neoarchean to paleoproterozoic high-pressure mafic ... to... · minor intrusions of mesozoic...

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Neoarchean to Paleoproterozoic high-pressure mafic granulite from the JiaodongTerrain, North China Craton: Petrology, zircon age determination andgeological implications

Shoujie Liu a,b,⁎, Bor-ming Jahn b, YushengWan a, Hangqiang Xie a, ShijinWang c, Shiwen Xie a, ChunyanDong a,Mingzhu Ma a, Dunyi Liu a

a Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, Chinab Department of Geosciences, National Taiwan University, Taipei, Taiwanc Shandong Geological Survey Institute, Jinan 250013, China

⁎ Corresponding author at: Beijing SHRIMP Center,Academy of Geological Sciences, 26 Baiwanzhuang Roa+86 10 68999759.

E-mail address: [email protected] (S. Liu).

http://dx.doi.org/10.1016/j.gr.2014.07.0061342-937X/© 2014 International Association for Gondwa

Please cite this article as: Liu, S., et al., NeoaCraton: Petrology, zircon age determination

a b s t r a c t
a r t i c l e i n f o
Article history:Received 1 May 2014Received in revised form 21 July 2014Accepted 29 July 2014Available online xxxx

Handling Editor: M. Santosh

Keywords:High-pressure granuliteNeoarcheanNorth China CratonSHRIMPPseudosection modeling

The North China Craton is an ideal place for studying the transition of the Earth's thermal structure and tectonicsat the Archean–Proterozoic boundary due to its good preservation of the ~2.5 Ga tectono-thermal events. Wereport the discovery of a high-pressure mafic granulite from the Jiaodong Terrain in the North China Craton.The mafic granulite occurs as garnet–clinopyroxene–orthopyroxene–hornblende gneiss enclaves within a late-Archean trondhjemite–tonalite–granodiorite (TTG) gneiss. Typical high-pressure mineral assemblage ofgarnet–clinopyroxene–plagioclase–quartz ± rutile has been identified. Plagioclase + clinopyroxene ±orthopyroxene ± hornblende symplectite surrounding garnet (“white eye”) is also observed. Using theconventional geothermobarometry and the pseudosection modeling, a clockwise metamorphic P–T path withthe peak conditions at ~17 kbar and ~880 °Cwas determined. ZirconU–Pb analyses (SHRIMP) on the overgrowthrim of zircon grains of two samples from the same outcrop yielded ametamorphic age of 2473±6Ma (MSWD=0.8). The analyses on magmatic core gave a probable magmatic age of 2527 ± 12Ma (MSWD= 1.9). The high-pressure granulite faciesmetamorphismcorresponds to a collisional event between the ~2.5 Ga crust and ~2.9 Gacrust at the dawn of Paleoproterozoic in the North China Craton. It also represents a new but rare case of asubduction–collision tectonics at the Archean–Proterozoic transition and provides insight into the change ofthe Earth's thermal structure.

© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The secular change of the Earth's thermal structure throughout thegeological history is a topic of broad interest and is considered to beresponsible for the change of metamorphic style and tectonic settingof the continental crust (Brown, 2007a,b; Condie and Kröner, 2013).High P/T (low thermal gradient, b20 °C/km) metamorphism rarelyoccurred till the end of Paleoproterozoic. The only known cases include:(1) a garnet-bearing mafic granulites from the Lewisian Complex ofnorthwestern Scotland (Sajeev et al., 2013), (2) a kyanite–garnet-bearing felsic granulite (2.49 Ga) from the Salem block of southernIndia (Anderson et al., 2012), (3) a chlorite- and phengite-bearinggneiss (2.2–2.0 Ga) from the Birimian Terrain of theWestAfrican Craton(Ganne et al., 2012), (4) the Meso-Neoarchean Belomorian eclogiteprovince in the Kola Peninsula (Mints et al., 2010, 2012), (5) the

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rchean to Paleoproterozoic hand geological implications, G

garnet–albite-bearing para-amphibolites from the MesoarcheanBarberton granitoid–greenstone terrain (Moyen et al., 2006), and(6) the garnetwebsterites from the early Paleoproterozoic SharyzhalgaiComplex, Siberia Craton (Ota et al., 2004). Ultrahigh-pressure meta-morphic (UHP) rocks occur commonly in the Phanerozoic, and theoldest UHP eclogites have only been found in the Neoproterozoic(Jahn et al., 2001). The rare occurrences of UHP–HP metamorphicrocks have provoked the controversy about (1) the tectonicsetting(s) in which the rocks were formed (O'Brien and Rötzler, 2003;Brown, 2007a; Sizova et al., 2010); (2) the mechanism andmode of ex-humation of the rocks (vanHunen and van den Berg, 2008); and (3) theproblem of preservation of HP–UHP index minerals (Anderson et al.,2012).

The North China Craton has become a highlight in the study of earlyPrecambrian geology due to the followingmain research progress in thepast decades (Zhai and Santosh, 2011 and references therein): (1) theidentification of Eoarchean rocks including the 3.8 Ga TTG gneisses inAnshan, Liaoning Province (Liu et al., 1992; Song et al., 1996; Wanet al., 2001, 2005, 2011c) and ~3.8 Ga detrital zircons in the supracrustalrocks in Caozhuang, eastern Hebei Province (Huang et al., 1986; Jahn et

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igh-pressure mafic granulite from the Jiaodong Terrain, North Chinaondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.07.006

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2 S. Liu et al. / Gondwana Research xxx (2014) xxx–xxx

al., 1987; Liu et al., 1992, 2007, 2013;Wuet al., 2005;Wilde et al., 2008);(2) the identification of significant crustal growth at ~2.9, ~2.7 and2.5 Ga; (3) the still-debated issue on the time of cratonization and itstectonic framework; and (4) the discovery of late-Paleoproterozoichigh-pressure granulites and ultrahigh-temperature metapelites (Guoet al., 2006; Santosh et al., 2006; Liu et al., 2007, 2008; Santosh et al.,2007; Tsunogae et al., 2011; Guo et al., 2012; S.J. Liu et al., 2012;Zhang et al., 2012) and the related discussion on the Paleoproterozoicorogenic belts.

The high-pressure granulites documented so far are mostly pro-duced at the end of Paleoproterozoic; for example, (1) garnet-bearingmetabasic dykes in the Hengshan area (Zhao et al., 2001), Sangganarea (Guo et al., 2002, 2005), and Chengde area (Li et al., 1998; Maoet al., 1999), and high-pressure pelitic granulites in the Sanggan area(Ma andWang, 1995), which are related to the formation of the CentralOrogenic Belt (Zhao et al., 2001); (2) kyanite–K-feldspar-bearing high-pressure pelitic granulites in the Qianlishan–Helanshan area (Zhouet al., 2010) during the formation of the Khondalite Belt (Zhao, 2001)or the Inner Mongolia Suture Zone (Santosh, 2010); and (3) high-pressure mafic granulites and high-pressure pelitic granulites in theJiaobei Massif or Jiaodong Terrain during the formation of Jiao–Liao–Jibelt (Liu et al., 1998; Zhou et al., 2004; J.B. Zhou et al., 2008; X.W.Zhou et al., 2008; Liu et al., 2010; P.H. Liu et al., 2011; Tam et al., 2011;Tam et al., 2012a,b). However, high-pressure granulites of Archeanages are rarely documented in the North China Craton. The only casefound is from the Jianping Complex, western Liaoning Province. Themetamorphism was characterized by an anti-clockwise P–T path with

Fig. 1.A. Contour of theNorth China Craton (after Zhao, 2001) and locality of Eastern Shandongstudy area.

Please cite this article as: Liu, S., et al., Neoarchean to Paleoproterozoic hCraton: Petrology, zircon age determination and geological implications, G

the peak PT condition at 10.5 ~ 11.7 kbar and 785 ~ 820 °C (Cui et al.,1991; Wei et al., 2001).

In this study, we report the discovery and result of study of a maficgranulite from the Jiaodong Terrain. The mafic granulite occurs asenclaves in a TTG gneiss in the Hexikuang Reservoir of Qixia City. Themetamorphic evolution is traced based on the mineral assemblagesand reaction textures. The metamorphic P–T condition is calculatedusing the conventional geothermobarometry and the pseudosectionmodeling. The timing of the metamorphism will be constrained by adetailed zircon dating using the SHRIMP analytical technique, and thegeological implications will be discussed.

2. General geology in Jiaodong Terrain

The Jiaodong Terrain is an important part of the North China Craton(Fig. 1A). It is composed of early Precambrian rocks, covered by sedi-mentary sequences and intruded by Mesozoic granitoids. The terrainis bounded by the Yantai–Wulian Fault to the southeast against theSulu UHPmetamorphic belt (Fig. 1B). The early Precambrian rocks com-prise (1) the Paleoproterozoic Fenzishan “Group” composed of pelitic–psammitic metasediments and marble with subordinate greenstones;(2) the Paleoproterozoic Jinshan “Group” composed of sillimanite–bio-tite–quartz schist, biotite leptite and gneiss, andmarblewithminor am-phibolite; and (3) and Archean basement made up of TTG gneisses,granitoids and related plutonic rocks with minor supracrustal rocks. Azircon geochronological study established three major tectonothermalevents at ~2.9, ~2.7 and ~2.5 Ga (Jahn et al., 2008).

Province. B. The simplified geological map of Eastern Shandong Province and locality of the


image of Fig.�1


3S. Liu et al. / Gondwana Research xxx (2014) xxx–xxx

The Archean TTG gneisses and associated rocks were emplaced intwo stages at ~2.9 and ~2.7 Ga (Jahn et al., 2008; J.H. Liu et al., 2011;Xie et al., under review). The TTG gneisses of the two stages are similarregarding the field occurrence, petrography, whole-rock chemistry,metamorphism, deformation, and local anatectic feature (Jahn et al.,2008; J.H. Liu et al., 2011; Xie et al., under review). Note that theyhave been subjected to the same 2.5 Ga metamorphic event. The twostages of TTG gneisses are so similar that they can only be distinguishedby geochronology (Xie et al., under review). Aside from TTG gneisses,granitoids of 2.5 Ga of age, which occur mainly as high-Si trondhjemiticrocks (Tanggezhuang suite), have also been identified within this areaand they are probably produced by partial melting of older TTG sources(J.H. Liu et al., 2011, 2012). In general, the deformation of the 2.5 Gagranitoids is not as strong as the older counterparts with few excep-tions. Consequently, it is difficult to distinguish them in the field.Furthermore, some of the 2.5 Ga granitoids underwent metamorphismat the end the late Paleoproterozoic (Wan et al., 2011a).

The ~2.9 Ga TTG gneisses from the Huangyandi locality are found tocontain metamorphosed supracrustal rocks. These rocks occur as smallenclaves of varied rock types from amphibolite to leptite. Zircon age

Fig. 2. Geological map of the southeast of Qixia Ci


study revealed that these rocks formed at ~2.9 Ga and underwentmetamorphism at 2.5 Ga (Jahn et al., 2008; J.H. Liu et al., 2011). Anothersuite of supracrustal rocks, named the “Jiaodong Group”, was probablyformed during the Neoarchean, but no zircon U–Pb ages have beenobtained for confirmation (J.H. Liu et al., 2011; Wan et al., 2012a).Note that their lithological characteristics are similar to that of theolder (~2.9 Ga) supracrustal rocks.

3. Local geology and sampling

In a recent field study around the Qixia City, mafic granulites werediscovered at a locality near the southern part of the Hexikuang waterreservoir (Fig. 2). The rocks occur as enclaves within NeoarcheanTTG gneisses. In fact, this region is dominated by TTG gneisses ofMesoarchean to Neoarchean ages. The supracrustal rocks in this areaare mainly biotite–plagioclase gneisses and leptite with minor inter-layers of amphibolite of Jiaodong “Group”. In addition to the Archeanrocks, Proterozoic mafic rocks are also found to intrude into the TTGgneisses. The terminal magmatic activities are represented by the

ty (after Jahn et al., 2008; Wan et al., 2011b).


image of Fig.�2



minor intrusions of Mesozoic granite and eruption of Tertiary basalt insome areas.

At the Hexikuang reservoir, the building of the dam has providedfresh outcrops. To the north of the dam, a gneissic quartz diorite and atonalite of 2.9 Ga have been identified (Xie et al., under review). Tothe south of the dam, TTG gneisses of 2.5 Ga with enclaves of Grt-bearing mafic granulites are the dominant rock types. The Grt-bearingmafic granulites have a regional NW-trending foliation, which is similarto that of the surrounding TTG gneisses (Fig. 3A). The size of the maficgranulite enclaves ranges from several to more than 10 m. Thus, theyare giant enclaves.

The mineral assemblages and grain size show a gradual change inthe outcrop. The fine-grained domain is mainly composed of garnet,clinopyroxene, hornblende, orthopyroxene and plagioclase with minorquartz (Fig. 3B) whereas garnet, clinopyroxene and orthopyroxenebecome less abundant in the coarser domains. In some domains, tinygrains of garnet are surrounded by very thin light-colored rims whichare further identified as the symplectite of plagioclase + clinopyroxeneor plagioclase+ hornblende (“white eye”). A complete retrogression ofgarnet into plagioclase and hornblende is also observed as shown bywhite dots of garnet pseudomorphs (Fig. 3C and D).

We collected eight samples with different grain sizes numberedfrom QX12118-1 to QX12118-8. All the samples are garnet–clinopyroxene–orthopyroxene–hornblende gneisses. The onlydifference between them is the grain size and phase proportion.The mineral assemblages and reaction textures are best presentedin sample QX12118-6, which was subject to the most detailedstudy in petrography, mineral chemistry and petrology. Two largesamples, S1205 and S1207, were collected from the same outcropfor zircon age determination. Sample S1205 is gneissic garnet-bearing amphibolite whereas sample S1207 is gneissic amphibolitewith garnet pseudomorph. A host gneissic trondhjemite sample,QX12117 was also collected for age determination. Leucosomeveins are avoided when collecting the samples.

Fig. 3. Field photos of themafic rocks at the southern dam of Hexikuang reservoir. A. Gneissic gagarnet grains inside the garnet amphibolite. C. Gneissic amphibolitewith garnet pseudomorphsdots”). (For interpretation of the references to color in this figure, the reader is referred to the


4. Analytical methods

4.1. Mineral chemistry

Mineral chemistry of sample QX12118-6 was analyzed using a JXA-8230 electron microprobe analyzer (EPMA) at the Institute of MineralResources, Chinese Academy of Geological Sciences. Mineral structuralformulae were calculated for fixed oxygen. Fe3+was calculated by stoi-chiometric charge balance. Typical mineral assemblages and textureswere also documented by backscattered electron (BSE) imaging.

4.2. Whole rock chemistry

The major element composition of sample QX12118-6 was ana-lyzed using an X Ray Fluorescence spectrometer (XRF, PW4400),whereas FeO and Fe2O3 contents were determined by the wetchemical method at the National Research Center of Geoanalysis,Chinese Academy of Geological Science following the standard pro-cedures (Zhang and Ye, 1987).

4.3. Zircon geochronology

Zircon grains were extracted from samples S1205, S1207 andQX12117 following the standard procedures at the Beijing SHRIMPCenter, Chinese Academy of Geological Sciences. Zircon grains weremounted on a double-sided adhesive tape and enclosed in epoxy resindisks together with reference zircon standards TEMORA1 (206Pb/238Uage = 417 Ma) and M257 (U = 840 ppm) (Black et al., 2003; Nasdalaet al., 2008). The disks were then polished and gold coated for the fol-lowing cathodoluminescence (CL) imaging and U–Th–Pb isotopeanalyses.

The morphology, internal texture and structure of zircon grainswere examined by a joint imaging process using a transmitted andreflected light microscope and a HITACHI S3000-N Scanning

rnet amphibolite (Sample S1205) enclaveswith felsic veins inside the TTG gneiss. B. Small(Sample S1207) next to the gneissic garnet amphibolite. D. Garnet pseudomorphs (“whiteweb version of this article.)


image of Fig.�3



Electron Microscope (SEM) equipped with a ROBINSON backscatter probe and a Gatan Chroma CL probe at the Beijing SHRIMPCenter.

Zircon U–Th–Pb isotope analyses were performed using SHRIMPII following the standard procedures (Williams, 1998). The primaryion beam of O2− at the intensity of 6 nA was used to impinge zircongrains with spot size of 25 μm. Following 150 s of rastering for eachanalytical spot, 5 scans were made through the nine mass stationsincluding 196Zr2O, 204Pb, background, 206Pb, 207Pb, 208Pb, 238U,248ThO and 254UO. Reference zircon M257 was analyzed for ele-mental abundance calibration and TEMORA1 was analyzed for cali-bration of 206Pb/238U after every 3–4 analyses on unknowns. Themeasured 204Pb abundances were used for the common lead cor-rection. Data reduction was done using the SQUID and ISOPLOTsoftware (Ludwig, 2001, 2003). Errors on individual analysis arebased on counting statistics and at 1σ (one standard deviation),whereas the pooled analyses are quoted at 2σ or 95% confidencelevel.

5. General petrography

Sample QX12118-6 is a fine-medium grained mafic granulite con-sists of hornblende (25–30%), plagioclase (18–20%), garnet (13–15%),clinopyroxene (13–15%), orthopyroxene (8–10%), quartz (3–5%),opaque minerals (3–5%, mainly magnetite and ilmenlite) with accesso-ryminerals including rutile, zircon, chlorite and epidote. The other sam-ples have the same mineral assemblages, only with different phaseproportions. According to the inclusion relationship and reaction tex-tures of mineral phases, we have recognized three stages of metamor-phism, as described below.

5.1. Pre-peak stage (M1)

The pre-peak metamorphism (Fig. 4 A and B) is indicated by thecore of porphyroblast of garnet and its mineral inclusions, whichcomprise rutile (Rti), clinopyroxene (Cpxi), plagioclase (Pli), quartz(Qtzi), ilmenite (Ilmi) with or without epidote (Epi). The inclusionminerals are usually tiny with length smaller than 50 μm. Rutile issometimes associated with ilmenite (Fig. 4A). The composite inclu-sion of clinopyroxene and plagioclase is also observed (Fig. 4B).The core of garnet is regarded as growing in equilibrium with theseinclusion minerals.

5.2. Peak stage (M2)

The fine- to medium-grained matrix minerals represent thepeak stage metamorphism, including rim or mantle of garnetporphyroblast, matrix plagioclase (Plm), clinopyroxene (Cpxm)and quartz (Qtzm). Minor fine-grained rutile (Rtm) is also observedin association with these minerals. The rim or mantle of garnetporphyroblast is inclusion-free (Fig. 4B) and the boundary of garnetgrains are always embayed indicating later modification of thegrain. Anhedral matrix plagioclase and clinopyroxene grains havesizes from ca. 100 to 1000 μm, whereas anhedral quartz grains aresmaller. The peak mineral assemblage is the same as other typicalhigh-pressure mafic granulites (Green and Ringwood, 1967;O'Brien and Rötzler, 2003; O'Brien, 2008; Liu et al., 2010; Tamet al., 2012b).

5.3. Post-peak stage (M3)

The post-peak metamorphism is indicated by the reaction orbreakdown of the peak metamorphic minerals. The typical reactionduring retrograde metamorphism is the breakdown of garnet in thepresence of quartz with or without matrix clinopyroxene and O2

into symplectite of plagioclase and clinopyroxene or orthopyroxene


through the following reactions (Harley, 1989; Carswell andO'Brien, 1993; Zhao et al., 2001; Guo et al., 2002; O'Brien, 2008):

garnetþ clinopyroxenem þ quartz� O2→orthopyroxeneþplagioclase�magnetite

ð1Þ

garnetþ quartzþ O2→orthopyroxeneþ plagioclase�magnetite ð2Þ

garnetþ quartz→orthopyroxeneþ plagioclase: ð3Þ

In the case of the Hexikuang mafic granulite in this study, thesymplectites of clinopyroxene (Cpxs) + plagioclase (Pls) and clino-pyroxene (Cpxs)+ orthopyroxene (Opxs)+ plagioclase (Pls) surroundgarnet grain, forming a typical “white eye” texture (Fig. 4C and D). Thisreaction texture was regarded as the record of the post-peak metamor-phism with an isothermal decompression. Matrix orthopyroxene(Opxm) also occurs as anhedral grains but their sizes are smaller thanCpxm. The Opxm grains are closely associated with the symplectite orcorona surrounding the relic garnet grains and are regarded as post-peak stage mineral. Aside from the Cpxs + Opxs + Pls, the symplectiteof hornblende (Hbs) and plagioclase (Pls) also surrounds some garnetgrains. The Hbs + Pls symplectite may directly touch the garnet grain(Fig. 4D, E and F, left grain) or together with Cpxs + Pls (Fig. 4C, D).Some garnet grains are surrounded by plagioclase corona (Plc) withsmall grains of hornblende inside (Fig. 4E and F, middle and rightgrain). Hornblende also occurs as anhedral to subhedral matrix grains(Hbm) in pyroxene-free domain. Furthermore, exsolution of plagioclaselamella (Ple) is also very common in thematrix clinopyroxene (Fig. 4C),probably resulting from breakdown of Al-rich clinopyroxene after thepeak metamorphism.

It is noticed that, in certain domains of the same thin section,clinopyroxene and orthopyroxene are absent (Fig. 4G); onlymatrix gar-net, hornblende and plagioclase with minor quartz are present. Minorchlorite (Chl) is also found to intercept hornblende grains. This mineralassemblage is the same as the examined portion of sample S1205(Fig. 4H). However, this sample is slightly weathered, and garnet issurrounded by dirty rims of clay minerals, suggesting a breakdown ofgarnet to symplectite or corona (Fig. 4H).

6. Mineral chemistry

In response to the well represented mineral assemblages and re-action textures, we examined the mineral chemistry using the samethin section of sample QX12118-6.

6.1. Garnet (Table 1)

Based on the textural characteristics, garnet could be divided into aninclusion-bearing core domain and an inclusion-free mantle domain.The chemical compositions of three garnet grains are shown inTable 1. In some grains, rim shows a composition different from themantle (Table 1, grain A). All the examined garnet grains are dominatedby the almandine component (XAlm= 0.50–0.62)withminor grossular(XGrs = 0.14–0.28) and pyrope (XPrp = 0.16–0.2). The contents of an-dradite (XAdr= 0–0.06) and spessartine (XSps= 0.004–0.01) are ratherlow. Most garnet grains are characterized by weak compositional zon-ing which indicates that the grains have been homogenized during thepeak metamorphism. However, some grains show a significant compo-sitional change from core through mantle to rim. For example, the coreof grain A in Table 1 is characterized by an increase in the content ofgrossular and pyrope and a decrease in almandine from core to mantle.This indicates that the composition of the mantle domain correspondsto the peak metamorphism, whereas the core survived and maintainedits composition of the pre-peak stage. Furthermore, this grain also



Fig. 4. Representative microphotos and backscattered electron (BSE) images of samples QX12118-6 (A–G) and S1205 (H). A. BSE image showing rutile inclusion inside a garnetporphyroblast which is surrounded by symplectite of clinopyroxene and plagioclase. B. BSE image showing inclusions of quartz and clinopyroxene inside a garnet grain which issurrounded by corona of plagioclase; bar a–b marks the analytical line for the chemical composition of garnet grain with the results showing in Fig. 5; red dashed line marks the core–mantle boundary of the garnet grain. C. Polarized image showing the typical mineral assemblages and the symplectite surrounding garnet (“white eye”). D. Polarized image showingthe detail of the symplectite surrounding garnet from Fig. 4C. E. Polarized image showing garnet grains surrounded by corona of plagioclase; one relic garnet almost retrograded into pla-gioclase and hornblende (garnet pseudomorph). F. Same as Fig. 4E, cross nicols. G. Polarized image showing pyroxene-free domain which is consists of matrix garnet, hornblende andplagioclase. G. Microphoto showing similar mineral assemblages of the sample S1205 to the pyroxene-free domain of sample QX12118-6; possible corona surrounding the garnet ispresent; cross nicols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


Please cite this article as: Liu, S., et al., Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong Terrain, North ChinaCraton: Petrology, zircon age determination and geological implications, Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.07.006

image of Fig.�4


Table 1Chemical composition of representative garnet in associated with texture by EPMA. The calculation of garnet end members and composition are: XAdr = Fe3+/2; XGrs = (Ca − 3 ∗ XAdr)/(Fe2+ + Mn + Mg + Ca); XAlm = Fe2+ / (Fe2+ + Mn + Mg + Ca); XPrp = Mg / (Fe2+ + Mn + Mg + Ca); XSps = Mn / (Fe2+ + Mn + Mg + Ca); XFe = Fe2+ / (Fe2+ / Mg);XCa = Ca / (Fe2+ + Mg + Ca).

Grain A A A A B B B C C

Texture Core Mantle Mantle Rim Core Mantle Mantle Core Mantle

SiO2 38.43 37.93 38.17 37.75 38.27 39.30 39.17 38.71 38.11TiO2 0.06 0.08 0.09 0.02 0.07 0.17 0.11 0.03 0.09Al2O3 21.51 21.62 21.95 21.44 21.65 21.30 21.32 21.55 21.70Cr2O3 0.03 0.02 0.03 0.01 0.03 0.01 0.01 0.00 0.00FeO 27.90 24.33 25.27 29.70 25.00 24.38 25.10 24.94 25.68MnO 0.55 0.19 0.14 0.59 0.32 0.37 0.23 0.37 0.52MgO 4.72 5.11 5.05 4.23 5.29 5.07 4.77 4.68 4.31CaO 7.54 10.24 9.69 7.09 10.17 10.52 10.19 9.82 9.44Na2O 0.02 0.02 0.02 0.02 0.00 0.02 0.04 0.02 0.01K2O 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 100.73 99.53 100.38 100.84 100.78 101.13 100.91 100.13 99.85O 12 12 12 12 12 12 12 12 12Si 2.993 2.957 2.955 2.954 2.950 3.021 3.023 3.013 2.985Ti 0.003 0.005 0.005 0.001 0.004 0.010 0.006 0.002 0.005Al 1.977 1.988 2.005 1.979 1.969 1.933 1.941 1.978 2.005Cr 0.002 0.001 0.002 0.000 0.002 0.000 0.001 0.000 0.000Fe3+ 0.032 0.090 0.076 0.116 0.122 0.009 0.005 0.000 0.017Fe2+ 1.785 1.496 1.560 1.828 1.490 1.558 1.614 1.623 1.665Mn 0.028 0.010 0.007 0.030 0.016 0.019 0.012 0.019 0.027Mg 0.547 0.594 0.583 0.494 0.608 0.581 0.548 0.542 0.503Ca 0.629 0.855 0.804 0.595 0.840 0.867 0.842 0.819 0.792Na 0.003 0.003 0.003 0.003 0.000 0.003 0.006 0.003 0.000K 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000Zn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total cation 7.999 7.999 8.000 8.001 8.001 8.001 7.998 7.999 7.999XAdr 0.016 0.045 0.038 0.058 0.061 0.005 0.003 0.000 0.009XGrs 0.194 0.244 0.234 0.143 0.222 0.282 0.277 0.273 0.257XAlm 0.597 0.506 0.528 0.620 0.504 0.515 0.535 0.540 0.557XPrp 0.183 0.201 0.197 0.168 0.206 0.192 0.182 0.180 0.168XSps 0.009 0.003 0.002 0.010 0.005 0.006 0.004 0.006 0.009XFe 0.765 0.716 0.728 0.787 0.710 0.728 0.747 0.750 0.768XCa 0.212 0.290 0.273 0.204 0.286 0.288 0.280 0.274 0.268

Fig. 5. Compositional zoning of the garnet grain on Fig. 4B. Calculation formula of thegarnet end members is given in Table 1.

Table 2Chemical composition of representative plagioclase in associated with texture by EPMA.The calculation of plagioclase end members and composition are: An = (Ca / (Ca +Na + K)) ∗ 100; Ab = (Na / (Ca + Na + K)) ∗ 100; Or = (K / (Ca + Na + K)) ∗ 100.

Texture I M-rim M-core M-core S-Hb S-Cpx

SiO2 52.85 54.85 57.93 58.03 49.18 51.46TiO2 0.01 0.03 0.04 0.03 0.02 0.03Al2O3 29.90 28.75 25.88 25.73 31.04 30.53Cr2O3 0.04 0.00 0.01 0.00 0.00 0.02FeO 0.13 0.30 0.40 0.39 0.18 0.15MnO 0.00 0.00 0.06 0.00 0.00 0.01MgO 0.01 0.02 0.00 0.01 0.01 0.00CaO 12.37 10.68 8.34 8.60 15.76 13.38Na2O 4.45 5.32 6.85 6.67 2.46 3.79K2O 0.02 0.04 0.07 0.07 0.03 0.05ZnO 0.00 0.00 0.00 0.00 0.00 0.00Total 99.73 99.99 99.57 99.53 98.66 99.38O 8 8 8 8 8 8Si 2.398 2.473 2.605 2.614 2.284 2.352Ti 0.000 0.001 0.001 0.001 0.001 0.001Al 1.601 1.530 1.374 1.368 1.701 1.647Cr 0.002 0.000 0.000 0.000 0.000 0.001Fe3+ 0.000 0.000 0.000 0.000 0.000 0.000Fe2+ 0.005 0.011 0.015 0.015 0.007 0.006Mn 0.000 0.000 0.002 0.000 0.000 0.000Mg 0.000 0.001 0.000 0.001 0.000 0.000Ca 0.602 0.516 0.402 0.415 0.784 0.655Na 0.391 0.456 0.597 0.582 0.221 0.335K 0.001 0.002 0.004 0.004 0.001 0.003Zn 0.000 0.000 0.000 0.000 0.000 0.000Total cation 5.000 4.990 5.000 5.000 4.999 5.000An 61 53 40 41 78 66Ab 39 47 60 58 22 34Or 0 0 0 0 0 0

Note: S and M mean symplectite and matrix in texture respectively.


Please cite this article as: Liu, S., et al., Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong Terrain, North ChinaCraton: Petrology, zircon age determination and geological implications, Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.07.006

image of Fig.�5


Table 3Chemical composition of representative clinopyroxene in associated with texture by EPMA. XMg = Mg / (Mg + Fe2+).

Cpx I M-core M-core M-rim M-rim M-rim S S

SiO2 52.70 51.66 51.96 53.37 51.90 51.66 53.95 51.98TiO2 0.02 0.24 0.20 0.12 0.19 0.14 0.03 0.05Al2O3 1.12 2.39 1.82 1.32 1.62 1.73 0.83 0.80Cr2O3 0.02 0.00 0.02 0.02 0.01 0.06 0.03 0.00FeO 11.53 11.15 10.18 8.97 12.88 10.63 9.79 14.18MnO 0.06 0.12 0.10 0.00 0.12 0.10 0.10 0.09MgO 12.94 12.13 12.08 13.26 10.73 12.58 12.96 10.99CaO 22.00 20.96 21.71 22.60 21.92 22.32 22.34 21.63Na2O 0.34 0.43 0.35 0.31 0.36 0.32 0.26 0.31K2O 0.00 0.06 0.01 0.01 0.00 0.02 0.01 0.01ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 100.71 99.13 98.41 99.95 99.72 99.50 100.26 100.03O 6 6 6 6 6 6 6 6Si 1.966 1.959 1.983 1.993 1.978 1.947 2.017 1.979Ti 0.001 0.007 0.006 0.003 0.006 0.004 0.001 0.001Al 0.049 0.107 0.082 0.058 0.073 0.077 0.037 0.036Cr 0.000 0.000 0.000 0.001 0.000 0.002 0.001 0.000Fe3+ 0.043 0.000 0.000 0.000 0.000 0.043 0.000 0.026Fe2+ 0.317 0.353 0.325 0.280 0.410 0.292 0.307 0.426Mn 0.001 0.003 0.002 0.000 0.003 0.002 0.002 0.002Mg 0.719 0.685 0.687 0.738 0.609 0.707 0.722 0.624Ca 0.879 0.852 0.888 0.904 0.895 0.901 0.895 0.882Na 0.025 0.031 0.026 0.023 0.027 0.023 0.019 0.023K 0.000 0.003 0.001 0.000 0.000 0.001 0.000 0.000Zn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total cation 4.000 4.000 4.000 4.000 4.001 3.999 4.001 3.999XMg 0.694 0.660 0.679 0.725 0.598 0.708 0.702 0.594

Note: I, S and M mean inclusion, symplectite and matrix in texture respectively.


shows outward decrease in grossular andpyrope and increase in alman-dine, suggesting the composition is in associationwith theproduction ofthe neighboring symplectite of clinopyroxene and plagioclase. A cross-section EPMA analysis was performed on the garnet grain B (Fig. 4B).The result shows a similar trend as in garnet grain A (Fig. 5).

Table 4Chemical composition of representative orthopyroxene in associated with texture byEPMA. XMg = Mg / (Mg + Fe2+); XFe = Fe2+ / (Mg + Fe2+).

Texture M M S S

SiO2 51.66 52.37 51.58 51.77TiO2 0.04 0.07 0.06 0.11Al2O3 0.98 1.05 1.06 1.01Cr2O3 0.00 0.03 0.00 0.00FeO 29.81 28.78 30.33 30.90

6.2. Plagioclase (Table 2)

The plagioclase could be divided into four groups as: (1) inclusioninside the garnet core (Pli), (2) matrix grain (Plm), (3) symplectite pla-gioclase with clinopyroxene, orthopyroxene and hornblende (Pls) and(4) plagioclase lamella inside the clinopyroxene (Ple). The inclusionplagioclase shows amediumAn content (61). The core ofmatrix plagio-clase has the lowest content of An (40) and its rim is slightly rich in An(53). The symplectite plagioclase has the highest content of An and thatassociated with hornblende has even higher An (78) content than thatwith clinopyroxene (66).

MnO 0.25 0.22 0.20 0.23MgO 17.22 18.70 16.78 16.96CaO 0.32 0.33 0.43 0.36Na2O 0.03 0.00 0.02 0.00K2O 0.00 0.00 0.01 0.00ZnO 0.00 0.00 0.00 0.00Total 100.31 101.52 100.47 101.34O 6 6 6 6Si 1.987 1.974 1.986 1.978Ti 0.001 0.002 0.002 0.003Al 0.045 0.047 0.048 0.046Cr 0.000 0.001 0.000 0.000Fe3+ 0.000 0.000 0.000 0.000Fe2+ 0.958 0.907 0.977 0.988Mn 0.006 0.005 0.005 0.006Mg 0.987 1.051 0.963 0.965Ca 0.013 0.013 0.018 0.015Na 0.002 0.000 0.001 0.000K 0.000 0.000 0.000 0.000Zn 0.000 0.000 0.000 0.000Total cation 3.999 4.000 4.000 4.001XMg 0.507 0.537 0.496 0.494XFe 0.493 0.463 0.504 0.506


6.3. Clinopyroxene (Table 3)

Similar to garnet and plagioclase, the clinopyroxene could also bedivided into three groups according to their textures: (1) inclusion in-side the core of garnet (Cpxi), (2) matrix clinopyroxene (Cpxm) and(3) symplectite clinopyroxene with plagioclase (Cpxs). Note thatclinopyroxene is known to be more Al-rich with increasing meta-morphic pressure (Anovitz, 1991; Liu et al., 2010; Tam et al., 2012b).The core of matrix clinopyroxene has the highest Al content, up to2.38 wt.%, whereas the rim has 1.32–1.73 wt.%. The inclusion clino-pyroxene at the core of garnet has a low Al content of 1.12 wt.%. Thesymplectite clinopyroxene has an even lower Al content of 0.80–0.83 wt.%. In a study of high-P granulite from the same study area, asignificant difference in FeOT andXMg has been noticed in clinopyroxenewith different textures (Liu et al., 2010; Tam et al., 2012a,b). Theclinopyroxene of this study also shows a strong textural relevance(Table 3). They have FeOT contents ranging from 8.97 to 12.88 wt.%and XMg from 0.594 to 0.725.


6.4. Orthopyroxene (Table 4)

Orthopyroxene occurs as matrix mineral (Opxm) in associationwith hornblende and clinopyroxene, and as symplectite (Opxs) withplagioclase and clinopyroxene. In both cases, it is a product of garnetbreakdown during the post-peak metamorphism. No significant com-positional difference between the two types of orthopyroxene has



Table 5Chemical composition of representative hornblende in associated with texture by EPMA.Aliv, Alvi, Fe3+ and Fe2+ are reduced by using Probe-Amph 3.0 (Tindle and Webb, 1993).XMg = Mg / (Mg + Fe2+); XFe = Fe2+ / (Mg + Fe2+).

Amp S-Pl–Cpx M M S-Pl M

SiO2 42.08 42.50 43.13 42.84 43.57TiO2 1.37 1.69 1.37 0.81 1.44Al2O3 11.88 11.38 11.54 11.84 11.39Cr2O3 0.02 0.00 0.01 0.00 0.00FeO 16.35 18.01 16.59 16.71 17.86MnO 0.04 0.12 0.04 0.01 0.04MgO 9.50 9.65 10.08 10.00 9.74CaO 11.43 11.28 11.46 11.54 11.38Na2O 2.17 2.12 2.08 1.95 2.08K2O 0.54 0.64 0.54 0.64 0.58ZnO 0.00 0.00 0.00 0.00 0.00Total 95.36 97.37 96.81 96.32 98.08O 23 23 23 23 23Si 6.436 6.371 6.467 6.454 6.469Ti 0.158 0.190 0.154 0.092 0.161Aliv 1.564 1.629 1.533 1.546 1.531Alvi 0.577 0.382 0.506 0.556 0.462Cr 0.003 0.000 0.002 0.000 0.000Fe3+ 0.173 0.506 0.329 0.392 0.418Fe2+ 1.918 1.752 1.752 1.714 1.799Mn 0.005 0.015 0.005 0.001 0.005Mg 2.167 2.155 2.254 2.247 2.155Ca 1.873 1.811 1.841 1.863 1.811Na 0.644 0.617 0.603 0.569 0.599K 0.105 0.122 0.103 0.122 0.110Zn 0.000 0.000 0.000 0.000 0.000Total cation 15.623 15.549 15.548 15.554 15.520XMg 0.530 0.552 0.563 0.567 0.545XFe 0.470 0.448 0.437 0.433 0.455



been discerned (Table 4). All analyzed orthopyroxene grains have theXMg values from 0.494 to 0.537 and XFe values from 0.463 to 0.506.

6.5. Hornblende (Table 5)

Similar to orthopyroxene, hornblende could also be divided into twotypes: (1) symplectite hornblende (Hbs) in association with plagioclaseand clinopyroxene, and (2) matrix grains (Hbm). The two types do notshow a significant difference in composition. However, a singlesymplectite hornblende with plagioclase shows a drop in Ti content(Table 5). The examined hornblende crystals are all calcic hornblendewith Ca ranging from 1.811 to 1.873 and Na + K from 0.691 to 0.749.They have XMg values from 0.530 to 0.567. According to the nomencla-ture of amphiboles by Leake et al. (1997), most calcic hornblende ispargasite, and the rest belongs to magnesiohastingsite.

7. Metamorphic P–T conditions

The metamorphic P–T conditions and evolution of the mafic granu-lites are constrained using two complementary techniques: the conven-tional geothermobarometry and pseudosectionmodeling. The results ofsample QX12118-6 are shown below.

7.1. Conventional geothermobarometry

According to the mineral assemblages and mineral chemistry, themetamorphism is divided into three stages. The P–T conditions weredetermined using the Grt–Cpx–Pl–Q thermobarometry of Eckert et al.(1991) and Newton and Perkins (1982), and Grt–Cpx thermometry ofEllis and Green (1979) for the pre-peak, peak and post peak metamor-phism. For the post-peak metamorphism, we also used the Grt–Opx–Pl–Q thermobarometry of Bhattacharya et al. (1991) and the Grt–Hb–Pl–Q thermobarometry of Dale et al. (2000) and Holland and Blundy


(1994). The results are projected onto a phase diagram constructed bypseudosectionmodeling which is described in detail in the next section(Section 7.2).

For the pre-peakmetamorphism, the composition of the garnet core,inclusion phases of clinopyroxene and plagioclase was used in the Grt–Cpx calculation (Ellis and Green, 1979) as well as in the Grt–Cpx–Pl–Qthermobarometry (Eckert et al., 1991). The results indicate that the P–T condition of the pre-peak metamorphism was at ca. ~9.7 kbar and~720 °C.

The garnet mantle, matrix clinopyroxene and plagioclase are themineral assemblages representing the peak metamorphism. The sameGrt–Cpx thermometry (Ellis and Green, 1979) and Grt–Cpx–Pl–Qthermobarometry (Eckert et al., 1991) was used for calculating the P–T conditions. One mineral assemblage shows that the metamorphicpressure reached as high as ~17 kbar and ~880 °Cwhile another assem-blage yielded P = ~15.5 kbar and T = ~810 °C. However, the applica-tion of the Grt–Cpx–Pl–Q thermobarometry of Newton and Perkins(1982) yielded a significantly lower pressure at ~13 kbar.

For the post-peak and retrograde metamorphism, the combined useof the Grt–Cpx thermometry and Grt–Cpx–Pl–Q thermobarometry re-veals that the clinopyroxene + plagioclase symplectite surrounding thegarnet rim has recorded a P–T condition of ~8.7 kbar and ~760 °C. Theapplication of the Grt–Opx–Pl–Q thermobarometry of Bhattacharyaet al. (1991) yielded a result of ~8.7 kbar and ~690 °C whereas the Grt–Hb–Pl–Q thermobarometry of Dale et al. (2000) and Holland andBlundy (1994) gave a result of ~7.8 kbar and ~780 °C.

7.2. Pseudosection modeling

Unlike the conventional geothermobarometry, P–T pseudosectionmodeling is employed to construct the stable mineral assemblages at acertain P–T condition with a given bulk-rock composition (Hollandand Powell, 1998; Powell and Holland, 1998; White et al., 2001, 2003,2007; Clark and Hand, 2010). Here we chose the same sample,QX12118-6, for the pseudosectionmodeling because itsmineral assem-blages and reaction textures arewell presented. The bulk chemical com-position (inwt.%) from the XRF analysis is SiO2= 45.06, Al2O3= 16.11,CaO = 11.23, MgO = 6.35, Fe2O3 = 3.12, FeO = 12.38, Na2O = 2.07,TiO2 = 1.65, K2O = 0.35, MnO = 0.26 (total = 98.75). The systemNCFMASHTO (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) waschosen because K2O and MnO are very minor components. Melt is alsoignored due to the presence of hornblende (Poli and Schimidt, 2002).The fluid phase was regarded as pure water and set to be in excess.The bulk composition in wt.% was then recalculated to mol% asSiO2 = 48.37, Al2O3 = 10.19, CaO = 12.91, MgO = 10.15, FeO =13.63, Na2O = 2.15, TiO2 = 1.33, O = 1.26 (total = 100).

The pseudosection modeling was carried out using THERMOCALC3.33 (Powell et al., 1998) with the internal thermodynamic data set,tcds55.txt (Holland and Powell, 1998). The phases considered in the cal-culation included: garnet (G), plagioclase (Pl), clinopyroxene (shown asdiopside; Di), hornblende (Db), ilmenite (Ilm), rutile (Ru), magnetite(Mt), epidote (Ep), quartz (Q) and pure water (H2O). Among thesephases, rutile, quartz and pure water are considered as pure end-member phases. The activity–composition (a–x) models for garnet arefrom White et al. (2007), plagioclase from Holland and Powell (2003),clinopyroxene from Green et al. (2007), orthopyroxene from Whiteet al. (2002), hornblende fromDiener et al. (2007), ilmenite andmagne-tite from White et al. (2000), and epidote from Holland and Powell(1998).

The calculated P–T pseudosection of the sample within the P–Trange of 6–18 kbar and 650–950 °C is shown in Fig. 6, in which the re-sults of conventional geothermobarometry for the pre-peak, peak andpost-peak metamorphic conditions are also shown. The pseudosectionis dominated by fields with variance of 3 to 5; only two fields withvariance of 2 are shown. The clinopyroxene (as di) and pure waterare stable in all the fields. The garnet-out line is nearly horizontal at



Fig. 6. P–T pseudosection of sample QX12118-6 in the NCFAMSHTO system; the P–Tconditions by conventional geothermobarometry is also shown. A. P–T conditions byapplication of Grt–Cpx thermometry and Grt–Cpx–Pl–Qtz thermobarometry to pre-peakassemblages. B. P–T conditions by application of Grt–Cpx thermometry and Grt–Cpx–Pl–Qtz thermobarometry to peak assemblages. C. P–T conditions by application of Grt–Cpxthermometry and Grt–Cpx–Pl–Qtz thermobarometry to post-peak assemblages. D. P–Tconditions by application of Grt–Hb–Pl–Qtz thermobarometry to post-peak assemblages.E. P–T conditions by application of Grt–Opx thermometry and Grt–Opx–Pl–Qtzthermobarometry to post-peak assemblages. Bold line refers to the P–T path.


pressure of 7–8 kbar when temperature is higher than 670 °C, whereasit is much more temperature-dependent in lower temperature con-ditions. Quartz is stable in most fields but disappeared in HT/LP con-ditions. By contrast, orthopyroxene is stable in HT/LP conditions. Thecoexisting fields of quartz and orthopyroxene are very narrow and al-most parallel to the orthopyroxene-in and quartz-out line. Hornblendeis stable toward low temperature and low pressure conditions. Rutileis much more sensitive to pressure and disappeared below ca. 10.2–12.8 kbar.

The pre-peak assemblages correspond to the stabilityfield of Hb–Di–Pl–Ep–Ru–G–Q–H2O or Hb–Di–Pl–Ru–G–Q–H2O. The Grtc–Cpxi–Pli–Qthermobarometry gave the conditions below the rutile-out line. This iscontroversial to the fact that rutile does occur as an inclusion phase ofgarnet. Therefore, we suggest that the recorded P–T condition for thepre-peakmetamorphism is near the rutile-out linewithin the two fieldsmentioned above, even though they have relatively large P–T ranges.

The peak metamorphic mineral assemblages correspond to thestability field of Di–Pl–Ru–G–Q–H2O at pressure of N14.5 kbar andtemperatures of N800 °C. The P–T conditions for the stable peak assem-blages in the pseudosection are broadly consistent with that calculatedby the conventional geothermobarometry of Grtm–Cpxm–Plm–Q.

The post peak metamorphism is marked by (1) the breakdownof garnet into symplectite of clinopyroxene (Cpxs) + plagioclase(Pls) ± orthopyroxene (Opxs) ± hornblende (Hb), (2) the presenceof matrix hornblende, ilmenite, magnetite and (3) the absence of rutile.Although it is not easy to identify the production of the symplectite ofCpxs + Pls on the P–T pseudosection, the presence and absence ofmineral phases indicate that the P–T conditions shifted toward lowerpressure conditions crossing the ilmenite-in, hornblende-in, rutile-out, magnetite-in and orthopyroxene-in lines. The presence of quartz


indicates that the P–T conditions did not shift to the quartz-out line.The P–T path is also controlled by the results from the conventionalgeothermometry calculation (Grtr–Cpxs–Pls–Q and Grtr–Hbs–Pls–Qthermobarometry) although the calculated temperature is lower thanindicated by the P–T pseudosection. However, the calculated P–T condi-tion by using Grtr–Opxs–Pls–Q is not in agreement with the phasediagram in which the condition lies in the Opx-absent field.

The complete breakdown of garnet into plagioclase + quartz indi-cates that the P–T path shifted into the garnet-absent fields when themetamorphic pressure and temperature further decreased.

7.3. P–T evolution

Integrating the results from the conventional geothermobarometryand pseudosectionmodeling, a clock-wise P–T path could be construct-ed (Fig. 6). The recorded P–T condition for the pre-peakmineral assem-blages was ca. 10 kbar and 730 °C. With increasing pressure andtemperature, the P–T path shifted into the stability field of G–Di–Pl–Q–Ru–H2O, corresponding to the peak metamorphism at ~17 kbar and880 °C. Subsequently, the P–T condition dropped along a near isother-mal decompression path into the stability field of Q–Hb–Mt–Opx–Ilm–

Pl–Di–G–H2O at ca. 7.5 kbar and 800 °C. Finally, the P–T path crossedthe garnet-out line with further decreasing pressure and temperatureduring the retrograde metamorphism.

8. Zircon geochronology

Zircon dating was performed on two samples (S1205 and S1207).The zircon grains show similar morphology (Fig. 7) and yielded identi-cal ages (Fig. 8). A host gneissic trondhjemite sample (QX12117) wasalso dated.

The “magmatic” zircon grains separated from the two samples havesizes from 80 to 250 μm, and length/width ratios of 4 to 1. Most grainsdisplay blurred planar zoning, suggesting partial to strong recrystalliza-tion during the high grade metamorphism (sample S1205, grain b andc). Somegrains show light grayhomogeneous overgrowth rims (sampleS1205, grain a; sample S1207 grain c), but some individual grains havedark gray overgrowth rims (sample S1207, grain a). In addition, a fewgrains with core show oscillatory zoning (sample S1207, grain b),which is not common for zircon from mafic rocks. We suggest thatthese grains have probably been captured from TTG gneisses whenthe mafic rock intruded. Sample QX12117 has similar zircon grainsizes but different internal structure. Blurred oscillatory zoning withovergrowth are the main characteristics, indicating reworking of themagmatic zircon.

Seven spotswere analyzed on six zircon grains for sample S1205. Fourspots were made on the light gray recrystallized magmatic domainsand two spots on the light gray overgrowth rims (Table 6). The zircongrains show low U content (27 to 61 ppm) and intermediate Th/U ratios(0.11–0.74). The four spots on the recrystallized magmatic domains, in-cluding 1.1RC, 2.1RC, 3.1RC and 4.1RC, yielded a mean 207Pb/206Pb ageof 2518 ± 23 Ma (MSWD = 2.5). This is considered as the minimumage of the mafic intrusion. The other two spots on the overgrowth rimsyielded similar ages of 2488 ± 17 Ma (1.2R) and 2450 ± 26 Ma (6.1R).They are interpreted as the time of metamorphism.

For sample S1207, ten spots were analyzed on eight zircon grains;among them, six were on light gray magmatic domains and four onthe light to dark gray overgrowth rims. The results are similar to sampleS1205. The five spots on the “magmatic” cores (1.1RC, 2.1RC, 4.1RC,5.1RC and 6.2RC) yielded a mean 207Pb/206Pb age of 2531 ± 15 Ma(MSWD= 1.7). The four analyses on the overgrowth rims (2.2R, 3.1R,6.1R and 7.1R) yielded a mean 207Pb/206Pb age of 2474 ± 6 Ma(MSWD= 0.75).

Twenty two spots were analyzed on twenty one zircon grains ofsample QX12117. The ten spots on “magmatic” cores (2.1RC, 5.1MA,6.1MA, 7.1RC, 8.1MA, 9.1RC, 11.1RC, 15.1RC, 16.1RC and 21.1RC) yielded


image of Fig.�6


Fig. 7. CL images showing representative zircon morphology of sample S1205, S1207 and QX12117. Analytical spots with 207Pb/206Pb ages are also shown.


amean 207Pb/206Pb age of 2540±8.3Ma (MSWD= 1.7). The five spotson strongly recrystallized domain or overgrowth (1.1RC, 4.1R, 12.1RC,19.1, 20.1R) gave metamorphic age of 2480 ± 11 Ma (MSWD = 2.1).

If the data of the two mafic granulites samples (S1205 and S1207)are combined (Fig. 8D), the nine spots on “magmatic” core yielded themean 207Pb/206Pb age of 2527 ± 19 Ma (MSWD = 1.9), which isinterpreted as the lower limit of the mafic intrusive age. All the spotson the strongly recrystallized domain or overgrowth rim from thethree samples yielded the mean 207Pb/206Pb age of 2477 ± 5.5 Ma(MSWD= 1.6), which is regarded as the metamorphic age.

9. Discussion

9.1. P–T evolution of the high-pressure mafic granulites

The typical indicator of high-pressure granulite metamorphism inmafic rocks is the mineral assemblages of garnet + clinopyroxene +plagioclase and the reaction texture “white eye” produced by break-down of garnet into the symplectite of clinopyroxene + plagioclase +orthopyroxene ± hornblende (Green and Ringwood, 1967; Guo et al.,1993; Zhao et al., 2001; O'Brien and Rötzler, 2003; O'Brien, 2008; Liuet al., 2010; Tam et al., 2012a,b). The mineral assemblages and reactiontextures of the studied samples are the same as other high-pressuremafic granulites worldwide, so we assume that they have undergonehigh-pressure granulite facies metamorphism. In fact, this assumptionhas been confirmed by the mineral chemistry, conventional geothero-barometry and pseudosection modeling.


It has been noticed that in high-pressure mafic granulites, garnetbecomes Ca-rich, plagioclase Na-rich and clinopyroxene Al-rich underhigh pressure conditions (Liu et al., 2010; Tam et al., 2012b). Earlierstudies onmafic granulites from Jiaodonghave shown that the grossularcontent in garnet at peak metamorphism is generally higher than 0.30and reaching 0.38 (Liu et al., 2010; Tam et al., 2012b). In the presentstudy, the grossular content is found to be slightly lower (up to 0.28;Table 1). Note also that the garnet compositions in different samplesat peak metamorphism vary significantly, indicating that the mineralchemistry is also controlled by whole-rock composition (Liu et al.,2010). Since sample QX12118-6 has a bulk-rock CaO content lowerthan the sample studied by Tam et al. (2012b), the lower grossular con-tent in our garnet may be explained by this bulk-rock effect. Similarly,the clinopyroxene of our samples has Al content up to 2.8 wt.% whereasthat from late Paleoproterozoic samples has Al content up to 4.2 wt.%.Likewise, the plagioclase of this study has Na content similar to that ofLiu et al. (2010), which showed an Ab content of ~60, about 10 higherthan the examples of Tam et al. (2012b). All the above shows thebulk-rock effect on mineral chemistry.

The application of the conventional geothermobarometry andpseudosection modeling further confirmed the high-pressure granulitemetamorphism. Applying the Grt–Cpx thermometry (Ellis and Green,1979) and Grt–Cpx–Pl–Q thermobarometry (Eckert et al., 1991), thehighest P–T condition of ~17 Kbar and ~880 °C was calculated usingthe compositions from the mantle domain of a garnet grain and thecore domains of a nearby plagioclase and clinopyroxene grain. How-ever, the same mineral assemblage (Gt–Plag–Cpx) was also used toobtain the P–T condition using the Newton and Perkins' (1982)


image of Fig.�7


Fig. 8.Concordia diagrams showing the SRHIMPzircon age results. A. Results from sample S1205; B. Results from sample S1207. C. Results from sampleQX12117. D. Combined average ageof the three samples.


Grt–Cpx–Pl–Q thermobarometry. It yielded a similar temperaturebut a lower pressure at ~13 kbar. We chose the former because itwas generally consistent with the results from the pseudosectionmodeling. This P–T condition reaches the eclogite facies meta-morphism. However, as indicated by the mineral assemblagesand pseudosection modeling, plagioclase is stable at the peakmetamorphism.

A significant discrepancy between the results from the conven-tional geothermobarometry and pseudosection for pre-peak andpost-peak metamorphism is also noticed. Especially, the results bythe Grt–Opx–Pl–Q thermobarometry (Bhattacharya et al., 1991)shifted away from the Opx-in fields toward lower temperatures inthe pseudosection. This could be explained as follows. (1) theorthopyroxene selected for calculation might have not been in equi-librium with garnet and plagioclase, leading to an uncertainty in theresult of the geothermobarometry; (2) the error of the results fromthe conventional geothermobarometry is large but not shown onthe diagram (Liu et al., 2010); (3) although theMnO content is ratherlow in bulk chemistry, it could affect the stability of garnet; and (4)the “melt” phase was not considered during the construction of theP–T pseudosection due to the presence of hornblende (Poli andSchimidt, 2002). Besides, no mixing model is available for melt-


bearing mafic rocks in the pseudosection (Tam et al., 2012b). How-ever, this may lead to an error.

9.2. Timing of the high-pressure granulite metamorphism

The two mafic granulite samples used for zircon SHRIMP geochro-nology study are collected fromexactly the same outcrop as the samplesfor the petrological study. Although the samples for the age study con-tain less clinopyroxene and orthopyroxene, they have both recordedthe same metamorphism. It is also noticed that garnet grains aresurrounded by a dirty rim in the slightly weathered sample S1205(Fig. 4H), indicating the post-peak metamorphism was recorded. Thus,the results from zircon SHRIMP study might be the age of post-peakmetamorphism.

Furthermore, both the high-pressure mafic granulite and surround-ing TTG gneisses were not affected by the late-Paleoproterozoic meta-morphism, which is broadly recorded in other areas of the JiaodongTerrain. In another recent work, a ~2.9 Ga TTG gneiss was reported(Xie et al., under review) at the northern dam of the Hexikuang re-servoir, about 100 m from the present study locality. The ~2.50 Gametamorphic event was also recorded but the rock shows no evidenceof the late Paleoproterozoic reworking. These authors discussed that


image of Fig.�8


Table 6SHRIMP U–Th–Pb zircon data of the garnet amphibolite (S1205) and amphibolite with garnet pseudomorph (S1207).

Spot name 206Pbc(%)

U(ppm)

Th(ppm)

Th/U 206Pb⁎(ppm)

207Pb⁎/206Pb⁎ ±% 207Pb⁎/235U ±% 206Pb⁎/238U ±% Err.corr.

206Pb/238Uage (Ma)

207Pb/206Pbage (Ma)

Discordance(%)

S1205: garnet amphiboliteS1205-1.1RC 0.32 73 8 0.12 30 0.1632 1.0 10.78 1.7 0.4791 1.4 0.816 2523 ±29 2488 ±17 −1S1205-1.2R 0.77 51 6 0.11 21 0.1583 1.5 10.32 2.2 0.4729 1.6 0.716 2496 ±32 2437 ±26 −2S1205-2.1RC 0.42 43 31 0.74 18 0.1700 1.1 11.34 2.0 0.4842 1.6 0.822 2546 ±34 2557 ±19 0S1205-3.1RC 1.21 30 20 0.70 12 0.1669 2.4 10.98 3.1 0.4772 1.9 0.622 2515 ±40 2527 ±40 0S1205-4.1RC 0.91 27 18 0.71 11 0.1654 2.1 10.94 2.9 0.4796 2.0 0.679 2526 ±41 2512 ±36 −1S1205-5.1R 64.39 0 0 0.32 0 0.1320 44.0 798 ±340S1205-6.1R 0.51 61 10 0.16 24 0.1594 1.5 10.10 2.1 0.4595 1.5 0.701 2437 ±30 2450 ±26 0

S1207: amphibolite with garnet pseudomorphS1207-1.1RC 0.22 69 70 1.05 30 0.1680 1.1 11.56 1.9 0.4993 1.5 0.809 2611 ±33 2538 ±19 −3S1207-2.1RC 0.33 82 37 0.47 35 0.1668 0.9 11.47 1.6 0.4987 1.4 0.832 2608 ±29 2525 ±15 −3S1207-2.2R 0.12 328 44 0.14 135 0.1609 0.5 10.61 1.2 0.4784 1.1 0.904 2520 ±23 2465 ±8.6 −2S1207-3.1R 0.02 527 56 0.11 214 0.1620 0.4 10.56 1.2 0.4730 1.1 0.950 2497 ±24 2476 ±6.3 −1S1207-4.1RC 0.38 79 37 0.48 33 0.1638 1.0 10.93 1.7 0.4841 1.4 0.815 2545 ±29 2495 ±17 −2S1207-5.1RC 0.31 102 120 1.22 44 0.1692 0.9 11.68 2.0 0.5007 1.8 0.900 2617 ±38 2550 ±14 −3S1207-6.1R 0.06 910 62 0.07 370 0.1619 0.3 10.55 1.2 0.4729 1.1 0.974 2496 ±24 2475 ±4.4 −1S1207-6.2RC 0.52 91 49 0.56 39 0.1684 1.1 11.44 1.8 0.4930 1.4 0.798 2584 ±31 2541 ±18 −2S1207-7.1R 0.15 447 48 0.11 184 0.1617 0.4 10.66 1.2 0.4780 1.1 0.940 2519 ±23 2474 ±6.7 −2S1207-8.1MA 0.16 104 28 0.28 44 0.1706 1.0 11.44 1.7 0.4862 1.4 0.805 2554 ±29 2564 ±17 0

QX12117: gneissic trondhjemiteQX12117-1.1RC 0.01 64 22 0.35 26 0.1647 1.0 10.53 1.7 0.4639 1.4 0.821 2457 ±28 2504 ±17 2QX12117-2.1RC 0.04 66 26 0.41 26 0.1685 0.7 10.54 1.5 0.4535 1.3 0.886 2411 ±26 2543 ±12 5QX12117-3.1 RC – 46 22 0.48 19 0.1711 1.3 11.21 2.4 0.4752 2.0 0.843 2506 ±42 2568 ±22 2QX12117-4.1R – 762 49 0.07 316 0.1633 0.4 10.86 1.2 0.4822 1.2 0.958 2537 ±24 2490 ±5.9 −2QX12117-5.1MA 0.00 172 30 0.18 73 0.1672 0.4 11.29 1.3 0.4895 1.2 0.939 2569 ±25 2530 ±7.4 −2QX12117-6.1MA – 77 15 0.20 33 0.1672 1.0 11.57 1.7 0.5020 1.3 0.793 2622 ±28 2530 ±17 −4QX12117-7.1 RC – 368 176 0.49 156 0.1697 0.3 11.55 1.2 0.4935 1.2 0.968 2586 ±25 2554 ±5.0 −1QX12117-8.1MA 0.01 172 23 0.14 72 0.1667 0.5 11.12 1.3 0.4837 1.2 0.938 2543 ±25 2525 ±7.6 −1QX12117-9.1RC 0.03 81 22 0.28 35 0.1687 0.6 11.50 1.5 0.4947 1.3 0.908 2591 ±28 2545 ±10 −2QX12117-10.1RC 0.54 36 15 0.42 14 0.1600 1.3 10.05 2.8 0.4560 2.4 0.932 2422 ±49 2455 ±23 1QX12117-12.1RC 0.01 558 27 0.05 215 0.1616 0.3 9.99 1.2 0.4483 1.1 0.977 2388 ±23 2473 ±4.3 3QX12117-13.1 R – 14 4 0.31 5 0.1610 1.8 9.72 2.7 0.4378 2.0 0.789 2341 ±38 2467 ±31 5QX12117-11.1 RC 0.14 46 12 0.27 19 0.1683 0.9 10.98 1.7 0.4733 1.4 0.872 2498 ±29 2541 ±15 2QX12117-14.1R 0.52 14 4 0.27 6 0.1498 2.2 9.51 2.9 0.4602 1.9 0.756 2440 ±39 2344 ±37 −4QX12117-15.1RC 0.07 162 12 0.08 65 0.1675 0.5 10.73 1.3 0.4648 1.2 0.942 2461 ±25 2533 ±7.8 3QX12117-16.1RC 0.01 59 21 0.36 24 0.1683 0.7 11.07 1.5 0.4769 1.3 0.877 2514 ±28 2541 ±12 1QX12117-16.2RC 0.03 73 33 0.47 30 0.1657 0.6 10.74 1.4 0.4702 1.3 0.897 2484 ±26 2514 ±10 1QX12117-17.1RC – 13 2 0.14 5 0.1621 1.8 9.71 2.7 0.4344 2.1 0.783 2326 ±41 2477 ±30 6QX12117-18.1RC 0.22 31 19 0.63 13 0.1789 1.1 12.13 1.9 0.4919 1.6 0.854 2579 ±33 2643 ±18 2QX12117-19.1RC 0.08 87 22 0.26 34 0.1617 0.6 10.01 1.4 0.4487 1.2 0.912 2390 ±25 2474 ±10 3QX12117-20.1R – 449 38 0.09 177 0.1625 0.3 10.28 1.2 0.4587 1.2 0.975 2434 ±23 2482 ±4.4 2QX12117-21.1RC 0.01 62 16 0.26 25 0.1676 0.7 10.99 1.5 0.4757 1.3 0.881 2509 ±27 2534 ±12 1

Note: (1) R, RC and MA refer to rim, recrystallization and magmatic domain of the zircon grains respectively. (2) Measured 204Pb was used for the common lead correction.⁎ Means radiogenic Pb.


the drymetamorphic condition during late-Paleoproterozoic disfavoredthe recrystallization or re-growth of the zircon grains (Xie et al., underreview). However, hydrous minerals, such as hornblende and chlorite,would be unstable in high-pressure granulite metamorphism. Thebreakdown of such minerals would result in hydrous fluids whichwould favor the re-growth of zircon grains. On the other hand, if theseminerals were formed during late-Paleoproterozoic, the retrogrademetamorphism would be related to fluid-rich conditions. This wouldalso facilitate the zircon growth. Thus, the late-Paleoproterozoicmetamorphic event should be well recorded in the zircon grains. Inthis case, we suggest that the proto-continent represented by the ~2.9and 2.5 Ga TTG associations in the study area had behaved as a relativelyrigid block during the tectono-thermal event during the late-Paleoproterozoic.

The magmatic age of the high-pressure mafic granulite could not bewell constrained directly from the zircon analyses because all the mag-matic cores are partially or strongly recrystallized. Therefore, the ob-tained magmatic age might be a mixed age. Specifically, a capturedzircon grain in sample S1207 has the same age of the host TTG gneisses.

Wemay conclude that themafic rocks intruded into the TTG and thenthe ensemble was subjected to high-pressure granulite metamorphism.


However, it is also probable that themafic rocks andTTG formed contem-poraneously because their similar magmatic ages.

9.3. Implications for the crustal evolution of the North China Craton at theend of Neoarchean

The tectono-thermal event at ~2.5 Ga of the North China Craton wascharacterized by the amalgamation of several micro-continental blocksand large scale crustal growth (Zhai and Santosh, 2011; Wan et al.,2012b). The Neoarchean greenstone belts represent the remnants ofthe supposed sutures (Zhai and Santosh, 2011).

The ~2.47 Ga high-pressure mafic granulites are characterized by aclockwise P–T path, which is distinguished from the generally counter-clockwise P–T paths of Archean metamorphic rocks in the North ChinaCraton (Cui et al., 1991; Zhao et al., 1998; Ge et al., 2003; Wu et al.,2012). The clockwise P–T path was commonly regarded as a result ofsubduction–collision related process (Harley, 1989; Brown, 1993, 2001,2007a,b). In combination with its occurrence inside the ~2.5 Ga TTGgneiss against the ~2.9 Ga TTG rocks to the north (Xie et al., underreview and references therein), it has probably recorded a collisionalevent between the newly formed ~2.5 Ga crust and the ~2.9 Ga crust.




9.4. General implications for the crustal evolution

The high-pressure mafic granulites at the southern dam of theHexikuang reservoir might have been formed in an over-thickenedcontinental crust (ca. 50 km) with a relatively low thermal gradient ofca. 19 °C/km; but this condition is known to be rare in the Archean.However, the difference between Archean and post-Archean crust hasbeen noticed due to the dominating rock and metamorphic types(Brown, 2007a,b and references therein). The dominant TTG suite andmetamorphism with uniformly high apparent thermal gradients indi-cate a higher thermal regime, whereas the appearance of paired HProcks (eclogite to high-pressure granulite) and HT/UHT granulites inthe Mesoarchean–Neoarchean time might signify the transition fromdouble-sided to one-sided subduction (Sizova et al., 2010; Brownet al., 2013; Sizova et al., 2014). Numerical modeling also suggested ahigher mantle heat flow and a greater radiogenic crustal heat produc-tion during the Archean, which is responsible for a weaker continentallithosphere and double-sided subduction (Sizova et al., 2010; 2014).The discovery of the high-pressure mafic granulites with peak P–T con-ditions up to ~17 kbar and 880 °C in the Jiaodong Terrain, may serve asan indicator of a global transition toward a cooler thermal regime and athicker continental crust. In any case, the present study provides a newand rare case of subduction–collision tectonic scenario at the boundaryof Archean and Proterozoic.

10. Conclusion

(1) An unusual late Archean high-pressure mafic granulite was dis-covered in the Qixia area of the Jiaodong Terrain. The diagnosticmineral assemblage is garnet + clinopyroxene + plagioclase +quartz ± rutile. Application of the conventional geothermo-barometry and pseudosection modeling revealed a clockwiseP–T path with the peak metamorphic condition at P =~17 kbar and T = ~880 °C.

(2) SHRIMP U–Pb analyses on the overgrowth rim of the zircongrains yielded a metamorphic age of ca. 2.47 Ga. Analyses onrecrystallized magmatic cores gave a minimum magmatic ageof ~2.53 Ga.

(3) The high-pressure mafic granulites have probably recorded ahighly significant collisional event at the end of Archean. Theoccurrence of the granulites may serve as an indicator of a globaltransition toward a cooler thermal regime and a thicker con-tinental crust.

Acknowledgment

We thank Prof. Shuwen Liu and M. Santosh for constructive com-ments. Guangshen Ni, Baoying Zheng, and Siyu Zheng performed zirconseparation, Chun Yang and Weilin Gan made the zircon mounts, andLiqin Zhou, Ning Li and Xiaochao Che took the zircon CL and SE images.Their assistance is highly appreciated. We are also grateful to tech-nicians Yuhai Zhang, Zhiqing Yang and Jianhui Liu for the maintenanceof the SRHIMP II. Driver Shusen Liu is thanked for kind help in thefield work. Dr. Jiahui Qian at Peking University is thanked for discus-sion in pseudosection modeling. This research was supported by theMajor State Basic Research Program of the People's Republic of China(2012CB416600), the National Natural Science Foundation of China(40672127) and the Key Program of the Ministry of Land and Re-sources of China (12120113013700, 1212010811033). The seniorauthor (Shoujie Liu) is kindly supported by the NSC Taiwan for apost-doctoral research fellowship at NTU-Geosciences. Bor-mingJahn acknowledges the support of NSC Taiwan (NSC 101-2116-M-002-003).


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neoarchean to paleoproterozoic high-pressure mafic ... to... · minor intrusions of mesozoic...

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