High-grade metamorphism during Archean–Paleoproterozoic ... · paths. However, this arbitrary model waschallenged by detailedpetro-logic and phase equilibria studies on the Paleoproterozoic

Lithos 263 (2016) 101–121

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High-grade metamorphism during Archean–Paleoproterozoic transitionassociated with microblock amalgamation in the North China Craton:Mineral phase equilibria and zircon geochronology

Qiong-Yan Yang a,b, M. Santosh a,b,c,⁎, Toshiaki Tsunogae d,e

a School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, Chinab Department of Earth Sciences, University of Adelaide, SA 5005, Australiac Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japand Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japane Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa

⁎ Corresponding author at: School of Earth Sciences andGeosciences Beijing, 29 Xueyuan Road, Beijing 100083, Ch

E-mail address: [email protected] (M. Santosh)

http://dx.doi.org/10.1016/j.lithos.2015.11.0180024-4937/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 September 2015Accepted 17 November 2015Available online 30 November 2015

Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth.The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recentmodels propose that thecratonic architecture was built through the assembly of several Archean microcontinental blocks into largercrustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery ofthe Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblageof garnet+ clinopyroxene+ orthopyroxene+magnetite + plagioclase+ quartz ± biotite ± ilmenite. Mineralphase equilibria computations using pseudosection and geothermobarometry suggest peak P–T condition of800–830 °C and 7–8 kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in anothersample containing the peakmineral assemblage of garnet + orthopyroxene+ quartz +magnetite ± fluid yieldpeak P–T conditions of 860–920 °C and 11–14 kbar. Geochemical data show tonalitic to granodioritic compositionand arc-related tectonic setting for themagmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562 ± 20 Ma (MSWD = 0.94) and 2539 ± 21 Ma (MSWD =0.59) which is correlated with the timing of emplacement of themagmatic protolith. A younger group of zirconswith upper intercept ages of 2449±41Ma (MSWD=0.83); N=6 as 2449±41Ma (MSWD=0.83; N=6) and2480 ± 44 Ma (MSWD = 1.2; N = 9) constrains the timing of metamorphism. Zircon Lu–Hf data show domi-nantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC ) in the range of 2529 to2884 Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature andmedium to high pressure metamorphism is considered to have resulted from the subduction–collision tectonicsassociated withmicroblock amalgamation in the NCC at the end of Archean. Together with the evidence for highpressure metamorphism from an adjacent locality, our results correlate with models that predict paired meta-morphism at the Archean–Proterozoic transition with the onset of modern style plate tectonics.

© 2015 Elsevier B.V. All rights reserved.

Keywords:Petrology and phase equilibriaGeochemistryZircon U–Pb geochronologyTectonicsNorth China Craton

1. Introduction

Metamorphic rocks have provided important keys to understandsecular changes in our planet including thermal gradients and rheology,with the general consensus of a hotter Earth during Archean andPaleoproterozoic (e.g., Brown, 2007, 2014). Petrological studies andthermo-mechanical numerical modeling have also highlighted thecontrast in the style of collisional orogens between the Phanerozoicand the Precambrian (e.g., Sizova et al., 2014), with a transition in the

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geodynamic regime during Neoproterozoic. However, examples forcoolermetamorphic conditions in the early Earth have also been report-ed in recent studies (e.g., Anderson et al., 2012;Ganne et al., 2012;Mintset al., 2010), attesting to the changing thermal structure of the Earthinto more modern style. It has also been demonstrated that during theNeoarchean, the integrated strength of the lithosphere increased dra-matically enabling greater crustal thickening with elevated topography(Rey and Coltice, 2008). Anderson et al. (2012) reported high pressuremetamorphism corresponding to continental crustal thickness of≥45–50 km near the Archean–Proterozoic boundary, suggesting thatmodern style tectonics might have operated early in the Earth history.

The North China Craton preserves a prolonged history of geologicaland tectonic events from early Archean to late Paleoproterozoic prior

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102 Q.-Y. Yang et al. / Lithos 263 (2016) 101–121

to the final amalgamation of the continental blocks and constructionof the stable cratonic architecture (Wilde et al., 2002; Yang andSantosh, 2015; Zhai and Santosh, 2011; Zhao and Cawood, 2012; Zhaoand Zhai, 2013; Zhao et al., 2005, among others). Previous studiesattributed the Archean metamorphism to plume tectonics and thePaleoproterozoic events to plate tectonics, following the global conceptsof secular change (Zhao and Zhai, 2013 and references therein). Meta-morphic P–T paths from preliminary and imprecise estimates weresought in support of this argument with the Archean metamorphicrocks proposed to be characterized by anti-clockwise paths whereasthe Paleoproterozoic counterparts are considered to have clockwisepaths. However, this arbitrary model was challenged by detailed petro-logic and phase equilibria studies on the Paleoproterozoic ultrahigh-temperature (UHT) granulites from the ‘Khondalite Belt’ of the NorthChina Craton, developed within a subduction–collision regime whichshows clearly defined anti-clockwise P–T paths (Santosh et al., 2012and references therein). There are various global examples of diverseP–T paths in both Archean and Proterozoic terranes. As summarized inthe petrological and numeral modeling studies of Sizova et al. (2014),some Proterozoic orogens with post-extension thickening wouldgenerate counter-clockwise metamorphic P–T paths followed bynear-isobaric retrograde cooling whereas others are characterized byclockwise P–T paths.

In this paper we present petrology and mineral phase equilibriacomputations, whole rock chemistry and zircon U–Pb and Lu–Hfisotopes from garnet-bearing charnockitic rocks from the margin of anArchean microblock in the North China Craton. We evaluate our resultsin understanding the nature of protoliths and the tectonic setting oftheir formation, the P–T conditions of metamorphism, and the timingsof emplacement and metamorphism with a view to understand thetectonic processes and thermal regimes in a convergent margin duringthe Archean–Proterozoic transition.

Fig. 1. Tectonic framework of the North China Craton showing themajor crustal blocks and inteby box.After Zhao et al., 2005; Santosh, 2010.

2. Geological background

2.1. Tectonic framework of the North China Craton

The popular models of tectonic sub-division of the North ChinaCraton (NCC, Fig. 1), identify two major crustal blocks, the WesternBlock and the Eastern Block with three major Paleoproterozoic suturezones welding these blocks and their sub-blocks termed as the InnerMongolia Suture Zone (IMSZ; also known as the Khondalite Belt),Trans-North China Orogen (TNCO) and the Jiao-Liao-Ji Belt(e.g., Santosh, 2010; Yang and Santosh, 2015; Yang et al., 2014; Zhaoand Cawood, 2012; Zhao and Zhai, 2013; Zhao et al., 2005). However, al-ternate models identify that the NCC is a collage of several ancient cra-tonic nuclei, with some of these microcontinents preserving rocks thatdate back to the Eoarchaean (Yang et al., 2015; Zhai, 2014; Zhai andSantosh, 2011), confirming earlier proposals (Bai et al., 1996; Shenand Qian, 1995; Wu et al., 1998; Zhai and Bian, 2001). A major crustbuilding event in the NCC is considered to have occurred at ca. 2.7 Ga,and the rock suites generated during this period constitute the core ofat least seven micro-blocks such as the Jiaoliao (JL), Qianhuai (QH),Ordos (OR), Jining (JN), Xuchang (XCH), Xuhuai (XH) and Alashan(ALS) Blocks (Zhai and Bian, 2001; Zhai and Santosh, 2011, andreferences therein). These ancient tectonic blocks are bound bygranite–greenstone belts that might mark the sites of closure of theintervening ocean basins (Zhai and Santosh, 2011). These includethe Zunhua greenstone belt located between the JN and QH Blocks,the Wutaishan greenstone belt between the OR and QH Blocks, theYanlingguan greenstone belt between the JL and QH Blocks, theDongwufenzi greenstone belt between the JN and OR Blocks, theXuchang greenstone belt between the XCH and QH Blocks, amongothers (Zhai and Santosh, 2011). The identification of these micro-blocks also derives support from geochemical features (Liu et al.,

rvening suture zones. The location of the presented study area in Qianxi Complex is shown

103Q.-Y. Yang et al. / Lithos 263 (2016) 101–121

1998), as well as geophysical boundaries of the deep crust (Guan et al.,1987).

The microblocks in the NCC also preserve the history of multiplemetamorphic events, as deduced from combined petrological andgeochronological studies by various workers. The JL Block is consideredto have experienced two phases of metamorphism; a granulite to upperamphibolite facies event at ca. 2.52–2.51 Ga and an upper to lower am-phibolite facies event at ca. 1.9–1.82 Ga (Zhai, 2014 and referencestherein). Those in the QH Block experienced three stages of metamor-phism: a granulite facies event at ca. 2.6–2.56 Ga, granulite to upperamphibolite facies event at ca. 2.52–2.51 Ga, and an upper to loweramphibolite facies event at ca. 1.9–1.82 Ga (Zhai and Santosh, 2011,and references therein). The ca. 2.7 Ga and ca. 2.6–2.5 Ga basementcomplexes of the XCH Block carry granulite to upper amphibolite faciesrocks.

2.2. Geology of the study area

Our present study area is located within the Jiaoliao Block,sandwiched between the TNCO to the west and Jiao-Liao-Ji Belt to theeast (Fig. 1). This region is also known as the Longgang Block (Zhaoand Cawood, 2012) or the Yanliao Block (Santosh, 2010). Origin of theNeoarchean rocks in this region has been variably interpreted by differ-ent workers. The short time span between the dominant TTG rocks andmafic volcanics of continental tholeiite affinity in the region, the anti-clockwise P–T paths, the presence of komatiites and bimodal volcanicassemblages, and the diapiric structural features were correlated tomantle plume model (Wilde et al., 2002; Zhai and Santosh, 2011;Zhao and Zhai, 2013). However, the geochemical similarities of theTTG gneisses with calc-alkaline magmas in modern continental marginarcs ledmany others to propose supra-subduction origin for these rocks(Li et al., 2010; Martin, 1999; Martin et al., 2005; Wang et al., 2012,2013; Wu et al., 1998; Zhai, 2014; Zhai and Santosh, 2011; Zhai et al.,2005).

Three distinct complexes have been identified in the region inprevious studies: the Eo- to Paleoarchean Caozhuang Complex, theMesoarchean Shuichang (Qianxi) Complex, and the NeoarcheanZunhua Complex. Among these, the Qianxi Complex (also known asQian'an Complex or traditional Qianxi Group), fromwhere our sampleswere collected, is known to carry vestiges of Eoarchaean rocks (e.g., Liuet al., 1990). Themetamorphic rocks in this region show granulite faciesassemblages (two pyroxene granulite, pyroxene-plagioclase granulite;hypersthene bearing charnockites and migmatites). Previous geochro-nological studies of the hypersthene granulite from the Qianxi complexshow ages of ca. 2.6 Ga–2.5 Ga based on Rb–Sr whole rock, Sm–Ndwhole rock, and U–Pb zircon techniques (Jahn and Zhang, 1984a,b;Jahn et al., 2008; Pidgeon, 1980).

Samples for this study were collected from the Santunying–Taipingzhai area (Fig. 2; Table 1) and belong to the Qianxi complex ineastern Hebei province. The oldest ages from the Qianxi Complex arein the range of 3.65–3.67 Ga (Huang et al., 1986; Jahn et al., 1987) andare considered to mark the beginning of crustal growth in this region.The evolution of Qianxi complex spanned ca. 1 Ga, culminating in 2.7–2.5 Ga tectonothermal events, followed by amphibolite to granulitefacies metamorphism at about 2.5 Ga ago (Compston et al., 1983; Jahnand Zhang, 1984a, 1984b; Pidgeon, 1980). Geochronological investiga-tions of hypersthene granulites from theQianxi region show concordantages of about 2.5 Ga based on Rb–Srwhole rock, Sm–Ndwhole rock, andzircon techniques (Jahn and Zhang, 1984b; Pidgeon, 1980). Some olderages have also been recorded from amphibolite facies rocks (N2.5 Ga,and up to 3.8 Ga; e.g., Pidgeon, 1980).

The granulite facies rocks in our study area are composed of hyper-sthene-bearing charnockites, two pyroxene-granulite, and garnet(sillimanite)-bearing felsic gneisses (Huang et al., 1986; Jahn et al.,1987; Liu et al., 1990; Lv et al., 2012; Wang, 1992; Wang and Peng,1994; Wang et al., 1985). Enclaves of supracrustal rocks occurring

within the orthogneisses vary in size from a few centimeters to a fewhundred meters. Isotopic ages of the gneisses concentrate around 2.6to 2.4 Ga (Jahn et al., 1987; Liu et al., 1990; Lv et al., 2012; Wang andPeng, 1994). Supracrustal rocks, including banded iron formations(BIF) and acid metavolcanics in association with amphibolites, granu-lites, ultrabasic rocks and charnockites, have also been reported fromthe study area (Wang and Peng, 1994; Wang et al., 1985). These rocksare overlain by Mesoproterozoic to Neoproterozoic sedimentary rocksand Phanerozoic strata that were later intruded by Mesozoic plutons.

3. Sampling and analysis methods

3.1. Sampling

Representative samples of the garnet-bearing charnockitic rockswere collected for this study (Table 1). A brief summary of the field oc-currence (Fig. 3) of the samples used for zircon dating is given below.Representative thin section photomicrographs showing themineral as-semblages are shown in Fig. 4.

Locality TP-1 is a road cutting near Qianxi county where medium tocoarse grained greasy greenish charnockitic rocks are exposed (Fig. 3a).Biotite flakes in the rocks define a distinct foliation. Garnet occursas tiny grains or granular aggregates along compositional bands.Orthopyroxene occurs as dark clots in association with bluish-greenquartz or with garnet. The charnockites intrude into the surroundingleucocratic quartzo-feldspathic gneisses with a sharp contact. Samplesfor this study were collected from garnet- and orthopyroxene-bearingdomains ofmedium to coarse grained charnockite. Locality TP-2 is an ac-tive quarry in Fanjiayu village where fresh vertical sections of massivegrayish green and medium to coarse grained charnockitic rocks areexposed (Fig. 3b). Garnet occurs as medium- to coarse-grained grainsor as granular aggregates along compositional layers. Distinctly foliatedlayers of garnet- and biotite-bearing felsic gneisses with compositionalbanding are exposed in the lower levels of the quarry. The charnockitesample collected for this study is medium grained charnockite withgarnet and orthopyroxene. In Locality TP-4, garnet-bearing charnockiteoccurs in association with amphibolites and garnetiferous metagabbroicrocks in an open-cast mine in the Zhaojiacun village. Charnockite is thedominant rock type in this locality, which occurs in close associationwith magnetite-rich amphibolite which carry disrupted bands, boudinsand blocks of metaggabros. Garnet-bearing charnockite in Locality TP-7(Fig. 3c,d), is medium grained and shows greasy green texture withabundant clots of coarse garnet crystals, particularly in the quartz-richdomains of the rock. Greenish orthopyroxene occurs in associationwith K-feldspar and plagioclase. Brown biotite laths define weakfoliation.

3.2. Analytical methods

3.2.1. Petrography and mineral chemistryPolished thin sections were prepared for petrographic study at

Peking University, China. The petrographic studies were carried out atthe China University of Geosciences, Beijing. Mineral chemical analyseswere carried out using an electron microprobe analyzer (JEOLJXA8530F) at the Chemical Analysis Division of the Research FacilityCenter for Science and Technology, the University of Tsukuba. The anal-yseswere performed under conditions of 15 kV accelerating voltage and10 nA sample current, and the data were regressed using an oxide-ZAFcorrection program supplied by JEOL.

3.2.2. Whole rock geochemical analysisThe least altered and homogeneous portions of the rock samples in

this studywere crushed andpowdered to 200mesh for geochemical anal-yses after petrographic observation.Major and trace (including rare earthelements) elements analyses were conducted in the National ResearchCenter for Geoanalysis, Beijing. The major elements were determined by

Fig. 2. Geological map of the Qianxi Complex in the North China Craton showing Taipingzhai charnockites and surrounding rocks. The sample locations for this study are also shown.


X-ray fluorescence (XRF model PW 4400), with an analytical uncer-tainties ranging from 1 to 3%. Loss on ignition was obtained using about1 g of sample powder heated at 980 °C for 30 min. The trace elementswere analyzed by Agilent 7500ce inductively coupled plasma mass spec-trometry (ICP-MS). About 50 mg of powder was dissolved for about7 days at ca. 100 °C using HF–HNO3 (10:1) mixtures in screw-top Teflonbeakers, followed by evaporation to dryness. The material was dissolved

Table 1Locations and details of charnockitic rocks used in this study from the North China Craton.

No Sample no Rock type GPS co-ordinates

1 TP-1/2 Charnockite N40° 10′19.71″; E118° 20′ 43.76″; Height: 112 TP-2/2 Charnockite N40° 12′ 10.48″; E118° 26′ 47.90″; Height: 13 TP-4/2 Charnockite N40° 13′ 33.78″; E118° 11′ 06.71″; Height: 14 TP-4/3a Charnockite N40° 13′ 33.78″; E118° 11′ 06.71″; Height: 15 TP-7/1 Charnockite N40° 14′ 10.49″; E118° 30′ 48.15″; Height: 16 TP-7/2 Charnockite N40° 14′ 10.49″; E118° 30′ 48.15″; Height: 1

Mineral abbreviations: Opx—orthopyroxene; Cpx—clinopyroxene; Grt—garnet; Bt—biotiteMt—magnetite; Ap—apatite; Zr—zircon; ilm—ilmenite; Py—pyrite.

in 7 N HNO3 and taken to incipient dryness again, and then was re-dissolved in 2%HNO3 to a sample/solutionweight ratio of 1:1000. The an-alytical errors vary from 5 to 10% depending on the concentration of anygiven element. An internal standard was used for monitoring drift duringanalysis. Trace and rare earth elementswere analyzedwith analytical un-certainties 10% for elements with abundances b 10 ppm and approxi-mately 5% for those N10 ppm (Gao et al., 2008).

Mineralogy

4 m Plg + Qtz + Bt + Opx + Grt + Apth + Grt + ilm + Py + Ap + Zr26 m Plg + Qtz + Opx + Grt + Mt + Cpx + Kfs + Py + Ap + Zr41 m Grt + Cpx + Opx + Mt + Hbl + Kfs + Bt + Qtz + Zr41 m Cpx + Plg + Opx + Mt + ilm + Grt + Hbl + Bt + Qtz + Kfs + Ap + Zr38 m Qtz + Grt + Opx + Mt + Pl + Kfs + Bt + Ap + Zr + Py38 m Qtz + Grt + Opx + Mt + Pl + Kfs + Bt + Ap + Zr + Py

; Hbl—hornblende; Kfs—K-feldspar; Pl—Plagioclase; Apth—antiperthite; Qtz—quartz;

Fig. 3. Representative field photographs of the garnet- and orthopyroxene-bearing charnockites from the study area. (a) Locality TP-1; (b) locality TP-2; (c) and (d) locality TP-7. Garnetcoexists with orthopyroxene, and the modal proportion of garnet varies from place to place. Large garnet porphyroblasts and coarse pristine grains of orthopyroxene also occur in somedomains.


3.2.3. Sample preparation and imaging for U–Pb dating and Lu–Hf analysesZircons grains were separated using standard procedures for U–Pb

dating and Lu–Hf analyses at the Yu'neng Geological and MineralSeparation Survey Centre, Langfang city, Hebei Province, China.Cathodoluminescence (CL) imaging was carried out at the BeijingGeoanalysis Centre using scanning electron microscope (JSM510)equippedwith Gantan CL probe, and transmitted and reflected light im-ages were examined by a petrological microscope. Individual grainswere mounted along with the standard TEMORA 1, with 206Pb/238Uage of 416.75±0.24Ma (Black et al., 2003), onto double-sided adhesivetape and enclosed in epoxy resin discs. The discs were polished to a cer-tain depth and gold coated for CL imaging and U–Pb isotope analysis.

3.2.4. Zircon U–Pb analysisZircon U–Pb analysis was performed on laser ablation inductively

coupled plasma spectrometry (LA-ICP-MS) housed at Tianjin Instituteof Geology and Mineral Resources. The zircon U–Pb dating analyseswere conducted using a Neptune MC-ICP-MS equipped with a 193 nmGeolas Q Plus ArF exciplex laser ablation, with spot sizes of 35 μm. Zir-con GJ-1 was used as an external standard for U–Pb dating analyses(published thermal ionization mass spectrometry normalizing ages of207Pb/206Pb = 607.7 ± 4.3 Ma, 206Pb/238U = 600.7 ± 1.1 Ma, and207Pb/235U=602.0± 1.0Ma; Jackson et al., 2004). Common-Pb correc-tions were made using the method of Anderson (2002). U, Th and Pbconcentrations were calibrated by using the standard silicate glassNIST SRM 610 as the external standard and 29Si as an internal standard.Every 8 analyses were followed by 2 analyses of standard zircon GJ-1and every 24 analyses were followed by measurements of GJ-1 andNIST SRM 610. 207Pb/206Pb, 206Pb/238U, 207Pb/235U and 208Pb/232Th ra-tios were calculated using GLITTER 4.0 (Macquarie University, Sydney,Australia). Concordia diagrams and weighted mean U/Pb ages wereprocessed using ISOPLOT 3 (Ludwig, 2003). Age data and concordiaplots were reported at 1σ error, whereas the uncertainties for weighted

mean ages were at 95% confidence level. Details of the technique aredescribed by Li et al. (2009).

3.2.5. Zircon Lu–Hf analysisIn situ zircon Hf isotopic analyses were conducted by using a

Neptune MC-ICP-MS equipped with a 193-nm laser at the Institute ofGeology and Geophysics, Chinese Academy of Sciences (IGGCAS), witha spot size of 60 μm and a laser repetition rate of 10 Hz at 100 mJ. Thedetailed analytical procedure and correction for interferences are simi-lar to those described by Wu et al. (2006). The 176Hf/177Hf ratios of thestandard zircon (GJ-1) and standard zircon (Mud Tank) during analysiswere 0.282000 ± 0.000030 (2σ, n = 200) and 0.282500 ± 0.000030(2σ, n = 200), respectively. The 176Hf/177Hf ratio of GJ-1 is very similarto the commonly accepted 176Hf/177Hf ratio of 0.282015 ± 0.0000019(2σ, n = 15) reported by Elhlou et al. (2006). While the 176Hf/177Hfratio of Mud Tank is almost identical to the values based on long-termextensive LA-MC-ICP-MS analyses, which are 0.282523 ± 0.000043(2σ, n = 2190; Griffin et al., 2006) and 0.282504 ± 0.000044 (2σ,n = 158; Woodhead and Hergt, 2005), respectively.

4. Results

4.1. Petrology and mineral phase equilibria

4.1.1. PetrographyThe petrographic features of five representative garnet-bearing

charnockite samples analyzed for zircon geochronology and geochemis-try are summarized below. Representative photomicrographs areshown in Fig. 4 and BSE images in Fig. 5. Mineral abbreviations areafter Spear (1993).

4.1.1.1. TP2/2. The sample is composed of plagioclase (25–30%), quartz(20–25%), orthopyroxene (15–20%), garnet (10–15%), magnetite(10–15%), clinopyroxene (5–10%), and K-feldspar (5–10%) with


accessory pyrite, apatite, and zircon (Fig. 4a–e). Garnet shows two differ-ent occurrences. The first variety is a coarse-grained (1.6–2.5 mm),porphyroblastic and subhedral garnet (Grt1) coexisting with coarse-grained clinopyroxene (~2.5 mm) and magnetite (~2.7 mm) withplagioclase, orthopyroxene, and quartz. In some domains, coarse garnetoccurs in association with clinopyroxene, plagioclase and magnetite(Fig. 4a). Based on this texture the peak mineral assemblage of the rockis inferred as garnet (Grt1) + clinopyroxene (Cpx1) + orthopyroxene+

Fig. 4. Photomicrographs showing the occurrence and assemblages of minerals discussed inclinopyroxene coexistingwith plagioclase andmagnetite in sample TP2/2. (b) Corona of garnetress of reaction (1) during cooling (sample TP2/2). (c) Coarse-grained orthopyroxene and clinmorphism (sample TP2/2). (d) Retrograde clinopyroxene (Cpx2) filling the matrix of fine-grwith irregular grain margin suggesting recrystallization during prograde to peak stage (sampl2. (g) Fine-grained aggregates of retrograde Grt2+ quartz corona overgrowing around inclusioning garnet (sample TP1/2). (h) Texturally retrograde biotite mantling orthopyroxene grains poorthopyroxene–garnet aggregates formingweak foliation of sample TP1/2. (j) Coarse-grained, spyroxenes in sample TP4/3a. (k) Grt2+ quartz aggregates associatedwithmagnetite–ilmenitethin exsolution lamellae of orthopyroxene. The clinopyroxene is locally replaced by greenorthopyroxene–plagioclase assemblage with retrograde greenish-brown calcic amphibole (orthopyroxene in the matrix in sample TP7/1. (o) Quartz, magnetite, and apatite inclusions wand quartz-rich portions of sample TP7/1.

magnetite + plagioclase + quartz. The coarse-grained Grt1 is nearlyinclusion-free, although it contains minor clinopyroxene and apatite.The second and the dominant variety is irregular-shaped aggregates offine-grained (b0.1 mm) garnet (Grt2) forming garnet + quartz +clinopyroxene corona around magnetite (Figs. 4b, 5a). The individualgarnet grains forming the corona show polygonal euhedral grain shape,and contain tiny inclusions of irregular quartz and K-feldspar (Fig. 5b).As discussed later, fine- to medium-grained rounded orthopyroxene

the text. Mineral abbreviations are after Spear (1993). (a) Porphyroblastic garnet and(Grt2)+quartz± clinopyroxene betweenmagnetite and plagioclase, suggesting the prog-opyroxene which are also regarded as the stable minerals during prograde to peak meta-ained orthopyroxene as a retrograde mineral (sample TP2/2). (e) Coarse-grained quartze TP2/2). (f) Garnet + biotite + orthopyroxene + plagioclase assemblage in sample TP1/-free prograde Grt1. Biotite occurs as inclusions in garnet or as thematrix phase surround-ssibly formed by hydration reaction of orthopyroxene (sample TP1/2). (i) Aligned biotite–ubhedral, and inclusion-free porphyroblastic garnet (Grt1) in thematrix of plagioclase andand clinopyroxene in sample TP4/3a. (l) Coarse-grained and subhedral clinopyroxenewithish calcic amphibole by later hydration event (sample TP4/3a). (m) Clinopyroxene–sample TP4/3a). (n) Coarse-grained porphyroblastic garnet associated with quartz andithin garnet (sample TP7/1). (p) Aggregates of euhedral to subhedral garnet in garnet-

Fig. 4 (continued).


adjacent to Grt2 is mantled by clinopyroxene (Fig. 5c). These texturesmight suggest the progress of the following reaction (1).

Mag þ Opx þ Pl ¼> Grt2 þ Qtz þ Cpx1 þ O2 ð1Þ

This reaction probably progressed during cooling and/or infiltrationof reduced fluid.

Orthopyroxene is present either as a coarse-grained (~3.1 mm)porphyroblast withminor inclusions of magnetite and apatite, probablyas a prograde to peak mineral (Fig. 4c), or as medium- to fine-grained(0.2–0.6 mm) rounded mineral in magnetite-rich and relatively fine-grained portion of the rock (Fig. 4d). Clinopyroxene shows two genera-tions; coarse-grained (~2.5 mm) porphyroblast mineral (Cpx1; Fig. 4a)associated with the peak mineral assemblage including garnet (Grt1),magnetite, plagioclase, and quartz, or a retrogrademineral (Cpx2) fillingthe matrix of fine-grained orthopyroxene (Figs. 4d, 5c). K-feldspar

dominantly occurs as thin films between coarse-grained matrix plagio-clase and pyroxenes/garnet (Figs. 5a,c), which is interpreted to havebeen formed by crystallization of melt phase formed by partial melting(e.g., Touret and Huizenga, 2012) or high-temperature metasomatismby infiltration of low H2O-activity fluid (e.g., Harlov et al., 1998;Rajesh and Santosh, 2012; Tsunogae and van Reenen, 2014). K-feldspar also occurs as irregular elongated grains surrounded by aggre-gates of retrograde garnet (Grt2). The mutual grain margins betweenthe garnet and K-feldspar is irregular and protrude along the length ofK-feldspar (Fig. 5b). Quartz is present either as coarse-grained(~2.6 mm) subhedral mineral in thematrix with irregular grain marginsuggesting recrystallization during high-grademetamorphism (Fig. 4e),or as fine-grained (b0.1 mm) irregular grains with garnet (Grt2) proba-bly formed by the progress of retrograde reaction (1) (Figs. 4b, 5a,b).Magnetite varies in grain size from 0.01 mm to up to 2.7 mm withrounded to irregular grain shape. The coarse grains are mostly mantledby Grt2 + quartz corona by the progress of reaction (1) (Figs. 4b, 5a),

Fig. 4 (continued).

Fig. 5. Back-scattered electron (BSE) images showing detailed textures discussed in the text. (a) Corona of garnet (Grt2) + quartz ± clinopyroxene between magnetite and plagioclase,suggesting the progress of reaction (1) (sample TP2/2). K-feldspar occurs as thin films between the garnet and matrix plagioclase. (b) Irregular-shaped quartz and K-feldspar grainssurrounded by aggregates of retrograde garnet (Grt2) (sample TP2/2). (c) Retrograde clinopyroxene (Cpx2) filling the matrix of fine-grained orthopyroxene as a retrograde mineral(sample TP2/2). Thin K-feldspar film occurs between the Cpx2 and matrix plagioclase. (d) Grt2 + quartz corona overgrowing around prograde Grt1 (sample TP1/2).



although magnetite grains completely enclosed in pyroxenes and feld-spars do not show any reaction texture (e.g., Fig. 4c). With respect tothe fine-grainedmagnetite, it occurs as fine-grained (b0.05mm) aggre-gates distributed along grain boundaries among pyroxenes and garnetpossibly formed by retrograde re-oxidation event after the progress ofreduction reaction (1).

4.1.1.2. TP1/2. This biotite-bearing and clinopyroxene-free variety ofgarnet-bearing granulite is composed of plagioclase (30–35%), quartz(10–15%), biotite (10–15%), orthopyroxene (10–15%), and garnet

Fig. 6. Compositional diagrams showing chemistry of representativeminerals. (a) Ca/(Fe+Mg+versus XMg diagram showing garnet chemistry. (c) Triangular diagram showing pyroxene chemshowing compositions of calcic amphibole. (f) XMg versus TiO2 (wt.%) diagram showing biotite

(5–15%) with accessory K-feldspar, magnetite, ilmenite, pyrite, and ap-atite (Fig. 4f–i). Garnet in the sample shows two varieties: coarse-grained (~1.1 mm) inclusion-free grains, which are nearly equivalentto Grt1 in sample TP2/2, and fine-grained (b0.2 mm) aggregates ofeuhedral grains intergrown with quartz, which are texturally similarto Grt2 (Figs. 4f,g, 5d). In this sample, the Grt2 + quartz intergrowthovergrows around the inclusion-free Grt1, showing compositegrains with irregular grain margins (Figs. 4g, 5d). Biotite (0.1–0.9 mm)occurs as reddish-brown flakes either as inclusions in garnet andorthopyroxene, or in the matrix of the minerals (Fig. 4f,g). Texturally

Ca+Mn) versusXMg diagram showing garnet chemistry. (b)Mn/(Fe+Mg+Ca+Mn)istry. (d) Triangular diagram showing feldspar chemistry. (e) Si (pfu) versus XMg diagramchemistry.


retrograde biotite mantling orthopyroxene grains, possibly formed bythe following hydration reaction (2) is also observed (Fig. 4h).

Opx þ Kfs þ H2O ¼> Bt þ Qtz ð2ÞModal abundance of magnetite in this sample is less than 2%, which

is significantly lower than that in sample TP2/2 possibly suggestinglower bulk FeO content or consumption of magnetite by the progressof reaction (1). Orthopyroxene is subhedral, medium to coarse grained(0.3–1.6 mm), and occur as aggregates with biotite and garnet(Fig. 4f,h). The aggregates defineweak foliation in the rock (Fig. 4i). Pla-gioclase (0.2–1.9 mm) in the matrix shows semi-granoblastic texturewith curved to irregular grain margins. Quartz occurs either as roundedto oviodal inclusions in plagioclase (Fig. 4i) or asmatrix mineral aroundplagioclase in felsic portion of the rock. As described earlier quartz alsooccurs as vermicular grains intergrowing with retrograde garnet(Figs. 4f,g, 5d). K-feldspar is minor, but if present it occurs as interstitialgrainsmostly betweenmatrix plagioclase and quartz in felsic portion ofthe rock.

4.1.1.3. TP4/3a. This rock is a garnet-bearing mafic granulite dominantlycomposed of plagioclase (25–35%), clinopyroxene (35–30%),orthopyroxene (5–15%), magnetite-ilmenite (5–10%), garnet (2–5%),hornblende (2–3%), and biotite (1–2%), with accessory quartz, apatiteand zircon (Fig. 4j–m). The occurrence of garnet is similar to the previ-ous two samples; coarse-grained (~3.3 mm), subhedral, and inclusion-free homogeneous grains (Grt1; Fig. 4j) or medium to fine aggregates ofgarnet + quartz grains (Grt2; Fig. 4k). Fine-grained orthopyroxene,biotite, plagioclase, and quartz occur with the aggregates. Clinopyroxeneis medium to coarse grained (0.3–3.2 mm), and often very coarse(~6.2 mm), subhedral, and contains thin exsolution lamellae of

Fig. 7. P–T diagram showing calculated pseudosection for themineral assemblages discussed infield of the peak assemblage is defined by Grt–Opx–Cpx–Pl–Qtz–Mag–Liq (the area outliclinopyroxene–plagioclase–quartz geothermobarometry, which is about 50 °C lower than theshaded area (800–830 °C, 7–8 kbar) as discussed in the text. (b) P–T pseudosection in CFMAGrt–Opx–Qtz–Mag–H2O (the area outlined by red line). The temperature range estimated bpeak P–T condition of the sample is defined by the shaded area (860–920 °C, 11–14 kbar) as d

orthopyroxene (Fig. 4l). Orthopyroxene is relatively smaller in size(~2.1 mm) than clinopyroxene, subhedral, and dominant in garnet-poor portion of the rock (Fig. 4m). Plagioclase is medium grained(0.3–1.2 mm) and subhedral with curved to irregular grain margin(Fig. 4m).Magnetite and ilmenite occur as aggregates forming compositegrains, which is a common texture of Fe–Ti oxide in charnockite. Biotite(0.1–0.2 mm) and calcic amphibole (0.2–0.7 mm) occur as fine-grainedaggregates around orthopyroxene (Fig. 4j,i,m), possibly formed by retro-grade hydration reactions.

4.1.1.4. TP7/1 and TP7/2. These samples are coarse-grained, massive,garnet- and quartz-rich granulite. Clinopyroxene is absent in these rocks.They are composed of quartz (40–50%), garnet (20–30%), orthopyroxene(15–20%), andmagnetite (2–5%)with accessory apatite, zircon, and pyrite(Fig. 4n–p). No obvious reaction texture and retrograde mineral occur inthe rock except minor chlorite replacing garnet rim. The rock thereforecontains simple peak mineral assemblage of garnet + orthopyroxene +magnetite + quartz.

The rocks are characterized by coarse-grained (~4.4 mm)poikiloblastic garnet (Fig. 4n) with minor fine-grained inclusions ofquartz, magnetite, and apatite (Fig. 4o). It is mostly subhedral, butgarnet with euhedral grain surface is also present in garnet- andquartz-rich portions of the rock (Fig. 4p). Medium- to coarse-grained(~2.2 mm) orthopyroxene occurs as subhedral grains with irregular tocurved grain margins (Fig. 4n). Quartz filling the matrix of garnet andorthopyroxene shows irregular grain margins possibly suggesting re-crystallization of smaller grains during high-grade metamorphism.The quartz aggregates contain rare fine-grained inclusions of pyrite.Magnetite and ilmenite (0.2–0.8 mm) are subhedral and scatteredalong grain boundaries, although it sometimes occur as anhedral grainssurrounding garnet and orthopyroxene.

this study. (a) P–T pseudosection in NCKFMASHTO system for sample TP2/2. The stabilityned by red line). Grt–Cpx–Pl–Qtz implies the P–T condition calculated using garnet–P–T field of the peak assemblage. The peak P–T condition of the sample is defined by theSHTO system for sample TP7/1. The stability field of the peak assemblage is defined byy Grt–Opx geothermometry of Lee and Ganguly (1988) is also shown in the figure. Theiscussed in the text.


4.1.2. Mineral chemistryRepresentative compositions of minerals obtained by electron

microprobe analyses are given in Supplementary Table 2, plotted inFig. 6, and briefly discussed below.

4.1.2.1. Garnet. Garnet in the granulites is essentially a solid solution ofpyrope, almandine, grossular and spessartine, with compositional vari-ation depending on the assemblage and core-rim positions (Fig. 6a,b).Garnet in magnetite-rich granulite (TP2/2) shows the lowest XMg

(=Mg/(Mg + Fe)) content of 0.08–0.10 and almandine-rich composi-tion of Alm69–72Prp6–8Grs19–21Sps1–3. Although they are compositional-ly nearly homogeneous, prograde and peak garnet (Grt1) is slightenriched in spessartine component (2.0 to 2.5 mol.%) than retrogradeGrt2 and rim of Grt1 (0.14–0.18 mol.%). Garnet in sample TP7/1 is alsoFe-rich (XMg = 0.17–0.18), and depleted in spessartine component asAlm69Prp14–15Grs15–16Sps0–1. Garnet in sample TP1/2 is also Fe-rich(XMg = 0.24–0.28) and shows the lowest grossular component ofAlm62–65Prp22–24Grs10–11Sps2. Core of porphyroblastic Grt1 and retro-grade Grt2 with quartz in the sample are slightly Mg-rich (XMg =0.27–0.28) than rim of the porphyroblastic Grt1 (XMg = 0.24–0.26).Garnet in sample TP4/3a shows the lowest almandine and the highestspessartine contents of Alm55–57Prp21–24Grs18–19Sps4. The garnet alsoshows slightly Mg-enriched Grt1 core and Grt2 (XMg = 0.29–0.30)than Grt1 rim (XMg = 0.27–0.28).

Fig. 8. (a) SiO2 vs. Na2O+ K2O diagram. The compositional fields are after Le Bas et al. (1986). (and Rollinson, 1993). (c) An–Ab–Or. (d) A/CNK [Al2O3/(CaO+Na2O+ K2O)] vs. A/NK [Al2O3/(et al. (2001). The shaded regions represent the composition of charnockitic rocks in Qianxi Com

4.1.2.2. Pyroxene. Clinopyroxene (classified as augite) in sample TP4/3ais Mg-rich (XMg = 0.67–0.73) is compositionally nearly homogeneousin terms of Fe–Mg ratio (Fig. 6c). Clinopyroxene inclusion inorthopyroxene shows higher Mg content of XMg = 0.70–0.73 thanthose in other occurrences. Clinopyroxene in sample TP2/2 is Mg-rich(XMg = 0.37–0.45) probably reflecting higher bulk MgO content(Table 3, Supplementary data). Core of coarse-grained porphyroblasticclinopyroxene (Cpx1) shows higher Fe content of XMg = 0.37–0.38than the retrograde Cpx2 around orthopyroxene in the same sample(XMg = 0.43–0.45).

Orthopyroxene in magnetite-rich samples (TP2/2 and TP7/1) showsferrosilite-rich compositions of XMg= 0.28–0.32 and 0.41–0.45, respec-tively, whereas orthopyroxene in samples TP1/2 and TP4/3a showsnearly consistent Mg-rich composition of XMg = 0.52–0.58. Ca contentof orthopyroxene is generally very low (CaO ~ 0.67 wt.%), whereasporphyroblastic orthopyroxene in sample TP2/2 shows higherCaO content of 0.8–1.3 wt.%. Al2O3 content of orthopyroxene variesfrom 0.5 wt.% (fine-grained Opx in sample TP2/2) to 2.6 wt.%(porphyroblastic Opx core of sample TP1/1).

4.1.2.3. Feldspar. Plagioclase in the granulites is albite-rich as An27 toAn38 (Fig. 6d). Coarse-grained plagioclase in sample TP2/2 shows slightincrease of albite component from core (An35–37) to rim (An27–29).Plagioclase grains in samples TP1/2 andTP4/3a are nearly homogeneous

b) R1 [4Si-11(Na+ K)-2(Fe+ Ti)] vs. R2 [6Ca+ 2Mg+Al] (after De la Roche et al., 1980Na2O+ K2O)] plots. The fields of (c) and (d) are after Maniar and Piccoli (1989) and Frostplex as reported by Yang et al. (2015).

Fig. 10. (a) Chondrite-normalized REE patterns and (b) Primitive mantle-normalized spi-der diagrams. Chondrite normalization values and Primitive mantle values are after Sunand McDonough (1989). The shaded regions represent charnockitic rocks in QianxiComplex as reported by Yang et al. (2015).

Fig. 9. (a) Zr vs. Y plots (fields after Pearce and Norry, 1979). The shaded region represents the composition of charnockitic rocks in Qianxi Complex as reported by Yang et al. (2015).(b) Fe*/(Fe* + MgO) vs. SiO2 (fields after Frost et al., 2001). Late Archean charnockites from Ma et al. (2013), post-collisional charnockites from Yang et al. (2014), arc magmaticcharnockites from Yang and Santosh (2014), ridge subduction-related charnockite from Yang and Santosh (2015), and charnockite in Qianxi Complex from Yang et al. (2015).


as An31–32 and An36–38, respectively. K-feldspar is orthoclase-rich asOr80–98. Thin K-feldspar film around garnet and pyroxenes in sampleTP2/2, which is possibly a product of fluid infiltration or partial melting,is orthoclase-rich (Or91–94) than K-feldspar inclusion (Or80–81) in gar-net in the same sample. K-feldspar film in samples TP1/2 and TP4/3ais also orthoclase-rich (Or93–97) than the inclusion phase in garnet(Or82–90).

4.1.2.4. Calcic amphibole. The calcic amphibole in sample TP4/3a isenriched in Mg and (Na + K)A (XMg = 0.55–0.57, Si = 6.33–6.38 pfu,CaB = 1.84–1.88 pfu, NaA = 0.29–0.37 pfu, KA = 0.33–0.36 pfu, andTiO2 = 1.6–1.9 wt.%), and compositionally classified as pargasitebased on the classification of Leake et al. (1997) (Table 2c, Supplemen-tary data; Fig. 6e).

4.1.2.5. Biotite. The biotite in samples TP1–2 and TP4/3a exhibit high XMg

of 0.54–0.68 and 0.75 (Fig. 6f), respectively. Its TiO2 content is higher inTP1–2 (5.4–6.2 wt.%) than in TP4/3a (4.2–4.4 wt.%). Biotite inclusion ingarnet, which probably is a prograde mineral, shows higher XMg andTiO2 content (XMg = 0.67–0.68 and TiO2 = 6.1–6.2 wt.%) than that ofretrograde biotite (XMg = 0.54–0.60 and TiO2 = 5.4–5.9 wt.%).

4.1.3. P–T conditionsThe Grt–Cpx geothermometer was applied to porphyroblastic

garnet and clinopyroxene in garnet-bearing granulites. The estimatedtemperature ranges for the pairs are 750–770 °C for sample TP2/2 and700–760 °C for sample TP4/3a based on the method of Ellis and Green(1979) which relies on experimental calibration of Fe–Mg fractionationbetween garnet and clinopyroxene at 750 °C to 1350 °C and 24 to30 kbar, and is a widely applied thermometer for granulite terranes.Calculated temperature ranges using the method of Ganguly et al.(1996),which is based on revised solutionmodel of garnet, are also con-sistent, 730–750 °C (sample TP2/2) and 700–760 °C (sample TP4/3a)are also consistent with the results of Ellis and Green (1979). Thetemperatures were calculated at 7 kbar, a reference pressure inferredfrom the peak pressure condition of the samples estimated by phaseequilibrium modeling as discussed below.

Metamorphic pressure for the garnet-bearing granulite was calcu-lated using Grt + Cpx + Pl + Qtz assemblages in the samples basedon experimental calibration of Perkins and Newton (1981). The esti-mated results are 5.0–5.3 kbar for sample TP2/2 and 6.0–6.6 kbar forsample TP4/3a at 750 °C. These results are significantly low if comparedto the results of phase equilibrium modeling discussed in the nextsection (P = ~7 kbar). We therefore adopted the method of Moecheret al. (1988), which is based on updated thermodynamic and

experimental data, and yielded pressure ranges of 7.2–7.4 kbar for sam-ple TP2/2 and 8.8–9.6 kbar for sample TP4/3a at 750 °C, suggestingslightly higher-pressure conditions.

The garnet-orthopyroxene geothermometer was applied toporphyroblastic garnet and orthopyroxene in samples TP2/2, TP4/3a,and TP7/1. The estimated temperature ranges for garnet–orthopyroxene


pairs are 730–760 °C (TP2/2), 830–860 °C (TP4/3a), and 740–800 °C(TP7/1) based on the experimental thermometer of Lee and Ganguly(1988). Application of the method of Aranovich and Berman (1997)gave nearly consistent temperature ranges of 710–760 °C (TP2/2),760–800 °C (TP4/3a), and 720–800 °C (TP7/1). The calculated tempera-ture ranges are consistent with those of garnet–clinopyroxenegeothermometry.

Metamorphic pressure for the Grt + Opx + Pl + Qtz assemblageshave been calculated based on experimental calibration of Perkins andNewton (1981). The estimated results are 5.2–5.6 kbar for sampleTP2/2 and 8.6–8.8 kbar for sample TP4/3a at 750 °C. Application ofthe geobarometer of Moecher et al. (1988) yielded slightly higher-pressure conditions of 7.3–7.6 kbar for sample TP2/2 and 9.8–10.1 kbarfor sample TP4/3a.

4.1.4. Phase equilibrium modelingMetamorphic P–T conditions based on the stability of peak mineral

assemblages in granulite samples TP2/2 and TP7/1 were constrainedusing THERMOCALC 3.33 (Powell and Holland, 1988, updated October2009) with an updated version of the internally consistent data set ofHolland and Powell (1998; data set tcds55s, file created November2003). We selected these samples for the calculation because sampleTP2/2 contains the representative reaction texture whichmight be use-ful for evaluation of P–T path, and sample 7/1 is a typical dry granulite inthe study area. Calculations were undertaken in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) for sam-ple TP2/2 (White et al., 2003, 2007), which provides the most realisticapproximation to model the granulite sample. On the other hand,CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (CFMASHTO) systemwas adopted for sample TP7/1 because of very low concentration ofNa2O and K2O in the sample. The phases considered in the modeling

Fig. 11. (a)Nb vs. Y diagram, (b) Rb vs. Y+Nbdiagramand (c)Nb/Zr vs. Zr diagram. Fields are afas reported by Yang et al. (2015).

and the corresponding a–x models used are garnet, biotite, and melt(White et al., 2007), cordierite (Holland and Powell, 1998), plagioclase(Holland and Powell, 2003), clinopyroxene (Green et al., 2007),orthopyroxene (Powell and Holland, 1999), amphibole (Diener et al.,2007), spinel andmagnetite (White et al., 2002), and ilmenite–hematite(White et al., 2000). The aluminosilicates, quartz, andH2O are treated aspure end-member phases. For the analysis, slabs of relatively homoge-neous domains of the granulite sample were used for thin-section prep-aration, and the counterpart of the same slabs was used for chemicalanalysis. Whole rock compositions for the rocks were determined byX-ray fluorescence spectroscopy for major elements and by titrationfor FeO–Fe2O3 ratio, at Activation Laboratories, Canada. MnO isneglected in the modeling because the MnO content of these samplesis generally low. Fe2O3 is taken into account for the calculations becausemagnetite is present in some samples.

Fig. 7a is a P–T pseudosection for sample TP2/2 which contains thepeakmineral assemblage of garnet+clinopyroxene+orthopyroxene+plagioclase + quartz + magnetite ± liquid (melt). The stability field ofthe assemblage plotted in the pseudosection suggests a wide P–T rangeof 770–1150 °C and 5.2–9.8 kbar (shaded area in Fig. 7a). The upper pres-sure stability limit is constrained by Pl-out line, whereas the lower limitby Grt-out line as both garnet and plagioclase are stable in the sample.The lower-temperature limit is defined by the absence of biotite in thesample. The estimated P–T condition is N50 °C higher than the result ofGrt–Cpx–Pl–Qtz geothermobarometry (Ellis and Green, 1979; Moecheret al., 1988; 770–800 °C and 5.2–7.9 kbar).

Fig. 7b is a P–T pseudosection for sample TP7/1 which contains thepeakmineral assemblage of garnet+orthopyroxene+quartz+magne-tite ± fluid. The stability field of the assemblage plotted in thepseudosection suggests a wide P–T range of N750 °C and 9 kbar. Thelower pressure stability limit is constrained by Pl-in line, whereas the

ter Pearce et al. (1984). The shaded regions represent charnockitic rocks inQianxi Complex


lower temperature limit by Ath-in line as both plagioclase and anthophyl-lite are stable in the sample. In order to further constrain the P–T conditionof the peak mineral assemblage, we estimated isopleths of XMg oforthopyroxene (0.41–0.42) and XCa (=Ca/(Fe + Mg + Ca)) of garnet(0.150–0.157), and inferred the peak P–T conditions are 860–920 °C and11–14 kbar. The estimated temperature is about 100 °C higher than thatobtained from Grt–Opx geothermometry (Lee and Ganguly, 1988; 780–830 °C at 13 kbar), although the method of Aranovich and Berman(1997) gave higher temperatures of 890–975 °C at 13 kbar.

4.2. Whole-rock geochemistry

The whole rock geochemical data of the samples analyzed in thisstudy are given in Table 3 (Supplementary data) and the salient featuresare briefly described below.

The rocks show SiO2 contents in the range of 42.82 to 55.32 wt.%,moderate TiO2 contents of 0.31–0.81 wt.%, moderate to high Al2O3

(4.51–17.26 wt.%), high FeOt (up to 34.34 wt.%), and moderate MgO(2.95–5.01 wt.%) and CaO (2.52–7.18 wt.%) contents. Total alkalicontents of these rocks are markedly low and in the range of 0.15 to1.77 wt.%, except for the samples TP1/2 and TP4/2 showing 7.05 and3.70. Their Mg# values show a wide range from 16.70 to 25.01 withtwo exceptions of 43.10 and 53.22 from samples TP1/2 and TP4/2.

In the total alkali vs. silica diagram (Le Bas et al., 1986; Fig. 8a), therocks plot outside or on the boundaries various fields showing subalkaliccomposition except for sample TP1/2 with alkalic feature. In the diagramusing the parameters R1 (R1 = 4Si-11(Na + K)-2(Fe + Ti)) and R2(R2 = 6Ca + 2 Mg + Al) (after De la Roche et al., 1980 and Rollinson,1993), all the rocks plot in the fields of granodiorite and tonalite(Fig. 8b). In the An–Ab–Or diagram (Frost et al., 2001; Maniar andPiccoli, 1989), the rocks show tonalitic to granodiorite composition(Fig. 8c). In terms of A/NK vs. A/CNK relationships (Frost et al., 2001;

Fig. 12. CL images of zircons from Sample TP2/2. Age inMa (numerator), εHf(t) value (marked bblue circle indicates the Lu–Hf analytical spot and the smaller dashed pink circle represents th

Maniar and Piccoli, 1989), the rocks showmetaluminous to peraluminouscomposition (Fig. 8d).

Immobile trace element relationships of Zr and Y suggest that therocks have the tholeiite and transitional affinities (Fig. 9a). The exceptionsare samples TP1/2 and TP4/2 which fall in the calc-alkaline field (Fig. 9a,fields after Pearce and Norry, 1979). In the SiO2 vs. Fe*/(Fe* +MgO) dia-gram (Frost et al., 2001; Fig. 9b), the rocks fall in the ferroan field withtholeiite affinity except one sample (TP1/2) that shows magnesian andcalc-alkaline affinities, similar to the Paleoproterozoic transitional mag-matic charnockites with ferroan features in the Lüliang complex ofTNCO (Yang and Santosh, 2015) and the post-collisional ferroancharnockites from the Chengde area of TNCO (Yang et al., 2014). Howev-er, they are distinct from the Neoarchean charnockites of the YinshanBlock (Ma et al., 2013), the post-collisional magnesian charnockitesfrom the Chengde area of Trans-North China Orogen (Yang et al., 2014),the Paleoproterozoic arc magmatic charnockites from the Xinghe area ofTNCO (Yang and Santosh, 2014) and the other suites of charnockiticrocks from the Qianxi Complex (Yang et al., 2015).

These rocks show awide range in transitional trace element compo-sitions (Ni: 29.2–140 ppm; Cr: 8–71.8 ppm; Co: 8.73–38 ppm). They arecharacterized by prominent and variable LREE enrichment on chondritenormalized REE patterns, with no evidence for the fraction of HREE(Fig. 10a). They display clear negative Eu anomaly (δEu = 0.34–0.89),with one exception of sample TP1/2 in the absence of Eu anomaly(δEu = 1.00). The primitive mantle normalized trace elementabundances for the rocks (Fig. 10b) show positive Pb anomaly andnegative Nb, Ta, Sr and Ti anomalies. Furthermore, they exhibit positiveanomalies of Nd, and Gd and dominantly Ba positive anomalies, andslightly negative anomalies at Ce and Y.

The salient geochemical features of these rocks suggest magma der-ivation in subduction-related settings. In the Y–Nb diagram (Fig. 11a),all these rocks are plotted in the VAG + syn-COLG field (Pearce et al.,

y yellowhighlight) and spot number (parentheses) are shown against each spot. The largee U–Pb analytic spot. All the spot ages are zircon 207Pb/206Pb ages.


1984)with two exceptions of TP7/1 andTP7/2 falling in theORGfield. Inthe Y+Nb vs. Rb diagram, they all fall in the VAG area (Fig. 11b; Pearceet al., 1984) with one exception of TP7/2 that falls in the field of ORG. Inthe Nb/Zr vs. Zr diagram, all the plots fall in the field of rocks generatedin subduction-related setting (Fig. 11c; Pearce et al., 1984).

4.3. Zircon U–Pb geochronology and Lu–Hf isotopes

Representative cathodoluminescence (CL) images of the zircongrains from the rocks analyzed in this study are given in Figs. 12–13.The U–Pb analytical data are given in Table 4 (Supplementary data),and the data are plotted in concordia diagrams together with age datahistograms andprobability curves in Figs. 14–15. Lu–Hf isotope analyseswere performed in the same magmatic domains from where U–Pb agedata were obtained (Fig. 16; Table 5, Supplementary data). A briefdescription of the zircon characteristics, age results and Lu–Hf isotopesfrom individual samples is given below.

4.3.1. Zircon morphologyZircons from the charnockitic rock of sample TP2/2 show prismatic

to stumpy morphology, and some grains are partly rounded. Most ofthe zircon grains from these samples are colorless and some are lightbrownish. The grains range from 50 to 200 μm × 30 to 180 μm in sizewith aspect ratios of 3:1 to 1:1. In CL images, all the zircons displayclear oscillatory zoning except few zircons that are structureless, andsome grains showing core-rim textures with variably sized rims(Fig. 12).

The zircons from the charnockitic sample TP7/1 display prismatic tostumpy morphology and a few of the grains show partly roundedshapes (Fig. 13). Only few zircons show clear oscillatory zoning and

Fig. 13. CL images of zircons from Sample TP7/1. Age inMa (numerator), εHf(t) value (marked bblue circle indicates the Lu–Hf analytical spot and the smaller dashed pink circle represents th

most of them display core-rim textures. Some of the zircon grains arestructureless. The zircons are colorless or slight brownish, with a sizerange of 50–150 μm × 30–100 μm and aspect ratios of 3.5:1 to 1.2:1.

4.3.2. U–Pb data

4.3.2.1. TP2/2. Thirty four zircon grains from charnockitic rock sampleTP2/2 were analyzed for U–Pb age dating and data can be divided intothree groups (see Fig. 14a,b). Twenty five zircon grains define anupper intercept age of 2562 ± 20 Ma (MSWD = 0.94; N = 25) andshow a 207Pb/206Pb mean age of 2543 ± 9.6 Ma (MSWD = 1.02, N =22) (Fig. 14a,c). Six analyses including 2 concordant spots (spot 1 andspot 19) and 4 discordant ones showing lead-loss define an upper inter-cept age of 2449 ± 41 Ma (MSWD = 0.83, N = 6) (Fig. 14a). The twoconcordant spots (spot 1 and spot 19) yield 207Pb/206Pb ages of ca.2458 Ma and ca. 2475 Ma with a 207Pb/206Pb mean age of 2466 ±32 Ma (MSWD = 0.29, N = 2), which is similar to the upper interceptage of this zircon group. These data are interpreted to represent thethermal event associated with metamorphism. Three zircons (spots 3,7 and 14) display older 207Pb/206Pb ages of ca. 2632 Ma, 2622 Ma and2626 Ma, respectively, suggesting inherited grains. The 2562 ± 20 Maage is taken to represent the crystallization age of this rock.

4.3.2.2. TP7/1. Thirty three zircon spots were analyzed from this sample(TP7/1), and the age data can be divided into four groups (seeFig. 15a,b). Nineteen spots including 18 concordant spots and onediscordant spot showing lead loss define an upper intercept age of2539 ± 21 Ma (MSWD = 0.59; N = 19) and show 207Pb/206Pb meanage of 2544± 12Ma (MSWD=0.47; N=19) (Fig. 15a,c,d), suggestingthat magmatic crystallization of this rock at ca. 2544 Ma. Nine zircon

y yellowhighlight) and spot number (parentheses) are shown against each spot. The largee U–Pb analytic spot. All the spot ages are zircon 207Pb/206Pb ages.

Fig. 14. Zircon U–Pb concordia plots (a,c,d) and age data histograms with probability curves (b) for the Sample TP2/2.


grains yield younger 207Pb/206Pb ages of ca. 2497–2391 Ma and definean upper intercept age of 2480 ± 44 Ma (MSWD = 1.2; N = 9) with207Pb/206Pb mean age of 2452 ± 19 Ma (MSWD = 1.3; N = 9)(Fig. 15a), representing the thermal event at ca. 2480 Ma associatedwith metamorphism. The remaining four zircon spots (spots 1, 2, 3and 35) are concordant and display older ages ranging from ca. 2678to 2643 Ma and show the 207Pb/206Pb mean age of 2656 ± 27 Ma(MSWD = 0.35; N = 4) (Fig. 15a). The 2544 ± 12 Ma age is taken asthe best estimate of the crystallization age of the rock, and the minorgroups of older zircons (207Pb/206Pb mean age: 2656± 27Ma) are con-sidered to be inherited grains. One analysis (spot. 9; Fig. 13) is concor-dant and preserves a 207Pb/206Pb age of 1865 ± 30 Ma (Fig. 15a,b),which is interpreted to reflect the ca. 1.85 Ga Paleoproterozoic thermalevent reported throughout the NCC.

4.3.3. Lu–Hf isotopesEleven zircons from sample TP2/2 were analyzed for Lu–Hf isotopes

(Table 5, Supplementary data) and yield initial 176Hf/177Hf valuesbetween 0.281324 and 0.281386. Among these, 8 zircon grains showpositive εHf(t) values ranging from6.0 to 8.5 (Fig. 16a), when calculatedbased on the upper intercept age of 2562 Ma. They show crustal resi-dence ages (TDMC ) ranging from 2566 to 2680 Ma (Fig. 16b). However,two zircon grains (spots 9 and 32) in this group show crustal residenceages (TDMC ) of 2529 Ma and 2540 Ma, slightly younger than the crystal-lization age of this rock (upper intercept age: 2562 Ma). The remaining3 zircon grains with younger 207Pb/206Pb ages also display positiveεHf(t) values of 3.8–5.3 and show crustal residence ages (TDMC ) rangingfrom 2641 to 2730 Ma, calculated based on the upper intercept age of2449 Ma. The data indicate that the zircons in the rock were derivedfrom the Neoarchean juvenile components.

Only five zircons from sample TP7/1 could be selected for Lu–Hfisotopic analysis. The result show that their initial 176Hf/177Hf valuesare between 0.281228 and 0.281288 (Table 5, Supplementary data).Three zircons show positive εHf(t) values between 2.5 and 4.6(Fig. 16a), when calculated based on 207Pb/206Pb mean age of 2544 Ma.Their crustal residence ages (TDMC ) range from 2754 to 2884 Ma(Fig. 16b). Two inherited zircons (spots 17 and 35) also show positiveεHf(t) value of 5.5 and 5.8, when calculated by the 207Pb/206Pb meanage of 2656 Ma, with crustal residence ages (TDMC ) of 2766 Ma and2783 Ma, The data indicate that the magma was sourced from Meso-Neoarchean juvenile components.

5. Discussion

5.1. Implications of petrological and geochronological data

The garnet-bearing charnockites discussed in this study contain peakmineral assemblages of garnet + clinopyroxene + orthopyroxene +magnetite + plagioclase + quartz (sample TP2/2), garnet +orthopyroxene + biotite + magnetite + plagioclase + quartz (sampleTP1/2), clinopyroxene+orthopyroxene+garnet+plagioclase+mag-netite + ilmenite with retrograde and biotite (sample TP4/3a),and garnet + orthopyroxene + quartz (samples TP7/1 and TP7/2).Peak P–T conditions of these rocks calculated using conventionalgeothermobarometers show 750–770 °C and 7.2–7.4 kbar (sampleTP2/2), 700–760 °C and 8.8–9.6 kbar (sample TP4/3a), and 740–800 °C(sample TP7/1), suggesting granulite facies. Application of phase equilib-rium modeling on a representative sample (TP2/2) yielded a wide P–Tstability range of 770–1150 °C and 5.2–9.8 kbar for the peak mineral as-semblage (garnet + clinopyroxene + orthopyroxene + magnetite +

Fig. 15. Zircon U–Pb concordia plots (a,c,d) and age data histograms with probability curves (b) for the Sample TP7/1.


plagioclase + quartz). Considering the possible effect of post-peak Fe–Mg exchange between garnet and clinopyroxene, and also the evidencethat the quantity ofmelt phase possibly present during high-grademeta-morphism was very small (e.g., only very thin film of K-feldspar occursbetween plagioclase and garnet/clinopyroxene in the sample), we inferthat the peak P–T condition of the sample is around 800–830 °C and7–8 kbar (shaded area in Fig. 7a). Pseudosection analysis of Grt–Opx–Pl–Mag assemblage in sample TP7/1 indicates peak P–T condition of860–920 °C and 11–14 kbar (shaded area in Fig. 7b), which is slightlyhigher than the estimate from Grt–Opx geothermometry of Lee andGanguly (1988) (780–830 °C at 13 kbar), although it is nearly consistentwith the result obtained from the computation of Aranovich and Berman(1997) (890–975 °C at 13 kbar).

The rocks analyzed in this study are dominantly subalkalic incomposition and fall in the fields of granodiorite and tonalite. Thecharnockites show broadly tonalitic to granodiorite composition. Theyshow variable LREE and HREE enrichment with clear negative Euanomaly. The primitive mantle normalized trace element features areconsistent with arc derivation. Trace element discrimination diagramsalso suggest volcanic arc granite (VAG) affinity with magma generationin a subduction-related setting.

The age data obtained in this study are consistentwith the ages fromthe charnockite suite reported in Yang et al. (2015). The magmatic zir-cons show upper intercept ages of 2562 ± 20 Ma to 2539 ± 21 Maand 207Pb/206Pb mean ages of 2544 ± 12 Ma to 2543 ± 9.6 Ma, corre-sponding to the timing of emplacement of the protoliths at ca. 2.56–2.54 Ga and marking major late Neoarchean magmatism. The slightlyyounger zircon rims suggest that metamorphism closely followed themagmatic emplacement, during the Archean–Paleoproterozoic transi-tion. The presence of Paleoproterozoic zircon (ca. 1.86 Ga) correlateswith the tectonothermal event at this time reported from elsewhere

in the NCC which is considered to represent the final collision betweenthe unified Eastern and Western Blocks (Wilde, 2014; Yang andSantosh, 2014, 2015; Zhai and Santosh, 2011; Zhao and Zhai, 2013).Some inherited zircons in these rocks show slightly older 207Pb/206Pbages of 2678 ± 27 Ma to 2622 ± 23 Ma suggesting capture from olderbasement rocks.

The age data from present study are compiled in Fig. 17a, where the207Pb/206Pb major age peak of 2543 ± 7.6 Ma (N= 41; MSWD= 0.75)is defined. Among the other two minor peaks, the 2641 ± 18 Ma (N=7; MSWD = 0.59) age is from the inherited zircons and 2461 ± 19 Ma(N= 15; MSWD= 1.7) is correlated to the timing of high-grade meta-morphism. The majority of Th/U values is higher than 0.1, and evenrange up to 3.35 (Fig. 17b), suggesting a magmatic origin for most zir-cons. The data indicate a major late Neoarchean magmatic event at ca.2.55–2.54 Ga.

The Lu–Hf data from zircon grains in the two samples show positiveεHf(t) values ranging from 2.5 to 8.5 with all the plots above the CHURline, andmost of them close to the depletedmantle and new crust lines.The depleted mantle model ages (TDM) of 2542 to 2752 Ma and thecrustal residence ages (TDMC ) of up to 2884 Ma (Fig. 16a,b). The dataindicate a major juvenile crust formation event in the Meso- toNeoarchean.

The geochemical and geochronological data on charnockites fromTaipingzhai area suggest a major phase of late Neoarcheanmagmatism in an arc-related subduction setting, followed by meta-morphism at ca. 2.50–2.4 Ga, possibly marking the assembly of themicroblocks within the cratonic domain (Yang et al., 2015). The ca.1.86 Ga metamorphic age corresponds with the thermal event asso-ciated with the collision between the Eastern and Western Blocks inthe NCC (Yang and Santosh, 2014, 2015; Zhai and Santosh, 2011;Zhao and Zhai, 2013).

Fig. 16. Zircon Lu–Hf isotope plots for charnockitic rocks of sample TP2/2 and sample TP7/1. (a) εHf(t) vs. age (Ma) and (b) TDMC (Ma) vs. age (Ma). The shaded regions represent data onzircons fromArchean to Paleoproterozoic rocks in various regions of theNorth China Craton as compiled by Geng et al. (2012) and Yang et al. (2015). “Newcrust” line is fromDhuime et al.(2011).


5.2. Correlation with previous studies

Several previous studies have reported the pressure–temperatureconditions of metamorphism from different domains in the Easternpart of the NCC. Mineral assemblages and textures in a wide variety ofrocks including mafic, intermediate and felsic granulites, BIFs andmetasedimentary sequenceswere employed to trace the P–T conditionsduringdifferent stages of prograde, peak and retrogrademetamorphism(e.g., Lu et al., 1995; Shen et al., 1992; Zhao et al., 1993).

Evaluated the metamorphic pressures and temperatures ofNeoarchean granulites from different localities and reported, 7–8 kbarand 740–840 °C in the area near the junction of Hebei–Shanxi InnerMongolia, 5–7 kbar and 700–750 °C in Huadian, NE China, and4–7 kbar and 650–810 °C in central-western Inner Mongolia.

Shen et al. (1992) evaluated the metamorphic pressures andtemperatures of Neoarchean granulites from different localities in theEastern Block of the NCC. They identified 6.5–8 kbar and 750–850 °Cin Eastern Hebei and 5–7 kbar and 700–750 °C in the Huadian area inNE China. Sills et al. (1987) reported metamorphic P–T of ca. 700–750 °C and 7–8 kb from Archean high-grade gneisses in the Taipingzhaiarea. They estimated 650–700 °C and 5–6.5 kb for granulites in theShuichang area and 4.5–6.0 kb and 600–650 °C for those in theCaozhuang area where retrogression associated with shear zones andintrusion of pegmatites are common. Zhao et al. (1998) traced themulti-stagemetamorphic evolutionary history of theArcheanbasementrocks in the eastern Hebei domain, and reported temperatures of 650°to 770 °C and pressures of 5.5 to 6.5 kbar for the prograde (M1) stage,

Fig. 17. (a) Combined age data histogram with probability curve and (b

based on garnet cores and enclosed hornblende and plagioclase grains.For the peak (M2) stage, they estimated 850 °C to 900 °C and pressuresof 8.0 to 8.5 kbar, based on the core compositions of garnet, matrixorthopyroxene, clinopyroxene, and plagioclase. The retrograde (M3)stage was constrained at 700 °C to 750 °C and 7.5 to 8.0, based on therim compositions of hypersthene + clinopyroxene + plagioclase andthe compositions of coronitic garnet. They also traced a near-isobariccooling and counterclockwise P–T path.

He et al. (1992) reported metamorphic conditions of 650 °C to700 °C and 6.5 to 7.5 kbar for the M1 stage of granulites in the Miyun–Chengde domain. The M2 stage was constrained as 800 °C to 900 °Cand 10.0 to 12.0 kbar (Chen et al., 1994). The post-peakmineral assem-blage (M3) yielded 670 °C to 770 °C and 9.0 to 10.0 kbar (Chen et al.,1994). The metamorphic textures and their P–T estimates define anear-isobaric cooling for this domain. Garnet-bearing mafic granulitesin the western Liaoning domain preserve the prograde (Ml), peak(M2), post-peak (M3), and retrograde (M4) mineral assemblages. TheM1 stage in these rocks defines 750° to 800 °C and 5.5 to 6.7 kbarwhere as the M2 stage shows 880° to 990 °C and 9.0 to 11.5 kbar (Cuiet al., 1991). The M3 stage is constrained at 750° to 850 °C and 9.5 to11.5 kbar. The retrograde M4 stage occurred under 650° to 750 °C and8.5 to 10.5 kbar.

From the slightly lower grade rocks of western Shandong domain,Zhao (1995) reported thermal peak at ~500° to 600 °C and 5.0 to6.0 kbar, followed by two stages of retrogression at 450° to 500 °C and~5.0 kbar, and ~400 °C and b5.0 kbar. From the eastern Shandongdomain, Li (1993) traced up to 780 °C and 5.0 kbar for the M1

) Th/U ratios vs. Age (Ma) for all the zircons analyzed in this study.


stage. The M2 stage is constrained at 850° to 900 °C and 8.0 to 8.5 kbar,followed by the retrograde M3 stage at ~650 °C and 7.0 kbar. Fromgarnet-bearing amphibolites in the Xinjin area of the southern Liaoningdomain, Zhao et al. (1998) traced M1 to M4 stages of metamorphismand counterclockwise P–T path. Mafic granulites in the northern Liao-ning domain were used by Sun et al. (1993) to constrain the multiplestage of metamorphism. In their study, theMl stage assemblage recordsP–T conditions of ~680° to 730 °C and 5.5 to 6.5 kbar, M2 stage shows800° to 850 °C and 7.5 to 8.5 kbar, M3 stage experienced 700° to750 °C and 7.0 to 8.0 kbar, and M4 stage was constrained at 560° to625 °C and 5.2 to 6.1 kbar. Ge et al. (1994) studied mafic granulitesfrom the southern Jilin domain and reported temperatures of 575° to600 °C and pressures of 5.5 to 6.0 kbar for the M1 stage. The M2 stageis constrained at 800° to 850 °C and 8.0 to 8.5 whereas the M3 stage ischaracterized by 600° to 620 °C and 7.0 to 7.5 kbar. These authors alsotraced an isobaric cooling path from M1 to M3 stages.

Zhao et al. (1998) argued that the counterclockwise P–T–t paths inthe Eastern Block reflectmetamorphism related to the intrusion andun-derplating of voluminous mantle derived magmas, and argued againstsubduction–collision models. Zhao et al. (1998, 2001) favored amantle-plume (hotspot) model to explain the origin of the basementrocks of the eastern zone of the North China Craton. Their models donot support arc accretion and collision in the Eastern Block. In contrast,Li (1999) and Zhai et al. (2005) who investigated the mafic granulitesfrom Zunhua and Taipingzhai area that are characterized by anticlock-wise P–T paths suggested that these units jointly constitute aNeoarchean island arc terrain, representing the upper part and rootpart, respectively. According to their interpretation, the Eastern Hebeihigh-grade region is a composite terrane of at least three island arcterranes that formed during the Paleoarchean, Mesoarchean andNeoarchean. Their models thus strongly suggest island arc accretionthrough arc–arc or arc–microcontinent collision (Zhai and Liu, 2001).

The granulite facies rocks from Taipingzhai area in Qianxi Complexalong the western margin of Jiaoliao microblock located within thecomposite Eastern Block in our present study carry peak mineralassemblage of garnet+clinopyroxene+orthopyroxene+magnetite+plagioclase + quartz ± biotite ± ilmenite. Mineral phase equilibriacomputations using pseudosection and geothermobarometry suggestpeak P–T condition of 800–920 °C and up to 14 kbar. Our computationsprovide more precise estimates than those given in previous studies,with the peak P–T conditions suggesting high temperature andmoderateto high pressure metamorphism in the Archean.

5.3. Tectonic implications

Zhai et al. (2000), Zhai and Santosh (2011, 2013) and Zhai (2014)proposed that the NCC is composed of several microblocks and that be-tween 2.6 and 2.45 Ga, around seven microblocks were amalgamatedtogether by continent–continent, continent–arc or arc–arc collision.The new geochemical and geochronological data presented in ourstudy indicate that late Neoarchean magmatism occurred dominantlyin an arc-related subduction tectonic setting, and was shortly followedby metamorphism during Archean–Proterozoic transition possiblyassociated with the collisional assembly of the microcontinents. Overthe globe, the Archean–Proterozoic boundary witnessed cratonizationand formation of large continents (Condie et al., 2001; Rogers andSantosh, 2004; Windley, 1995). In a recent study, Yang et al. (2015) in-vestigated a suite of charnockites, amphibolites, metagabbros andorthogneisses at the periphery of the Jialiao microblock and presentedpetrological, geochemical and zircon U–Pb geochronological data. Theirresults show dominantly magnesian composition and arc-related fea-tures for the charnockites with trace element features suggestingsubduction-related origin. The zircon data show peak 207Pb/206Pb meanage of 2554±3.2Ma attesting tomajor late Neoarchean arcmagmatism.The western margin of the Jiaoliao microblock is adjacent to a major Ar-chean greenstone belt developed through oceanic subduction and the

magmatic suite reported by Yang et al. (2015) is interpreted to havebeen generated in an active convergent margin with oceanic lithospheresubduction beneath thewesternmargin of the Jialiao block. The rocks in-vestigated in our present study also show arc-related tectonic setting forthe parent magma, with the upper intercept ages of 2562 ± 20 Ma of2539 ± 21 Ma suggesting latest Neoarchean magma emplacement.Metamorphism is constrained as 2449 ± 41 Ma and 2480 ± 44 Ma.We correlate the high temperature and medium pressure metamor-phism obtained in the present study with the subduction–collision tec-tonics associated with microblock amalgamation in the NCC at the endof Archean. The late Neoarchean arc magmatism and collisionalmetamorphism are analogous to modern style plate tectonic processesin Phanerozoic terranes. Except for rare examples (see Anderson et al.,2012 and references therein) high-pressure metamorphism is uncom-mon in the Archean rock record, a feature which is correlated with theabsence of thickened crust and low apparent thermal gradients, andconfirmswith themodels of secular change of the Earth's thermal struc-ture (Brown, 2007). The latest Neoarchean high temperature andmedi-um to high pressure metamorphism associated with the assembly ofmicrocontinents is another exception to this model. In a recent review,Brown (2014) noted that both eclogite–high-pressure granulite meta-morphism, with apparent thermal gradients of 350–750 °C/GPa, andgranulite–ultrahigh temperature metamorphism, with apparent ther-mal gradients of 750–1500 °C/GPa, appeared in theNeoarchean rock re-cord. He interpreted this paired metamorphism to mark the onset ofone-sided subduction leading to an asymmetric thermal structure atthe convergent plate margins characterized by lower T/P in the subduc-tion channel and higher T/P in the overriding plate. Brown (2014) cor-related the onset of such paired metamorphism to a transition fromstagnant lid to subduction and initiation of a global plate tectonics re-gime by ca. 2.5 Ga. A rare example of high pressure metamorphismwas reported by Liu et al. (2014) from mafic granulites in the Jiaodongterrane at the southernmargin of Jiaoliaomicroblock. Thesemafic granu-lites are composed of garnet–clinopyroxene–orthopyroxene–hornblendeenclaves within late-Archean trondhjemite–tonalite–granodiorite (TTG)gneisses and display typical high-pressure mineral assemblage compris-ing garnet–clinopyroxene–plagioclase–quartz ± rutile. Through the ap-plication of conventional geothermobarometry and pseudosectionmodeling, the authors demonstrated a clockwise P–T path withthe peak conditions at ~17 kbar and ~880 °C. The high pressure rocks re-ported by Liu et al. (2014) and the medium pressure granulites reportedin our study are consistent with the paired metamorphism at theArchean–Proterozoic transition as envisaged by Brown (2014).

6. Conclusion

➢ The garnet-bearing charnockites from the Santunying–Taipingzhaiarea along the western margin of the Jialiao microblock in theastern Block of the North China Craton define peak P–T conditionsof around 800–920 °C and up to 14 kbar employing phase equilibriacomputations.

➢ Zircon geochronology reveals that the magmatic protoliths wereemplaced at ca. 2.56–2.54 Ga, marking major late Neoarcheanmagmatism. A younger group of zircons constrain that metamor-phism closely followed at ca. 2.48 Ga.

➢ Zircon Lu–Hf data show dominantly positive εHf(t) values (up to8.5), and yield crustal residence ages (TDMC ) in the range of 2529 to2884 Ma, suggesting magma sources from Meso-Neoarcheanjuvenile components.

➢ The high temperature andmedium to high pressure metamorphismis correlatedwith the subduction–collision tectonics associatedwithmicroblock amalgamation in the NCC at the end of Archean.

➢ Combined with a recent report of high pressure metamorphismfrom an adjacent locality, our results are consistent with the modelthat predicts paired metamorphism at the Archean–Proterozoictransition with the onset of modern style plate tectonics.


Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2015.11.018.

Acknowledgments

We thank Dr. Tamer Abu-Alam, Guest Editor and two referees fromthe Journal for constructive comments which helped in improving ourmanuscript. This study forms part of the PhD research of Qiong-YanYang at the China University of Geosciences Beijing. The study was sup-ported by the Foreign Expert Fund fromChinaUniversity of Geosciences(Beijing) and the Professorial grant toM. Santosh from the University ofAdelaide.

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