u-pb zircon ages for a collision-related k-rich...

Geochemical Journal, Vol. 37, pp. 35 to 46, 2003

35

*Corresponding author (e-mail: [email protected])

U-Pb zircon ages for a collision-related K-rich complex at Shidaoin the Sulu ultrahigh pressure terrane, China

J. F. CHEN,1* Z. XIE,1 H. M. LI,2 X. D. ZHANG,3 T. X. ZHOU,1 Y. S. PARK,4 K. S. AHN,5

D. G. CHEN1 and X. ZHANG1

1Department of Earth and Space Sciences, Laboratory for Chemical Geodynamics,University of Science and Technology of China, Hefei 230026, China

2Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China3Regional Geological Party, Department of Land and Resources of Shandong Province,

Weifang, Shandong 261021, China4Department of Mineral Resource Engineering, Chosun University, Gwangju 501-759, South Korea

5Department of Earth Science, Chosun University, Gwangju 501-759, South Korea

(Received March 18, 2002; Accepted July 15, 2002)

The Shidao complex at the eastern extremity of the Sulu ultrahigh pressure (UHP) terrane is composedof oldest pyroxene syenite, quartz syenite and youngest granite intrusives of the K-rich shoshonitic series.U-Pb zircon dating yields nearly concordant ages of 225 ± 2, 211 ± 3 and 205 ± 5 Ma for pyroxene syenite,quartz syenite and granite, respectively. The ages closely postdate the 240 to 220 Ma UHP metamorphismand correspond to the rapid cooling and exhumation of UHP rocks. A close genetic relation may existbetween the formation of the Shidao intrusives and the continental collision and UHP metamorphism.However, the K-rich Shidao intrusive rocks are different from common syn-collisional granites in associa-tion of K-rich and granitic magmatism. A breakoff model is postulated to explain the formation of thecomplex. The breakoff of the subducting slab caused the rapid exhumation of the UHP terrane. Mantleupwelling resulted in basaltic magmatism and formation of the K-rich complex. Formation of the Shidaocomplex marked the cessation of the UHP metamorphism and the oldest ages of 225 ± 2 Ma of the com-plex is the minimum timing for the UHP metamorphism.

et al., 1997). Much research work has been car-ried out on the tectonic evolution of the orogenicbelt (e.g., Cong et al., 1999; Dong et al., 1998;Hacker et al., 1995; Liou et al., 1996; Maruyamaet al., 1994; Wang et al., 1992). Collision-relatedintrusions are one of the main components in manyorogenic belts (e.g., Atherton and Ghani, 2002;Davies and von Blanckenburg, 1995; Hansmannand Oberli, 1991; von Blanckenburg, 1992). Studyof such intrusions provides important constraintsto interaction between subducted continental slaband upper mantle rocks, timing of cessation ofsubduction and upwelling of the mantle.

Granitic rocks are widely distributed in the

INTRODUCTION

The Dabie-Sulu terrane in east central Chinais known to contain the largest distribution ofultrahigh pressure (UHP) rocks in the world.Coesite-bearing and other UHP rocks are widelydistributed in the two terranes (Liou and Zhang,1995; Okay et al., 1989; Xu et al., 1992; Yang etal., 1993; Ye et al., 2000a). Available age dataindicate that the continental collision and UHPmetamorphism took place in Early Triassic, around240 to 220 Ma (e.g., Ames et al., 1993, 1996;Chavagnac and Jahn, 1996; Hacker et al., 1998,2000; Li et al., 1993a, 1996, 1997, 2000; Rowley

36 J. F. Chen et al.

Qinling-Dabie-Sulu orogenic belt. Most of themare post-collisional, emplaced during Early Cre-taceous, around 140 to 120 Ma (Chen et al., 1995;Eide et al., 1994; Jahn et al., 1999; Ma et al., 1998;Xie et al., 1996, 2001). Syn-collisional graniteshave only been identified in South Qinling. Zir-cons from six high-K calc-alkaline granites gaveconcordia Triassic U-Pb ages of 220 to 206 Ma(Sun et al., 2002) whereas rapakivi granites weredated at 217 to 210 Ma using zircon U-Pb, biotiteAr-Ar and Rb-Sr mineral isochron (Lu et al.,1999). However, syn-collisional intrusion is rarein the Dabie-Sulu terranes. Lin et al. (1992) datedShidao complex from eastern Sulu terrane andgave whole rock Rb-Sr ages of 220 and 217 Ma.However, they did not relate the origin of the com-plex to the UHP belt. Guo, J. H. et al. (2001) re-ported two zircon U-Pb ages for one intrusion ofthe Shidao complex.

In this paper, we report new zircon U-Pb agesfor three units of the Shidao complex in the Suluregion and discuss their origin and geologicalimplications.

GEOLOGIC SETTING

The Qinling-Dabie-Sulu orogenic belt in thecentral China was formed by collision between theNorth China and Yangtze blocks during theTriassic. The Sulu terrane was offset by the sinis-tral Tanlu Fault by about 500 km to the north (Xuand Zhu, 1994; Fig. 1). The terrane is generallydivided into a high-pressure blue schist unit in thesouth and an UHP unit in the north (inset of Fig.1). Similar to the Dabie terrane, the Sulu UHP unitis represented by a metamorphic complex com-posed mainly of granitic gneiss, granulite and sub-ordinate eclogite, schist, amphibolite, marble andquartzite. These rocks were subjected to TriassicUHP metamorphism (Carswell et al., 2000; Ye etal., 2000b). Eclogites, including coesite-, andcoesite-pseudomorph-bearing eclogites, andserpentinized peridotites occur as “enclaves” ingneisses or as nodules in marbles. The Sulu terraneis intruded by abundant Mesozoic granitic plutons,most of which are Late Jurassic to Early Creta-ceous in age (Shandong, 1991).

Fig. 1. Geologic sketch map of the Shidao K-rich complex with the sampling locations. Inset shows the locationof the Shidao complex with respect to the Qinling-Dabie-Sulu orogenic belt. The hatched area represents theQinling-Dabie-Sulu orogenic belt.

Collision-related K-rich complex in Sulu UHP terrane 37

The Shidao complex is located in the easternextremity of the Sulu terrane (Fig. 1) and intrudedinto the granitic gneisses. The complex is com-posed of three intrusions: the Jiazishan pyroxenesyenite, the Renheji quartz syenite and theChashan granite (Fig. 1).

The pyroxene syenite body is situated in thenortheastern part of the complex (Fig. 1). Thepyroxene syenite is composed of K-feldspar (55–70%), plagioclase (18–27%), small amount ofquartz (less than 4%), biotite (2–7%) and variableamount of amphibole and pyroxene of up to 14%(Lin et al., 1992; Shandong, 1991). Rocks croppedout in the central part of the intrusion is porphyriticwith coarse-grained matrix of K-feldspar,plagioclase and small amount of pyroxene andbiotite, containing K-feldspar phenocryst as largeas 1 × 2 cm. Pyroxene syenite with trachytoidstructure occurs in the northeastern part of the in-trusion where feldspars have a subparallel dispo-sition, parallel to the contact of the intrusion andthe granitic gneiss. The relation between the twokinds of pyroxene syenite with different texturesis unknown because the contact between them iscovered by Quaternary sediments. The samplestudied here (97CK28) is a porphyritic pyroxenesyenite collected from the central part of the in-trusion (Fig. 1).

The quartz syenite intrusion is in the middleof the complex (Fig. 1). The rocks are composedof alkaline feldspar (up to 75%), plagioclase(10%), quartz (10–15%), and minor biotite,amphibole of up to 2% (Lin et al . , 1992;Shandong, 1991). Sample 97CK29 collected in theeastern periphery of the body (Fig. 1) isporphyritic with phenocrysts mainly of medium-grained K-feldspar, sodic plagioclase and quartz,mafic minerals are rare. Some K-feldspar has beenreplaced by albite.

The granite is located in the southeastern partof the complex (Fig. 1) and composed of perthite(up to 66%), plagioclase (7–26%), quartz (25%)and minor biotite and amphibole (about 1–2%).Miarolitic texture is common (Lin et al., 1992;Shandong, 1991). Sample 97CK30 from the south-ern part of the intrusion (Fig. 1) is coarse-grained

and composed of K-feldspar, plagioclase, quartzand small amount of amphibole, biotite andtitanite.

Garganite and syenite-aplite dykes occur in thepyroxene syenite and quartz syenite intrusions.Granite-aplite, syenite-porphyry and granite-por-phyry dykes cut quartz syenite and granite.Eclogite xenolith in these dykes was reported byLin et al. (1992).

Intrusive relations are well established in thefield. The quartz syenite clearly intrudes intoporphyritic pyroxene syenite along the southwest-ern part of the later, with sharp contact (Fig. 1).Numerous dykes of quartz syenite cut pyroxenesyenite, and the later forms many xenoliths withthe long dimension ranging from tens of metersnear the contact to only several centimeters faraway from the contact. Quartz decreases in thequartz syenite dykes along the contact withpyroxene syenite. Xenoliths of pyroxene syeniteare “digested” by the quartz syenite magma to dif-ferent degrees. Contact between quartz syenite andgranite is covered by Quaternary sediments andcan not be observed. However, xenoliths ofpyroxene syenite and quartz syenite can be foundin the granite. These xenoliths are also “digested”by the granite magma. These features are used toestablish the relative chronology of intrusions:from pyroxene syenite to quartz syenite to gran-ite. Gabbro xenoliths were found in pyroxenesyenite and quartz syenite, the biggest one is about300 m long, while the small ones are a fewcentimeters across, that suggested an even earliergabbroic magmatism.

According to classification of igneous rocksusing major elements (Middlemost, 1994), gabbroenclaves plot near the junction of foid gabbro,monzogabbro, foid monzodiorite andmonzodiorite fields; pyroxene syenite inmonzonite and syenite fields; quartz syenite insyenite and quartz monzonite fields and granitein quartz monzonite and granite fields (Fig. 2a;Lin et al., 1992; Shandong, 1991; Xie, 1998). Thethree rock units have high K2O contents of 4.22to 8.65% at 48.60 to 60.85% SiO2, 5.80 to 6.66%at 61.60 to 69.18% SiO2 and 5.15 to 6.07% at


67.22 to 73.54% SiO2 for the pyroxene syenite,quartz syenite and granite, respectively (Fig. 2b;Lin et al., 1992; Shandong, 1991; Xie, 1998).Thus, they belong to the shoshonitic series.

METHODS AND RESULTS

Zircons were extracted from samples of about10 kg by table concentrator, panning and handpicking. U-Pb isotope analysis was done at theTianjin Institute of Geology and Mineral Re-sources. A modified chemical procedure of Krogh(1973) was used to dissolve the zircons in con-

centrate HF and HNO3 for about 48 hours at 195°Cin teflon pressure bombs. The solutions were thenspiked with a mixed 205Pb-235U tracer for isotopedilution and Pb isotope ratio measurements. Pband U isotopic ratios were measured on a VG-354mass spectrometer with a Daly-type detector in adynamic mode. Pb isotopic ratios were correctedfor fractionation, total analytical blank, initialcommon Pb and the spike. The total blank of theprocedure is less than 30 pg for Pb and 1 pg for U.The Pb isotopic ratios of the blank are 206Pb/204Pb = 17.97, 207Pb/204Pb = 15.55, 208Pb/204Pb =37.71, which are typical of Pb found in the lab

Fig. 2. (K2O + Na2O) vs. SiO2 (a) and K2O vs. SiO2 (b) plot for the three rock units of the Shidao complex (AfterMiddlemost, 1994; Rickwood, 1989). Data were taken from Lin et al. (1992), Shandong (1991) and Xie (1998).FMS = Foid monzosyenite; FMD = Foid monzodiorite; FG = Foid gabbro; M = Monzonite; MD = monzodiorite;MG = Monzogabbro; QM = Quartz monzonite; GD = gabbroic-diorite.


reagents. Isotopic compositions of the initial Pbare estimated from feldspars of Mesozoic gran-ites in the Dabie-Sulu terranes (206Pb/204Pb =16.66 ± 0.64, 207Pb/204Pb = 15.41 ± 0.07, 208Pb/204Pb = 37.68 ± 0.70; Zhang, 1995). The data were

reduced following the procedure of ISOPLOT ver.2.92 (Ludwig, 1994). The uncertainties reportedfor the ages are 95% confidence limits. The U-Pbanalytical results are listed in Table 1 and shownas isotope plots in Fig. 3. In Table 1, only the 206Pb/

Fig. 3. U-Pb concordia diagram and weighted averages of 206Pb/238U ages for zircons from the pyroxene syenite97CK28 (a), (b), quartz syenite 97CK29 (c), (d) and granite 97CK30 (e), (f).


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238U ages are listed since the 207Pb/235U and 207Pb/206Pb ages are not applicable to young, lowerconcordia intercepts.

The zircons separated from the pyroxenesyenite (97CK28) are transparent and brown incolor. They are subhedral to euhedral with thelength to width ratios of 1 to 2. Some grains arecrystal fragments. The grain size ranges fromabout 200 to 400 µm. Corrosion pits can be seenin some surfaces. Five fractions analyzed are con-cordant or nearly concordant (Fig. 3a). Byweighted averaging of 206Pb/238U ages (t206), aweighted average t206 age of 225 ± 2 Ma withMSWD value of 1.5 was obtained (Fig. 3b). Thisapparent age is interpreted as the crystallizationtime of the pyroxene syenite intrusion.

The zircons isolated from the quartz syenite(97CK29) are transparent, yellow, brown to lightpink in color, subhedral to euhedral with the lengthto width ratios of about 1.5 to 2.5. The lengths ofcrystal range from about 120 to 350 µm and a fewgrains are fragments of crystals. Six fractions wereanalyzed. Five fractions are concordant and theother plotted close to the concordia (Fig. 3c). The206Pb/238U ages of the 6 fractions form two groups:220 ± 2 Ma (MSWD = 1.3) and 211 ± 1 Ma(MSWD = 0.2; Fig. 3d). The apparent ages do notcorrelate with the color and crystal form of zir-cons. The older age of 220 ± 2 Ma is close to the225 ± 2 Ma for the pyroxene syenite (97CK28)and is thus interpreted as the zircon xenocryst fromthe pyroxene syenite. Abundant pyroxene syenitexenoliths being digested to different degrees andof different sizes in quartz syenite supports thisexplanation. The younger age of 211 ± 1 Ma isinterpreted as the crystallization age of the quartzsyenite intrusion.

The zircon separates from the granite(97CK30) are transparent, yellow to brown incolor, euhedral with the length to width ratios ofabout 1.5 to 4. The lengths of crystal range fromabout 150 to 350 µm. Six fractions were analyzedand are all concordant (Fig. 3e). The 206Pb/238Uages of the 6 fractions form two groups: 222 ± 2Ma (MSWD = 1.3) and 205 ± 5 Ma (MSWD =5.4; Fig. 3f). Similar to the 97CK29, the older

apparent age of 222 ± 2 Ma agrees with the age of225 ± 2 Ma for the pyroxene syenite (97CK28)within analytical uncertainty and is thus inter-preted as the captured zircon from the pyroxenesyenite. The younger age of 205 ± 5 Ma is inter-preted as the crystallization age of the granite in-trusion.

The ages obtained here agree with the intru-sive sequence observed in the field.

DISCUSSION

Comparing with previous workOur U-Pb ages agree with existing Rb-Sr re-

sults by Lin et al. (1992) who dated two bodies ofthe Shidao complex by using Rb-Sr technique.They report a whole rock-biotite isochron age of220 ± 13 Ma for pyroxene syenite intrusion. Al-though the isochron has a large scatter withMSWD of 60 (re-calculated using ISOPLOT;Ludwig, 1994), it is nevertheless compatible withthe U-Pb zircon age of this work. The Rb-Sr gran-ite data give a linear array in 87Sr/86Sr against 87Rb/86Sr plot that would correspond to an age of 217 ±3 Ma (Lin et al., 1992; re-calculated usingISOPLOT, Ludwig, 1994). This is close to the U-Pb zircon age of the intrusion.

Guo, J. H. et al. (2001) reported zircon U-Pbages of 212 ± 1 and 209 ± 7 Ma for Jiazishanpyroxene syenite. These ages are about 15 Mayounger than the present result for the same in-trusion by unknown reason.

Slab breakoff modelThe location just in the middle of the UHP belt

and formation closely after UHP metamorphismled us to conclude that Shidao complex is geneti-cally related to the Triassic continent-continentcollision taken place at the Dabie-Sulu orogenicbelt and is a syn-collisional intrusive complex.However, most syn-collisional granites show char-acteristics reflecting metasomatism caused byfluid from subducted slab and crustal melting. K-rich Shidao intrusives do not resemble such rocks.

Davies and von Blanckenburg (1995) put for-ward a slab breakoff model to explain the syn-


collisional magmatism associated with deep sub-duction of continental crust and UHP metamor-phism. According to this model, when the conti-nental lithosphere was subducted, opposing buoy-ancy force led to an extensional deformation inthe subducted slab, resulting in a narrow mode ofdeformation in the subducted slab and slab break-off. Following the slab breakoff, theasthenospheric mantle upwelled into the narrowrift, resulting in partial melting of the previouslymetasomatised overriding mantle lithosphere, pro-ducing mafic magmatism (Davies and vonBlanckenburg, 1995). Experimental studies pre-dicted that deeply subducted crustal rocks woulddevelop K-rich fluids. Such fluids could causemetasomatism of the overriding lithospheric man-tle (Schreyer et al., 1987). The melts could be al-kaline to ultrapotassic from small degree meltingor calc-alkaline from slightly higher degree melt-ing of more fertile or hydrated peridotite (Athertonand Ghani, 2002). These magmas will rise intothe crust and induce crustal melting to producegranitic magma (Davies and von Blanckenburg,1995). Contemporaneous to the mafic-graniticmagmatism, the UHP rocks in the subducted slabstarted to exhume and the terrane uplifted becauseof the loss deeply subducted part of the slab andthe increasing buoyancy (Davies and vonBlanckenburg, 1995). Rapid cooling “froze” theisotopic composition equilibrated in the high tem-perature, which recorded the time of peak UHPmetamorphism and the beginning of exhumationof the UHP rocks and uplifting of the terrane.

The origin of syn-collisional magmatism in theAlps (Davies and von Blanckenburg, 1995; vonBlanckenburg and Davies, 1995), in SouthKarakorum and South Tibet (Maheo et al., 2002),and in Scotland and Donegal, Ireland (Athertonand Ghani, 2002) was successfully explained byusing this model. Except for timing of magmatism,the most important feature of the magmatic ex-pression of slab breakoff is association of basal-tic (lamprophyric to high-K calc-alkaline) andgranitic magmatism (Atherton and Ghani, 2002).

Spatial and temporal relation with respect to theorogeny

In the Alps, continental subduction at ca. 55–45 Ma followed by lamprophyric-graniticmagmatism between 43 and 25 Ma (Davies andvon Blanckenburg, 1995; von Blanckenburg andDavies, 1995). While in the Scotland and Ireland,magmatism of 435–390 Ma in age (maxima at410–400 Ma) occurred after closure of IapetusOcean (Atherton and Ghani, 2002).

Detailed Sm-Nd and zircon U-Pb dating ofeclogites and other UHP rocks from the Dabie-Sulu terranes show that peak UHP metamorphismtook place around ca. 240 to 220 Ma (Ames etal., 1993, 1996; Chavagnac and Jahn, 1996; Li etal., 1993a, 2000; Hacker et al., 1998). However,a few results gave younger or older ages (Ames etal., 1993; Hacker et al., 1998; Okay et al., 1993).From Sm-Nd and Rb-Sr dating of the UHP rocksfrom Shuanghe, Dabie terrane, Li et al. (2000) sug-gested that the UHP rocks experienced two epi-sodes of rapid cooling which correspond to fastexhumation of the terrane. They suggested that thefirst rapid cooling took place immediately afterUHP metamorphism and the second at 180 to 170Ma. Most of the age determinations for the UHProcks in Dabie-Sulu terranes have been done onrocks from the Dabie terrane. There are only a fewages for the Sulu region. Ames et al. (1996) datedan eclogite from western part of the Sulu terraneand obtained a zircon U-Pb age of 217.1 ± 8.7 Ma.A gneiss at Yankou in central Sulu gave a zirconU-Pb age of 220 Ma (Li et al., 1993b, 1997). Thesedata suggest that the peak metamorphism tookplace at the same time in both Dabie and Suluterranes. Presence of coesite and other UHP min-erals suggests rapid cooling after the peak meta-morphism for most Sulu terrane. Therefore, UHProcks from the Sulu and Dabie terranes probablyshare the similar first cooling histories.

The formation of the pyroxene syenite closelypostdated the regional UHP metamorphism. Thecrystallization ages of the quartz syenite and gran-ite were about 10 to 20 Ma later than the peak


UHP metamorphism and corresponded to the firststage of fast cooling of the UHP rocks and rapidexhumation of the terrane.

K-rich syenitic and granitic magmatismIn the Alps, the duration of the Tertiary

lamprophyric-granitic magmatism was 17 Ma,while in the Scotland and Ireland, syn-collisionalmagmatism lasted for about 45 Ma. At Shidaomagmatism lasted for about 20 Ma. This age dif-ference is consistent with slab breakoff model(Davies and von Blanckenburg, 1995).

K-rich pyroxene syenite was formed either bymelts from metasomatised lithospheric mantle orby calc-alkaline basaltic magma that crystallizedunder high pressure (10 kbar) and water-deficientconditions (Meen, 1987). Preliminary work showsa very negative εNd(T) values of –16.0 forpyroxene syenite (Xie, 1998), suggesting no com-ponent of asthenosphere and supporting a deriva-tion from enriched old lithosphere of the NorthChina craton, the overriding plate (Guo, F. et al.,2001; Xu, 2001). Quartz syenite and granite alsogive negative εNd(T) values of –15.6 and –15.7,respectively (Xie, 1998). This suggests that thethree intrusions of the Shidao complex probablyshared a common source. Quartz syenite and gran-ite were possibly formed by fractional crystalli-zation of the syenitic magma because of the iden-tical εNd(T) values. On the other hand, basalticmagma that intruded into the crust could also leadto granitic magmatism at the crustal depths(Huppert and Sparks, 1988; Davies and vonBlanckenburg, 1995). The possibility that the gran-ite of Shidao was formed by crustal melting cannot be ruled out since εNd (220 Ma) values of re-gional gneisses of the Dabie terrane concentratedin the –10 to –25 range (Chen and Jahn, 1998).The detail model on the formation of the Shidaocomplex merits further detailed investigation.

As discussed above, the overall feature of theShidao complex fit well into the scenario of slabbreakoff model (Davies and von Blanckenburg,1995; Li et al., 2001).

Formation of the Shidao complex signals the

cessation of the continental collision and thecompressive tectonics and marks the onset of theextensional tectonics and exhumation of the UHProcks. The oldest age of the K-rich complex (225± 2 Ma for the pyroxene syenite) provides theminimum estimate for the age of the UHP meta-morphism.

Comparison with other parts of the orogenic beltSyn-collisional granites have been found in the

Qinling terrane (Lu et al., 1999; Sun et al., 2002).They are high-K calc-alkaline and posses charac-teristics of crustal melts (Sun et al., 2002; Zhang,1994). Using breakoff model, Sun et al. (2002)suggested that these intrusions could form bymelting of the crust induced by intrusion of ba-saltic magma due to upwelling of theasthenosphere. Existence of large syn-collisionalgranite belt suggests that breakoff happened at ashallow depth which disturbed the asthenospheregreatly and led to the large scale melting of thecrust.

Since there was no report of syn-collisionalintrusions in the Dabie-Sulu orogenic belt, Daviesand von Blanckenburg (1995) suggested thatbreakoff in Dabie-Sulu belt took place at greatdepths (>130 km) and the upwelling asthenospheremight not lead to a significant thermal perturba-tion of the distant lithosphere of the overridingplate. The new U-Pb zircon ages reported here forthe Shidao complex in the Sulu region suggeststhat breakoff may not take place in such a greatdepth for the Sulu terrane, at least in the Shidaoregion.

According to the breakoff model, heat pulseby upwelling asthenosphere would produce sepa-rate intrusions along the strike of the orogenic beltat surface (Atherton and Ghani, 2002). Other slabbreakoff related syn-collisional intrusions mightexist in the Sulu region. Search for such intrusionsin the Dabie and Sulu terranes is being carried out.Priority should be given to K-rich rock-graniteassociation(s) similar to the Shidao complex(pyroxene syenite, amphibole-bearing syenite,quartz syenite and granite) in the Sulu region.


CONCLUSIONS

Located in the most eastern corner of the SuluUHP terrane, the Shidao complex gave U-Pb zir-con ages ranging from 225 ± 2 to 205 ± 5 Mawhich are very close to the ages of the UHP meta-morphism and correspond to the first stage of rapidcooling and exhumation of the UHP rocks in theDabie-Sulu orogenic belt. The Shidao complex isthe first identified collision-related intrusive com-plex in the eastern Sulu terrane although the syn-collisional granites have been observed in theQinling region. The K-rich Shidao intrusives wereformed in an extensional environment by partialmelting of the lithosphere of the overriding slaband was induced by upwelling of theasthenosphere following the breakoff. Formationof the Shidao complex marks the cessation of thecontinental collision and UHP metamorphism. Theoldest intrusive ages of 225 ± 2 Ma are similar tothe minimum ages of the UHP metamorphism.

Acknowledgments—We would like to thank Prof.Chang Y. F. (Department of Land and Resources ofAnhui Province), Prof. Deng J. F. (China University ofGeosciences, Beijing), Prof. Zheng Y. F. (Universityof Science and Technology of China) and Prof. Jahn B.M. (University of Rennes 1, France) for constructivediscussion. Thoughtful journal reviews by Dr. Liou J.G. and Dr. Turec A. helped us clarify many ambigui-ties. This work is supported by the former Ministry ofGeology and Mineral Resources of China (grant9501102-3-2), the Major State Basic Research Programgrant of China (G1999075503) to Chen J. F. and ChosunUniversity Funds (2000) to Park Young Seog.

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