2. The Eighth Institute of Shandong Geological Exploration, Rizhao 276000, China;
3. Qinghai Provincial Bureau of Ferrous Metal and Geological Exploration, Xining 810007, China
The east Kunlun orogenic belt (EKOB) is a part of the western segment of the central orogenic belt, China, and is one of the primary tectono-magmatic belts of the northern Tibetan Plateau. Multistage tectono-magmatic events have affected the region, including the closure of ancient oceans, continental convergence, accretionary, collisional and superimposed orogenies, and changes in tectonic regime (Chen et al., 2018; Zhao et al., 2018; Deng et al., 2015, 2014a, b, 2012; Dai et al., 2013). The EKOB is situated in a pivotal location at the junction between the Tethyan and Pan-Asian tectonic domains (Jiang et al., 2013). From the Early Paleozoic to Late Triassic, it was affected by the Proto- and Paleo-Tethys orogenies (Chen et al., 2015; Yang et al., 1996). Multistage tectono-magmatic events occurred during subduction of the Proto-Tethys oceanic lithosphere. The EKOB was subsequently affected by the Paleo-Tethys orogeny, which performed a key role in the generation of Permian–Triassic granitic magmatism (Yang et al., 1996). The Paleozoic tectonic evolution of the EKOB has been the focus of much research (Zhu C B et al., 2018; Liu et al., 2012; Zhang et al., 2010; Chen et al., 2008; Zhao et al., 2008; Li et al., 2006; Zhu Y H et al., 2006, 2005; Pan et al., 1996), although uncertainties remain. For example, Mo et al. (2007) put forward that subduction of the Proto-Tethys continued until the Late Ordovician, while Dong et al. (2017) considered that the Proto-Tethys Ocean has closed during the Devonian. Neither view can emphasize the post-collision magmatism, while the granites and mafic-ultramafic complexes associated with post-collision extension can provide an effective method to deal with these contradictions.
The Maxingdawannan area in the Qimantagh region records the early tectonic evolution of the region and contains voluminous magmatic rocks. Therefore, an understanding of the geology of the area is vital in elucidating the tectonic evolution of the EKOB. Although some detailed studies of the Qimantagh region have been undertaken (Feng et al., 2012; Wang et al., 2012, 2009; Cui et al., 2011; Ma et al., 2010; Liu et al., 2006; Lu et al., 2006; Tan et al., 2004; Shen et al., 1999), its Paleozoic evolution remains unclear. Here we report new data for monzogranites and mafic-ultramafic complexes of the Maxingdawannan area, with the aim of establishing an evolutionary model of Early Devonian magmatism in the EKOB and further constraining the tectonic evolution of the EKOB.1 GEOLOGICAL BACKGROUND AND SAMPLE DESCRIPTION
The EKOB is situated in the north section of Tibetan Plateau (Ju et al., 2017; Mo et al., 2007), bordered by the Qaidam Basin to the north and the Songpan-Ganzi Basin to the south (Fig. 1), extending 1 500 km from east to west and south-north of 50–200 km (Yang et al., 1996; Jiang et al., 1992). Sun et al. (2009) regarded the EKOB as a collage of blocks that experienced multiple stages of orogeny and accumulated northward to the Qaidam Block. Major E-W-trending faults delineate several belts, which from north to south are the Caledonian back-arc rift belt (north EKOB), the basement uplifted and granite belt (middle EKOB), and the composite collage belt (south EKOB)(Fig. 1; Sun et al., 2009). The northern EKOB comprises mainly basic volcanic rock that is locally overlain by Devonian sedimentary molasse, and the middle EKOB is characterized by intense granitic magmatism that covers an area of ~48 400 km2. In the southern EKOB, exposed strata range from Paleoproterozoic to Triassic in age (Jiang et al., 1992), are generally intensely deformed, and are overlain by extensive Triassic strata. Sun et al. (2009) considered the marine basic volcanic rocks of the Mesoproterozoic Anbaogou Group to constitute an "Ocean Basalt Plateau". The middle and northern EKOB are located at the northern end of the middle Kunlun fault, and consist of a Paleoproterozoic crystalline basement (Jinshuikou Group). The Meso-Neoproterozoic southern EKOB comprises an oceanic plateau and a belt of tectono-stratigraphic terranes.
The study area is located in the middle EKOB (Fig. 1) 91°27′39″E–91°29′41″E and 36°55′23″N–36°58′01″N. Quaternary sediments are mainly exposed strata (Fig. 2). Northwest-southeast- and NE-SW-trending faults dominate the area. The NW-SE faults are strike-slip faults. Intrusive rocks are widespread and are dominated by monzogranite, granodiorite and diorite, as well as mafic-ultramafic complexes. The monzogranites are mainly Mesozoic and Paleozoic in age, and exposed area is ~6 km2. The Mesozoic monzogranites occur in the northwestern part of the area, while the Paleozoic monzogranites occur in the eastern and middle parts. The granodiorites and diorites are found in the southwest and central parts of the area, respectively. The mafic-ultramafic complexes occur in the southern end of the district, where they outcrop over an area of ~0.73 km2. Dikes, including granite porphyry, diorite and gabbro dikes, are also present in the area (Fig. 2).
The samples used for study were gained from outcrops in the southern and eastern Maxingdawannan area (Fig. 2). The monzogranites, collected from 36°55′56″N, 91°29′45″E (Figs. 2, 3a, 3c), are pale red, medium- and coarse-grained, massive and contain quartz (~35%), plagioclase (~27%), orthoclase (~30%), biotite (~5%), and minor zircon and titanite (~3%) (Figs. 4a, 4b). Samples from mafic-ultramafic complexes, collected at 36°55′43″N, 91°29′09″E (Figs. 2, 3b), are olivine gabbros and olivine pyroxenites. The olivine gabbros are pale-grey, fine- to medium-grained, massive, and contain plagioclase (~40%), clinopyroxene (~30%), olivine (~25%), and minor talc and biotite (~5%)(Figs. 3d, 4c). The olivine pyroxenites are dark-grey, fine- to medium-grained, massive, and contain clinopyroxene (~65%), olivine (~25%), plagioclase (~5%), and minor talc and biotite (~5%) (Figs. 3e, 4d). Early crystallized clinopyroxene was later transformed into talc by autometamorphism.2 ANALYTICAL METHODS 2.1 U-Pb Zircon Geochronology
Zircons were separated and extracted at the Langfang Regional Geological Survey, Hebei Province, China, according to the standard procedures. All zircons were examined carefully and their internal structures were revealed by cathodoluminescence (CL) images. Zircon U-Pb isotope analysis were implemented at the Yanduzhongshi Geological Analysis Laboratories, Beijing, China, following the method of Yuan et al. (2004), corrected method seen Andersen (2002). Isotopic data were calculated by the ICP-MS-DATECAL program (Liu Z Q et al., 2011; Liu Z X et al., 2008). Age calculations and generate concordia plots used Isoplot program (Ludwig, 2003). These data are listed in Table S1.2.2 Whole-Rock Geochemistry
Whole-rock samples were performed in the Yanduzhongshi Geological Analysis Laboratories Ltd., Beijing, China. After removing altered surfaces, fresh samples were chosen and then grind to less than 200 mesh (0.520 0±0.000 1 g) for analysis. Sample powders were performing V8C automatic fusion machine procedure. The precision of major and trace elements are better than 1% and 5%, respectively. These data are shown in Table S2.2.3 Zircon Hf Isotopes
Zircon Hf isotope analyses were carried out using a NewWave UP213 laser system attached to a Neptune multi-collector ICP-MS, at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. The spot size of instrument is 55 μm. Analytical methods and process, data acquisition techniques see Guo et al. (2012) and Wu et al. (2006). Data are presented in Table S3.3 RESULTS 3.1 U-Pb Zircon Geochronology 3.1.1 Monzogranites
Zircons from monzogranite sample MXDWN-16-DB-N4 are 100–150 μm long with aspect ratios of 1 : 1.5 to 1 : 3, and exhibit oscillatory zoning (Fig. 5). They have variable Th (58 ppm–249 ppm) and U (139 ppm–599 ppm) contents, with Th/U ratios of 0.34–0.79 (Table S1), consistent with a magmatic origin (Corfu et al., 2003; Hoskin and Schaltegger, 2003). The 24 analyses yield a mean age of 399±1 Ma (MSWD=1.07; Fig. 6a), interpreted as the crystallisation age of MXDWN monzogranites.3.1.2 Mafic-ultramafic complexes
Cathodoluminescence images reveal that the zircon crystals from a gabbro sample (MXDWN-16-DB-N12) are mostly subhedral with noinherited cores. Most crystals are stubby prisms or sub-rounded with aspect ratios of 1 : 1.5 to 1 : 2.5 (Fig. 5). The samples have variable Th (384 ppm–2 191 ppm) and U (431 ppm–2 109 ppm) contents, and Th/U ratios of 0.54–1.4, which, together with internal textures, indicate a magmatic origin (Corfu et al., 2003; Hoskin and Schaltegger, 2003). Thirty-two analyses yielded ages of 402–393 Ma, with a weighted-mean age of 397±1 Ma (MSWD=0.42; Fig. 6b), interpreted as the crystallization age of MXDWN gabbros.3.2 Whole-Rock Geochemistry 3.2.1 Monzogranites
The monzogranites contain 64.8 wt.%–66.8 wt.% SiO2, 14.1 wt.%–14.6 wt.% Al2O3, 2.56 wt.%–3.28 wt.% Na2O, 4.20 wt.%–5.95 wt.% K2O (Na2O/K2O=0.43–0.78), 1.97 wt.%–2.76 wt.% CaO, 0.76 wt.%–0.81 wt.% TiO2, 4.59 wt.%–5.22 wt.% TFeO, 0.92 wt.%–1.13 wt.% MgO, and Mg#=28.0–30.0 (Mg#=100×MgO/(MgO+TFeO)). The monzogranites are related to quartz monzonite series in the TAS diagram (Fig. 7a), and within the shoshonite series in the K2O vs. SiO2 diagram (Fig. 7b), with A/CNK ratios of 0.94–1.06. The samples have right-declining REE patterns (Fig. 8a) with (La/Yb)N ratios of 3.67–6.94 and negative Eu anomalies (Eu/Eu*=0.36–0.48). Primitive-mantle-normalized variation patterns (Fig. 8b) indicate Rb, U and Sr enrichment and strong Nb, Ta and Ti depletion.3.2.2 Mafic-ultramafic complexes
The loss-on-ignition (LOI) values of 1 wt.%–5 wt.% indicate significant weathering and/or alteration of the olivine gabbros and olivine pyroxenites (Li et al., 2018). The olivine gabbros contain 47.3 wt.%–52.0 wt.% SiO2, 16.1 wt.%–21.3 wt.% Al2O3, 0.32 wt.%–3.15 wt.% TiO2, 6.07 wt.%–11.82 wt.% TFe2O3, 7.54 wt.%–12.7 wt.% CaO, 1.80 wt.%–4.59 wt.% Na2O+K2O, 5.00 wt.%–8.87 wt.% MgO, and Mg#=46.88–77.33. The olivine pyroxenites contain 41.3 wt.%–42.9 wt.% SiO2, 7.26 wt.%–9.13 wt.% Al2O3, 0.45 wt.%–0.82 wt.% TiO2, 11.7 wt.%–12.7 wt.% TFeO, 4.96 wt.%–6.05 wt.% CaO, 0.99 wt.%–1.38 wt.% Na2O+K2O, 23.0 wt.%–24.7 wt.% MgO, and Mg#=79.5–81.5. The LREE contents and (La/Yb)N ratios of the olivine gabbros are 25.20 ppm–73.57 ppm and 4.44–5.03, respectively, and those of the olivine pyroxenites are 17.39 ppm–35.52 ppm and 2.96–3.18, respectively (Fig. 8c). Both rock types are depleted in Nb and Ta (Fig. 8d) and show weak Eu anomalies (Eu/Eu*=0.85–1.16; Fig. 8c; Sun and McDonough, 1989).3.3 Zircon Hf Isotopes
Fifteen spot analyses were gained from the MXDWN monzogranite samples, yielding176Yb/177Hf ratios of 0.016 555–0.039 955 and 176Lu/177Hf ratios of 0.000 584–0.001 388. 176Hf/177Hf ratios are ranging from 0.282 343 to 0.282 560, which corresponds to εHf(t) values of -6.68 to 1.11 (weighted mean -1.99; Fig. 9). TDM2 model ages vary from 1.32 to 1.81 Ga (weighted mean 1.52 Ga; Table S3).
Twelve spot analyses on MXDWN gabbro samples yielding 176Yb/177Hf ratios of 0.019 772–0.030 968 and 176Lu/177Hf ratios of 0.000 680–0.001 117. 176Hf/177Hf ratios are 0.282 480– 0.282 711 with εHf(t) values of -1.81 to 6.29 (Fig. 9). TDM1 model ages are 1 087–769 Ma (weighted-mean 865 Ma; Table S3).4 DISCUSSION 4.1 Early Devonian Magmatism in the EKOB
There is a general consensus that intrusive rocks in the EKOB were mostly emplaced in Early Paleozoic and Early Mesozoic (Dai et al., 2013; Feng C Y et al., 2012, 2010; Cao et al., 2011; Cui et al., 2011; Chen et al., 2006). However, voluminous intrusive rocks within the Maxingdawannan area have not yet be precisely dated. Based on rock associations and lithostratigraphic relationships with surrounding rocks, it was initially identificated as monzogranites and mafic-ultramafic complexes by the China Aero Geophysical Survey and Remote Sensing Center (AGRS) during regional geological mapping. This study firstly provides precise U-Pb dating of the monzogranites and mafic-ultramafic complexes in the Maxingdawannan area.
The ages of the Early Paleozoic Maxingdawannan monzogranites and mafic-ultramafic complexes (399 and 397 Ma, respectively) are consistent with those of Early Paleozoic magmatic activity in other parts of the EKOB (Fig. 10; Wang et al., 2013; Liu et al., 2012; Chen et al., 2006). For example, ~407 and 406 Ma granodiorites and gabbros are found in the Yuejinshan area (Liu et al., 2012), ~406 Ma websterite and ~405 Ma gabbronorite in the Xiarihamu (Song et al., 2016), ~391 Ma syenogranites occur in the Xiarihamu area (Wang et al., 2014, 2013), and ~403 and 394 Ma gabbros and syenogranites are present in the Kayakedengtage area (Chen et al., 2006). The new U-Pb ages for the Maxingdawannan area, together with previously published data, indicate a widespread Early Devonian igneous event in the EKOB.4.2 Magma Sources and Petrogenesis 4.2.1 Petrogenesis 18.104.22.168 Monzogranites
The geochemical data of the Early Devonian monzogranites collected from the Maxingdawannan area can be utilized to trace their magma sources. The monzogranites have elevated SiO2, Al2O3, and alkali element concentrations. In contrast, the concentrations of TFe2O3, MgO (Mg#=28.04–30.00), P2O5, and TiO2 are relatively low. The monogranites have high Zr+Nb+Ce+Y contents (> 698 ppm) and 10 000 Ga/Al ratios (> 2.46), consistent with A-type granites (Whalen et al., 1987). In the (Zr+Nb+Ce+Y)×10-6 vs. (K2O+NaO)/CaO, (Zr+Nb+Ce+Y)×10-6 vs. TFeO/MgO, 10 000Ga/Al vs. (K2O+NaO)/CaO, and 10 000Ga/Al vs. TFeO/MgO diagrams (Fig. 11; Whalen et al., 1987), most samples plot in the A-type granite field. Furthermore, in the Nb vs. Y vs. Ce and Y/Nb vs. Rb/Nb diagrams (Fig. 12), the Maxingdawannan A-type granites plot in the A2 subgroup, indicating their formation in a post-collisional setting. Zircon saturation thermometry (Watson and Harrison, 1983; Watson, 1979) can be used to estimate the temperatures of felsic magma. The zircon saturation temperature (827–865 ℃; mean 842 ℃) is close to the A-type granite temperature of 839 ℃ (Chen et al., 2018).22.214.171.124 Mafic-ultramafic complexes
It is unavoidable that mantle-sourced magma will undergo contamination before the magma was emplaced. LILEs in enrichment and HFSEs in depletion, combined with the REE patterns of the Maxingdawannan complexes, unlike those of N-MORB originated from asthenospheric mantle source (Fig. 8c). Some elements ratios would not be affected during fractional crystallization and partial melting, due to the similar distribution coefficient (MacDonald, 2001; Campbell and Griffiths, 1993). Among these elements and their ratios (e.g., K2O/P2O5, Ce/Pb, Ta/Yb, Nb/Ta and Th/Yb), their covariant relationships can efficiently detect whether crustal components were incorporated into the magma. Positive correlation between the Th/Yb-Nb/La, and La/Yb-Ce/Yb are indicated in Figs. 13a, 13b. Nb/U and Ce/Pb ratios will remain constant during magmatic evolution; accordingly, implying their origin. Hofmann (1988) suggest that Nb/U ratios of MORB and OIB are 47±10. By comparison, Maxingdawannan mafic-ultramafic complexes have Nb/U ratio range from 3.57 to 14.62, which is close to the mean value of the net continental crust (~9.7; Campbell, 2002). In addition, Ce/Pb ratio in the mantle is 25±5, as for the continental crust is less than 15 (Furman et al., 2004). The Maxingdawannan maficultramafic complexes have Ce/Pb ratio range from 1.24 to 3.52 (mean of 2.35), suggesting that incorporated significant crustal material. (La/Nb)PM and (Th/Ta)PM value for the upper and lower crust are markedly different, meaning these trace element ratios can be utilized to ascertain the contaminants (Neal et al., 2002). In Fig. 14, samples either plot near the upper crust or near the lower crust, implying that both the upper and lower crust were the sources of contamination.4.2.2 Magma sources 126.96.36.199 Monzogranites
A-type granites commonly originate from partial melting of felsic crust under low pressure and have negative Eu anomalies and relatively flat HREE patterns. Their Rb/Sr and Rb/Nb values differ from those of coeval mafic-ultramafic complexes, and they contain no mafic enclaves. Based on field and microscopic observations and geochemical analyses, we consider that the Maxingdawannan A-type granites are in this category. Moreover, the Rb/Sr ratios (0.50–0.86) and Rb/Nb ratios (6.0–7.6) of monzogranites are higher than the mean values for global continental crust (0.32 and 4.5, respectively), which also supports a crustal origin for the magmas. Besides, the monzogranites are LILEs in enrichment and HFSEs in depletion, suggesting that these magma was originated from partial melting of the crust (Wu et al., 2000). Depletions in Ti, Nb and Ta indicate that the source for the magma are contained residual ilmenite, rutile and titanite (McKenzie, 1989; Sun and McDonough, 1989). The negative Eu anomalies indicate that these magmas either fractionated plagioclase or were derived from a source containing residual plagioclase.
The zircons from the monzogranites have εHf(t) values of -6.68 to 1.11 (mean value of -1.99; Table S3) and 176Hf/177Hf ratios of 0.282 343 to 0.282 560, reflecting that the zircons didn't crystallize from a homogeneous magma (Andersen et al., 2007). Negative εHf(t) values are consistent with crustal melt, while positive values may indicate mantle components or juvenile crust (Wang et al., 2014, 2013). The older TDM2 ages of ca. 1.81 Ga for the monzogranites resemble the formation age of the Baishahe Group (Wang et al., 2007), indicating that older continental crust materials might do duty for the principal source. The positive Hf for monzogranites suggest that the source might have incorporated into a high-proportioned juvenile materials. In conclusion, we conclude that the monzogranites are probably originated from partial melting of ancient lower crust during the Paleo-Mesoproterozoic, including juvenile crustal materials.188.8.131.52 Mafic-ultramafic complexes
The εHf(t) values of zircons are intermediate between those of chondrites and depleted mantle (Fig. 9), implying the gabbros were mainly associated with partial melting of depleted mantle. Except for two samples, Zr/Nb and Sm/Nd ratios are 22.31–29.74 and ~0.26, respectively, similar to MORB values (10–60 and ~0.32, Davidson, 1996; Anderson, 1994). But Hf model ages TDM1 (1 087–769 Ma) of the zircons are much older. If the primitive magma of the zircons came from depleted mantle, Hf model ages would represent the time of separation of zircon from source. Under the circumstances, the crystallization ages (397 Ma) should be comparably equal to the Hf model ages (Wu et al., 2007, 2004). Therefore, we consider that the enriched materials have contaminated the magma source.
Nb/U ratios are generally defined to reflect the mantle source characteristics, on account of uniform partition coefficient during magmatic crystallization process. MORB and OIB have Nb/U ratios of ~50 (McDonough and Sun, 1995) and continental upper crust ~9 (Rudnick and Fountain, 1995), but arc volcanic rocks have lower ratios of 0.3–9 (Chung et al., 2001). If mantle rocks are generated by contamination of the crust materials, Nb/U ratios should be between ~9 and ~50. We've found that three rocks have Nb/U ratios of 9.97–14.64, this may be the result of contamination. The other three rocks have lower Nb/U ratios (3.57–4.93) than those of continental upper crust but fall into the range of arc volcanic rocks. This signifies that these rocks aren't formed by contamination, but stemmed from a metasomatic mantle source.
Previous researches have suggested that the bulk of fluid-soluble elements would be affected by an influx of water from the subducting plate in the mantle wedge, but HFSEs are relatively depleted (Regelous et al., 1997). Thus, the LILEs in enrichment and HFSEs in depletion are due to the metasomatism of the mantle wedge (Peng et al., 2016), as observations of the Maxingdawannan mafic-ultramafic complexes. In the Th/Yb vs. Nb/Yb diagram, the mafic-ultramafic complexes, plotting close to the volcanic arc array (Fig. 15a), which reflects the input of subduction materials (Pearce and Peate, 1995). Ba/Th and Th/Nb ratios can effectively distinguish between slab fluid and sediment melts. Maxingdawannan mafic-ultramafic complexes have Th/Nb and Th/Yb ratios of 0.20–1.27 and 0.32–3.64, respectively. Figure 15b shows that large volumes of fluids were injected into the source region of the parental magmas (Hanyu et al., 2006). In conclusion, we consider that the Maxingdawannan mafic-ultramafic complexes are resulted from magma originated from a depleted mantle source, which had previously been metasomatized by slab fluids.4.3 Tectonic Implications 4.3.1 Post-collision magmatism in the EKOB
A-type granites are normally considered to form in an extension setting (Eby, 1992; Whalen et al., 1987), with A2-type granites representing a post-orogenic setting. Mafic-ultramafic complexes are also considered A-type granites. Granites in the western EKOB have received little attention, and study of their petrogenesis may help constrain the dynamical evolution of the Proto-Tethys Ocean. As mentioned above, we suggest that Maxingdawannan monzogranites have a A2-type origin. The Maxingdawannan A2-type granites (399 Ma) and associated mafic-ultramafic complexes (397 Ma) indicate the onset of an Early Devonian post-collision setting in the EKOB.
Many granitic rocks in the EKOB have been generated in a post-collision extensional setting (419.0±1.7 Ma, Yan et al., 2016; 420.6±2.6 Ma, Hao et al., 2015; 407±1.7 Ma, Liu et al., 2012; 432 Ma, Gao et al., 2010; 420±4 Ma, Hao et al., 2003). Massive magmatic Ni-(Cu) sulfide deposits linked with mafic-ultramafic complexes (424–394 Ma; Peng et al., 2016; Song et al., 2016; Wang et al., 2014; Li et al., 2012) and 391 Ma post-orogenic A2-type granites in the Xiarihamu area (Wang et al., 2013) are indicative of strong extension. The ages of the Maxingdawannan monzogranites and associated mafic-ultramafic complexes overlap those of the Xiarihamu Ni-(Cu) Deposit (394–424 Ma; Peng et al., 2016; Song et al., 2016; Wang et al., 2014; Li et al., 2012), which is consistent with post-collisional extension. Given the facts above, the authors inclined to support the view of Dong et al. (2017), that is, EKOB was in the post-collisional setting during the Early Devonian.4.3.2 Regional geodynamics
Systematic studies of the opening and expansion of the Early Paleozoic oceans related to the rifting and prolonged break-up of the Rodinia supercontinent at 860–570 Ma in the EKOB were presented by Feng J Y et al. (2010), Li et al. (2008), Lu (2001) and Yang et al. (1996). Early Paleozoic ophiolites represent Proto-Tethyan oceanic crust along the central Kunlun suture (Liu et al., 2011; Yang et al., 1996) that formed mainly at 525–509 Ma (Kong et al., 2017; Liu et al., 2011; Lu, 2002). Subsequently, along the middle Kunlun fault, the Proto-Tethys oceanic lithosphere began subducting to the north, meaning that the southern margin of Qaidam Block switched from passive to active property. A series of volcanic events may have occurred along the middle Kunlun active epi-continental arc in this period (Liu et al., 2013a, b ; Cui et al., 2011; Chen F et al., 2002; Chen N S et al., 2002). Gabbro from Qingshuiquan area (452±5 Ma; Sang et al., 2016), basalt from the Kuangou-Xiaolangyashan area (440±2 Ma; Wang et al., 2012) and basalt from Qimantagh area (439±1.2 Ma; Li et al., 2008) were the products of northward subduction of Proto-Tethys oceanic lithosphere. Along the middle Kunlun suture zone, these mafic plutons and dikes may represent the earliest stage of magmatism relevant to the Early Paleozoic subduction of oceanic crust (Peng et al., 2016; Liu et al., 2013a, b ). Monzogranite and rapakivi were formed in continental collision setting that yield crystallization ages of 430.8±1.7 and 428.5±2.2 Ma in the EKOB, respectively (Cao et al., 2011). Moreover, eclogites with ages of ~428 Ma were formed in the course of closure of the Proto-Tethys Ocean (Meng et al., 2013). Furthermore, the deposition of molasse sediments of Maoniushan Formation (423–400 Ma; Lu, 2002, 2001) indicate that subduction did not continue to Devonian. Collectively, the ages of these rocks indicate that continent-continent collision, including post-collision extension and closure of the Proto-Tethys, finished at ~430 Ma.
Northward subduction of the Proto-Tethys oceanic plate was resulted in metasomatism and enrichment of the subcontinental lithospheric mantle by slab fluids during the Early Paleozoic. At the same time, the Proto-Tethys oceanic lithosphere was accreted to the Qaidam Block and then the closure of Proto-Tethys Ocean. By reason of its great thickness (25–35 km), subduction of the Proto-Tethys oceanic lithosphere was impeded, whereas the previously subducted oceanic crust continued sinking into the mantle. Thus, the subducting slab broke due to the asymmetrical tensional forces, resulting in the formation of a slab window. Asthenospheric mantle upwells through the slab window and then partial melting. Overlying enriched lithospheric mantle would be incorporated into the primary magmas during their uprise. Fractional crystallisation plus crustal contamination was resulted in S saturation and sulphide liquation, and then magmatic differentiation. These differentiated magmas ascended into the crust under buoyancy, forming the mafic-ultramafic complexes. Subsequently, the overlying felsic crust was heated by upwelling and melting of the asthenosphere, resulting in partial melting of the felsic crust at low pressure and formation of the monzogranites (Fig. 16).5 CONCLUSIONS
(1) U-Pb zircon dating shows that the monzogranites (398.8±1.4 Ma) and mafic-ultramafic complexes (396.65±0.92 Ma) were formed during the Early Devonian.
(2) The monzogranites were probably generated by partial melting of ancient lower crust during the Paleo-Mesoproterozoic, involving juvenile crustal melt. The mafic-ultramafic complexes were probably originated from a depleted mantle source that had previously been modified by slab fluids. Crustal materials were incorporated into the primitive magmas during their ascent.
(3) The Early Devonian intrusions were associated with northward subduction of the Proto-Tethys oceanic lithosphere.ACKNOWLEDGMENTS
We thank the staff of the Yanduzhongshi Geological Analysis Laboratories Ltd., Institute of Mineral Resources, Chinese Academy of Geological Sciences, for helping in the analysis. This work was supported by the National Natural Science Foundation of China (No. 41272093), and China Geological Survey (No. 12120114080901). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1203-8.Electronic Supplementary Materials
Supplementary materi-als (Tables S1, S2, S3) are available in the online version of this article at https://doi.org/10.1007/s12583-018-1203-8.
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