Sr-Bearing Zoisite and Epidote in Ultra-High Pressure

American Mineralogist, Volume 83, pages 240-247, 1998

Sr-bearingzoisite and epidotein ultra-highpressure (UHP) metamorphic rocks from the Su-Luprovince, eastern China: An importantSr reservoirunder UHP conditions

Axrxo Nlclslxr nno Mnslrr ENlvrr

Departmentof Earth and Planetary Sciences,Graduate School of Science,Nagoya University, Nagoya 4@-01, Japan

Ansrnlcr Sr-bearingzoisite and epidote are common constituentsof eclogites and associatedpar- aschiststhroughout the Su-Lu ultra-high pressure(UHP) province, easternChina. The SrO content of prograde zoisite and epidote reaches 3.2 wtTo in crystal cores and generally decreasestoward crystal margins. Retrogradeepidote is poorer in SrO (<0.1 wtTo). Pre- liminary rare earth element (REE) analysesof epidote give LarO. (up to 2.9 wtTo), CerO, (5.9 wtVo), and Nd"O. (3.0 wt%o).REE contents of zoisite are distinctly lower (La,O, up to 0.16 wt%o,Ce,O, up to 0.26 wt%o,and Nd,O,up to 0.16 wt%o)thn coexistingepidote. Apatite is always more depletedin SrO (0.10-0.59 wtEoon average)than coexisting zoisite and epidote, and Sr-Ca partition coefficients for zoisite and epidote and apatite [(Sr/Ca),.,"o"o]range from 5 to 20. SrO content of K-white mica (0.012-0.O44 wtTo) is an order of magnitude lower than that of apatite. An evaluation of the SrO content in zoisite and epidote and their modal abundancesin seven samples indicates that >'70Voof the whole-rock SrO is contained in these minerals. Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Zoisite and epidote are thus regarded as the most important Sr reservoirs at UHP conditions where calcic plagioclaseand titanite are unstable.

INrnonucrroN portance of zoisite and epidote as Sr reservoirs under Monoclinic epidote-group minerals have two distinct UHP_conditionswhere calcic plagioclaseand titanite are sites commonly bccupied by Ca: ninefold-coordinatedA1 unstable. and tenfold-coordinatedA2 sites. The A2 site is slishtlv ^ AND PETROGRAPHY larger than the A1 site, and its size increaseswith iicoi- GEOLOGICALBACKGROUND poration of Fe3*and Mn3* in octahedralMl and M3 sites. The Sr-bearing zoisite and epidote studied here occur The A2 site is too large for Ca (1.23 A1 llollase _1971); in eclogites and associatedparaschists from the Su-Lu however this site readily incorporates Sr (1.36 Al and UHP province of easternChina, which is an easternex- larger cations.Sr-rich epidote (up to 8.5 wtTo SrO: Grapes tension of the Dabie UHP province (Fig. 1). Twenty-five and Watanabe 1984; Brastad 1985), piemontite (up to samplescollected from the sevenlocalities shown in Fig- l5.l wtVo:Mottana 1986; Perseil 1990), and allanite (1.5 ure 1 were examined in detail; zoisite, epidote, or both wtVo:Exley 1980) have been reported in some metamor- in all sampleswere confumed to have >0.5 wtTo SrO. phic rocks, manganesedeposits, and hydrothermally al- The Su-Lu province is bounded by the Yantai-Qing- tered rocks. In contrast, orthorhombic zoisite has two Ca dao-Wulian (YQW) fault in the northwest (Yu 1990), sites of .essentially the same size as the A1 site of the where it is in contact with middle to late Proterozoic or- monoclinic epidote-group minerals (Dollase 1968). tho- and paragneisses(730-1620 Ma; Enami et aL.1993a; Therefore zoisite contains very little Sr and rare earth Ishizaka et al. 1994). The Su-Lu province is underlain elements(REE) when found in normal rock compositions chiefly by orthogneissesand amphibolite, in which coe- metamorphosedunder P-Z conditions where calcic pla- site-eclogite, garnet-peridotite,and marble blocks occur gioclase and other phasesthat favor Sr and REE are sta- sporadically. The UHP eclogites and associated para- ble. However, Sr-rich zoisite (up to 7.4 wt%oSrO) has schists occur as lenticular blocks 1 to 300 m long, elon- been reported from Sr-metasomatizedeclogite (up to 2.4 gate parallel to the foliation of the enclosingcountry rock wt%owhole rock SrO) in the Western Gneiss Region of gneisses(e.g., Yang l99l:' Zhang et a7. 1995; Wallis et Norway (Brastad 1985). This study (1) reports the com- al. 1991). The peak metamorphic conditions of the UHP 'C mon and widespreadoccurrence of Sr-rich zoisite and ep- rocks are estimated at >2.6 GPa and 700-890 (e.9., idote (up to 3.2 wtVo SrO) in ultra-high pressure (UHP) Enami et al. 1993b; Zhang et al. 1995). The sequenceof metamorphicrocks of the Su-Lu province, easternChina, UHP rocks and the surrounding country rocks were both (2) examines the partitioning of Sr among zoisite and subjectedto greenschistto albite-epidoteamphibolite fa- epidote and other phasesthat could incorporate Sr (e.g., cies metamorphism during the Mesozoic (Cong et al. apatite and K-white mica), and (3) emphasizesthe im- 1992; Ishiwatari et al. 1992). In the country rocks, no 0003-{04)v98l0304-0240$05.00 240 NAGASAKI AND ENAMI: ST-BEARING ZOISITE AND EPIDOTE IN UHP ROCKS 241

+ + + + + + + + + +

+++++

JiaonanGroup Eclogiticblock Fenzishan& E PenglaiGroups Fault Paleozoicsediments z Cenozoiccover JiaobeiGroup r Mesozoicsediments TaishanGroup w Mesozoicgranites

Frcunr 1. Geological sketch map showing sample localities in the northern Jiangsu and southernShandong Provinces, eastern China. Localities (l), (2), (3), and (4) in the Donghai area correspondto Hetang, Caihu, Quiglongshan,and Maobei, respectively. The other localities include Yangzhuang(Junan), Taohang (Zhucheng), and Xianguling (Rongcheng).Tan-Lu Fault : Tancheng- Lujian fault; YQW Fault - Yantai-Qingdao-Wulian fault. The Jiaobei and Jiaonan Groups are tentative names for the middle Proterozoic and early Mesozoic gneisses,respectively clear evidence exists for UHP metamorphismbefore the sions in garnet and omphacite, and therefore they appear greenschistto albite-epidoteamphibolite facies event. to have formed during the same UHP stage. In a Junan Petrological and mineralogical data for UHP rocks in sample (SJY02), fine-grained epidote forms an aggregate the Su-Lu province are presentedby Enami et al. (1993b), with hornblende and quartz included in garnet, whereas Zhang et al. (1995),'and others. The eclogites studied zoisite occurs in the matrix. The epidote-bearingaggre- consist mainly of garnet, omphacite, silica minerals gates might have recrystallized before the matrix zoisite. (quartz -f coesite), zoisite and epidote, paragonite,phen- In a paraschistfrom the Donghai area(9IHZI7), epidote gite, kyanite, calcic-subcalcicamphiboles, and rutile. Ac- always surrounds zoisite (Fig. 2a). In this case, zoisite cessory minerals are apatite, titanite, plagioclase,and il- formed before epidote during the UHP metamorphism. menite. Paragoniteand titanite are retrogradephases that Retrograde epidote occurs as veinlets cutting prograde form rims on kyanite and rutile, respectively. Most pro- zoisite in some Junan samples(SD0307, SD0315) (Fig. grade amphiboles occur as inclusions in garnet. Retro- 2b). grade amphiboles surround garnet and form symplectites Modal abundancesof the principal minerals in seven with plagioclase. The paraschistshave mineral assem- samplesfrom Yangzhuangwere estimatedusing Al, Ca, (35 x 25 mm: 3.5 x 10'pix- blages similar to those of the eclogites, although they and K X-ray mappingdata els). Garnet and omphacite abundancesare 19-46 and contain only a little omphacite but large amounts of 14-65 volVo,respectively. Zoisite and epidote have non- quartz (possibly after coesite). Garnet in the paraschists overlapping abundancesof 21-28 and 0-8 vol7o, respec- is porphyroblastic and reaches3 mm in diameter. Some tively. K-white mica, kyanite, and apatite constitute less garnet and omphacite crystals include coesite and quartz than 9, 6, and 0.6 vol%oof the rocks, respectively. pseudomorphs(Zhang et al. 1995). Prograde zoisite and epidote are common interstitial Mrnnnar- cHEMrsrRY phases between garnet, omphacite, and other prograde All sampleswere analyzedon JEOL electron-probemi- UHP minerals. In addition, both minerals occur as inclu- croanalyzers,the JCXA-733 (WDS), and the JXA-8800R 242 NAGASAKI AND ENAMI: ST-BEARNG ZOISITE AND EPIDOTE IN UHP ROCKS

incorporatesREE (detectionlimit: 0.03 wtVo)by the substitution REE (Mg, Fe'z*,Mn)Ca ,Al , (see below); thus some iron is FeO and Fe2* : REE - (Mg + Mn). All iron in zoisite is assumedto be incorporatedas Fe,O.. Sr concentrationsof K-white mica were determinedby isotope dilution mass spectrometryusing the Finnigan MAT THQ at Nagoya University.

Zoisite and epidote Valuesof X.. [: (Fe3* + Fet*)/(Al + Fe3* + Fe2* + Mn + Mg)l for zoisite and epidote are relatively uniform within each sample. Zoisite is always a prograde phase and its X.. is 0.04-0.05. Prograde epidote that coexists with zoisite has Xo.that rangesfrom 0.1I to 0. 14. Epidote in zoisite-free eclogites has variable Xo., ranging from O.l2 to 0.24. Retrograde epidote is distinctly richer in Fe,O. (Xo.: 0.19 -r 0.01) comparedwith progradeepidote in the samesample (0.12 t 0.02). The MnO contents of zoisite and epidote are generally <0.05 and 0.2 wt%o, respectively. The MgO content reaches 1.3 wtVo in epidote and is typically less than 0.1 wtvo in zoisite. The sum of divalent cations (Ca + Sr) in the A1 and A2 sites equals 1.99 -+ O.O2per formula unit (pfu; O : 12.5) for zoisite and 1.94 -+ 0.04 pfu for epidote, indicating the simple substitution SrCa ,. Some epidotes having low (Ca + Sr) are enriched in Mg + Fe2t + Mn (see above). REE are higher in epidote than in coexisting zoisite (up to 2.9 vs. 0.16 wtVoLarO., up to 5.9 vs. 0.26 wt%oCerO., and up to 3.0 vs. 0.16 wt%oNdrO,). Figure 3 showsvariations of SrO in zoisite and epidote. Most zoisite and epidote crystals are zonedwith decreas- ing SrO toward crystal margins (Fig. a). The cores of zoned grains contain up to 3.2 wtTo SrO in zoisite and up to 2.7 wtva SrO in epidote.The SrO content of epidote correlates with its mode of occurrence;e.g., in samples SD0307 and SD0315 primary epidote (0.1 -+ 0.3 wt%o) contains more SrO than secondary epidotes from veins (0.06 -r 0.Ol wt%o;Fig. 5).

Other minerals Sr was not detectedin garnet and omphacite by EPMA Gamets are Mn-poor almandine-pyrope-grossulartypes, and FIcunn 2. Backscattered-electronimages showing texfural their averagecompositions are Almo.r,Spso,-rkp,o .oGsr.,, relationshipsbetween zoisite and epidote.(a) Progradeepidote in eclogites and Alrn*-nSpso,Prp,o.,Grsr..n in paraschists (b) as a rim on zoisite Retrogradeepidote as a veinletcutting [abbreviations follow Kretz (1983)]. The gamets typically progradezoisite. Numbers enclosed in polygonalboxes indicate contain up to 0.03 wtTo NarO implying an NaTiCa ,Al , the Sro-content(wt%a) of zoisiteand epidote. Abbreviations are: substitution (cf. Enami et al. 1995; Kato et al. 1991). Zo: zoisite;Ep - epidote;Ky : kyanite; : quartz; - Qtz Ab poor albite. Omphacites in most samples are relatively in FerO. (generally less than 1.0 wt%o), and their average compositions are Aug., ,,Jd*-urAcmo, in eclogites and (WDS + EDS) at Nagoya University. Accelerating volt- AugorJdr,Acmooin paraschists.In one Donghai sample age, specimencurrent, and beam diameterfor quantitative (91Q03), acmite-rich omphacite (Aug..Jdo,Acm,,) coex- analyseswere 15 kY, 12 nA on the Faraday cup, and 3 ists with FerO.-richepidote (X,": 0.24 t 0.01). pm, respectively. The Bence and Albee (1968) method Prograde amphiboles are mostly pargasite with Si : was used for matrix corrections. Representativemicro- 6.6-6.7 pfu (O : 23) and X,, [:Mg/(Mg + Fe'z*)] : probe analysesof Sr-bearing zoisite, epidote, and coex- 0.59-0.80. Nybciite also occurs as a prograde phase in isting apatite are given in Table 1. The detection limit of zoisite-free eclogites. Some prograde amphiboles in the SrO is 0.02-0.O3 wt%o(2o level) for all minerals.Epidote Donghai samples contain up to 2.8 wt%oCl. Retrograde NAGASAKI AND ENAMI: ST-BEARING ZOISITE AND EPIDOTE IN UHP ROCKS 243

Traue 1. Representativeanalyses of zoisite,epidote, and apatite

Donghai Rongcheng 91CH08b(Ec) 91H217(Ps) SD0307(Ec) SJY02 (Ec) R1-7(Ec) tr^ Ap Ep Ep Ap Ep Ap Ep Ap Weight percentages sio, 38.4 37 6 385 37.9 38.8 37.2 3A.7 37.8 562 Tio, 0.11 0.06 o17 0.25 0 00 0.05 0.08 0.14 008 Alro3 31 1 28 4 JI J 27 5 31.4 27.1 31.3 28.7 26.5 CrrO" 02 004 0.13 0.01 0 00 0.00 o 14 0.07 000 FerO3* 214 5.09 0 101 2.14 7.64 2.O5 6.22 0.121 1.91 6.06 0.131 5 66 0.071 MnO 0 03 0 04 O02 0.00 0 15 0.00 0 00 002 0.00 006 000 0.14 0 00 MgO 000 014 008 002 0.06 0.00 0 38 0.00 0.04 0 11 0.05 0.83 0.00 SrO 120 266 0 14 154 064 131 0.94 015 304 1.67 043 0.93 0 66 CaO 231 220 55.1 233 233 234 21 1 54.8 22.4 22 6 54.1 19.3 55 3 La,O" b d. bd b.d. b.d. o 71 b d. 0 08 0.06 1 49 0.04 CerO. 0 14 b.d 006 012 160 0 05 0 15 b.d. 3.62 0.O7 Nd"o" 0 08 bd bd 006 094 008 013 b.d 219 014 PrOu 41 I 41 6 41 8 40.9 F 3.03 2.77 3.75 3.73 ct 001 013 006 0.00 Less O - F+Ct 12A 120 159 1.57 Toral 96 5 96 0 99 1 97.1 97 5 97 1 96.2 98 4 97.7 97 6 98 7 96.9 99 3 Formula proportionS Si 2.996 3 000 2.988 2979 3.006 3 008 3.003 2970 2.984 Ti 0 006 0 004 0 010 0 015 0.000 0 003 0 005 0 008 0.005 AI 2 860 2671 2.863 2548 2A67 2 583 2 863 2 658 2.574 Cr 0 012 0 003 0 008 0 001 0 000 0 000 0 009 0 004 0 000 Fe3tt 0 126 0.306 0125 0452 0119 0329 o 112 0.358 0 244 Fe2' + 0 000 0.007 0 000 0.050 0.009 0.000 0.009 0 102 0.005 Mn 0 002 0 003 0.001 0 000 0 010 0 000 0 000 0.001 0 000 0.004 0.000 0 010 0.000 Mg 0.000 0 017 0.010 0 002 0 007 0 000 0 046 0.000 0.005 0 013 0.006 0 102 0.000 Sr 0.054 0.123 0.007 0.069 0 029 0 059 0.044 0.007 o 137 0.076 0.021 o044 0.032 Ca 1.931 1 881 4.982 1 938 1 962 1 942 1.828 4.990 1 863 1.903 4944 1 704 5.021 o.o bd. bd. bd 0.021 b.d 0.002 0 002 0 045 0.001 Ce 0.004 0.001 0.002 0 003 0.047 0.001 0.004 b d. 0 109 0.002 Nd 0.002 b d b.d 0 002 0.027 0.002 0.004 b d 0.064 0 004 P 2.993 2 993 3.018 2.935 F 0 809 o 745 1 012 1 000 cl 0 002 0 019 0 009 0 000 Note:AbbreviationsareEc-eclogite;Ps-paraschist; Zo:zoisite; Ep-epidote;Ap-apatite; bd.-belowdetectionlimit. . Total Fe as Fe,O. t Total Fe as FeO + Calculatedvalues (see text). S Basedon 12 5 O atomsfor zoisiteand epidote,and I cationsfor apatite

amphiboles have variable compositions. Pargasite from DrscussroN : symplectiteshas Si 6.1-6.3 pfu and X-": 0.62-0.82, SrO contentsof zoisite and epidote garnet whereas amphibole replacing is characteristically Epidote is generally enriched in Sr comparedwith co- (5.6-5.8ptu). low in Si existing zoisite, as shown by Brastad (1985). However, Potassium-whitemicas are phengitic, and their average the Sr/Ca values of coexisting zoisite and epidote in the Si contentsand X." [:Na/(Na + K + Ba + Ca)] are 3.3- samples of the present study (Fig. 5) are more complex : 3.4 pfu (O 11) and 0.07-0.10, respectively.SrO con- than expectedfrom crystal chemistry, which is most like- centrations of K-white mica in five samplesfrom Yang- ly due to their variable timing of crystallization. Epidote zhuang are 0.012-0.044 wt%o.Apatite is F-rich (-2.0- in sample 9lLlZI1, for example, has a slightly lower -0.10-0.59 3.8 wt%oF and <0.7 Cl wtvo) and contains Sr/Ca value than zoisite. The Sr-poorerepidote occurs as wtTo SrO. La"O,, CerO., and NdrO. of apatite are <0.06, a rim on the zoisite and is inferred to be younger. This 0.12, and O.l4 wt%o,respectively. Retrograde plagioclase indicates that Sr was preferentially incorporated into the is albitic, (-An* to An,r). The ayerageSrO content of zoisite during the prograde stage and that epidote crys- plagioclase is 0.05-0.15 wt%o,which is distinctly lower tallized later under Sr-depletedconditions. The gradualSr than that of zoisite and epidote. Retrogradetitanite con- decreasetoward the margin of the zoisite and epidote tains a small amount of Al,O, (up to 0.5 wt%), and its supports this interpretation. In sample 91CH08b, the contents of SrO and F are <0.1 and 0.2 wtVo. Sr/Ca value of the epidote varies from grain to grain even respectively. within a single thin section,and some epidote grains have 244 NAGASAKI AND ENAMI: ST-BEARING ZOISITE AND EPIDOTE IN UHP ROCKS

(a) Donghai area (Eclogite) (b) Donghai area (Paraschist) 6 samples ZA Epidote 3 samDles o tc ffi Zoisite 6 6 610 o z

SrO(wP/d SrO (wt"/")

1 2 3 012 sro 1*ez4 SrO (wtr/d Frcunr 3. Histograms of SrO concentrations(wt%o) in zoisite and epidote from eclogite and paraschist. lower Sr/Ca than zoisite. This Sr heterogeneityof epidote those earlier assemblages,and thus the Sr-poor epidote in and zoisite might reflect local variations in the SrO con- SJY02 might be a relic of that earlier stage.Orthorhombic tent of the protolith. Epidote in SJY02 occurs as aggre- zoisite is a higher-temperaturepolymorph of the mono- gate inclusions with hornblendein garnet and has a lower clinic phase (Enami and Banno 1980), and the more Sr- Sr/Ca value than the zoisite in the matrix. Epidote-am- rich zoisite in SJY02 might have been formed by the phibolite and amphibolite facies assemblagesformed pri- following pressure-sensitive,plagioclase-consuming re- or to the UHP stageare preservedas inclusions in garnet actions during high P-Z prograde stage: (e.g., Enami et al. 1993b, Zhang et al. 1995). Sr is in- t"i*H:,o. + 3Ca.Al,Sio,, corporated preferentially into the calcic plagioclase of

+ Ca,(Mg,Fe)aAl,Si,O,,(OH), + 2H2O

: 6Ca,Al,Si,O,,(OH) + 4Ca(Mg,Fe)Si,O"

4CaAl,Si,O, + H,O

:2ca"Al,si,o,,(oH). olL:3, .:*

Sr reservoirs Sr-partitioning among minerals in medium- to high- pressuremetamorphic rocks has been discussedby Sor- ensen and Grossman (1989), Getty and Selverstone (1994), Hickmott et al. (1992), and others. Sorensenand Grossman (1989) studied clinopyroxene-bearinggarnet- amphibolites migmatized to varying degreesin the Cat- alina Schist, southern California. Their data clearly indicate that Sr preferentially concentratesin zoisite and epidote of non-migmatized amphibolites,but is enriched in calcic plagioclase, apatite, and titanite by the breakdown Frcunn 4. Strontium X-ray map of zoisite in eclogite. of zoisite and epidote during migmatization. Geochemical Brighter shadesindicate higher Sr-content.Numbers enclosedin studies on banded eclogites from the Tauern Windows, rectangularboxes indicate SrO contents(wt7o). eastern Alps, by Getty and Selverstone (1994) showed NAGASAKI AND ENAMI: ST-BEARING ZOISITE AND EPIDOTE IN UHP ROCKS 245

O Donghai I q,r_r. Junan E l;;;;"-''"'"'-" Rongchsng--t Q inclusion(prograde) I m SanlaCatalinaisland O inclusion& matrix E (Soaens€n & Gro$man,1989) (prograde) a 0.04 O matrix(prograde) 'aE E vein (retrograde) I o o o a ()(! ;om x E 6 ae SJYO2 o o 02 o 06 o'08 sr/ca#zoisite 0 0 002 0 0&l 0 006 0.008 Frcunr 5. Sr-Capartitioning between coexisting zoisite and Sr,Caot apatib epidote.Zoisites are all progradecrystals in matrix.Data points representaverage values * lo Frcunr 6. Sr-Ca partitioning between coexisting zoisite and epidote and apatite. Numbers enclosedin rectangularboxes indicate the Sr-Ca partition coefficient. Data points representav- that zoisite and epidote contain up to 0.25 wtTo SrO and erage values :t lo. that there is little systematicpartitioning of trace elements (including Sr) among garnet, omphacite, and zoisite and epidote. They emphasizedthat mineral-fluid interactions along grain boundarieslargely control trace element be- havior during eclogite facies metamorphism. The SrO analysesof major minerals in the Su-Lu UHP rocks demonstratethat zoisite and epidote, and apatite, 0.2 (a) are also the major Sr-bearingphases under UHP condi- Junaneclogites a (1) tions. Zoisite and epidote are always richer in SrO than E (2)' coexisting apatite, and Sr-Ca partition coefficients be- J tween these phases [(Sr/Ca)"",.*o] range from 5 to 20 V -g 0.1 (Fig. 6). SrO contents of K-white mica (0.012-0.044 o wt%o)are an order of magnitude lower than those of ap- 3 o(3) atite (0. 10-0.59 wt%o).The estimated(Sr/Ca),","-,o values in the Su-Lu samples are consistent with those reported o U) (4) for the CatalinaSchist (5.2-5.3; Sorensenand Grossman r 1989). The ionic radius of Sr might be too large to be 10 20 30 40 readily and ninefold-coordi- accepted in the sevenfold- Modalabundance of zoisite/epidote(%) nated Ca sites of apatite (Brastad 1985). A positive re- lationship betweenwhole-rock SrO content and the modal abundance of zoisite and epidote in eclogites from an (b)l______w outcrop at Yangzhuang,Junan area,(Fig. 7a) implies that SrO content of the eclogites is strongly controlled by the abundanceof zoisite and epidote. Mass-balancecalculations for seven samplesreveals that >70Vo of the whole- rock SrO is bound in these minerals (Fig. 7b). Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Sorensen and Grossman (1989) estimated that (1) about 85Voof the SrO in the Catalina Schist am- phengiteapatite phibolites is concentratedin zoisite and (2) the contri- bution of apatite to the SrO budget of the whole rock is Frcunr 7. Relationshipsbetween whole-rock SrO contents zoisite and epidote (a) and calculated about one thirtieth that of zoisite. The higher SrO con- and modal abundances of strontium "budgets" (b) of eclogites from an outcrop at Yang- tents and modal abundancesof zoisite and epidote in zhuang,Junan area (see Fig. 1). Bar lengthsrepresent the fraction comparison with apatite and K-white mica imply that (wt%o)of the whole rock concentrationof SrO present in each zoisite and epidote are the most important Sr reservoirs mineral. The SrO "budget" of sample (2) is over 1007o,which under HP and UHP conditions where calcic plagioclase is a result of uncerfaintiesin the mass-balancecalculations. SrO and titanite are unstable.Although no reports exist of Sr- content of whole rocks was determined by XRF analysis at Na- bearing minerals from the Su-Lu and Dabie UHP prov- goya University (Morishita and Suzuki 1993) 246 NAGASAKI AND ENAMI: ST.BEARING ZOISITE AND EPIDOTE IN UHP ROCKS

inces, Ca-poor, possibly Sr-rich zoisite and epidote have RrrunnNcns crrED been reported (e.g., Hirajima et al. 1992;Zhang and Liou Bence, A.E and Albee, A L. (1968) Empirical correction factors for 1994; Okay 1995). Thus, the occurrencesof Sr-bearing the electron microanalysisof silicatesand oxides Journal of Geology, 76, zoisite and epidote in the UHP provinces may be wide- 382-403 spread,and the presenceor absenceof these phasesmay Brastad,K (1985) Sr metasomatism,and partition of Sr betweenthe min- strongly control the cycling of Sr in deep crustal eral phasesof a meta-eclogitefrom Bjorkedalen,west Norway Tscher- envronments. maks Mineralogischeund PetrographischeMitteilungen, 34, 87-103 Cong,B., Zhang,R ,Li,S,Zhai, M, Wang,S, Chen,C, andIshiwatari, A recent experimental study in the CaO-AlrO.-SiOr- A. (1992) Preliminary study of isotopechronology of eclogitesin north- HrO system under high-pressureconditions indicatesthat em Jiangsuand easternShandong Provinces, China. In Commemorative zoisite and lawsonite are stable at up to 6.7 and 9.2 Gpa, Association for ProfessorYukio Matsumoto, Ed , Exploration of Vol- 'C canoesand Rocks in p respectively, at about f : 1000 (Schmidt and Poli Japan,China and Antarctica, 411-417. Semura Furoppi Press,Yamaguchi. 1994).Thus, zoisite and lawsonite appearto be major Ca- Dollase,W.A. (1968) Refinementand comparisonof the structuresof zois- Al silicates within subduction zones, even at depths of ite and clinozoisite.American Mineralogist, 53, 1882-1898. >150 km. Lawsonite coexisting with epidote in high p- -(1971) Refinementof the crystal structuresof epidote,allanite and Z Sanbagawametamorphic rocks contains up to 0.3 wt%o hancockite American Mineralogist, 56, 447-464. Enami, M. and Banno, S (1980) Zoisite-clinozoisiterelations in low- SrO with Sr-Ca partition coefficients between these to two medium-gradehigh-pressure metamorphic rocks and their implications phases[(Sr/Ca),"**o] ranging from 0.7 to 1.1 (M. Enami, MineralogicalMagazine, 43, 1005-1013 unpublished data). These data imply that lawsonite is a Enami, M., Suzuki, K , Zhai, M, and Zheng, X. (1993a) The chemical possible alternative Sr reservoir at depths of >200 km Th-U-total Pb isochron ages of Jiaodong and Jiaonan metamorphic where zoisite is unstable.Hickmort et al. (1992) pointed rocks in the Shandong Peninsula,eastern China The Island Arc, 2, 104-113 out that dehydration and melting of epidote-group min- Enami, M., Zang, Q., and Yin, Y (1993b) High-pressureeclogites in erals occurs in a depth range similar to that of the Wada- northern Jiangsu-southernShandong province, easternChina. Journal chi-Benioff zone underlying island-arc volcanic fronts of Metamorphic Geology, 11, 589 603 (typically -100 km), and that the released Sr-rich fluid Enami, M, Cong, B , Yoshida, T, and Kawabe, I (1995) A mechanism for Na incorporationin gamet: An example from garnet leads to the high Sr in orthogneiss abundancesof arc magmas.The high- from the Su-Lu terane, easternChina American Mineralogist,80,475 pressure stability of zoisite and lawsonite, however, im- 482 plies that these minerals could contribute considerablyto Exley, R A. (1980) Microprobe studiesof REE-rich accessoryminerals: the Sr budget of subducted slabs and that Sr could be Implications for Skye granite petrogenesisand REE mobility in hydro- thermal systems Eath and Planetary Letters, transportedefficiently to depths of more than 200 Science 48,9'7-I1O km. Getty, S R and Selverstone,J. (1994) Stable isotopic and trace element evidence for restricted fluid migration in 2 GPa eclogites.Journal of Implications for Rb-Sr isotope systematics Metamorphic Geology, 12, 74'7 -7 60 Grapes,R and Watanabe,T, (1984) Al-Fe3* and Ca-Sr,* epiilotes rn me- Zoisite and epidote contain little Rb, which has a dis- tagreywacke-quartzofeldspathicschist, Southern Alps, New Zealand tinctly large ionic radius (1.66 A for tenfold-coordination: American Mineralogist, 69, 490-498. Shannon 1976), and have very low Rb/Sr values (about Griffin, W.L and Brueckner,H K. (1985) REE, Rb-Sr and Sm-Nd studies 10 a; Griffin and Brueckner 1985). Consequently, the of Norwegian eclogites.Chemical Geology, 52,249-271 Hickmott, D.D, Sorensen,S S., and Rogers,PS Z (1992) Metasomatism 875r/865rvalue of Sr-rich zoisite and epidote should in a subductioncomplex: Constraintsfrom microanalysisof trace ele- change very little from at its initial value. The 875r/865r ments in minerals from garnet amphibolite from the Catalina Schist of Sr-rich zoisite and epidote may therefore be a good Geology, 20, 347-350 indicator of initial 875r/865rand a useful check on the Hirajima, T., Zhang, R., Li, J J, and Cong, B (1992) Nybdite from the Donghai area, Jiangsu Province, eastem China Mineralogical Maga- heterogeneity (or homogeneity) of whole rocks during 15r/865r zine. 56.37-46 metamorphism. Moreover, the values of zoisite Ishiwatari, A., Hirajima, T., Enami, M, Hiramatsu,N., Zhao, Z-Y.,Wang, and epidote should also be important for the calculation Q., Zhai, M., Cong, B , Zhang, R , Banno, S., and Ishizaka,K (1992) of Rb-Sr mineral isochron ages. Eclogitesin ShandongProvince and JiangsuProvince, China: 1991 sur- vey report In CommemorativeAssociation for ProfessorYukio Mat- sumoto, Ed., Exploration of Volcanoesand Rocks in Japan,China and Acxnowr,nncMENTS Antarctica, p 393-409. Semura Furoppi Press, Yamaguchi (in Japanese) This paper is the result of a cooperativeproject between Kyoto Uni- Ishizaka, K, Hirajima, T,, and Zheng, X (1994) Rb-Sr dating for the versity, Japan,and the Institute of Geology and Academia Sinica, China; Jiaodonggneiss of the Su-Lu ultra-high pressureprovince, eastem Chi- funded by the Japmese Society for the Promotion of Science We are na The Island 4rc.3.232-24I grateful to T Hirajima, A Ishiwatari, M. Zhat, and other membersof the Kato, T, Enami, M , and Zhai, M (199'7)The ultrahigh-pressure(UHP) project for their frank discussionsduring the field work Laboratory works marble and eclogite in the Su-Lu UHP terrane,eastern China. Journal were supported in part by Grants in Aid for Scientific Researchfrom of Metamorphic Geology, 15, 169-182 Ministry of Educationof Japan(Nos 06302031and 08304034) This pa- Kretz, R (1983) Symbols of rock-forming minerals. American Mineral- per benefitedfrom the critical reviews of B.R. Hacker and an monymous ogist,68, 277-2'79 referee and constructivecomments of S R Wallis and G.E Bebout. Our Morishita, T and Suzuki, K. (1993) XRF analysesof the Mitsuhashigran- thanks also to S Yogo for making many thin sectionsand to y Asahara ite in the Shitara area, Aichi Prefecture Bulletin Nagoya University and T, Morishita for their help in chemrcal analysesby isotope dilution Furukawa Museum, No 9, 77-90 (in Japanesewith English abstract). and XRF methods.The loan of four samples(SD0307, SD03l5, SJy02, Mottana, A. (1986) Blueschist-faciesmetamorphism of manganiferous and Rl-7) by Q. Zang is gratefully acknowledged. schists: A review of the alpine occunences.In B.W. Evans and E.H. NAGASAKI AND ENAMI: ST-BEARING ZOISITE AND EPIDOTE IN UHP ROCKS 247

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