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Precambrian Research 222– 223 (2012) 312– 324

Contents lists available at SciVerse ScienceDirect

Precambrian Research

j o ur nal homep age : www.elsev ier .com/ locate /precamres

etrology and P–T path of the Yishui mafic granulites: Implications forectonothermal evolution of the Western Shandong Complex in the Eastern Blockf the North China Craton

eiling Wua,∗, Guochun Zhaoa, Min Suna, Changqing Yina, Sanzhong Lib, Pui Yuk Tama

Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong KongCollege of Marine Geosciences, Ocean University of China, Qingdao 266003, China

r t i c l e i n f o

rticle history:eceived 22 March 2011eceived in revised form 4 August 2011ccepted 25 August 2011vailable online 6 September 2011

eywords:ate Archeanafic granuliteetamorphism

–T pathorth China Craton

a b s t r a c t

Mafic granulites from the Yishui Group of the Western Shandong Complex in the Eastern Block of theNorth China Craton occur as enclaves or boudins within Late Archean TTG gneisses, and are composedmainly of garnet, clinopyroxene, orthopyroxene, plagioclase, hornblende, and minor quartz, ilmenite, andmagnetite. Petrographic examination has revealed three distinct metamorphic mineral assemblages:the pre-peak prograde assemblage (M1) of hornblende + plagioclase + quartz + ilmenite + magnetiteoccurring as inclusions within garnet and pyroxene grains, peak assemblage (M2) of orthopyrox-ene + clinopyroxene + plagioclase + garnet + hornblende + quartz + ilmenite + magnetite, and post-peakassemblage (M3) represented by garnet + quartz and garnet + ilmenite/magnetite symplectites. Pseudo-section modeling using THERMOCALC in the NCFMASHTO model system for a representative sampleconstrains the P–T conditions of M1, M2 and M3 stages at 660–730 ◦C/<6.6 kbar, 800–820 ◦C/8.0–8.5 kbarand 686–710 ◦C/7.6–8.6 kbar, respectively. The results of petrology and quantitative P–T pseudosec-tion modeling define an anticlockwise P–T path involving near-isobaric cooling following the peakmedium-pressure granulite-facies metamorphism, suggesting that the metamorphism of the YishuiGroup was most likely related to the intrusion and underplating of mantle-derived magmas. Although theunderplating of voluminous mantle-derived magmas leading to granulite-facies metamorphism with ananticlockwise P–T path involving isobaric cooling may occur in continental magmatic arc regions, abovehot spots driven by mantle plumes, or in continental rift environments, a mantle plume model is favored

because this model can reasonably interpret many other geological features of Late Archean basementrocks from the Western Shandong Complex in the Eastern Block of the North China Craton as well astheir anticlockwise P–T paths involving isobaric cooling. The relatively cooler mantle-plume head heatedthe crust initially, causing amphibolite-facies metamorphism (M1). Subsequently, the relatively hottermantle-plume tail heated the crust, causing granulite-facies metamorphism (M2). Finally, a near-isobariccooling process (M3) occurred when the mantle plume ceased to heat the crust.

. Introduction

In the last two decades, metamorphic researchers have car-ied out extensive investigations on metamorphic P–T paths ofarious metamorphic terrains in the world. These investigationsave led to a broad consensus that clockwise P–T paths, espe-

ially for those involving isothermal decompression (ITD), areonsistent with metamorphism operative in subduction zonesr continent–continent collisional environments (Bohlen, 1991;

∗ Corresponding author at: Department of Earth Sciences, James Lee Scienceuilding, The University of Hong Kong, Pokfulam Road, Hong Kong.el.: +852 28598913; fax: +852 25176912.

E-mail address: h0992074@hku.hk (M. Wu).

301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2011.08.008

© 2011 Elsevier B.V. All rights reserved.

Brown, 1993), whereas anticlockwise P–T paths, especially for thoseinvolving isobaric cooling (IBC), reflect metamorphism related tothe intrusion and underplating of mantle-derived magmas, whichmay occur in intra-continental magmatic arc regions (Wells, 1980;Bohlen, 1987, 1991), hot spots related to mantle plumes (Bohlen,1991) and incipient rift environments (Sandiford and Powell,1986). Thus, combined with lithological, structural, geochemicaland geochronological data, metamorphic P–T paths can be usedto recognize terranes and their tectonic boundaries – subduc-tion zones or collisional belts. Mafic granulites are of particularimportance in this regard as they often preserve mineral assem-

blages suitable for estimating the P–T conditions of metamorphismand textural evidence used to infer metamorphic reaction rela-tions, which are particularly useful in determining metamorphicP–T paths.

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M. Wu et al. / Precambrian R

Mafic granulites have a widespread distribution in theorth China Craton (NCC). Zhao et al. (1999) have classi-ed the mafic granulites of the NCC into A- and B-types,f which the former displays garnet + quartz symplectic coro-as and mainly crops out in the late Archean basement ofhe NCC, whereas B-type mafic granulites exhibit orthopyrox-ne + plagioclase ± clinopyroxene symplectites or coronas and arenly exposed in linear tectonic belts. In the last few years, extensivetudies have been carried out on the spatial distribution, meta-orphic ages and tectonothermal evolution of the B-type mafic

ranulites in the NCC (Zhao et al., 2000a,b, 2001a). The results ofhese studies show that the B-type mafic granulites in the NCCre limited to three Paleoproterozoic tectonic belts, named thehondalite Belt, Jiao-Liao-Ji Belt and Trans-North China Orogen

Fig. 1), and their metamorphic evolution is exclusively character-zed by clockwise P–T paths involving isothermal decompression,

hich reflect continent–continent collisional environments. Basedn their distinct clockwise P–T paths as well as the presence of high-ressure mafic and pelitic granulites, ancient oceanic fragmentsnd ophiolitic mélange, linear structural belts defined by strike-lip ductile shear zones, large-scale thrusting and folding, sheatholds and mineral stretch lineations, Zhao et al. (2001b, 2005) inter-reted the three Paleoproterozoic tectonic belts as collisional belts,f which the Khondalite Belt is considered to have formed byhe amalgamation of the Yinshan and Ordos Blocks to form the

estern Block, the Jiao-Liao-Ji Belt formed by the amalgamationf the Longgang and Langrim Blocks to form the Eastern Block,nd finally the Western and Eastern Blocks collided to form therans-North China Orogen (Fig. 1). Now there is a coherent

WESTERN BLOCK

TRANS-NORTCHINA OROG

Central China Orogen

Central China Orogen (Qinlin

Khondalite Belt

Yinshan Block

Ordos Block

Jiayuguan

Songpan

Xi an

Taiyuan

Bayan Obo

Wuha

Exposed Archean to Paleoproterozoic basement Hidden basement in the Eastern and Western blocks

Exposed basement in the Paleoproterozoic belts

Hidden basement in the Paleoproterozoic orogens

Major fault

40 N

35 N

11110 E105 E100 E

0

ig. 1. Tectonic subdivision of the North China Craton (revised after Zhao et al., 2005). Ahandong; SL, Southern Liaoning; NL, Northern Liaoning; WL, Western Liaoning; SJ, South

h 222– 223 (2012) 312– 324 313

outline of the timing and tectonic processes involved in the col-lisions between the micro-continental blocks along these threebelts.

Comparatively, no much work has been done on thetectonothermal evolution of the A-type mafic granulites in theNorth China Craton. Zhao et al. (1998, 1999) overviewed the majorpetrological features and P–T paths of the A-type mafic granulitesfrom some metamorphic complexes in the Eastern and WesternBlocks, and concluded that the metamorphic evolution of theseA-type mafic granulites is characterized by anticlockwise P–Tpaths involving isobaric cooling. However, the previous studieson metamorphic P–T conditions and P–T paths of the A-typemafic granulites from the Western and Eastern Blocks werebased on calculations of inconsistent or out-of-date traditionalgeothermobarometry. Thus, it still remains unclear whether theseanticlockwise P–T paths really reflect the metamorphic evolutionof the A-type mafic granulites in the NCC or are just artifacts ofthe traditional geothermobarometers. Moreover, not all metamor-phic complexes with the A-type mafic granulites in the NCC havebeen well studied in terms of metamorphic evolution, which hashindered the further understanding of formation and evolution ofArchean terrains within the micro-continental blocks of the NCC.

In this paper, we present detailed petrographic, mineral chem-ical and pseudosection modeling studies on the A-type maficgranulites from the Yishui Group in the Western Shandong Com-

plex of the Eastern Block, whose metamorphic evolution has notbeen studied before. The inferred P–T path of the mafic gran-ulites, in combination with available lithological, geochemical andgeochronological data, places important constraints on the late

H EN

EASTERN BLOCK

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Changchun

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n

WS

ES

SL

NL

SJ

WL

EH

AB

MYNangrim

TLF

YSF

40 N

35 N

30 N

125 E

125 E 130 E120 E5 E

200 400 km

Yishui Complex

bbreviations: MY, Miyun; EH, Eastern Hebei; WS, Western Shandong; ES, Easternern Jilin; AB, Anshan-Benxi; TLF, Tancheng-Lujiang fault; YSF, Yi-Shu fault.

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14 M. Wu et al. / Precambrian R

rchean evolution of the Western Shandong Complex in the Easternlock of the NCC.

. Geological setting

The Precambrian basement of the North China Craton hasecently been divided into the Archean to Paleoproterozoic East-rn and Western Blocks and three Paleoproterozoic orogenic belts,alled the Jiao-Liao-Ji Belt, Khondalite Belt and Trans-North Chinarogen (Fig. 1; Zhao et al., 2003, 2005), of which the Jiao-Liao-Jielt and Khondalite Belt are located within the Eastern and West-rn Blocks, respectively (Fig. 1). An extensive overview on theseontinental blocks and Paleoproterozoic orogenic belts has beeniven by Zhao et al. (2003, 2005).

The Archean basement of the Eastern Block consists of high-rade terrains and low-grade granite–greenstone belts, whichrop out in the Southern Jilin, North Liaoning, Anshan-Benxi,outhern Liaoning, Western Liaoning, Eastern Hebei, Miyun, West-rn and Eastern Shandong complexes (Fig. 1). The basementocks are composed predominantly of the early to late Archeanigh- and low-grade tonalitic–trondjemetic–granodioritic (TTG)neisses and ca. 2.5 Ga syntectonic granitoids, with minor raftsf supracrustal rocks including ultramafic (komatiitic) to felsicolcanic and sedimentary rocks. All of these rocks experiencedolyphase deformation and widespread greenschist- to granulite-acies metamorphism at ca. 2.5 Ga (Bai and Dai, 1998; Ge et al.,003; Zhao et al., 1998). Previous metamorphic studies showedhat the metamorphic evolution of some granulite-facies ter-ains in the Eastern Block is characterized by anticlockwise P–Taths, which is generally considered to reflect the intrusion ornderplating of large amounts of mantle-derived magmas (Zhaot al., 1998). The structural styles of the Archean basementf the Eastern Block are dominated by oval domes of variouscales composed by 2.6–2.5 Ga TTG gneisses, separated by lin-ar supracrustal rocks belts (Zhao et al., 2001b). Many domiformatholiths are composed chiefly of TTG gneisses that generallynderwent upper greenschist- to granulite-facies metamorphism,ith quartz monzonites in amphibolite-facies areas or charnock-

tes in granulite-facies areas as the cores of domes locally (Zhaot al., 2001b).

The Western Shandong Complex covers an area of more than5,000 km2 and is bordered by the Liaochen-Lankao fault to theest and the Yi-Shu fault (YSF) to the east (Fig. 1). The Precambrian

asement rocks in this complex have been traditionally considereds an Archean granite–greenstone belt (Bai and Dai, 1998; Shent al., 1993), of which the greenstones consist of ultramafic to felsicocks metamorphosed in greenschist facies to lower-amphiboliteacies, while the granitic rocks are composed of pre-tectonic TTGneisses and syntectonic massive monzonitic granites. Due to latexhumation or uplift along the Tancheng-Lujiang fault (TLF), minormounts of Archean granulite-facies basement rocks are exposedn the Yishui area, which lies in the Yishui-Tangtou fault zone to theast of the Yishui County, Shandong Province (Fig. 2) and consists ofgneous plutons which make up 70–75% of the total exposure, and

etamorphic supercrustal rocks which were traditionally calledhe Yishui Group.

The igneous plutons in the Yishui area have been well dated.hen et al. (1997) reported a single grain zircon age of 2507 ± 3 Maor the Linjiaguanzhuang pluton, and Gu and Chen (1997) reported

zircon U–Pb concordant age of 2537 ± 5 Ma for the Niuxin-uanzhuang pluton. Recent zircon U–Pb SHRIMP dating has

evealed that the metamorphosed Caiyu, Dashan, Mashan, Xue-han and Yinglingshan plutons were emplaced at 2563 ± 14 Ma,545 ± 10 Ma, 2538 ± 6 Ma, 2532 ± 9 Ma and 2530 ± 7 Ma, respec-ively (Shen et al., 2004, 2007; Zhao et al., 2008). Metamorphic

ch 222– 223 (2012) 312– 324

zircons from the Caiyu and Dashan plutons yielded ages of2518 ± 13 Ma and 2508 ± 5 Ma respectively, indicating that theregional metamorphism of the Yishui Group and the plutonsoccurred at ca. 2500 Ma (Shen et al., 2004).

The Yishui Group is composed of amphibolites, mafic granulites,minor magnetite-quartzites and Al-rich metapelites, of which themafic granulites are commonly closely associated with charnock-ites. These supracrustal rocks usually occur as irregular ribbons andboudins within the host granitic plutons with sharp boundariesand the total exposure is up to 60 km2. The Yishui Group is subdi-vided into three distinct lithological subgroups from south to north:the Linjiaguanzhuang subgroup, the Shishanguanzhuang subgroup,and the Beixiazhuang subgroup (Shen et al., 2000). The Linji-aguanzhuang subgroup consists mainly of pyroxene amphibolites,garnet-diopside amphibolites, and garnet granulites with minorfelsic gneisses in local places, and crops out around Jiucengling Vil-lage in the north and Linjiaguanzhuang Village in the south (Fig. 2).The Shishanguanzhuang subgroup is dominated by granulites andfelsic gneisses, with minor pyroxenites, which are distributed inthe Shishanguanzhuang, Yangjuan, Caiyu and Hujiazhuang areas(Fig. 2). The Beixiazhuang subgroup is composed of amphibolites,fine-grained biotite gneisses, and biotite–garnet–sillimanite–k-feldspar paragneisses. This subgroup is only exposed in an areanorth of Beixiazhuang Village (Fig. 2).

3. Petrography and mineral chemistry

Both garnet-bearing and garnet-free mafic granulites areobserved in the Yishui Group and locally they occur on the sameoutcrop. In this study, we focus only on the garnetiferous maficgranulites as they contain mineral assemblages that are suitablefor P–T estimation, and reaction textures that can be used to infermetamorphic processes.

The studied mafic granulite samples were collected from areaaround Linjiaguanzhuang Village to those south of Yishui County(Fig. 2). These granulites are dark grey on the fresh surfaces, andare mostly massive but locally show weak banded structures. Theyconsist mainly of clinopyroxene, orthopyroxene, plagioclase, horn-blende, garnet and quartz, with minor ilmenite, magnetite andapatite or sphene.

Selected minerals from the studied samples were analyzedwith a Link EDS system connected to a JEOL JXA-8100 elec-tron probe microanalyzer at Institute of Geology and Geophysics,Chinese Academy of Sciences, Beijing. Analysis was performed witha 15 kV accelerating voltage, ∼20 nA beam current, counting timeof 10–20 s and ca. 1–3 �m spot size. Natural minerals were usedas standards for major elements and synthetic minerals were usedfor some minor elements. A representative selection of the mineralcompositions used for P–T calculations of the Yishui mafic gran-ulites is listed in Tables 1–5.

3.1. Garnet

Two textural types of garnet are present in granulites: por-phyroblast (Fig. 3c and d) and symplectic corona (Fig. 3e–h). Theporphyroblastic garnet grains are coarse (1–3 mm), subhedral, andgranular, containing mineral inclusions such as hornblende, pla-gioclase, and quartz. The symplectic garnet coronas intergrownwith quartz or ilmenite/magnetite occur surrounding the matrixminerals of pyroxene and plagioclase (Fig. 3e–g). Chemically,porphyroblastic garnet is dominated by almandine (58.9–60.4%),

grossular (20.8–22.2%) and pyrope (16.2–17.8%), with minor andra-dite (0–0.7%) and spessartine (1.3–1.7%). Most porphyroblasticgarnet grains do not show pronounced zoning in all end-members.Symplectic garnet grains have higher grossular (23.4–28.9%) and

M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324 315

hui Co

ac

3

g

Fig. 2. Geological sketch map of the Yis

ndradite (1.7–4.2%) contents but lower pyrope (8.0–9.2%) contentsompared to those of porphyroblastic garnet grains (Table 1).

.2. Clinopyroxene

Clinopyroxene is subhedral or anhedral, occurring in variousrain sizes (0.5–2.5 mm) in the matrix (Fig. 3a, b, e and f). Most

mplex (revised after Shen et al., 2000).

clinopyroxene grains are inclusion-free except a few containingplagioclase and/or quartz inclusions. Some clinopyroxene grainsare rimmed by hornblende or surrounded by garnet symplectic

coronas (Fig. 3f). In chemical composition, clinopyroxene can beassigned to diopside and its compositions vary little from core torim. The values of XMg (= Mg/(Mg + Fe2+)) range from 0.549 to 0.606(Table 2).

316 M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324

Fig. 3. Microphotographs (plane-polarized light) showing representative mineral assemblages and textures of the Yishui mafic granulite rocks. (a) Pre-peak hornblendeinclusions within pyroxene. (b) Peak assemblage of orthopyroxene + clinopyroxene + plagioclase + hornblende and granular texture. (c) Peak garnet porphyroblast sur-rounded by clinopyroxene and hornblende. (d) A garnet porphyroblast containing plagioclase inclusions. (e) Post-peak garnet + quartz symplectites on clinopyroxene.(f) Garnet + quartz symplectic coronas surrounding clinopyroxene. (g) Garnet + quartz symplectic coronas around opaque minerals. (h) Garnet + ilmenite/magnetite symplec-tites. Mineral symbols are after Kretz (1983).

M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324 317

Table 1Representative garnet analyses (stoichiometry is calculated on the basis of 12 oxygens).

Sample Porphyroblastic-type Symplectic-type

10SD05 10SD06 10SD06 10SD06 10SD05 10SD05 10SD05 10SD05

SiO2 37.72 38.63 39.04 38.78 37.76 37.84 37.76 37.74TiO2 0.221 0.149 0.049 0.071 0.000 0.000 0.044 0.145Al2O3 19.65 20.74 20.76 20.87 20.43 20.66 20.58 20.10Cr2O3 0.007 0.015 0.034 0.006 0.019 0.000 0.005 0.031FeO 29.26 27.76 27.82 26.68 28.35 28.29 27.27 27.05MnO 0.318 0.676 0.572 0.763 1.768 0.697 0.892 1.048MgO 1.834 4.184 4.505 4.374 2.187 2.161 2.341 2.033CaO 10.803 7.881 7.537 7.860 9.413 10.051 10.964 11.016Na2O 0.014 0.000 0.022 0.018 0.017 0.000 0.008 0.021K2O 0.000 0.017 0.007 0.002 0.000 0.008 0.006 0.000NiO 0.000 0.000 0.000 0.000 0.000 0.023 0.000 0.000Total 99.83 100.0 100.3 99.42 99.94 99.72 99.87 99.18

Si 3.037 3.039 3.057 3.060 3.008 3.013 2.994 3.021Ti 0.007 0.009 0.003 0.004 0.000 0.000 0.003 0.009Al 1.929 1.923 1.916 1.941 1.918 1.939 1.924 1.896Cr 0.003 0.001 0.002 0.000 0.001 0.000 0.000 0.002Fe3+ 0.000 0.000 0.000 0.000 0.068 0.035 0.084 0.047Fe2+ 1.803 1.826 1.822 1.761 1.820 1.849 1.725 1.764Mn 0.043 0.045 0.038 0.051 0.119 0.047 0.060 0.071Mg 0.537 0.491 0.526 0.515 0.260 0.257 0.277 0.243Ca 0.635 0.664 0.632 0.665 0.803 0.858 0.932 0.945Na 0.004 0.000 0.003 0.003 0.003 0.000 0.001 0.003K 0.000 0.002 0.001 0.000 0.000 0.001 0.001 0.000Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Alm 0.597 0.603 0.604 0.589 0.606 0.614 0.576 0.584Grs 0.210 0.220 0.210 0.222 0.234 0.268 0.269 0.289Prp 0.178 0.162 0.174 0.172 0.086 0.085 0.092 0.080Sps 0.014 0.015 0.013 0.017 0.040 0.016 0.020 0.024Adr 0.000 0.000 0.000 0.000 0.034 0.017 0.042 0.023XFe(g) 0.771 0.788 0.776 0.774 0.875 0.878 0.862 0.879XCa(g) 0.214 0.223 0.212 0.226 0.279 0.289 0.318 0.320

Adr, Fe3+/2; Grs, (Ca-3Adr)/(Fe2+ + Mg + Mn + Ca); Prp, Mg/(Fe2+ + Mg + Mn + Ca); Sps, Mn/(Fe2+ + Mg + Mn + Ca); Alm, Fe2+/(Fe2+ + Mg + Mn + Ca); XFe(g), Fe2+/(Fe2+ + Mg);XCa(g), Ca/(Fe2+ + Ca + Mg). The amount of Fe3+ was calculated from stoichiometric constraints using the program AX (Powell et al., 1998). Mineral symbols are after Kretz(1983).

Table 2Representative clinopyroxene analyses (stoichiometry is calculated on the basis of 6 oxygens).

Sample 10SD02 10SD02 10SD05 10SD05 10SD06 02s013-4 02s013-4 02s013-3

SiO2 51.53 51.81 51.51 50.92 52.41 51.39 51.33 51.97TiO2 0.189 0.153 0.110 0.184 0.050 0.131 0.183 0.099Al2O3 1.714 1.534 1.603 1.872 1.775 1.701 1.847 1.813Cr2O3 0.026 0.016 0.011 0.000 0.042 0.013 0.026 0.031FeO 15.16 13.91 14.42 15.16 12.60 15.37 14.19 13.91MnO 0.336 0.256 0.397 0.204 0.306 0.504 0.672 0.260MgO 10.41 10.72 10.47 9.82 10.84 9.438 10.33 10.99CaO 20.54 21.25 21.25 21.74 21.52 20.91 20.56 20.88Na2O 0.496 0.515 0.592 0.559 0.577 0.784 0.635 0.617K2O 0.000 0.000 0.006 0.009 0.000 0.000 0.000 0.008NiO 0.028 0.000 0.000 0.031 0.000 0.006 0.000 0.000Total 100.4 100.2 100.4 100.5 100.1 100.2 99.77 100.6

Si 1.959 1.967 1.954 1.937 1.983 1.962 1.960 1.961Ti 0.005 0.004 0.003 0.005 0.001 0.004 0.005 0.003Al 0.077 0.069 0.072 0.084 0.079 0.077 0.083 0.081Cr 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.001Fe3+ 0.029 0.026 0.057 0.074 0.000 0.049 0.032 0.037Fe2+ 0.453 0.416 0.401 0.408 0.399 0.441 0.421 0.402Mn 0.011 0.008 0.013 0.007 0.010 0.016 0.022 0.008Mg 0.590 0.607 0.592 0.557 0.611 0.537 0.588 0.618Ca 0.837 0.864 0.864 0.886 0.873 0.855 0.841 0.844Na 0.037 0.038 0.044 0.041 0.042 0.058 0.047 0.045K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000

XMg 0.566 0.593 0.597 0.577 0.605 0.549 0.583 0.606

XMg, Mg/(Fe2+ + Mg). The amount of Fe3+ was calculated from stoichiometric constraints using the program AX (Powell et al., 1998).

318 M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324

Table 3Representative orthopyroxene analyses (stoichiometry is calculated on the basis of 6 oxygens).

Sample 10SD02 10SD02 10SD02 10SD02 10SD06 10SD06 10SD06 10SD06

SiO2 49.44 50.14 49.15 50.41 50.01 50.07 49.97 49.79TiO2 0.045 0.067 0.101 0.000 0.000 0.054 0.034 0.068Al2O3 0.981 0.638 1.368 0.986 1.011 0.986 0.980 0.992Cr2O3 0.029 0.000 0.035 0.044 0.000 0.002 0.014 0.015FeO 33.66 33.70 33.79 32.75 32.73 33.11 33.00 33.01MnO 0.829 0.847 0.830 0.721 1.002 0.986 0.979 0.884MgO 14.26 14.32 14.32 14.95 14.28 14.28 14.29 14.16CaO 0.589 0.578 0.471 0.363 0.482 0.407 0.517 0.606Na2O 0.011 0.011 0.013 0.002 0.000 0.003 0.009 0.005K2O 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.009NiO 0.000 0.005 0.001 0.020 0.039 0.019 0.015 0.020Total 99.85 100.3 100.1 100.2 99.56 99.91 99.81 99.56

Si 1.950 1.969 1.933 1.971 1.976 1.972 1.970 1.968Ti 0.001 0.002 0.003 0.000 0.000 0.002 0.001 0.002Al 0.046 0.030 0.063 0.045 0.047 0.046 0.046 0.046Cr 0.001 0.000 0.001 0.001 0.000 0.000 0.000 0.000Fe3+ 0.052 0.029 0.065 0.011 0.002 0.006 0.013 0.013Fe2+ 1.058 1.079 1.047 1.059 1.080 1.084 1.074 1.078Mn 0.028 0.028 0.028 0.024 0.034 0.033 0.033 0.030Mg 0.839 0.838 0.840 0.872 0.841 0.839 0.840 0.835Ca 0.025 0.024 0.020 0.015 0.020 0.017 0.022 0.026Na 0.001 0.001 0.001 0.000 0.000 0.000 0.001 0.000K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 4.000 4.000 4.000 3.999 3.999 3.999 4.000 3.999

XMg 0.442 0.437 0.445 0.451 0.438 0.436 0.439 0.436X(opx) 0.558 0.563 0.555 0.549 0.562 0.564 0.561 0.564

X stoich

3

ogtatC0

TR

A(

Mg, Mg/(Fe2+ + Mg); X(opx), Fe2+/(Fe + Mg). The amount of Fe3+ was calculated from

.3. Orthopyroxene

Like clinopyroxene, medium- to coarse-grained (0.5–3 mm)rthopyroxene is present as subhedral prismatic or anhedralranular matrix minerals (Fig. 3b). Some orthopyroxene grains con-

ain hornblende, plagioclase, magnetite and ilmenite inclusions,nd in places they are surrounded by garnet + quartz symplec-ic coronas. Orthopyroxene has low Al2O3 (0.638–1.368 wt.%) andaO (0.363–0.606 wt.%) contents, and the XMg value is around.436–0.451 (Table 3).

able 4epresentative plagioclase analyses (stoichiometry is calculated on the basis of 8 oxygen

Sample Inclusion-type

10SD02 10SD03 10SD05 10SD06

SiO2 58.38 61.42 57.56 59.05

Al2O3 26.40 24.86 27.06 25.85

MgO 0.000 0.006 0.028 0.000

Na2O 6.764 7.687 6.55 7.077

K2O 0.202 0.355 0.27 0.108

CaO 8.44 6.392 8.995 7.759

BaO 0.000 0.000 0.000 0.000

Total 100.2 100.7 100.5 99.85

Si 2.607 2.710 2.570 2.639

Al 1.390 1.293 1.424 1.361

Mg 0.000 0.000 0.002 0.000

Ca 0.404 0.302 0.430 0.371

Ba 0.000 0.000 0.000 0.000

Na 0.586 0.658 0.567 0.613

K 0.012 0.020 0.015 0.006

Total 4.997 4.983 5.009 4.990

An 0.403 0.308 0.425 0.375

Ab 0.585 0.671 0.560 0.619

Or 0.011 0.020 0.015 0.006

XCa(pl) 0.408 0.315 0.431 0.377

b, Na/(Na + K + Ca); An, Ca/(Na + K + Ca); Or, K/(Na + K + Ca); XCa(pl), Ca/(Ca + Na). The amoPowell et al., 1998).

iometric constraints using the program AX (Powell et al., 1998).

3.4. Plagioclase

Two textural types of plagioclase are present in granulites:inclusion-type and matrix-type. The inclusion-type plagioclase(Fig. 3c and d), together with hornblende and quartz, is mainlyenclosed in the porphyroblastic garnet and matrix-type orthopy-roxene and clinopyroxene grains. The matrix-type plagioclase

(Fig. 3a–d) coexist with matrix-type orthopyroxene and clinopy-roxene and porphyroblastic garnet grains. The matrix-typeplagioclase has dominantly andesine compositions (An = 40.7–43.5,

s).

Matrix-type

10SD02 10SD05 10SD05 10SD06

58.40 57.16 57.29 57.7825.95 26.71 26.60 26.010.000 0.001 0.000 0.0006.738 6.592 6.331 6.6540.203 0.088 0.262 0.1888.59 8.755 9.072 8.4120.024 0.000 0.005 0.00099.91 99.30 99.6 99.1

2.617 2.578 2.580 2.6101.370 1.420 1.412 1.3850.000 0.000 0.000 0.0000.412 0.423 0.438 0.4070.000 0.000 0.000 0.0000.585 0.576 0.553 0.5830.012 0.005 0.015 0.0114.997 5.003 4.998 4.995

0.409 0.421 0.435 0.4070.580 0.574 0.550 0.5820.011 0.005 0.015 0.0110.413 0.423 0.442 0.411

unt of Fe3+ was calculated from stoichiometric constraints using the program AX

M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324 319

Table 5Representative hornblende analyses (stoichiometry is calculated on the basis of 23 oxygens).

Samples Inclusion-type Matrix-type

02s013-3 02s013-3 10SD06 02s013-3 10SD02 10SD06 10SD06 10SD06

SiO2 42.70 41.99 42.36 42.91 40.73 43.44 42.43 41.83TiO2 1.631 1.906 1.834 1.294 1.431 1.833 1.623 1.819Al2O3 12.16 12.72 12.09 12.01 13.66 11.58 11.78 11.69Cr2O3 0.000 0.017 0.028 0.000 0.004 0.029 0.018 0.027FeO 19.30 18.38 17.10 19.08 19.20 15.76 19.21 19.81MnO 0.180 0.109 0.024 0.137 0.164 0.054 0.191 0.166MgO 8.850 8.873 9.495 9.077 7.786 10.061 8.259 7.802CaO 10.85 11.11 11.67 10.95 11.32 11.82 11.71 11.50Na2O 1.760 1.636 1.769 1.644 1.461 1.761 1.540 1.731K2O 1.003 1.387 1.332 1.002 1.527 1.400 1.107 1.062NiO 0.000 0.041 0.000 0.000 0.018 0.000 0.000 0.000Total 98.43 98.16 97.70 98.09 97.30 97.73 97.87 97.45

Si 6.155 6.056 6.103 6.199 5.953 6.231 6.177 6.129Ti 0.177 0.207 0.199 0.141 0.157 0.198 0.178 0.201Al 2.066 2.162 2.053 2.044 2.352 1.958 2.021 2.019Cr 0.000 0.002 0.003 1.921 0.000 0.003 0.002 0.003Fe3+ 1.947 2.023 2.060 0.384 2.125 1.890 1.907 2.009Fe2+ 0.380 0.195 0.000 0.017 0.222 0.000 0.432 0.419Mn 0.022 0.013 0.003 0.000 0.020 0.007 0.024 0.021Mg 1.902 1.908 2.039 1.955 1.696 2.151 1.793 1.704Ca 1.675 1.716 1.802 1.694 1.773 1.816 1.827 1.806Na 0.492 0.458 0.494 0.460 0.414 0.490 0.435 0.492K 0.184 0.255 0.245 0.185 0.285 0.256 0.206 0.199

15.00

T am AX

Apc(go

3

agpgattn

4

smhpoTagctc

pmAg

5. P–T pseudosection modeling

Total 15.00 15.00 15.00

he amount of Fe3+ was calculated from stoichiometric constraints using the progr

b = 55.0–58.3, Or = 0.5–1.5) and commonly exhibits no com-ositional zoning. The inclusion-type plagioclase is less cal-ic (An = 31–43) than the matrix-type plagioclase (An = 41–44)Table 4). The relative large variations in An of inclusion-type pla-ioclase might be caused by different degrees of Calcium removalut of plagioclase during later re-equilibration.

.5. Hornblende

Hornblende displays two textural types: the inclusion-typend matrix-type. The inclusion-type hornblende coexists with pla-ioclase and quartz enclosed within porphyroblastic garnets andyroxene (Fig. 3a and b). The matrix-type hornblende is coarse-rained and in physical contact with other matrix minerals (Fig. 3bnd c). There are no pronounced variations in chemical composi-ions between the two textural types of hornblende, and both ofhem are pargasite – tschermakite solid solution according to theomenclature of Leake et al. (1997) (Table 5).

. Metamorphic stages and metamorphic reactions

The term “metamorphic stage (M1, M2, etc.)” used in thistudy refers to different metamorphic episodes in a single meta-orphic event, since evidence for discrete metamorphic events

as not been found in the Yishui area. Based on the aboveetrographic observations, three metamorphic stages can be rec-gnized: pre-peak (M1), peak (M2) and post-peak (M3) stages.he pre-peak M1 stage is represented by inclusion-type miner-ls of hornblende + plagioclase + quartz enclosed in porphyroblasticarnet and coarse-grained matrix-type minerals of orthopyroxene,linopyroxene and plagioclase. This mineral assemblage indicateshat the M1 metamorphism occurred under amphibolite-faciesonditions.

The peak M2 stage is represented by the formation of garnet

orphyroblasts and growths of relatively coarse-grained matrixinerals of clinopyroxene, orthopyroxene, plagioclase and quartz.s described above, some coarse hornblende (green-brownish)rains are in physical contact with other matrix minerals, indicating

15.00 15.00 15.00 15.00

(Powell et al., 1998).

hornblende was still stable at the peak metamorphic stage. There-fore, the characteristic assemblage of the peak M2 stage is orthopy-roxene + clinopyroxene + plagioclase + garnet + quartz ± horn-blende, which developed under medium-pressure granulite-faciesconditions. The mineral assemblage of the peak stage (M2) mayhave been produced from the inclusion-type minerals of thepre-peak stage (M1) through the following metamorphic reactions(Mengel and Rivers, 1991):

Hornblende + quartz → orthopyroxene + clinopyroxene

+ plagioclase + H2O (1)

Hornblende + plagioclase + quartz + magnetite →clinopyroxene + garnet + H2O + O2 (2)

The post-peak M3 stage is represented by the developmentof garnet-bearing symplectites or symplectic coronas surround-ing peak minerals such as garnet, pyroxene and opaque minerals(Fig. 3e–h). The symplectic structures consist of intergrowths offine-grained, worm-like quartz or opaque minerals within garnet(Fig. 3e–h). These assemblages are often considered to be relatedto isobaric or near-isobaric cooling metamorphism processes afterpeak metamorphism (Appel et al., 1998; Bohlen et al., 1985; Harley,1985, 1988, 1989). The garnet + quartz symplectic coronas of thepost-peak stage (M3) may have formed from the clinopyroxene,orthopyroxene and plagioclase of the peak stage (M2) throughthe following possible metamorphic reactions (Harley, 1988; Thostet al., 1991):

Orthopyroxene + clinopyroxene + plagioclase → garnet

+ quartz (3)

On the basis of the mineral assemblages and compositionsmentioned above, a model system NCFMASHTO (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) was selected to calculate

3 esear

PTettrcipnpfmNtaeFttbprSm1lmfwsciwMPM

CuAf((deM

sdvTa8(OeGwiipPatI

20 M. Wu et al. / Precambrian R

–T pseudosection for the mafic granulites from the Yishui Group.he P–T pseudosection diagram not only yields quantitative P–Tstimations, but also helps to reveal the paragenetic evolution ofhe rock. K2O and MnO were neglected in the chosen model sys-em as they only constitute very small proportions of the wholeock. Quartz and the fluid phase assumed to be pure H2O wereonsidered to be in excess. The melt phase was also ignored ast would only make up very small amount of the rocks with theresence of hornblende (Poli and Schmidt, 2002) and there hasot been a suitable mixing model for the melt in mafic rocks atresent. The high-temperature fields in the pseudosections there-ore may be metastable with respect to the assemblages involving

elt or at least coexist with melt (Daczko and Halpin, 2009).onetheless, experimental work, natural observations and calcula-

ions suggest that the topology of the phase relationships is similarnd the field boundaries may not move significantly when min-ral assemblages coexist with fluids or melts (Pattison, 2003). Ase2O3 was not determined separately by wet chemistry, 12% ofhe total Fe was assumed to be present as Fe2O3 correspondingo a typical value for wet-chemical analyses of mid-ocean ridgeasalts (e.g. Diener et al., 2007). The bulk rock composition for thehase equilibrium calculation was obtained directly from whole-ock bulk XRF analysis performed in the Department of Earthciences, the University of Hong Kong, and then normalized intoole proportions in the model system. A representative sample

0SD01 collected from an area south of the Linjiaguanzhuang Vil-age (GPS: N35◦41.901′, E118◦41.235′) was used for pseudosection

odeling. This homogeneous massive sample is dark grey on theresh surface and consists mainly of Cpx + Opx + Grt + Hb + Pl + Qtzith minor ilmenite, magnetite and apatite. All the three repre-

entative metamorphic mineral assemblages can be distinguishedlearly in sample 10SD01 and its bulk composition is sim-lar to others. The bulk composition of Sample 10SD01, in

t.%, is SiO2 = 48.765, TiO2 = 2.148, Al2O3 = 14.025, Fe2O3 = 17.925,nO = 0.238, MgO = 5.406, CaO = 8.932, Na2O = 2.229, K2O = 0.409,

2O5 = 0.193, and in mol.% is SiO2 = 53.52, Al2O3 = 9.07, CaO = 10.5,gO = 8.84, FeO = 13.03, Na2O = 2.37, TiO2 = 1.77, O = 0.89.Pseudosection calculations were performed with THERMO-

ALC 3.33 (Powell et al., 1998), using the November 2003pdated data set (file tcds55.txt) of Holland and Powell (1998).ctivity–composition relationship (a–x) models used were updated

rom those presented for garnet (White et al., 2007), clinopyroxeneGreen et al., 2007), orthopyroxene (White et al., 2002), hornblendeDiener et al., 2007), plagioclase (Holland and Powell, 2003), epi-ote (Holland and Powell, 1998), and ilmenite, magnetite (Whitet al., 2000). Quartz, rutile, and H2O are pure end-member phases.ineral symbols are after Kretz (1983).The pseudosection for representative sample 10SD01 is pre-

ented in Fig. 4, ranging from 550–900 ◦C and 5–12 kbar. The phaseiagram is characterized by trivariant, quadrivariant and quini-ariant fields with minor divariant and sextivariant fields (Fig. 4).he temperature-dependent hornblende-out line lies from 5 kbart 775 ◦C heading through 11.3 kbar at 885 ◦C towards 12 kbar at65 ◦C, and accordingly hornblende occurs in most of the diagramFig. 4), which is consistent with observation under microscope.rthopyroxene-in line is also mainly temperature-dependent andxtends from 5 kbar at 745 ◦C upwards to 11.5 kbar at 900 ◦C (Fig. 4).arnet is stable above 7.2 kbar over a wide temperature range,ith the slope of garnet-out line negative below 760 ◦C and pos-

tive above (Fig. 4). Clinopyroxene-in line (diopside in the diagram)s strongly temperature-dependent below 610 ◦C above 7 kbar butressure-dependent from 610 ◦C to 750 ◦C below 7 kbar (Fig. 4).

lagioclase begins to disappear in high-pressure low-temperaturerea, and epidote appears in low-temperature area and isemperature-dependent at pressures higher than 8.3 kbar (Fig. 4).lmenite-out and rutile-out lines are both pressure-dependent

ch 222– 223 (2012) 312– 324

at the high-pressure areas (above 10 kbar), and the coexistenceof ilmenite and rutile is restricted to a narrow band zone (Fig. 4).Magnetite is stable at relative high-temperature medium-pressureareas (Fig. 4).

The pseudosection is contoured with compositional isoplethsof XCa(g) and XFe(g) in garnet, XFe(opx) in orthopyroxene and XCa(pl)in plagioclase for the relevant mineral assemblages in order toconstrain the conditions of metamorphic stages. The XCa(g) iso-pleths have very steep positive slopes in the fields of hb-di-g-pl-ilm(-q-H2O) and hb-di-g-pl-ilm-mt (-q-H2O) as well as moderatelypositive slopes in the fields of hb-di-g-opx-pl-ilm (-q-H2O) andhb-di-g-opx-pl-ilm-mt (-q-H2O). These steep XCa(g) isopleths arestrongly temperature-controlled and their values decrease as tem-perature increases in the large quadrivariant field of hb-di-g-pl-ilm(-q-H2O). On the contrary, the XFe(g) isopleths have steep negativeslopes in the fields of hb-di-g-pl-ilm (-q-H2O), hb-di-g-pl-ilm-mt(-q-H2O) and hb-di-g-opx-pl-ilm (-q-H2O) but very steep positiveslopes in the hb-di-g-opx-pl-ilm-mt (-q-H2O) fields. As the tem-perature goes up, XFe(g) decreases in all the four fields describedabove. Isopleths of the anorthite content of plagioclase (XCa(pl)) arestrongly pressure-dependent and their values decrease as pres-sure increases in the field of hb-di-g-pl-ilm (-q-H2O), whereas ingarnet-absent fields, the XCa(pl) isopleths are mainly temperature-dependent and their values increase with temperature increasing.The steep sloped XFe(opx) isopleths in the fields of hb-di-opx-pl-ilm-mt (-q-H2O) and hb-di-g-opx-pl-ilm-mt (-q-H2O) show a sig-nificant temperature-dependence and their values decrease withtemperature increasing. In the hornblende-absent fields, however,XFe(opx) is mainly pressure-dependent. In the fields of hb-di-g-opx-pl-ilm-mt (-q-H2O) and hb-di-g-opx-pl-ilm (-q-H2O), XFe(opx)decreases continuously as both temperature and pressure increase.

As discussed earlier, the observed pre-peak (M1) assemblagerepresented by the hornblende + plagioclase + quartz + ilmenite +magnetite mineral inclusions matches the modeled stable fieldof hb-pl-ilm-mt (-q-H2O) whose temperature ranges between660 and 730 ◦C and pressure is lower than 6.5 kbar (Fig. 4). Inthis field, the XCa(pl) isopleths are mainly controlled by temper-ature. However, the measured XCa(pl) values (0.315–0.431) of theinclusion-type plagioclase do not match the calculated XCa(pl) val-ues which increase from 0.55 to 0.58 in this field. This may haveresulted from the re-equilibration of the inclusion-type plagioclaseduring the peak or post-peak metamorphism (e.g. Wei et al., 2007).Relatively fast rates of ion diffusion during the peak metamorphismmake the prograde minerals difficult to preserve their original com-positions. Therefore, the P–T conditions of the pre-peak (M1) stagecan be approximated at 660–730 ◦C and <6.5 kbar, which is equiv-alent to the upper-amphibolite facies.

The peak (M2) assemblage of orthopyroxene + clinopyroxene +plagioclase + garnet + quartz ± hornblende corresponds to thedivariant stability field of hb-di-g-opx-pl-ilm-mt (-q-H2O),which is a narrow band with P–T conditions of 760–860 ◦C and7.1–9.6 kbar (Fig. 4). In this field, the temperature-dependent XFe(g)decreases from 0.83 to 0.71 as temperature rises, while XCa(g)decreases in a very small range from 0.21 to 0.19 with increasingtemperature and pressure (Fig. 4). XFe(opx) also decreases from0.63 to 0.48 with increasing temperature but changes little withpressure (Fig. 4). The measured XFe(g) values (0.771–0.788) ofporphyroblastic garnet and the XFe(opx) values (0.549–0.564) oforthopyroxene fall in a narrow area, which defines P–T conditionsof 800–820 ◦C and 8.0–8.5 kbar. Generally, the Fe–Mg exchange ofgarnet is much more sensitive to a temperature variation than thatof orthopyroxene, and thus, orthopyroxene is more common to

preserve the peak metamorphic conditions than garnet (Aranovichand Berman, 1997). Taken together, the P–T conditions of the peak(M2) stage can be approximately estimated at 800–820 ◦C and8.0–8.5 kbar.

M. Wu et al. / Precambrian Research 222– 223 (2012) 312– 324 321

Fig. 4. P–T pseudosection calculated for the Yishui mafic granulites (sample 10SD01) in the NCFMASHTO (+ quartz + H2O) system. The pseudosection is contoured withi ferentP anulia

unwpagcTtc(

6

o

sopleths of XCa(g), XFe(g), XFe(opx) and XCa(pl) for corresponding mineral assemblages. Dif–T path involving near-isobaric cooling (IBC) is reconstructed for the Yishui mafic grnd mineral chemistry in the sample. Mineral symbols are after Kretz (1983).

After the peak granulite-facies stage, orthopyroxene becamenstable and reacted with plagioclase, forming the gar-et + quartz ± opaque minerals symplectic coronas, which coexistith clinopyroxene, hornblende and plagioclase. Therefore, theost-peak (M3) assemblage is consistent with the wide quadrivari-nt field of hb-di-g-pl-ilm (-q-H2O). In this field, as temperatureoes down, both XCa(g) and XFe(g) increase with the addition of cal-ium and iron mainly from plagioclase and magnetite, respectively.he measured compositions of symplectic garnet match well withhe modeled XFe(g) and XCa(g) values, which define equilibrated P–Tonditions of 686–710 ◦C and 7.6–8.6 kbar for the post-peak stageM3).

. P–T path and tectonic implications

As shown in Fig. 4, the synthesis of metamorphic petrol-gy and quantitative P–T pseudosection modeling defines an

field shading indicates different variances of mineral assemblage. The anticlockwisetes by comparing the modeled assemblages and isopleths with mineral assemblages

anticlockwise P–T path involving near-isobaric cooling for theYishui mafic granulites. The establishment of such a P–T path isbased on the assumption that the mineral compositions and reac-tion textures used to define the P–T path resulted from a singlemetamorphic cycle rather than two or more unrelated metamor-phic events. A single-cycle granulite-facies model is favored in thispaper because we have not found any petrographic and geochrono-logic evidence for the existence of a second regional metamorphicevent in this area.

The pre-peak metamorphism (M1) represented by the mineralinclusions of hornblende + plagioclase + quartz + ilmenite + mag-netite within garnet porphyroblasts suggests that the Yishuimafic granulites underwent amphibolite-facies metamorphism

before the peak granulite-facies metamorphism. Because ofre-equilibrium during the peak (M2) and post-peak (M3) meta-morphism, the measured compositions of the inclusion-typeminerals (plagioclase) do not match the modeled isopleths of the

3 esearch 222– 223 (2012) 312– 324

cmieiaf

eve8o+goAati

(sscntc1

twprmn

lEti(tagapmppsptraiPt(Aen

aom

0

2

4

6

8

10

12

500 600 700 800 900

Temperature ( C)Pr

essu

re (

kbar

)

1

2

3

45

6

7

8

14

400 1000

Fig. 5. Metamorphic P–T paths of Late Archean basement complexes in theEastern Block of the North China Craton (modified after Zhao et al., 1998).

22 M. Wu et al. / Precambrian R

orresponding minerals, and thus the P–T conditions of the M1etamorphism cannot be quantitatively estimated by using the

sopleths of the inclusion-type minerals. However, a qualitative P–Tstimation can be made based on the modeled stable field of hb-pl-lm-mt (-q-H2O) in the NCFMASHTO system, which matches the M1ssemblage and defines P–T conditions of <6.5 kbar and 660–730 ◦Cor the M1 stage.

Despite some uncertainties caused by the Fe–Mg exchangequilibria during the later down-temperature processes, The XFe(g)alues of peak garnet and the XFe(opx) values of orthopyrox-ne define the medium-pressure granulite-facies conditions of00–820 ◦C and 8.0–8.5 kbar for the peak (M2) assemblage ofrthopyroxene + clinopyroxene + plagioclase + garnet + quartz

hornblende + ilmenite + magnetite. Such medium-pressureranulite-facies conditions suggest that the peak metamorphismf the Yishui mafic granulites occurred at the level of lower crust.s shown in Fig. 4, the metamorphic process from M1 to M2 is char-cterized by both temperature- and pressure-increasing, indicatinghat the metamorphism of the Yishui mafic granulites was not onlynvolved in the input of heat but also a crustal thickening process.

The compositions of symplectic garnet of the post-peak stageM3) record relatively lower temperatures of 686–710 ◦C but pres-ures of 7.6–8.6 kbar similar to that of the peak stage (M2). Thisuggests that the Yishui mafic granulites underwent a near-isobaricooling process following the peak M2 stage (Fig. 4). Similar gar-et + quartz symplectic coronas recorded in other granulite-facieserrains are considered to be related to isobaric or near-isobaricooling metamorphic processes (Appel et al., 1998; Bohlen et al.,985; Harley, 1985, 1988, 1989; Zhao et al., 1999).

In summary, the P–T path of the mafic granulites suggests thathe Yishui mafic granulites underwent metamorphic processes thatere initiated by the upper amphibolite-facies prograde metamor-hism (M1), and then underwent the peak metamorphism (M2)eaching granulite facies, followed by near-isobaric cooling meta-orphism (M3), which defines an anticlockwise P–T path involving

ear-isobaric cooling.As summarized by Zhao et al. (1998) and Zhao et al. (2001a,b),

ate Archean metamorphic rocks from other complexes in theastern Block of the NCC also underwent regional metamorphismhat is characterized by an anticlockwise P–T path involving near-sobaric cooling. These metamorphic rocks underwent the pre-peakM1), peak (M2) and post-peak (M3) metamorphic stages, of whichhe pre-peak stage (M1) is represented by inclusion-type mineralssemblages of hornblende + plagioclase + quartz ± biotite in maficranulites, chlorite + actinolite + epidote + plagioclase + quartz inmphibolites, and biotite + plagioclase + quartz ± andalusite inelitic gneisses. The peak stage (M2) is indicated by matrix-typeineral assemblages of orthopyroxene + clinopyroxene + garnet +

lagioclase + quartz ± hornblende in mafic granulites, hornblende +lagioclase + quartz ± garnet in amphibolites, and garnet +illimanite + plagioclase + quartz + biotite in pelitic gneisses. Theost-peak stage (M3) is shown by garnet + quartz symplec-ic coronas in mafic granulites, garnet + actinolite retrogressiveims surrounding hornblende or garnet grains in amphibolites,nd kyanite replacing sillimanite or staurolite replacing silliman-te + garnet in pelitic gneisses. These mineral assemblages and their–T estimates, mainly based on traditional geothermobarome-ers, define anticlockwise P–T paths involving near-isobaric coolingZhao et al., 1998; Fig. 5). It is amazing that metamorphism of laterchean basement rocks over the ∼800 km wide Eastern Block isxclusively characterized by an anticlockwise P–T path involvingear-isobaric cooling.

Generally, anticlockwise P–T paths of metamorphic rocks reflectn origin related to the intrusion and underplating of large amountsf mantle-derived magmas that not only provide heat for the meta-orphism but also add a large volume of mostly mafic material

1, Taishan greenstone in Western Shandong; 2, Eastern Hebei; 3, Western Liaoning;4, Northern Liaoning; 5, Eastern Shandong; 6, Miyun-Chengde; 7, Southern Jilin;8, Yishui mafic granulite (this study).

to the base of the crust (Bohlen, 1987, 1991). Large volumes ofunderplating magma leading to metamorphism with an anticlock-wise P–T path may occur in continental rift settings (Sandiford andPowell, 1986), continental magmatic arc regions (Bohlen, 1991),and mantle-plume tectonic regimes (Condie, 1997; Jayananda et al.,1998, 2000; Zhao et al., 1999). Among these three potential tectonicsettings in which substantial magmatic activity occurs, a conti-nental rift environment is inappropriate for the formation of LateArchean basement rocks in the Eastern Block of the NCC, as it can-not reasonably explain: (1) the extensive exposure of ca. 2.6–2.5 Gagranitoids over a width of more than 800 km; (2) predominantdomal structures; and (3) lack of abundant alkali intrusions whichare generally associated with rifting (Zhao et al., 1998).

Both the mantle-plume and continental magmatic arc modelsare suitable to explain the close temporal relationship betweenthe primary emplacement of granitoid magmas and subsequentregional metamorphism involving anticlockwise P–T paths in theEastern Block of the NCC. However, a mantle-plume model hasbeen favored (Zhao et al., 1998, 2001b; Ge et al., 2003; Geng et al.,2006), because this model can reasonably explain: (1) the excep-tionally large exposure of granitoid intrusions that formed over ashort time period (2.6–2.5 Ga), without systematic age progressionacross a ∼800 km wide terrain; (2) affinities of mafic rocks to con-tinental tholeiitic basalts; (3) dominant diaprism-related domalstructures; and (4) bimodal volcanic assemblages (basaltic rockson the one hand, and dacitic, rhyodacitic and rhyolitic rocks on theother), which are different from Phanerozoic magmatic arc assem-blages where andesites are the predominant volcanic-rock types(Hamilton, 1998).

Available petrological and geochemical, data for the WesternShandong Complex are consistent with a mantle plume model.

Petrologically, the protoliths of the Yishui Group are principallybasic and acid, similar to the Archean bimodal volcanics (Shen et al.,1992), which are considered to have formed in an extensional set-ting, not a compressive environment like a magmatic arc (Hamilton,

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M. Wu et al. / Precambrian R

998, 2011). In the Western Shandong Complex, the Taishan Group,hich is exposed in an area west of the Yishui Complex, contains

omatiites with typical spinifex textures that have been interpreteds products of mantle plumes based on their geochemical featuresnd extremely high rock-forming temperatures (Polat et al., 2006;heng and Kusky, 2007). The Yishui Group also contain komatiiticocks, though their rare earth elements patterns suggest possiblerustal contamination, which might be caused by the interactionetween the mantle plume and the lower crust (Song et al., 2009;hao et al., 2009). Therefore, a mantle plume model can well explainhe rock assemblages of the Western Shandong Complex and theireochemical features as well as the metamorphic evolution of theishui mafic granulites characterized by an anticlockwise P–T path

nvolving isobaric cooling. Similar mantle plume models have alsoeen applied to the origins of mafic granulites with anticlockwise–T paths in other Archean terrains, including the South Indianlock (Raith et al., 1983; Jayananda et al., 1998, 2000) and the Pik-itone granulites from the Superior Province (Mezger et al., 1990;

omlinson et al., 1998).Based on all those considerations above and the result of this

tudy, we present the following brief scenario for the late Archeanectonothermal evolution of the Western Shandong Complex in theastern Block of the NCC.

At 2.6–2.5 Ga, a mantle plume occurred beneath the Westernhandong area. The plume heads uplifted the mantle lithospherend overlying continental crust and caused lithospheric stretch-ng, leading to extensive eruption of ultramafic to mafic volcanicocks (komatiites and basalts), represented by the Yishui Groupnd the Taishan greenstone belt. The heat transfer from the plumeo the upper mantle or lower crust resulted in extensive par-ial melting of basaltic or amphibolitic rocks, forming enormousTG plutons in the Western Shandong area. Meanwhile, the heatransfer from the plume to the crust also led to regional metamor-hism. At first, the relatively cooler plume head heated the crust,ausing prograde metamorphism (M1) of greenschist to amphi-olite facies of which the former occurred in the crust fartherway from the head, whereas the latter occurred in the crust adja-ent to the plume head. As large volumes of magma were addedo the base of the crust and some intruded to higher levels, therustal was slightly thickened with both temperature and pressurencreasing. Subsequently, the hot plume “tail” continued rising andeated the crust, causing the peak metamorphism (M2) at amphi-olite to granulite facies, depending on the distance to the plume.t also resulted in widespread anatexis of TTG gneisses, formingyn-tectonic charnockites in the Yishui area and monzogranitesn the Taishan area. Finally, when the effect of heating ceasedhrough the termination of plume activity, the heated crust experi-nced near-isobaric cooling (M3). These tectonothermal processesre consistent with the anticlockwise P–T path involving isobaricooling recorded in the Yishui mafic granulites from the Westernhandong Complex in the Eastern Block of the North China Craton.

cknowledgments

This research was financially funded by Chinese NSFC Grants40730315, 40872123 and 41072152), and Hong Kong RGC GRFrants (7066/07P and 7053/08P). We are grateful to Prof. Zeminghang and another anonymous reviewer for their valuable com-ents and corrections on this manuscript.

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