Early Paleozoic Granulite-Facies Metamorphism and Magmatism in the Northern Wulan Terrane of the Quanji Massif: Implications for the Evolution of the Proto-Tethys Ocean in Northwestern China

0. INTRODUCTION

The Proto-Tethys Ocean originated from the breakup of the supercontinent Rodinia is an east-west trending oceanic basin, which existed from Neoproterozoic to the end of the Early Paleozoic (Li et al., 2017, 2016a, b, c, d; Zhang et al., 2017, 2015). The Paleo-Qilian Ocean (Li et al., 2017) in northwestern China is considered as a branch of the Proto-Tethys Ocean during the Early Paleozoic. Many microcontinents (such as Alxa, Qilian, Quanji and Qaidam) and continents (i.e., Tarim, North China and Yangtze) were located in and near the Paleo-Qilian Ocean, generating a complex archipelagic ocean (Li S Z et al., 2017; Song et al., 2017, 2015, 2014, 2013, 2006; Zhang et al., 2017, 2015; Pan et al., 2012, 1997; Chen et al., 2009; Lu et al., 2009, 2008, 2006; Li Z X et al., 2008; Gao et al., 2005). The Qilian-Qaidam area recorded complex Early Paleozoic subduction-collision processes (Li et al., 2017, 2016a, b, c, d; Zhang et al., 2017, 2015; Xia et al., 2016). In particular, the North Qaidam tectonic belt (NQTB), bounded by the Qilian Block in the north and the Qaidam Block in the south (Fig. 1), is characterized by the occurrences of abundant high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks as well as ophiolites and subduction-accretionary complexes, thus resulting in long-standing questions about Early Paleozoic orogenesis types (accretion or collision), subduction polarity, and the time-span and closure of the Paleo-Qilian Ocean and more broadly the Proto-Tethys Ocean in northwestern China.

Figure  1.  (a) Geological map of the Quanji Massif and the North Qaidam HP-UHP metamorphic belt (modified after Chen et al., 2013a; Wang et al., 2009). Also, the study area (shown in Fig. 2) in the northern Wulan terrane is outlined. Inserts (b) showing the tectonic location of (a), and (c) simplified geological units in the North Qaidam tectonic belt (NQTB).

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The North Qaidam tectonic belt (NQTB) (Fig. 1b), in particular, has been the focus of enormous studies over the past 30 years for its tectonic evolution (Song et al., 2018, 2017, 2014, 2013, 2006, 2005, 2003; Li et al., 2017; Zhang et al., 2017, 2015; Yu et al., 2015, 2014, 2011; Chen et al., 2013a, b, 2009; Wu et al., 2011, 2009, 2005; Lu et al., 2008, 2006; Yang and Powell, 2008; Yang et al., 2005, 2003; Lu, 2002; Yang and Deng, 1994; BGMQ, 1991). It consists of three tectonic units: the Quanji Massif (QM) to the north, the Tanjianshan volcanic zone in the middle (suture zone), and the North Qaidam HP-UHP metamorphic belt to the south (Figs. 1a and 1c). The latter two units are collectively referred to as the North Qaidam (NQD). Early Paleozoic magmatic and metamorphic events related to the closure of the Proto-Tethys Ocean have been reported to occur in the NQD, but poorly documented in the dominantly Proterozoic QM (He et al., 2018; Ma et al., 2018a, b; Li X C et al., 2018, 2015; Wang L et al., 2016; Wang Q Y et al., 2015). This contribution reports on new results of zircon U-Pb ages and petrological/geochemical data of Early Paleozoic granulites and associated granites and gabbroic dikes in the northern Wulan terrane of the QM (Fig. 1). These results are used to better constrain the low P/T metamorphism and contemporaneous magmatism in the QM, especially their relationships to the HP-UHP metamorphism in the NQD, thus providing evidence for Early Paleozoic paired metamorphic belts and subduction polarity as well as constraints on the closure of the Proto-Tethys Ocean in northwestern China.

1. GEOLOGICAL SETTING AND SAMPLE FEATURES

1.1 The North Qaidam

The North Qaidam (NQD) is composed of three subunits in lithology: a medium pressure (MP) metamorphic belt, a HP-UHP metamorphic belt and the Tanjianshan volcanic zone (e.g., Zhang et al., 2015 and references therein). The NQD HP-UHP metamorphic belt is characterized by abundant HP-UHP metamorphic rocks such as garnet peridotites and eclogites, which formed at 500–420 Ma with pressures up to 2.8 GPa (Song et al., 2018, 2017, 2014, 2006, 2005; Zhang et al., 2017, 2015, 2010; Yang et al., 2005, 1998; Yang and Deng, 1994). Zhang et al. (2017) and Song et al.(2014, 2006) suggested an earlier oceanic subduction at 460–440 Ma and a later continental subduction stage at 440–420 Ma. The MP metamorphic belt in the northern margin of the Qaidam Block (Zhang et al., 2017, 2007, 2005; Yu et al., 2015, 2014, 2011, 2010) is characterized by typical Barrovian zones of kyanite and sillimanite as well as syn-collisional Iand S-type granitoids at 450–420 Ma (Zhang et al., 2017, 2015, 2009, 2008; Wu C L et al., 2011, 2009, 2006, 2004a, b; Meng and Zhang, 2008; Meng et al., 2005; Wang et al., 2005, 2001). The Tanjianshan volcanic zone (Peng et al., 2016; Sun et al., 2015; Zhang J X et al., 2015; Gao et al., 2011; Shi et al., 2006, 2004; Zhang G B et al., 2005; Wang et al., 2003; Yuan et al., 2002; Lai et al., 1996a, b) spreads on the northern side of the NQD and adjacent to the southern margin of the QM (Fig. 1c). It is mainly composed of ultramafic rocks, gabbros, and intermediate-basic volcaniclastic rocks, with minor ophiolitic complexes and islandarc volcanic rocks, and diachronous formation during 540–500 Ma (Gao et al., 2011; Shi et al., 2006, 2004; Wang et al., 2003; Lu, 2002; Yang et al., 1998; BGMQ, 1991). The Tanjianshan volcanic zone at 540–500 Ma has been suggested to have formed from volcanic island-arc magmatism related to oceanic subduction (Song et al., 2018, 2014; Zhang J X et al., 2017, 2014) before Middle Ordovician and the disappearance of the South Qilian Ocean between the Qaidam Block and the Quanji Massif by collision orogeny.

1.2 The Quanji Massif

The Quanji Massif is a nearly NW-SE-trending (locally E-Wtrending) long and narrow small remnant continental fragment sandwiched between the South Qilian orogenic belt and the North Qaidam. The QM is characterized by component of double-layer structure: Early Paleoproterozoic granitoid gneisses and mediumto high-grade metamorphic rocks constitute its basement (He et al., 2018; Liao et al., 2018a, b, 2014, 2012; Gong et al., 2014, 2012; Zhang L et al., 2014, 2011; Chen N S et al., 2013a, b, 2012, 2009, 2008, 2007; Wang et al., 2009, 2008; Lu et al., 2008, 2006; Wan et al., 2006, 2001), and Mesoproterozoic–Phanerozoic sedimentary rocks form the cover (Ma et al., 2018a, b; Wang et al., 2016; Zhang et al., 2012; Lu et al., 2008, 2006; Zhao et al., 2000). Previous researches focused on the Proterozoic rocks and interested in the role that the QM played in the assembly and breakup of the supercontinents Columbia and Rodinia (He et al., 2018; Wang L et al., 2016; Chen et al., 2013a, b, 2012, 2009, 2008; Wang Q Y et al., 2009, 2008; Lu et al., 2008, 2006; Wan et al., 2006). However, Paleozoic assemblages formed from magmatism and metamorphism are also known to occur in the QM but have not been investigated in detail. For example, an island-arc magmatic zone with ages from 514 to 440 Ma occurs on the southern margin of the QM and mainly consists of Ordovician diorite and granite intrusions (Xia et al., 2016; Zhu et al., 2014; Wu C L et al., 2011, 2009, 2006, 2004a, b; Xu et al., 2006). In particular, island-arc granitoids emplaced at 475–460 Ma formed significantly after magmatism in the Tanjianshan volcanic zone but were broadly coeval with the earlier oceanic subduction in the NQD HP-UHP metamorphic belt. It has been proposed that the QM was located within the South Qilian Ocean during the Early Paleozoic, closer to the Qilian Block than to the Qaidam Block. The QM and Qaidam Block were finally welted in subduction-collision by the Tanjianshan volcanic zone. Both the UHP eclogites and the MP granulites associated with the Ordovician granitoids in the NQD at the Luliangshan and Xitieshan areas record two distinct groups of ages (470–445 and 438–420 Ma) (Song et al., 2018; Zhang J X et al., 2017, 2015; Zhang G B et al., 2005). However, it remains unclear whether the MP granulites in the NQD recorded regional Barrovian metamorphism or multiple episodes of oceanic crust subduction and how the QM involved in the evolution of the Proto-Tethys Ocean.

1.3 Geology of the Northern Wulan Terrane

This study was carried out in the northern Wulan terrane on the northern side of the Quanji Massif (Fig. 2). This terrane consists of three litho-tectonic units: an Early Mesoproterozoic unit in the middle, a Late Mesoproterozoic to Early Neoproterozoic unit on the northeastern and southwestern sides of the Early Mesoproterozoic unit, and an Early Paleozoic unit on the most northeastern margin in fault-contact with the Late Mesoproterozoic to Early Neoproterozoic unit (Wang, 2016; Wang et al., 2016).

Figure  2.  Detailed geological map showing the northern Wulan terrane in the Quanji Massif and the sample locations (modified after Wang et al., 2016).

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The Early Mesoproterozoic unit consists of dominantly Al-rich pelitic gneisses and schists, intruded by ~1.5 Ga trondhjemite, ~1.1 Ga mega-porphyritic granodiorite and ~1.0 Ga monzogranite. This unit experienced amphibolite-facies metamorphism, locally up to granulite-facies conditions with pervasively developed anatectic leucosomes at ~1.1 Ga and intensively reworked at ~0.47 Ga (Wang, 2018). The deposition of this unit has been constrained to occur between ~1.67 and 1.5 Ga by the youngest ages of detrital zircon and the ~1.5 Ga trondhjemitic intrusions. The Late Mesoproterozoic to Early Neoproterozoic unit is composed of two subunits with depositional ages of ~1.1 and ~0.94–0.90 Ga (Wang, 2016; Yu et al., 1994), respectively. The unit, which consists mainly of meta-sandstones, metavolcanic rocks, amphibolites and calc-silicate rocks, is characterized by the development of large volumes of thick-layered marbles, 860–810 Ma garnet-bearing granites (Ma et al., 2018a, b), and amphibolite-facies metamorphism.

The northern Wulan terrane was collided to and welted with on the northern margin of the QM at the Early Neoproterozoic (Wang, 2018). The Early Paleozoic unit in the northern Wulan terrane includes a series of gneissic granitoids, pelitic gneiss and amphibole-biotite gneiss, with minor mafic granulite lenses in granitic and pelitic gneisses (Da et al., 2017; Li et al., 2017, 2015; Wang et al., 2015; Dong, 2014; Guo et al., 2009). The Early Paleozoic metamorphic rocks recorded lowto medium-pressure granulite-facies metamorphism (Li et al., 2015; Wang et al., 2015). Early Paleozoic to Mesozoic igneous rocks also have been identified in the northern Wulan terrane, such as granitoids (Wu et al., 2016; Sun et al., 2015; Wang et al., 2015) and gabbros (Cheng et al., 2015; Guo et al., 2009). However, relationships between Early Paleozoic magmatism and granulite-facies metamorphism in the QM remain unclear.

1.4 Petrography of Magmatic and Metamorphic Rocks in the Northern Wulan Terrane

Six representative samples investigated in this study were collected from a granitic pluton (11WL-18), a gabbroic dike (11WL-23), granulites (11WL-1, 11WL-2 and 11WL-20) and a granitic leucosome (11WL-19) from the southern part of the Late Mesoproterozoic to Early Neoproterozoic unit in the northern Wulan terrane. Specifically, sample 11WL-18 is a monzogranite (Fig. 2), which occurs as small apophyses intruding the Late Mesoproterozoic to Early Neoproterozoic metamorphic strata (Figs. 3a–3b). The granitic pluton contains variable sized xenoliths of granulites and marbles. The monzogranite of a gneissic texture (Fig. 3b) contains mainly of K-feldspar (Kfs), quartz (Qtz), and plagioclase (Pl), with minor biotite (Bt) and accessory minerals of titanite (Ttn), allanite (Aln), magnetite (Mag), rutile (Rt) and zircon (Zrn) (Fig. 4a). The Kfs crystals are mostly subhedral plates of ~3 cm in length, while Qtz grains are commonly anhedral.

Figure  3.  Photographs showing the field occurrences of granitoids and associated granulites in the northern Wulan terrane. (a) The granitic pluton intruded into the Late Mesoproterozoic to Early Neoproterozoic unit and granulites as xenoliths in the granitic pluton; (b) banded granitic leucosome associated with the granite; (c) pelitic granulite; (d) felsic granulite; and (e) a gabbroic dike in the northern Wulan terrane.

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Figure  4.  Photomicrographs showing microtextural relationships of minerals in the samples from the northern Wulan terrene. (a) Monzogranite (11WL-18); (b) gabbro (11WL-23); (c) felsic granulite (11WL-1); (d) felsic granulite (11WL-2); (e) pelitic granulite (11WL-20); and (f) granitic leucosome (11WL-19).

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Sample 11WL-23 was collected from a gabbroic dike of 3–5 m wide (Fig. 3e). The gabbro has a gabbro-diabasic texture (Fig. 4b), and is composed mainly of plagioclase and clinopyroxene (Cpx), with minor amounts of orthopyroxene (Opx), hornblende (Hb) and Bt, and accessory Mag, ilmenite (Ilm) and Zrn. Both Cpx and Opx usually have exsolution lamellae of Ilm, while Hb occurs as a narrow retrograde rim around Cpx (Fig. 4b). Some fined grained Opx are enclosed by large lath-shaped Pl crystals, forming the typical ophitic texture.

Two felsic granulites (samples 11WL-1 and 11WL-2) occur as xenoliths in the granitic pluton (Figs. 3a, 3d). Sample 11WL-1 shows a porphyroblastic texture (Fig. 4c) and consists of dominantly Pl, garnet (Grt), Bt, Opx and Qtz, minor Cpx and Kfs, and accessory Mag, Ilm and Zrn. The Grt porphyroblasts (GrtI) are commonly 0.5–2 mm in diameter and have abundant inclusions of Qtz (Qtz₀), Bt (Bt₀), chlorite (Chl₀) and Pl (Pl₀) in the core (GrtI-c) and an inclusion-free rim (GrtI-r). These porphyroblasts often have embayed edges, suggesting that resorption occurred along the margins. The matrix consists of finegrained Grt (GrtII), Pl, Opx, Bt, Kfs and Qtz. GrtII is 0.01–0.2 mm in diameter and commonly occurs close to the edges of large Opx grains (OpxI). Most plates of Bt define the main gneissosity and appear to be part of the peak metamorphic generation (BtI). Fine-grained aggregates of Cpx, Qtz, PlII and BtII locally occur as replacement after OpxI. Therefore, sample 11WL-1 has three generations of mineral assemblages: with n1 of Pl₀+Qtz₀+Bt₀+Chl₀ occurring as inclusions in the cores of Grt porphyroblasts (GrtI-c); n2 of OpxI+PlI+Kfs±GrtI-r±BtI±Qtz; and n3 of GrtII+Qtz+Cpx+PlII±BtII.

Another felsic granulite (sample 11WL-2) shows similar characteristics as 11WL-1 (Fig. 4d) and consists dominantly of Grt, Pl and Opx, minor Cpx, Bt, actinolite (Act), Chl and epidote (Ep), and accessory Mag, Ilm and Zrn. Sample 11WL-2 also has two prograde mineral assemblages (c1 of OpxI+PlI+ GrtI±Qtz and c2 of GrtII+Qtz+Cpx±OpxII±PlII), and one retrograde metamorphic assemblage (c3 of Act+Chl+Ep). In particular, Cpx occurs as corona around OpxI and is partially replaced by fine-grained aggregates of Act+Chl+Ep (Fig. 4d).

The pelitic granulite (sample 11WL-20) with a strong NW-SE gneissosity displays a porphyroblastic texture (Fig. 4e), and consists of dominantly Grt, sillimanite (Sil), Bt, Kfs and Pl, with minor cordierite (Cord), Opx, Chl, Qtz and muscovite (Mus), and accessory Mag, Ilm and Zrn. Garnet occurs in two generations with GrtI as porphyroblasts (5–8 mm) and GrtII in the matrix (< 0.5–1.0 mm). The GrtI porphyroblasts commonly contain abundant inclusions Qtz₀, Chl₀, Bt₀ and Pl₀ in the core (GrtI-c) and a narrow inclusion-free rim (GrtI-r). The edges of GrtI porphyroblasts commonly show embayment, indicative of resorption. GrtII occurs locally with fine-grained Cord and Kfs (and less commonly Opx) close to prismatic Sil (SilI) (Fig. 4e). K-feldspar (KfsI), CordI and PlI are locally replaced by late SilII, PlII and BtII. Therefore, the pelitic granulite is interpreted to contain three distinct generations of mineral assemblages: m1: GrtI-c+Chl₀+ Qtz₀±Mus₀±Bt₀±Pl₀, m2: GrtI-r+SilI+KfsI+CordI+BtI±PlI±Qtz, and m3: GrtII±SilII±PlII+BtII+Qtz.

The granitic leucosome (sample 11WL-19) occurs as an irregular dike of 10–50 cm wide (Fig. 3f), crosscutting the main gneissosity in the granitic pluton. The granitic leucosome has a massive to pegmatitic texture (Fig. 4f) and consists mainly of microperthite, Qtz and albite with minor Mus and Bt, as well as trace Zrn.

2. ANALYTICAL METHODS

2.1 Whole-Rock Geochemical Analysis

All six representative samples were used for whole-rock geochemical analysis at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (Wuhan). All details about the sample preparation, analytical methods and uncertainties of whole-rock geochemical analyses are similar to those described in Wang et al. (2014) and Liu et al.(2008a, b). The analytical data are listed in Table 1.

Table  1.  Whole-rock bulk (wt.%) and trace elements and REE (ppm) compositions of samples from the northern Wulan terrane

Sample No.	11WL-18	11WL-19	11WL-1	11WL-2	11WL-20	11WL-23
Rock type	Monzogranite	Granitic leucosome	Felsic granulite	Felsic granulite	Pelitic granulite	Gabbro
SiO2	62.8	73.4	59.4	67.6	63.3	48.9
TiO2	1.38	0.046	2.16	1.05	0.76	1.17
Al2O3	13	15.1	11.4	10.1	17.3	16.7
Fe2O3*	7.4	0.28	14.5	9.01	8.46	9.51
FeOt	6.66	0.25	13.01	8.11	7.61	8.56
MnO	0.1	0	0.09	0.07	0.11	0.11
MgO	2.01	0.12	5.87	5.76	2.85	8.3
CaO	4.76	1.72	2.11	1.48	0.63	7.19
Na2O	2.27	2.9	0.65	0.43	1.1	3.06
K2O	3.46	5.45	2.91	2.99	3.64	1.32
P2O5	0.58	0.09	0.3	0.16	0.05	0.27
LOI	1.69	0.74	0.38	1.06	1.58	3.15
Total	98.8	99.7	98.3	98.8	98.9	98.7
Na2O/K2O	0.66	0.53	0.22	0.14	0.30	2.32
MgO/FeO	0.27	0.43	0.41	0.64	0.34	0.87
Al2O3/TiO2	9.45	327	5.27	9.64	22.7	14.3
MgO/CaO	0.42	0.07	2.78	3.89	4.52	1.15
P2O5/TiO2	0.42	1.98	0.14	0.15	0.07	0.23
A/CNK	0.80	1.09	1.41	1.52	2.51	0.85
δ	1.66	2.29	0.77	0.48	1.11	3.25
Cr	3.15	0.71	97.6	53.7	69.9	190
Ni	3.14	0.88	34.9	31.3	33.4	92
Co	17.2	0.4	35.9	28.7	21.3	40.4
V	191	3.91	398	194	101	124
Sc	19.3	1.3	32.8	24.4	20.4	21
Rb	109	177	75.2	82.7	156	61.9
Sr	349	184	33.5	22.6	98.4	308
Ba	2089	614	493	385	603	348
Th	5.54	20.1	7.78	7.49	20.1	3.17
U	1.24	3.46	1.15	1.38	2.47	0.75
Nb	42.5	7.31	16.6	9.99	14.9	7.48
Ta	1.8	1.21	1.15	0.68	1.01	0.45
Pb	11	36.5	5.97	3.66	9.91	6.61
Y	36.8	10.8	30.9	34.8	44.5	28.7
Zr	264	49.7	198	161	170	149
Hf	6.55	2.92	5.39	4.62	5.3	3.93
Cr/Ni	1.00	0.81	2.80	1.72	2.09	2.07
Sr/Y	9.48	17.04	1.08	0.65	2.21	10.73
La	61.9	20.5	29.5	27.5	50.7	18.8
Ce	138	44.6	61.4	56	105	39.9
Pr	16.2	5.36	7.76	6.86	12	5.35
Nd	61.3	19.2	31.8	27.2	45.7	22.5
Sm	10.5	5.42	6.68	5.64	9.02	5.14
Eu	2.71	0.74	1.69	1.25	1.47	1.53
Gd	8.71	4.53	6.62	5.63	8.08	5.22
Tb	1.3	0.69	1.04	0.96	1.38	0.89
Dy	7.27	2.67	5.86	6.08	7.96	5.31
Ho	1.35	0.35	1.19	1.3	1.64	1.05
Er	3.5	0.72	3.13	3.78	4.8	2.87
Tm	0.53	0.096	0.49	0.63	0.8	0.46
Yb	3.1	0.51	2.93	3.95	4.78	2.84
Lu	0.49	0.069	0.49	0.67	0.82	0.42
ΣREE	317	105	161	147	254	112
LREE	290.6	95.8	138.8	124.5	223.9	93.2
HREE	26.25	9.635	21.75	23	30.26	19.06
LR/HR	11.07	9.94	6.38	5.41	7.40	4.89
(La/Yb)N	14.33	28.85	7.23	5.00	7.61	4.75
EuN/EuN*	0.87	0.46	0.78	0.68	0.53	0.90
The δ value is defined as (K2O+Na2O)2/(SiO2–43) (Rittmann, 1962). Calc-alkaline rocks, δ < 3.3; alkaline rocks, δ=3.3–9; peralkaline rocks, δ > 9. EuN/EuN*=EuN/SQRT (SmN×GdN).

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2.2 Zircon U-Pb Dating

The procedures for the separation and preparation of zircon grains used for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis are similar to those described in Wang et al. (2014). Zircon cathodoluminescence (CL) images were taken on an LEO1450VP SEM at the Institute of Geology and Geophysics, Chinese Academy of Science in Beijing. In situ zircon U-Pb dating and trace element analyses were made by LA-ICP-MS at GPMR, China University of Geosciences (Wuhan). The diameter of individual spots for LA-ICP-MS analysis is 32 µm with a laser pulse rate of 6 Hz at 40 mJ/cm² energy density. The analytical procedure and operating conditions for the laser ablation system and the ICP-MS instrument as well as data reduction are similar to those described in Wang et al. (2014), which followed methods developed by Liu et al.(2010a, b, 2008a), Hu et al.(2008a, b) and Ludwig (2003). The analytical data of zircon U-Pb dating are given in Tables S2 and 2.

Table  2.  Summary of zircon data in samples from the northern Wulan terrane of the Quanji Massif

Sample No. (rock type)	Zircon morphology	Magmatic zircon	Inherited or detrital zircon	Metamorphic zircon
11WL-18 (monzogranite)	Euhedral columnar prism 40–200 μm in length most have bright CL and oscillatory zoning, minor with a core-rim texture	476.7±2.8 Ma (N=28) from bright cores		443±9 Ma (N=2) from dark rims
11WL-23 (gabbro)	Subhedral-anhedral prism dark CL with oscillatory zoning but no obvious core-rim texture	423±2 Ma (N=30)
11WL-1 (felsic granulite)	Subeuhedral to anhedral 30–120 μm in length two populations: bright cores and dark rims (narrow and wide)		2 431±51 (N=5); 2 047±85 (N=8); 1 752±50 (N=3); 1 130±46 Ma (N=7) from bright cores	472±4.8 Ma (N=6) from dark wide rims
11WL-2 (felsic granulite)	Similar to those in 11WL-1		1 909±30 (N=17); 1 721±73 Ma (N=12) from bright cores	lower intercept age of 474±32 Ma (N=17)
11WL-20 (pelitic granulite)	Subhedral short or round prism 40–120 μm in length; some with a core-rim texture		2 698±87 (N=6); 2 309±87 Ma (N=1) from bright cores	496±6 (N=10); 474±3.7 (N=10); 446±3.5 Ma (N=11) dark rims and cores
11WL-19 (granitic leucosome)	Subhedral round prism 50–100 μm in length two populations: most have dark CL and minor with a core-rim texture	474.8±2.3 Ma (N=28) from bright cores		443±6 Ma (N=1) from a dark rim

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2.3 Electron Microprobe Analysis

Chemical compositions of minerals such as garnet, orthopyroxene, plagioclase and biotite in the granulites have been determined using a JEOL JXA-8230 electron probe microanalyzer (EPMA) at the Materials Research and Testing Center, Wuhan University of Technology. Analytical conditions included an accelerating voltage of 15 kV, a beam current of 20 nA, and beam sizes of ≤5 µm in diameter. Natural minerals and synthetic oxides were used as standards for the analyses of major and minor elements, and data reduction used the ZAF correction procedures. Chemical formulas included Fe³⁺ were calculated using the method of Droop (1987). Compositional data of representative minerals in the granulites are given in Table 3, and additional EMPA data of minerals in the granulites are available in Tables S1-1–S1-5 as supplementary information.

Table  3.  Compositions (wt.%) of selected minerals in granulites from the northern Wulan terrane of the Quanji Massif

Sample	11WL-20 (pelitic granulite)										11WL-1 (felsic granulite)
Metamorphic stage	Early prograde metamorphism m1			Peak metamorphism m2			Late metamorphism m3				Peak metamorphism n2			Late metamorphism n3
Mineral	GrtI-c	Bt0	Pl0	GrtI-m	BtI	PlI	GrtII	BtII	PlII		GrtI	OpxI	PlI	GrtII	OpxII	PlII
SiO2	37.0	36.8	61.4	38.8	35.8	60.7	37.9	35.9	61.6		39.4	50.8	47.5	38.9	50.6	47.3
TiO2	0.00	1.82	0.03	0.00	4.63	0.00	0.00	3.77	0.01		0.00	0.03	0.00	0.00	0.05	0.00
Al2O3	20.4	18.8	23.9	20.8	16.9	23.6	19.4	17.3	24.1		21.5	3.61	31.9	21.2	3.13	31.9
Cr2O3	1.35	0.04	0.03	0.18	0	0.47	1.35	0.06	0.04		0.01	0.13	0.02	0.07	0.14	0.00
FeO	33.1	16.8	0.02	31.9	18	0.01	31.8	17.1	0.06		27.2	23.8	0.13	28.4	24.1	0.14
MnO	1.82	0.01	0.01	1.27	0.06	0.00	1.24	0.00	0.03		1.60	0.46	0.02	1.67	0.39	0.00
MgO	3.41	10.6	0.00	5.28	8.97	0.02	4.59	10.2	0.00		7.53	19.2	0.00	6.08	19.7	0.02
CaO	0.92	0.00	5.73	1.09	0.00	5.99	1.08	0.00	5.84		2.32	0.20	15.6	2.61	0.27	16.4
Na2O	0.13	0.32	8.04	0.33	0.13	7.48	0.08	0.21	7.87		0.00	0.01	2.05	0.03	0.01	1.98
K2O	0.03	9.06	0.21	0.06	9.08	0.06	0.06	9.11	0.14		0.01	0.00	0.01	0.01	0.00	0.02
Total	98.2	94.2	99.3	99.8	93.7	98.3	97.5	93.6	99.6		99.6	98.3	97.2	99.1	98.3	97.7
Structural formula
Si	3.03	2.88	2.75	3.10	2.88	2.76	3.10	2.85	2.76		3.07	1.95	2.24	3.08	1.94	2.22
Ti	0.00	0.11	0.00	0.00	0.28	0.00	0.00	0.23	0.00		0.00	0.00	0.00	0.00	0.00	0.00
Al	1.97	1.73	1.26	1.87	1.61	1.26	1.87	1.62	1.27		1.98	0.16	1.77	1.98	0.14	1.77
Cr	0.09	0.00	0.00	0.09	0.00	0.02	0.09	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00
Fe3+	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	2.27	1.10	0.00	2.18	1.21	0.00	2.18	1.13	0.00		1.78	0.76	0.01	1.88	0.77	0.01
Mn	0.13	0.00	0.00	0.09	0.00	0.00	0.09	0.00	0.00		0.11	0.02	0.00	0.11	0.01	0.00
Mg	0.42	1.24	0.00	0.56	1.07	0.00	0.56	1.21	0.00		0.88	1.10	0.00	0.72	1.12	0.00
Ca	0.08	0.00	0.28	0.10	0.00	0.29	0.10	0.00	0.28		0.19	0.01	0.79	0.22	0.01	0.82
Na	0.02	0.05	0.70	0.01	0.02	0.66	0.01	0.03	0.68		0.00	0.00	0.19	0.01	0.00	0.18
K	0.00	0.90	0.01	0.01	0.93	0.00	0.01	0.92	0.01		0.00	0.00	0.00	0.00	0.00	0.00
Total	8.00	8.00	5.00	8.00	8.00	5.00	8.00	8.00	5.00		8.00	4.00	5.00	8.00	4.00	5.00
Structural formulas of biotite, garnet, orthopyroxene and plagioclase are based on 11, 12, 6 and 8 oxygen atoms, respectively. Mineral symbols to denote different mineral assemblages are described in the text.

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3. RESULTS

3.1 Whole-Rock Analytical Results

The two granitic samples (11WL-18 and 11WL-19) show variations in K₂O (3.46 wt.%–5.45 wt.%), Na₂O (2.27 wt.%–2.90 wt.%), and TiO₂ (0.05 wt.%–1.38 wt.%). They are high-K calc-alkaline rocks (Le Maitre et al., 1989), featured by high values of 1.6 to 2.3 (Rittmann, 1962) and the A/CNK values of 0.81 to 1.09 (Table 1). The variation in total rare earth element (∑REE) is large (105 ppm–317 ppm) (Table 1). The chondrite-normalized REE patterns show significant negative Eu anomalies with Eu_N/Eu_N^* from 0.46 to 0.87 (Table 1, Fig. 5a), and are characterized by strong fractionation between LREE and HREE ((La/Yb)_N=14.3–28.8). Strong enrichments in large ion lithophile elements (LILE) over high field strength elements (HFSE) are also evident (Fig. 5b). These geochemical data suggest that monzogranite and granitic leucosome probably formed from melting of crustal sources as well. These rocks are similar to those from Andean-type arcs or continental margin arcs (Stern, 2002; MacDonald et al., 2000).

Figure  5.  Whole-rock geochemical data for six representative samples from the northern Wulan terrane. (a) Chondrite-normalized REE patterns and (b) MORBnormalized trace element spider diagrams. Chondrite and MORB normalization values are from Sun and McDonough (1989) and Pearce (1983), respectively.

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The felsic granulites (11WL-1 and 11WL-2) are characterized by high abundances of Si, Fe, Mg and K but low Al and Na contents. The pelitic granulite (11WL-20) has high SiO₂ (63.3 wt.%), relatively high K₂O (3.64 wt.%) and Al₂O₃ (17.3 wt.%) but low Na₂O (1.1 wt.%), and is similar in composition to khondalites occurred widely in the Quanji Massif (Wan et al., 2006). All three granulite samples have relatively high K₂O (2.91 wt.%–3.64 wt.%) (Table 1), the elemental contents and ratios (e.g., Na₂O/K₂O < 1, MgO/Fe < 0.7, Al₂O₃/TiO₂ < 30) combined with the discrimination of MgO/CaO vs. P₂O₅/TiO₂ are all indicative of para-metamorphic origins. Moreover, these granulites have high LILE concentrations (Table 1, Fig. 5b), high Cr/Ni values, but low ∑REE (112 ppm to 254 ppm) and (La/Yb)_N ratios (4.8 to 7.6), and negative Eu anomalies (Eu_N/Eu_N^*=0.53–0.90) (Table 1, Fig. 5a). Their REE contents and ratios are comparable with those of post-Archean sedimentary rocks around the world (McLennan, 1989).

The gabbro (11WL-23) has low SiO₂ (48.9 wt.%), high CaO (7.19 wt.%) and high loss on ignition (LOI: 3.15 wt.%) values. This rock is also enriched in LREE (Fig. 5a) and most of the LILE, such as Rb, Sr and Ba, but anomalously depleted in Nb-Ta-Ti (Fig. 5b), indicative of island arc magmatism (e.g., Kelemen et al., 1993).

3.2 Zircon U-Pb Geochronological Data

3.2.1   Monzogranite (11WL-18)

Zircon grains from sample 11WL-18 are mostly euhedral prismatic in shape, with a combination of the {100}+{110} prisms and the {111} dipyramid. Grain sizes vary from 40–200 µm in length, with the length-to-width ratios mainly from 1.5 : 1 to 5 : 1. These zircon grains are highly luminescent in CL and have well-developed oscillatory zoning (Fig. 6a), typically crystallized from magmas (Corfu et al., 2003; Belousova et al., 2002; Rubatto, 2002). All 30 analyses from 27 zircon grains are characterized by high Th/U values from 0.40 to 1.31 (Table S2). Excluding two analyses from the rims, all remaining concordant analyses give a weighted mean ²⁰⁶Pb/²³⁸U age of 476.7±2.8 Ma (MSWD=0.42, N=28) (Fig. 7a). The two analyses on the rims (spots 12 and 13) give a weighted mean ²⁰⁶Pb/²³⁸U age of 443±9 Ma (Fig. 7a). On the basis of the morphological and luminescent features and high Th/U values, the ~477 Ma is interpreted to represent the crystallization age of the monzogranite (Table 2). The ~443 age obtained from zircon rims probably records a late metamorphic/deformation event (Table 2).

Figure  6.  Zircon CL images from the six samples investigated in this study showing the zircon morphology, analytical spots and corresponding U-Pb ages.

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Figure  7.  Zircon U-Pb concordia diagrams showing the ages from the six samples investigated in this study. (a) Monzogranite (11WL-18); (b) gabbro (11WL-23); (c) felsic granulite (11WL-1); (d) felsic granulite (11WL-2); (e) pelitic granulite (11WL-20); and (f) granitic leucosome (11WL-19).

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3.2.2   Gabbro (11WL-23)

Zircon grains from the gabbro dike are mostly subhedral to anhedral prismatic in shape. They are 80–200 µm long, with length-width ratios from 2 : 1 to 3 : 1. Most crystals display dark gray luminescence with distinct oscillatory zoning (Fig. 6b), indicative of magmatic origin (Corfu et al., 2003; Belousova et al., 2002; Rubatto, 2002). Thirty spot analyses on 30 zircon grains yielded highly concordant data with a weighted mean ²⁰⁶Pb/²³⁸U age of 423±2 Ma (MSWD=0.03, N=30) (Fig. 7b). These zircon grains have Th/U values from 0.47 to 1.51 (Table S2), further supporting a magmatic origin (Belousova et al., 2002).

3.2.3   Felsic granulite (11WL-1)

Zircon grains from sample 11WL-1 are mostly subhedral to anhedral and exhibit apparent core-rim textures (Fig. 6c). The crystal sizes range from 30 to 120 µm in length, and the length-to-width ratios are mainly 1.5 : 1~2.5 : 1. These zircon grains can be divided into two types. The first type is characterized by a core of bright CL (e.g., spots 9, 16, 17 and 20 in Fig. 6c) and a narrow rim of dark luminescence. The second type has small cores of high CL and wide rims of dark luminescence (e.g., spots 2–3, 4–5, 27–28, and 29–30 in Fig. 6c). Thirty analyses on 25 zircon grains from sample 11WL-23 are listed in Table S2. Three analyses from the bright CL cores plot on a concordant line (i.e., spots 2, 20, and 21), yielding a weighted mean age of 1 752±50 Ma (Fig. 7c). However, twenty analyses from the bright cores are discordant and plot roughly along three discordant lines (Fig. 7c) with the upper intercept ages of 2 431±51 (MSWD=0.88, N=5), 2 047±85 (MSWD=0.84, N=8) and 1 130± 46 Ma (MSWD=2.1, N=7). One analysis from a bright core (i.e., spot 14) exhibits apparent Pb loss. The remaining six analyses from the dark wide rims of the second type are all concordant and give a weighted mean ²⁰⁶Pb/²³⁸U age of 472±4.8 Ma (MSWD=0.062, N=6). The dark rims show weak or no zoning (Fig. 6c) and have low Th/U values from 0.01 to 0.04 (Table S2), indicative of metamorphic overgrowth. Therefore, the age of ~472 Ma is interpreted to represent the timing of the granulitefacies metamorphism (Table 2).

3.2.4   Felsic granulite (11WL-2)

Zircon grains from sample 11WL-2 have morphological features similar to their counterparts from sample 11WL-1. They are 40–150 µm in length, with the length-to-width ratios from 1 : 1 to 2.5 : 1. The zircon grains have bright CL cores with occasionally oscillatory zoning and narrow rims of dark CL (Fig. 6d). Thirty analyses on the cores of 27 zircon grains are given in Table S2. The data define two discordant lines, with upper intercept ages of 1 909±30 (MSWD=0.45, N=17) and 1 721±73 Ma (MSWD=0.23, N=12) (Fig. 7d), excluding one point (04) for apparent Pb loss. These two ages from zircon cores are interpreted to represent the crystallization ages of detrital zircon (Table 2). This interpretation is supported by the high Th/U values (Table S2), which are indicative of magmatic origin. Unfortunately, the dark rims of zircon are too narrow for LA-ICP-MS analysis. It is interesting to note that the detrital grains of zircon with an upper intercept age of ~1 909 Ma also define a lower intercept age of 474±32 Ma (Fig. 7d), which is identical to the age of the granulite-facies metamorphism obtained from sample 11WL-1 (Fig. 7c, Table 2).

3.2.5   Pelitic granulite (11WL-20)

Zircon grains from sample 11WL-20 can be divided into three types on the basis of morphology. Type 1 is characterized by subto euhedral prismatic shapes and is 50–120 µm in length, with the length-to-width ratios from 1.5 : 1 to 3 : 1. The CL images reveal a core-mantle-rim texture, with highly luminescent cores of probably detrital origin (i.e., spot 28 in Fig. 6e), dark mantles and grey rims. Type 2 is mostly subhedral prismatic in shape and is 40–80 µm in length, with the length-to-width ratios from 1.5 : 1 to 2 : 1. Grains of Type 2 are characterized by dark CL cores and grey rims (i.e., spots 6 and 20 in Fig. 6e), which are interpreted to represent inherited remnants and metamorphic overgrowth, respectively. Type 3 without a core-rim texture is generally dark in CL and often exhibits a weak oscillatory zonation (i.e., spots 12 and 15 in Fig. 6e). A few grains of Type 3 with dark CL do not have any apparent oscillatory zonation.

Of the forty analyses on 40 zircon grains (Table S2), six analyses on the cores of types 1 and 2 define one discordant line with an upper intercept age of 2 698±87 Ma (MSWD=1.3, N=6) (Fig. 7e), whereas another spot on the concordant line gives an age of 2 309±87 Ma (Table 2). The remaining 33 analyses roughly along the discordant line yield ²⁰⁶Pb/²³⁸U ages between ~402 and 499 Ma, with the exception of two analyses (i.e., spots 9 at 513 Ma and 38 at 574 Ma). These discordant ²⁰⁶Pb/²³⁸U ages from zircon with low Th/U values (0.01–0.09) were interpreted to represent metamorphic ages and can be further divided into three populations with the mean ages at 496±6 Ma (MSWD=1.9, N=10) with Th/U values (0.01–0.12), 474±3.7 Ma (MSWD=0.04, N=10) with Th/U values (0.01–0.19), and 446±3.5 Ma (MSWD=0.64, N=10) with Th/U values (0.01–0.06), excluding spot 11 (Tables S2 and 2). Interestingly, the second population at ~474 Ma is comparable to those (~472 and ~474 Ma) obtained from metamorphic zircon in the felsic granulites (Table 2).

3.2.6   Granitic leucosome (11WL-19)

Zircon grains from sample 11WL-19 are mostly subhedral prismatic in shape. The crystal sizes range from 50 to 200 µm in length, and the length-to-width ratios are mainly 1 : 1~3 : 1. They are characterized by low luminescence and no apparent oscillatory zonation (Figs. 6f, spots 10 and 16). However, a few grains have a distinct core-rim texture with bright CL cores and dark rims (Fig. 6f, spots 12–13). Thirty analyses on 27 zircon grains (Table S2) define a concordant line with a weighted mean ²⁰⁶Pb/²³⁸U age of 474.8±2.3 Ma (MSWD=0.04, N=28) (Fig. 7f), excluding one spot 13 for its apparent Pb loss and another spot 16 that has a concordant age of 443±6 Ma. This ~475 Ma age from zircon grains with Th/U values from 0.02 to 0.24 (Table S2) is interpreted to record anatexis, whereas the ~443 Ma age probably represents a late metamorphic/deformation event (Table 2).

3.3 Mineral Compositions, Metamorphic P-T Conditions and P-T Paths

3.3.1   Mineral compositions

3.3.1.1   Garnet

Garnets in the pelitic granulite (11WL-20) are richer in the almandine component but poorer in the pyrope component than those in the felsic granulites (11WL-1 and 11WL-2) (Fig. 8a; Table 3 and Tables S1-1–S1-5). The texturally different generations of garnet in all three granulite samples are also distinct in chemical composition (Fig. 8b; Table 3). For example, GrtI and GrtII in sample 11WL-1 have compositions of Alm_0.61Prp_0.29 Grs_0.07Sps_0.04 and Alm_0.64Prp_0.26Grs_0.07Sps_0.04, respectively, whereas GrtI and GrtII in sample 11WL-2 are Alm_0.54–0.58 Prp_0.29–0.33Grs_0.07–0.13Sps_0.04 and Alm_0.58–0.59Prp_0.29–0.30Grs_0.07 Sps_0.04–0.05, respectively (Table S1-1). In sample 11WL-20, GrtI-c has a composition of Alm_0.78Prp_0.14Sps_0.04Grs_0.03, distinct from GrtI-r at Alm_0.73Prp_0.21Sps_0.03Grs_0.03 (Fig. 8b). Fine-grained GrtII in sample 11WL-20 is homogeneous in individual grains but exhibits small compositional variations from grain to grain, with an average composition of Alm_0.75Prp_0.19Sps_0.03Grs_0.03 (Table S1-1).

Figure  8.  (Alm+Sps)-Grs-Prp triangular plots showing (a) compositional differences of garnet between the felsic granulites (11WL-1 and 11WL-2) and the pelitic granulite (11WL-20), and (b) compositional differences among texturally distinct varieties of garnet in the pelitic granulite (11WL-20).

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3.3.1.2   Orthopyroxene

Orthopyroxene in the felsic granulites (11WL-1 and 11WL-2) is compositionally homogeneous in individual grains and is dominated by the enstatite-ferrosilite series, in the range of En_0.59–0.64Fs_0.35–0.41 (Table S1-2). Orthopyroxene in sample 11WL-1 at En_0.59–0.60Fs_0.39–0.41 is slightly richer in Fe than that in sample 11WL-2 at En_0.59–0.64Fs_0.35–0.41 (Table S1-2). Minor compositional differences between OpxI and OpxII in sample 11WL-2 are also notable (Table 3).

3.3.1.3   Plagioclase

Two or three generations of plagioclase can be distinguished on the basis of their occurrences in the cores or rims of garnet porphyroblasts and in the matrix (Figs. 4c, 4d–4e). The compositions of plagioclase differ significantly between the felsic granulites (An_0.81–0.85Ab_0.13–0.19) and the pelitic granulite (An_0.28–0.31Ab_0.69–0.71) (Tables 3 and S1-3).

3.3.1.4   Biotite

The three generations of biotite in the pelitic granulite differ significantly in the TiO₂ and X_Mg contents (Tables 3 and S1-4). Also, the X_Mg values of 0.61–0.71 for biotite in the felsic granulites are notably higher than those (0.43–0.57) of its counterpart in the pelitic granulite (Table S1-4).

3.3.1.5   Other minerals

Representative compositions of K-felspar, muscovite, cordierite, and sillimanite in the granulites are given in Table S1-5.

3.3.2   Metamorphic P-T conditions and P-T paths

The P-T conditions for the three stages of regional metamorphism have been estimated using conventional geothermobarometry. Specifically, the GB-GASP geothermobarometry (Wu et al., 2006; Holdaway, 2001, 2000) and the GB-GBPQ geothermobarometry (Wu C M et al., 2004; Holdaway, 2001, 2000) have been used for the mineral assemblages in the pelitic granulite. The Grt-Opx (or Grt-Cpx) geothermometer (Taylor, 1998; Brey and Köhler, 1990) and the Grt-Opx-Pl-Qtz geobarometer (Nimis and Taylor, 2000; Perkins and Chipera, 1985; Harley, 1984) were used for mineral assemblages in the felsic granulites. These calculations show that the granulite-facies metamorphism recorded peak P-T conditions of 729 C and 0.46 GPa from the pelitic granulite, and 718 C and 0.53 GPa from the felsic granulites (i.e., points m2 and n2 in Fig. 9; Table 4). The early stage of the prograde metamorphism constrained from the pelitic granulite occurred at 598 C and 0.28 GPa (Table 4).

Table  4.  Metamorphic mineral assemblages and P-T conditions in granulites from the northern Wulan terrane

Sample (rock type)	Mineral assemblages	Metamorphic reactions	P-T conditions
11WL-20 (pelitic granulite)	m1: GrtI-c+Mus0+Bt0+Pl0±Chl±Qtz	Bt+Pl=GrtI-c+H2O or Chl+Qtz=GrtI-c+H2O	598 ℃, 0.28 GPa
	m2: GrtI-r+SilI+KfsI+CordI+BtI±PlI±Qtz	Ms+Qtz=Sil+Kfs +H2O Sil+Bt+Qtz=Grt+Cord+Kfs+H2O	729 ℃, 0.46 GPa Isobaric heating
	m3: GrtII+SilII±PlII±BtII+Qtz	Kfs+Cord=Sil+Bt	669 ℃, 0.43 GPa Isobaric cooling
11WL-1 (felsic granulite)	n1: GrtI-c±Pl0+Bt0+Qtz	Bt+Pl=GrtI-c
	n2: Opx+Kfs+PlI±GrtI-r±BtI±Qtz	Bt+Qtz=Opx+Kfs+H2O Grt+Qtz±Cpx=Opx+Pl	718 ℃, 0.53 GPa Isobaric heating
	n3: GrtII+Qtz+Cpx+PlII ±BtII	Opx+Pl(An)=Grt +Qtz	583 ℃, 0.44 GPa Isobaric cooling
11WL-2 (felsic granulite)	c1: GrtI-c±Pl0+Bt0+Qtz (?)
	c2: OpxI+GrtI+PlI±Qtz	Grt +Qtz=Opx+Pl	Isobaric heating
	c3: OpxII+GrtII+PlII+Cpx±Qtz	Opx+Pl=Grt+Cpx+Qtz	Isobaric cooling
	Retrograde: Act+Chl +Ep		Decompressional cooling

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The mineral assemblages m3, n3 and c3 (Figs. 4e, 4c–4d) suggest a late episode of near-isobaric cooling after the peak granulite-facies metamorphism. Mineral assemblages of m3 and n3 yielded the post-peak conditions of 583–669 C and 0.43–0.44 GPa (Table 4; Fig. 9). For example, the replacements of biotite and sillimanite (SilI) by garnet (GrtII) and cordierite, and then Kfelspar and cordierite by sillimanite (SilII) and biotite (BtII), (Sil+ Bt+Qtz=Grt+Cord+Kfs+H₂O and Kfs+Cord=Sil+Bt) are observed in the pelitic granulite (Fig. 4e). These characteristic reactions are indicative of nearly isobaric heating followed by isobaric cooling, which is commonly attributable to uplifting or collapse after crustal thickening and excess heating. The anticlockwise P-T path (Fig. 9) in the pelitic granulite is characterized by the metamorphic reactions of Bt+Pl+Qtz±Chl→GrtI-c (m1) and Mus+Qtz→SilI+Kfs (m2), via Sil+Bt→Cord+Kfs and Kfs+Cord→SilII+Phl, to GrtII+ SilII+Phl+PlII±BtII±Qtz (m3) (Table 4).

Figure  9.  Metamorphic evolution in the pelitic granulite (from m1 to m2 and m3) and the felsic granulites (from n2 to n3), showing the anticlockwise P-T paths (after Winter, 2014).

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Similarly, the mineral assemblages in the felsic granulites suggest the following reactions: (1) Bt₀+Pl₀=GrtI-c, (2) BtI+Qtz= OpxI+Kfs, and (3) GrtI-r+Qtz±Cpx=OpxI+PlI. Reaction (2) is characteristic of granulite-facies metamorphism, and reaction (3) is variable depending on the occurrence of orthopyroxene or garnet (Figs. 4c–4d). The anticlockwise P-T path in the felsic granulite is constrained by the metamorphic reactions of Bt+Qtz→Opx+Kfs (n2) and Opx+Pl→Grt+Qtz±Cpx (n3) (Fig. 9). The replacement of orthopyroxene and garnet by retrograde minerals such as actinolite, chlorite and epidote further supports an anticlockwise P-T path involving decompressional cooling (Table 4).

4. DISCUSSION

4.1 Timing of Early Paleozoic Magmatism and Regional Metamorphism

The whole-rock Sm-Nd isochron age of ~3 450 Ma for the granulites reported by Wang et al. (2000) is difficult to explain, because the oldest U-Pb age of detrital zircon in the same rocks is only ~2.70 Ga (Table 2). The five populations of U-Pb ages (~2.70, 2.43–2.31, 2.05–1.91, ~1.73, and ~1.13 Ga; Table 2) from detrital zircon suggest that the protoliths of the granulites have complex Precambrian sources. However, the detrital zircon age of ~1.13 Ga is similar to that of magmatic zircon from granitoids (Wang, 2016) and constrains the maximum deposition age for the protoliths of the granulites (Wang et al., 2016).

Our U-Pb dating of magmatic, metamorphic and inherited or detrital zircon from monzogranite, gabbro, granulites and granitic leucosome (Tables S2 and 2) yielded at least two pulses of magmatism at ∼477 and ∼423 Ma, as well as three episodes of regional metamorphism at ∼496, ~474, and ∼443 Ma. The crystallization age of 476.7±2.8 Ma from magmatic zircon in the monzogranite constrains the peak timing of island arc magmatism. The younger age at 443±9 Ma from zircon in the monzogranite is attributable to late metamorphic/deformation overprint, although a ∼443 Ma magmatism has also been reported to occur in the Yutuan Mountain (Wu et al., 2008). The zircon U-Pb age of 423 Ma from the gabbroic dike recorded an episode of mafic magmatism.

Zircon grains of metamorphic origin from the granulites yielded U-Pb ages of 496±6, 474±3.7, 472±4.8, and 446±3.5 Ma (Table 2). The ~496 Ma age from the pelitic granulite is interpreted to represent the earliest episode of Paleozoic metamorphism in the northern Wulan terrane. The ages of 474±3.7 and 472±4.8 Ma, together with the lower intercept age of 474±32 Ma from detrital zircon in the felsic granulite, all constrain the peak granulite-facies metamorphism to have occurred at ~475 Ma. These U-Pb ages for the peak granulite-facies metamorphism are also within analytical uncertainties to that (474.8±2.3 Ma) from the granitic leucosome, confirming that anataxis accompanied the peak metamorphism. Interestingly, the younger age of 443±6 Ma from a zircon rim with dark CL in the granitic leucosome is similar to those in the monzogranite (443±9 Ma) and the pelitic granulite (446±4 Ma) (Table 2), apparently representing a late episode of metamorphism/deformation in the northern Wulan terrane.

4.2 Early Paleozoic Paired Metamorphic Belts in the NQTB

Previous studies recognized two stages of subductioncollision in the North Qaidam HP-UHP metamorphic belt, with an earlier oceanic subduction at ~470 Ma and a late continental collision at ~440 Ma (Zhang et al., 2017; Song et al., 2014, 2013, 2006; Xiong et al., 2012, 2011). For example, Song et al. (2018) documented early HP lawsonite-bearing eclogites formed from "cold" subduction at 470–445 Ma and late "hot" and "dry" UHP kyanite eclogites related to continental subduction/collision at 438–420 Ma. Several studies showed that the exhumation and retrograde metamorphism in the North Qaidam HP-UHP metamorphic belt involved isothermal decompression along a clockwise P-T path (Song et al., 2018, 2014; Zhang et al., 2017). In contrast, the low-P/T metamorphism in the northern Wulan terrane is characterized by an anticlockwise P-T path involving nearly isobaric cooling during retrograde metamorphism.

Cold subduction of an oceanic lithosphere beneath a continental plate usually causes arc magmatism and low P/T metamorphism on the continental arc margin or island arc region, such as in the Andean Range and Japanese Islands. One characteristic feature at such convergent margins is the occurrences of paired metamorphic belts with a high P/T metamorphic belt on the ocean side and a low to medium P/T metamorphic belt on the continental side (Brown, 2010; Miyashiro, 1973). The present study shows that the peak granulite-facies metamorphism and associated anatexis at ~475 Ma were contemporaneous with the island arc magmatism in the northern Wulan terrane. Moreover, the low P/T granulite-facies metamorphism, anataxis and island arc magmatism in the northern Wulan terrane are broadly coeval with the early HP metamorphism in the North Qaidam HP-UHP metamorphic belt, providing compelling evidence for Early Paleozoic paired metamorphic belts (Fig. 10). In particular, the anticlockwise P-T path observed in the northern Wulan terrane is characteristic of low P/T metamorphism associated with subduction-induced arc magmatism in continental arcs such as the Quanji Massif. This clearly suggests that the northern Wulan terrane of the Quanji Massif was on the side of the overriding plate and participated in the subsequent continentcontinent collision process in the Early Paleozoic.

Figure  10.  Schematic N-S cross-sections of the North Qaidam tectonic belt illustrating the closure of the South Qilian Ocean (i.e., the Proto-Tethys Ocean) and Early Paleozoic tectonic evolution.

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4.3 Implications for the Closure of the Proto-Tethys Ocean

Early Paleozoic has long been proposed to be a key time in the final amalgamation of various components to form the Gondwana supercontinent and the initiation of subduction for the closure of the Proto-Tethys Ocean along the peri-Gondwana margin (Li et al., 2017; Cawood and Buchan, 2007; Meert, 2003). Several studies have suggested an Early Paleozoic Andean-type orogeny along the peri-Gondwana Proto-Tethys margin (Li et al., 2017; Zhang J X et al., 2014; Cawood et al., 2007).

Our new data from granulites and granitoids in the northern Wulan terrane, including recognition of Early Paleozoic paired metamorphic belts in the North Qaidam tectonic belt, provide further support for the Andean-type orogeny between ~540 and 440 Ma (see also Li et al., 2017), responsible for the closure of the South Qilian Ocean, which was part of the larger ProtoTethys Ocean (Fig. 10). We envision that the 540–500 Ma Tanjianshan volcanic zone represented an early volcanic island arc formed from the northward subduction of the Proto-Tethys oceanic plate. Continuing subduction and HP metamorphism in the lower plate between 500 and 440 Ma (Song et al., 2018) gave rise to further island-arc magmatism in the Quanji Massif as a continental arc, which in turn was responsible for low P/T granulite-facies metamorphism and associated anataxis in the upper plate (Fig. 10). Therefore, the Proto-Tethys Ocean in northwestern China was most likely completely closed by ~440 Ma (Fig. 10), when the continent-continent collision became dominant in the North Qaidam tectonic belt. The closure of the Proto-Tethys Ocean at this time is further supported by the intrusion of the post-collisional gabbroic dike at ~423 Ma.

5. CONCLUSIONS

The low P/T pelitic and felsic granulites in the northern Wulan terrane are characterized by an anticlockwise P-T path and have metamorphic ages of 500–440 Ma. A granitic pluton of dominantly monzogranite intruded at 476.7±2.8 Ma and is characteristic of island arc magmatism with high potassium calc-alkaline metaluminous compositions. Magmatic zircon from a granitic leucosome yielded a crystallization age of 474.8±2.3 Ma. The contemporaneous low P/T granulite-facies metamorphism, anatexis and island-arc magmatism in the northern Wulan terrane of the Quanji Massif were coeval with the early lawsonite-bearing eclogites in the North Qaidam HP-UHP metamorphic belt to the south, providing evidence for Early Paleozoic paired metamorphic belts with a northward subduction of the Proto-Tethys oceanic plate and the closure of the Proto-Tethys Ocean in northwestern China.

ACKNOWLEDGMENTS

This paper is dedicated to the celebration of Prof. Zhendong Youʼs 90th birthday. In particular, Qinyan Wang and Yuanming Pan both benefited directly from Prof. Youʼs metamorphic petrology courses. We also thank three journal reviewers as well as Profs. Neng-Song Chen, Qunke Xia, Yixian Xu and Xianhua Li for constructive reviews and helpful suggestions. This research supported by the National Natural Science Foundation of China (Nos. 41072044, 41130315 and 41530319). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0881-6.

Electronic Supplementary Materials: Supplementary materials (Tables S1 and S2) are available in the online version of this article at https://doi.org/10.1007/s12583-018-0881-6.