Geoscience Frontiers 4 (2013) 305e320

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Research paper

Metamorphism of the northern Liaoning Complex: Implications for the tectonicevolution of Neoarchean basement of the Eastern Block, North China Craton

Kam Kuen Wu a, Guochun Zhao a,*, Min Sun a, Changqing Yin b, Yanhong He c, Pui Yuk Tama

aDepartment of Earth Sciences, James Lee Science Building, The University of Hong Kong, Pokfulam Road, Hong KongbDepartment of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, CanadacDepartment of Geology, North West University, 229 North Taibai Road, Xi’an 710069, China

a r t i c l e i n f o

Article history:Received 7 November 2012Received in revised form22 November 2012Accepted 26 November 2012Available online 16 December 2012

Keywords:AmphibolitesNorthern Liaoning complexNorth China CratonLate ArcheanMetamorphic evolution

* Corresponding author. Tel.: þ852 28572192; fax: þE-mail address: gzhao@hkucc.hku.hk (G. Zhao).

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a b s t r a c t

As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North ChinaCraton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic (TTG)gneisses, massive granitoids and supracrustal rocks. The supracrustal rocks, named the Qingyuan Group,consist of interbedded amphibolite, hornblende granulite, biotite granulite and BIF. Petrological evidenceindicates that the amphibolites experienced the early prograde (M1), peak (M2) and post-peak (M3)metamorphism. The early prograde assemblage (M1) is preserved as mineral inclusions, represented byactinotiteþhornblendeþplagioclaseþ epidoteþquartzþ sphene,within garnet porphyroblasts. Thepeakassemblage (M2) is indicated by garnet þ clinopyroxene þ hornblende þ plagioclase þ quartz þ ilmenite,which occur as major mineral phases in the rock. The post-peak assemblage (M3) is characterized by thegarnet þ quartz symplectite. The PeT pseudosections in the NCFMASHTO system constructed by usingTHERMOCALC define the PeT conditions of M1, M2 and M3 at 490e550 �C/<4.5 kbar, 780e810 �C/7.65e8.40 kbar and 630e670 �C/8.15e9.40 kbar, respectively. As a result, an anticlockwise PeT path involvingisobaric cooling is inferred for the metamorphic evolution of the amphibolites. Such a PeT path suggeststhat the late Archean metamorphism of the northern Liaoning Complex was related to the intrusion andunderplatingofmantle-derivedmagmas. Theunderplatingof voluminousmantle-derivedmagmas leadingto metamorphism with an anticlockwise PeT path involving isobaric cooling may have occurred incontinental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift envi-ronments. A mantle plume model is favored because this model can reasonably interpret many othergeological features of late Archean basement rocks from the northern Liaoning Complex in the EasternBlock of the North China Craton as well as their anticlockwise PeT paths involving isobaric cooling.

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1. Introduction

The field- and thermodynamics-based metamorphic investi-gations, combined with lithological, structural and geochronolog-ical studies, can be directed toward understanding the tectonic

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settings and evolution of metamorphic terrains (England andThompson, 1984; Thompson and England, 1984; Sandiford andPowell, 1986; Ellis, 1987; Harley, 1989; Bohlen, 1991; Brown,1993; Vernon, 1996). The PeT path of a metamorphic terrain isof particular significance in this regards because the variations ofpressure and temperature that characterize a metamorphic eventare a function of the tectonic setting and processes that wereactive during metamorphism (England and Thompson, 1984;Thompson and England, 1984; Bohlen, 1987; Harley, 1989; Brown,1993). Generally, clockwise PeT paths involving isothermaldecompression are considered to develop in continental subduc-tion and collisional environments in the context of plate tectonics(England and Thompson, 1984; Thompson and England, 1984;Bohlen, 1991; Brown, 1993; Maruyama et al., 2010), whereasanticlockwise PeT paths involving isobaric cooling are interpreted

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K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320306

to be related to the intrusion and underplating of mantle-derivedmagma which may occur in intra-continental magmatic arcregions (Wells, 1980; Bohlen, 1987, 1991), hot spots related tomantle plumes (Bohlen, 1991) and incipient rift environments(Sandiford and Powell, 1986). Thus, combined with lithological,structural, geochemical and geochronological data, metamorphicPeT paths can be used to infer the tectonic settings and evolutionof metamorphic terrains. This is the case with the North ChinaCraton where extensive investigations of metamorphic PeT pathshave been carried out, especially on the two Paleoproterozoictectonic belts, named the Khondalite Belt (also called the InnerMongolia Suture Zone; Santosh, 2010) and the Trans-North ChinaOrogen (Jin, 1989; Cui et al., 1991; Jin et al., 1991; Zhai et al., 1992;Li, 1993; Sun et al., 1993; Chen et al., 1994; Ge et al., 1994; Mei,1994; Liu, 1995, 1996; Zhao et al., 2000, 2007a, b, 2008a, b; Liuet al., 2006, 2011a, b, 2012a, b, c, d, e; Yin et al., 2009, 2011). Theresults of these investigations have shown that the metamorphicevolution of the Khondalite Belt and the Trans-North China Orogenare mainly characterized by clockwise PeT paths involvingisothermal decompression, which reflect continent-continentcollisional environments. This has led Zhao et al. (2001, 2005) topropose that the Khondalite Belt and Trans-North China Orogenrepresented two distinct continent-continent collisional belts, ofwhich the former formed by the amalgamation of the Yinshan andOrdos Blocks to form the Western Block, which then collided withthe Eastern Block to form the Trans-North China Orogen (Fig. 1).Now there is a coherent outline of the tectonic processes involvedin the formation and evolution of the Paleoproterozoic KhondaliteBelt and Trans-North China Orogen in the North China Craton,though it still remains controversial regarding the subductionpolarity and timing of final collision in these belts (Guo et al., 2002,

Figure 1. Tectonic subdivision of the No

2005, 2012; Wilde et al., 2002; Kusky and Li, 2003; Kröner et al.,2005a, b, 2006; Santosh et al., 2006, 2007a, b, 2008, 2009a, b,2010; Zhao et al., 2006a, b, 2009, 2010, 2011; Faure et al., 2007;Kusky et al., 2007; He et al., 2009, 2010a, b; Santosh, 2010; Santoshand Kusky, 2010; Kusky, 2011; Trap et al., 2011, 2012; Zhaiand Santosh, 2011; Zhai et al., 2011; Sun et al., 2012; Yang et al.,2012).

Comparatively, few investigationsofmetamorphicPeTpathshavebeen carried out on Archean blocks separated by the Khondalite Beltand Trans-North China Orogen in the North China Craton. AlthoughZhao et al. (1998, 1999) summarized the major petrological featuresand PeT paths of some Archean complexes in the Eastern andWestern Blocks, and concluded that their metamorphic evolution ischaracterized by anticlockwise PeT paths involving isobaric cooling,previous studiesweremainly based on calculations of inconsistent orout-of-date traditional geothermobarometry. Thus, it still remainsunclear whether these anticlockwise PeT paths really reflect themetamorphic evolution of Archean complexes in the North ChinaCraton or are just artifacts of the traditional geothermobarometers.Furthermore, not all Archean complexes in the North China Cratonhave been well studied in terms of their metamorphic evolution,which has hampered the further understanding of formation andevolution of Archean blocks in the North China Craton.

In this paper, we present detailed petrographic, mineral chem-ical and pseudosection modeling studies on the Neoarcheangarnet-bearing amphibolites from the northern Liaoning Complexof the Eastern Block, whose metamorphic evolution has not beenstudied before. The inferred PeT path, in combination with avail-able lithological, geochemical and geochronological data, placesimportant constraints on the late Archean evolution of the EasternBlock of the North China Craton.

rth China Craton (Zhao et al., 2005).

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320 307

2. Regional geology

The Archean to Palaeoproterozoic basement of the North ChinaCraton has been divided into four Archean to Palaeoproterozoicmicrocontinental blocks, named the Yinshan, Ordos, Longgang andNangrim Blocks, of which the Yinshan and Ordos Blocks were fusedalong the EW-trending Khondalite Belt to form the Western Blockat 1.95e1.92 Ga (Dong et al., 2007; Santosh et al., 2007a, b; Yin et al.,2009, 2011; Li et al., 2011a; Peng et al., 2011, 2012a; Tsunogae et al.,2011; Dan et al., 2012; Zhao and Zhai, in press; Zhang et al., 2012a,b; Zheng et al., 2013), the Longgang and Nangrim Blocks amal-gamated along the Jiao-Liao-Ji Belt to form the Eastern Block atw1.90 Ga (Li et al., 2004a, b, 2005, 2006, 2011b, 2012; Luo et al.,2004, 2008; Lu et al., 2006; Li and Zhao, 2007; Zhou et al., 2008;Tam et al., 2011, 2012a, b, c; Zhao and Cawood, 2012; Zhao and Guo,2012), and finally theWestern and Eastern Blocks collided along theTrans-North China Orogen to form the coherent basement of theNorth China Craton at w1.85 Ga (Zhao et al., 2001, 2003a, b, 2005;Guo et al., 2002, 2005; Kröner et al., 2005a, b, 2006; Zhang et al.,2006, 2007, 2009, 2011, 2012a; Lu et al., 2008; Li et al., 2011c;Zhao et al., 2012). Detailed lithological, geochemical, structural,metamorphic and geochronological differences between thesemicrocontinental blocks and associated Palaeoproterozoic colli-sional belts have been summarized by Zhao et al. (2001, 2005) andare not repeated here.

Located in the Archean Longgang Block, the northern LiaoningComplex occupies w15,000 km2 and covers the Fushun, Qingyuanand Xinbin areas in Liaoning Province. It appears as a compositedome composed by oval structures with various scales (Fig. 2).The oval structures are defined by foliation or supracrustal lenses.The dome is principally made up of three major rock assemblages,including pretectonic TTG gneisses which account for morethan 80%, syntectonic granitoids and supracrustal rocks. The

Figure 2. Geological map of the northern Liaoning Complex

supracrustal rocks are composed of ultramafic to felsic volcanicrocks and sedimentary rocks metamorphosed from amphibolite togranulite facies (Zhai et al., 1985; Shen et al., 1994; Zhao et al., 1998).Besides, it holds plentiful gold, copper, zinc, iron and other mineralresources (Li et al., 1998, 1999).

In the 1980s and 1990s, the Archean basement of northernLiaoning Complex was thought to be composed of a late Archeangranite-greenstone belt and a mid Archean high-grade meta-morphic terrain. The late Archean granite-greenstone belt consistsof the supracrustal Qingyuan Group and TTG granitic rocks in theareas north of the Hunhe Fault, while the mid Archean high-grademetamorphic terrain is composed of the supracrustal HunnanGroup and TTG gneisses in the areas south of the Hunhe Fault(Wang et al., 1987; Shen et al., 1994; Wu et al., 1998).

However, according to Wan et al. (2005a, b), the ages, rockassemblages and geochemistry of the Hunan and Qingyuan Groupsare very similar. Both of them are dominated by TTG rocks andmetamorphic volcanic-sedimentary formations at amphibolitefacies to granulite facies. Besides, zircons of magmatic origin fromamphibole granulites of the Hunnan Group yielded SHRIMP UePbages of w2.52 Ga. Therefore, the Hunnan Group should not bea mid-Archean terrain as previously thought (Wan et al., 2005a, b).As a result, Wan et al. (2005a, b) combined the two groups into one,called the Qingyuan Group, and interpreted it as a late Archeancomponent.

The supracrustal rocks of the newly established Qingyuan Groupare mainly exposed in the Xiaolaihe, Tangtu, Tonghe and northernQingyuan areas. They consist of interbedded plagioclase amphib-olite, hornblende granulite, biotite granulite and BIF, which origi-nated in ultramafic to felsic volcanic rocks and relevant volcanic-sedimentary rocks. Metasedimentary rocks are dominantlydistributed in the Tongtu and Hongtoushan areas, and arecomposed of various granulites, including, biotite granulite, mica

(modified from Sun et al., 1993; Wan et al., 2005a, b).

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320308

granulite, garnet-kyanite-mica granulite, sillimanite-biotite gran-ulite, etc (Zhai et al., 1985; Shen et al., 1994; Zhao et al., 1998; Wanet al., 2005a, b).

The ages of the rocks and metamorphic events in the NorthernLiaoning Complex have been poorly constrained until recently.Previous ages were mainly based on Ar-Ar and Sm-Nd analyses ofminerals and whole-rock samples and conventional multigrainU-Pb zircon analyses (Zhai et al., 1985; Pecutt and Jahn,1986; Wanget al., 1987; Shen et al., 1994; Li et al., 2000; Wan et al., 2005a).Amphibolites in the northern Liaoning Complex were dated at2.99 Ga by using the amphibole 40Ar/39Ar dating technique (Wanget al., 1987) and at 3.02 � 0.02 Ga by using the SmeNd whole-rockisochron method (Li et al., 2000), which were regarded as the mainevidence for the existence of themid-Archean rocks in the northern

Figure 3. Photomicrographs showing the mineral assemblages and textures of the amphinclusions preserved in garnet (plane-polarized); (b) Clinopyroxene grains in matrix (cross-ppolarized); (e) M2 garnet surrounded by the M3 garnet (plane-polarized); (f) Clinopyroxen

Liaoning Complex. Moreover, the intrusive and eruptive phasesrepresented by the pre-tectonic TTG gneisses and mafic volcanicrocks, respectively, were dated at 2880 � 170 Ma by using theconventional zircon U-Pb methods (Zhai et al., 1985) and at2844 � 48 Ma by using the Sm-Nd whole-rock isochron method(Shen et al., 1994), respectively. Furthermore, syntectonic graniteswere dated at 2551 � 1 Ma (Pecutt and Jahn, 1986) and2519 � 77 Ma (Shen et al., 1994) by using single-zircon U-Pbmethods, which were interpreted as reflecting the time of theregional metamorphism of the northern Liaoning Complex.Recently, Wan et al. (2005a) applied the SHRIMP U-Pb zircon datingtechnique to the basement rocks of the northern Liaoning Complex,and the results show that the supracrstal rocks weremainly formedat 2.56e2.51 Ga, followed by the intrusion of TTG granites. During

ibolites from the northern Liaoning Complex. (a) Plagioclase, quartz and hornblendeolarized); (c) Matrix minerals (cross-polarized); (d) Garnet-quartz symplectite (plane-e with hornblende retrograde rims at the rims of garnet (plane-polarized).

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320 309

2.52e2.47 Ga, the supracrustal rocks and TTG gneisses experienceda regional metamorphic event. These dates confirm that the majorlithologies of the northern Liaoning Complex were formed andmetamorphosed in the late Archean. The timings of the aboveevents are also consistend to the latest geochronological study ofthe Paleoproterozoic Jianping Complex in the western LiaoningProvince which is just located at the east of the northern LiaoningComplex by Liu et al. (2011).

3. Petrography

Amphibolites in the northern Liaoning Complex consist mainlyof plagioclase, hornblende, garnet with occasional clinopyroxeneand epidote and minor sphene and ilmenite. Three distinct meta-morphic stages represented by different mineral assemblages havebeen distinguished based on mineral interrelationships and reac-tion texture.

3.1. Pre-peak stage (M1)

Mineral inclusions preserved within porphyroblastic garnet in theamphiboites of the northern Liaoning Complex mark the pre-peakstage of metamorphism (M1). The mineral inclusions are mainlyquartz, plagioclase and hornblende with minor amount of actinolite,epidote and sphene (Fig. 3a). Thus, themineral assemblage of the pre-peak metamorphism is quartz þ plagioclase þ actinotite þhorbblende þ epidote þ sphene.

3.2. Peak stage (M2)

The M2 stage is marked by the growth of garnet porphyroblasts.Following the pre-peak stage of metamorphism, garnet startedgrowing and enclosed theminerals formedduring the pre-peak stage,like quartz, plagioclase and hornblende. Apart from garnet, the peakmetamorphism is also indicated by the appearance of coarse-grainedclinopyroxene, together with plagioclase, hornblende, quartz andminor ilmenite in the matrix (Fig. 3b, c). Therefore, the mineralassemblage of thepeak stage ofmetamorphismcanbe representedbygarnetþ clinopyroxene þ plagioclaseþ hornblendeþ quartz þ ilmenite. The garnet porphyroblasts might be produced through thefollowing reaction:

epidoteþ quartz ¼ plagioclaseþ garnetþ fluid (1)

The appearance of clinopyroxene could be explained by thefollowing metamorphic reaction (Zhao et al., 1998, 1999):

plagioclaseþ hornblende ¼ garnetþ diopsideþ quartzþ H2O(2)

3.3. Post-peak stage (M3)

The M3 stage is represented by symplectitic intergrowths ofgarnet and quartz around the peak-stage minerals such as clino-pyroxene and plagioclase (Fig. 3d). The garnet-quartz symplectite isthought to have formed due to the isobaric or near-isobaric coolingmetamorphic processes subsequent to the peak metamorphism(Zhao et al., 1998, 1999).

Apart from the garnet-quartz symplectites, as shown in Fig. 3e,the inclusion-free M2 garnet is found to be surrounded by the M3garnet which shows more quartz inclusions and looks like thegarnet þ quartz symplectites nearby. This structure also suggeststhe existence of garnet forming during the post-peak stage.

In addition, some clinopyroxene grains in the matrix have horn-blende reaction rims (Fig. 3f),whichare considered tohave formedbythe retrogression of clinopyroxene into hornblende during the post-

peak metamorphism. Furthermore, amphibole þ epidote over-printing the peakminerals is also observed in some of the samples. Itis believed that amphibole þ epidote were formed during the post-peak stage of metamorphism and so they overprinted the peakminerals.

The formation of garnet þ quartz symplectites can be explainedby the following reaction (Zhao et al., 1998, 1999):

diopsideþ plagioclase ¼ garnetþ quartz (3)

The retrogression of clinopyroxene into hornblende could bedeveloped through the following reaction (Spear, 1993):

clinopyroxeneþplagioclaseþH2O¼hornblendeþquartz (4)

Thus, the representative assemblage of the post-peak meta-morphism should be garnet þ quartz þ clinopyroxene þplagioclase þ hornblende þ epidote þ sphene.

4. Mineral chemistry

Seven samples were chosen for analyzing mineral compositionsused for PeT calculations by using Electron Probe MicroAnalysis(EPMA). The EPMA was carried out by a JOEL JXA-8100 electronmicroprobe at two laboratories, Guangzhou Institute of Geochem-istry, Chinese Academy of Sciences and Key Laboratory of OrogenicBelts and Crustal Evolution, MOE, School of Earth and SpaceSciences, Peking University. The analysis done at GuangzhouInstitute of Geochemistry was performed with an acceleratingvoltage of 15 kV, a beam current of 20 nA, counting time of about20e30 s and ca. 2 mm spot size, while the analysis at the PekingUniversity was performed with an accelerating voltage of 15 kV,a beam current of 10 nA, counting time of about 40 s and ca. 1 mmspot size.

4.1. Garnet

In the northern Liaoning amphibolites, four textural types ofgarnet have been distinguished. Type I is the core of garnetincluding the M1 prograde minerals of plagioclase, quartz, epidoteand sphene. The Type II garnet is the rim of garnet in contact withthe matrix minerals such as plagioclase, quartz and hornblende.The Type III garnet is symplectitic gartnet (or garnet-quartz sym-plectite). The Type IV garnet is the rim of garnet in contact withinclusion-type clinopyroxene with hornblende retrograde rims.However, the Type III and IV garnets can be only found in threesamples. Representative chemical compositions of the garnet arepresented in Table 1.

For the samples which contain Type I, Type II, Type III and TypeIV, the garnet is dominated by almandine (48.0e51.1%) and gros-sular (31.3e42.5%), with minor spessartine (4.9e7.9%), pyrope(2.7e4.0%) and andradite (0e8.1%). However, no essential differ-ence in chemical composition is observed among these fourtextural types. In contrast, for those samples which only containType I and Type II, the garnet is dominated by almandine(65.3e74.1%), with minor grossular (9.5e19.3%), pyrope(5.7e19.4%), spessartine (1.5e9.1%) and andradite (0e2.5%). Simi-larly, no difference in chemical composition is found between TypeI and Type II.

From the above results, two groups of garnet can be defined. AsType III and Type IV are the textures which reflect the post-peakand retrograde metamorphic stages, respectively, it is believedthat the chemical composition of garnet in the group whichconsists of Type III and Type IV (Group 1) was homogenized duringthe post-peak or retrograde metamorphismwhile the garnet in thegroup which only Type I and Type II can be found (Group 2) just

Table 1Representative analysis of garnet.

Group 2

Type I II

Sample L72-2 L72-2 L86-16 L86-16 L86-17 L86-17 L72-2 L72-2 L76-11 L76-11 L86-16 L86-16

Texture Core Core Core Core Core Core Rim Rim Rim Rim Rim Rim

SiO2 37.23 37.38 37.27 37.79 37.66 37.98 36.97 37.22 37.69 37.19 37.46 37.53TiO2 0.050 0.040 0.020 0.030 0.07 0.03 0.08 0.060 0.000 0.010 0.000 0.080Al2O3 21.31 21.71 21.27 21.50 21.56 21.60 21.36 21.23 21.75 21.29 21.19 21.24Cr2O3 0.030 0.000 0.030 0.140 0.00 0.04 0.01 0.050 0.010 0.010 0.000 0.000Fe2O3 0.246 0.000 0.696 0.569 0.86 0.00 0.00 0.000 0.000 0.181 0.000 0.173FeO 28.76 28.11 29.24 29.78 29.09 29.43 28.84 29.80 31.97 32.71 31.00 30.54MnO 3.500 3.880 2.250 2.250 1.30 1.21 3.76 3.230 0.820 0.650 2.740 2.660NiO 0.050 0.060 0.000 0.000 0.00 0.02 0.00 0.000 0.000 0.020 0.000 0.000MgO 1.390 1.530 3.480 3.560 4.79 4.90 1.71 1.500 3.760 3.540 2.640 2.610CaO 6.730 6.470 5.280 5.220 4.27 4.32 6.33 6.490 3.340 3.480 4.580 5.050Na2O 0.200 0.080 0.020 0.020 0.14 0.00 0.06 0.040 0.060 0.070 0.040 0.120K2O 0.060 0.010 0.000 0.010 0.01 0.01 0.00 0.000 0.020 0.000 0.010 0.010Total 99.6 99.3 99.6 100.9 99.7 99.5 99.1 99.6 99.4 99.1 99.7 100.0Si 2.997 3.014 2.977 2.980 2.98 3.01 2.99 3.000 3.013 2.990 3.008 2.999Ti 0.003 0.002 0.001 0.002 0.00 0.00 0.00 0.004 0.000 0.001 0.000 0.005Al 2.021 2.063 2.003 1.998 2.01 2.01 2.04 2.017 2.049 2.018 2.006 2.001Cr 0.002 0.000 0.002 0.009 0.00 0.00 0.00 0.003 0.001 0.001 0.000 0.000Fe3þ 0.015 0.000 0.042 0.034 0.05 0.00 0.00 0.000 0.000 0.011 0.000 0.010Fe2þ 1.936 1.895 1.954 1.964 1.92 1.95 1.95 2.009 2.137 2.199 2.082 2.042Mn 0.239 0.265 0.152 0.150 0.09 0.08 0.26 0.221 0.056 0.044 0.186 0.180Ni 0.003 0.004 0.000 0.000 0.00 0.00 0.00 0.000 0.000 0.001 0.000 0.000Mg 0.167 0.184 0.414 0.418 0.56 0.58 0.21 0.180 0.448 0.424 0.316 0.311Ca 0.580 0.559 0.452 0.441 0.36 0.37 0.55 0.560 0.286 0.300 0.394 0.432Na 0.031 0.013 0.003 0.003 0.02 0.00 0.01 0.006 0.009 0.011 0.006 0.019K 0.006 0.001 0.000 0.001 0.00 0.00 0.00 0.000 0.002 0.000 0.001 0.001Total 8.000 8.000 8.000 8.000 8.00 8.00 8.00 8.000 8.000 8.000 8.000 8.000Alm 0.663 0.653 0.657 0.660 0.655 0.655 0.658 0.676 0.730 0.741 0.699 0.689Sps 0.082 0.091 0.051 0.051 0.030 0.027 0.087 0.074 0.019 0.015 0.063 0.061Prp 0.057 0.063 0.139 0.141 0.192 0.194 0.070 0.061 0.153 0.143 0.106 0.105Grs 0.191 0.193 0.131 0.131 0.097 0.123 0.185 0.189 0.098 0.095 0.132 0.141Adr 0.007 0.000 0.021 0.017 0.025 0.000 0.000 0.000 0.000 0.005 0.000 0.005XFe 0.921 0.912 0.825 0.824 0.773 0.771 0.904 0.918 0.827 0.838 0.868 0.868XCa 0.216 0.212 0.160 0.156 0.127 0.127 0.203 0.204 0.100 0.103 0.141 0.155

Group 1

Type III IV

Sample L73-4 L73-4 L73-5 L73-5 L73-5 L73-3 L73-4 L73-3

Texture Rim Rim Rim Rim Rim Rim Rim Rim

SiO2 37.54 37.62 37.62 37.88 37.36 36.81 37.62 37.55TiO2 0.030 0.040 0.170 0.060 0.040 0.097 0.100 0.210Al2O3 20.52 20.93 21.57 21.12 21.27 20.72 20.67 21.10Cr2O3 0.006 0.000 0.020 0.000 0.030 0.006 0.070 0.000Fe2O3 2.159 0.609 0.000 0.657 1.026 2.696 1.323 0.524FeO 21.90 21.53 22.58 22.61 21.89 21.70 22.14 21.83MnO 3.004 2.190 2.160 2.890 3.030 3.424 2.960 3.470NiO 0.121 0.010 0.060 0.070 0.010 0.000 0.000 0.000MgO 0.807 0.680 0.890 0.820 0.980 0.804 0.720 0.700CaO 14.30 15.54 13.89 14.09 13.65 13.62 14.21 14.27Na2O 0.009 0.010 0.130 0.050 0.090 0.008 0.070 0.040K2O 0.021 0.030 0.030 0.000 0.030 0.003 0.040 0.010Total 100.4 99.2 99.1 100.2 99.4 99.9 99.9 99.7Si 2.977 2.999 2.996 2.996 2.976 2.939 2.991 2.986Ti 0.002 0.002 0.010 0.004 0.002 0.006 0.006 0.013Al 1.917 1.966 2.025 1.969 1.997 1.950 1.937 1.978Cr 0.000 0.000 0.001 0.000 0.002 0.000 0.004 0.000Fe3þ 0.129 0.037 0.000 0.039 0.062 0.162 0.079 0.031Fe2þ 1.452 1.435 1.504 1.496 1.458 1.449 1.472 1.452Mn 0.202 0.148 0.146 0.194 0.204 0.232 0.199 0.234Ni 0.008 0.001 0.004 0.004 0.001 0.000 0.000 0.000Mg 0.095 0.081 0.106 0.097 0.116 0.096 0.085 0.083Ca 1.215 1.327 1.185 1.194 1.165 1.165 1.211 1.216Na 0.001 0.002 0.020 0.008 0.014 0.001 0.011 0.006K 0.002 0.003 0.003 0.000 0.003 0.000 0.004 0.001Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000Alm 0.490 0.480 0.511 0.502 0.495 0.493 0.496 0.486Sps 0.068 0.049 0.050 0.065 0.069 0.079 0.067 0.078Prp 0.032 0.027 0.036 0.032 0.040 0.033 0.029 0.028Grs 0.345 0.425 0.403 0.381 0.364 0.313 0.368 0.392Adr 0.064 0.018 0.000 0.020 0.031 0.081 0.040 0.016

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320310

Table 1 (continued )

Group 1

Type III IV

Sample L73-4 L73-4 L73-5 L73-5 L73-5 L73-3 L73-4 L73-3

Texture Rim Rim Rim Rim Rim Rim Rim Rim

XFe 0.938 0.947 0.934 0.939 0.926 0.938 0.945 0.946XCa 0.440 0.467 0.424 0.429 0.425 0.430 0.437 0.442

Group 1, Garnets in the samples which consist of Type I, II, III and IV; Type III, symplectitic gartnet; Type IV, rims of garnet in contact with clinoproxene with hornblenderetrograde rims. Cations are calculated based on 12 oxygens. Alm ¼ Fe2þ/(Mn þ CaþMg þ Fe2þ); Prp¼Mg/(Mnþ CaþMgþ Fe2þ); Grs¼ (Ca e 3dr)/(Mnþ CaþMgþ Fe2þ);Sps ¼ Mn/(Mn þ Ca þ Mg þ Fe2þ); Adr ¼ Fe3þ/2; XFe ¼ Fe2þ/(Mg þ Fe2þ); XCa ¼ Ca/(Ca þ Mg þ Fe2þ).Group 2, Garnets in the samples which only consist of Type I and II; Type I, cores of garnet including prograde mineral inclusions, including plagioclase, quartz, epidote andsphene; Type II, rims of garnet in contact with the matrix, such as plagioclase, quartz and hornblende. Cations are calculated based on 12 oxygens. Alm ¼ Fe2þ/(Mnþ CaþMgþ Fe2þ); Prp¼Mg/(Mnþ CaþMgþ Fe2þ); Grs¼ (Ca e 3dr)/(Mnþ CaþMgþ Fe2þ); Sps¼Mn/(Mnþ CaþMgþ Fe2þ); Adr¼ Fe3þ/2; XFe ¼ Fe2þ/(Mgþ Fe2þ);XCa ¼ Ca/(Ca þ Mg þ Fe2þ).

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320 311

recorded the peak metamorphism. Therefore, the data of Types Iand II and the data of Types III and IV used for PeT calculationsafterward are adopted from Group 2 and Group 1, respectively.

In terms of end members of garnet, the composition variation ofgarnet from Types I and II to Types III and IV is characterized bya significantly decrease of almandine and pyrope from 65.3e74.1%to 48.0e51.1% and from 5.7e19.4% to 2.7e4.0%, respectively, and anapparent increase of grossular from 9.5e19.3% to 31.3e42.5%. Onthe other hand, compared with Types I and II, Types III and IV showan obvious rise in both XFe and XCa from 0.771e0.921 to0.926e0.947 and from 0.100e0.216 to 0.424e0.467 respectively(XFe ¼ Fe2þ/(Mg þ Fe2þ); XCa ¼ Ca/(Ca þ Mg þ Fe2þ)).

4.2. Clinopyroxene

There are two major types of clinopyroxene, which are Type I:matrix-type clinopyroxene and Type II: clinopyroxene with

Table 2Representative analysis of clinopyroxene.

Type I

Sample L73-3 L73-3 L73-3 L73-4 L73-4 L76-12

Texture Core Core Core Core Core Core

SiO2 49.31 49.67 49.70 49.79 49.69 50.19TiO2 0.076 0.032 0.104 0.000 0.064 0.039Al2O3 0.904 0.557 0.992 0.677 0.835 0.757Cr2O3 0.004 0.000 0.000 0.040 0.062 0.000Fe2O3 3.299 3.321 3.215 2.972 2.885 1.395FeO 14.71 14.14 13.66 15.03 14.33 16.15MnO 0.290 0.219 0.244 0.238 0.225 0.309NiO 0.072 0.000 0.000 0.090 0.097 0.020MgO 6.520 7.008 6.956 6.535 6.912 6.394CaO 24.26 24.47 24.53 24.43 24.33 24.13Na2O 0.270 0.253 0.385 0.253 0.283 0.268K2O 0.000 0.013 0.000 0.018 0.005 0.004Total 99.72 99.68 99.79 100.10 99.72 99.65Si 1.938 1.947 1.941 1.950 1.946 1.971Ti 0.002 0.001 0.003 0.000 0.002 0.001Al 0.042 0.026 0.046 0.031 0.039 0.035Cr 0.000 0.000 0.000 0.001 0.002 0.000Fe3þ 0.098 0.098 0.095 0.088 0.085 0.041Fe2þ 0.484 0.464 0.446 0.492 0.469 0.530Mn 0.010 0.007 0.008 0.008 0.007 0.010Ni 0.002 0.000 0.000 0.003 0.003 0.001Mg 0.382 0.410 0.405 0.382 0.404 0.374Ca 1.022 1.028 1.027 1.025 1.021 1.015Na 0.021 0.019 0.029 0.019 0.021 0.020K 0.000 0.001 0.000 0.001 0.000 0.000Total 4.000 4.000 4.000 4.000 4.000 4.000XMg 0.441 0.469 0.476 0.437 0.462 0.414

Type I, matrix-type clinopyroxene; Type II, the rim of the matrix-type clinopyroxene with(Mg þ Fe2þ).

hornblende retrograde rims in the matrix. All types of clinopyrox-ene are diopside. Some clinopyroxene grains have been retrogradedinto hornblende during the retrograde metamorphism followingthe post-peak metamorphism (M3). The clinopyroxene grains withretrograded hornblende are all surrounded by the garnet-quartzsymplectites or appear at the rim of garnet. As shown in Table 2,the variation in chemical compositions between Type I and Type IIhas an apparent difference. Compared with Type I, the value of XMg(¼Mg/(Mg þ Fe2þ)) of Type II show a great increase from0.414e0.476 to 0.0.497e0.533.

4.3. Hornblende

Hornblende occurs as three different textural types in theexamined samples, including: inclusion-type hornblende enclosedin garnet (Type I), matrix-type hornblende (Type II), hornblendeformed at the rims surrounding the matrix-type clinopyroxene

II

L86-11 L73-3 L73-4 L73-4 L73-5 L73-5

Core Rim Rim Rim Rim Rim

49.79 50.30 52.37 50.66 51.99 51.090.000 0.052 0.028 0.035 0.080 0.0800.677 0.477 0.431 0.574 0.450 0.5300.040 0.025 0.000 0.000 0.000 0.0202.972 2.516 0.000 1.980 0.000 1.20915.03 13.84 14.35 13.48 14.17 14.710.238 0.261 0.227 0.302 0.350 0.3600.090 0.077 0.033 0.000 0.070 0.0006.535 7.667 8.347 8.146 9.090 8.34024.43 24.49 24.96 24.31 23.35 23.390.253 0.204 0.190 0.248 0.240 0.2700.018 0.010 0.000 0.000 0.020 0.000100.07 99.92 100.90 99.74 99.81 100.001.950 1.958 2.001 1.966 2.003 1.9780.000 0.002 0.001 0.001 0.002 0.0020.031 0.022 0.019 0.026 0.020 0.0240.001 0.001 0.000 0.000 0.000 0.0010.088 0.074 0.000 0.058 0.000 0.0350.492 0.451 0.459 0.438 0.456 0.4760.008 0.009 0.007 0.010 0.011 0.0120.003 0.002 0.001 0.000 0.002 0.0000.382 0.445 0.475 0.471 0.522 0.4811.025 1.021 1.022 1.011 0.964 0.9700.019 0.015 0.014 0.019 0.018 0.0200.001 0.000 0.000 0.000 0.001 0.0004.000 4.000 4.000 4.000 4.000 4.0000.437 0.497 0.509 0.519 0.533 0.503

hornblende retrograde rims. Cations are calculated based on 6 oxygens. XMg ¼Mg/

Table 3Representative analysis of hornblende.

Type I II III

Sample L73-3 L73-4 L73-3 L73-3 L73-3 L73-4 L73-4 L73-3 L73-4 L73-5

Texture Inclusion Inclusion Core Core Core Core Core Rim Rim Rim

SiO2 43.52 44.82 44.44 43.85 44.89 43.49 43.17 43.94 44.21 44.30TiO2 0.414 0.224 0.402 0.432 0.390 0.562 0.581 0.300 0.450 0.410Al2O3 10.74 10.23 11.14 11.51 10.24 12.40 12.74 11.12 11.15 10.93Cr2O3 0.062 0.002 0.010 0.000 0.012 0.021 0.020 0.000 0.000 0.000FeO 23.07 22.99 23.06 23.91 22.79 23.35 22.71 22.84 23.38 21.19MnO 0.211 0.201 0.216 0.246 0.210 0.261 0.206 0.187 0.189 0.300NiO 0.000 0.150 0.060 0.000 0.016 0.137 0.000 0.000 0.141 0.000MgO 6.105 6.842 6.696 6.351 6.706 6.167 6.153 6.083 6.217 6.870CaO 12.61 12.53 12.20 12.31 12.72 12.39 12.41 12.46 12.55 11.48Na2O 0.984 0.977 1.063 1.188 0.819 1.394 1.263 1.054 1.044 0.940K2O 0.715 0.575 0.661 0.666 0.753 0.824 0.878 0.768 0.852 0.600Total 98.43 99.54 99.94 100.5 99.54 101.0 100.13 98.75 100.2 97.02Si 6.864 6.975 6.880 6.762 6.996 6.651 6.652 6.895 6.845 7.046Ti 0.049 0.026 0.047 0.050 0.046 0.065 0.067 0.035 0.052 0.049Al 1.995 1.876 2.032 2.092 1.880 2.235 2.313 2.057 2.034 2.049Cr 0.008 0.000 0.001 0.000 0.001 0.003 0.002 0.000 0.000 0.000Fe2þ 3.043 2.992 2.986 3.084 2.970 2.986 2.927 2.997 3.027 2.819Ni 0.000 0.019 0.007 0.000 0.002 0.017 0.000 0.000 0.018 0.000Mg 1.435 1.587 1.545 1.460 1.558 1.406 1.413 1.423 1.435 1.629Ca 2.131 2.089 2.023 2.034 2.123 2.031 2.049 2.094 2.082 1.956Na 0.301 0.295 0.319 0.355 0.247 0.413 0.377 0.321 0.313 0.290K 0.144 0.114 0.131 0.131 0.150 0.161 0.173 0.154 0.168 0.122Total 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00XMg 0.320 0.347 0.341 0.321 0.344 0.320 0.326 0.322 0.322 0.366

Type I, inclusion-type enclosed hornblende in the garnet; Type II, matrix-type hornblende; Type III, the rims of retrograded hornblende surrounding the matrix-typeclinopyroxene. Cations are calculated on 23 oxygens. XMg ¼ Mg/(Mg þ Fe2þ).

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320312

(Type III). Nevertheless, according to Table 3, no obvious dissimi-larity is found among the chemical composition of different typesof hornblende. The XMg value of the three textual types range from0.320 to 0.366 (XMg ¼ Mg/(Mg þ Fe2þ)).

Table 4Representative analysis of plagioclase.

Type I II

Sample L86-17 L86-17 L86-17 L73-3 L73-5 L76-11

Details Inclusion Inclusion Inclusion Core Core Core

SiO2 59.00 59.26 58.37 56.30 56.37 57.07TiO2 0.000 0.020 0.000 0.000 0.000 0.000Al2O3 25.48 25.71 26.55 26.70 26.75 27.44Cr2O3 0.000 0.000 0.000 0.030 0.010 0.000Fe2O3 0.030 0.040 0.270 0.020 0.070 0.040MnO 0.000 0.000 0.040 0.000 0.000 0.030NiO 0.000 0.030 0.000 0.000 0.000 0.010MgO 0.010 0.000 0.000 0.000 0.000 0.030CaO 7.230 7.070 7.430 9.290 9.320 8.930Na2O 7.900 7.930 7.180 6.700 6.540 6.930K2O 0.060 0.030 0.170 0.120 0.110 0.100Total 99.71 100.10 100.00 99.17 99.17 100.60Si 2.629 2.631 2.605 2.554 2.555 2.533Ti 0.000 0.001 0.000 0.000 0.000 0.000Al 1.338 1.345 1.397 1.427 1.429 1.435Cr 0.000 0.000 0.000 0.001 0.000 0.000Fe3þ 0.001 0.001 0.010 0.001 0.003 0.001Mn 0.000 0.000 0.002 0.000 0.000 0.001Ni 0.000 0.001 0.000 0.000 0.000 0.000Mg 0.001 0.000 0.000 0.000 0.000 0.002Ca 0.345 0.336 0.355 0.451 0.453 0.425Na 0.683 0.683 0.621 0.589 0.575 0.596K 0.003 0.002 0.010 0.007 0.006 0.006Total 5.000 5.000 5.000 5.000 5.000 5.000Ab 0.662 0.669 0.630 0.562 0.556 0.581An 0.335 0.330 0.360 0.431 0.438 0.414Or 0.003 0.002 0.010 0.007 0.006 0.006XCa 0.336 0.330 0.364 0.434 0.441 0.416

Type I, inclusion-type plagioclase preserved within garnet; Type II, matrix-type. CationsOr ¼ K/(Ca þ Na þ K); XCa ¼ Ca/(Ca þ Na).

4.4. Plagioclase

Plagioclase can be divided into two textural types of which Type Iis inclusion-type plagioclase preserved within garnet and Type II is

L76-11 L86-16 L86-16 L86-16 L86-17 L86-17 L86-17

Core Core Core Core Core Core Core

57.34 57.06 56.77 56.65 56.76 56.84 57.180.000 0.010 0.070 0.010 0.000 0.020 0.01027.05 26.34 27.14 26.83 26.91 26.88 27.070.010 0.000 0.020 0.000 0.000 0.000 0.0400.100 0.030 0.000 0.020 0.040 0.060 0.0700.010 0.000 0.000 0.000 0.030 0.020 0.0000.000 0.020 0.030 0.000 0.000 0.000 0.0100.010 0.020 0.000 0.000 0.050 0.010 0.0008.630 8.590 8.680 9.120 8.360 9.120 9.0406.860 6.920 6.700 6.540 7.650 6.660 6.7400.090 0.030 0.040 0.070 0.240 0.130 0.120100.10 99.02 99.44 99.23 100.00 99.74 100.302.560 2.574 2.553 2.556 2.518 2.550 2.5500.000 0.000 0.002 0.000 0.000 0.001 0.0001.423 1.400 1.438 1.426 1.407 1.421 1.4230.000 0.000 0.001 0.000 0.000 0.000 0.0010.004 0.001 0.000 0.001 0.001 0.002 0.0030.000 0.000 0.000 0.000 0.001 0.001 0.0000.000 0.001 0.001 0.000 0.000 0.000 0.0000.001 0.001 0.000 0.000 0.003 0.001 0.0000.413 0.415 0.418 0.441 0.397 0.438 0.4320.594 0.605 0.584 0.572 0.658 0.579 0.5830.005 0.002 0.002 0.004 0.014 0.007 0.0075.000 5.000 5.000 5.000 5.000 5.000 5.0000.587 0.592 0.581 0.563 0.616 0.565 0.5700.408 0.406 0.416 0.434 0.372 0.428 0.4230.005 0.002 0.002 0.004 0.013 0.007 0.0070.410 0.407 0.417 0.435 0.377 0.431 0.426

are calculated based on 8 oxygens. An ¼ Ca/(Ca þ Na þ K); Ab ¼ Na/(Ca þ Na þ K);

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320 313

matrix-type. However, only few plagioclase inclusions are found.Manymatrix-type plagioclase grains showa typical twinning opticalproperty. Representative chemical compositions of plagioclase arelisted inTable 4. Both types of plagioclase are dominatedbyanorthiteand albite,withminor orthoclase. However, comparedwith the TypeII plagioclase (Ab ¼ 55.6e61.6, An ¼ 37.2e43.8, Or ¼ 0.2e1.3), theType I plagioclase has higher contents in albite but lower contents inanorthite (Ab ¼ 63.0e66.9, An ¼ 33.0e36.0, Or ¼ 0.3e1.0).

4.5. Epidote

Similar to plagioclase, two textural types of epidote are observedin the amphibolites: pre-peak inclusions preserved in garnet (TypeI) and post-peak minerals (Type II). Epidote can be only found insome of the samples and epidote inclusions are rare in thosesamples. According to the chemical compositions of epidote pre-sented in Table 5, there is no essential difference in chemicalcompositions between the two types of epidote. The values of XAlrange from 0.801 to 0.823 (XAl ¼ Al/(Al þ Fe3þ)).

5. Pseudosection modeling

5.1. Methods of pseudosection modeling

Pseudosection can be built to approximate the phase relation-ships in rocks (Powell et al., 1998), by calculating the equilibrium ofminerals which involves solid solutions in complex systems (Spearand Cheney, 1989; Powell and Holland, 1990; Mahar et al., 1997) onthe basis of the enormous internally-consistent thermodynamicdatasets (e.g. Helgeson et al.,1978; Holland and Powell,1985; Powelland Holland, 1985; Berman, 1988; Holland and Powell, 1990;Gottschalk, 1997). In this paper, the PeT pseudosections for theamphibolites from the northern Liaoning Complex are calculatedbased on the NCFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) system. K2O and MnO are ignored in this system

Table 5Representative analysis of epidote.

Type I II

Sample L73-4 L73-4 L73-3 L73-3

Texture Inclusion Inclusion Core Core

SiO2 38.22 38.91 38.83 38.82TiO2 0.110 0.032 0.000 0.117Al2O3 26.69 26.48 25.98 26.36Cr2O3 0.020 0.004 0.029 0.000Fe2O3 8.670 9.135 9.093 8.964MnO 0.090 0.070 0.059 0.032NiO 0.020 0.000 0.165 0.097MgO 0.040 0.009 0.015 0.013CaO 23.03 24.99 24.99 24.90Na2O 0.090 0.006 0.019 0.000K2O 0.030 0.000 0.017 0.010Total 97.01 99.64 99.19 99.32Si 2.996 2.975 2.983 2.978Ti 0.006 0.002 0.000 0.007Al 2.466 2.386 2.353 2.383Cr 0.001 0.000 0.002 0.000Fe3þ 0.568 0.584 0.584 0.575Mn 0.006 0.005 0.004 0.002Ni 0.001 0.000 0.010 0.006Mg 0.005 0.001 0.002 0.001Ca 1.934 2.047 2.058 2.047Na 0.014 0.001 0.003 0.000K 0.003 0.000 0.002 0.001Total 8.000 8.000 8.000 8.000XFe 0.813 0.803 0.801 0.806

Type I, pre-peak inclusions preserved in garnet; Type II, post-peak minerals. Cations are

since theamphibolites are just composedof very little amountof K2Oand MnO.

The bulk rock composition of the samples necessary for mineralequilibrium calculations was acquired by using X-Ray Fluorescence(XRF) whole rock analysis. It was conducted in the Department ofEarth Science, the University of Hong Kong. A representative sample(L73-7) is chosen for the analysis and its bulk rock composition isSiO2 ¼ 51.359, TiO2 ¼ 1.237, Al2O3 ¼ 15.165, FeOt ¼ 11.277,MnO ¼ 0.163, MgO ¼ 4.688, CaO ¼ 11.212, Na2O ¼ 2.385,K2O¼ 0.430, P2O5 ¼ 0.310 (inwt %). As it is found that the bulk-rockcomposition stated above is quite similar to the typical mid-oceanicbasalt (MORB) composition (sample 104-16 of Sun andMcDonough,1989) which was also used for PeT pseudosection construction byDiener et al. (2007). Therefore, 12% of FeO is adopted to assume theproportion of Fe2O3 present in the amphibolites as is a typical valuefor wet-chemical analyses of MORB. After recalculation, the bulkrock composition in mol% is SiO2 ¼ 56.519, TiO2 ¼ 1.024,Al2O3 ¼ 10.224, FeO ¼ 8.218, MgO ¼ 7.690, CaO ¼ 13.220,Na2O ¼ 2.544, O ¼ 0.560. It is noted that all the 8 samples havesimilar bulk rock composition and so only one sample is used for theconstruction of PeT pseudosections.

The PeT pseudosections in this paper were calculated byTHERMOCALC 3.33 (Powell et al., 1998) with the DATASET 5.5(Holland and Powell, 1998; Nov. 2003). The thermodynamicmodelsand a-X relationships for the minerals used in the calculation areadopted from different references and they are listed as follows:amphibole (Diener et al., 2007), clinopyroxenes (Green et al., 2007),chlorite (Holland et al., 1998), garnet (White et al., 2007), epidote(Holland and Powell, 1998), plagioclase (Holland and Powell, 2003),orthopyroxene (White et al., 2002), magnetite, ilmenite andhematite (White et al., 2000). The calculation for the PeT pseu-dosections was done on the basis of quartz and the fluid phasewhich is assumed to be pure H2O to be set in excess. Furthermore,due to the presence of hornblende, the melt phase would forma very small proportion of the rocks and so it is neglected duringcalculation (Poli and Schmidt, 2002). Besides, there is no suitable

L73-3 L73-4 L73-4 L73-4 L73-4

Core Core Core Core Core

38.88 38.35 38.30 38.76 38.860.048 0.039 0.012 0.090 0.14026.50 26.83 25.87 27.08 26.840.002 0.014 0.000 0.000 0.0008.710 8.360 8.866 8.230 8.4500.021 0.062 0.994 0.050 0.0600.000 0.039 0.000 0.040 0.0100.029 0.012 0.006 0.030 0.02025.06 24.95 24.88 23.28 23.310.023 0.044 0.062 0.000 0.0300.014 0.008 0.014 0.020 0.04099.29 98.71 99.00 97.58 97.762.979 2.952 2.950 3.019 3.0230.003 0.002 0.001 0.005 0.0082.393 2.433 2.349 2.486 2.4610.000 0.001 0.000 0.000 0.0000.558 0.538 0.571 0.536 0.5500.001 0.004 0.065 0.003 0.0040.000 0.002 0.000 0.003 0.0010.003 0.001 0.001 0.003 0.0022.057 2.058 2.053 1.943 1.9430.003 0.007 0.009 0.000 0.0050.001 0.001 0.001 0.002 0.0048.000 8.000 8.000 8.000 8.0000.811 0.819 0.804 0.823 0.817

calculated based on 12.5 oxygens. XAl ¼ Al/(Al þ Fe3þ).

Figure 4. PeT pseudosection in the NCFMASHTO system calculated for an amphibolites sample (L73-7) with bulk composition SiO2 ¼ 56.519, TiO2 ¼ 1.024, Al2O3 ¼ 10.224,FeO ¼ 8.218, MgO ¼ 7.690, CaO ¼ 13.220, Na2O ¼ 2.544, O ¼ 0.560 (in mol%). It is assumed that quartz and H2O are in excess.

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320314

model for calculating the pseudosection of mafic rocks withinvolvement of melt.

5.2. PeT pseudosections

PeT pseudosection for the representative sample (L73-7) inthe NCFMASHTO system is shown in Fig. 4. It ranges fromtemperatures of 450e850 �C and pressures of 4e12 kbar. Garnetis stable at relatively high temperature and high pressureconditions. Garnet-in line is obviously temperature-dependentabove 8.1 kbar at 650 �C but it becomes pressure-dependentabove 650 �C. Clinopyroxene-in line which is shown as epidotein the pseudosection is pressure-independent and it only rangesfrom 540 �C to 560 �C. Hornblende is stable over a very wide PeTrange. It starts appearing above 470 �C. Plagioclase also largelyappears in the pseudosection. The plagioclase-in line starts from6.8 kbar at 400 �C and move upward to 12 kbar at 600 �C. Thewidely appearance of hornblende and plagioclase observed in thethin section under microscope wholly agrees with the pseudo-section. Epidote-in line is quite straight in general with a negativeslope. It begins at 520 �C at 3 kbar and ends at 735 �C at 12 kbar.Ilmenite-in line and sphene-out line are very close together

below 715 �C at 9.9 kbar and both of them start at around 550 �Cat 3 kbar. However, above 715 �C, the sphene-out line migratestoward lower temperature and finally ends at 610 �C at 12 kbarwhile the ilmenite-in line carries on moving upward until itreaches 800 �C at 11 kbar. Furthermore, the pseudosection ismainly occupied by trivariant, quadrivariant and quinquivariantfields while only few divariant and sextivariant fields areobserved and they just make up very small area of thepseudosection.

5.2.1. Pseudosection for the pre-peak stage (M1)As discussed above, the pre-peak stage of metamorphism is

characterized by the mineral inclusions of actinotite þhorbblende þ plagioclase þ epidote þ quartz þ sphene enclosedwithin garnet. The mineral assemblage matches the field of act-hb-pl-ep-sph-q-H2O in the pesudosection. It is stable at temperature of490e550 �C and pressure below 4.5 kbar. In the field of act-hb-pl-ep-sph-q-H2O, the isopleths of XCa for plagioclase (XCa ¼0.375e0.600) indicates that the values of isopleths increase withtemperature with high sensitiveness.

Due to the absence of garnet in the mineral assemblage of M1,only the values of XCa for plagioclase are applicable. The data of

K.K. Wu et al. / Geoscience Frontiers 4 (2013) 305e320 315

plagioclase inclusions are used to calculate the values as M1 ismarked by the presence of mineral inclusions. However, thesevalues range from 0.330 to 0.364, which do not fit the isopleths inM1. This can be explained by the unequilibrium between theminerals of M1. As the mineral composition of the M1 assemblageswas greatly influenced by the peak and post-peak metamorphism,the equilibrium between the minerals could not be maintained.Therefore, the PeT conditions of M1 cannot be constrained basedon its mineral compositions.

5.2.2. Pseudosection for the peak stage (M2)The peak stage metamorphism (M2) is marked the mineral

assemblage of garnet þ clinopyroxene þ hornblende þ plagioclaseþ quartz þ ilmenite and it is equal to the field of g-di-hb-pl-ilm-q-H2O (where “di” is the abbreviation of diopside, which is an end-member of clinopyroxene) in the pseudosection. Compared withM1, the field of M2 is distinguished by increases of both tempera-ture and pressure from 490e560 �C to 670 �C onward and from4.2e9.75 to 7.6e10.55 kbar. In the field of g-di-hb-pl-ilm-q-H2O,the isopleths of XCa for plagioclase (XCa¼ 0.550e0.625) have similarpositive slopes with the field of M1. The isopleths of XFe for garnet(XFe ¼ 0.660e0.865) are all negative in slope with high steepnesswhich means that the isopleths are quite sensitive to temperatureand increase in value with decreasing temperature. On the otherhand, the isopleths of XCa for garnet (XCa ¼ 0.19e0.41) are pressure-dependent and the values decrease with increasing pressure.

The values of XCa for plagioclase were acquired from the EPMAresults of the plagioclase grains in the matrix. Unfortunately, sincethe values (0.377e0.441) are out of the isopleth range in the field ofM2, they cannot be plotted on the field of M2. It may be due to theinfluence on the M2 plagioclase caused by the post-peak meta-morphism. On the other hand, with the appearance of garnet in themineral assemblage of M2, the values of XFe and XCa for garnet canbe obtained. However, as the rims of garnet have a higher chance ofbeing influenced by the post-peak metamorphism, the values arecalculated by using the data of the cores of garnet. These XFe and XCavalues are within the range of 0.771e0.921 and 0.127e0.216respectively. Both of them fall in a narrow area in the field of M2.As a result, it gives very good constrained PeT conditions of M2 atthe temperature of 780e810 �C and the pressure of 7.65e8.40 kbar.

5.2.3. Pseudosection for the post-peak stage (M3)The post-peak stage of metamorphism is identified by the

garnet-quartz symplectites surrounding the peakminerals with themineral assemblage garnet þ clinopyroxene þ hornblendeþ plagioclase þ epidote þ quartz þ sphene. This assemblagecorresponds with the field of g-di-hb-pl-ep-sph-q-H2O in thepseudosection. It does not have a great change in pressure(8.1e11.9) but a drop of temperature from 670 �C onward to610e710 �C which indicates a nearly isobaric-cooling metamorphicprocess. In the field of g-di-hb-pl-ep-sph-q-H2O, unlike the field ofM1 and M2, the isopleths of XCa for plagioclase, which range from0.125 to 0.600, have very gentle slopes, which indicate that theisopleths change to pressure-dependent in high temperature. Theisopleths of XFe for garnet, which ranges from 0.80 to 0.90, havevery similar characteristics with those in the field of M2. However,the isopleths of XCa for garnet are totally different from those in thefield of M2. They change from pressure-dependent to temperature-dependent at high temperature. Besides, the spacing suddenlydecreases a lot from the field of g-di-hb-pl-sph-q-H2O to the field ofg-di-hb-pl-ep-sph-q-H2O, which has lower temperature. Similar tothe isopleths of XFe for garnet, the values of isopleths XCa forgarnet also increase with decreasing temperature.

As the plagioclase growing during M3 cannot be firmly distin-guished, so only the values of XFe and XCa for garnet are used to

constrain the PeT conditions of M3. The calculations of XFe and XCavalues are based on the data obtained from the symplectic garnet.The XFe and XCa values range from 0.926 to 0.947 and from 0.424 to0.467, respectively. Although the XFe values are not consistent withthe isopleths, the XCa values can make a good constrain on M3. Asa result, M3 is restricted to the PeT conditions of 630e670 �C and8.15e9.40 kbar. Compared with the PeTconditions of M2, the trendof M2 to M3 obviously indicates an isobaric cooling metamorphicprocess which is also consistent with the appearance of the garnet-quartz symplectites found in the samples.

6. Metamorphic PeT path and tectonic implications

Together with the petrographic observations, construction ofisopleths on the pseudosection and themineral compositions of theassemblages, the PeT path of the amphibolites from the northernLiaoning Complex can be well defined as follows:

The amphibolites initially experience the pre-peak metamorphicstage (M1) under the PeT conditions of <4.50 kbar and 490e550 �C,which is characterized by a mineral assemblage horn-blende þ plagioclase þ epidote þ quartz þ sphene occurring asinclusions within garnet. Subsequently, the amphibolites underwentthe peakmetamorphism (M2) with PeTconditions of 7.65e8.40 kbarand 780e810 �C, forming a mineral assemblage of garnetþ clinopyr-oxene þ hornblende þ plagioclase þ quartz þ ilmenite. Finally, theamphibolites experienced a nearly isobaric cooling process until thetemperatures of 630e670 �C at the pressure of 8.15e9.40 kbar werereached, forming the garnet þ quartz symplectites surrounding thepeak minerals such as clinopyroxene and plagioclase. As a result, ananticlockwise PeT path, as shown in Fig. 5, is defined for theamphibolites from the northern Liaoning Complex.

Such an anticlockwise PeT path involving nearly isobaric cool-ing defined for the amphibolites from the northern LiaoningComplex is consistent with those PeT paths constructed for otherlate Archean metamorphic complexes in the Eastern Block of theNorth China Craton, including Miyun, eastern Hebei, westernShandong, eastern Shandong, southern Liaoning and southern Jilincomplexes (Cui et al., 1991; Liu and Lin, 1992; Li, 1993; Sun et al.,1993; Wang and Cui, 1994; Chen et al., 1994; Ge et al., 1994), asillustrated in Fig. 6 (modified from Zhao et al., 1998). These PeTpaths were reconstructed from different rock types, includingpelitic gniesses, mafic granulites and amphibolites. Althoughdifferent methods were used to determine the above PeT condi-tions instead of the pseudosection modeling, they all show verysimilar metamorphic evolution that is characterized by an anti-clockwise PeT path involving near-isobaric cooling (Fig. 6). Asmentioned earlier, anticlockwise PeT paths involving near-isobariccooling is considered to reflect an origin of metamorphism relatedto the intrusion and underplating of large amounts of mantle-derived magmas that not only provide heat for the meta-morphism but also add a large volume of mostly mafic material tothe base of the crust (Bohlen, 1987, 1991). Underplating andintrusion of large volumes of mantle-derived magmas leading tometamorphism with an anticlockwise PeT path may occur in (1)continental rift settings (Sandiford and Powell, 1986), (2) conti-nental magmatic arc regions (Bohlen, 1991), and (3) mantle-plumetectonic regimes (Condie, 1997; Jayananda et al., 1998, 2000; Zhaoet al., 1999). First of all, continental rift environment is unlikely toexplain the metamorphism of the Archean basement rocks of theEastern Block because it cannot expound the broadly distribution of2.6e2.5 Ga granitoid intrusions over a large area with width ofmore than 800 km, the large amount of oval gneiss domes, and thedeficiency of abundant alkali rocks which should be commonlyassociated with rifting (Zhao et al., 1998). Albeit that the other twotectonic settings, i.e. hot spots related to mantle plume (Zhao et al.,

Figure 5. PeT pseudosection in the NCFMASHTO system calculated for the sample (L73-7) with the construction of isopleths, XFe for garnet, Xca for garnet and Xca for plagioclase, forthe metamorphic assemblages of M1eM3, and the constrained field of the corresponding mineral assemblages which represent the metamorphic stages M1eM3. The fieldsrepresenting assemblages M1 to M3 is highlighted. The anticlockwise PeT path involving isobaric cooling is reconstructed for the studied amphibolites. It is noted that the PeTpseudosection ranges from 450 �C to 850 �C from 4 kbar to 10 kbar.

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1998, 1999, 2001; Ge et al., 2003; Geng et al., 2012; Wu et al., 2012)and continental magmatic arc regions (Bohlen, 1987, 1991; Nutmanet al., 2011; Jian et al., 2012; Peng et al., 2012b; Wan et al., 2012;Wang et al., 2012; Zhang et al., 2012b, c, d; Shi et al., 2012), are alsopossible to elucidate the formation of the Archean basement rocksof the Eastern Block in terms of their PeT paths, lithology andspatial distribution, a mantle plume model is favored for theEastern Block due to the following reasons (Zhao et al., 1998, 1999,2001; Ge et al., 2003). First, the distribution of the komatiitic rocksand other ultramafic rocks, as well as their close spatial andtemporal relationships with mafic rocks can be more reasonablyexplained by a mantle plume model than by a magmatic arc model(Campbell et al., 1989). Second, based on geochemistry studies, it isfound that most of the mafic granulites exposed in the EasternBlock show affinities to continental tholeiitic basalts which arecommonly agreed in a relationship with the interaction ofasthenosphere and mantle plumes (Campbell et al., 1989). Third,the volcanic assemblages of the Archean basement rocks of the

Eastern Block are bimodal (Zhai et al., 1985; He et al., 1991; Sunet al., 1993; Bai and Dai, 1998), which is also the characteristic ofmost Archean granite-greenstone terranes in the world. It is notedthat most mature magmatic arcs are characterized by unimodalvolcanic assemblages (Hamilton, 1998). Forth, the huge exposure ofthe granitoids over a width of more than 800 km withoutsystematic age progression (Bai and Dai, 1998) indicates theimpossibility of the presence of migrating or successively-accretedarcs.

Furthermore, Wu et al. (1998) and Wan et al. (2005a, b)proposed a continental magmatic arc collision model to explainthemetamorphic evolution of the Archean basement of the EasternBlock. Although this model involves the development of conti-nental magmatic arc at 2.51e2.56 Ga which is possible to accountfor the intrusion and underplating of mantle magmas occurring inthe Eastern Block, it also suggests that a collision between a conti-nental block and a magmatic arc at 2.47e2.48 Ga occurred imme-diately after the formation of the magmatic arc. Besides, some

Figure 6. Summary of the metamorphic PeT paths of late Archean metamorphiccomplexes in the Eastern Block of the North China Craton. 1: MiyuneChengde Complex(Wang and Cui, 1994; Chen et al., 1994); 2: Eastern Hebei Complex (Zhao et al., 1998;Liu and Lin, 1992); 3: Western Shandong Complex (Zhao, 2009); 4: Eastern ShandongComplex (Li, 1993); 5: Western Liaoning Complex (Cui et al., 1991); 6: NorthernLiaoning Complex (Sun et al., 1993); 7 Southern Jilin Complex (Ge et al., 1994); 8:Northern Liaoning Complex (from this research).

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recent studies (e.g. Liu et al., 2011; Wang et al., 2011) also suggestedan active continental margin setting for the late Archean to Pale-oproterozoic tectonothermal evolution history in the northernmargin of the Eastern Block. However, as mentioned earlier, colli-sional environments should be represented by clockwise PeT pathswith nearly isothermal decompression (England and Thompson,1984; Thompson and England, 1984; Harley, 1989; Brown, 1993).It is clearly that only anticlockwise PeT paths have been found forthe basement rocks of the metamorphic complexes in the EasternBlock. Therefore, it seems that these models are inappropriate forthe formation of the Archean basement of the Eastern Block. Basedon all those considerations above and the result of this study, wepresent the following brief scenario for the late Archean tectono-thermal evolution of the northern Liaoning Complex in the EasternBlock of the North China Craton:

Due to the interaction between the upwelling mantle plumesand the lithosphere, a large amount of ultramafic to mafic volcanicrocks and TTG plutons were formed in the Eastern Blocks in theperiod 2.6e2.5 Ga. The mantle lithosphere was uplifted and thecontinental crust was overlain by the mantle plumes. The litho-sphere was also stretched by the mantle plumes. As a result, hugevolumes of ultramafic to mafic magma erupted, represented by theQingyuan Group. Besides, the enormous partial melting of basalticand amphibolitic rocks which led the formation of vast amount ofTTG magma was caused by the heat transfer from the mantleplumes to the upper mantle or lower crust. The TTG magma thenascended diapirically to the lower and upper crust and developedinto the TTG domes in the northern Liaoning area. Due to the heattransfer from the mantle plumes, the crust was metamorphosed,which resulted in the exposure of the mafic granulites, peliticgneisses and amphibolites in the northern Liaoning Complex. Atthe beginning, the pre-peak metamorphism (M1) was led by thecomparatively cooler plume head heating the crust. Meanwhile, thecrust was thickened because of adding of enormous volumes ofmagma to the crust and intrusion to shallow depth of the crust.Following the pre-peakmetamorphism, the peakmetamorphism atamphibolites- to granulite-facies was caused by the continuously

rising of the plume tail with higher temperature. Besides, theplume tail also led the broadly distributed anatexis of TTG gneissesand the subsequent formation of syntectonic charnockites in theHongtoushan area and quartz monzonites in the Dasuhe area.Eventually, the end of plume activity associated with the stop ofheating brought the near-isobaric cooling to the crust, resulting inthe post-peak metamorphism (M3). The overall metamorphicprocess perfectly fits the anticlockwise PeT paths constructed foramphibolites from the northern Liaoning Complex in the EasternBlock of the North China Craton.

Acknowledgments

The authors thank Prof. M. Santosh and an anonymous reviewerfor their constructive suggestions and criticisms. This research wasfinancially funded by Chinese NSFC Grants (41190075, 40730315,40872123 and 41072152), and Hong Kong RGC GRF grants (7066/07P and 7053/08P).

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