In situ LA-SF-ICP-MS U-Pb dating of metasomatic zircon growth during

retrogression of UHP eclogites, Sulu deep drilling hole, China

ASTRID RIEMANN1, ROLAND OBERHANSLI1,*, ANDREAS MOLLER2, AXEL GERDES3 and ZHIGIN XU4

1 Institut fur Geowissenschaften, Karl-Liebknechtstrasse 24, Haus 27, 14476 Potsdam, Germany*Corresponding author, e-mail: roob.ejm@geo.uni-potsdam.de

2 Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045, USA3 Institut fur Geowissenschaften, Mineralogie, Altenhoeferallee 1, 60438 Frankfurt am Main, Germany

4 Chinese Academy of geological Sciences, Institute of geology, 26 Baiwan & huang Street, Beijing 10003 , China

Abstract: Thermobarometry and in situ U–Pb dating of zircon in texturally and petrologically defined positions are used to assessexhumation accompanied by fluid-enhanced retrogression of ultra high pressure (UHP) eclogite core samples from the ChineseContinental Scientific Drilling (CCSD) project.

Eclogites from the CCSD project show discrete retrograde zones that can be attributed to exhumation processes. The retrogressionfrom UHP eclogite to HP eclogite, amphibolite and finally greenschist facies conditions is fluid-induced. The occurrenceof significant barium concentrations in two mica generations points to an external fluid source during retrogression. This Ba andF-bearing fluid mobilized zirconium from the eclogitic host, which was preferentially precipitated as overgrowths on pre-existinggrains in the retrograde zone.

Thermodynamic modelling with progressive addition of H2O to the bulk composition simulates the petrological developmentof the fluid-influenced retrograde zone. Growth of zircon precipitated from the fluid occurred between 730�–740 �C at 3.46 GPa and700 �C at 1.4 GPa.

LA-SF-ICPMS analyses point to a local onset of metasomatic zircon growth at 233.1 � 5.3 Ma ongoing to 207.4 � 1.6 Ma that isspatially limited to the retrograde zone.

Key-words: zircon, U–Pb dating, eclogite, retrogression, ultrahigh-pressure metamorphism.

1. Introduction

Despite the development of advanced methods for extract-ing time information from metamorphic zircons, themechanisms and dissolution-precipitation reactions ofmetamorphic zircon growth are poorly understood.Textural observations, however, give evidence of resorp-tion and precipitation of zircon along with fluid-inducedprocesses. Zirconium is mobile under hydrated conditions(Rubin et al., 1993) and released by breakdown of majorand accessory minerals such as garnet, omphacite andrutile as well as zircon. It is dissolved and transported influids and melts (e.g., Pan & Fleet, 1996; Bingen et al.,2001; Moller et al., 2002, 2003; Geisler et al., 2003;Hoskin & Schaltegger, 2003; Tomkins et al., 2005;Hermann et al., 2006). Combination of textural and ageinformation is important for understanding fluid andelement mobility during the exhumation history. A majorquestion in evaluating metasomatic zones as evidence offluid flow is their age in relation to the evolution of meta-morphic conditions. Several authors have previously

discussed origin, composition and development stages offluid in the Dabie Sulu orogen (e.g., Ferrando et al., 2005;Zheng et al., 2007). Fu et al. (2003a, 2003b) suggested thatfluids in equilibrium at metamorphic peak conditions areCa-rich brines containing CO2 and/or N2. Rare CH4-richfluid inclusions coexist with high-salinity brine inclusions(Fu et al., 2003a). Mixed CO2–H2O fluids (Shen et al.,2003), N2–CO2–CH4–H2O fluids, or CO2 fluids (Fu et al.,2003b) were present during the initial stages of exhuma-tion. CO2-rich fluids appear to be characteristic of HP-UHP rocks re-equilibrated under granulite-facies condi-tions (Fu et al., 2003a). These studies indicate the presenceof low-salinity fluids during late stages of retrogression (Fuet al., 2003a). Stable isotope studies of the Dabie-SuluUHP rocks reveal low dD values of hydroxyl-bearingminerals from eclogite, quartz schist, paragneiss, andorthogneiss, whereas the eclogites from Qinglongshanshow the lowest d18O values ever observed in eclogite-facies rocks (e.g., quartz from�7.7 toþ4.5 and rutile from�14.8 to �5.3%; e.g., Rumble & Yui, 1998). Such valuesare interpreted as due to interaction of the protolith with

25 years of UHP metamorphism

0935-1221/09/0021-2005 $ 6.30DOI: 10.1127/0935-1221/2009/0021-2005 # 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

Eur. J. Mineral.

2009, 21, 1251–1264

Published online January 2010

meteoric-hydrothermal water under cold climate condi-tions. The preservation of the original isotopic valuessuggests that during prograde metamorphism, fluid flowoccurred only at a small scale (e.g., Zheng et al., 2003).Mineral oxygen isotope studies of UHP metamorphicrocks from the Dabie-Sulu orogenic belt indicate that theamphibolite-facies retrogression caused mineral reactionsand O isotope disequilibria between some of the minerals(Zheng et al., 1999, 2003; Zhang et al., 2003; Li et al.,2004). Considerable changes occur in d18O and petrogra-phy at the contact between eclogite and gneiss in the CCSDcore samples (Chen et al., 2007), suggesting that thecontact between different lithologies is the most favour-able place for fluid activity. The stable-isotope studieshave demonstrated that the retrograde fluid is of deutericorigin and thus was derived from the decompression exso-lution of structural hydroxyl and molecular water (Zhenget al., 1999, 2003; Chen et al., 2007).

The drill core from the main hole of the CCSD project(Fig. 1) consists mainly of eclogites, ortho- and para-gneisses, ultramafic rocks, and quartzite (Zhang et al.,2004). Some of these rocks contain considerable amountsof zircons that were investigated in terms of geochronol-ogy and mineral inclusions (e.g., Liu et al., 2001, 2004;Rumble et al., 2002; Yang et al., 2003). From previousstudies, three different metamorphic events were recog-nized for the Dabie-Sulu orogen: a 244–236 Ma ‘‘precur-sor’’ UHP event, followed by a 230–220 Ma ‘‘main’’ UHP

event, which terminated with a 220–205 Ma amphibolite-facies overprint (Liu et al., 2004; Hacker et al., 2006;Wawrzenitz et al., 2006; Liu et al., 2008). Pioneeringwork on the CCSD core samples has focussed on generalpetrography and deformation analysis, geochemistry andzircon dating (Liu et al., 2004; Xu et al., 2004; Zhang et al.,2004). Whereas these studies provide a geochronologicalframework, the data are generally based on mineral sepa-rates. Detailed studies on CCSD rocks correlating petrol-ogy, deformation and in situ geochronology in texturalcontext are still missing. There is abundant evidence forfluid induced processes in the core material, but detailedstudies on discrete metasomatic zones on Sulu CCSD rockshad thus far not been conducted. Our study aims to assessthe petrology and timing of fluid-related retrogression ineclogites from the CCSD hole.

To link the textural association of zircon and surround-ing parageneses, we study a retrograde zone within theUHP metamorphic rocks. This sample shows fluid influ-enced parageneses from UHP eclogite to greenschistassemblages over a few centimetres. Growth of hydrother-mal zircon linked with textural observations and geochro-nology allows precise dating of the fluid event and thus theretrograde stage. The parageneses of different retrogradestages and their reaction textures are used to reconstructthe metasomatic history.

This study reports detailed petrographic investigationscombined with LA-SF-ICP-MS (laser ablation-sectorfield-inductively coupled plasma-mass spectrometry) insitu U–Pb dating on zircon in a fluid influenced zone.It aims to: (1) document the petrological evolution ofdistinct retrograde zones, (2) describe zircon (re)crystalli-sation in local (fluid-controlled) retrogressed zones in theCCSD core, and (3) provide a better resolved P–T-t historyof the post-UHP evolution of these eclogites.

2. Geological setting

The Sulu UHP metamorphic terrane is the eastern exten-sion of the Qinling-Dabie collision zone, with sinistraldisplacement to the northeast by the large scale Tan-Lufault in eastern China. The Sulu belt consists of orthog-neiss, paragneiss, amphibolite, and marble, in which abun-dant coesite-bearing eclogite, kyanite quartzite, and UHPgarnet peridotite/pyroxenite occur as lenses, layers, orblocks (Hirajima et al., 1990; Yang et al., 1993; Zhanget al., 1994, 1995, 2000, 2003; Liou et al., 1995; Cong,1996). A regional UHP metamorphism has been inferredon the basis of the extensive occurrence of coesite inclu-sions in country-rock gneisses (Ye et al., 2000; Liu et al.,2002, 2004, 2005). Results of U–Pb and Sm–Nd geochro-nology are interpreted to date UHP metamorphism of theSulu belt at 220–240 Ma, and the protolith ages of eclogiteand gneiss are 700–800 Ma (Ames et al., 1996; Li et al.,1996; Hacker et al., 1998; Ayers et al., 2002; Liu et al.,2004, 2005). UHP metamorphism of coesite-bearing eclo-gite in the Sulu UHP terrane has been considered to have

Fig. 1. Simplified geological map of the Dabie-Sulu orogen (mod-ified after Ratschbacher et al., 2000).

1252 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu

occurred at 700–890 �C and 2.8–4.0 GPa (Zhang et al.,1995, 2000). However, garnet peridotite and pyroxeniterecord higher peak P–T conditions of 750–950 �C and4.0–6.5 GPa, as indicated by various P–T calculationsand mineral exsolution textures (Yang et al., 1993; Zhanget al., 1994, 2000, 2005; Liou et al., 1998; Zhang & Liou,1999; Ye et al., 2000). Equilibrium temperatures of678–816 �C and pressures of 3.1–4.4 GPa are estimatedas UHP peak metamorphic conditions (Zhang et al., 2006)for the CCSD project eclogites. Katophorite-taramite bear-ing assemblages from a Sulu eclogite reach post-peakconditions at maximum conditions of 1.7 GPa and 750 �C(Ferrando et al., 2005). A garnet-plagioclase-amphibolefluid-infiltrated assemblage yields post-peak conditionsof 586 � 70 �C and 1.2 � 0.3 GPa (Yang, 2004).Decompression textures with amphibole-plagioclasesymplectites after clinopyroxene and biotite-plagioclaseon phengite as well as amphibole coronas after garnetand ilmenite on rutile are related to the post-peak stage,with estimated conditions of 530–560 �C and 0.7–0.9 GPa(Tong et al., 2007).

A lithological column (0–2000 m) of the main hole ofCCSD is divided into four petrotectonic units by Xuet al. (2004): Unit I (0–738 m) is rutile eclogite, inter-layered with ultramafic rock and rare gneiss; Unit II(738–1113 m) consists of paragneisses, interleavedwith some rutile eclogites and minor ultramafic rock;Unit III (1113–1600 m) is granitic orthogneisses, andminor paragneisses; Unit IV (1600–2000 m) is phengiteeclogite, with paragneisses and granitic orthogneisses asinterlayers.

3. Textures and mineral assemblages

The sample for this investigation was taken from anUHP eclogite at 382 m drill-hole depth in theCCSD main hole. A fifteen centimetre segment of coreshows macroscopic zoning with intense retrogression(Fig. 2a) by the conspicuous occurrence of whitebands (quartz) and very dark-blue green amphibole.Four thin sections were cut from this core sampleand used for detailed petrographic observations and insitu measurements (Fig. 2a þ b). The sample can besubdivided into: (i) non-retrogressed coesite-bearingeclogite; (ii) transition zone (predominantly eclogiticminerals, but first appearance of retrograde minerals)and (iii) retrograde zone (predominantly retrogrademinerals, few eclogitic relics).

On both sides of the reaction zone described here asa transition and retrograde zone, the eclogite is poorlyretrogressed and shows a marked UHP foliation definedby omphacite, phengite and garnet (Fig. 2a þ b). Theretrograde zone cuts this foliation at a low angle.Contacts between UHP and lower-grade regions are gra-dual. In the retrograde zone, the foliation is lost dueto neomineralization and recrystallization.

3.1. UHP eclogite

The weakly retrogressed eclogite (Fig. 2a) consistsof garnet I, omphacite (clinopyroxene I), phengite I,as well as accessory rutile, apatite I and zircon I.Unzoned garnet I and omphacite I are predominantlyfree of inclusions. The eclogite exhibits compositionallayering expressed by millimetre-scale variation inmodal proportions. Garnet I (XMg ¼ 0.22) alternates withomphacite layers. Omphacite grains are elongated andlarger than garnet (Fig. 2b). Omphacite is rimmed by acoronitic symplectite (II, albite þ diopside ¼ clinopyrox-ene II) and is oriented parallel to the layering.

3.2. Transition and retrograde zone

From the transition zone towards the retrograde zone, rimsof garnet I are progressively resorbed to form coronalpargasitic amphibole. Phengites I up to 500 mm in lengthare oriented parallel to omphacite. Phengite I þ II arezoned with high Si in cores, decreasing towards the rims,and an increasing barium and small fluorine contenttowards the rims (Fig. 3). Biotite-plagioclase-Ba-micasymplectites III form coronas around phengite I þ II.Amphibole occurs in two different types: light blue amphi-bole I is zoned from katophorite cores to taramite rims(Fig. 4). Pargasitic blue-green amphibole II can be foundforming coronas around garnet I.

The amount of amphibole and symplectitesincreases from the transition toward the retrograde zone

Fig. 2. (a) Photograph showing a part of CCSD core (depth 382 m)with increasing retrogression from eclogite- (right) to greenschist-facies (left). Black rectangles ¼ thin section sites, white boxes ¼areas of photomicrographs shown in B and C. (b) Eclogite (crossedpolar) with UHP foliation indicated by oriented phengite I (phg I),garnet I (grt) and omphacitic clinopyroxene (cpx). (c) Low-gradeassemblage of the retrogression zone (quartz (qz), phengite II (phgII), symplectite (symp); crossed polars).

In situ LA-ICP-MS dating of zircon in UHP eclogite, Sulu deep drill hole 1253

(Fig. 2a, c); absence of a well-defined foliation is inter-preted to indicate that the retrograde assemblages recrys-tallized under mostly static conditions. The phengite IIgeneration shows similar compositional zonating as phen-gite I (core high in Si and low in Ba; rim low in Si and highin Ba). Elongated apatite II grows parallel to the directionof the transition zone interpreted as evidence for a weakpost-UHP foliation. Under cathodoluminescence, apatitegenerally exhibits a yellow core and a red rim. Rutile isintergrown with ilmenite and in some places with titanite.Few rutile grains remain in the retrograde zone. Ilmenite ismainly situated in the transition zone.

Zircon II is found in symplectites II, quartz, phengite II,amphibole II, rutile, along grain boundaries and as inclu-sions in garnet II, with XMg ¼ 0.15. Few zircon inclusionswere identified by Raman spectroscopy and indicate firstzircon overgrowth on oscillatory cores. They occur at theresorbed outer boundaries of the cores (Fig. 7 and 9) at theboundary to the rims grown during the retrograde over-print. Phengite II flakes are often found together withquartz segregations.

Quartz is polycrystalline and surrounded by aegirine(clinopyroxene III). Barite grows always in the presence

of pyrite. Calcite forms small grains in the matrix aswell as in symplectites. Some grains are in contact withzircon II. Late idiomorphic allanite grows in the assem-blages that have the strongest retrograde overprint. Inthe retrograde zone, garnet II and omphacite are mainlyresorbed due to the progress of retrograde reactions.

3.3. Fluid indication

Progressive influence of fluid in the transition and retro-grade zones is marked by the occurrence of OH-bearingminerals (phengite II, amphibole II, biotite) and theirincreasing abundance as compared to UHP eclogite.Barium as a large ion lithophile element (LILE) isdescribed as an indication of fluids (Tracy, 1991; Zacket al., 2001). Therefore, barium zonation in phengitesI þ II, F content and the occurrence of barite are furtherindications for influx of a fluid phase. Small carbonatesindicate a mixed H2O–CO2 nature of the fluid inclusion.Their trails in recrystallized quartz indicate a late fluid. Thetransitional change from eclogite to the retrograde zone istherefore interpreted as a fluid reaction front.

Three of the H2O relevant reactions can be formulated asfollows in unbalanced reactions:

garnet Iþ omphaciteþ fluidþ rutile ¼pargasiteþ plagioclaseþ diopsideþ ilmenite

(1)

phengite ¼ biotiteþ plagioclase (2)

Ba-phengite ¼ biotiteþ plagioclaseþ Ba-mica (3)

3.4. P–T conditions

The core sample contains metamorphic mineral assem-blages ranging from UHP to greenschist-facies conditions(Fig. 5). The assemblage garnet-omphacite-coesite-phen-gite-rutile reflects UHP conditions. Albite-diopside sym-plectites appear in the first stage of retrograde overprint atthe transition from UHP to HP conditions as a response topost-peak decompression. The sequence of katophorite,taramite and pargasite (Fig. 4) signalises fluid influenceat decreasing pressure at amphibolite-facies conditions.Garnet in the transition zone is cracked and rimmedby amphibole, signalising fluid influence during amphibo-lite-facies conditions. The inner part of the retrogradezone is represented by an allanite and aegirine-bearinggreenschist-facies assemblage with breakdown of phengiteto biotite and albite.

In order to determine which minerals were in equili-brium, we modelled the P–T evolution with Gibbs-free-energy minimization methods (De Capitani & Brown,1987) by using the THERIAK software. For the mea-sured bulk composition (SiO2 48.67, TiO2 2.96, Al2O3

Fig. 4. Chemical composition of amphibole. Si vs. Na (per formulaunit; pfu) show the retrograde trend from katophorite (amphibole I)and taramite to pargasite (amphibole II).

Fig. 3. X-ray map of Ba-bearing phengite. Map is correlated withmicroprobe analyses (values in BaO wt%). Zonation pattern is inter-preted to be caused by fluid ingression.

1254 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu

15.7, FeO 12.52, MnO 0.25, MgO 4.70, CaO 9.32,Na2O 4.21, K2O 0.49, P2O 0.20 wt%) of fresh UHPeclogite we were able to reproduce the analysed mineralcompositions of garnet I and omphacite I (Table 1) thatshow equilibrium textures at 730–740 �C and around3.4 GPa. In a second step we modelled the mineralcompositions and modes in the retrogressed zone byprogressively adding H2O to the eclogitic bulk-rockcomposition.

The mineral compositions of garnet II and pyroxene IIwere reproduced in local equilibrium with the first addedfluid (1 wt% H2O) at conditions of � 700 �C at 1.4 GPa.Continued fluid addition led to the formation of additionalretrograde phases such as amphibole and phengite. Forthese, no equilibrium corresponding to the measured rockand mineral compositions could be computed. For mineralassemblages containing phengite, biotite, allanite andaegirine component in pyroxene, Tong et al. (2007) esti-mated conditions of ca. 530–560 �C and 0.7–0.9 GPa.Since the retrograde zone of our rock shows the sameminerals and similar compositions as those described byTong et al. (2007), we used their pressure and temperatureestimates.

4. Analytical methods

The thin section with the highest amount of zircons withmeasurable rims (�20 mm) from the retrograde zone was

chosen for in situ U–Th–Pb measurements. In particular,zircons at grain boundaries, in garnet, amphibole, andquartz and in symplectites were selected for the isotopemeasurements.

Thirty-five grains were dated by using a Thermo-Finnigan Element II sector field ICP-MS coupled to aNew Wave Research UP213 ultraviolet laser ablationsystem at Goethe University Frankfurt. Isotope data wereacquired in peak jumping mode using a 20 mm spot.A teardrop-shaped, low-volume laser ablation cell wasused to enable sequential sampling of heterogeneous grains(e.g., growth zones) during time resolved data acquisition.Raw data were processed offline using an Excel� spread-sheet program (Gerdes & Zeh, 2006). Laser-inducedelemental fractionation and instrumental mass discrimina-tion were corrected by normalizing to the reference zirconGJ-1, which was analysed under exactly the same condi-tions as the samples except for the spot size. Prior to thisnormalization, the change of elemental fractionation (e.g.,the Pb/Th and Pb/U ratios as a function of ablation time andthus crater depth) was corrected for each set of isotoperatios collected during the time of each single spot analy-sis. All reported uncertainties (2s) were propagated byquadratic addition of the external reproducibility (2 s.d.;standard deviation) obtained from the standard zircon GJ-1(n ¼ 18) during the analytical session and the within-runprecision of each analysis (2 s.e.; standard error). Thereproducibilities (1s) obtained from the GJ-1 are 0.5 %for 207Pb/206Pb, 0.9 % for 206Pb/238U and 2.8 % for208Pb/232Th. For further details see Gerdes & Zeh (2006,2009) and Frei & Gerdes (2008).

Cold cathodoluminescence (CL) imaging of zircon crys-tals was performed on polished thin sections using a visible– near infrared camera mounted on a LEICA microscope atthe University of Potsdam. Beam operating conditionsadjusted on the CITL 8200 Mk4 instrument were 20 kVand 300 mA. Contrast and brightness were adjusted indivi-dually for each grain in order to enhance the visualisationof internal structures.

Electron microprobe analysis was performed atGeoforschungszentrum Potsdam (GFZ) on a Cameca SX100 using standards at 15 kV and 20 nA. Spot sizes of 5 to15 mm were used, depending on mineral species and size.Natural and synthetic phases were used for calibration.

5. Zircon characteristics

Zircon from the CCSD rocks has traditionally beenextracted by conventional mineral separation techniquesfor U–Pb analyses. Their size, shape and zoning character-istics were documented by cathodoluminescence (CL) andbackscatter electron images prior to geochemical and geo-chronological analysis. However, much debate existsregarding the nature of zircon growth and modificationwithin polymetamorphic, multiply deformed or highly ret-rogressed and fluid-influenced metamorphic rocks (e.g.,Moller et al., 2002, 2003; Rubatto, 2002; Tomkins et al.,

Fig. 5. Mineral paragenesis and facies of UHP eclogite and HP-LPtransition – retrograde zone. Solid lines indicate the presence ofminerals through different metamorphic stages. Broken lines indi-cate probable presence.

In situ LA-ICP-MS dating of zircon in UHP eclogite, Sulu deep drill hole 1255

2005). This is due to the fact that zircon may grow or bemodified during prograde, peak and retrograde meta-morphic conditions (e.g., Harley et al., 2007) and shouldtherefore be investigated in situ, within the textural context.

In general, three different generations of the 35 zirconsstudied were distinguished which are named zircon I, IIand III. Depending on the type of zircon a further subdivi-sion is accomplished. A detailed thematic sketch is pre-sented in Fig. 7. Polished thin sections of the rock wereused to characterise the different zircon types by CL-tech-niques and to perform age determinations.

Zircon in polished thin sections was investigated usingCL imaging to distinguish among different generations(zircon I–III) and types (A–C) within the eclogite, thetransition and retrograde zones. Spot sites for LA-SF-ICPMS U–Th–Pb analyses were selected on the basis ofthese observations. Zircon generations (I–III) aredescribed in detail below.

5.1. Zircon I (type A) – UHP eclogite

The eclogite contains 1–2 zircon grains per cm2, which aresmaller than 20 mm. They are typically euhedral and showan oscillatory bluish luminescent core with a thin yellowluminescent rim. The host minerals are garnet I andomphacite. Zircon inclusions in garnet and omphacitetend to form clusters (Fig. 6a). Initial retrogression withineclogite occurs along grain boundaries, indicated by theoccurrence of zircons with a core and a very thin rim thatare situated on garnet-omphacite grain boundaries.

5.2. Zircon II (type B, Bb) – Transition andretrograde zone

Number and size of zircons increase with increasing retro-grade overprint. The most retrogressed part of the sample

Table 1. Electron microprobe analysis of selected minerals from the retrogressed sample. Mineral compositions are representative ofdifferent retrogression domains. For explanation, see text.

1256 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu

contains 10–12 zircons per cm2. The CL images in Fig. 9show yellow-greenish colours of the metamorphicdomains of zircon II from the retrograde zone. The shapeof the zircons relates to their position (Fig. 10), e.g. nearlyidiomorphic in symplectite III (Fig. 10a) or oriented alongthe cleavage of phengite II (Fig. 10b).

Zircon II occurs with and without cores of zircon I andis therefore divided into two types, Bb (with cores) andB (without cores). The size of the yellowish type B is,20 mm. It is mostly located in symplectites, occurs with-out magmatic cores (Fig. 7, 8) and is more homogeneousthan other textural positions. These small, core-free zir-cons (B) show the same luminescence as zircon with rimsaround the bluish luminescent magmatic cores (type Bb),suggesting nucleation and growth of zircon II on zircon I.

Type Bb zircons are irregular and indicate a complexgrowth history. They contain an oscillatory core similar totype A. This core is enclosed in a yellowish rim in differenttextural positions. Type Bb is situated directly on grainboundaries, in symplectite, quartz, and phengite,

amphibole II, garnet II and on rutile. The size is between30 and 150 mm (mostly 40–50 mm). Core domains of zirconI (type Bb) show corrosion and resorption patterns(Fig. 9d). Some of these resorbed areas contain coesiteclose to the boundaries to the cores (Fig. 9d). Rims directlyadjacent to the cores (Fig. 7) are interpreted as the firstmetamorphic growth event (zircon II). Thin protrusions ofzircon rims following grain boundaries (Fig. 9f) indicate alate zircon generation formed contemporaneously withretrograde phengites.

5.3. Zircon III (type C, Cc) – retrograde zone

A few zircons II have a second, yellowish luminescent rimthat is brighter and only a few micrometres thick. This isinterpreted as a third generation of zircon (zircon III, Fig. 7and 9c, e, f).

6. Results of U–Pb geochronology

Ninety-two spots on thirty-five zircons in the retrogradezone were analysed. Zircon I type A was not analysed inthe eclogite, because it is too small. Cores and rims of typeBb (zircon I þ II), C (zircon II) and Cc (zircon I þ II)were analysed in situ in a single thin section from theretrograde zone. Zircon III was too thin (,15 mm) to bedated by the applied laser ablation technique.

Figure 10 shows grains with location of the LA spots, theU–Pb age, and their textural position. Newly grown zircon

Fig. 7. Schematic view of three different zircon generations (I–III)found in the analysed sample. Zircon I (type A) generally is euhedraland prismatic with oscillatory zoning, interpreted to be magmatic.Zircon II is interpreted to be metamorphic and forms rims (type Bb)on partly resorbed magmatic cores of zircon I. Few form homoge-neous grains in symplectite. Zircon of generation III (types C and Cc)rims is rare and thin (less than 30 mm) and overgrows zircon II(blue ¼ resorption zones, red ¼ coesite locations between coresand overgrowth zones).

Fig. 6. Textural position of zircon in eclogite and the retrograde zone(with crossed polarizers). (a) Cluster of small zircons I in omphacitefrom eclogite. (b) Large zircon II situated in phengite II and at grainboundaries of garnet and quartz, rt ¼ rutile, phg ¼ phengite, qz ¼quartz, red circles ¼ zircon locations.

Fig. 8. X-ray maps of metamorphic zircon (generation II) withoutinherited magmatic core (see Fig. 7) showing F, Y, P, and Baabundances. The respective elements are homogeneously distribu-ted. A garnet inclusion is visible below the uppermost laser spot in Y,P and Ba maps; note that Ba is slightly enriched around zircon.

In situ LA-ICP-MS dating of zircon in UHP eclogite, Sulu deep drill hole 1257

II is predominantly represented by overgrowths on inher-ited cores of zircon I. Zircon was analysed in the followingpositions, included in: (a) garnet II (2�), (b) amphibole(7�), (c) symplectite (12�), (d) quartz (3�), and (e)on grain boundaries (11�). Concordant LA-SF-ICPMSdata on zircon (II) from metamorphic rims are listed inTable 2 in the Appendix (supplementary material freelyavailable from the GSW website of the journal, http://eur-jmin.geoscienceworld.org/).

The zircon types I þ II differ significantly in their U–Thcomposition. U in zircon I cores and metamorphic zircon IIrims ranges from 68 to 734 ppm and 39 to 181 ppm,respectively (Fig. 11). Th in cores and rims ranges from15 to 550 ppm and 0.8 to 19 ppm, respectively. Comparedto the oscillatory magmatic zircon generation I, the ana-lyses of zircon II (see Fig. 7) yield lower Th but overlap inU content (Fig. 11). Zircon II analyses align along the Th/Utrend of 0.1 (Fig. 11).

The Th/U of core analyses (zircon I, type Bb, Cc) variesfrom 0.19 to 1.12 (Fig. 12). The Th/U ratio increases

with increasing 206Pb/238U ratios for all analyses withTh/U higher than 0.041 (Fig. 12). Rim analyses (zircon II,Bb, C, Cc) with lower 206Pb/238U have significantly lowerTh/U ratios around 0.1. (Fig. 12a). The Th/U value has oftenbeen used to characterize metamorphic zircon growthinfluenced by fluids (e.g. Rubatto et al., 2002). For zirconswith such low Th/U ratios as found here, 206Pb/238U ratiosrange from 0.032 to 0. 039 with a s mall gap between 0.034 to0.035. A single point shows a 206Pb/238U value of 0.028.

Fig. 10. Representative cathodoluminescence and transmitted lightphotomicrographs taken after LA-SF-ICPMS analyses (note pits) ofzircon II in the retrogression zone. LA-SF-ICPMS results are indi-cated as 206Pb/238U dates with 2s error. (a) Zircon II without mag-matic core, rimmed by zircon III, in symplectite with garnetinclusion. (b) Zircon in biotite–albite symplectite as breakdownreaction of phengite. (c) Zircon in amphibole–plagioclase symplec-tite, (d) Zircon in garnet II.Fig. 9. CL images of anhedral to subhedral zircon typical from the

retrograde zone. (a) Zircon with magmatic core (weak blue lumines-cence) and yellow luminescent metamorphic rim. (b) Residual faintmagmatic core with a inhomogeneous metamorphic overgrowthwith a slightly brighter yellowish luminescence outermost rim. (c)Turbid core with narrow, lower luminescent metamorphic rim andnon-luminescent inclusion of garnet. (d) Strongly resorbed andcracked zircon with oscillatory bluish-grey luminescent oscillatorymagmatic core and thin, weakly yellowish luminescent metamorphicrim and annealed cracks. Carbonate with reddish luminescence isvisible on the upper left outside the zircon, white circle ¼ coesiteinclusion. (e) Zircon II and III cluster with yellow CL metamorphicrims and bluish magmatic cores. (f) Skeletal zircon rim, growthalong edges is interpreted as fluid enhanced rapid growth alonggrain boundaries.

zircon II (type Bb,C,Cc)zircon I (type Bb, Cc )

Th

[ppm

]

U [ppm]

1000

100

100 1000

10

1

Fig. 11. Differences in Th and U contents of the analysed zircongenerations I and II. Protolith-derived zircon I cores have high Th/Uclose to unity, whereas metamorphic rims of zircon II have distinctlylower Th/U. Partially recrystallized cores fall between those twogroups. Trend line is shown for the Th–U ratio of 0.1.

1258 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu

The U–Pb zircon analyses (zircon II, Bb, C, Cc) producea cluster of concordant Triassic ages (201–246 Ma) and atrend of discordant ages (zircon I, type Bb, Cc) that inter-sects at 784 � 21 Ma and 207.6 � 7 Ma with a MSWD of1.4 (Fig. 13a). Thirty-six concordant analyses range from201 to 246 with a gap between 214 Ma and 221 Ma (Fig.13b, d). The histogram shows the most frequent agesbetween 201 and 214 (21 analyses), 14 analyses between221 Ma and 246 Ma and one single analysis at 180 Ma.These zircon II analyses are restricted to the retrogradezone.

Since the distinct growth zones of zircon II are in partsvery small, one cannot rule out the possibility that mixedages were obtained for some of the analyses, which wouldbe responsible for the scatter in Fig. 13. This becomesapparent using the CL techniques, which in some casesdocuments an inhomogeneous zoning of zircon (Fig. 9c).However, such mixed analyses are making up less than10 % of the performed analyses.

7. Discussion

7.1. Zircons in eclogite and retrograde zone

The sizes and abundances of magmatic core (generation I)and newly grown metamorphic rim (generations II þ III)zircons differ with respect to their location within fresheclogite, transition and retrograde zones. Magmatic coresare most abundant in fresh eclogite, whereas metamorphicrims are largest and most abundant in the retrograde zone.These observations correlate well with an increasing retro-gression from eclogite to greenschist facies as indicatedby the mineral assemblages.

Three zircon generations (I–III) and five zircon types(A–Cc) were observed in the eclogite and retrograde zone.The first zircon I with oscillatory zoning grew in the pre-cursor of the eclogite. Zircon I is interpreted as magmaticzircon. The following overgrowths are metamorphic rims.The second zircon II grew in the transition and the retro-grade zones. It occurs as metamorphic overgrowth (typeBb) on zircon I and without zircon I cores (type B). A fewoutermost rims on zircon II were observed with and with-out zircon I core (zircon III, type C, Cc).

The irregularity of type Bb can be explained as follows.Zircons enclosed in host minerals were protected fromresorption or overgrowth. Others are located close to or atgrain boundaries of a reacting host mineral (Fig. 10b). Dueto reaction, the host grain is locally consumed and parts ofthe included zircons are not longer fully enclosed andshielded. At such zircon grain boundaries, growth of ametamorphic zircon (II) rim started, whereas the partsstill enclosed in the host mineral were not overgrownor resorbed.

The textural position of zircon II in retrograde minerals(amphibole II, garnet II), the homogeneous formation ofnewly grown zircon in symplectites, the growth of zirconrims with proceeding retrogression and the occurrence ofcoesite in along resorbed parts of zircon point to a single,extended growth period of zircon II.

Three mechanisms can in general be consideredfor zircon growth during high-grade metamorphism:(1) crystallization from melt during anatexis (e.g.,Roberts & Finger, 1997; Vavra et al., 1999; Keay et al.,2001); (2) nucleation and crystallisation due to Zr andSi release by metamorphic breakdown reactions of miner-als under subsolidus to solidus conditions (e.g., Fraseret al., 1997; Bingen et al., 2001; Degeling et al., 2001;Tomkins et al., 2005); (3) growth in the presence

Fig. 12. Th/U vs. 206Pb/238U plots of zircon I and II analyses. (a) Complete range of analyses: magmatic cores (generation I, full circles)show variable Th/U, while metamorphic rims (generation II, open triangles) scatter around Th/U � 0.1. (b) Enlarged portion of (a);concordant rim analyses are very similar in Th/U, but scatter around two age groups with a small gap inbetween at around206Pb/238U ¼ 0.0345 (see Fig. 13b).

In situ LA-ICP-MS dating of zircon in UHP eclogite, Sulu deep drill hole 1259

of aqueous fluids (e.g., Liati & Gebauer,1999; Rubatto &Hermann, 2003).

Modification of existing zircon may occur throughout themetamorphic cycle during high-grade conditions accordingto the scheme proposed by Harley et al. (2007). Zircon IImainly is accumulated within the retrograde zone and showslarge oscillatory cores of zircon I. Zircon I is assigned to aprimary magmatic feature of the protolith or to refractoryaccumulation and thus an early specification of the subse-quent fluid induced processes in the transition zone.

In contrast, zircons II indicate fluid interaction byresorption of zircon cores and by metamorphic overgrowthwithin the retrograde zone.

7.2. Petrology and geochronology

Based on Th–U–Pb composition, zircon II forms twogroups. Concordant analyses from the retrograde zonewith low Th/U and minimal influence of older inheritedcores yield a mean age of 233.1� 5.3 Ma. Therefore thesezircon (II) ages are interpreted to date onset of fluid fluxand thus the beginning of the exhumation at or shortly afterthe UHP event. This age is in good agreement with resultsof previous studies, linked to the UHP event (Liu et al.,2004). The second group of concordant ages marks theperiod of a more massif fluid influx, which occurredbetween 214 and 201 Ma, illustrated by the new in situU–Pb ages of this study. The advantage of the in situapproach used in this study is that texture, Th/U character-istics and age of different zircon generations could bedirectly correlated with textural relationships of mineralsindicative of changing P–T conditions from UHP eclogitefacies to the fluid influx during periods of retrograde over-prints. This fluid event started during exhumation at upperamphibolite conditions along a discrete fluid-inducedretrograde zone. P–T condition and age data overlapwith data from previous studies of amphibolitisation(Liu et al., 2008).

This points to an onset of amphibolitisation in localzones along a fluid pathway (1.4 GPa, 700 �C, 214 Ma)earlier than the main retrograde event, which takes placelater and at lower (amphibolite facies) P–T conditionsaccording to Liu et al. (2008) (Fig. 14). In the discreteretrograde zone this early amphibolitisation is locallypreserved. However, ongoing retrogression in the retro-grade zone also produced similar conditions and ages asfor the coherent amphibolite bodies (1.0 GPa, 650 �C,201 Ma; Liu et al., 2008). One concordant U–Pb age of180 Ma can be interpreted as the product of final zirconprecipitation during the greenschist-facies stage. This isconsistent with data from the associated gneisses of theCCSD drill hole (Zheng et al., 2008).

If we compare the P–T estimates made for the retro-grade zone with the estimates for the regional amphibo-lites from the Sulu area (Liu et al., 2008), small but

Fig. 13. (a) Concordia diagram for LA-SF-ICPMS U–Pb core andrim analyses of zircon I þ II from CCSD eclogite retrograde zone.Results are given with 2s error. (b) Concordia diagram for LA-SF-ICPMS U–Pb analyses of zircon II. First group of concordant ages(open ellipses), 201–214 Ma; second group (light grey ellipses),221–246 Ma. (c) Probability plot of zircon II U–Pb ages. Twoepisodes of zircon growth between UHP and HP conditions clusterat 201–214 Ma and 221–246 Ma.

1260 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu

significant differences are obvious. The UHP and eclo-gite conditions are identical, while the amphiboliticretrogression in the investigated fluid bands (retrogradezone) occurred at higher pressures and temperatures(�P . 4 kb; �T 50 �C) than reported for coherentamphibolite bodies (Liu et al., 2008) (Fig. 14). Weinterpret this to indicate an early onset of retrogressionand the start of exhumation along discrete zones.

7.3. Fluid (vs. melts)

Many studies have aimed to characterise the fluids(Ferrando et al., 2005; Zhang et al., 2005, 2006;Xu et al., 2006; Zheng et al., 2007), their geochronology(U–Pb) and petrology in the CCSD core (Liu, 2004, 2008).However, detailed studies combining petrology andgeochronology in a fluid-induced textural context aresparse (Zhang et al., 2005). We investigate a discrete15 cm wide zone of retrogressed UHP eclogite. This zonepreserves geochronological and textural evidence for loca-lized retrogression, presumably induced by externallyderived fluids between 214 to 201 Ma from upper tolower amphibolite-facies conditions. This zone, inter-preted as a fluid-induced retrograde zone, comprises strongevidence for external fluid influx (e.g. fluid inclusions,fluid bearing minerals, Ba-zonation). Since Ba is not avail-able from the eclogite host rocks, but from the surroundinggneisses (Zhang et al., 2006; Liu et al., 2007), this indi-cates an external source for fluids during retrogression.The Ba- and F- bearing fluids (characterized by Ba–F-rich mica and barite) influenced resorption and renewed

growth of zircon, while mineral reactions related to thebreakdown of garnet, omphacite and rutile have acted asinternal sources for zirconium to trigger metamorphiczircon growth.

Hermann et al. (2006) suggested that fluids involved inUHP metamorphism might stem from hydrous melt ofUHP felsic rocks. Similar fluid rock and melt interactionin UHP eclogites associated with metapelites werereported from Greenland (Lang & Gilotti, 2007).Sun et al. (2007) argued that omphacite and apatite-bearingquartz veins in eclogite (from the CCSD drill core) formedthrough deposition of silica rich fluids. Zhang et al. (2008)report that some complex veins hosted in Sulu eclogitescontain mineral assemblages, which crystallised directlyfrom silicate-rich supercritical fluids formed probablyby dehydration of hydrous minerals.

They suggested that many generally fluid-immobileelements might be mobilized during interaction betweensupercritical fluids and rocks under extreme UHP condi-tions. However, Hermann et al. (2006) argued that nearthe critical point (for gneisses � 700 �C, 2.5–3.5 GPa;. 700� for mafic rocks) solubility (transition from aqueousfluid to hydrous melts) changes drastically within atemperature range of 50–100 �C. In felsic rock thecooling path therefore affects the transition from meltto fluid within a very limited range of metamorphicconditions. At high metamorphic conditions, hydrousmelt (in the country-rock gneisses) may coexist with anaqueous fluid (in mafic to ultramafic lenses) (Hermannet al., 2006).

The mafic sample studied here does not show traces ofhydrous melts, but the interacting fluid was enriched inLILE such as Ba, pointing to an external fluid source. Suchan external Ba source is given in the surrounding (para-)gneisses (Zhang et al., 2006) for which also local partialmelting is discussed (Tong et al., 2007).

8. Conclusions

We analyzed a retrograde zone that does not show anyrelics of deformation. In situ dating of petrologically andtexturally well-constrained zircon rims in a retrogressedzone within the UHP eclogite was used to establish therelationships between the different generations of zirconand the degree of retrogression, triggered by fluid influx.This direct correlation of textural information (with impli-cations for the retrograde P–T path for the UHP eclogites)and geochronological data had been unavailable fromprevious geochronological studies carried out on mineralseparates.

Our data document that the beginning of the retrogres-sion already initiated at a deeper level (under UHP condi-tions, in the coesite stability field) than previously deducedfrom amphibolites (Zhang et al., 2006). In addition,geochronological results show that this retrogressionstarted 5 Ma earlier than previously described (Zhanget al., 2006). This documents the beginning of the

T h

e r i

a k - D o m i n o

500 600 700 800 900

Temperature [C]

5000

10000

15000

20000

25000

30000

35000

40000

Pre

ssur

e [B

ar]

219-210 Ma

239-225 Ma

180 Ma?

209-207 Ma

221 - 246 Ma

201 - 214 Ma

coesite

quartz

T h

e r i

a k - D o m i n o

500 600 700 800 900

Temperature [C]

5000

10000

15000

20000

25000

30000

35000

40000

Pre

ssur

e [B

ar]

219-210 Ma

239-225 Ma

180 Ma?

209-207 Ma

221 - 246 Ma

201 - 214 Ma

coesite

quartz

Fig. 14. Compilation of P–T and age estimates for the CCSD corematerial. Dashed boxes: age estimates by Liu et al. (2008). Blacksquares: this study. Fields mark the facies conditions (not the P–Tpath!): dark grey, eclogite facies; grey, amphibolite facies; lightgrey, greenschist facies.

In situ LA-ICP-MS dating of zircon in UHP eclogite, Sulu deep drill hole 1261

exhumation history of UHP eclogites from their site offormation at depth. Our investigation supports the onsetof exhumation during the UHP metamorphic event alongpre-existing zones of rheologic weakness. That this onsetof exhumation occurred under UHP conditions is docu-mented by the few coesite inclusions we found at the edgesof the resorbed zircon cores.

The HP retrogression and amphibolitisation of theselocal fluid movement zones indicate an earlier initiationand different P–T conditions compared to large amphibo-lite bodies with massive fluid infiltration in the Suluregion. Fluids that infiltrated the retrograde zone arecharacterized by F and high Ba contents and thus derivedfrom the surrounding Ba-rich gneisses. Such F-bearingfluids were probably also instrumental in remobilizingZr from pre-existing zircon grains, producing distinctzoning patterns that can texturally be linked to the retro-grade overprint and characteristically low Th–U ratios,typical for fluid-related zircon growth. Additional zirco-nium for retrograde growth was likely made available byretrograde breakdown of eclogite-facies garnet, rutile andomphacite, of which the first two have demonstratedpotential to incorporate elevated concentrations of zirco-nium at high T (e.g., Zack et al., 2004; Tomkins et al.,2005, 2007).

In combination, we interpret our results to indicaterelatively rapid exhumation from UHP to amphiboliteconditions between 233.1 � 5.3 Ma and 207.4 �1.6 Maand possibly subsequent slower exhumation (between207.4 � 1.6 Ma and 180 � 5 Ma) to greenschist-faciesconditions (Fig. 14). This study thus exemplifies theusefulness and facility of in situ analyses within thinsections for detailed correlation of geochronological andpetrological evidence.

Acknowledgements: Research was supported by DGFgrants to RO. We thank J. Hermann and the guest editorsJ. Gilotti and H.-P. Schertl for their suggestions, whichsignificantly helped to improve the manuscript.

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Received 30 January 2009

Modified version received 23 October 2009

Accepted 9 November 2009

1264 A. Riemann, R. Oberhansli, A. Moller, A. Gerdes, Z. Xu