Tectonic erosion related to continental subduction: An example from the eastern North Qaidam Mountains, NW China

2.1 North Qaidam UHP metamorphic belt

The basement of the North Qaidam UHP metamorphic belt is mainly composed of orthogneiss, paragneiss, with lesser amphibolite, mafic granulite, marble, and locally eclogite or garnet-peridotite lenses, similar to most continental-type UHP terranes worldwide. They are thrust over, or exhibit a detachment contact with the Early Palaeozoic volcanic sedimentary rocks of the Tanjianshan Group, and are intruded by Silurian granites (Mattinson, Menold, Zhang, & Bird, 2007; Song, Yang, Xu, Liou, & Shi, 2003; Song et al., 2006, 2014; Yang et al., 2002; Yin et al., 2007; Zhang, Mattinson, et al., 2008; Zhang, Song, Zhang, & Niu, 2008; Zhang et al., 2005, 2017). The magmatic protolith ages of most orthogneisses from the NQM UHP metamorphic belt are Late Mesoproterozoic–Early Neoproterozoic (1,030–890 Ma; Mattinson, Wooden, Liou, Bird, & Wu, 2006b; Song et al., 2012; Wan et al., 2001, 2006; Yu et al., 2013; Zhang, Mattinson, et al., 2008; Zhang et al., 2012). Some paragneisses record an almost concurrent metamorphism at 945–895 Ma (Yu et al., 2013; Zhang, Mattinson, et al., 2008; Zhang et al., 2012). However, a few gneisses in the Dulan area, eastern NQM UHP metamorphic belt have Palaeoproterozoic protolith ages of (see below), suggesting that they may have a different origin. Most eclogites of the NQM UHP metamorphic belt have protolith ages ranging from 850 to 750 Ma (Xu et al., 2016; Zhang, Mattinson, Yu, Li, & Meng, 2010; Zhang, Mattinson, et al., 2008; Zhang et al., 2005, 2017 and references therein). Minor eclogite blocks from the Dulan UHP terrane yielded Cambrian protolith ages (c. 516 Ma; Zhang, Song, et al., 2008; Zhang et al., 2016), and were interpreted as part of an ophiolite sequence (Zhang, Song, et al., 2008), implying possible preservation from oceanic subduction that preceded continental subduction and collision (Song et al., 2006, 2014; Zhang, Song, et al., 2008; Zhang et al., 2016).

A large number of zircon U–Pb ages have been reported for NQM UHP metamorphic rocks. Most SHRIMP and LA-ICPMS U–Pb data yield ages ranging from 460 to 420 Ma. Combining these ages with petrological and geochemical data, Song et al. (2014) suggested an earlier oceanic subduction event (460–440 Ma), followed by a continental subduction event (440–420 Ma). Although an earlier oceanic subduction or subduction of continent–ocean transition zone cannot be excluded (Song et al., 2006; Yin et al., 2007; Zhang, Zhang, & Christy, 2013; Zhang et al., 2016), most published data show that eclogite and host gneisses (continental material) have a similar metamorphic age range. We, therefore, prefer to explain that the NQM may represent diachronous or protracted (prolonged) (U)HP metamorphism during early continental subduction and collision (e.g. Mattinson et al., 2006b; Zhang et al., 2010, 2017.

2.2 Oulongbuluke Block

The OB forms a narrow, WNW-ESE oriented belt in the Dakendaban–Quanji Mountain–Delingha–Wulan area (Figure 1). It is separated from the Qaidam block by the NQM UHP metamorphic belt to the south, and from the Qilian block by the Zongwulong Fault (tectonic belt) to the north (Figure 1). The basement of the OB includes the Delingha Complex and Dakendaban Group (Lu, Li, Zhang, & Niu, 2008). They are overlain unconformably by low-grade metamorphosed Wandonggou Group, unmetamorphosed Neoproterozoic Quanji Group and Lower Palaeozoic Oulongbuluke Group (Chen, Zhang, Sun, Wang, & Kusky, 2012; Chen et al., 2013; Lu et al., 2008). The Delingha Complex is mainly composed of amphibolite, monzogranitic gneiss and migmatite. Amphibolite (locally mafic granulite) is exposed as enclaves at various scales (Gong et al., 2012; Lu et al., 2008). The granitic gneisses have been dated at 2.40–2.20 Ga (Gong et al., 2012; Lu et al., 2008). The Dakendaban Group mainly comprises high-grade metamorphic supercrustal rocks including paragneiss, micaschist, quartzite and marble (Chen et al., 2013). Published geochronological data suggest that both the Dakendaban Group and the Delingha Complex record a 1.95–1.80 Ga tectonothermal event (Chen et al., 2013; Gong et al., 2012; Liao et al., 2014; Yu et al., 2017).

In addition, an Early Palaeozoic arc complex belt, which mainly consists of high-grade metamorphic rocks and spatially associated mafic and granitic plutons, is newly recognized in northern part of the OB (Figure 2). It was previously considered as part of the Palaeoproterozoic metamorphic basement. Recently, new data show that this unit experienced Early Palaeozoic (500–460 Ma) low-P/high-T metamorphism and associated magmatism (Li et al., 2015; J. Zhang et al., unpublished data). This also suggests an earlier oceanic subduction prior to continental subduction along the North Qaidam Mountains (see discussion below).

3.1 Oulongbuluke Block

Three felsic orthogneisses (AQ12-12-1.1, AQ12-12-4.2 and AQ14-16-8.1) and one paragneiss (AQ12-9-2.1) from OB were sampled for zircon U–Pb dating (Figure 2). These felsic orthogneisses display clear gneissic structures and preserve locally migmatitic textures characterized by quartzofeldspathic leucosomes and dark melanosomes with a higher concentration of biotite (Figure 3a). The amphibolite (partly mafic granulite) can be observed as lenses or interlayers within the felsic gneisses (Figure 3a). The homogenous felsic orthogneisses consist of K-feldspar (~35%), plagioclase (~25%), quartz (~25%) and biotite (8%–12%), with minor apatite, titanite, monazite and zircon (Figure 4a,b). Feldspar shows a certain degree of sericitization, and the biotite is partially replaced by chlorite. The quartz shows dynamic recrystallization related to ductile deformation.

Migmatitic paragneiss exhibits a mixture of light-coloured quartzofeldspathic leucosomes interpreted to represent the former sites of melt segregation, and grey-coloured palaeosomes and dark-coloured melanosomes which are interpreted to represent the residuum from which melt was extracted (Figure 3b). The leucosomes generally occur as thin layers parallel to the foliation in the gneiss, and are mainly composed of feldspar and quartz, with very minor biotite, titanite, apatite and zircon. Leucosome layers are locally folded by tight to open folds on the centimetre to metre scale. Sample AQ12-9-2.1 is sourced from grey-coloured palaeosomes with thin leucosome layers. It is mainly composed of feldspar (K-feldspar+plagioclase, 30%–35%), quartz (40%–45%), biotite (~10%) and epidote (~10%), with zircon, titanite and monazite as accessory minerals (Figure 4b,c).

3.2 Dulan UHP metamorphic unit

Four samples of felsic gneiss (Q08-8-1.5, Q08-9-3.3, AQ13-7-4.2 and D73) and two eclogite samples (AQ16-22-2.1 and AQ16-22-6.2) were collected from the south of the Shaliu River and north of the Aercituo Mountain. These felsic gneisses enclose eclogite lenses or boudins from 1 to 10 m in width (Figure 3c,d and Figure S1). Eclogite lenses commonly are rimmed by foliated amphibolites parallel to the foliation of the enclosing gneisses. The four gneiss samples have similar texture and mineral composition. They display a porphyroblastic texture and weak gneissic structure. Their mineral assemblage is quartz+plagioclase+K-feldspar+garnet+biotite+amphibole, with minor titanite, epidote and zircon (Figure 4c). This mineral assemblage suggests an amphibolite facies metamorphism related to retrogression. No clear HP-UHP evidence is recognized although some titanite grains contain rutile inclusions, possibly representing high-P metamorphic relicts (Figure 4c).

The mineral assemblage of eclogites (AQ16-22-2.1 and AQ16-22-6.2) is garnet+omphacite+rutile+quartz±phengite (Figure 4d). Amphibole+plagioclase symplectite around omphacite records minor amphibolite-facies retrogression. Garnet–clinopyroxene–phengite thermobarometry yields a P–T condition ~700°C and 2.8 GPa (J. Zhang et al., unpublished data), falling in the coesite stability field although relict coesite has not been observed.

4.1 Analytical procedures

The zircon U–Pb dating of sample D73 was carried out on the Stanford/USGS SHRIMP-RG. The zircon U–Pb and Hf isotopic analysis was carried out using laser ablation sampling and multiple collector inductively coupled plasma mass spectrometry at the Tianjin Institute of Geology and Mineral Resources. Analytical procedures are presented in Appendix S1. The detailed data are shown in Tables S1–S3.

4.2 U–Pb dating results

4.2.1 Oulongbuluke Block

Felsic orthogneisses

Zircon grains from felsic orthogneisses are 150–300 μm long and show irregular or prismatic morphologies. In the CL images, some crystals (sample AQ12-12-1.1) are characterized by irregular inherited zircon cores, oscillatory zoning inner rims, and structureless and CL-dark outer rims (Figure 5). The inherited zircon cores from AQ12-12-1.1 yield ²⁰⁷Pb/²⁰⁶Pb ages ranging from 2,937 ± 16 Ma to 2,443 ± 15 Ma. The oscillatory-zoned inner rims gave ²⁰⁷Pb/²⁰⁶Pb ages between 2,405 ± 16 Ma and 2,354 ± 15 Ma, combining to a weighted mean age at 2,378 ± 12 Ma (n = 11, MSWD = 1.3), which we interpret as the magmatic crystallization age of the protolith of the felsic orthogneiss. Five analyses of structureless outer rims gave ²⁰⁷Pb/²⁰⁶Pb ages between 1,935 ± 19 and 1,888 ± 20 Ma, combining a weighted mean age of 1,914 ± 16 Ma (Figure 6), which we interpret to reflect Late Palaeoproterozoic metamorphism.

Zircon grains from felsic orthogneisses, AQ12-12-4.2 and AQ14-16-8.1, show oscillatory-zoned domains surrounded by structureless rims with varying width (Figure S2). The analyses from oscillatory-zoned domains yield upper intercept ages of 2,364 ± 10 Ma (MSWD = 1.4) and 2,390 ± 9 Ma (MSWD = 0.62) for AQ12-12-4.2 and AQ14-16-8.1, respectively (Figure S3), which are interpreted as the magmatic crystallization age of the protolith. Seven analyses from structureless zircon rims of AQ12-12-4.2 yield a weighted mean age of 1,889 ± 13 Ma (MSWD = 0.23), and one analysis from a zircon rim of AQ14-16-8.1 gives a ²⁰⁷Pb/²⁰⁶Pb age of 1,886 ± 39 Ma (Table S1), which are interpreted as timing of a metamorphic and anatectic event.

Migmatitic paragneiss

Zircon grains from migmatitic paragneiss AQ12-9-2.1 are 100–250 μm in diameter and show oval and nearly spherical morphologies. CL images reveal that most grains contain CL-bright grey and oscillatory-zoned cores, and CL-grey and structureless or fir-tree zoned rims (Figure 5). A few zircon grains do not have cores but consist entirely of CL-grey grains with patchy or fir-tree patterns (Figure 5 and Figure S2), similar to those of the rims surrounding the cores. The 37 cores analysed yield ²⁰⁷Pb/²⁰⁶Pb ages ranging from 2,708 ± 15 Ma to 2,318 ± 19 Ma, which are interpreted as ages of detrital zircon from the sedimentary protolith of the paragneiss. The 24 analyses from zircon rims yield an upper intercept age of 1,917 ± 17 Ma (MSWD = 0.92; Figure 6).

4.3 Hf isotopic results

4.3.1 Oulongbuluke Block

Of the 77 Hf isotope analyses obtained for orthogneiss samples AQ12-12-1.1 and AQ12-12-4.2, 69 of those from inherited cores and magmatic rims with oscillatory zoning have ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.281210 to 0.281564 (mainly between 0.281300 and 0.281400; Table S3). Their age-corrected ε_Hf(t) values are between −8.47 and 8.52 with two-stage model ages (T_DM2) ranging from 2.91 to 2.55 Ga (Figure 7; Table S3). Eight analyses on c. 1.9 Ga outer rims yield ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.281371 and 0.281481, age-corrected ε_Hf(t) values of −4.10 and −7.32, with T_DM2 between 2.77 and 2.59 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf ratios of this metamorphic zircon overlap those of the inherited and magmatic zircon.

Of the 40 Hf isotope analyses obtained for migmatitic paragneiss AQ12-9-2.1, 23 analyses from detrital cores have ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.281080 to 0.281431 (Table S3). Their age-corrected ε_Hf(t) value is between −2.17 and 6.68, corresponding to T_DM2 values between 3.16 and 2.54 Ga. Seventeen analyses from metamorphic rims have ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.281296 to 0.281413, overlapping those from the detrital zircon cores. Their age-corrected ε_Hf(t) values are between −5.31 and −11.30, and corresponding T_DM2 between 2.94 and 2.70 Ga.

5.1 Palaeoproterozoic magmatic and metamorphic ages of the OB

In this study, three orthogneisses from the Delingha Complex of the OB yield ages of 2,364 ± 10 Ma (AQ12-12-4.2), 2,378 ± 12 Ma (AQ12-12-1.1) and 2,390 ± 9 Ma (AQ14-16-8.1) from zircon cores which exhibit clear oscillatory zoning with high Th and U contents, and high Th/U ratios. These features suggest that these zircon domains are magmatic in origin and thus 2.39–2.36 Ga represents the magmatic protolith age of the orthogneisses. Similar Early Palaeoproterozoic magmatic ages (from 2.4 to 2.3 Ga) have been commonly reported in the Delingha Complex of the OB (Figure S4; Gong et al., 2012; Yu et al., 2017 and references therein). Two orthogneisses give weighted mean ages of 1,914 ± 16 Ma (AQ12-12-1.1) and 1,889 ± 13 Ma (AQ11-12-4.2), one granitic orthogneiss (AQ14-16-8.1) gives a ²⁰⁷Pb/²⁰⁶Pb age of 1,886 ± 39 Ma, and one migmatitic paragneiss (AQ12-9-2.1) yields an upper intercept age of 1,917 ± 17 Ma. Most of these data were obtained from CL structureless rims, similar to typical metamorphic zircon. A few analyses from oscillatory-zoned rims possibly imply an anatectic origin. We thus interpret the c. 1.9 Ga as the age of metamorphism and possible anatexis. Previous studies gave similar Late Palaeoproterozoic metamorphic ages ranging from 1.95 to 1.80 Ga (Chen et al., 2013; Yu et al., 2017 and references therein, also see Figure S4). Chen et al. (2013) suggested that the OB underwent multiple Late Palaeoproterozoic metamorphic events: medium-P/T type upper amphibolite facies metamorphic event at 1.96–1.90 Ga; low-P/T amphibolite facies metamorphism at c. 1.85–1.82 Ga and a subsequent medium-P/T metamorphism at c. 1.82–1.80 Ga. SHRIMP U–Pb analyses of one mafic granulite (enclosed in orthogneiss AQ12-12-4.2) also give multiple metamorphic ages at c. 1.91, 1.86 and 1.83 Ga, which are interpreted as granulite facies metamorphic timing and subsequent amphibolite facies overprinting ages (Chen et al., 2013). However, a detailed discussion of multiple Palaeoproterozoic metamorphic events is outside the scope of this paper. Generally, the present data provide unambiguous evidence that the OB underwent Early Palaeoproterozoic magmatism and subsequently Late Palaeoproterozoic high-grade metamorphic and anatectic event(s).

5.2 Palaeoproterozoic rocks in the Dulan UHP metamorphic unit: slices from the OB?

Although Palaeoproterozoic and Archean ages have been reported from inherited or detrital zircon from orthogneisses and paragneisses in the NQM UHP metamorphic belt as well as in the Qaidam block (e.g. Mattinson, Wooden, Zhang, & Bird, 2009; Yu et al., 2013), metamorphic rocks with Palaeoproterozoic protolith ages have not previously been recognized. In this study, four orthogneiss samples from the Dulan UHP metamorphic unit give U–Pb ages of 2,307 ± 32, 2,388 ± 58, 2,329 ± 33 and 2,364 ± 20 Ma from analyses on zircon cores with oscillatory zoning. Combined with petrographic observations, we interpret that 2.39–2.31 Ga represents the magmatic crystallization age of the orthogneiss protoliths, similar to those from the OB orthogneisses. The zircon rims (inner rims) from four gneiss samples yield ages of 1,833 ± 19, 1,839 ± 36, 1,882 ± 20 and 1,849 ± 17 Ma. These ages are obtained mainly from CL structureless rims and we thus interpret them to record Late Palaeoproterozoic metamorphism, consistent with the age of Late Palaeoproterozoic metamorphism recorded in the OB. In addition, the analyses on CL structureless zircon cores from one eclogite sample enclosed within gneiss give an age of 1,916 ± 10 Ma, similar to the age obtained from metamorphic mafic rocks enclosed within orthogneisses in the OB (Yu et al., 2017). These data demonstrate that some gneisses and enclosed mafic rocks from both the NQM UHP metamorphic belt and Oulongbukuke experienced similar Early Palaeoproterozoic magmatism and Late Palaeoproterozoic metamorphism (Figure S4). In addition, the Palaeoproterozoic magmatic and metamorphic zircon from the two terranes have consistent Hf isotopic features (Figure 7). Despite variable Pb-loss, Palaeoproterozoic zircon of the gneisses from both units shows similar Hf isotopic evolution patterns and correspondingly consistent Hf model ages (Figure 7). Therefore, we suggest that these rocks with Palaeoproterozoic protolith age in the NQM UHP metamorphic belt may be derived from the Palaeoproterozoic metamorphic basement of the OB rather than from the Qaidam block. Up to now, no Palaeoproterozoic magmatic and metamorphic rocks have been reported in the Qaidam block although zircon with Palaeoproterozoic ages has been observed as detrital or inherited grains in Neoproterozoic–Palaeozoic metasedimentary rocks or magmatic rocks from the basement of the Qaidam Basin (e.g. Cheng et al., 2017).

5.3 Tectonic models for Early Palaeozoic NQM UHP metamorphic belt

Several contrasting models have been proposed to explain the formation and geodynamic evolution for the NQM UHP metamorphic belt (Song et al., 2006, 2014; Yang et al., 2002; Yin et al., 2007; Zhang et al., 2017). The debates concern continental v. oceanic subduction and the subduction polarity. Yang et al. (2002) explained the NQM UHP metamorphic belt as an Early Palaeozoic suture, which resulted from the north-dipping subduction of the Qaidam block. Song et al. (2006) proposed a single, north-dipping subduction model to incorporate both the North Qilian oceanic subduction belt and NQM continental subduction belt. This model implied that the NQM UHP metamorphic belt is not a suture, and the NQM UHP rocks travelled ~350 km after originating as oceanic crust from the northern margin of the Qilian–Qaidam block basement, and were then tectonically emplaced within the continental crust of the coherent Qaidam–Qilian block (see review by Song et al., 2014). However, in this model it is difficult to explain the Early Palaeozoic arc magmatic and metamorphic events in the northern part of the North Qadiam Mountains (Li et al., 2015; Shi, Wu, & Yang, 2004). Yin et al. (2007) proposed a “diapiric flow” model, in which the NQM UHP metamorphic eclogites originated from south-dipping subducted palaeo-Qilian oceanic crust via diapiric flow across the mantle wedge. Subduction of continental–oceanic transition zone sediments was invoked to explain the occurrence of the continental UHPM materials. However, in this model it is difficult to interpret the dominant continental material in the NQM UHP metamorphic belt, which mainly consists of Late Mesoproterozoic–Early Neoproterozoic gneisses, similar to Precambrian crystalline basement of the Qaidam block.

Based on the existing and our new data, we explain the NQM UHP metamorphic belt as a result of north-dipping continental subduction of the Qaidam block following earlier oceanic subduction (Zhang et al., 2016, 2017; and references therein). The earlier oceanic subduction along the North Qaidam Mountains is also supported by a newly recognized arc magmatic–metamorphic belt in the northern part of the North Qaidam Mountains (Figure 2). This arc unit is proposed to be related to oceanic subduction that preceded continental subduction (Li et al., 2015). The spatial overlapping of UHP rocks and arc sequences (Tanjianshan Group) along the NQD can be related to Mesozoic–Cenozoic deformation (Yu et al., 2017; and references therein).

5.4 Tectonic erosion related to continental subduction

Tectonic erosion can be classified into two distinct processes, that is, frontal erosion and basal erosion (Vannucchi et al., 2008; Von Huene & Scholl, 1991). The former can tectonically erode pre-existing accretionary wedge/slope sediments. On the other hand, the latter works at much deeper levels where basement material from the upper plate is tectonically removed into the subduction channel (e.g. Clift & Hartley, 2007; Clift & Vannucchi, 2004).

As mentioned above, northward subduction of the Qaidam block is preferred to explain the formation of the continental-type UHP metamorphic rocks along the North Qaidam Mountains. If this subduction geometry is correct, gneiss and enclosed eclogite with Palaeoproterozoic protolith ages in the NQM UHP metamorphic belt would be derived from the upper plate (OB) rather than the lower plate (Qaidam block). As mentioned before, the Qaidam block mainly consists of Meoproterozoic–Neoproterozoic crystalline basement, and no Palaeoproterozoic rocks have been reported. LA-ICMPS and SHRIMP U–Pb dating on the metamorphic rims of zircon from these gneisses and eclogites yields an Early Palaeozoic age. These data suggest that the Palaeoproterozoic gneisses and enclosed eclogites underwent a common Early Palaeozoic UHP metamorphism; the scarcity of (U)HP assemblages in these gneisses might be explained by the more extensive retrograde recrystallization, compared with the enclosed eclogites.

Based on the discussion above, we propose a tectonic erosion model: When the Qaidam block subducted towards the north under the OB during the Early Palaeozoic, slices of Palaeoproterozoic basement from the upper plate (OB) were tectonically buried to more than 80 km depth in the subduction channel, and experienced UHP metamorphism during the Early Palaeozoic continental subduction prior to their exhumation (Figure 8). This also implies that a basal erosion process occurred in the North Qaidam Mountains during the Early Palaeozoic. However, given the lack of an accretionary complex in the eastern North Qaidam Mountains (the Dulan region), frontal erosion cannot be excluded during oceanic subduction prior to continental subduction (Yin et al., 2007).