Journal of Earth Science  2019, Vol. 30 Issue (3): 494-509   PDF    
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Confirmation of the Major Grenville Event in the Qinling Complex of the Qinling Orogenic Belt, Central China
Chong Li 1, Liudong Ren 1, Shi Zong 1, Yanbin Wang 1, Miao Li 2     
1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
ABSTRACT: The Qinling Complex from the Qinling orogenic belt was generally considered to be part of the Caledonian orogeny, however information of the Grenville event of the Qinling Complex has been poorly recognized. Two granite samples of greenschist-facies and two paragneiss samples of amphibolite- faices are identified from the Qinling Complex. The granites occur along the regional gneissosity of the Qinling Complex, thus it is suggested that their magmatic zircon ages (~970 Ma) mark the lower boundary of the main metamorphic age (amphibolite-granulite facies). In addition, some early-formed metamorphic zircons (~1 000 Ma) are distinguished in the granites, which may reflect the information about the source area of the granites. Therefore the major metamorphism of amphibolite-granulite facies in the Qinling Complex is constrained at Early Neoproterozoic (~1 000 Ma), not Early Paleozoic as conventionally considered. In the Early Paleozoic, the Qinling Complex was characterized by multiple extension- shear activities, overprint of greenschist-facies metamorphism and emplacement of extensive granites. These granites and related thermal events could reset the U-Pb isotopic system of the early-formed zircons, leading to the apparent zircon ages younger than their protolith age. As a result, the Qinling Complex is a modified Early Neoproterozoic orognenic belt or an independent block, which became a continental margin arc during the Early Paleozoic, being accompanied by metamorphism, deformation, and emplacement of continental arc granites. The Erlangping, Kuanping, and Taowan groups to the north of the Qinling Complex show more intensive deformation of the Caledonian, and their oblique subduction towards the Qinling Complex caused the formation of eclogites. Afterwards, the Qinling Complex was amalgamated with the Erlangping, Kuanping, and Taowan groups, which all experienced the same Caledonian orogeny, and possible later orogenies.
KEY WORDS: Neoproterozoic    Caledonian    metamorphism    granite    zircon chronology    Qinling Complex    
0 INTRODUCTION

The Qinling orogenic belt is located between the North China Block (NCB) and the South China Block (SCB) (Fig. 1). Being a part of the belt, the Qinling Complex shows a complex appearance of migmatization and ultrahigh-pressure (UHP), ultrahigh-temperature (UHT) metamorphism and multiple granite activities (Liu et al., 2014; Xiang et al., 2012; Dong et al., 2011a, b; Zhang et al., 2011, 2009; You et al., 1991). Controversies exist about the formation and main metamorphic age of the Qinling Complex, as well as the tectonic attribution of the North Qinling Block. Although Archean to Paleoproterozoic formation age can be obtained, the Sm-Nd, Rb-Sr whole rock analyses only gave the poorly constrained results (Zhang et al., 1996).

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Figure 1. Geological map of the Qinling Complex (modified after Lu et al., 2003). SDF. Shangdan fault; ZXF. Zhuxia fault.

As to the metamorphism of the Qinling Complex, some authors believed that it involved into multiple stages of metamorphism and deformation (Pei et al., 1999, 1998; Zhang et al., 1996; An et al., 1985), which were also known as the Jinning and Caledonian metamorphism (Pei et al., 1998; Dong et al., 1997a, b; Wang et al., 1997; Chen and Zhang, 1993). The earlier Jinning event was supported by the ambiguous Rb-Sr whole rock technique (Chen et al., 1993). Recent studies, however, emphasized that the whole Qinling Complex experienced Early Paleozoic metamorphism which consists of the early eclogite-facies metamorphism, the following 450 Ma granulite-facies metamorphism and the later amphibolite-facies metamorphism at 420 Ma (Xiang et al., 2014; Zhang et al., 2011; Liu et al., 2009). In addition to the widely accepted point of view that the early UHP and high- pressure (HP) granulite metamorphism occurred at ~500 Ma (Zhang et al., 2011; Liu et al., 2009; Yang et al., 2003), there are still some major debates on the characteristics of the main metamorphisms, such as the metamorphic types and forming age of the medium-low pressure granulite and amphibolite-facies metamorphism, and the relationship between granite and metamorphism. The debates come from the dating method itself and interpretation of the age data, because early dating methods were mainly Rb-Sr and Sm-Nd dating which are not accurate enough, and the interpretation of recent zircon age data is complex and problematic considering the diversity of zircon genesis and degree of closure of U-Pb isotope system (Harley et al., 2007; Corfu et al., 2003).

Through detailed observations and analyses both from the outcrops and under microscope, we notice that the Qinling Complex has experienced multiple stages of deformation, metamorphism and granite emplacement (Fig. 1). The multiple deformation, metamorphism and granite activities are very likely to obscure the structure of pre-existing zircons, leading to the complex structure appearances of zircons, such as the dark zircon (rim) in the cathodoluminescence (CL) images, and resetting of the isotope system of these zircons. Most of the age data of these zircons will certainly contribute diversity and complexity to interpretation of the metamorphic zircon ages. From all of which it will raise a question: how to distinguish magmatic activities, metamorphic events or zircon reset events from each other through the information recorded by zircon age data? Through a detailed study on the distribution, composition of the major rocks and the structural relationship of minerals, we have analyzed the distribution pattern of the metamorphic belt in the Qinling Complex, and constrained the metamorphic characteristics and age of the Qinling Complex. Thus we emphasize that the time of metamorphic events could be constrained by the emplacement age of granites which are closely related to metamorphism.

1 GEOLOGICAL SETTING

The Qinling orogenic belt, located between the NCB and SCB, is a very important tectonic belt of China. It has been involved in multiple tectonic, metamorphic and magmatic activities, and has complex material compositions and structures (Fig. 1).

The China continent became a united continent during the Indosinian Period, marked by amalgamating of the NCB and SCB (Zhang et al., 2001). The Qinling orogenic belt is divided into the North Qinling and South Qinling belts by the Shangdan suture zone, while the North Qinling belt mainly consists of several metamorphic units such as the Taowan Group, Kuanping Group, Erlangping Group, Qinling Group and Danfeng Group from north to south. The Taowan Group is made up of metamorphic conglomerate and sandy slate, showing a lower-greenschist facies feature (Liu et al., 1993). The Kuanping Group comprises chiefly basic volcanic rocks, biotite marbles, phyllites and thin interlayer of quartzites, which underwent a lower greenschist- to amphibolite-facies metamorphism. The Erlangping Group is composed of an ophiolitic unit and clastic sediment, with greenschist- to amphibolite-facies metamorphism. The Qinling Group comprises clastic rocks, and marbles with little volcanic rocks (Shi et al., 2009). The Danfeng Group is composed of metamorphosed clastic rocks, limestones and basic volcanic rocks, which underwent an amphibolite-facies metamorphism.

The Qinling Group extending for thousands kilometers from east to west, is mainly distributed continuously in lenticular blocks (Zhang et al., 2001). In general, You et al. (1991) proposed that the Qinling Complex consisted of several tectonic lenticular blocks, such as the Yaozhuang-Shewei lenticular block at the boundary area of Henan and Hubei provinces, and each of the blocks could be regarded as a metamorphic core complex, or as described by Wang et al. (1997), the Qinling Complex was a huge lenticular block constrained between the Zhuxia fault and Shangdan fault, which was also called 'Qinling Group' including the Qinling Group Complex, Xiahe Group Complex, orthogneisses, marbles and ophiolitic unit.

Former researchers suggested that the Qinling Complex was a series of supracrustal rocks which chiefly comprise metasedimentary rocks rich in aluminum, carbon, and carbonate. The major types of rocks are gneisses, quartz schists, quartzites, amphibolites, marbles, calc-silicate rocks, and leptynites. Biotite, graphite, sillimanite, garnet are common minerals. The Qinling Complex has experienced multiple stages of metamorphism, deformation and magmatism, and has undergone medium to high grade metamorphism which is dominated by amphibolite facies, locally reaching granulite facies, and accompanied by anatexis (Zhang H F et al., 1995; You et al., 1991; Zhang G W et al., 1988). The multiple-stage deformation and magma intrusion are featured by deep plastic rheological flow and ductile shear zones (Zhang et al., 2001; Wang and Yang, 1993; You et al., 1991). In recent years, a series of HP and UHP rocks with coesite pseudomorphs and diamond inclusions in zircon has been reported at the north margin of the Qinling Complex (Yang et al., 2003; Hu et al., 1995). The UHT metamorphic rocks have also been found in the eastern part (Xiang et al., 2012). In addition, there have been multiple magma activities of Neoproterozoic, Early Paleozoic and Mesozoic in the complex. The granites intruding in the Qinling Complex were mainly formed between 490–400 Ma (Wang X X et al., 2013; Wang T et al., 2009). Through statistical analysis of a large number of age data, Zhang et al. (2013) suggested that there were three stages of major magmatism activities of 500, 450 and 420 Ma occurring in the North Qinling belt in the Early Paleozoic, which are in accordance with the HP-UHP metamorphism (500 Ma), the medium-pressure granulite-facies metamorphism (450 Ma) and the amphibolite-facies retrograde metamorphism (420 Ma) (Diwu et al., 2014; Zhang et al., 2011, 2009; Liu et al., 2009), respectively.

As to the forming age of the Qinling Complex, there are three major points of views, such as Paleoproterozoic, Mesoproterozoic and Neoproterozoic (Huang et al., 2018; Shi et al., 2009; Lu et al., 2006; Yang et al., 2002; Zhang Z Q et al., 1996; Zhang H F et al., 1995; Kröner et al., 1993; Liu et al., 1993; You et al., 1991; Zhang G W et al., 1988; An et al., 1985).

2 ZIRCON U-PB DATING 2.1 Analytical Techniques

Separating and picking of zircons were carried out in the Mineral and Rock Analysis Laboratory of Institute of Geological Survey of Hebei Province. The picked zircons were paved on the epoxy target and polished. Optical photographs, CL and BSE (back scattered electronic) images were taken successively. Then the suitable grains were chosen for age measuring. The zircon age dating was operated with the LA-MC-ICPMS instrument in the Isotope Laboratory of Tianjin Center of China Geological Survey. The properties, analytical method and measuring procedures were after Li et al. (2009). The laser beam is 35 μm in diameter, frequency at 8–10 Hz and energy density of the laser instrument is 13–14 J/cm3. Take the TEMORA as the out age standard of zircon and the ICPMSDataCal and ISOPLOT program (Liu et al., 2010), for data processing, and the common lead is corrected with 208Pb. The NIST612 glass standard is used for calculating the Pb, U and Th contents of the samples. For ages older than 1 000 Ma, the 207Pb/206Pb age, and those younger than 1 000 Ma, the 206Pb/238U age is adopted, respectively. Error for each spot is 1σ, and the confidence level for the average ages is 95%. Here, we selected two gneissic granites (samples XX25-2 and SN50-1) and two paragneisses (samples SN50-2 and NX16-1) from the Qinling Complex for zircon U-Pb dating in order to clarify its formation age (Fig. 1).

2.2 Sample Description

Sample XX25-2 is a gneissic biotite-plagioclase granite which was collected from Shuanglong Section from Xixia County, Henan Province. It occurs along the regional gneissosity (Fig. 2a), and mainly consists of biotite (5%), plagioclase (60%), quartz (20%) and K-feldspar (10%), in which the quartz grains show a cataclastic appearance (Fig. 3a). It has undergone greenschist facies metamorphism.

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Figure 2. Field Photographs showing the outcrops of the Qinling Complex. (a) Gneissic Bt-Pl granite (XX25-2), Shuanglong Section, Xixia County; (b) gneissic monzonitic granite (SN50-1) occurring along the gneissosity of paragneiss (SN50-2), Shima Town, Shangnan County; (c), (d) Bt-Pl gneiss (paragneiss) (NX16-1) with pegmatite vein, Neixing County. Bt. Biotite; Pl. plagioclase.
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Figure 3. Photomicrographs showing the rocks of the Qinling Complex. (a) Gneissic Bt- Pl granite (XX25-2) with subhedral plagioclase and cataclastic quartz (Qtz); (b) gneissic monozonitic granite (SN50-1) with coarse-grained euhedral to subhedral feldspar; (c) paragneiss (SN50-2), biotite and fine-grained sillimanite (Fib) define the gnessosity; (d) Bt-Pl paragneiss (NX16-1), kyanite (Ky) and garnet (Grt) often occur with biotite. Kfs. K-feldspar.

Sample SN50-2 is a paragneiss (Fig. 2b) belonging to the Guozhuang Formation of the Qinling Complex, and mainly consists of garnet (10%), sillimanite (Sil, 5%), biotite (10%), plagioclase (20%), K-feldspar (10%) and quartz (40%) (Fig. 3c). It has undergone amphibolite facies metamorphism.

Sample SN50-1 is a gneissic monzonitic granite (non- metamorphic sample) which was collected from Shangcangfang, Shima Town, from Shangnan County, Shaanxi. It is next to the Sil-Grt-Bt-Pl gneiss (SN50-2) and Grt-amphibolite (SN50-3), and mainly consists of biotite (5%), plagioclase (40%), K-feldspar (35%), quartz (15%) and myrmekite. It occurs as lenticular lenses along the regional gneissosity (Fig. 2b) indicating that the emplacement was later than the formation of regional gneissosity.

Sample NX16-1 is a Bt-Pl gneiss which was collected from Neixiang County, Henan Province. It mainly consists of kyanite (5%), garnet (10%), biotite (10%), muscovite (5%), K-feldspar (10%), plagioclase (20%), sillimanite (3%) and quartz (40%). The mineral assemblage and zircon morphology indicate that it is a paragneiss (Fig. 3d). Pegmatite veins are visible inside the rocks (Figs. 2c, 2d). It has undergone a higher-amphibolite facies metamorphism.

3 ANALYTICAL RESULTS 3.1 Sample XX25-2 (Gneissic Granite)

Zircon sizes range in length of 100–200 μm, with length to width ratios of 1 : 1 to 2 : 1. All grains exhibit well-developed prismatic faces and are relatively uniform in grain sizes (Fig. 4a). The 33 spots from 24 zircon grains have been analyzed (Table S1). Core-mantle-rim structure (Fig. 4a, spot 19) is usually developed in zircons according to CL images, and four domains can be identified. Domain 1 often occurs as anhedral zircon core with oscillatory zoning (Fig. 4a, spot 13) and gives an unconcordant age (Fig. 5a); Domain 2 usually occurs as zircon mantle (Fig. 4a, spot 19) or core (Fig. 4a, spot 2) with clear oscillatory zoning (magmatic origin), ages obtained from domain 2 spread along the concordia (Fig. 5b), and the oldest concordant 206Pb/238U age is 970±9 Ma (spot 4, Th/U=0.23), the 6 relatively clustered ages give a weighted mean age of 904±24 Ma (MSWD=6.5) (Fig. 5b); Domain 3 generally occurs as zircon rim (Fig. 4a, spots 1, 14), the oldest concordant 206Pb/238U age is 921±10 Ma (Th/U=0.11); Domain 4 displays a distinctive soccer-like appearance (Fig. 4a, spot 22), spot 4 gives a 206Pb/238U age of 910±9 Ma (Th/U=0.11).

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Figure 4. Representative CL images of zircons of the Qinling Complex with analyzed spots and zircon ages
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Figure 5. Concordant diagrams and histograms of LA-ICP-MS zircon U-Pb data from the Qinling Complex.
3.2 Sample SN50-1 (Gneissic Granite)

Zircons are between 80 and 200 μm in length, with length to width ratios of 2 : 1 to 3 : 1. Fractures are often developed in the grains which are marked by white threads in CL images (Fig. 4b, spot 4). Most of zircon grains exhibit an euhedral habit of crystallization, but the margins are often rounded. The 102 spots from 90 zircon grains have been analyzed (Table S2). Core-mantle-rim structure often occurs in the zircons, but is more complex than that of Sample XX25-2. Four zircon domains can be recognized according to CL images. Domain 1 occurs as anhedral zircon core with weak or blurred oscillatory zoning (Fig. 4b, spot 46), ages from this domain are often older than those of the other domains, which suggests it belongs to inherited zircon, spot 46 gives the oldest 207Pb/206Pb age of 1 552±22 Ma (Th/U=0.02); Domain 2 is the zircon with Grenville age, which is minor in the sample, the zircons are often euhedral to subhedral with typical oscillatory zoning (Fig 4b, spots 4, 14) (magmatic origin), four analyzed spots (spots 4, 14, 17 and 47) give a weighted mean age of 963±46 Ma (MSWD=4.4); Domain 3 can occur as zircon core or rim, with gray to dark gray CL images (Fig. 4b, spot 73), the majority of zircon ages belongs to Early Paleozoic, which can be subdivided into three groups giving the weighted mean ages of 565±10 (N=8, MSWD=2.3), 483±6 (N=5, MSWD=0.79), and 439±17 Ma (N=7, MSWD=6.0), respectively. There are also some Domain 3 zircons of light gray to dark gray CL images, giving the 206Pb/208U age of 304±4.6 and 245±5 Ma with Th/U=0.001–0.08. Counted by 206Pb/208U age, these zircons give a strong peak of 576 Ma, sub-strong peaks of 600, 485, 441, 317, 306 Ma, and weak peaks of 967, 791, 679, 379, 291, 279, and 244 Ma; Domain 4 shows a soccer-like appearance without oscillatory zoning which indicates its metamorphic origin (Fig. 4b, spots 18, 98, 90), spot 18 gives the oldest 207Pb/206Pb age of 1 090±166 Ma with Th/U ratio=0.43 (206Pb/238U age of 1 150±16 Ma, Fig. 4b), and this age could be older because of the existence of inherited zircon in the analyzed area (Fig. 4b, spot 18), spot 22 with gray CL image gives a 206Pb/238U age of 790.7±13.4 Ma (Th/U=0.02), spot 44 with gray CL image gives a 206Pb/238U age of 703.1±13.4 Ma (Th/U=0.002), the other analyzed spots are clustered at 602±11 (N=3, MSWD= 0.000 5), 566±6.5 (N=6, MSWD=0.74), and 449±23 Ma (N=8, MSWD=13) (Fig. 5c). Counted by 206Pb/208U age, these zircons give a strong peak of 576 Ma, sub-strong peaks of 603, 485, 447 Ma, and weak peaks of 306 and 244 Ma (Fig. 5d).

3.3 Sample SN50-2 (Paragneiss)

Zircons range in length from 50 to 150 μm, with length to width ratios of 1 : 1 to 3 : 1, it is worth noting that there are over half of the zircons having a length to width ratios of 1 : 1 (Fig. 4c, spot 20), which makes it quite different from other samples. The 43 spots from 39 zircon grains have been analyzed (Table S3). Core-rim structure is developed in the zircons according to CL images, and the zircon core can make up most of the zircon volume (Fig. 4c, spots 38, 20), two domains could be identified from the zircons. Domain 1 often occurs as subhedral to anhedral zircon cores with blurred or clear oscillatory zoning (Fig. 4c, spots 20, 38), spot 18 gives the oldest 207Pb/206Pb age of 2 446.6±16.7 Ma, 4 spots with blurred oscillatory zoning give a weighted mean 207Pb/206Pb age of 1 890± 52 Ma (MSWD=8.7), 6 spots with clear oscillatory zoning give a weighted mean 207Pb/206Pb age of 1 767±11 Ma (MSWD= 0.73), another 10 spots give a weighted mean 207Pb/206Pb age of 1 551±34 Ma (MSWD=11.7), spot 34 of which shows a prismatic shape with clear oscillatory zoning (Fig. 4c, spot 34) gives a 207Pb/206Pb age of 1 589±11 Ma (Th/U=0.96), the youngest concordant 207Pb/206Pb zircon age, which is obtained from spot 38 with white CL image and oscillatory zoning, is 1 154.6±16.7 Ma (Th/U=0.96) (Fig. 4c, spot 38); Domain 2 occurs as zircon rims and is characterized by zircon ages of Neoproterozoic and Early Paleozoic, the rims have dark gray CL images and do not have oscillatory zoning. Five spots give ages of Neoproterozoic, among them a weighted mean age is 743±26 Ma (MSWD=0.13) (Fig. 5e) given by the 4 spots with similar age (Fig. 4e). Two spots give ages of Early Paleozoic, which are spot 39 (506±10 Ma, Th/U=0.06) (Fig. 4e) and spot 17 (434±9 Ma, Th/U=0.02). Counted by 207Pb/206Pb age, zircon ages are clustered at 1 878, 1 770, 1 696, 1 621, 1 590, 1 526, and 746 Ma (Fig. 5f).

3.4 Sample NX16-1 (Paragneiss)

Zircons showing complicated appearances with euhedral to anhedral shape, range in length from 50 to 150 μm, with length to width ratios between 1 : 1 and 3 : 1, core-rim structures are developed in the zircons. Four groups can be recognized according to the CL images. Domain 1 occurs as euhedral zircon cores (Fig. 4d, spot 26) or single crystals with rounded shape, the oscillatory zoning of which can be blurred, the ages dating from the domain can be up to 1 943±20 Ma (Fig. 4d, spot 26), which indicates that they are inherited zircons, the most reliable inherited zircon has a oscillatory core and a dark rim (zircon from volcanic rock with widen oscillatory zoning and significant change in brightness, e.g., Zhang et al., 2013), gives a 207Pb/206Pb age of 1 169±21 Ma (Th/U=0.48); Domain 2 often occurs as euhedral crystal with prismatic face and clear oscillatory zoning (magmatic origin), the 206Pb/238U ages of which have a large range of variations (954–859 Ma), the oldest magmatic age of 954±5 Ma with Th/U=0.37 is obtained from spot 2 (Fig. 4d) which shows a prismatic face and has oscillatory zoning, spot 16 displays a perfect prismatic shape with oscillatory zoning, gives 206Pb/238U age of 933±5 Ma (Th/U=0.62), 11 spots give a weighted mean age of 939±7 Ma (MSWD=3.7) (Fig. 5g), among which the three older ages (spots 2, 29, 34) clustered at weighted mean age of 952.3±6.4 Ma (MSWD=0.075) (Fig. 5g); Domain 3 is zircons with subhedral shape and blurred oscillatory zoning, 10 spots give a weighted mean age of 850±3 Ma (MSWD=0.93) which could reflect the age of reset; Domain 4 occurs as zircon rim (Fig. 4d, spot 32), among which spot 22 (zircon with oscillatory zoning) gives a 206Pb/238U age of 849±5 Ma (Th/U=0.12), spot 23 of 886±5 Ma (Th/U=0.12), spot 27 of 911±5 Ma (Th/U=0.29), spot 32 of 630±3 Ma (Th/U=0.05), spot 35 of 830±5 Ma (Th/U=0.11), all of these rims are similar to the gray rims of sample SN50-2. The youngest age is 434±2 Ma (Th/U=0.01) given by spot 6. Counted by 206Pb/238U age, the ages are clustered at a strong peak of 850 Ma, sub-strong peaks of 952, 935, 914 Ma and a weak peak of 435 Ma (Fig. 5h).

4 INTERPRETATION OF ZIRCON U-PB AGES

The pre-existing zircon is prone to recrystallize through overprinting of later magmatic and metamorphic events. Two general mechanisms are available for the recrystallization: Fluid-dominated and solid-state recrystallization (Hoskin and Black, 2002). Fluid-dominated mechanisms involve partial dissolution and re-precipitation of the crystal, leading to a spongy texture. Both ends of zircon are prone to recrystallize and lead to a pyramid shape of the zircon, and the pyramid shape is known to be energetically favorable sites for trace element incorporation (Vavra, 1990), some authors suggest that trace element-rich oscillatory-zoned zircon is relatively unstable at low temperatures due to lattice strain and more susceptible to recrystallize at both ends (Köppel and Sommerauer, 1974) or along the crystal edge (Hoskin and Black, 2002). Recrystallized zircons would show blurred and curved oscillatory zoning in CL images, the zircon U-Pb isotopic system is disturbed during recrystallization giving a completely different age from the initial crystallized age, and the Th/U ratio decreases as a result of Th and Pb* expulsion during recrystallization (Hoskin and Black, 2002). Due to the difference in the degree of recrystallization of zircon, the apparent age may spread along the concordia, the minimum age given by the fully-recrystallized zircon represents the age of magmatic or metamorphic event that caused the recrystallization.

5 ANALYSIS OF ZIRCON AGES

The relatively older crystallization age of 970±9 Ma (Fig. 5b) given by the gneissic Bt-Pl granite (XX25-2) indicates the magmatic age (i.e., the protolith age); the oscillatory zircon rims with darker CL images and some soccer-ball zircons give younger ages of 904±24 and 910±9 Ma (Fig. 4a), respectively, which could be related to the metamorphic origin.

Zircons from the leucosome (XX29-7) occurring along the gneissosity give a relatively older mean 206Pb/238U age of 894±16 Ma with one spot (spot 7) giving the oldest age of 942±8 Ma (Th/U=0.16) (Ren et al., 2016), in contrast, the core domain, having blurred oscillatory zoning, gives a relatively younger age of 792±30 and 717±20 Ma which may indicate a reset zircon age as result of zircon recrystallization. That is to say, the zircon U-Pb system has been reset (Kröner et al., 2014). The zircon U-Pb system could be opened even though it has a high close-temperature (Cherniak and Watson, 2003).

It is believed that sample SN50-1 is a gneissic granite according to its field appearance and mineral assemblage (Fig. 3b). The euhedral magmatic zircons give a mean age of 963±46 Ma indicating the magmatic age, and the rarely inherited zircons give an older age of 1 552±22 Ma. The dark core and rim in CL images may indicate that the zircons have been affected by later events (Fig. 4b), isotopic data from those zircon domains give different zircon age, the mean age of 565±10, 483±6.3, and 439±17 Ma represent the events of Late Neoproterozoic and Early Paleozoic, the 206Pb/238U age of 304.5±4.6, 245.0±5.0, 314.5±5.5 Ma and mean age of 292±24 Ma may reflect the events of Late Paleozoic. More importantly, there are some gray to white zircons in CL images which have polycrystalline-face and lack of oscillatory zoning (Fig. 4b, spots 18, 90, 98), most of its Th/U ratios are between 0.001 and 0.08, which are consistent with the typical metamorphic zircon. Isotopic data from those zircons give three sets of relatively concentrated 206Pb/238U ages, 449±23, 566±6.5, and 602±11 Ma, respectively (Fig. 5c), and some concordant age of 703±13, 790±13 Ma. The scattered ages may represent different resetting of the isotopic system of the metamorphic zircons, constraining the metamorphic age older than 790 Ma.

The gneissic monzonitic granite (SN50-1) and gneissic Bt-Pl granite (XX25-2) were collected from different locations, but are identical in field appearance and lithology. The spherical metamorphic zircon failed to give an accurate metamorphic age due to the overprint and modification of later events, but the information of the age trends is sufficient to prove that the metamorphic zircons were formed much older than Early Paleozoic (500 Ma), which could be formed at the Early Neoproterozoic. The concentrated age of ca. 560 Ma may suggest the impact of the Pan-African event.

The Bt-Pl gneiss (SN50-2) is thought to be a paragneiss according to the combination of field appearances, mineral assemblages (Fig. 3c), and characteristics of detrital zircons (Fig. 5e), and also does not rule out the possibility of recycling. Except minor age of ca. 2 447 and 2 198 Ma, the zircon ages are clustered at several peaks of 1 878, 1 770, 1 696, 1 621, 1 590, and 1 526 Ma, respectively (Fig. 5f). For the zircons of Early Mesoproterozoic which give a mean age of 1 551±34 Ma (N=10), their prismatic shape and clear oscillatory zoning (spot 34, 207Pb/206Pb age 1 588.9±11.1 Ma, Th/U=0.96) suggest that they are closer to the sedimentary source. The isotopic data giving Neoproterozoic and younger ages are all from the zircon rims with their 206Pb/238U age ranging from 832 to 580 Ma (Th/U lower to 0.05), and the relatively concentrated age of 749±25 Ma (N=5), which may be related to the metamorphism or thermal resetting. The minor age of Early Paleozoic age ca. 506–434 Ma (Th/U=0.02–0.06) from zircon rims could also be related to the disturbance of later thermal events.

The different detrital zircons of sample SN50-2 with identical zircon rims, which lack of oscillatory zoning (Fig. 4c, spots 28, 39) indicate that they experienced the same event and share the same origin. The process involved not only deformation but also significant metamorphism and/or migmatization. We suggest that the age are between 832 and 434 Ma which are obtained from the zircon rims, and the dark gray to dark appearance of zircon in CL images suggests they has been involved in severely resetting and modification, leading to the younger age of the zircons (Wan et al., 2011).

The Sil-Ky-Grt mica gneiss (NX16-1) is believed to be a paragneiss according to the field appearance (Fig. 2d) and metamorphic mineral assemblage (Fig. 3d) (i.e., presence of graphite). Some detrital zircons give an unconcordant age of Proterozoic, while the zircons (spot 1) with prismatic shape and oscillatory zoning give two concordant ages of 1 169±21 (207Pb/206Pb) and 1 150±8 Ma (206Pb/238U), which are similar to the minimum age of other two low-grade metamorphic paragneisses (NX14-1 and XX102-1, unpublished data) in the study area. We suggest that the two ages represent the youngest age of the detrital zircons, but it does not rule out the possibility that they are volcanic tuff zircons. Most of the zircons from sample NX16-1 are magmatic zircon with prismatic shape and clear oscillatory zoning, and give 206Pb/238U age ranging of 952–939 Ma and clustering at two peaks of 940 and 850 Ma. According to the previous studies and the analysis above, granites of Early Neoproterozoic (even older than 980 Ma) can be found near the sample, it is believed that the zircons giving 952–939 Ma age should be magmatic zircon, not detrital zircon. Actually, the pegmatite (NX16-2, unpublished data) next to the paragneiss gives some zircon age clustered at 900 and 800 Ma. Considering the possibility of resetting, it is likely that some newly-born zircons could be present in the paragneiss (NX16-1) mentioned above. It is inferred that these zircons crystallized from the injected magma during the amphibolite-facies metamorphism and migmatization process which had occurred in the paragneiss. Similar phenomena are common in the migmatitic area (Ren et al., 2013, 2012; Hasalová et al., 2008). The clustered age of 850±3 Ma is thought to be the result of resetting, considering the anhedral shape, unclear oscillatory zoning of the dating zircons. The ca. 844 Ma Caiwa granite of the second stage granites showed A-typed granite geochemical affinity, implying the post-collisional collapse and extension at this period (Wang et al., 2013).

As for the zircon rims, the oscillatory zoning is either absent or darkened (Fig. 4d, spots 22, 32), and the Th/U ratios are lower to 0.05. These features resemble those of the metamorphic zircons (Corfu et al., 2003), their 206Pb/238U ages range from 849 to 630 Ma. There are also some zircons with dark gray CL images, of which the minimum 206Pb/238U age of 434±2 Ma (spot 6, soccer-like) is consistent with the metamorphic age of Caledonian extensively existing in the studying area (Bader et al., 2013; Liu B X et al., 2013b; Wang et al., 2011; Zhang et al., 2011; Liu L et al., 2009), but the distribution of these zircons is quite rare. Therefore, sample NX16-1 mainly shows the age of magmatism or metamorphism in Early Neoproterozoic, and the Caledonian metamorphism is weakly developed.

In conclusion, besides the detrital zircons, zircons in the paragneisses of the Qinling Complex, especially in the medium- to high-grade metamorphic areas can develop obvious metamorphic zircon rims. In contrast, the Early Neoproterozoic granites which intruded after the main stage of deformation and metamorphism could carry some zircons with typical soccer-like appearance of metamorphic origin (Fig. 4b, spots 18, 98, 90), which were vulnerable to later modification in isotopic system.

6 DISCUSSION 6.1 Age of the Qinling Complex 6.1.1 The forming age of the Qinling Complex

As for the forming age of the Qinling Complex, three opinions have been proposed, namely, Paleoproterozoic, Mesoproterozoic and Neoproterozoic (Shi et al., 2009; Lu et al., 2006; Yang et al., 2002; Zhang Z Q et al., 1996; Zhang H F et al., 1995; Liu et al., 1993; You et al., 1991; Zhang G W et al., 1988; An et al., 1985). Dong et al. (2003) suggested that the North Qinling belt was an independent terrane, derived from oceanic islands and lateral accretion which occurred around 2 000 Ma. Lu et al. (2006) reported a paragneiss from the Qinling Complex whose detrital zircons had age peak of 1.9–1.5 Ga by SHRIMP and LA-ICP-MS U-Pb methods, and also suggested that the sedimentary age of the paragneiss was between 1 500 and 960 Ma, possibly at the end of Mesoproterozoic, according to the extensively existing orthogneiss of 960–900 Ma in the Qinling Complex.

Wan et al. (2011) proposed that the age of protolith of the Qinling Complex could be much younger according to the age of some detrital zircons ranging from 900 to 660 Ma, but they also mentioned that the zircon age could become younger because of the modification of multiple tectono-thermal events. We have analyzed the zircons of several meta-sandstone samples (unpublished data) which show low-grade metamorphism from the Qinling Complex, and the data can constrain the sedimentary age of the protolith at 1 136–980 Ma, according to the youngest detrital zircon age groups and oldest intrusive ages of the granitic rocks.

6.1.2 The metamorphic age of the Qinling Complex— previous studies

It was suggested that the North Qinling belt had been involved into the multiple stages of deformation and metamorphism (Pei et al., 1999, 1998, 1995; Zhang et al., 1996; An et al., 1985), which consisted of Proterozoic fold deformation, large gneisses doming of Early Paleozoic, Late Paleozoic superimposed fold deformation and Mesozoic to Cenozoic uplifting, fault depression and fragmentation (An et al., 1985). By Rb-Sr dating method, Chen et al. (1991) analyzed six gneisses and migmatites in the Guozhuang Formation from Mashankou of the Qinling Complex giving the Rb-Sr isochron age of 990±0.4 Ma, analyzed the schist of Zhaigen Formation from the Xiahe Group giving the Rb-Sr isochron age of 973±34 Ma, analyzed the Shuanglong (Shewei) gneiss which was affected by the Caledonian granite giving the Rb-Sr isochron age of 424±48 Ma. Chen et al.(1993, 1991), Chen and You (1990) proposed that the North Qinling belt might have undergone two stages metamorphism of the Early Neoproterozoic amphibolite facies (~990 Ma) and the Paleozoic greenschist facies (~420 Ma). Zhang et al. (1994) reported Rb-Sr isochron age of 996±76 Ma and zircon 207Pb/206Pb age of 891±7 Ma for gneisses from the Shewei area, Xixia County, and Sm-Nd isochron age of 1 169±58 Ma for gneisses from the Yongyu area, Danfeng County. Dong et al. (1997a) gave a Sm-Nd isochron age of 1 030±58 Ma for an amphibolite from Songshugou. Based on these age data, Pei et al. (1998) argued that a severely tectonic- magmatic-metamorphic geological event had been developed in the North Qinling belt during the Jinning Period, the North Qinling belt was an old Neoproterozoic orogenic belt that had experienced a severe modification of Caledonian Period and later superimposed by Late Variscan to Indosinian tectonic deformation. Wang et al. (1997) also proposed that the Qinling Complex suffered from two stages of deformation and metamorphism during the Jinning Period (780–740 Ma) and Caledonian– Variscan Period (415–286 Ma), respectively.

Isotopic chronology studies in recent years show that there has no reliable data to prove the existence of the Neoproterozoic metamorphism in the Qinling Complex in addition to some granites whose zircon ages were in the Early Neoproterozoic. Lu et al. (2009) reported a zircon SHRIMP age of 512±9 Ma from a basic granulite and two zircon SHRIMP ages of 504±7 and 506±3 Ma from felsic HP granulites in Xigou, Shangnan County. Zhang et al. (2011) regarded these three ages as metamorphic age. It has been confirmed by zircon dating that the peak metamorphism of eclogite was between 500 and 485 Ma, and it began to fold back into the shallow crust at 480 and 470 Ma, respectively (Bader et al., 2013; Liu L et al., 2013, 2009; Wang et al., 2011; Yang et al., 2003). However, the HP granulite and retrograde eclogite (garnet-amphibolite) also occurred in the Songshugou area from the south of the Qinling Complex, the metamorphic age was similar to that of the eclogite in the north. The metamorphic age of both HP granulite and eclogite were 505 Ma, while the metamorphic age of low-pressure granulite was 440 Ma, and the amphibolite facies metamorphism was 426 Ma (Zhang et al., 2011). Xiang et al. (2014) suggested that the Qinling Complex widely suffered 500–480 Ma HP-UHP metamorphism, and subsequently overprinted by the 440–400 Ma medium-low P/T metamorphism, anatexis and magmatism, that is, the northward subduction of Shangdan Ocean lithosphere during Ordovician causes the orogeny of the Qinling micro-continent which was characterized by medium-pressure high-temperature metamorphism, anatexis and magmatism.

Bader et al. (2013) reported that the Early Paleozoic magmatic age of the Qinling Complex had a bimodal feature with mean age of 483±9 and 429±8 Ma, although the metamorphic age was clustered at 473 and 414 Ma. By contrast, Yu et al. (2016) thought that the first metamorphism peak was 501–486 Ma, and the retrograde peak was 470–454 Ma, which meant that the peak feature of the age was not obvious. They also suggested the Qinling Complex experienced two stages of metamorphic events during Early Paleozoic, which were the HP-UHP metamorphism from Late Cambrian to Early Ordovician and the high- temperature metamorphism from Middle Ordovician to Silurian, respectively. It was suggested that there should be magmatic activities after the peak period of metamorphism.

As for the anatexis and migmatization of the Qinling Complex, some authors suggested that they were related to the main metamorphism of Early Paleozoic, which had happened during 455–400 Ma (Liu et al., 2014) with time span of 55 Ma. Dong et al. (2011a) reported two ages of 517±14 and 455±4.5 Ma from a migmatite gneiss sample and one age of 455±4.5 Ma from a leucosome sample, which were all from the Qinling Complex, south of the Huichizi granite from more than 20 km northwest of Shangnan County. They interpreted the age of 517–455 Ma as the time span of the migmatization. Thus, there had two interpretations about the duration of migmatization which were 517–455 (Dong et al., 2011a) and 455–400 Ma (Liu et al., 2014), and the time span was quite different. In contrast, the duration of the Barrovian metamorphism occurred in the Scottish Highlands was 473–465 Ma with time span of only 8 Ma which was also involved with metamorphism and migmatization (Viete et al., 2013). The migmatization time span of the Qinling Complex seems too large.

6.1.3 The age of the main metamorphism—this study

It is suggested that the granite veins occur after the peak period of metamorphism according to the structure and texture relationship observed both from the field and under the microscope. Bader et al. (2013) analyzed the pegmatite occurring between the amphibolite boudins in the North Qinling belt from Danfeng County and obtained the older magmatic zircon age of 476±6 Ma, Yang et al. (2010) reported a zircon (dark CL image) LA-ICP-MS age of 473±10 Ma for a Grt-Pl gneiss in the Qinling Complex from Yuling Village, Danfeng County, Shaanxi Province. Bader et al. (2013) recalculated the LA-ICP-MS zircon age of the Piaochi granite which had been reported by Wang et al. (2009) with the age of 495±6 Ma, and gave a concordant magmatic age of 477±3 Ma. Qin et al. (2014) reported that the magmatic zircon age of 473±4 Ma from the Piaochi granite and suggested that some residues of gneiss inclusion could be found in the Piaochi granite which indicated that the main deformation period could be older than that of 473 Ma. Ren et al. (2016) reported a pegmatite with ages of 479±5 and 484±3 Ma which occurred after the main metamorphic stage. Lu et al. (2009) analyzed a leucosome within a Sil-Bt-Pl gneiss and obtained the age of 499±4 Ma which suggested the main metamorphic age could be older.

Former metamorphic structures could be affected by later granite formed in different tectonic settings, zircons thus may be affected by the fluid activities (Kröner et al., 2014). The former magmatic zircons, especially the metamorphic zircon, are easy to become recrystallized leading to the darken of CL images which are generally developed at the edge of zircons. Therefore, the dark edges are considered to be metamorphic rims conventionally. According to Yang et al. (2010), the analyzed samples had a large number of Neoproterozoic magmatic zircons (950–850 Ma) with typical magmatic feature and the zircon rims which were dark gray and black with low Th/U ratios (< 0.03) were considered to be metamorphic origin. The 206Pb/238U ages were between 465 and 510 Ma with mean age of 477±18 Ma which was considered as the main metamorphic age of Caledonian. However, the U-Pb isotopic system of this type of zircons was often reset and thus their zircon ages were generally younger than the real age (Wan et al., 2011; Corfu et al., 2003). That is to say that the measured data usually failed to give the real age of the former metamorphic events. Ren et al. (2016) suggested that the minimum metamorphic age of the Qinling Complex was 463±3 Ma which was given by the zircon rim with black CL images from a leucosome. The metamorphic age of the eclogite (500 Ma) in the Qinling Complex was actually the resetting age of the magmatic zircon rim (Chen et al., 2015). The metamorphic age of garnet-amphibolite (496±9 Ma) was also in the same situation (Qian et al., 2013) from which the 496±9 Ma was a resetting age of the zircon rim with oscillatory zoning. Zhang et al. (2011) emphasized the age of 500 Ma given by the typical metamorphic zircon in the HP granulite of the Qinling Complex represented the metamorphic age, actually there were also some older zircon ages in that paper, such as 521±9, 539±4, and 610±9 Ma with Th/U ratios lower to 0.02.

Due to the multiple stages of modification by later tectono- thermal events, it is hard for the possible metamorphic zircon to preserve the record of early age. However, the typical magmatic zircon has a greater possibility to preserve its isotopic system closed (Corfu et al., 2003). Therefore, the magmatic intrusion is suggested to constrain the metamorphic age. Some gneissic granite veins or lenses intruding into the gneisses indicate that the age of emplacement is younger than that of the main stage gneissosity, thus the age recorded by the magmatic zircon which is related to the metamorphism should be younger than the age of peak metamorphism (Finger et al., 1997). Thus the zircon age of 963±46 Ma (N=4), given by the gneissic granite (SN50-1), the older zircon age of 970±9 and 942±8 Ma given by the gneissic granite (XX25-2) and the leucosome (XX29-7) (Ren et al., 2016) along with the magmatic zircon age of 952–939 Ma which reflect the crystallized age of the leucosomes or injected granites along the gneissosity constrain the age of the main gneissosity to the age no later than ~970 Ma. Wang T et al. (2005) analyzed the Niujiaoshan granite (gneissic peraluminous S-type granite) and a weakly deformed granite vein intruding into the granite, obtained SHRIMP zircon 206Pb/238U age of 955±13 and 929±25 Ma, respectively, thus constrained the age of the syn-collisional deformation (metamorphism not mentioned), which was recorded by the gneissic granite, at 955–929 Ma, and the syn-collisional deformation also caused the emplacement of the Dehe granite, Laoyu granite, Niujiaoshan granite, and Zhaigen granite, which all belonged to VAG and COLG, at the same period from 980 to 900 Ma (Lu et al., 2004). The conclusion is consistent with our study. The Dehe granite and Zhaigen granite of the Qinling Complex formed at Early Neoproterozoic (Lu et al., 2003), however the metamorphic grade of the granite is only up to greenschist-facies which is far from the upper amphibolite to granulite-facies.

On the other hand, the commonly occurred non-magmatic zircon rims in detrital zircons of the paragneiss indicate that they were affected by the metamorphism or magmatic activities relating to the emplacement of granites, such as the clustered zircon age of 850±3 and 743±26 Ma for the paragneiss (NX16-1) and (SN50-2) with their zircon age ranging of 849–630 and 832–580 Ma, respectively. Also the groups of zircon ages (792±30, 717±20 Ma) given by the leucosome (XX29-7) (Ren et al., 2016) are related to the metamorphism or magmatic activities (Ren et al., 2013, 2012). These zircon ages may all represent the resetting age, that is, the zircon is partially dissolved and the isotopic system is reset. The widely spread age of 735–705 Ma dioritic- granitic intrusions in the South Qinling orogenic belt or the north margin of the Yangtze Craton are consistent with that of the Qinling Complex during the period (Yu et al., 2015; Diwu et al., 2014). Diwu et al. (2014) suggested that the Qinling Complex was a part of the Rodinia and separated at ca. 830–740 Ma. Recently, Liu et al. (2013a) reported a U-Pb zircon SIMS age of an amphibolite dyke intruding into the Qinling Complex in the Xixia area, with an upper intercept age of 845±69 Ma and a lower intercept age of 517±71 Ma, suggesting that the dyke intruded during the Neoproterozoic and suffered metamorphism during a later Paleozoic tectono-thermal event.

The soccer-like or ellipsoidal zircon in the monzonitic granite (SN50-1) shows metamorphic characteristics, but the rock itself has no evidence of high-grade metamorphism, which indicates that the metamorphic zircon might come from the deep source of the granite, such as the HP granulite nearby (Zhang et al., 2011), and the zircon age is between 1 000 and 450 Ma which indicates the modification degree of isotopic system differs from grain to grain of zircon. The existence of the similar soccer-like zircons (910±9 Ma) in the gneissic Bt-Pl granite (XX25-2) suggests that they could be formed in the same mechanism. These metamorphic zircons can not give the accurate metamorphic age because of the overprinting and modification of the later tectono-thermal events, however the existence of different metamorphic zircon ages can at least suggest that the metamorphic zircons were formed in Early Neoproterozoic. The Qinling Complex has large amount of Early Neoproterozoic granites (Wang X X et al., 2013; Pei et al., 2007; Wang T et al., 2005; Lu et al., 2003), and the same type of granite near West Qinling of Tianshui City gave an older zircon LA-ICP- MS U-Pb age of 979±5 Ma (Pei et al., 2007). According to our study, these granites formed after the main stage of deformation and metamorphism, thus constrained the age of the main metamorphism older than ~980 Ma, which is equivalent to the Grenville orogeny (Dong and Santosh, 2016; Yu et al., 2015; Dong et al., 2014; Liu et al., 2013a), not that of Early Paleozoic (ca. 500 or 450–420 Ma). You et al. (1991) suggested that the near-horizontal ductile system of Proterozoic was embroiled into the recumbent fold with gneissosity. Most of the Early Neoproterozoic granitic gneisses show concordant contact with the foliation of the wall rock, but the Caiwa granite (889±10 Ma) cut foliations of the Qinling Complex (Zhang et al., 2004), implying the intrusive relations with the complex, and the major foliation must have formed prior to the Caiwa granite.

6.2 Deformation and Metamorphism of Caledonian 6.2.1 Previous studies

Recent researches emphasized that the entire Qinling Complex was mainly a Caledonian orogenic belt (Dong and Santosh, 2016; Zhang et al., 2011; Liu et al., 2009; Lu et al., 2006, 2003), with amphibolite-granulite facies metamorphism, and even UHT metamorphism (Li et al., 2016; Shi et al., 2013; Xiang et al., 2012; Zhang et al., 2011; Yang et al., 2010; Liu et al., 2009). Zhang et al. (2011) suggested that the HP granulite facies (ca. 500 Ma), LP granulite facies (ca. 450 Ma) and amphibolite facies (ca. 420 Ma) metamorphism occurred at Early Paleozoic. The main metamorphism, which consisted of early eclogite facies, subsequent granulite facies and final amphibolite facies metamorphism, occurred at Early Paleozoic (Xiang et al., 2014).

6.2.2 Constraint from the metamorphic intrusions

However, the characteristics and forming ages of metamorphism-deformation (discussed above) of the Qinling Complex indicate the time of the main metamorphism could be much earlier. The early Middle Neoproterozoic intrusions of the Dehe granite, Zhaigen granite and Niujiaoshan granite all occurred in the Qinling Complex, but none of these granites has been subjected to the granulite facies metamorphism of ca. 500 Ma, the metamorphism of these granites is only up to greenschist facies.

The Early Paleozoic intrusion of the Qinling Complex, such as the Fushui basic complex, shows severe modification of deformation, however it preserves a clear gabbro texture and a certain degree of hydrothermal alteration, and no obvious dynothermal metamorphism is observed. Age determination showed that the Fushui basic compelx emplaced at about 500 Ma and surged at 490–480 Ma with the latest zircon crystallized at about 476 Ma (Zhang et al., 2015). Li et al. (2006) realized the complexity of the metamorphic zircon in dating, emphasized the method of using baddeleyite to constrain the emplacement age, and gave the age of 501±1.2 Ma for the basic rocks. The gabbro (484.3±1.8 Ma) in the Qinling Complex was significantly amphibolitized (Zhao et al., 2019) which was not consistent with the report that their country rock experienced HP granulite facies metamorphism (Liu L et al., 2013; Zhang et al., 2011). These features of the gabbro seem to be inconsistent with that of the country rocks of HP granulites.

The typical granitic texture, weakly deformed appearance, free of metamorphism combined with the residue of gneiss xenoliths in the Piaochi granite (zircon LA-ICP-MS age of 495±6 Ma, Wang et al., 2009; or 477.6±2.8 Ma, as recalculated by Bader et al., 2013; 473±4 Ma, Qin et al., 2014) suggest that the age of main deformation-metamorphism is older than ~470 Ma.

According to You et al. (1991), the small Caledonian gabbro-diorites and red granitic rocks are linearly distributed along the shear zone system of the Qinling Complex, and show some degree of plastic deformation.

6.3 Constraint from the Metamorphic Sediment Rocks

However, Early Paleozoic events can be observed in various types of rocks, such as the ca. 506–434 Ma zircon age in paragneiss (SN50-2), the minor zircon age of 434±2 Ma in paragneiss (NX16-1), the clustered age of 483±6.3 and 439±17 Ma given by the black and dark gray zircons of the gneissic granite (SN50-1) and the mean age of 463±3 Ma (N=10) from the leucosome (XX29-7) (Ren et al., 2016), all of these zircon ages indicate the modification of existing rocks by overprinting of the Caledonian tectono-thermal events, which may be related to the emplacement of Early Paleozoic granites. The thermal modification age of the nearby country rocks is determined by the emplacement age of the granites and the intensity of the thermal modification varies with space. These cause the resetting intensity and resetting age of zircons to vary greatly with locations and make the rocks lack of metamorphic age peak (Bader et al., 2013). In general, greenschist to lower amphibolite facies metamorphism are dominant in the south, especially along the locations of the Xiahe Group (Chen and Zhang, 1993), while the Caledonian modification and overprinting to the Qinling Complex are dominant in the north especially the areas where many Caledonian granites occur.

On the other hand, Lu et al. (2006) analyzed a Sil-Bt-Qtz schist of the Qinling Complex, the Mesoproterozoic detrital zircon ages and Early Neoproterozoic magmatic zircon ages were obtained, but no Early Paleozoic ages were obtained. This suggested from another angle that the main metamorphism which caused the formation of the sillimanite occurred in Early Neoproterozoic, rather than Early Paleozoic.

Dong et al. (2013) reported that the detrital zircon age of Liuling Formation clustered at several peaks, which are ~1 050, ~950, 850–710, and 528–466 Ma, respectively, the 850–710 Ma belonged to the Yangtze Craton, and the other peaks were related to the Qinling Complex. But these age peaks are not the same as the granite peaks of ~950, 500, 450, and 420 Ma in the Qinling Complex. It also suggested that there were large amount of Early Paleozoic granites in the Qinling Complex and most of their ages were clustered at 528–466 Ma. Thus at least a part of the granites are older than the eclogite facies metamorphism (ca. 500 Ma, Yang et al., 2003) which suggests indirectly that the main stage of metamorphism should be earlier.

6.4 Comparison of the Metamorphic Conditions

As for P/T condition of the metamorphism, using the Grt-Cpx-Pl-Qtz geobarometry and Grt-Cpx geothermometer, Zhang et al. (2009) gave the peak P/T condition of the high pressure granulite: P=1.45–1.80 GPa, T=850–925 C, which is similar to the results of Liu et al. (1995). The Fushui basic rock consists of the early magmatic mineral of pyroxene, euhedral plagioclase and the later metamorphic mineral of actinolite and chlorite, which indicates that the later metamorphic condition is only up to the greenschist facies, and should not exceed the amphibolite facies. Amphibolites were widely distributed in the Shujigou ultrabasic rocks, the metamorphic condition was P=0.6–0.7 GPa, T=700–750 C (Wang X B et al., 2005) which are much lower than that of the country rocks (granulite facies) (Zhang et al., 2009; Liu et al., 1995). The early retrograde mineral assemblage (green amphibole, margarite, chlorite, etc.) of the basic granulite suggests that the later metamorphic condition is up to greenschist to amphibolite facies with P=0.6–1.0 GPa and T=680–820 C (Zhang et al., 2009).

If the amphibolite-granulite facies metamorphism occurred during the Caledonian of ca. 500 or 450–420 Ma, it is quite difficult to explain why the intrusions of the Early Neoproterozoic (980–850 Ma) and Early Paleozoic (510–450 Ma) escaped from it. By contrast, if the main metamorphism occurred at the Early Neoproterozoic (Wang et al., 2003; Chen et al., 1991; this study), the metamorphism would certainly not affect the intrusions mentioned above. The UHP metamorphism may occur in a special way, possibly in Early Paleozoic of ca. 500 Ma (Chen and Liu, 2011; Zhang et al., 2011; Liu et al., 2009; Yang et al., 2003).

You et al. (1991) suggested that the locally overprinting of Caledonian metamorphism would lead to the forming of new garnet and kyanite and retrograding of the early amphibolite facies metamorphism to the lower-amphibolite facies and even greenschist facies metamorphism. The meta-granodiorite (Grt-Bt-Pl gneiss, with SiO2 content of 63.21 wt.%, zircon mean age of 485±5.9 Ma, Ren et al., 2016) of the Qinling Complex in the eastern part of the Tongbai area consists of typical greenschist facies mineral assemblage of Grt-Pl-Bt-Chl- Qtz with clearly schist or gneissosity, while another granodiorite gneiss in the same area only developed weakly gneissosity, gave a LA-ICP-MS magmatic zircon mean age of 476±3 Ma (Liu et al., 2011) which constrained the grade of the Caledonian stage of metamorphism.

The metamorphic P/T condition is similar to that of the Fushui Complex and Songshugou ultrabasic rocks. The ultrabasic rocks did not experience the granulite facies metamorphism, only underwent the metamorphic modification during the Caledonian, the forming age of the ultrabasic rocks should be younger than 1 000 Ma, not 983±140 Ma (whole-rock and minerals Sm-Nd isochron age, Li et al., 1991). A conclusion from Liu and Sun (2005) suggested that the Songshugou ultramafic rocks had intruded into the Qinling Complex during a tectono-thermal event of 0.52 Ga is possible.

6.5 Summary of the Caledonian Events

The Qinling Complex in the northern Qinling orogenic belt is a Neoproterozoic orogenic belt or terrane (Fig. 6a) which is dominated by Early Neoproterozoic movement and is subjected to the modification of Caledonian tectono-thermal events. However, the northern margin of the Yangtze Craton and the southern margin of the North China Craton, which are adjacent to the Qinling Complex, are devoid of the evidence of the Grenville event (Dong and Santosh, 2016; Ratschbacher et al., 2003). Thus the Qinling Complex belongs to, if it has to belong to, an independent terrane. The detrital zircon age peaks of 1.0 Ga and 598–512 Ma from the Kuanping Group (Li et al., 2018) are not found in that of the Qinling Complex (characterized by the magmatic zircon age peak of 450 Ma, Zhang et al., 2013; Wang et al., 2013), which indicates that the sediments of the Kuanping Group are not from the North China Craton or Yangtze Craton (Diwu et al., 2010), and even not from the Qinling Complex, and they are separated from each other.

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Figure 6. Evolution schematic diagrams-cartoons showing evolution of the Qinling Complex. (a) The main deformation and metamorphism of the Qinling Complex is more closely related to the formation of Rodinia (RD?); (b) the Qinling Complex separated from Rodinia or Gondwana (GW), and may be independent of North China Craton (NC) or Yangtze (YZ); (c) the Qinling terrane subducted towards the oceanic crust between NC and YZ, accompanied with shear activities; (d) the Qinling terrane amalgamated with YC and NC, accompanied with shear activities.

The North Qinling terrane was characterized by large amount of Lower Paleozoic gabbroic and granitic intrusions, which were related to the subduction along the Shangdan suture zone (Dong et al., 2011b). Except a few S-type granitoids exposed in the Piaochi, Danfeng and Erlangping areas, most of the granitic plutons in the Danfeng-Shangnan region were assigned to I-type series (Wang et al., 2009), and the 460–422 Ma granitoids in the North Qinling belt were possibly emplaced in a collisional setting (Wang et al., 2013; Zhang et al., 2013). While the adakitic and related geochemical features of the 460–422 Ma granitoids throughout the NQB imply the involving of the mantle source in the formation process (Zhang et al., 2015), and originated from the lower crust with the involvement of mantle-derived magma in a collisional setting, therefore was thought to be representative of arc magmatism (e.g., Dong et al., 2011b; Ratschbacher et al., 2003; Zhang et al., 2001). Several gabbroic intrusions with typical island-arc geochemical signatures were distributed along the island-arc terrene, such as the Fushui gabbro, which shows enrichment of LREE, LILE and depletion of Nb-Ta, as well as low initial 143Nd/144Nd ratios (0.511 43–0.511 62) and ɛNd values (-3.5 to +2.0), and high ɛSr values ranging from +81 to +135, suggesting the magma was derived from a mantle wedge source above the subducted slab (Dong et al., 1997c). According to Zhang et al. (2015), the isotopic feature implied that the Fushui Complex was originally derived fromenriched lithospheric mantle, which was metasomatized by melts from ancient continental sediments and altered MORB basalts. Isotopic geochemistry indicates that these plutons can be attributed to the northward subduction of the Shangdan ocean crust (Chen et al., 1995). Based on the lithological assemblage and available geochronological data, other workers also regarded the Qinling Complex as a Precambrian micro- continent split from the Yangtze Craton, which evolved into a magmatic arc during the Ordovician–Silurian (Hacker et al., 2004; Ratschbacher et al., 2003; Kröner et al., 1993).

The Qinling Complex was a continental arc in the Early Paleozoic with deformation, metamorphism, and intrusions of continental arc granites (Fig. 6c). The granites and tectono-thermal events in the Qinling Complex have different degrees of reset on the early zircon isotopic system, the Pan-Africa zircon age recorded in the Qinling Complex (e.g., 560 Ma in SN50-1) suggest its relation with the Gondwana continents (Fig. 6b) (to be discussed in another paper). But the metamorphic age was affected by the later granites (~420 Ma). The Early Paleozoic magmatism (ultrabasic rocks, basic and granite) was overprinted by the heterogeneous deformation and metamorphism of Caledonian.

At the same time, the Erlangping Group, Kuanping Group and Taowan Group had been involved into a new orogenic movement simultaneously. Liu et al. (2011) suggested that the main metamorphic age of the Qinling Complex, Erlangping Group and Kuanping Group was basically the same (~450 Ma). Moreover, different from the Qinling Complex, these groups mainly showed the orogenic movement of Caledonian, the southward subduction of the three groups caused the formation of the Caledonian eclogite and inner arc igneous rocks.

7 CONCLUSION

The leucosome and Early Neoproterozoic granite could be formed along the regional gneissosity during the main stage of deformation and metamorphism, most of which were overprinted and modified by later deformation and metamorphism. The zircon ages of these granites constrained the age of the main metamorphism in the Qinling Complex to Early Neoproterozoic, rather than Early Paleozoic.

The main metamorphism can reset the U-Pb isotopic system of zircons, form different zircon rims in the paragneiss, and lead to the formation of typical metamorphic zircons in some granites, and the age of the main metamorphism is likely around 1 000 Ma.

In Early Paleozoic, the Qinling Complex was mainly characterized by a strong overprint of multiple extension-shear activities and basically greenschist-facies metamorphism. The deformation was accomplished by emplacement of extensive granites. The granites and related thermal events can reset the U-Pb isotopic system of early-formed zircons, thus rejuvenate the zircon ages.

The Qinling Complex was a continental arc in the Early Paleozoic with deformation, metamorphism, and intrusions of continental arc granites (Fig. 6c); the Erlangping Group, Kuanping Group in the north showed stronger deformation, their oblique subduction to the Qinling Complex leads to the formation of eclogites. The Qinling Complex, together with the Erlangping and Kuanping groups, had been involved into the Caledonian orogeny.

Therefore, it is necessary to re-evaluate the influence of the Caledonian tectonics on the Qinling Complex. The Qinling Complex may belong to a reconstructed Neoproterozoic orogenic belt or terrane which involves the reactivation of the old block, leading to the elongation of the North Qinling terrane.

ACKNOWLEDGMENTS

We are grateful to Prof. Laixi Tong and an anonymous reviewer for their constructive comments, which greatly improved this paper. This work was supported by the China Geological Survey (Nos. DD20190358, 20190370 and 20190448) and the National Natural Science Foundation of China (No. 41472172). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1218-9.

Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1218-9.


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