0 Introduction

The Eastern Kunlun Orogen (EKO) is divided into the fault-bound North Kunlun belt (NKL), Middle Kunlun belt (MKL), and South Kunlun belt (SKL) (Fig.1b, Jiang et al., 1992). The Wulonggou area, where granites contribute to about 80% of the exposed rocks, is located in the central-eastern segment of the MKL in Qinghai Province in NW China (Fig.1b). So it is an ideal area for studying granitic intrusions. Granites play a major role in the evolution of the continental crust and the enrichment of economic minerals, making them a hot topic of research (Wu et al., 2011; Jiang et al., 2012). However, many researchers only focused on granitic batholiths or stocks in the Wulonggou area (Lu, 2011; Ding et al., 2014; 2015). The Huanglonggou granodiorite is a small scale intrusive stock, which is cut by an orogenic gold-bearing fault in the Huanglonggou gold mining area. Although an LA-ICP-MS zircon age of ～221 Ma was reported by Zhang et al. (2017), no geochemical and isotopic data of this granodiorite were completed. Apparently, such granitic intrusive rock is very important not only to constrain the age and petrogenesis of granites themselves, but also to better understand the tectonic settings for the genesis of mineralization in the EKO.

In this study, we present precise LA-ICP-MS zircon U-Pb ages, together with major and trace elements and Hf isotopic data for the Huanglonggou granodiorite, in order to better constrain the ages of magmatism and related mineralization and understand the geodynamic evolution history of the Late Triassic EKO.

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1 Geological background and local geology

The EKO, located in the north of Qinghai-Tibet plateau of Northwest China (Bian et al., 2002; Chen et al., 2007), represents a composite orogen which can be divided into three structural belts (Fig.1b), i.e., NKL, MKL and SKL (Jiang et al., 1992). Two ophiolite belts lay along the Middle Kunlun Fault (MKLF) and South Kunlun-Aryan Maqin Fault (SKLF-AMF), which represent suture zones of the Proto-Tethys and Paleo-Tethys oceans, respectively (Jiang et al., 1992; Yang et al., 1996). The NKL is characterized by thick Ordovician detrital and carbonate rocks, which are unconformably overlain by Devonian molasse deposits. Both sedimentary units are intruded by widespread Late Paleozoic granites (Jiang et al., 1992). The MKL consists of a Precambrian metamorphic complex of gneiss, schist and phyllite (Yang et al., 1996; Yuan et al., 2009). The metamorphic unit was later intruded by widespread Neoproterozoic and early to Late Mesozoic granites (Jiang et al., 1992; Liu et al., 2005). The oldest rocks exposed in the SKL are metamorphosed Meso- Neoproterozoic and Ordovician units. Triassic and Jurassic marine sedimentary rocks unconformably overlay these sequences in the SKL (Yuan et al., 2009; Liu et al., 2005).

The Wulonggou study area (Fig.2) is located in the central segment of the MKL. Paleoproterozoic Jinshuikou Group, Mesoproterozoic Xiaomiao Formation and Neoproterozoic Qiujidonggou Formation are all Precambrian rocks in the Wulonggou area (FIQGS, 2010). Some skarn-type Cu-Pb-Zn deposits are located at the contact between Middle Triassic granitic dykes and marble within Jinshuikou Group (Ding et al., 2014). Ductile faults are the most important structures in the Wulonggou mining area (Qian et al., 1998; Yu et al., 1999). Most of the ductile faults strike northwest, and a few strike east-west or northeast. Many orogenic gold deposits formed within the northwest-trending faults and are among the most important mineral resources in this area (Fig.2, Ding et al., 2014; 2015). The Wulonggou area is characterized by widespread granitic bodies, which include Cambrian, Silurian, Devonian and Permian and Triassic granites (Ding et al., 2014; 2015). The Early Triassic (ca. 246--248 Ma) granites intruded into the Jinshuikou Group in the northern part of the area, and it was suggested that the Triassic granitic magma was generated during the northward subduction of the Palaeo-Tethys oceanic plate between the EKO and BH-SG-Bayan Har-Songpanganzi Terrane (Ding et al., 2015).

The Late Triassic (～221 Ma) granodiorite (Zhang et al., 2017), here named Huanglonggou granodiorite, was emplaced in the Huanglonggou orogenic gold mining area (Fig.3). It was also intruded by the Huanglonggou diorite dyke (215--220 Ma, Ding et al., 2014; Zhang et al., 2017). Gold-bearing Fault XI cut both of the Huanglonggou diorite dyke and Huanglonggou granodiorite, with few pyritization, indicating the orogenic gold mineralization within Fault XI formed after both of the Late Triassic intrusive rocks (Ding et al., 2014). The Huanglonggou granodiorite is an approximately 1 km wide porphyritic-like intrusive stock, which consists of medium-grained plagioclase (～10%) and hornblende (～5%) phenocrysts in the matrices of plagioclase (45%--55%), k-feldspar (10%--15%), quartz (30%--40%), hornblende(～10%)andbiotite(<5%). Accessory minerals include titanite, magnetite, apatite and zircon etc.

2 Sampling and analytical methods

2.1 Sampling

Six samples for the Huanglonggou granodiorite (Sample HLG-03-B02, HLG-03-B03, HLG-04-B01, HLG-05-B01, HLG-05-B02 and HLG-06-B01) have been collected on the north bank of the Huanglonggou Valley in the Wulonggou area (Fig.3).

2.2 CL imaging of zircons

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2.3 LA-ICP-MS zircon U-Pb dating

The LA-ICP-MS zircon U-Pb analyses for samples were carried out at Analysis Center in the Shandong Bureau of China Metallurgical Geology Bureau using a Thermo Xseries2 ICP-MS equipped with a Geolas Pro 193 nm laser ablation system. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are as described by Liu et al. (2010b, 2010c). Off-line selection and integration of background and analytic signals, time-drift correction and quantitative calibration for U-Pb dating were performed by ICPMSDataCal (Liu et al., 2008; 2010b). Zircon 91500 was used as external standard for U-Pb dating. Common Pb contents were evaluated using the method described by Andersen (2002). The age calculations and plotting of Concordia diagrams were made using Isoplot Ver.3.23 (Ludwig, 2003).

Chondrite-normalized REE patterns for the Huanglonggou granodiorite invariably show REE (ΣREE=66.17--374.45) enrichment and slightly negative Eu anomalies (Eu/Eu*=0.68--0.86) and relatively flat HREE patterns (Fig.7a). The rocks are quite enriched in LREE relative to HREE (LREE/HREE=11.07--16.25) with apparent REE pattern slopes ((La/Yb)N =13.20--25.25) (Table 3).

2.4 Lu-Hf isotopes in Zircon

In situ Hf isotope analyses of zircons that have been determined LA-ICP-MS zircon U-Pb ages were carried out using the Newwave UP213 laser-ablation microprobe, attached to a Neptune II MC-ICP-MS at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Instrumental conditions and data acquisition were comprehensively described by Wu et al. (2006) and Hou et al. (2007). A stationary spot was used for the present analyses, with a beam diameter of 35 μm. The mass 176 isobaric interference was described by Yang et al. (2011). Zircon MT was used as one standard reference, with a weighted mean 176Hf/177Hf ratio of 0.282 485±0.000 007 (2σ, n=28) during our routine analyses. Zircon Plai was used as another reference standard, with a weighted mean 176Hf/177Hf ratio of 0.282 914±0.000 014 (2σ, n=19).

The crossplots of major element versus SiO2 variation for the Huanglonggou granodiorite show a general decrease in TiO2 (Fig.6c), Al2O3 (Fig.6d), Fe2O3T (Fig.6e), MgO (Fig.6g), CaO (Fig.6h), and P2O5 (Fig.6j) with increasing SiO2, a general increase in K2O (Fig.6b) with increasing SiO2, and no apparent correlate in Na2O (Fig.6i) and MnO (Fig.6f) with increasing SiO2.

2.5 Major and trace elements of whole-rocks

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3 Results

The Huanglonggou granodiorite falls in granodiorite/quartz diorite field on the TAS diagram in Fig.6a. The granite is metaluminous with an ASI of 0.93--1.00 (Fig.8a, 8b), which is consistent with I-type granitic classification of Chappell (1999). The granodiorite has a high alkalis content of 6.37%--8.86% (Fig.7a), low Ga/Al values with low (K2O+Na2O)/CaO ratios and low Fe2O3T/MgO ratios, which are characteristic of non-fractionated granites. In addition, the high Sr contents of (426--475)×10-6, relatively high Sr/Y ratios (35.50--42.94), high (La/Yb)N values (13.20--25.25) and low HREE [e.g. Yb of (1.00--1.20)×10-6] and Y (10.90--13.10)×10-6 are different from a normal island-arc magma, suggesting an adakite-like source (Fig.8c, 8d), as defined by Castillo (2012). Furthermore, high SiO2 content of 64.30%--66.87% are consistent with a high-SiO2 adakite (HAS, SiO2>60%), as defined by Martin et al. (2005).

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3.1 LA-ICP-MS zircon U-Pb dating

Zircons extracted from the Huanglonggou granodiorite are typically gray or colourless, euhedral to subhedral and elongate to stubby in shape (100 to 200 μm long, mostly with length-to-width ratios of 1∶1 to 2∶1). The zircons are concentrically zoned (typical of magmatic oscillatory zonation; Hoskin & Black, 2000; Belousova et al., 2002). The Th/U ratios of zircons vary from 0.51 to 1.13 (clustering around 0.6--0.8), which is also consistent with a magmatic origin (Hoskin & Black, 2000; Belousova et al., 2002). Thus, the dates determined from the magmatic zircons represent the formation age of the granites.

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The U-Pb concordia diagrams for the Huanglonggou granodiorite are shown in Fig.4. The spots analyzed from the Sample HLG-03-B01 cluster is a basically coherent group on the concordia diagram, yielding a weighted mean 206Pb/238U age of 221±2 Ma (2σ), which is interpreted as the crystallization age of the granodiorite.

Major elements for all samples were determined by X-ray fluorescence spectroscopy (XRF) at the ALS Chemex (Guangzhou) Co., Ltd. Detailed operating conditions and procedures are described by Ding et al. (2015). The precision for major elements is better than 5%, and is 5%--10% for trace elements (Gou et al., 2013).

3.2 Zircon Hf isotopes

In situ Hf isotope analyses have been carried out on zircons with LA-ICP-MS zircon U-Pb dates. All spots were analyzed from the dated magmatic zircons in the Huanglonggou granodiorite. 25 spots have got reliable Hf isotope data for the Huanglonggou granodiorite.

The Hf isotope results (Table 2) indicate that all the magmatic zircons from the Huanglonggou granodiorite have close initial εHf(t) values (age corrected using U-Pb age for individual grains) ranging from -4.4 to +1.1 with an arithmetic mean of -2.9 (Fig.5a), corresponding to tDM2 two-stage model ages of 1 297 to 1 502 Ma with an arithmetic mean of 1 409 Ma (Fig.5b).

3.3 Major and trace elements

The whole-rock geochemical data (Table 3) indicate that the Huanglonggou granodiorite contains 64.30%--66.87% SiO2, and is metaluminous with an ASI of 0.93--1.00 and a low Mg# value of 0.45--0.47. It has high alkalis content (total K2O+Na2O) of 6.37%--8.86% (Fig.6a) and high K2O contents of 2.89%--3.19% (Fig.6b) with K2O/Na2O ratios between 0.77 and 0.89, which correspond to high-K calc-alkaline granitic series (Morrison, 1980).

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LOI = Loss on ignition; ASI = Molecular Al2O3/(CaO+Na2O+K2O); Mg#=Molecular MgO/(MgO+ Fe2O3T); Fe2O3T are total iron; Eu/Eu* is a measure of the europium anomaly when compare to Sm and Gd. Eu/Eu*=EuN/[(SmN)×(GdN)]; TZr = 12 900/(2.95+0.85M+lnDZr, zircon/melt) (Watson and Harrison, 1983), where DZr, zircon/melt is the ratio of Zr concentrations (～0.496) in zircon to that in the saturated melt, M=cation ratio (Na+K+2·Ca)/(Al·Si); εHf (t) denotes a weighted mean primary zircons Hf isotopic composition.

In the primitive mantle-normalized variation spidergrams for the Huanglonggou granodiorite the rocks are slightly enriched in Rb, Th, U and K [e.g. Rb=(69.80--110.00)×10-6; Th=(6.50--11.40)×10-6], and strongly depleted in Nb, Ta, P, Zr, and Ti [e.g. Nb=(6.20--7.20)×10-6; Ta=(0.61--0.76)×10-6; Zr=(46.20--65.80)×10-6] (Fig.8b). Such elements are often strongly controlled by individual minerals, such as that Ti, Nb and Ta are by ilmenite, rutile or titanite, and Zr by zircon (Rollison, 1993).

4 Discussion

4.1 Geochronology of the Huanglonggou granodiorite

One sample of the Huanglonggou granodiorite yielded a LA-ICP-MS U-Pb zircon mean age of 221±2 Ma, which is identical within analytical errors as the age of 220.6±1.7 Ma reported by Zhang et al. (2017). This indicates that the Huanglonggou granodiorite are intruded in the Late Triassic (Carnian), which suggests the orogenic gold mineralization within Fault XI in Huanglonggou gold deposit formed after 221 Ma as the gold-bearing ductile fault cut the Huanglonggou granodiorite (Fig.3). This conclusion is further strengthened by the age of ～215 Ma of the pre-mineralization Huanglonggou diorite in the Huanglonggou gold deposit in the Wulonggou area (Ding et al., 2014). Therefore, we argued that the gold mineralization in the Wulonggou gold deposit, or at least the Huanglonggou gold mineralization, must have formed after 215--221 Ma.

4.2 Petrogenesis for the Huanglonggou granodiorite

4.2.1 Petrogenetic type

The analytical data for the LA-ICP-MS zircon U-Pb dating are included in the Table 1. Results of the in situ Hf isotope analyses have been carried out on the zircons with LA-ICP-MS U-Pb dating are presented in Table 2. The whole-rock geochemical data is included in Table 3.

4.2.2 Origin and petrogenesis

Adakites can be divided into two subclasses, i.e. high-SiO2 adakites (HSA) and low-SiO2 adakites (LSA). HSA is considered to represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge, while LSA is thought to have formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts (Martin et al., 2005). The Chondrite-normalized REE patterns for the Huanglonggou High-SiO2 adakitic granodiorites show a weak negative Eu anomaly (Eu/Eu*=0.68--0.86) and a relatively large REE slope ((La/Yb) N=13.20--25.25), indicating that the source was garnet-bearing (Table 3, Fig.7a). The high Al2O3 concentration of 15.41%--16.19%, which indicates the pressure is >16 kbar if the melts have >15% Al2O3 (Rapp et al., 1991), implies that the source region of the Huanglonggou granodiorite was relatively deep (>30 km, Jiang et al., 2010). The relative depletion of Y and Yb in Fig.7b and the high Sr/Y and La/Yb values are also consistent with a source within the garnet stability field under eclogite-facies conditions (Defant & Drummond, 1990). Significantly negative Nb-Ta and Ti anomalies are commonly recognized as fingerprints for a subduction process (Fig.7b; e.g. Jiang et al., 2012). Moreover, the granodiorite has higher Mg# (0.49--0.54) values than those of pure crustal melts (Fig.9a), suggesting that the granitic magmas was interacted with mantle peridotite or mixed with mantle-derived melts (Jiang et al., 2009; 2010). However, magmatic zircons in the Huanglonggou granodiorite with a weighted mean 206Pb/238U age of ～221 Ma are characterized by initial εHf(t) values of -4.4 to +1.1 with clustering -4 to -2 (Fig.5b), which suggests that their origins were crustal materials rather than pure mantle-derived material. This means that the partial melting of subducted oceanic slab could be accepted for the origin of the Huanglonggou granodiorite, as high-SiO2 adakites are commonly considered to represent subducted basaltic slab-melts that have reacted with peridotite during their ascent through a mantle wedge (Martin et al., 2005).

The samples were processed through crushing, conventional magnetic and heavy liquid separation methods to extract zircons for U-Pb dating and Hf isotope analyses. The zircon grains were handpicked under a binocular microscope. The CL images for LA-ICP-MS zircon U-Pb dating and Hf isotope analyses were taken using a Gatan MiniCL detector attached to a scanning electron microscope of JEOL JSM6510 at Beijing Geoanalysis Co., Ltd.

As mentioned above, the Huanglonggou granodiorite decreases in TiO2 (Fig.6c), Al2O3 (Fig.6d), Fe2O3T (Fig.6e), MgO (Fig.6g), CaO (Fig.6h), P2O5 (Fig.6j) with increasing SiO2 on binary major element versus SiO2 variation diagrams, suggesting fractional crystallization of ferromagnesian minerals (biotite±hornblende), plagioclase, Ti-bearing phases and apatite (Rollison, 1993). The granodiorite is also depleted in Nb, Ta and Ti (Fig.7b), indicating fractional crystallization of Ti-bearing minerals. Moreover, the rough linear correlation between TiO2 (Fig.6c), Al2O3 (Fig.6d), Fe2O3T (Fig.6e), MgO (Fig.6g), CaO (Fig.6h), P2O5 (Fig.6j) with increasing SiO2 also suggests that the ascending of the adakitic magmatic source of the granite had different degrees of crustal assimilation or magmatic mixing.

To summary, the Huanglonggou granodiorite represents a melt generated from the assimilation combined with fractional crystallization (AFC) of a magma, which has reacted with peridotite during their ascent through a mantle wedge, was derived from the subducted basaltic oceanic slab. The composition of the source, degrees of partial melting and crustal assimilation may control the compositional variations observed in these rocks. The calculated zircon saturation temperatures (TZr℃) for the granite range between 670℃ and 699℃, which are interpreted as the minimum melt temperature for its magma.

4.3 Implications for the Late Triassic geodynamic evolutions of the EKO

As HSA have come from active subduction zones (Martin et al., 2005). The Huanglonggou granodiorite samples fall exclusively in the volcanic arc and syn-collision granite (VGA+Syn-COLG) fields on the Nb vs. Y plot (Fig.9b) and exclusively in the volcanic arc granite (VGA) field on the Y+Nb vs. Rb plot (Fig.9c), suggesting a magmatic arc setting. On the R1 vs. R2 plot (Fig.9d), they exclusively fall in pre-plate-collision granitoids, which suggests a subduction setting. This view is reinforced by their zircon Hf isotopic compositions and geochemical compositions, which suggest that these granodiorites were derived from the subducted basaltic oceanic slab that have reacted with peridotite during their ascent through a mantle wedge. Because the EKO terrane was a continental arc that formed by the northward subduction of the Paleo-Tethys oceanic crust in the Triassic, as was discussed above, the existence of the HSA can be used as an indicator of the northward subduction of the Paleo-Tethys oceanic crust in the Late Triassic, and indicating that Paleo-Tethys did not close until ～221 Ma (Carnian), which is close to ～215 Ma of the pre-plate-collision Huanglonggou High-Mg diorite as previously reported (Ding et al., 2014). In a word, both of the Huanglonggou granodiorite and Huanglonggou diorite dyke are intruded in a pre-plate-collision continental arc related to the Late Triassic northward subduction of the Paleo-Tethys oceanic crust.

Tentatively, we argued the subduction of Paleo-Tethys oceanic crust did not cease by the Carnian to Norian because the closure of Paleo-Tethys is a soft collision (Yin & Zhang, 1997). Alternatively, the break-off of the oceanic plate perhaps happened in the Late Triassic (e.g. Carnian to Norian) following the soft collision. But the EKO was still affected by the northward subduction of the oceanic slab under the EKO whose geodynamic setting is similar to that of pre-plate-collision. Therefore, the partial melting of the oceanic slab directly formed the Huanglonggou granodiorite in such setting (Fig.10), whereas partial melting of the slab’s surface sediments induced partial melting of the hybridized peridotitic mantle wedge, which further generated the magma of the Huanglonggou diorite dike (Ding et al., 2014). After the period of final closure of the Paleo-Tethys, the EKO was relatively stable until the Cenozoic, when K-rich volcanism was extensive in the northern Tibetan Plateau (Deng, 1998; Jiang et al., 1992; Yang et al., 2002).

5 Conclusions

Zircon U-Pb dating results and zircon Hf isotopic compositions, together with whole-rock geochemical analyses for the Huanglonggou granodiorite suggest the following conclusions:

(1) Precise LA-ICP-MS zircon U-Pb dating shows that the Huanglonggou granodiorite was emplaced during ～221 Ma.

(2) Geochemistry and Hf isotopic compositions indicate that the Carnian Huanglonggou granodiorite is adakite type granite and was probably derived from the partial melting of subducted oceanic slab.

(3) Detailed geochronological and isotopic data suggest that the Carnian granodiorite was emplaced during the northward subduction of Paleo-Tethys oceanic slab between the EKO and BH-SG. Orogenic gold mineralization in the Wulonggou area formed after the emplacement of such Late Triassic (221--215 Ma) intrusive rocks.

Thanks are also due to the First Geological Prospecting Institute of Qinghai Geology Survey, China. We are grateful to Dr. LIN Peijun, Dr. YANG Tao, Dr. LI Shijin, Mr. DENG Yuanliang and Mr. HAN Yu et al. for their assistances during field and analytical works.

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