Timing of formation of the Western Jinshajiang and Ganzê-Litang sutures: evidence from the Duocai Granite in the Zhiduo region, West China

Tuofei Zhao · Fengyue Sun · Bo Peng · Chao Wang,3

Abstract The timing of the closure of the Paleo-Tethys Ocean in West China remains debated. To investigate this problem, we examined the geochemical characteristics,zircon U-Pb chronology,and Hf isotopes of monzogranites and quartz diorites from the Duocai granite in the Zhiduo region, at the intersection of the Jinshajiang and Ganzê-Litangsutures. The monzogranites have the chemical characteristics of calc-alkaline I-type granites and yield a weighted mean zircon U-Pb age of 234.6 ± 0.9 Ma[mean square weighted deviation (MSWD) = 0.36]. Initial εHf(-t) values are high and positive, ranging from + 7.9 to+ 13.6 with a mean of+ 10.7,corresponding to two-stage Hf isotope model ages (TDM2) of 762-395 Ma. Geochemical and isotopic data indicate that the source magma of the monzogranites formed from mantle-derived magmas mixed with the overlying crustal materials. The quartz diorites,which also have compositional characteristics of calcalkaline I-type granites, yield a weighted mean zircon U-Pb age of 209.1 ± 0.7 Ma (MSWD = 0.29). Initial εHf(-t)values range from- 2.5 to+ 0.6 with a mean of- 1.5,with the corresponding TDM2 of 1402-1210 Ma. Geochemical and isotopic data indicate that the primary magma of the quartz diorites was derived mainly from partial melting of the mafic lower crust and small amount mantle-derived magma involved. Combining these results with regional data,the studied granites are inferred to have formed as a result of continuous subduction of plates underlying the Western Jinshajiang Ocean-Ganzê-Litang Ocean from 234 to 209 Ma, and were unrelated to subduction of the South Jinshajiang oceanic plate.We suggest that the Western Jinshajiang Ocean-Ganzê-Litang Ocean closed by the end of Late Triassic.

Keywords Triassic granite · Zircon U-Pb dating ·Jinshajiang suture · Ganzê-Litang suture · Yidun arc ·Paleo-Tethys tectonics

1 Introduction

The Qinghai-Tibet Plateau forms an important part of the Tethyan-Himalayan tectonic domain, which records a complex tectonic setting including multiple ocean basins,continental terranes, island arcs, subduction zones, and collisional structures (Pan et al. 2012; Xu et al. 2013).

From south to north, the major tectonic units in the Qinghai-Tibet Plateau are the Himalayan, Lhasa, Qiangtang, the Songpan-and East Kunlun blocks, which are separated by the Indus-Yarlung-Zangbo, Bangong-Nujiang, Jinshajiang, and Ayimaqin-Kunlun-Muztagh sutures,respectively(Dewey et al.1988;Yin and Harrison 2000; Pan et al. 2001; Zhu et al. 2013)

The Qiangtang Block is divided into the South Qiangtang Block and the North Qiangtang Block by the Longmu Co-Shuanghu Suture. Earlier studies suggested that the formation of the Jinshajiang Suture was related to the evolution of the Paleo-Tethys Ocean (Li 1987; Li et al.1995), whereas later studies reported that the Longmu Co-Shuanghu Suture zone, rather than the Jinshajiang Suture,was related to the evolution and formation of the Paleo-Tethys Ocean (Zhai et al. 2011a, b, 2013, 2016; Zhang et al. 2006a, b, 2011).

The Jinshajiang Suture is located between the North Qiangtang and Songpan-Ganzêblocks. It trends in north-south and then gradually turns into northwest-southeast in Yushu area of Qinghai province and then into the west-east direction to the west of Yushu area.Where the trend of the Jinshajiang Suture changes, it merges with the Ganzê-Litang Suture to the east. The Ganzê-Litang Suture represents a branch of the Paleo-Tethys Ocean and evolved through oceanic plate subduction and consequent closure of the Ganzê-Litang Ocean. The Ganzê-Litang and Jinshajiang sutures are subparallel to each other. Previous studies have suggested that expansion of the Ganzê-Litang Ocean occurred together with Late Permian magmatic activity associated with the Emei mantle plume that separated the Yidun arc from the Songpan-GanzêBlock. The Yidun arc has a podiform shape and is situated between the Jinshajiang and Ganzê-Litang sutures (Fig. 1b).

Previous studies of the Jinshajiang Suture mainly focused on the part of the Southern Jinshajiang Suture(SJS) between the Yidun arc and the North Qiangtang Block, with few on the Western Jinshajiang Suture (WJS).Research on the Yidun arc and the Ganzê-Litang Suture has been focused in the central and southern regions, with studies on rocks of the northern region being conducted mainly in the Changtai region, and a few reports on the geology of Zhiduo county, Qinghai, in the northernmost part of the Yidun arc. The scarcity of data limits our understanding of the tectonic evolution of the region.Recent studies have found evidence for Triassic magmatism in the northern margin of the Qiangtang Block and near the Jinshajiang Suture (Fu et al. 2010; Chen et al.2014; Wang et al. 2013a, b) Manyepisodes of Permian to Triassic magmatism have been reported from the North Qiangtang Block and the western Yidun arc (Peng et al.2008;Kong et al.2012;Jian et al.2003,2009;Wang et al.2010, 2011a; Zhu et al. 2011) There is also abundant evidence for Triassic magmatism in the eastern Yidun arc and near the Ganzê-Litang Suture(Weislogel 2008;Reid et al.2007; Wang et al. 2011a, b, c; Pang et al. 2009; Lai et al.2007).

Although the Jinshajiang and Ganzê-Litangsutures have been investigated in the studies listed above,the geological evolution of these sutures remains unresolved, and there has been no clear evidence supporting any genetic connection between them. As the present study area contains the intersection of the Jinshajiang and Ganzê-Litangsutures, as well as the junction between the North Qiangtang Block, the Yidun Arc, and the Songpan-Ganzê Block,there is potential to obtain key evidence on the evolution of the Paleo-Tethys Ocean. Accordingly, we sampled intermediate to felsic intrusive rocks from outcrops in the study area.Using the results of petrography,zircon LA-ICP-MS U-Pb chronology, zircon Lu-Hf isotopes, and major- and trace-element geochemistry, we discuss the petrogenesis and tectonic setting of these rocks. Our data constrain the dynamic evolution of the Paleo-Tethys Ocean in the study area, and the evolution of the Paleo-Tethys Ocean in the regions of the Jinshajiang and Ganzê-Litangoceans.

2 Regional geology

The study area is located in Zhiduo country, Qinghai province, within the northern section of the Sanjiang mineralization belt in West China.It is an important part of the Tethys-Himalaya tectonic domain, which represents the long-lived collision zone between Gondwana and Eurasia(Zhang and Hou 2015)(Fig. 1).In detail,the study area is located at the northern margin of the North Qiangtang Block, close to the intersection between the Yidun Arc, the North Qiangtang Block, and the Songpan-GanzêBlock, at the junction of the Jinshajiang Suture and the Ganzê-Litang Suture.

The region hosts mainly Paleozoic and Mesozoic strata,with Cenozoic strata being scarce.The Paleozoic strata are composed mainly of rocks of the Middle to Lower Permian Kaixinling Group and the Lower Carboniferous Zaduo Group. The Mesozoic strata, which are dominant, include the Upper Triassic Batang and Jiezha groups,the Middle to Upper Triassic Bayankara Group, the Middle Triassic Jielong Group,and the Middle and Upper Jurassic Yanshiping Group. Faults are abundant and are divided into three groups: WNW-, NE- and E-trending faults. The dominant WNW-oriented faults are hundreds of kilometers long and record displacements of 2-8 km. The NE- and E-oriented faults formed at a relatively late stage as part of the Himalayan collisional event (Wang et al. 2018).

The magmatic rocks in the study area comprise volcanic rock and younger intrusive rocks, the volcanic rocks are mainly submarine eruptive rocks with Permian to Middle-Late Triassic forming ages,and are dominated by andesite,tuff, and basalt. The intrusion is mainly Middle to Late Triassic rocks that range in composition from mafic to felsic but are dominated by intermediate to felsic rocks.The Cenozoic intrusive rocks occur as stocks and veins.The area also contains two WNW-trending ophiolite mélange belts: the early to Middle Permian Duocai ophiolite mélange belt in the south, and the Triassic Chayong ophiolite mélange belt in the north (Zhao et al. 2015).These ophiolite belts provide direct evidence for the existence of the Jinshajiang Ocean and Ganzê-Litang Ocean,respectively (Fig. 2).

3 Sample and petrography

Samples were collected from the Chayong-Galonggema and Sanalongwa mining areas, between the Chayong ophiolite mélange and the Duocai ophiolite mélange. The dikes and ophiolite mélanges show a similar WNW-ESE trend. The area containing outcrops istens of kilometers long and 2-5 km wide and ispartially covered by quaternary strata. Four samples of monzogranite were collected from a granitic dike in the Sanalongwa mining area(sample SNLW1-1-SNLW1-4) and five samples of quartz diorites were collected from a site located 10 km southeast of the town of Duocai, in Yushu Zhiduo county (sample DC1-1-DC1-5) (Fig. 2).

Monzogranite is gray-white, a massive, medium- to fine-grained granitic texture,and consists of alkali feldspar(～ 40%), plagioclase (～ 30%), quartz (～ 25%), amphibole (～3%), and accessory minerals (2%) (Fig. 3a, b).Colorless,anhedral quartz is 1.0-2.5 mm in size and shows undulose extinction. Simple- or multiple-twinned alkali feldspar occurs as mainly subhedral or tabular crystals of 3-5 mm in size. Plagioclase grains are 2-4 mm in size,mainly subhedral or euhedral, and show polysynthetic twins. Biotite is 0.5-1.5 mm in size. Accessory minerals include titanite, magnetite, and apatite.

Quartz diorite shows a massive granitic texture and comprises plagioclase (～ 50%), amphibole (～ 15%),quartz (～ 10%), orthoclase (～ 15%), feldspar (～ 8%),and accessory minerals (Fig. 3c, d). Feldspar occurs as tabular grains of 0.2-2.0 mm in size and shows polysynthetic twinning. Quartz occurs as irregular grains of 0.5-1.5 mm in size. Accessory minerals include titanite,magnetite, and apatite.

4 Analytical techniques

4.1 Zircon LA-ICP-MS U-Pb geochronology

Samples for zircon U-Pb dating were collected from fresh rock outcrops. The zircon grains are euhedral, and those with reflective and clean surfaces were selected under a binocular microscope for U-Pb dating. Transmitted-light,reflected-light, and cathod oluminescence (CL) images of zircon samples were taken at the Laboratory of Langfang Regional Research Institute, Hebei Provincial Geological Survey Bureau, Hebei, China.

Zircon U-Pb dating and trace-element analyses were under taken using laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at Yanduzhongshi Geological Analysis Laboratories, Beijing,using a New Wave UP213laser with a beam diameter of 30 μm and an ablation depth of 20-40 μm.The carrier gas(He) and the compensation gas used to regulate sensitivity(Ar) were mixed using a homogenizer before entering the ICP-MS. Analyses of each datum used a blank signal lasting ～ 20-30 s and a sample signal lasting ～ 50 s.The international standard zircons 91,500 and Plesovice were used as external standards for age calibration(Wiedenbeck 1995), and reference material SRM610 was used as an external standard for elemental analyses.29Si was used as the internal standard. Further details of the experimental procedures are provided by Hou et al.(2009).

Fig. 2 Regional geological map of Zhiduo region(based on Wang et al.2018)1.Quaternary;2.Eocene-Miocene;3.Jurassic;4.Upper Triassic Batang Group; 5. Upper Triassic Jiezha Group; 6. Middle to Upper Triassic Bayankara Group; 7. Middle Triassic Jielong Group; 8. Chayong Ophiolitic Mélange;9.Duocai Ophiolitic Mélange;10.Middle to lower Permian Kaixinling Group;11.Lower Carboniferous Zaduo Group;12.Mesoproterozoic Ningduo Group;13.Paleogene granite;14.Late Jurassic granite;15.Late Triassic granite;16.Fault;17.Polymetallic deposit;18. County; 19. Sample location

4.2 Zircon Hf isotopes

In situ Lu-Hf isotope measurements were performed using a Thermo Finnigan Neptune-plus MC-ICP-MS instrument fitted with a J-100 femtosecond LA system from Applied Spectra, housed at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences(CAGS), Beijing, China. The analytical procedures and calibration methods used were similar to those described by Wu et al. (2006).

Zircons were ablated for 31 s at a repetition rate of 8 Hz at 16 J/cm2, for which ablation pits were ～ 30 μm in diameter. During analysis, the isobaric interference of 176Lu on 176Hf was negligible owing to the extremely low176Lu/177Hf in zircon (normally ＜0.002). The mean173Yb/172Yb value of individual spots was used to calculate the fractionation coefficient(βYb)and then to calculate the contribution of176Yb to176Hf. Calculations assumed a173Yb/172Yb ratio of 1.35274 (Yuan et al. 2007).

4.3 Whole-rock geochemistry

Samples for whole-rock major-and trace-element analyses were collected from fresh rock outcrops. Analyses were undertaken at the laboratory of ALS Minerals-ALS Chemex (Guangzhou), Guangdong, China. The samples were first dried at low temperature, then crushed and sievedthrough 200 mesh,and 300 g of each sample was obtained for analysis.Major-element analyses were performed using X-ray fluorescence spectrometry (XRF), whereas traceelement analyses (including rare-earth elements or REEs),were conducted using an Elan6000 ICP-MS instrument.Uncertainties on the major-element analyses were below 5% and the trace-element and REE analyses were below 10%. Detailed analytical procedures are provided by Qu et al. (2004).

Fig. 3 Representative photomicrographs of a,b monzogranite, and c, d quartz diorite in the Zhiduo region.Afs alkali feldspar, Pl plagioclase,Qtz Quartz, Am amphibole

5 Results

5.1 LA-ICP-MS U-Pb zircon dating

Results of LA-ICP-MS U-Pb zircon dating of the monzogranite and quartz diorite from the Duocai area are presented in Table 1.

Most zircons in the monzogranite are euhedral colorless,transparent, stubby prisms and, less commonly, more acicular and granular examples, and range from 100 to 130 μm in length and 50-80 μm in width. In CL images,the zircon crystals show broad oscillatory zoning(Fig. 4a),and have Th and U contents of 91-762 ppm and 161-378 ppm, respectively, with high Th/U values of 0.56-1.21 that are consistent with a magmatic origin(Belousova et al. 2002; Hoskin 2003).

We selected 30 representative zircon crystals from monzogranite sample SNLW1 for zircon U-Pb dating and analyzed one spot per crystal. The zircon data fall on or close to Concordia, and define a weighted mean age of 234.6 ± 0.9 Ma(n = 30;MSWD = 0.36),within error of a calculated Concordia age of 234.6 ± 0.7 Ma (MSWD =1.09) (Fig. 5a), which is interpreted to represent the crystallization age of the monzogranite, indicating that the monzogranite formed during the early Late Triassic.

Most zircons in the quartz diorite are colorless and transparent, with crystals exhibiting mostly a long columnar crystal habit with some short columnar and granular specimens of 100-150 μm in length and 40-60 μm in width.Most zircons are euhedral and some are fractured.In CL images the zircon crystals show wide oscillatory zoning, indicating a magmatic origin (Fig. 4b). Th and U contents in the zircon are 90-543 ppm and 352-854 ppm,respectively, with high Th/U values of 0.22-0.64, again indicating a magmatic origin for the zircons (Belousova et al. 2002; Hoskin 2003).

We selected 24 representative zircon crystals from quartz diorite sample DC1 for zircon U-Pb dating and analyzed one spot per crystal. The zircon data fall on or close to Concordia, and define a weighted mean age of 209.2 ± 0.4 Ma(n = 24;MSWD = 0.37),within error of a calculated Concordia age of 209.1 ± 0.7 Ma (MSWD =0.29) (Fig. 5b), which is interpreted to represent the crystallization age of the quartz diorite, indicating that the quartz diorite formed during the Late Triassic.

Table 1 LA-ICP-MS U-Pb zircon analytical data of the monzogranites and the quartz diorites

5.2 Zircon Hf isotopes

Zircons from the two samples analyzed for U-Pb dating(SNLW1and DC1)werealsoanalyzedfor Lu-Hfisotopes on,or adjacent to,the same analysis sites(Fig. 9;Table 2).Fifteen spot analyses from monzogranite sample SNLW1 yieldedfLu/Hf values from - 0.90 to - 0.95. Fourteen spot analyses from quartz diorite sample DC1 yieldedfLu/Hf values from- 0.93 to- 0.98.These values are much lower than average values of mafic and siliceous crust(- 0.34 and- 0.72, respectively; Amelin et al. 1999; Vervoort et al.1996). Therefore, two-stage model ages derived from these data may reflect either the time when the source material was extracted from the depleted mantle or the average age of a crustal source(Diwu et al.2007).176Hf/177Hf ratios of zircon grains from the monzogranite vary from 0.282856 to 0.283025, corresponding to εHf(t) values of 7.9-13.6(mean = 10.7) and two-stage Hf model ages (TDM2) of 762-395 Ma.Zircon grainsfromthe quartzdioritehavelower εHf(t) values (- 2.5 to 0.6) and older TDM2model ages(1402-1210 Ma)than those from the monzogranite.

5.3 Whole-rock major- and trace-element geochemistry

Four samples of monzogranite and five samples of quartz diorite were selected for major-and trace-element analyses(Table 3).

Fig. 4 Representative CL images of zircon grains from a monzogranite and b quartz diorite from the Zhiduo region

Fig. 5 Zircon U-Pb ages concordia diagram for a monzogranite and b quartz diorite in the Zhiduo area

5.3.1 Major-element geochemistry

SiO2contents of the monzogranite samples are 62.19-65.70 wt%with a mean of 64.51 wt%.These samples have K2O contents of 2.05-2.64 wt%,total alkali contents of 8.09-8.22 wt%,and Na2O/K2O ratios of 2.06-2.98,indicating sodic characteristics. In a K2O versus SiO2diagram(Fig. 6a),the samplesbelongto thecalc-alkalineseries.Al2O3contents of the samples range from 15.44 to 16.33 wt%.In an A/CNK-A/NK diagram (Fig. 6b), the samples fall into the metaluminous field (A/CNK 0.89-1.00). In QAP and TAS diagrams (Fig. 6c, d), all the samples are classified as subalkaline quartz monzonite.

SiO2contents of the quartz diorite samples range from 61.21 to 62.43 wt%. These samples have 1.45-2.37 wt%K2O, total alkali contents of 4.22-5.07 wt%, and Na2O/K2O ratios of 1.14-1.92,indicating sodic characteristics.In a K2O versus SiO2diagram (Fig. 6a), the sample data plot in the calc-alkaline field.Al2O3contents of the samples are 16.59-16.86 wt%, and the samples fall in the metaluminous to weakly peraluminous fields in an A/CNK versus A/NK diagram (A/CNK 0.97-1.01) (Fig. 6b). In a QAP diagram (Fig. 6c), the samples are classified as granodiorite. In a TAS diagram (Fig. 6d), they are classified as sub-alkaline diorite.

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5.3.2 Trace-element geochemistry

Total REE abundance in the monzogranite samples ranges from 92 to 133 ppm with a mean of 116 ppm. Chondritenormalized La/Yb ratios (La/Yb)Nare 6.1-6.5, indicating enrichment in light REEs (LREEs) (Fig. 7a). The samples exhibit negative Eu anomalies (Eu/Eu* = 0.53-0.76). Total REE abundance in the quartz diorite samples ranges from 92 to 155 ppm with a mean of 119 ppm. The samples have fractionated patterns, with (La/Yb)Nratios of 4.5-9.9, and display negative Eu anomalies (Eu/Eu* = 0.69-0.83;Fig. 7a).

A primitive-mantle-normalized trace-element spider diagram for the monzogranite and quartz diorite samples(Fig. 7b)shows enrichment in large-ion lithophile elements(LILEs;e.g.,Cs,Ba,Rb,and K),Th,and U,and depletion in high-field-strength elements(HFSEs;e.g.,P,Ti,Nb,and Ta).

Table 2 In-situ zircon Lu-Hf isotopic data of the monzogranites and the quartz diorites

6 Discussions

6.1 Petrogenesis and magmatic source

6.1.1 Monzogranite

The monzogranite samples are sodium-rich calc-alkaline rocks(Fig. 6),with A/CNK ≤1.Minerals characteristic of peraluminous S-type granites, such as muscovite, garnet,and cordierite,were not found.P2O5contents are also very low(0.11-0.14 wt%),FeOT/MgO contents(1.20-1.90) are significantly lower than the average contents of A-type granite (13.4) but close to that of I-type granite (2.27;Whalen et al. 1987). Contents of Zr + Nb + Ce + Y are 286-361 ppm (mean = 327 ppm), and values of 10,000 × Ga/Al is 2.15-2.33 (mean = 2.25), all below the lower limits of A-type granite (350 ppm and 2.6, respectively Whalen et al. 1987). In geochemical classification diagrams,all of the monzogranite samples fall in the fields of undifferentiated I- and S-type granite (Fig. 8). Samples have Na2O/K2O ratios of 2.06-2.98, and low Ce contents(31.1-45.9). In Na2O versus K2O (Fig. 9a) and Ce versus SiO2(Fig. 9b) diagrams, all of the monzogranite samples fall in the fields of I type granite. The rocks comprise mainly quartz,amphibole, and plagioclase,consistent with the monzogranite samples representing calc-alkaline I-type granites.

Generally,I-type granites form through partial melting of basaltic rocks or by mixing of crust- and mantle-derived magmas (Salters and Hart 1991; Rapp and Watson 1995;Hofmann and Welin 1988). The high MgO contents(1.63-3.60 wt%) and Mg#values (48.40-59.70) of the monzogranite samples are significantly higher than those of magmas formed by partial melting of basaltic rocks(Mg#＜40).The Nb/Ta ratios of the samples(14.81-16.36)are close to those of magmas derived from a mantle source(17.5 ± 2; Green 1995; Saunders et al. 1988), but significantly higher than crust-derived magmas(11-12,Saunders et al.1988),indicating a significant role of a mantle source during the formation of the monzogranites.However,La/Nb ratios are 1.61-1.87,intermediate between continental crust(2.20) and mantle (0.94) (Saunders et al. 1988; Weaver 1991). The monzogranite samples are enriched in silica,alkalis,alumina,and LILEs,and are depleted in HFSEs o in addition, the Th/Nb (0.92-1.00) and Th/La (0.50-0.58)ratios of the samples are characteristic of crustal sources(continental crust and primitive mantle have Th/Nb ratios of 0.440 and 0.177 and Th/La ratios of 0.204 and 0.125,respectively;Saunders et al.1988;Weaver 1991),indicating a mixed magma source.

Table 3 Major, REE and trace element contents and parameter of the monzogranites and the quartz diorites

Fig. 6 Discrimination diagrams for monzogranite and quartz diorite from the Zhiduo region.a K2O versus SiO2 diagram(Peccerillo and Taylor 1976); b A/NK versus A/CNK diagram (Maniar and Piccoli 1989); c QAP modal diagram following the IUGS classification (Le Maitre et al.2002); d Total alkalis versus silica diagram (Middlemost 1994)

Fig. 7 a Chondrite-normalized REE diagram (Taylor and Mc Lennan 1985) and b primitive- mantle-normalized spider diagram (Anderson 1983) for samples of monzogranite and quartz diorite from the Zhiduo region

Hf isotopes in zircon are useful in characterizing magmatic source rocks(Amelin et al.1999;Scherer et al.2000;Griffin et al. 2000;Wu et al. 2007a,b). The monzogranite samples have initial176Hf/177Hf ratios of 0.282856-0.283025, εHf(-t) values of 7.9-13.6, and TDM2ages of 762-395 Ma. With one exception, all of the samples plot between the depleted mantle and chondrite lines,indicating a dominantly juvenile source (Fig. 10). Two main roles have been identified for mantle-derived magmas in the formation of granite. One is that partial melting of crustal materials may be driven by emplacement of mantle magma (Fernandez and Barbarin 1991).The other is that mantle-derived magmas dominate the juvenile lower crust,and mix with the overlying crust to form granitic magmas during subsequent thermal events (Pitcher 1985;Wu et al.2003).Magmas formed by mixing of mantlederived magma that induced partial melting of deep-crustal rocks are characterized byεHf(t)valuesthatshowa wide range of Hf isotopic compositions and which vary from positive to negative(Qiu et al.2008).The εHf(t)compositions of zircon grains from the studied samples are all positive and show little variation. The two-stage model ages are relatively young.Therefore, we suggest that the monzogranite formed from mantle-derived magmas mixed with the overlying crustal materials.

6.1.2 Quartz diorite

Hornblende,which is present in the quartz diorite samples,is a characteristic mineral of I-type granite. P2O5contents are also very low (0.10-0.11 wt%), FeOT/MgO contents(2.02-2.14) are significantly lower than the average contents of A-type granite (13.4) but close to that of I-type granite (2.27; Whalen et al. 1987). The rocks contain Zr + Nb + Ce + Y contents of 137.1-210.0 ppm,(mean = 196.7 ppm), and 10,000 × Ga/Al of 1.98-2.13(mean = 2.31), except for one sample (10,000 × Ga/Al =3.47), below the lower limits of A-type granite [350 ppm and 2.6, respectively; Whalen et al. 1987)]. Samples have Na2O/K2O ratios of 1.14-1.92, and low Ce contents(36.8-61.7).In K2O versus Na2O(Fig. 9a)and SiO2versus Ce(Fig. 9b)diagrams,all of the monzogranite samples fall in the fields of I type granite. In conclusion, the quartz diorites in the Duocai region are calc-alkaline I-type granites.

The quartz diorites are enriched in LILEs and depleted in HFSEs,which are characteristics of arc magmatic rocks.Arc magmatic rocks may be formed in four ways: (1)partial melting of subducting lithosphere (Defant and Drummond 1990); (2) partial melting of a metasomatized mantle wedge(Rogers and Hawkesworth 1989);(3)partial melting of mafic lower crust (Atherton and Petford 1993);and (4) magmatic mixing (Barbarin 1999). As the rocks contain lower Sr contents (203.5-339.9 ppm) and higher Yb (1.83-3.82) and Y (17.77-33.15) contents in comparison with typical adakites (Defant and Drummond 1990;Drummond and Defant 1990), formation of the quartz diorites through partial melting of subducting lithosphere is unlikely. The ranges of particular trace-element ratios in the quartz diorite (Nb/Ta = 9.77-10.91, La/Nb =1.87-4.57, Th/Nb = 1.15-3.11, Th/La = 0.54-0.68) are clearly different from the mean values of primitive mantle(Nb/Ta = 17.5 ± 2.0, La/Nb = 0.94, Th/Nb = 0.177, Th/La = 0.125) but are relatively close to the average values of continental crust (La/Nb = 2.2, Th/Nb = 0.44, Th/La =0.204) (Green 1995; Saunders et al. 1988; Weaver 1991).Therefore, it is unlikely that the quartz diorites formed directly from the partial melting of the mantle.

Fig. 8 Geochemical classification diagrams for monzogranite and quartz diorite from the Zhiduo region. (after Whalen et al. 1987). a FeOT/MgO versus Zr + Nb + Ce + Y;b(K2O + Na2O)/CaO versus Zr + Nb + Ce + Y.FG Fractionated felsic granites,OGT unfractionated M-,I-, and S-type granites

Fig. 9 a Na2O versus K2O diagram(after Collins et al.1982)and Ce versus SiO2 diagram for monzogranite and quartz diorite from the Zhiduo region

Fig. 10 Zircon Hf isotopic compositions for monzogranite and quartz diorite from the Zhiduoregion

Most of the εHf(t) values of the quartz diorite samples fall between the chondritic Hf isotope evolution line and the 1.8 Ga mean crustal evolution line (Fig. 10). The twostage model ages range from 1.21 to 1.66 Ga. These data indicate that the quartz diorites formed mainly from the partial melting of the mafic lower crust.But one sample fall between the depleted mantle and chondrite lines, maybeit implies an origin through magma mixing. Mean while, the quartz diorites show low SiO2contents and relatively higher Na2O contents than that of K2O. These characteristics suggest that the quartz diorites could not derive from pure crust melting, the involvement of the mantle-derived magma during the formation of the quartz diorite should be definite.

In conclusion, the magmas that produced the quartz diorites were likely derived mainly from partial melting of the mafic lower crust and small amount mantle-derived magma involved.

6.2 Tectonic implications

The studied samples are Triassic metaluminous calc-alkaline I-type granites that have geochemical characteristics typical of arc magmatic rocks. In tectonic discrimination diagrams(Fig. 11),the samples fall in the field of volcanic arc granites (VAG), indicating formation in a subduction setting(Pearce et al.1984;Kapp et al.2003;Condie 2005).However,the source areas of the two samples are markedly different. The early Late Triassic monzogranite samples show high positive εHf(t) values (7.9-13.6), whereas the Late Triassic quartz diorite samples show much lower values (- 2.5 to 0.6). This indicates a crustal source material and thickening of the continental crust. We suggest that the Zhiduo area was in a subduction environment during the early Late Triassic.Subduction ended at the end of the Late Triassic, and crustal melting resulted in magmatism during the initial stages of collision.

Studies utilizing paleontological, geochemical, and geochronological data from the region have suggested that the Jinshajiang Ocean was initiated during the Early to Middle Devonian, developed into an oceanic basin during the Early Carboniferous (Pan 1996; Yan et al. 2018), and continued to expand until reaching its maximum extent during the Permian (Jian et al. 1999; Zeng et al. 2018),after which it began to narrow and close. However, there are different views on the direction of the subduction of the Jinshajiang oceanic plate. On the basis of paleomagnetic,geochronological, and isotopic data, Roger et al. (2003)proposed that the Jinshajiang oceanic plate subducted eastward under the Yidun Island Arc. However, recent research on intrusive and sedimentary rocks of the Yidun Arc suggests that the western part of the Yidun Arc was a passive continental margin during the Triassic,whereas the eastern part contains back-arc volcanic rocks (Yang et al.2011, 2012, 2014; Wu 2015; Wu et al. 2017). The distribution of the magmatic rocks is inconsistent with the eastward subduction of the Jinshajiang oceanic crust.Therefore, we propose that the Jinshajiang oceanic crust was subducted southwestward beneath the Qiangtang Block.

Some researchers have recently regarded the intersection between the Jinshajiang and Ganzê-Litang sutures as representing the boundary between the western and southern sections of the Jinshajiang Ocean, thus subdividing the ocean into western and southern parts. Moreover,owing to collision between the Yidun Arc and the Qiangtang Block, the timing of closure of the Southern Jinshajiang Ocean and Western Jinshajiang Ocean is also different (Mo et al. 1994; Reid et al. 2005; Roger et al.2010;Zi et al.2012a,b,2013;Wang et al.2014;Liu et al.2018).The closure time of the Southern Jinshajiang Ocean has been proposed as Late Permian to Early Triassic (Mo et al. 1994; Yang et al. 2011, 2014; Liu et al. 2018), or Middle Triassic (Sun et al. 1997; Wang et al. 2014; Zeng et al.2018).However,previous studies have shown that the subduction of the Western Jinshajiang Ocean lasted until the Late Triassic. Zhang et al. (2012) studied the ophiolite mélange from the Wandao lake region in the WJS and considered that initiation of subduction of the Western Jinshajiang oceanic plate occurred at 232 Ma, during the early Late Triassic. Li et al. (2003) argued that initial collision as recorded in the WJS occurred during the Late Triassic,between the Carnian and Norian stages,according to geochronological data from a collisional granite, and a stratigraphic unconformity in the western section of the Jinshajiang Suture zone. Based on the occurrence of 209 Ma granites in Zhiduo county, Jin (2006) proposed that the WJS Zone was still undergoing subduction during the Late Triassic or was the locus of plate collision.On the basis of the aforementioned studies, we consider that subduction at the WJS Zone occurred during the Middle to Late Triassic, and that collision may have occurred during the Late Triassic.

Fig. 11 a Plots of Y versus Nb,b Yb versus Ta, c Y + Nb versus Rb, and d Yb + Ta versus Rb (after Pearce 1984)showing the compositions for monzogranite and quartz diorite from the Zhiduoregion. Syn-COLG syn-collisional granites,WPG within-plate granites,ORG oceanic ridge granites,VAG volcanic arc granites

Fig. 12 Schematic diagrams showing the evolution of the Jinshajiang Ocean, the Ganzi-Litang Ocean, and the Yidun Arc from the Late Permian to Late Triassic

The Ganzê-Litang Ocean Basin formed during the Late Permian owing to the activity related to the E’mei mantle plume (Wu 2015). In the Early-Middle Triassic, the ocean basin expanded to the maximum and subsequently began to subduct. The large amount of Triassic arc magmatic rocks in the eastern part of Yidun Island Arc provide strong evidence for the tectonic evolution of the Ganzê-Litang Ocean. Studies of granite from the Yidun island arc have shown that active subduction occurred mainly during the period 237-206 Ma (Hou et al. 2001). Previous research has confirmed that the Ganzê-Litang oceanic plate was subducting westwards beneath the Yidun Island Arc at this time (Yang et al. 2011, 2012, 2014; Wu et al. 2017). The widespread Middle to Late Triassic arc magmatic rocks in the Yidun Island Arc indicates that the peak of the subduction of the Ganzê-Litang oceanic plate was ～ 215 Ma(Wang et al. 2013a, b).Qin et al. (2019)studied a 206 Ma collisional granite on the east side of the Yidun Island Arc and concluded that closure of the Ganzê-Litang Ocean occurred at the end of the Late Triassic. An analysis of detrital zircon grains from Triassic flysch in the Yidun arc and the Songpan-GanzêBlock suggests that the Ganzê-Litang Ocean had completely closed by the end of the Triassic (Jian et al. 2019). Wang et al.(2011a,b,c,2013a, b)studied dacites and andesites of the Yidun Arc and concluded that subduction of the Ganzê-Litang oceanic crust occurred in the Late Triassic and that the ocean had closed by the end of the Late Triassic. Zhu et al.(2019)proposed that I-type granites and intermediate to felsic volcanic rocks in the Yidun Arc formed in relation to the subduction of the Ganzê-Litang oceanic crust at 230-206 Ma.

Previous studies have indicated that the timing of slab subduction and closure of the Western Jinshajiang Ocean differed from that of the Southern Jinshajiang Ocean, but that it was similar to that of the Ganzê-Litang Ocean.During the Late Permian, owing to the extension of the Ganzê-Litang Ocean Basin,the Jinshajiang Ocean,and the Ganzê-Litang Ocean co-existed. At this time, the Jinshajiang oceanic plate was subducting beneath the North Qiangtang Block to the south and west (Fig. 12a, b).During the Middle Triassic, the existence of the Yidun Island Arc resulted in diachronous closure of the Jinshajiang Ocean. The southern Jinshajiang Ocean closed first because of the collision between the Yidun arc and the North Qiangtang Block. The Western Jinshajiang Ocean and the Ganzê-Litang Ocean merged (Fig. 12c, d). At the end of the Late Triassic, the Jinshajiang-Ganzê-Litang oceanic plates were subducted beneath the North Qiangtang Block and Yidun Island Arc, resulting in ocean closure and collision between the Songpan-GanzêBlock and the Yangzi Block in the north and between the Qiangtang Block and the Yidun Island Arc in the southwest(Fig. 12e,f). This was also the final closure time of the entire Jinshajiang-Ganzê-Litang Ocean.

On the basis of the above results,we consider that in the early Late Triassic(234 Ma)the Zhiduo region was located in a subduction setting to the southwest of the West Jinshajiang-Ganzê-Litang Ocean. During the Late Triassic(～209 Ma), subduction ended and collision commenced.

7 Conclusions

The following conclusions can be drawn from our study of the geochemical characteristics, zircon U-Pb chronology,and zircon Hf isotopes of the Duocai granite:

1. The original magmatic source of the monzogranites was most likely derived from mantle-derived magmas mixed with the overlying crustal materials. The primary magma of the quartz diorite probably formed from partial melting of the juvenile lower crust.

2. The monzogranites formed at 234 Ma, and the quartz diorites formed at 209 Ma.Both granites,representing a continuous subduction-to-collision cycle.

3. The granites in the study area are related to oceanic plate subduction and associated closure of the Western Jinshajiang-Ganzê-Litang Ocean. Final closure of the Western Jinshajiang-Ganzê-Litang Ocean occurred soon after 209 Ma.

AcknowledgementsThis research was funded by the Geological Survey Project (12120113098300) of China Geological Survey and the National Natural Science Foundation of China (41272093). We would like to thank the Laboratory of Langfang Regional Research Institute, Hebei Provincial Geological Survey Bureau, Hebei, China;the Yanduzhongshi Geological Analysis Laboratories, Beijing,China and the laboratory of ALS Minerals-ALS Chemex (Guangzhou) Co., Ltd., Guangdong, China for helping in the analyses.