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Mineralogical and geochemical constraints on contribution of magma mixing and fractional crystallization to high-Mg adakite-like diorites in eastern Dabie orogen, East China
State Key Laboratory of Geological Processes and Mineral Resour ces, Faculty of Earth Sciences, China University of Geoscienc es, 430074 Wuhan, China
Institut für Mineralogie, Leibniz Universitt Hannover, Callinstr. 3, D-30167 Hannover, Germany
Instit ut für Mineralogie, Westflische Wilhelms-Univers it
t Münster, Corrensstr. 24, D-48149 Münster, Germany
abstractarticle i nfo
Article histo ry:
Received 16 January 2013 Accepted 13 April 2013 Available online 20 April 2013
Magma mixing Fractional crystallizatio n High-Mg adakite Dabie orogen Tan-Lu fault MLYR belt
The Liu jiawa p lu to n which is l ocate d n e ar the e ast ern b ou ndary o f t he Dabie orogen is c om po sed of mul ti ple lith- ologic units including mainly gabbronor ites, d iorites, granod iori tes and hornblende gabbros. Gabbronori te s and hornble nde gabbros occur as enclave s in di oritic hosts wh ich show gradu al contact w ith granodiorites. Zi rcon U–Pb dati ng i ndicate s t hat gabbronori tes and diori tes fo rmed co evall y at ~ 128 Ma, but they have distinct zircon Hf isotope s wi th εHf (t) o f −26 to −23 (gabbronorite ) and of −32 to −27 (diorite) respectively. Petrographic o b se rv at io ns and r ock-fo rming mine ral co mpo s it ions cl ea rly sho w mi xing b e tw een ma ﬁc a nd f elsic magma end-members, which might have formed the homogeneous whole-rock Sr–Nd isot opes with εNd ( t ) o f −17 to
Sr of 0. 70 7 to 0 . 709. As re vealed by zircon Hf isotope s, F concentrations in am phibole and biotite and thermodynamic modeling of crystallization, the gabbro norites r epresent enriche d l ithospheric mantle-derived magmas which evolved by fractional crystalli za ti on of o r th op y r ox en e, clino pyro xene, m agne tit e and/or amphibol e, whe re as the granodiori tes may be derived f ro m the Dabie Archean basem ent. M ineralogical and ge ochem ic al data as wel l as m ajo r and trace e lem ent m odeli ng show that the o rigin o f di orites, previously interpre ted as h i gh-Mg adaki te s, can be ex plai ned b y magm a mixi ng be tw een th e crust -deri ve d granod ioriti c m agmas and t he dif ferentiation p roducts o f mantle-derived gabbronor itic magm as. As a resul t, t he high-Mg ad akit e-lik e g eo chemi st r y of the di or it es is a co n sequ ence o f magm a d iffe re ntiat io n at a c ru stal de pt h, invo lving fracti on al crys talli za ti on and magm a m ixing, r athe r t ha n a nintrinsicfeatureofprimitive melts.Themantleupwell- ing in the adjacent central Middle-Lower Yangtze River metallogenic (MLYR) belt during Late Jurassic–Ear ly Cretaceous belt m ight h ave acted as a precurs or and tri gge red the p artial me lt ing o f litho sphe ri c mantle b eneath the e astern Dabi e oroge n and the f urthe r me lt ing o f oroge nic basem ent, consistent wi th the mode l of Zhang et al. (2010) suggesti ng a magmatic li nk betwee n t he ML YR bel t and the so utheaste rn D abi e o rogen. © 2013 Elsevier B.V. All rights reserved.
In arc environments, adakitic andesites have been frequently interpreted as products of direct partial melting of subducted oceanic crust (Defant and Drummond, 1990; Kay, 1978; Martin et al., 2005; Yogodzinski et al., 2001). The adakitic signatures (e.g. high Sr but low Y concentrations) can be formed by high-pressure melting of basaltic rocks leaving garnet as a stable phase. Interaction between melt and mantle wedge (peridotite) can substantially promote the Mg content of ascending melts ( Kelemen, 1995; Liu et al., 2012; Tsuchiya et al., 2005; Wood and Turner, 2009). Comparatively, in intra-continental environments where oceanic subduction is absent, high-Mg adakitic intrusive or volcanic rocks have been widely be- lieved to represent partial melts of delaminated lowermost part of thickened continental crusts at great depths ( Gao et al., 2004; Guo et al., 2006; Huang et al., 2008; Qin et al., 2007; Xu et al., 2002, 2006, W. Xu et al., 2008b; Zhang et al., 2010 ). However, if the high-Mg adakitic magmas observed in arc or intra-continental envi- ronments are not primitive, as evidenced by numerous studies using mineralogical and geochemical ( Barboni et al., 2011; Castillo et al., 1999; Chiaradia et al., 2009; Dessimoz et al., 2012; Gao et al., 2009; Guo et al., 2007; Li et al., 2009; Macpherson et al., 2006; Richards and Kerrich, 2007; Streck et al., 2007; Zhang et al., 2011b) as well as experimental studies (e.g., Alonso-Perez et al., 2009; Müntener and Ulmer, 2006), their high-Mg adakitic signatures might be related to other processes including magma mixing and/or fractional crystallization of a variety of minerals (e.g., pyroxene, garnet and amphibole). These rocks and corresponding magmas can be termed as “adakite-like ”or “pseudo-adakite”and their geological implications are thus distinct from those based on assumptions that they are primitive magmas (e.g., Chiaradia et al., 2009; Li et al., 2009). For cases where magma mixing and/or crystal accumulation are evident from ﬁeld and microscopic observations, it is critical to identify the involved end-members and their origins using multiple approaches prior to interpret their pristine geochemical signatures properly. In th e T an -L u f ault zone of East Ch in a, wh ic h is a dj ac en t t o the ea st- er n b ou ndary of the Dabi e oroge n, several L ate Mesozoic m ag am tic in trusions (inclu ding th e C hilu ti ng diorit e) have been sugge st ed to be high-M g a da ki tes resul ting fr om partial m elting of dela minated c on ti- nental crust a nd subsequent melt/mantl e i nteraction (Huang et a l. , 2008; Wang e t a l., 2006, 2004 a). However, up to now d etailed petro- gr aphical a nd mineralogical i nves tiga tion s t o c heck wh et her t hey rep - re sent pr im it iv e m agmas o r n ot are l ac king. F or exampl e, based on t he bulk geochemist ry of se le cted samples, th e Chilu ti ng dior it es (i.e. p art of the L iujiawa plutonic c om plex) h av e b een i nterpreted as primitive high-M g a da kites a nd a lit hospheric d elamin at ion m od el alon g t he Tan-Lu fault z on e h as be en prop os ed accord in gly (Huang et a l. , 2 008). Neve rt heless, o ur prelim in ary petrogr aphical o bs erva tion and geo- chemic al mode ling in dicate that the d io ritic m em be rs of th e L iu jiawa pluton mi ght have b een a ffected t o a l a rge extent b y mag ma mixing and f racti onal c rystal lizati on ( Zhang e t al., 2012a, 2 011b). In this paper, quantit a ti ve constr aints on t he cont ribut i on of mag ma m ixing and f rac- ti onal c rystall izati on p roce sses t o t h e high- Mg adakiti c ge ochemist ry of th e L i ujiawa pluto n a re prov ide d. W e c om bi ne pet rog raphic o bser va- ti ons, dat a o n who l e- rock geoch emistry ( major and t race e l ement , Sr –Nd isotopes), U–Pb and Hf i sot opes of z i rcon, major a nd t race ele ment compositi ons o f mine rals (pyroxe ne, amphibole , biot ite , apat it e a nd pla- gi oclase) a s well a s g eoche m ical modeli ng t o wor k o ut the pe tr ogen es is of the p lut on. We demo nst rate that the diori tic r ocks fro m the Liujiawa plut on are “pseudo high-M g adakites”and that the magmas ne it her deri ved di rect l y b y hig h-pressur e melt ing o f lo wer c on tine ntal cr ust nor reﬂect c rust de laminati on.
be termed as “adakite-like ”or “pseudo-adakite”and their geological implications are thus distinct from those based on assumptions that they are primitive magmas (e.g., Chiaradia et al., 2009; Li et al., 2009). For cases where magma mixing and/or crystal accumulation are evident from ﬁeld and microscopic observations, it is critical to identify the involved end-members and their origins using multiple approaches prior to interpret their pristine geochemical signatures properly. In th e T an -L u f ault zone of East Ch in a, wh ic h is a dj ac en t t o the ea st- er n b ou ndary of the Dabi e oroge n, several L ate Mesozoic m ag am tic in trusions (inclu ding th e C hilu ti ng diorit e) have been sugge st ed to be high-M g a da ki tes resul ting fr om partial m elting of dela minated c on ti- nental crust a nd subsequent melt/mantl e i nteraction (Huang et a l. , 2008; Wang e t a l., 2006, 2004 a). However, up to now d etailed petro- gr aphical a nd mineralogical i nves tiga tion s t o c heck wh et her t hey rep - re sent pr im it iv e m agmas o r n ot are l ac king. F or exampl e, based on t he bulk geochemist ry of se le cted samples, th e Chilu ti ng dior it es (i.e. p art of the L iujiawa plutonic c om plex) h av e b een i nterpreted as primitive high-M g a da kites a nd a lit hospheric d elamin at ion m od el alon g t he Tan-Lu fault z on e h as be en prop os ed accord in gly (Huang et a l. , 2 008). Neve rt heless, o ur prelim in ary petrogr aphical o bs erva tion and geo- chemic al mode ling in dicate that the d io ritic m em be rs of th e L iu jiawa pluton mi ght have b een a ffected t o a l a rge extent b y mag ma mixing and f racti onal c rystal lizati on ( Zhang e t al., 2012a, 2 011b). In this paper, quantit a ti ve constr aints on t he cont ribut i on of mag ma m ixing and f rac- ti onal c rystall izati on p roce sses t o t h e high- Mg adakiti c ge ochemist ry of th e L i ujiawa pluto n a re prov ide d. W e c om bi ne pet rog raphic o bser va- ti ons, dat a o n who l e- rock geoch emistry ( major and t race e l ement , Sr –Nd isotopes), U–Pb and Hf i sot opes of z i rcon, major a nd t race ele ment compositi ons o f mine rals (pyroxe ne, amphibole , biot ite , apat it e a nd pla- gi oclase) a s well a s g eoche m ical modeli ng t o wor k o ut the pe tr ogen es is of the p lut on. We demo nst rate that the diori tic r ocks fro m the Liujiawa plut on are “pseudo high-M g adakites”and that the magmas ne it her deri ved di rect l y b y hig h-pressur e melt ing o f lo wer c on tine ntal cr ust nor reﬂect c rust de laminati on.
2. Geological background
The L iujiawa p lut on i s l ocate d near the eastern boundary of the Dabie or ogen, i n t he Tan- Lu f ault z on e a s well a s a djace nt to t he Middl e–Lower Yang tze Riv er me tall oge nic belt (Fi g. 1 ). In or de r t o under st and the ge olog ical conte xt o f the ori gin of th e L i ujiawa p luto n, we summar ize some key i nfo rmatio n of t h e D abi e oro gen, the Tan- Lu faul t a nd t h e Mi ddle- L ower Yangt z e R ive r met a llog enic bel t as fol l ows.
2.1. Dabie orogen
The N or th China Blo c k a nd Yang tze Bl ock are s ut ur ed by t h e Tr i assic col lision- type Qi nling–Dabi e–Su lu oro geni c belt ( Ames et al., 1993), which i s c ut by t he T an-Lu fault i n t he e ast to s hift t he Sulu orog en nor th- ward ~ 530 km relative to the Dabie or ogen (Okay and Şengِr, 1992 ; Fi g. 1 ). Ac cording to the m et amorphic g rades, the Dabie or ogen c an be divided into ﬁve pet rot ect onic unit s which a re outli ned by fault s (Fig. 1 )(You e t al., 1996; Z hang et al., 1996), i. e. t he No rth Huaiyang Unit (NHU) of l o w-gr a de met a mo rphic ﬂysch deposits the N orth Dabie Unit (NDU) of hig h-te mpe rature m et am o rph ic cor e co mple x, t h e Cen tral Dabi e Unit (CDU) of me di um-t emperat ur e ult ra high- pr essure me ta- morp hic roc ks, th e S ou th Dabie l o w-te m pe ratu re u l t ra h i gh-p re ssu re met a mo rphic ro cks, an d t he Su-S ong U nit (SS U) of l ow- te mper a ture hig h-pressur e blueschists. T he curr ent a ppear a nce o f the Dabie or ogen is predominantly shaped by Lat e Mesozoic e xten sion struct ur es ( Wang et al., 1998), as well as massive grani toid intr usions ( Ma et al., 1998; Zhang et a l., 2 002 ) w hich a re ge ochemical l y a daki tic a nd no n-adak it ic (He et al., 2011; Xu e t al., 2007, 2013) a nd sporadic ma ﬁc–ultramaﬁc stocks (Jahn et al., 1999). Crustal-deri ved magmatism occurred mainly in the p er iod b et wee n 150 and 1 30 Ma ( Wang et al. , 2007a; Zhang and Ma, 2008), wher eas m a ﬁc mag ma t ism de riv ed fr om a n e nriche d lit ho- sphe ric m antl e was sy st emati c ally l a ter wit hin 130 –12 0 M a ( Da i e t a l. , 2012; Jahn et al., 1999; Zhao et al., 2005 ). This extensive crust- and mantle -deri ved magmat ism, as we ll as rapi d exte n si on, exhumat ion and c r ustal t hinni ng (Hacker et al., 2000; Rats chbac her et al., 2000; Zhou et al. , 2003 ), might h ave be en ini tiat ed by upwel ling of ho t a s- the n osphe ric mant le during the Early Cre tace ous t i me ( Huang e t al., 2007; Jahn et al., 1999; Wang et al., 2007a; Xie et al., 2011b; Xu et al., 2007; Zhang e t al., 2010; Zhao et al., 2005) a nd ge ody n ami c ally rel a ted to t h e n ort h wes t -ward s u bd u c tio n o f the P al eo- P aci ﬁc P late bene ath East China (Li and Li, 2007; Zhang et al., 2011a; Zhou and Li, 2000).
2.2. Tan-Lu fault zone
The continental-scale Tan-Lu fault striking NE to NNE in East China is characterized b y a complex activity history, which might have orig- inated from the collision between the North China Block and the Yangtze Block in the Middle Triassic ( Li, 1994; Yin and Nie, 1993; Zhu et al., 2009) and continued to be active through Mesozoic to Cenozoic and e ven to m odern tim es (Xu et al., 1987). Althou gh be in g in- ve st ig ated for d ec ad es , a full pictur e of its long hist or y of movem en t i s stil l vague and c on tr oversial. Tecto no-chr onolo gical data of Ratschbacher et al . (2000) indi cate that the major deformation a long the T an-Lu f aul t occ urr ed du ri ng a ~ 110–90 Ma per i od, w hich c le arly po st dat es th e str ong e xt ension a nd exhu mat i on of t h e c entr al Dabie orog en du ring t h e Early C ret aceou s . Structural and a patite ﬁssi on-track d ati ng e vidences of Grimmer e t a l. (2002) fur the r indicate t hat s inistr al transpressive mo vement along t he Tan-Lu fault t ook place during the Late C re taceous –Ce nozoic (younge r than ~ 9 0 Ma). Faulti ng-related b asi ns adjacent to t he Tan-Lu fault, such as t he Q ianshan b asin (K-E), t he Ji a shan basin (K
), have be en for m ed in t h e L at e Cr et aceou s or eve n lat er ( Zhu et al., 2001). In su mmary, t hese evidences sugg est that reg ional exte nsional (nor mal ) fault ing star ted in the mid- or lat e-C ret a ceo us ( ~ 110–90 Ma), whi c h f ollo wed t he Lat e Jurassi c –Early Cr et aceou s sini st ral d u c til e stri ke -s lip fault ing unde r a tr anspre ssiv e re - gime (Zhu e t al., 2004, 2005). Howe ver , Mer c ier et a l. ( 2007) and Zh u et al. (2010) proposed t hat the o nse t of e x tensio nal faulti ng was in t he Earl y C re taceo us a nd i t mi ght b e r elat ed t o t h e do me e xhu m ati o n o f th e D abie or oge n. We not e t hat th e e vi de nce of Lat e Cre tac eous –Ce noz o ic ext en si onal f ault ing i s much st ro nge r and p e rsuasi ve t han t hat of Earl y Cre tace ous ext ensio nal f ault ing , a n d we t hus sugg est t hat th e Tan- Lu faul t might have made little contribution to th e Dabie orog enic extension and e xhumati on a nd to th e Early Cretaceous massive magmatism in t he east er n D abi e o rog en.
2.3. Middle–Lower Yangtze River metallogenic belt
To t h e s out h an d east of t he Dabie o roge n a nd along th e st rike of t h e Yang tze R ive r ( Fig . 1), the Middle–Lower Y angtze Ri ver (MLYR) met al l og enic bel t ( compo se d o f s ev en l a rge m ining di st rict s; Zhai et al., 1996 ) i s c har a cter ized by widespread magmati sm-r elat ed Cu –Fe –Au d e - po si ts ( Chang et al., 1991; Li et al., 2010; Pan and D ong, 1999)wheregeo- ph ys ical data h ave revealed a deep fault zon e be neath ( Chang e t a l. , 1991 ) a nd uplifte d Mo ho ( Lü et al., 2005 ). The ev ent s of m iner a lizati on, no matt er t h eir o rigi ns are relat ed to skar n, p orp hyry or strat a , occu rre d int ensive ly at t he E arly Cr et ac eous (ca. 145 –136 Ma) a nd ar e con tempo- rane ous wit h the ages of the i r h o st m ag matic r ocks ( Li et al ., 2008; Mao et al., 2006). The d eposit- h osted m agmat i c r ocks are c ompo si tion al ly di orit ic to grano dio rit i c a nd v a rious m odels h av e be en p ro posed t o inte r- pr et the i r o rig ins, i n cluding (1) part i al me lti n g of d elamin at ed lo wer cont inent al cr ust (Wang et al., 2004a, b, 2006), (2) part ial me lt ing of subducted oceani c sl ab ( Ling et al., 2009, 2011; Liu e t al., 2010; Sun et al ., 2007; Wa ng et al., 2013) a nd (3) d iffere ntiat ion of lit hosphe ric mantle -deri ve maﬁc m agmas a nd possible mixing wit h lo wer c rust - de riv ed f els i c m ag mas ( Du et al., 2004; Li et al., 2 008, 2009; X ie et al.,
Fig. 1. (a) G eological s ke tc h m ap sh ow in g the distribution of Late Meso zoic ma gm atic rocks i n the Dabie orogen a nd the n ei gh bo ri ng Middle–Lo we r Y an gtze River b el t, in wh ic h s ta r shows the posi ti on of the L iu ji awa p lu to n. (b) G eological m ap of th e Liujiawa p lu ton. No te that gabb ro norite and h or nb le nd e g ab br o o cc ur as enclaves in t he dior ite h os t ( se e Fig. 2a), and t he ir shapes an d l oc ations in the ﬁgure are o nl y schemat ic . E xc ep t f or the d yk es of diorite p or ph yr y w hi ch contact with t he ﬁne-grained diorite s ha rp ly , o th er lithological units c on tact grad ua ll y with e ac h other.
2011b; Xu e t al., 2004). Geochemical s tudi es i ndi cate an enrich ed sub- cont inen tal lit hosphe ric mant le under neat h th e MLYR met a llog enic belt , a nd par tial m elt ing of whic h h ave ge n er at ed t h e ma ﬁcmagmatism along t he be lt ( Li et al., 2009; Wang et al., 2006; Xie et al., 2011b,c; Yan et al., 2008). Geo c hronol ogic al ly, mant l e upwell ing and m e l ti ng induce d magmati sm in t his met a llog enic bel t star ted s yste ma t icall y earl ier than it s c o unt erpart in the Dabie orog en ( Jahn et al., 1999; Li e t al., 2 010; Wang et al., 2005, 2007b; Xie et al., 2011b; Zhao et al., 2005), which
infers a po ssible ge ody n amic l ink betw een the MLYR m etal loge nic belt and t he Dabie oro gen, e speci ally in the t erm of L ate Me s ozoic magmatism a nd c rust a l exhumat ion a nd thi nning (Zhang et al., 2010).
3. Field geology and petrography
At the current exhumation level, it can be observed that the Liujiawa pluton was a small (~ 2 k min area) magmatic complexintruding surrounding granite gneiss. According to petrography and aided by geochemical analysis, the Liujiawa pluton can be divided into several major lithological units, including hornblende gabbro, gabbronorite, diorite, diorite porphyry and granodiorite. The horn- blende gabbros and gabbronorites are both enclaves enclosed in dio- rite host, and they contact the host sharply or gradually at various locations ( Fig. 2a). The diorites are normally ﬁne-grained at locations without maﬁc enclaves, but become porphyritic-like in texture (coarse-grained) at locations where maﬁc enclaves are present. The granodiorites are mainly porphyritic and contact diorites gradually, showing continuous evolution of these two units. The hornblende gabbros are characterized b y aggregates of euhedral amphibole and tabular subhedral plagioclase, showing a typical gabbroic texture. Minor quartz is present interstitially, but no orthopyroxene, clinopyroxene o r biotite has been observed. Four generations of amphibole are identiﬁed from morphology and miner- al chemistry ( Zhang et al., 2012a), indicating a broad range of pressure and temperature of crystallization. The gabbronorites are featured by near-euhedral grains of orthopyroxene, clinopyroxene and apatite, which usually form cumulate-like aggregates surrounded by plagioclase environment ( Fig. 2b). Both pyroxene-rich (mode of pyroxene ~ 25%) and pyroxene-poor (mode of pyroxene ~ 15%) ga bb ronorites h ave b ee n o bs erve d. Ov ergrow th s of a mp hibole arou nd pyro xe nes a re often o bs erve d indic at in g late-stage r ea ct ions with hy- drou s m el ts (or ﬂuids). Large euhedral a pa tite grains ar e present only in py roxene-r ic h g ab br on or ites wh il e a ci cu la r m ic rocrys ta ls of apatit e oc cur only i n p yr ox en e-poor ga bb ronorites. In the diori te s, ortho- p yr o xe ne is abse nt and mi n or anhedral clinopyr oxene grai ns are pr esent as rel ics encir c led b y euhedral a mphibole s, sho wing a c l ear bre a kdown re ac ti on of clino py roxene to fo rm amphi bo l e ( Fig . 2 c). R ounde d grains of magnet it e, ilme nite , q uart z and a pat i t e are a lso p re se nt tog ethe r wit h cl inopyro xe ne re lics. Plag ioclase p henocr ysts o ft en sh ow a stro ng compositi onal z oni ng (for e xample , Fig. 2d s hows an abrupt decrease in An cont ent f rom c ore of ~ 55 An% t o mant le of ~ 25 An%), which might r eﬂect a n a br upt c hange i n c o existing me lt compositi on and/ or cry st a llizat ion c o n di tion (pr essure , t emperat ur e a nd H
4. Analytical methods
4.1. Whole-rock major and trace element
The analysis o f whole-rock major, trace and rare earth element composition was conducted at the Analytical Institute of the Hubei Bureau of Geology and Mineral Resources. Major element oxides were measured using a Regaku 3080E XRF spectrometer. Trace element and rare earth element (REE) were measured b y ICP-AES. Analytical procedures were reported in detail by Ma et al. (2000). Rel- ative standard deviation is b 5% for major elements, b 4% for REE and Y, and 5–10% for trace elements.
4.2. Whole-rock Sm –Nd and Rb–Sr isotopes
The whole-roc k Rb–Sr an d S m–Nd is ot op ic comp os it io ns of th e s e- le cted sa mp le s of the Liuj ia wa pl ut on were measured us in g a Finnigan MA T- 261 therm al ionization mass spec trom eter (TIMS) equi pp ed with 7 Faraday Cup C ollectors a t t he Isot op e L ab orator y of the Yi ch an g
Instit ute of Geo logy and Min eral Resources. Th e procedures of c hemical separa tion and m ea su re ment we re desc ribed in detail b y Ma et al. (2000). T he ratios of Nd we re determin ed to a prec is ion o f ± 0.7% and ± 0.2% , r es pectively
4.3. Zircon U–Pb dating and trace element
Zircons were separated after rock crushing using conventional techniques (i.e. heavy liquid and magnetic properties). They were then selected by examination with a binocular microscope and mounted in epoxy resin and polished to approximately half exhuma- tion. Afterwards, Cathodoluminescence (CL) images were taken, which served to study the internal structures of individual zircon grains and to guide in-situ U –Pb dating and Hf isotope analysis. Zircon U–Pb dating was conducted by means of an inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 1700a) coupled with a laser ablation (LA) system using GeoLas 2005 DUV 193-nm UArF laser, equipped at the State Key Laboratory of Geological Processes and Mineral Recourses (China University of Geosciences, Wuhan, China). The laser spot was 32 μm i n diameter. The applied procedure has been described in detail by Yuan et al. (2004) . Zircon 91,500 and the NIST610 glass were used as an external standard for U/Pb ratio and rare earth element concentration, respectively. The common lead correction was performed after the method of Andersen (2002), and the isotopic ratios and element concentrations were cal- culated using GLITTER (ver. 4.0, Macquarie University). The concordia diagrams and U–Pb ages were obtained using the ISOPLOT program of Ludwig (2003)..