Lower crustal processes leading to Mesozoic lithospheric thinning beneath eastern North China: Underplating, replacement and delamination

7) 36–54www.elsevier.com/locate/lithos

Lithos 96 (200

Lower crustal processes leading to Mesozoic lithosphericthinning beneath eastern North China: Underplating,

replacement and delamination

Mingguo Zhai a,b,d,⁎, Qicheng Fan c, Hongfu Zhang a,d, Jianli Sui a, Ji'an Shao e

a Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, Chinab Key Laboratory of Mineral Resources, Chinese Academy of Sciences, Beijing, 100029, China

c Institute of Geology, Chinese Seismological Bureau, Beijing, 100029, Chinad State Key Laboratory of Lithosphere Tectonic Evolution, Beijing, 100029, Chinae School of Earth and Space Sciences, Peking University, Beijing, 100871, China

Received 23 December 2005; accepted 13 September 2006Available online 11 January 2007

Abstract

Our recent studies show that the lower crust and lithospheric mantle in the eastern North China Craton (NCC) was thinned inthe Mesozoic. The lower crust played a key role in the tectonic mechanism that led to lithospheric thinning. Geochemical data fromvolcanic rocks and xenoliths support the idea that delamination of the lower crust and lithospheric mantle took place only along thenorthern and perhaps the southern margins of the NCC, and this was linked to post-collisional orogenic processes. However, it isdifficult for the delamination process to act on the whole eastern NCC and remove 80–120 km of the lithosphere. Lower crustalxenoliths in Mesozoic and Cenozoic basalts within the NCC are of two kinds. One is granulite-facies metamorphosed two pyroxenegabbros and eclogite-facies metamorphosed garnet pyroxenites formed at 140–120 Ma. The other is Precambrian granulites thatwere strongly overprinted by Mesozoic metamorphism. Large-scale lower crustal replacement took place in the Mesozoic beneaththe NCC. In other words, some or perhaps most part of the present lower crust of the NCC is not Precambrian. The presentlowermost crust is mostly composed of Mesozoic meta-gabbros and pyroxenites. Abundant late Jurassic, high-Sr granites in theNCC, which are compositionally similar to a dakite, probably formed by partial melting of the lowermost crust, leaving a residue ofeclogite or garnet amphibolite. A hot upwelling mantle was necessary to partially melt the pre-existing lowermost crust andunderplate magma in order to form the present lower crust. Although eclogites could sink into the mantle, it is hard to imagine thatthere was a thick enough eclogite layer that could drag 80–120 km of the lithosphere into the asthenosphere beneath the NCC.Magmatic underplating seems to be a better alternative mechanism. Thus, the formation of the present lower crust through theunderplating and replacement of the pre-existing one was closely related to lithospheric thinning, and represented a continuation ofthe same geodynamic process.© 2006 Elsevier B.V. All rights reserved.

Keywords: Lower crustal processes; Underplating; Delamination; Lithospheric thinning; North China

⁎ Corresponding author. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029.E-mail address: [email protected] (M. Zhai).

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37M. Zhai et al. / Lithos 96 (2007) 36–54

1. Introduction

In recent years, the extent and cause of Mesozoiclithospheric thinning beneath the NCC have beendiscussed extensively (e.g., Fan and Menzies, 1992;Menzies et al., 1993; Meyer et al., 1994; Harris et al.,1994; Deng et al., 1996; Zheng, 1999; Lu et al., 2000;Fan et al., 2000, 2001b; Zhang et al., 2002a, 2003a,b,2006). Our previous studies (Shao et al., 2000; Zhaiet al., 2001a; Fan et al., 2001a; Zhang et al., 2002a,2003a; Zhai et al., 2001a, 2004a,b; Fan et al., 2005)suggest that: (1) Mesozoic lithospheric thinning wascoupled with Mesozoic tectonic regime inversion in theNCC. This tectonic regime inversion in the NCC istermed the Yanshan Movement (Wong, 1927), and wasinitially identified where structural trends change froman EW framework to a NE–NNE-trending structuralframework (Zhao et al., 1994), and by changes from acompressive tectonic regime during the Paleozoic to anextensional regime during the Jurassic–Middle Creta-ceous (Davis et al., 1998; Shao et al., 2000; Zheng andDavis, 2000; Zhao et al., 2004). In other words, thelithospheric thinning was a deep process involvingcrust–mantle interaction, and the transformation of thestructural framework represents only its shallow re-sponse. (2) The lower crustal thinning was a large-scaleprocess that involved the lithosphere mantle. Volcanism,granite intrusion, compositional change of the mantle,and gold mineralization were intensive and extensive,and reached a peak at 140–120 Ma. (3) In the northernand southern margins, adakite and high-Mg andesiteprovide possible evidence of lower crustal delaminationright after continental collision, but not within theinterior of the NCC.

This paper emphasizes the key role of underplating ina better understanding of the mechanism of lithosphericthinning. Our research suggests that the present lowercrust beneath the NCC formed through magmaticunderplating in the late Mesozoic and replaced partlyor mostly the old lower crust that was formed in theEarly Precambrian. Both the lower crust thinning/replacement and lithospheric mantle thinning werecontrolled by the same dynamic mechanism, althoughthe detailed mechanism is not clear.

1.1. Scope of Mesozoic lithospheric thinning

The conclusion that the Mesozoic lithosphericthinning with a loss in thickness of 80–120 km, isbased on a suggestion that there was a stablePrecambrian lithospheric mantle and lower crust duringthe Paleozoic beneath the NCC (Fan et al., 2000). This

suggestion came from the study of Paleozoic kimberliteand Mesozoic–Cenozoic basalt and their magmasources. The occurance of diamond-bearing kimberliteswas the indication of the existence of a cold and thicklithosphere existed in the NCC, where as the presence ofdiamond and its mineral inclusions suggest the typicalcontinental craton nature of the Paleozoic lithosphericmantle (Menzies et al., 1993; Meyer et al., 1994; Harriset al., 1994). Another suggestion is that lithosphericthinning occurred in all eastern China (Wu et al., 2003;Niu, 2005) evidenced by the extensive distribution ofJurassic granites in southern China and the exposure ofkimberlite in the Yangtze Block. However, based on thegeochemistry of volcanic rocks and geophysical data,there is a big difference between the NCC and SouthernChina, and the evidence is not strong enough to supportthe lithospheric thinning beneath southern China.Seismological tomography imagery shows that apossible mantle upwelling exists between longitude110–120° E and latitude 34–44° N (Liu and Chang,2001; Zhao and Zheng, 2005). Similarly, Liang et al.(2004) revealed a thermal mantle dome in the NCCbased on the results for Pn velocity inversion (Fig. 1).We believe that these geophysical results record theMesozoic mantle state, and indicate the scope ofMesozoic lithospheric thinning.

1.2. Time-range of Mesozoic lithospheric thinning

Wong (1927) first recognized an unconformitybetween E–W striking middle–late Jurassic strata andthe overlying NW striking Zhangjiakou Formation, andproposed the occurance of an orogenic movement(Yanshan Movement) based on this unconformity.Based on a SHRIMP zircon age of volcanic rocks inthe Zhangjiakou formation, Niu et al. (2003) concludedthat the Yanshan Movement began at 136 Ma, repre-senting the initiation of Mesozoic tectonic regimeinversion, although opinions differ on the timing ofthis lithospheric thinning, for example, ranging fromearly Middle Jurassic (e.g. Xu et al., 2004) or from140 Ma to 120 Ma (e.g. Fan et al., 2005). Recently,many investigators came to the consensus that the peakperiod was at 130–110 Ma. However, the starting timewas different between northern and southern parts of theorogen (Zhang et al., 2003a; Zhang, 2005a).

Zhai et al. (2004a,b) suggested that the Mesozoiclithospheric thinning was coupled with the Mesozoictectonic regime inversion in the NCC, as a result of theexchange and reorganization of material and energy.Lithospheric thinning was a deep process in the lowercrust–mantle and the Yanshan Movement represents the

Fig. 1. Results for Pn velocity from an inversion that does not include the Pn anisotropic terms (after Liang et al., 2004).

38 M. Zhai et al. / Lithos 96 (2007) 36–54

response of the shallow crust. This possibly representsan intracontinental dynamic mechanism rather than asingle orogenic delamination. Therefore, the time rangeof the lithospheric thinning cannot be fixed by anunconformity. Our new results delineate the time rangeof thinning based on the results of many studies (Heet al., 2001; Zhai and Fan, 2002; Zhang et al., 2002b;Liu et al., 2003a,b; Meng, 2003; Li et al., 2003; Zhanget al., 2003a; Yang et al., 2004; Zhang, 2005a; Fan et al.,2005). Structural analyses along the northern andsouthern margins of the NCC and within the NCCreveal that tectonic inversion consisted of changes fromcompression to extension and of structural strikes from∼EW to NNE. Geothermic analysis of sedimentarybasins also reveals a significant change in the thermalhistory and regime. Basin analysis establishes basininversion from compression to extension and basinmigration from ∼EW to NNE. Petrological andgeochemical study of volcanic rocks, lower crustxenoliths and main metallogenic epochs suggest apeak period of mantle upwelling and intense interactionbetween mantle and crust. All studies converge on thesame time range from ∼160–140 Ma to∼110–100 Ma,peaking at ∼120 Ma.

The Mesozoic volcanic–magmatic activities can be,in general, subdivided into four stages (Zhang et al.,2002a; Liu et al., 2002a; Zhang et al., 2003a; Liu et al.,2003a; Zhang et al., 2004; Xu et al., 2004; Zhang et al.,2005; Chen et al., 2005): (1) 210–180 Ma, small-scalevolcanism and granite–diorite intrusion; (2) 160–150 Ma, alkali-volcanism and voluminous crust-melt-ing; (3) 140–110 Ma, bimodal volcanism and occa-

sional intrusions of igneous bodies with characteristicsof crust and mantle; (4) 95–75 Ma, alkali basalt magma-tism. The first stage belongs to the Indosinian orogeny.The second and third stages were directly related tolithospheric thinning and tectonic regime inversion. Thefourth stage marked the end of the lithospheric thinningprocess, with the alkali basalts derived from theasthenosphere. Accordingly, the Mesozoic lithosphericthinning beneath the eastern NCC was a prolonged andcontinuous process with the partial thinning related to apost-collisional collapse in the northern and southernmargins around 180 Ma and distinct from the large-scalethinning during 160–110 Ma.

1.3. Disruption and replacement of the lithospheremantle

Many petrological, mineralogical, isotopic andchronological studies of mantle xenoliths in volcanicrocks and exposed Precambrian rocks show that therewas Archean or early Proterozoic mantle beneath theNCC, and the old mantle still partly survives in thepresent (Mesozoic–Cenozoic) mantle. Metasomatismand replacement of the Mesozoic lithosphere mantlewere intensive, heterogeneous and irregularly-shaped(mushroom-like) (Zheng, 1999; Lu et al., 2000).

Recently, Re–Os and Lu–Hf isotopic data for mantlexenoliths are reported in the literature (Gao et al., 2003;Wu et al., 2003; Gao et al., 2004; Zhi and Qin, 2004;Zheng et al., 2004b). The Re–Os isotopic system canpossibly trace the mantle partial melting and formationage of SCLM. The mantle xenoliths in kimberlites from


Fuxian yielded 2.5 and 2.8 Ga TRD ages, while themantle xenoliths in basalt from Hannuoba yielded anisochron age of 1940+/−180 Ma and their Re–Osisotopic composition appears to be a closed system withno indication of effects of later disturbance. A samplefrom Qixia shows the same Re–Os isotopic compositionas that present mantle, but with a trace of thecomposition of the old mantle. The granulite xenolithshave been determined to be lower crustal rocks (Zhanget al., 1998; Fan et al., 1998; Zhang and Sun, 2002;Zheng et al., 2004a). The isotopic ages of the lowercrustal xenoliths are generally restricted to 240–220Ma,but some are 2500 and ∼1900 Ma, and a few are even3000–2800 Ma. The εHf value is +18.37 and Hf modelages are 2600–2500 Ma. These data indicate that themain crustal growth time was at 2800–2500 Ma and the2.5–2.6 Ga Archean mantle was replaced by aProterozoic mantle at 1.9 Ga, identical to the results ofinvestigations of the exposed Precambrian terranes(Zhai and Liu, 2003, 2004; Zhai, 2004a). Then, theNCC underwent another intense mantle replacement inthe Mesozoic.

The geochemistry of Mesozoic basalts implies thatthe SCLM beneath the NCC is different from bothPaleozoic craton-type mantle and Cenozoic ocean-typemantle (Zhang et al., 2002a). Xu (2006) came to a sameconclusion by using basalt trace element geochemistry toconstrain the evolution of the lithosphere beneath theNorth China Craton. The SCLM underwent majorchanges at least two times at Mesozoic. The Paleozoicharzburgitemantle with depletion inmajor elements (e.g.Ca and Al) and moderately enriched in isotopiccomposition changed to a middle–late Mesozoic mantleof peridotite and pyroxenite that were strongly enrichedin major elements and isotopic composition, and thelatter somehow was in turn replaced by a hot and thinCenozoic two-pyroxene peridotite mantle enriched inmajor elements and depleted in isotopic composition.Extensive and continuous basic-intermediate and inter-mediate-acid magmatic activity shows that both thebeginning and ending time of lithospheric thinningbeneath the northern margin of the NCC were earlierthan those beneath the southern margin of the NCC.Extensive basic-intermediate magma activity in thesouthern margin occurred only in the Cretaceous, andbasalts derived from the asthenosphere erupted in theTertiary. The Mesozoic SCLM is highly heterogeneousin chemical composition (Zhang et al., 2004): weaklyenriched mantle in western Shandong, EM1-type mantlein Taihangshan (87Sr/86Sri=0.7050–0.7066; εNd(t) =−17 to −10) (Chen et al., 2003; Chen and Zhai, 2003;Chen et al., 2004); EM2-type mantle in southern and

eastern Shandong (87Sr/86Sri is up to 0.7114); andmixture of both depleted and depleted and enrichedmantles in the northern NCC. The Mesozoic SCLM alsoshowed an evolving tendency of gradually becomingricher with time. Their isotopic composition indicatesthat the Mesozoic SCLM was metasomatized by silica-rich fluids to varying degrees, strongly in west-southernShandong and weakly in central Shandong and Taihang-shan. TheMesozoic mantle heterogeneity was mainly aninherited property from the old heterogeneous mantle(Zhai and Liu, 2003), and is partly attributed to thePaleozoic subducted lithospheric slab in the northern andsouthern margins.

Zhang H. F. et al. (2005) reported that clearly zonedolivines in the Fangcheng basalts from ShandongProvince contain clearly zoned olivines provided newevidence for the replacement of the lithospheric mantlefrom high-Mg peridotites to low-Mg peridotites throughperidotite–melt reaction. Zoned olivines have corecompositions (Mg#=87.2–90.7) similar to those ofolivines from the mantle peridotitic xenoliths entrainedin Cenozoic basalts in the NCC and rim compositions(Mg#=76.8–83.9) close to those of olivine phenocrystsof the host basalts (75.7–79.0). These compositionalfeatures as well as rounded crystal shapes and smallergrain sizes (300–800 mm) demonstrate that these zonedolivines are mantle xenocrysts, i.e. disaggregates ofmantle peridotites. Their core compositions mostprobably represent those of olivines of mantle perido-tites. The zoned texture of olivines was formed throughrapid reaction between the olivine xenocryst and fluidfrom the crust. This olivine–fluid reaction could havebeen ubiquitous in the Mesozoic lithospheric mantlebeneath the NCC, i.e. an important component of thereplacement of the lithospheric mantle. The reactionresulted in the transformation of the Paleozoic refractory(high-Mg) peridotites to the late Mesozoic fertile (low-Mg) and radioactive isotope-enriched peridotites, lead-ing to the loss of the old lithospheric mantle.

2. Lower crust thinning and replacement

An obvious difference from other cratons in theworld of the NCC is that it has no or little craton-typePrecambrian lower crust. This can be attributed to theintense crust–mantle interaction and metasomaticreaction of the mantle in the Mesozoic, leading to alarge-scale thinning of the lithosphere mantle and lowercrust beneath the NCC (Zhai and Fan, 2002; Kuskyet al., in press).

Xenoliths of granulites representing the lower crustin Mesozoic and Cenozoic basalts have been found. The


host rock basalts are Precambrian high-grade metamor-phic rocks that represent exposed Archean–Paleopro-terozoic lower crust.

2.1. Precambrian lower crust in the NCC

Traditionally, continental crust is divided into twolayers (granite and basalt) or three layers (upper,middle, lower). Lower crust (N20–25 km) is com-posed of granulites, including meta-gabbro andvoluminous quartz–diorite and trondhjemite (Bohlenand Mezger, 1989). Qian et al. (1985), Zhao et al.(1993) and Shen et al. (1992) proposed that granulite-facies terrane along the north margin of the NCCrepresents an oblique section through Archean lowercrust. The lower crust beneath the NCC has beendiscussed geochemically and geophysically (Ma,1989; Ma et al., 1991; Lu and Xia, 1992; Gao et al.,1998a,b). There is a debate whether the lower crust ofthe NCC is mainly mafic or intermediate in compo-sition (Kern et al., 1996).

Zhai et al. (1996, 2001b) reported an exposedPrecambrian lower crustal section in Manjinggou,northern Hebei Province. This exposed cross-section oflower crust is divided into five layers from south(lowermost crust) to north (upper–lower crust) by faults.Along a section from Manjinggou northward, the meta-morphic grade gradually decreases from high-pressuregranulite facies, middle-pressure granulite facies, mod-erate-low-pressure granulite facies to amphibolite facies.The metamorphic pressures of these five layers are,respectively, 12–14 kbar, 9 kbar, 7–8 kbar, 6 kbar and5 kbar. The petrology of the five layers changes fromgabbroic granulites, intermediate-felsic orthogneiss tometamorphosed supracrustal rocks. Geochemically, thelowermost crust (gabbro) and lower crust (intermediate-acid orthogneiss) are relatively poorer in Si and Alcompared to the middle–lower crust and upper–lowercrust. The cross-section demonstrates a decreasing trendof heat-producing elements and some large ion lithophile(LIL) elements. The rocks in the lowermost and lowercrust only contain CO2 fluid inclusions, whereas therocks in the middle–lower crust and upper–lower crustusually contain H2O fluid inclusions. Isotopic data ofgarnet-bearing mafic granulites, which represent thelowermost crust, indicate that the lower crust in this areaformed in Neoarchean, and was uplifted and exposed tothe surface in late Paleoproterozoic (Zhai et al., 1992;Guo et al., 1993; Zhai et al., 1995).

From the above discussions, it is evident that theexposed cross-section of the lower crust in the NCC issimilar to those in Ivrea, Kapuskasing and Dharwar

(Fountain and Salisbury, 1981; Percival et al., 1992) andhas a relative more stable basic layer of gabbroic rocksmetamorphosed to high-pressure granulite facies andeclogite facies (Percival, 1988; Fountain et al., 1990;Percival et al., 1992; Rudnick and Fountain, 1995; Liet al., 1998; Liu et al., 1998).

2.2. Lower crust xenoliths in volcanic rocks

Lower crustal xenoliths in Mesozoic–Cenozoicbasalts were discovered from Hannuoba, Nü shan,Xinyang and in a kimberlite from Fuxian (Zhou et al.,1992; Fan and Liu, 1996; Fan et al., 1998; Zhang et al.,1998; Chen et al., 1998; Huang et al., 2001; Yu et al.,2003; Zheng et al., 2003, 2004a,b; Huang et al., 2004;Liu et al., 2004; Zheng, 2005). Most xenoliths aregranulite facies rocks, while a few samples fromXinyang (Zheng et al., 2003) and Hannuoba (Fanet al., 2001a) are of eclogite facies.

2.2.1. Petrology and geochemistry of granulitexenoliths

Granulite xenoliths mainly are basic granulites andsome felsic granulites. Most samples have remarkablezonal and cumulate structure, including deformed zoneand mineral zoning. Basic granulites are classified intogarnet-bearing and non-garnet-bearing granulites, butcommonly contain two pyroxenes (Cpx, Opx), plagio-clase (Plg) and with or without quartz (Qtz). The mineralcompositions and their Mg–Fe percentage contents inbasic granulites from Hannuoba vary widely. Blasto-gabbroic texture and igneous cumulate texture are seenlocally in two pyroxenes granulites and garnet-bearingbasic granulites, indicating that their protolith rockswere gabbros. Hypersthene crystals contain a smallamount of quartz inclusions and/or clinopyroxeneinclusions. Clinopyroxene has higher contents of MgOand lower contents of FeO compared to that ofPrecambrian high-pressure granulite exposed as basalticcountry rock. Moreover, clinopyroxene inclusions arealso rich in MgO and poor in FeO and high in Al2O3,AlVI and Na2O (Fan et al., 2001a). Garnet has nosymplectitic texture suggesting a different metamorphichistory from the Precambrian high-pressure granulite(Zhai et al., 1992; Guo et al., 1993; Zhai et al., 2001b).Almandine is the major garnet in the granulite xenoliths.The granulite xenoliths are not significantly differentfrom the Precambrian granulites in bulk compositions,only with slightly lower SiO2 (47–49%). The estimatedmetamorphic temperatures are 900–1000 °C, higherthan that of the Precambrian high-pressure granulites,the pressures are 10–13 kbar, corresponding to about


33–45 km depth, and are similar to those of thePrecambrian high-pressure granulites. Zhai et al.(2001b) and Huang et al. (2001) reported the geochem-ical characteristics of granulite xenoliths from Han-nuoba and nearby exposed Precambrian basicgranulites. Geochemically, the granulite xenoliths canbe sub-divided into two groups (Fig. 2). One group is ofre-metamorphosed Precambrian granulites with inher-ited refractory elements. Another one includes youngmeta-gabbros, and their REE distribution patterns aredifferent from the Precambrian granulites. SystematicSr, Nd and Pb isotopic study shows they have mixedcharacteristics. The decoupling of the low Rb/Sr ratioand high 87Sr/89Sr ratio indicates that the granulitexenoliths have an intense mixing history, with compo-nents possibly formed in the Mezozoic or Cenozoic(Zhang et al., 1998; Chen et al., 1998).

2.2.2. Chronology of granulite xenolithsResult of chronological studies are identical to those

of Sr–Nd–Pb isotopic geochemical analyses. The zirconages of granulite xenoliths are 140.2+/−0.5Ma, 120.9+/

Fig. 2. Chondrite-normalized REE (A) and incompatible element (B)plots for Precambrian and basic granulite xenoliths.

−0.6 Ma and 124.2+/−0.5 Ma (Fan et al., 1998). Theseages suggest the following: underplating of gabbroicmagma derived from the mantle began in the lateJurassic, and was accompanied by crust–mantle mixingand granulite facies metamorphism. However, large-scale gabbroic magma underplating took place in earlyCretaceous, intruding the regional lowermost crust andreplacing the Precambrian lower crust. If takingcomplicated isotopic mixing characteristics into consid-eration, its forming process may become more compli-cated, including remelting of original basic lower crust,replacement by underplating gabbro, and mixing oldmantle and new upwelling mantle, and so on. Zheng(2005) reports a group of dating ages (in situ U–Pbzircon) for granulite xenoliths from Hannuoba, Fuxianand Xinyang using laser abrasion analysis, showing allsamples have inherited ages of 2700–2500 Ma and1900–1800 Ma. These ages also extensively recorded inexposed Precambrian rocks in the NCC (Kusky and Li,2003; Zhai and Liu, 2004; Zhao et al., 2004; Kusky et al.,in press). Considering Hf isotopic analyses, the zirconsof granulite xenoliths trace a complicated crust–mantleevolutionary history, multi-lower crust deformation andMesozoic magmatic process. The Hannuoba meta-gabbro xenoliths are ideal study samples for under-plating and replacement of lower crust (Fan et al., 2005;Zheng, 2005).

2.3. Garnet pyroxenite xenoliths and crust–mantletransitional zone

Xenoliths in basalts reveal that the present-day(Mesozoic) lower crust is divided into three layers:layer of felsic granulites, a layer of basic granulite(cumulate meta-gabbro) and the crust–mantle transi-tional zone (Zhai et al., 2001b; Zhai and Fan, 2002; Fanet al., 2005).

2.3.1. Petrology of crust–mantle transitional zoneThe crust–mantle transitional zone is composed

mainly of garnet pyroxenite belonging to eclogite faciesand mafic granulite, as well as pyroxenite and spinellherzolite. Granulite- and eclogite-facies rocks have atypical layered accumulate texture. Granulite-faciesrocks are pyroxenite (Cpx+Opx+/−Plg) with minorgarnet-bearing plagioclase pyroxenite and felsic granu-lites. Eclogite-facies rocks do not contain plagioclase(Grt+Cpx+/−Opx), and mostly garnet clinopyroxenite.

Most minerals of garnet pyroxenite of eclogite faciesshow characteristic elongation and mineral orientation.Garnets range in size from 1 mm to 8 mm andcommonly have undergone alteration. A few garnets

Fig. 3. Chondrite-normalized REE (A) and incompatible element (B)plots for crust–mantle transitional zone and upper mantle.


contain small rounded clinopyroxene inclusions. Esti-mated temperature and pressure for mafic granulites are800–1000 °C and 10–12 kbar, and for eclogite-faciesgarnet pyroxenites 1065–1080 °C and 13–15 kbar. Inother words, the granulites and eclogite-facies pyrox-enites originated from a depth corresponding to 33–40 km and 40–45 km. This indicates that the lower partof present lower crust and crust–mantle transitionalzone corresponds to 33–40 km and 40–45 km depth.The depth of 40–45 km is equal to the upper limit ofmantle spinel lherzolite (Fan et al., 2005).

2.3.2. Geochemistry of crust–mantle transitional zoneThe pyroxenites of granulites and eclogite-facies in

crust–mantle transitional zone contain Cpx+Opx ofN50% in volume, and have SiO2 contents of 45%–55%and Mg#N60 (64–81 for granulites and 63–90 foreclogitic rocks). Most samples (Mg#=80–90) are closeto upper mantle ultramafic rocks (Mg#∼90) in chemistry.

The REE and incompatible element patterns of crust–mantle transitional zone rocks are shown in Fig. 3.Clinopyroxenites have extremely enriched LREE (LaN-40) patterns without Eu anomalies. Two eclogite-faciesgarnet pyroxenites exhibit U-shaped REE patterns,showing the clinopyroxene LREE-enrichment andgarnet HREE-enrichment characteristics. Two pyroxe-nite samples (Cpx+Opx) have inverted U-shaped REEpattern, different from the garnet pyroxenites, in whichone composed mainly of orthopyroxene (90%) has verysimilar REE features to mantle peridotites, i.e. extremelylow REE concentrations and flat REE pattern.

For the various rocks in the transitional zone, thedegree of LREE enrichment and positive Eu anomalyvaries with the amount of plagioclase and pyroxene. Thexenoliths representing the lower crust, crust–mantletransitional zone and upper mantle, demonstrate a goodnegative correlation of Sr, Nd isotopic ratios. Mantleperidotite xenoliths have DMM characteristics. Eclogitefacies garnet-bearing (or not) pyroxenites in thetransitional zone also have DMM characteristics,indicating that they were derived from the mantlewithout crustal contamination. In contrast, granulitesrepresenting the lower crust have high 87Sr/86Sr and lowεNd, showing a transitional trend from EM1 to enrichedmantle, obviously mixed with crustal composition.

2.4. High-Sr and low-Y granitoid rocks

2.4.1. Distribution and geochemistry of high-Sr andlow-Y granitoid rocks

Voluminous Yanshanian granitoid intrusive bodieshave geochemical characteristics similar to those of

adakite. These rocks are extensively distributed in theeastern NCC (Fig. 4) including diorite, quartz diorite,granodiorite, monzonitic granite and granite (Li et al.,1994; Zhang et al., 2001a; Liu et al., 2002a). Althoughthere are geochemical differences among these intrusiveand adakitic volcanic rocks, these granitoid rocks aresimilar to adakite in most geochemical characteristics.Therefore, we named these rocks high-Sr granitoid rocks,and suggest that they were possibly derived from partialmelting of the lower part of lower crust (mostly mafic)(Zhang et al., 2001a; Liu et al., 2002b; Zhai, 2004b). Chenet al. (2004) suggested that the Mesozoic intrusive bodies(gabbroic diorites and quartz–monzonites) in Taihang-shan, which also have high-Sr and low-Y features,probably originated from the mixing between maficmagmas from enriched mantle (EM1) sources and crust-derived granitic melts followed by fractionation, ratherthan frommelting of the lower crust. Summarizing studiesof regional structure, sedimentary basin and geothermicdata during the Mesozoic tectonic inversion, Zhai et al.(2004a,b) and Zhai (2004b) preferred to explain that mosthigh-Sr granitoid rocks originated from partial melting of

Fig. 5. Sr/Y–Y diagram of adakite-like granitoid rocks.

Fig. 4. Distribution of high-Sr granitoid and volcanic rocks in the eastern NCC.


lower crust although some intrusive rocks could haveoriginated from mixing magma sources. The high-Kfeature of high-Sr granitoid rocks is probably attributed tocomposition of lower crust of the NCC that is dominantlydioritic (Gao et al., 1998a; Zhai and Fan, 2002).

These high-Sr granitoid rocks have high SiO2 andAl2O3 contents, high Na2O/K2O (N1), La/Yb and Sr/Y(Fig. 5), enriched LREE, and depleted HREE, Y andHFSE, belonging to high-K calc-alkalic rocks. (Li et al.,2001; Wang and Zhang, 2001; Liu et al., 2002a; Li et al.,2004). They have low Mg, Cr and Ni, moderate–lowHREE (Ho–Lu) and weakly negative-Eu anomalies,implying that amphibole or plagioclase were present inthe melting residue or that plagioclase-fractionalcrystallization processes occurred.

2.4.2. Mesozoic “East Plateau” and partial melting oflower crust

Conditions favorable to form adakite magmasinclude high temperatures and pressures (850–

1150 °C, 10–40 kbar) with fluids rich in H2O. Thesealso represent conditions, favorable to extract theelements Cu, Au, Mo, Ag and so on from mafic rocks


and mantle (Bissig et al., 2003). Adakitic magmaformed at 70–90 km depth in subduction zones, situatednear transitional boundaries between amphibolite faciesand eclogite facies (Defant and Drummond, 1990).Abundant fluid rich in H2O from amphibole dissolutionpossibly can decrease the melting temperature ofMORB. Similarly, base of thickened lower crust is asetting to form high-Sr magma if the temperature,pressure and fluid are appropriate, and high-Sr magmafrom lower crustal melting is possibly richer in K thanthat from MORB melting (Kay and Mpodozis, 2002).

High-Sr granitoid rocks were extensively distributedin eastern NCC during the middle–late Jurassic. It isdifficult to relate these intrusive bodies to subduction.High-Sr granitoid rocks were possibly derived frompartial melting of basic-intermediate granulites inthickened lower crust (Zhang et al., 2001b) or from thedelamination of lower crust and crust–mantle interaction(Xiao et al., 2004). Zhang Q. et al. (2001b,c) suggestedthat a “Mesozoic East Plateau” existed in the NCC. Theyalso suggested the following processes: Basaltic magmafrom lithospheric mantle intruded the base of craton-typelower crust, and the lower crust was thickened toN50 km. Then the thickened lower crust underwentgranulite facies metamorphism and was intruded bymafic cumulates and ultramafic rocks, finally leading topartial melting of the lower crust and formation of high-Sr magma. The melting residue is possibly eclogite orgarnet–hornblendite.

LiuH. T. et al. (2002b) grouped theMesozoic granitoidrocks in the northern NCC into five types: stronglyperaluminous leucogranites, normal calc-alkaline and

Fig. 6. 87Sr/86Sr–εNd-diagram of Mesozoic alka

high-K calc-alkaline granites, high-Sr granites, alkali A-type granites and peralkaline granites. The peraluminousleucogranites were derived from dehydration melting ofmetapelites and metagraywackes during uplift anddecompression of the thickened crusts. The high-Srgranites were derived from the over-thickened lowercrust by dehydration melting of metamorphosed interme-diate-acid igneous rocks. The alkali A-type granites andperalkaline granites were generated through regionalextension of the lithosphere, with the later indicatingrifting within the craton. The regional graniticmagmatismrepresented by high-Sr granites in the NCC demonstratesa significant tectonism at 160–145 Ma; the regionalextension and crust thinning are indicated by the A-typegranite intrusion and decrease of the high-Sr granitevolume at 140–110Ma; the alkali granite marked that thecrust reached a normal thickness (35–40 km). Thereforethe crust lost 15–20 km of its thickness.

2.5. Mesozoic volcanic rocks

2.5.1. Mesozoic volcanic activityMesozoic volcanic activity in the eastern NCC

evolved with time, especially along its northern andsouthern margins. Along the northern margin, earlyvolcanic rocks (late Jurassic–early Cretaceous) includemainly thick deposits of basaltic andesite, andesite,trachyte, and small amounts of high-Mg andesite(Zhang et al., 2003a). The rocks were derived mainlyfrom the lithospheric mantle contaminated by crust. Latevolcanic rocks (∼105 Ma) include basalt in Liaoningand Inner Mongolia derived from asthenospheric

line of Magma (from Zhang et al., 2004).


mantle. On the southern margin, intermediate-basicvolcanic rocks derived from the lithospheric mantleformed mainly at ∼120 Ma, whereas basalt derivedfrom the asthenospheric mantle formed at ∼80 Ma.These indicate that the beginning and ending oflithospheric thinning beneath the northern NCC areearlier than those beneath the southern NCC.

On the southern margin, for example in thesouthwestern Shandong, Sr isotopic composition ofMesozoic volcanic rocks rose higher through time (from180 Ma to 120 Ma), reaching a peak at 120 Ma. Thisevolution of Sr isotopic compositions is shown by thealkaline intrusive bodies in this area (Fig. 6; Guo et al.,2001, 2003; Zhang et al., 2004). In the Cenozoic,Tertiary basalts show weak enrichment of Sr isotopiccomposition, and Sr isotopic composition of LateTertiary–Quarternary basalts became depleted, imply-ing that the magma source gradually become deeper,thermal gradients of lithosphere lower, and lithosphericmantle thicker.

2.5.2. High-Mg andesite and adakiteBased on trace element and Sr–Nd–Pb isotopic

characteristics, Zhang Q. et al. (2001c), Li W.P. et al.(2001), Li X.Y. et al. (2004) and Xiao et al. (2004)suggested that a part of the Mesozoic volcanic rocks inthe eastern NCC (e.g. western Liaoning, westernShandong and eastern Shandong) are adakitic, althoughthey have higher K content. They proposed that theserocks are different from adakites related to subductedocean crust, because they were formed in an intraconti-nental setting. Gabbroic magma underplating took placeduring the Late Triassic and thickened the lower crust. Asmall proportion of melt from the asthenosphere causedthe thickened lower crust to become richer in K andLILE (McKenzie, 1989). The K-rich and Mg-poorvolcanic magma derived from partial melting of themetamorphosed thickened lower crust, or magmaformed by melting of the delaminated crust and thenmixed with mantle peridotite, formed these adakite-likevolcanic rocks.

High-Mg andesites were present in Fuxin, NE China(Zhang et al., 2003a); their Mg# are ∼67, Sr–Nd–Pbisotopic compositions have EM1 mantle characteristics,and K–Ar ages are ∼142 Ma. Their suggested magmasource is EM1 mantle, i.e. refractory lithosphere mantlemodified by a previously subducted slab. A character-istic adakite was reported from the XinglonggouFormation in northern Liaoning (Gao et al., 2004). TheXinglonggou Formation is composed of high-Mgandesite, adakite, dacite and rhyolite. All containcontents of high-Na2O (≤5.7 wt.%) and Sr (500–

1618 ppm), are depleted in HREE (Ybb1.8 ppm,Y≤18 ppm), and have high values of Sr/Y (36–135)and LaN/YbN (17–19). The Na2O/K2O value of adakiteis N2.0, and the Mg# of daci te is 53–65(Mg#=100⁎Mg/ (Mg+Fe). The SHRIMP and LA-ICP-MS zircon ages of the lower part rhyolite andupper part cryptomere are 159–144 Ma. Consideringthat the zircons yielded 2500 Ma inherited age and thepresence of granulite xenoliths in basalts from Xinyangin southern NCC, Gao et al. (2004) proposed that theXinglonggou lava formed by melting of founderedeclogite forming at the base of Archean lower crustthickened in the Triassic. Their suggested dynamicmodel relates the subduction of the Palaeo-Pacific Oceanto the collision of an amalgamated North China–Mongolian plate with the Siberian plate during theclosure of a Mongolo–Okhotsk ocean, or to more globalevents, like mantle overturn and upwelling of hotasthenosphere at the base of crust.

3. Lower crust process in lithospheric thinning

3.1. Replacement of the lower crust

(1) It can be inferred from the previous discussionsthat the Precambrian granulite terrane in the NCCrepresents the Precambrian lower crust, whichwas uplifted to the surface during ca. 1800 Ma,whereas the granulite xenoliths brought byMesozoic–Cenozoic basalts represent the presentlower crust.

(2) The two types of lower crustal granulites are verydifferent from each other with respect to mineralcomposition, metamorphic history and isotopiccharacteristics, and the present lower crust wasformed between 140–120 Ma.

(3) The upper part of present lower crust is mainlycomposed of felsic granulites and the lower part ofbasic granulite-facies cumulates (meta-gabbros).The crust–mantle transition zone is dominantlycomposed of eclogite-facies pyroxenite, garnetpyroxenite, as well as mafic granulite and spinellherzolite. The Precambrian lower crust under-went a long history of geological evolution andwas mostly replaced by the present lower crust inthe Mesozoic.

(4) Adakite and high-Mg andesite occur only in thenorthern margin of the NCC. The high-Srgranitoid rocks are extensively distributed in theeastern NCC, and formed mainly in 160–140 Ma,while calc-alkaline and alkali granites mainlyformed between 140–110 Ma. This change of


granitoid types implies a loss of 15–20 km in thethickness of the crust (Liu et al., 2002b).

3.2. Constraints from seismic wave velocity andpetrology of xenoliths lower crust

Two-dimensional seismic wave refraction profilescross the NCC (Sun et al., 1985; Liu et al., 1991; Kernet al., 1996; Zhu et al., 1997; Chen et al., 1998).Experimental study on the P wave velocity for lowercrust xenoliths and Precambrian granulites at hightemperature and high pressure (Zhang and Sun, 1998;Zhai et al., 2001b; Fan et al., 2002) reveal the presentlower crust petrology. The Vp of Precambrian basic–felsic granulites varies from 6.57–7.0 km/s, the Vp ofbasic granulite xenoliths from 7.17–7.29 km/s, Vp ofgarnet pyroxenites in the eclogite facies from 7.31–7.78 km/s (Holbrook and Mooney, 1992; Rudnick andFountain, 1995). The Precambrian lower crust repre-sented by the exposed Precambrian granulites iscomposed of basic–felsic granulites, however thepresent crust represented by xenoliths in volcanicrocks is composed of mafic granulites and mafic–ultra-mafic rocks, exhibiting textural features of the lowerpart of the lower crust and crust–mantle transitionalzone. In the two wave refraction profiles crossingnorthern Shanxi–Hebei and Inner Mongolia–Hebei,there is a stable velocity layer of 6.5–6.9 km/s and aclear and flat gradient at 6.9–7.0 km/s (41–42 km),followed by an unstable layer of 7.0–8.0 km/s materialthat is 1–3 km in thickness. Bohlen (1987) suggested theEarth commonly has a layer of mafic lower crust with awave velocity of 7.0–7.5 km/s. However the layer of6.5–6.9 km/s (32–41 km) in the NCC is stable; thus it is

Fig. 7. Crust–mantle two-dimensional velocity structure diagram in H

most probably and composed of intermediate-acidgranulite with intercalated basic granulite as indicatedby the petrology and geochemistry of lower crustxenoliths. However, there is no stable mafic lower crust(lowermost crust) as suggested by Bohlen. The 7.0–8.0 km/s layer indicates that a crust–mantle transitionalzone exists. The 6.9–7.0 km/s layer perhaps is theboundary between the lower crust layer of Precambrianintermediate-acid granulites with intercalated basicgranulites and the Present (Mesozoic) crust–mantletransitional zone.

Geophysical data from many geological profiles inthe world reveal that the Moho is not a clear boundary,but is a transition zone formed by ultramafic–maficmagmatic underplating (e.g. O'Reilly and Griffin,1994). Rudnick and Fountain (1995) considered theMoho as a petrological transition zone between the crustand mantle, i.e. petrological Moho, on the basis of thestudies of the Cenozoic volcanic-borne xenoliths.Recently, artificial seismic COCORP profiles, electro-magnetic sounding and topography reveal that the lowercrust consists of a high velocity layer and a crust–mantletransition zone. The Moho obtained from seismicreflection in the eastern NCC is at a depth of about41–42 km, for example, Fig. 7 is a seismic profileshowing crust–mantle two-dimensional velocity struc-ture in the Hannuoba–Hunyuan area. But the experi-mental petrological Moho inferred from the maficxenoliths is ∼45 km. Therefore, we suggest that theboundary at 41–42 km is not the Moho. The Mohobeneath the eastern NCC is a crust–mantle transitionzone between a range of 40–45 km. Fig. 8 is a sketchdiagram for composition and structure of crust–mantlein the Hannuoba area.

annuoba–Hunyuan area (after Chen, 1998; Zhai and Fan, 2002).


3.3. Disruption and replacement of the lower crust:magma underplating

Petrological and geophysical studies (Fan et al., 1998,2001b; Zhai and Fan, 2002; Fan et al., 2005) show thatthe present lower crust in the eastern NCC can be dividedinto three main parts. They are the upper part of the lowercrust (25–33 km depth), part of the lower crust (33–40 km depth) and the crust–mantle transitional zone(40–45 km depth). The upper part of the lower crust iscomposed mainly of Precambrian felsic granulites. Apart of the lower part of the lower crust is composed ofinterlayered Precambrian felsic and basic granulites witha few underplated gabbroic granulites. The crust–mantletransition zone is composed mainly of garnet pyroxeniteof the eclogite facies and mafic granulite, and partly ofproxenite and spinel lherzolite. Mineral chemistry showsthat there is clear temporal and spatial evolution trend inthe mineral compositions of the different layers (lowercrust→crust–mantle transition zone→upper mantle).Plagioclase is only present in the lower crust, and their

Fig. 8. Sketch diagram of the composition and structure of the crust an

compositions vary from bytownite to andesine (An77-40), showing a large compositional variation in theunderplated cumulates. Clinopyroxene is the mineralubiquitously present in all layers, showing a regularincrease inMgO and a decrease in FeO, corresponding tochanges from augite→ salite→ diopside (Cr2O3 -0.5%)→Cr-diopside (Cr2O3N0.5%). Clinopyroxene inthe granulite-facies underplated cumulates is high inTiO2 (0.7%–1.0%) and is distinct from clinopyroxenesin the Precambrian granulites (generally TiO2b0.5%).Orthopyroxene in the granulite-facies pyroxenites ishypersthene (En62–80), in the green-colored websterite(DM9843) is bronzite (En86), and in the mantle peri-dotites is enstatite (En88–90). Garnet in the Precambriangranulites has low percent pyrope (Pyr20), in thegranulite-facies plagioclase garnet pyroxenite and garnetpyroxenite xenoliths has relatively high percentage(Pyr33–59), and in the eclogite-facies garnet pyroxenitexenoliths has highest percentage (PyrN70). A few rutilesoccur in the granulite-facies plagioclase pyroxenitexenoliths, consistent with the occurrence of TiO2-rich

d mantle in the Hannuoba area (modified from Fan et al., 2005).


clinopyroxene in these types of rocks. A minor amountof spinel occurs in spinel websterites and peridotites,including Al-spinel and Cr-spinel, respectively. Thecompositional variation characteristics of minerals fromthe upper part of lower crust→ the lower part of lowercrust→crust–mantle transitional zone→upper mantleis, in general, that plagioclase, pyroxene and garnetincrease in CaO, MgO and Cr2O3 contents, reflecting thechanges of rocks from felsic→mafic→ultramafic.

Based on mineral assemblages, mineral chemistryand P–T estimates for xenoliths (Xu et al., 1996; Fanet al., 1998; Deng et al., 1999; Liu et al., 2004), themetamorphic temperature of the eclogite-facies plagio-clase websterites is 800–1000 °C, with the majority in850–950 °C and pressure 10–12 kbar. Temperature andpressure for the eclogite-facies garnet pyroxenites is1065–1080 °C and 13–15 kbar, respectively. In otherwords, the granulite- and eclogite-facies pyroxenitesoriginated from depths corresponding to 33–40 km and40–45 km. These indicate that the lower part of thelower crust and crust–mantle transitional zone corre-spond to 33–40 km and 40–45 km, respectively.

The above-mentioned studies dealing with mineralcomposition, P–T conditions, rock geochemistry andcumulate texture indicate that the present lower crustpartly replaced the Precambrian lower crust. This lowercrustal process included emplacement of magma derivedfrom the asthenosphere to the lithospheric mantle,formation of mafic–ultramafic cumulate rocks, heatingthe bottom of the original lower crust, partial melting ofthe lower crust and formation of high Sr granites, andinteraction and mixing between crust and mantle. Themafic–ultramafic cumulate rocks also underwent gran-ulite–eclogite facies metamorphism controlled mainlyby rock chemical compositions and fluids.

4. Discussion of lithospheric thinning: magmaunderplating and delamination

Various hypotheses have been proposed to explainlithospheric thinning beneath eastern North China,including delamination of the lithosphere (Gao et al.,1998b, 2004), continental root–plume tectonics (Denget al., 1996, 2004) and mantle thermal erosion (Xu,2001).

4.1. Understanding underplating and delamination

Rudnick and Fountain (1995) discussed underplatingand delamination and synthesized the following: Under-plating— intrusion of magmas near the base of the crust;Delamination — a process by which a dense segment of

the lower crust (and lithospheric mantle) sink into theconvecting asthenosphere as a result of their negativebuoyancy. In their opinion, these two terms are notdirectly related to a specific tectonic mechanism. Under-plating emphasizes a magmatic process. The magmashould be mafic–ultramafic and derived from the mantle,and emplaced in deep levels only near the base of thecrust. Delamination emphasizes a convective processwhere a dense segment of the lower crust sinks into theasthenosphere. Rudnick and Fountain (1995) alsosuggested that the rheological and compositional differ-ences between the upper and the lower crust in continent–continent collision zones can promote delamination of thelower crust, and therefore delamination necessarily drawsenergy from an external force. Kay and Kay (1993)suggested that the causes for density inversion werethermal, compositional, and phase changes. As anexample, they reported that highly dense eclogite formsas a residue from partial melting of subducted a mafic slaband caused a delamination in the southern Puna plateau.Gao et al. (2004) explained delamination of the lowercontinental crust in the NCC by adakite eruption innorthernmargin of the North China and suggested that theremnant eclogite sank into the mantle. Therefore, highlydense eclogites inducing lithospheric delamination shouldbe formed in orogenic settings.

4.2. Mesozoic tectonic regime inversion is controlled bya continental intraplate mechanism

Study of Mesozoic magmatic activity and basinevolution shows that there are no Jurassic–Cretaceousisland arc rock associations in eastern North China. Inother words, evidence for Paleo-Pacific subduction from180 Ma beneath North China (Isozaki, 1997) is scarce.The Mesozoic NNE-trending basin-range structures inNorth China are obvious in the upper–middle crust, theEW and SN-trending structures are present in themiddle–lower crust and mushroom-like structure inthe mantle.

Mesozoic structural deformation of the Yanshanstructural belt is complicated and has records of multi-compression-extension from Middle–Jurassic to MiddleCretaceous. However, the NCC was in an extensionalsetting at ∼140–120 Ma, showing a ∼ENE tectonictrend including basin and range development, andoverprinting of an older EW basin and range series onthe northern and southern margins. The unconformitybetween the Tiaojishan–Tuchengzi Formations andZhangjiakou Formation is suggested to be a long-distance effect of collision of the Siberian Block andNorth China–Mongolia Block (Meng, 2003) or

Fig. 9. Diagrammatic sketch showing Mesozoic tectonic relation between the NCC and surrounding geological terranes.


subduction of the Paleo-Pacific Plate (Zhao et al., 1994;Deng et al., 2004). However, Shao et al. (2000) and Shaoand Zhang (2002) argued that this so-called unconfor-mity was a result of stress adjustment and regionaldeformation during different sedimentary stages in acontinued extensional process, therefore the Yanshanstructure belt was not a traditional orogenic belt.

Some important Paleozoic tectonic events have beenrecognized, such as continental collision and UHPmetamorphism a long the Qinling–Dabie orogenic beltand the Paleo-Mongolia Ocean closure in the Middle-Asian orogenic belt, although these could have no directrelations to the Mesozoic tectonic regime inversion andlithospheric thinning. Ding and Lai (2003) suggested thatthe High-Himalayas and Tethys–Himalayas (southernmargin of Asian continent) were similar to the AndeanMountains in the South American continent in theMesozoic. They reported petrological and structuralevidence for crustal shorting and rapid uplift, indicatingthat the Neo-Tethys Ocean started subducting in an areawest of the NCC, at least, since late Jurassic. The sedi-mentary analysis and chronological study support that

Fig. 10. Seismic tomography profiles in the NCC alon

collision between Siberia and the North China–Mongoliablocks took place in the Middle Jurassic (∼170–150 Ma)(Meng, 2003). The subduction of Paleo-Pacific Plate(including Kula or Inazaki) is commonly used to explainthe Mesozoic tectonic regime inversion of the easternNCC (Hilde et al., 1977; Charvet et al., 1985; Hu et al.,1994; Zhao et al., 1994). Ren et al. (1997, 1999) proposedthat the Paleo-Pacific Ocean was an extension of thePaleozoic Tethys Ocean in the area east to the NCC, andclosed in the Jurassic (to ∼145 Ma). Therefore, interac-tions between surrounding geological terranes stronglyaffected the North China from the Jurassic to theCretaceous (Fig. 9), resulting in large-scale mantleupwelling and formation of the “ East Plateau”.Geophysically, Yuan (1996) recognized an upwellingbody of asthenospheric materials in the NCC (Fig. 10Aand B), possibly recording the Mesozoic mantle. Thecrust was thickened mainly by magmatic underplating(Shao et al., 2003; Zhang et al., 2006). The intensiveinteraction between crust and mantle induced thedisturbance, rotation and adjustment of temperature anddensity, resulting in major re-melting of crust and

g lat. 32–42° N (A) and long.100–125° E (B).


production of high-Sr granite, material exchange betweenand mixing of mantle–crust, and finally forming a newmagmatism–fluid–mineralization system. This tectonicmechanism is intracontinental and different from generalorogeny (Zhai et al., 2004a,b; Yang et al., 2004).

4.3. Summary and conclusion

1. The Mesozoic magmatism does not show zonalcharacteristics in temporal–spatial distribution, butmagmatic compositions changed with time, markingthe beginning and end of theMesozoic tectonic regimeinversion and lithospheric thinning in the region.

2. Adakite and high-Mg andesite along the northern andsouthern margins of the NCC, suggest lower crustaldelamination soon after continental collision, butthere is no evidence for this within the NCC.

3. Voluminous high-Sr granitoid bodies that intrudedthe NCC at 160–140 Ma were probably derived frompartial melting of the lower crust. Upwelling of hotmantle provided the heat for this lower crustalmelting (Xu, 2001; Zhai and Fan, 2002).

4. The present lower crust in the NCC has three layersincluding a thick crust–mantle transitional zone. Thebase of the lower crust was partly to mostly replacedby underplated magmatic rocks at 140–120 Ma.

5. The large-scale partial melting of the crust led tolower crustal thinning, and produced residues ofeclogite or garnet–amphibolite. The high-densityresidues were probably recycled into the lithospher-ic mantle or asthenosphere. However, the amount ofremnant eclogite was probably small, and founder-ing of this material was not the cause of theextensive lithospheric thinning (80–120 km) be-neath the NCC.

Therefore, we conclude that magmatic underplatingwas the most important mechanism of lithosphericthinning beneath the NCC. Lower crust processes, suchas large-scale melting of old lower crust and thereplacement of old lower crust by Mesozoic underplatingofmafic magmas, were important in lithospheric thinning.The formation of the present lower crust through under-plating and its replacement of the old one were closelyrelated to lithosphere mantle thinning. Therefore, both thelower crustal thinning and lithospheric mantle replace-ment were controlled by the same dynamic mechanism.

Acknowledgements

Weare grateful toNSFC (40234050, 40421202) and theChinese Academy of Sciences (KZCX1Q07) for support to

this project. The authors specially thank Tim Kusky, PaulRobinson, Min Sun, Shuchun Su, Yaoling Niu and Er-QiWang for their sincere help and rewriting the English of themanuscript, and thank CinQTy Lee, Bin Chen and PatCastillo for their constructive review comments.

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Lower crustal processes leading to Mesozoic lithospheric thinning beneath eastern North China: Underplating, replacement and delamination