post-collision granite suites are a case of multisource multiprocess magmatism.
Although the Late Silurian–Late Devonian granitoids are post-collision granites that formed during an intracontinental collision orogeny, according to the characteristics of major and trace elements and the rock sources, there are seemingly some differences between the Late Silurian and Devonian granites in evolution process of the tectonic setting; that is, the mechanism for the formation of the granites may have varied over time. As Xiao et al. (2002) suggested that the post-collision tectonic setting experienced a long and complicated process, including large-scale plate motion along the shear zone, closure, lithosphere delamination, subduction of small oceanic plates, and the formation of rifts coupled with various types of magmatism. On the R1–R2 tectonic setting discrimination diagrams (figure 9), the granitoids are associated with the orogenic environment. Although most of the granites are in the syn-collision area, they may have only been emplaced in the syn-collision stage and formed in a post-collision stretch tectonic setting. Moreover, most of the Late Devonian granites dropped into the late-orogenic stage area, indicating the end of the orogenic stage.
Therefore, the Late Silurian–Late Devonian granitoids are post-collision granites that formed during an intracontinental collision orogeny. This special tectonic setting is also in accordance with the complex composition sources.
Thus, based on the previous research results, the authors consider the evolution process of the subduction and even syn-collision have ended at least before the Late Silurian (420 Ma) (figure 10a). The paper concludes that the 420.6 ± 2.6 Ma (sample K5, biotite monzonitic granite) and 421.2 ± 1.9 Ma (sample K46-2, potassium granite) samples were formed during a conversion period involving syn-collision extruding orogenesis progressing towards a post-collision stretch tectonic regime (420 Ma). These two samples were generated by partial melting of mafic lower crust that was restored during the subduction and collision periods (figure 10b). Next, lithosphere delamination may have occurred in the Early Devonian under a layer of thickened continental crust. This process lead to the upward gushing of mantle magma, providing a heat (heats and melts crustal material) and material source (mixes the molten and mantle-derived magma that is infused into the melting crust source) and resulting in the formation of the 403.7 ± 2.9 Ma K13-1 (granodiorite) sample (figure 10c), which reflects a mixture of enriched subcontinental lithospheric mantle-derived and lower crustal-derived magmas. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. The trend from diorite to granite is also observed, especially the Early Devonian and Late Devonian. With the progressive lithosphere extension and thinning and the continued upward movement of the mantle magma, which provided uninterrupted heat and material sources to the crust until the Middle Devonian period, the middle-shallow crustal rocks partially melted, resulting in the formation of the 391.3 ±3.2 MaK21-2 granite porphyry (figure 10d), which underwent strong crystallisation differentiation, and belong to highly fractionated granites. This type of relatively loose and stretch post-collision tectonic setting lasted at least until the early Late Devonian, resulting in the formation of the 380.52 ± 0.92 Ma K36-2 (granodiorite) sample. In addition, there is a basic gabbro mass of similar age (380.3 ± 1.5 Ma) (Ren et al. 2012) in the same place that co-exists well with the granitoid. This observation confirms the existence of tectonic magmatic activity during this period; namely, the underplating of mantle-derived magma still existed in the Qimantagh area during the Late Devonian (380 Ma). Eventually, the development of the continental molasse in the Late Devonian marked the end of the Qimantagh area’s Early Paleozoic Caledonian orogenic cycle (figure 10e).
The Late Silurian–Late Devonian tectonic evolution model in the Qimantagh area. (a) The subduction and even syn-collision have ended at least before 420 Ma; (b) a conversion period from syn-collision extruding orogenesis to a post-collision stretch tectonic regime; (c) lithosphere delamination occurred under a layer of thickened continental crust; (d) lithosphere extension and thinning and the continued upward movement of the mantle magma, and the middle-shallow crustal rocks partially melted; (e) the underplating of mantle-derived magma still existed.
7 Conclusions
From our study of Late Silurian–Late Devonian granitoids in the Qimantagh area of the East Kunlun orogenic belt, we arrived at the following conclusions:
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Multiperiodic magmatic activity existed in the Qimantagh area during the Late Silurian (approximately 420 Ma)–Late Devonian (approximately380 Ma). The zircon U–Pb ages of 5 samples in the Qimantagh area are 420.6 ± 2.6 Ma (Nalingguole biotite monzogranite), 421.2 ± 1.9 Ma (Wulanwuzhuer potassium granite), 403.7 ± 2.9 Ma (Yemaquan granodiorite), 391.3 ± 3.2 Ma (Qunli granite porphyry), and 380.52 ± 0.92 Ma (Kayakedengtage granodiorite).
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The Late Silurian–Late Devonian granitoids belong to the subalkaline, high-K calc-alkaline, metaluminous or weakly or strongly peraluminous series. The rocks are right oblique types, having overall relative LREE enrichment and HREE depletion and different degrees of Eu anomalies, trending towards moderate Eu depletion at different times, and are enriched in LILE, such as Rb, Th and K, and HFSE, such as Zr and Hf, and depleted in Ba, Nb, Ta, Sr, P, Eu, and Ti.
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The Late Silurian–Late Devonian granitoids have complex composition sources. The Late Silurian granitoids are mainly crust-derived and mantle-derived material was involved in the magmatism starting from the Early Devonian, as well as the granitoid-contained characteristics of a crust-mantle mixed source. Most of the Devonian granitoids are crust-mantle mixed-source, and only some parts of them are crust-derived, especially the Middle Devonian granitoids.
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The Late Silurian–Late Devonian granitoids were formed in a post-collision tectonic setting. Lithosphere delamination may have occurred in the Early Devonian (407 Ma), and the study area subsequently experienced an underplating of the mantle-derived magma at least until the Late Devonian (380 Ma).
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Acknowledgments
The authors would like to express sincere thanks to Academician Mo Xuanxue of CUGB, Engineer Ma Zhongyuan, and Shu Xiaofeng of Third Institute of Qinghai Geological Mineral Prospecting for their encouragement and support. They also gratefully acknowledge the anonymous reviewers for their detailed reviews and constructive suggestions. Funding for this study is supported by the China Geological Survey (No. 2011-03-04-06), Found Project of the Geology and Mineral Exploration Development Authority of Qinghai Province (2013-103), and the National Natural Science Foundation of China (Nos. 41172088, 40872141 and 41072160).
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HAO, N., YUAN, W., ZHANG, A. et al. Evolution process of the Late Silurian–Late Devonian tectonic environment in Qimantagh in the western portion of east Kunlun, China: Evidence from the geochronology and geochemistry of granitoids. J Earth Syst Sci 124, 171–196 (2015). https://doi.org/10.1007/s12040-014-0531-z
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DOI: https://doi.org/10.1007/s12040-014-0531-z