Ž .Chemical Geology 175 2001 703–712www.elsevier.comrlocaterchemgeo

Ion microprobe U–Pb dating of hydrothermal xenotime from anepisyenite: evidence for rift-related reactivation

Jesper Petersson a,), Martin J. Whitehouse b,1, Thomas Eliasson c,2

a Earth Sciences Centre, Mineralogy–Petrology, Goteborg UniÕersity, Box 460, SE-405 30 Goteborg, Sweden¨ ¨b Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

c Geological SurÕey of Sweden, Guldhedsgatan 5A, SE-413 20 Goteborg, Sweden¨

Received 9 December 1999; accepted 13 June 2000

Abstract

Hydrothermally intergrown xenotime–monazite from an episyenite in the Bohus granite, southwest Sweden, has beenanalyzed in situ by ion microprobe. Although the analysed xenotime crystals are extremely small, sometimes less than the

Ž . 206 238ion beam diameter ;17 mm , they retain relatively undisturbed U–Pb systems, which yielded a Pbr U age of 252"8Ma. The evolution of this episyenite involved an initial stage of quartz dissolution accompanied by albitization, followed bya second stage of vug-infilling and replacement of igneous Fe–Ti phases. The obtained age shows that the second alterationstage, during which xenotime was precipitated, was coeval with the thermal anomaly associated with Permo-Carboniferouscrustal thinning and the development of the nearby Oslo Rift. Moreover, the development of the permeable episyenitestructure must be ascribed to a preceding high-temperature event, most likely the post-magmatic cooling of the host graniteat 920 Ma. This study shows that U–Pb dating of young xenotimes may address geochronological problems, though furtherwork is required to characterize possible matrix effects which presently limit the accuracy of the method. q 2001 ElsevierScience B.V. All rights reserved.

Keywords: Episyenite; Xenotime; Chronometry; Ion microprobe; UrPb; Bohus granite

1. Introduction

Episyenites are granite-hosted rocks formed bysubsolidus dissolution and leaching of quartz. Thehighly permeable structure that results from the de-quartzification process makes them ideal as loci for

) Corresponding author. Fax: q46-31-7732849.Ž .E-mail addresses: jesper@geo.gu.se J. Petersson ,

Ž .martin.whitehouse@nrm.se M.J. Whitehouse ,Ž .thomas.eliasson@sgu.se T. Eliasson .

1 Fax: q46-8-51954031.2 Fax: q46-31-200205.

subsequent hydrothermalism and mineral deposition,Ž .especially U-mineralization e.g. Cathelineau, 1986 .

Although episyenites have received a great deal ofŽattention in the last decade e.g. Dempsey et al.,

1990; Turpin et al., 1990; Casquet et al., 1992;Patrier et al., 1997; Recio et al., 1997; Hecht et al.,

.1999 , there remain a number of questions regardingtheir origin and evolution. Precise constraints on theabsolute timing of individual alteration stages iscrucial to understand the genesis of these systems.

Direct radiometric dating of barren episyeniteshas been restricted largely to Rb–Sr techniquesŽTurpin et al., 1990; Casquet et al., 1992; Caballero

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 00 00338-7

( )J. Petersson et al.rChemical Geology 175 2001 703–712704

.et al., 1993 . An alternative that may offer advan-tages is the recent discovery of xenotime precipitated

Žin some episyenites Cathelineau, 1987; Petersson.and Eliasson, 1997; Hecht et al., 1999 . However,

conventional U–Pb dating is hampered by their tex-tural complexity which hinders mechanical separa-tion. Thus, a technique which enables in situ extrac-tion of the isotopic information from small crystals isrequired. The ion microprobe is currently the onlymethod which provides the necessary spatial resolu-tion to resolve this problem. Its application to xeno-time is, however, very limited; only a few Archaeanand Paleoproterozoic 207Pbr206 Pb ages have been

Žreported Compston and Matthai, 1994; Zhu et al.,.1997; Kamber et al., 1998 , although a recent in

Ždepth study of diagenetic xenotime McNaughton et.al., 1999 illustrates the wide potential of this

chronometer. In Europe, where episyenites are fre-quently described, they essentially occur in Hercy-nian granites and are thus relatively young. Ionmicroprobe 207Pbr206 Pb ages from Phanerozoicminerals have unacceptably high uncertainties due tothe relatively slow change in this ratio within ‘young’rocks. As an alternative, U–Pb ages can be used, butthese require an external standard for UrPb ratiocalibration of the ion microprobe. We report here anattempt to obtain 206 Pbr238U ages of hydrothermallyintergrown xenotime–monazite from an episyenitewithin the Bohus granite, southwest Sweden. Thetiming of the episyenite forming event and imprint-ing alteration is further discussed in terms of thetectonothermal evolution of the region.

2. Geological setting

The Bohus granite is a large, elongated graniticcomplex, located along the west coast of SwedenŽ .Fig. 1 . The granite forms a slab-like body, dippinggently beneath Proterozoic polymetamorphic countryrocks, along its eastern and southern margins. Sev-eral distinct granite facies can be recognized; slightlyperaluminous biotite monzogranites with deutericmuscovite predominate. The emplacement of the

Žcomplex, dated at 920"5 Ma 2s ; Eliasson and.Schoberg, 1991 , has been interpreted as a conse-¨

quence of the successive filling of the space created

Fig. 1. Location map of the Bohus granite and the nearby OsloRift, with inset map showing position in Scandinavia. Also shownare the sampling sites of Fjb3 and 88102-5.

by transtensional tectonics accompanying the post-collisional collapse of the Sveconorwegian orogenŽ .Eliasson, 1992 . During the Permo-Carboniferousdevelopment of the nearby Oslo Rift, the Bohusgranite was intruded by a large number of coast-parallel, intermediate to ultramafic dykes, dated at

Ž .;300–275 Ma Sundvoll and Larsen, 1993 .Small bodies of episyenite have been found in the

southern part of the granite complex. Petrographicdetails of these episyenites are given by Petersson

Ž .and Eliasson 1997 . Briefly, the alteration processŽ .proceeded in two stages: 1 an initial dequartzifica-

tion, accompanied by albitization of primary plagio-Ž .clase, leaving a vuggy feldspathic rock, and 2 a

subsequent development of a finely crystalline illiteqhematite"ankerite"anatase assemblage as aprecipitate in the dissolution vugs and the concurrent

( )J. Petersson et al.rChemical Geology 175 2001 703–712 705

Ž . Ž .Fig. 2. Photomicrograph of a typical episyenite texture; an albite Ab qmicrocline Mc framework with a fine-grained interstitialassemblage of illiteqhematite"ankerite"anatase. 461.64 m depth, crossed polarizers.

Ž .breakdown of igneous biotite and oxides Fig. 2 .Hydrothermal overprinting of the deuteric mineral-ogy, including low-grade phases such as pumpel-lyite, implies that at least the second alterationepisode is disconnected from the emplacement and

Žcooling history of the host granite Petersson and.Eliasson, 1997 .

3. Sample description

The selected sample, denoted Fjb3, comes from asubsurface body of episyenite at the Fjallbacka Hot¨

Ž .Dry Rock HDR geothermal research site in theŽcentral part of the Bohus granite complex lat.

X X .58835 340N, long. 11817 400E; RT90-coordinates .The episyenite is intersected by a near vertical coredborehole at 456.9–462.0 m depth.

Ž .Xenotime occurs in two modes: 1 as purelyauthigenic microcrystalline precipitates, and less fre-

Ž .quently 2 as exsolutions in recrystallized monazite.This study involves only xenotime of the first type.

Generally, the authigenic precipitates consist oflath-shaped monazite crystals, 5–30 mm in length,that form polycrystalline aggregates with subordinate

amounts of similar sized sub- to euhedral xenotime.In some instances, however, xenotime is the domi-nant component. The aggregates of monazite–xeno-time are always closely associated with quartz, occu-pying the interstitial positions. These intergrowthsmay reach up to 200 mm in diameter. Small irregular

Ž .blebs -20 mm of apatite occur dispersed in thequartz, and physical contacts with the Th–REEphosphates have not been observed. Florencite isanother characteristic mineral of the assemblage. Mi-nor amounts of pyrite, less than 10 mm in size, aresporadically present in close association with theTh–REE bearing phases.

Ž .Backscattered electron BSE imaging and step-Ž .wise electron microprobe EMP scans of Yb, U and

Th across all crystals selected for ion microprobeanalyses were made using a Cameca Camebax SX50EMP in wavelength dispersive mode. This revealsthat all authigenic xenotime exhibits an internalstructure, with a vaguely zoned interior often sur-rounded by a distinct, ;5 mm wide rim which is

Ž .strongly enriched in REE illustrated by Yb and ThŽ .Fig. 3a and b . Such rims are absent at crystal edgesintergrown with cogenetic phases, such as monazite.Moreover, these rims are characterized by very fine

( )J. Petersson et al.rChemical Geology 175 2001 703–712706

Ž . Ž . Ž .Fig. 3. BSE images and EMP step scans of Yb solid line , U dashed line and Th dot-dashed line for authigenic xenotime crystals ofsample Fjb3. The EMP scans were measured along traverses marked A–AX. Two or three quantitative EMP spot analyses were made along

206 238 Ž .each traverse. Location of ion microprobe analysed areas are indicated by dotted outlines; Pbr U ages are shown with 1s errors. a( ) Ž . Ž .Euhedral xenotime from aggregate 132 intergrown with monazite white . Bright grey areas are enriched in REE i.e. Yb and Th. b

Ž . ( )Euhedral xenotime aggregate 85 with a very finely oscillatory-zoned rim bright , enriched in REE and Th. Note the bright, porous areaswith extreme Yb content. The anomalous 206 Pbr238 U date of 37"2 is possibly the result of recent Pb-loss associated with these structures.

oscillatory zoning parallel to the crystal faces andwith sharp transition to the relatively darker centraldomain. Two or three quantitative EMP spot analy-ses were done along the scan, one at the maximumBSE intensity and the other at the lowest intensity,where the Th content is typically below detection

Ž .limit ;700 ppm . Some of the larger grains containbright, porous ‘cores’, in which REE and Th reach

Ž .strikingly high values Fig. 3b . Thus, the BSE con-trast is directly attributable to the distribution of Thand, to some extent REE. The behaviour of U,however, is not coherent with the other elements,and its spatial variations is not correlated by BSE.

4. Analytical techniques

The grain mount initially examined by EMP wasre-polished and coated with ;30 nm of gold for ionmicroprobe U–Pb chronometry. The analyses wereperformed using the high-resolutionrhigh-sensitivity

Ž .CAMECA ims 1270 instrument NORDSIM facilityat the Swedish Museum of Natural History. Operat-

ing and data-reduction procedures followed thoseŽ .outlined by Schuhmacher et al. 1994 and White-

Ž .house et al. 1997a for zircon. In order to achieve aflat-bottomed spot with homogeneous primary beamdensity and a diameter less than 20 mm, as requiredfor the fine-grained xenotime, a 50-mm aperture wasinserted into the defocussed primary beam. The re-

Ž y.sulting ion current O was in the range 1.4–2.82

nA. Sample changing effects were minimised byoptimising the energy offset to maximum transmis-sion in the "15 eV energy window at the start ofeach analysis using the Y O reference peak at mass2

194; significant changes in inter-element ratios dur-ing individual analyses, which would be indicative ofcharging, were not observed. Molecular interferencesfrom HREEOq are sufficiently resolved at a mass2

resolving power of about 4500 at 1% peak heightŽ .Fig. 4 . To avoid intergrown monazite and locatexenotimes, the fine adjustment of the ion beamposition was done by mapping the CeO and YO,using the secondary ion imaging capability of theims 1270’s micro-channel plate.

( )J. Petersson et al.rChemical Geology 175 2001 703–712 707

Fig. 4. Mass spectra in the regions of the Pb masses for the 88102-5 xenotime. A mass resolving power of ;4500 at 1% peak height issufficient to resolve interferences from HREEOq.2

The procedure used for calibrating the 206 Pbr238Uratios in xenotime is analogous to that for zircon andis based on the observed correlation between206 q 238 q 270 q 238 q ŽPb r U and UO r U Claoue-Long et´2

.al., 1995; Whitehouse et al., 1997a . In Fig. 5, weplot the analytical data from three potential standards

Ž206 q 238 q. Ž270 q 238 q.in a log Pb r U vs. log UO r U di-2

agram, from which it is clear that a power law206 238 Ž270 qrelationship in the form Pbr Ura UO r2

238 q.b ŽU , previously adopted for zircon Claoue-Long´.et al., 1995; Whitehouse et al., 1999 , monazite

Ž . Ž .Williams et al., 1996 and apatite Sano et al., 1999is applicable also to xenotime. Using all of the data

Žpresented in Fig. 5 i.e. five separate data sets from.three samples , we applied the method of least

Žsquares fitting of multiple parallel lines Titterington.and Halliday, 1979 to obtain a value for the power

Ž . Ž .law exponent b of 1.24"0.03 2s . The rela-tively high MSWD value of 16 for this regressionprobably reflects scatter of one or more of the indi-vidual data sets. This number is the same, withinerror, as that obtained from a similar multiple data

Žset regression exercise performed on zircons M.J..Whitehouse, unpublished data and suggests that the

power law exponent is largely independent of matrix.We have not analysed xenotime over a sufficientlength of time to observe long-term stability of theexponent but, by analogy with far more extensivezircon U–Pb analyses undertaken in Stockholm andby other SIMS laboratories, we expect this value to

be unaffected by factors such as instrument tuningŽassuming that this is always optimised for transmis-

.sion , primary source maintenance, electron multi-

Fig. 5. Logarithmic calibration graph of 206 Pbqr238 Uq vs.270 UOqr238 Uq summarizing measurements for five data sets2

from 88102-5, AG9304 and FC-1 xenotimes. Conventional TIMSŽdata for reference samples AG9304 and FC-1 generously donated

.by G.N. Hanson and R.R. Parrish can be found in Sevigny andŽ . Ž .Hanson 1995 and Parrish 1990 , respectively. The range in

270 UOqr238 Uq was enhanced by varying the oxygen pressure in2Žthe sample chamber during the calibration exercise cf. Schuh-

.macher et al., 1994; Whitehouse et al., 1997b . Combined multi-ple regressions through the data yielded a power law exponent of

Ž .1.24"0.03 2s from the slopes.

( )J. Petersson et al.rChemical Geology 175 2001 703–712708

plier exchange or operator characteristics. For eachanalytical session, the value of this exponent wasapplied to the average of the standard analyses forthat session to yield a calibration line. Errors on thisaverage were propagated together with the greater ofthe observed or Poisson errors from the unknown togive an overall error for the PbrU ratio of eachanalysis.

A multi-crystal xenotime sample, 88102-5, col-lected from an intra-plutonic pegmatite–aplite dykein the southernmost Bohus granite, was selected asour primary 206 Pbr238U calibration standard. Con-ventional U–Pb analysis of this sample by Eliasson

Ž . 206and Schoberg 1991 yielded a concordant Pbr¨238 Ž .U age of 919"5 Ma 2s . All but two of the 20ion-microprobe analyses of this material had206 Pbr204 Pb ratios in excess of 105 and so perturba-tion of ages due to common Pb is negligible.Weighted average 207Pbr206 Pb ages yield 934"4

Ž .Ma 2s although the MSWD value of 1.8 suggestsan external source of error andror underestimatedanalytical errors. The latter is considered less likely,however, because throughout this study, we haveused observed analytical errors rather than countingstatistic errors. Bearing this in mind, we calculate arobust outlier independent Tukey’s Biweight mean

Ž .age value of 934"6 Ma 95% confidence whichŽ .ignores assigned analytical errors Ludwig, 1999 .

For the assessment of the 206 Pbr238U ages, we haveused an ‘internal’ calibration, treating each analysisin turn as an unknown with the remainder as stan-dards. This yields a weighted average age of 935"16

Ž . Ž .Ma 2s , MSWDs14 and a robust biweight meanŽ .age of 917"15 Ma 95% confidence . Although in

this paper we deal solely with U–Pb ages, whichwill be unaffected by instrumental mass fractionationandror 206 PbH interference on 207Pb, the closeagreement between this age and the conventional ageindicates that perturbation of the Pb isotopic ratiosdue to either of these effects is minimal. Using88102-5 as standard, we analysed xenotime sample

Ž .FC-1 with a known age of 54.5"0.3 Ma 2sŽ . 206Parrish, 1990 . A weighted mean of seven Pbr238U ion probe analyses defines an age of 58.4"1.1Ž .2s ; MSWDs0.45 . The age bias of ;7% mightbe related to heterogeneities of a multi-grain stan-

Ž .dard 88102-5 or more likely a matrix effect due tothe substantially different U content between the

Žstandard 88102-5 2944 ppm; Eliasson and Schoberg,¨. Ž .1991 and FC-1 1–2%; EMP analyses . The latter

explanation is strongly supported by the U–Pb cali-Žbration attempts of McNaughton et al. 1999; supple-

.mentary data , which disclose the existence of amatrix effect arising from differences in the U con-tent between standard and sample.

Corrections for common Pb, where made, assumethe present-day terrestrial average Pb-isotopic com-

Ž .position of Stacey and Kramers 1975 , i.e.207Pbr206 Pbs0.83, with a conservative and arbi-

Ž .trarily assigned error of "0.1 1s . All ages werecalculated using the decay constants recommended

Ž .by Steiger and Jager 1977 and the calculation¨Ž .routines of IsoplotrEx Ludwig, 1999 . Uncertain-

ties in ratios and ages obtained by the ion micro-probe are quoted at 1s level and 206 Pbr238U agesinclude the external error propagated from the stan-dard, which is generally the dominant factor in thiserror. Resulting regression and weighted average

Ž .ages are at 95% 2s confidence limits.

5. Results and interpretation

A total of 25 analyses were performed on xeno-Ž .time crystals from 18 aggregates Table 1 . Common

Pb contents may be estimated from the 206 Pbr204 Pbratio and are expressed in Table 1 by f , which is206

the percentage of 206 Pb in the xenotime that has notbeen produced by in situ decay. This values ranges

Ž .from 0 no common Pb to ;1.1%. The validity ofŽ .using the Stacey and Kramers 1975 composition of

modern terrestrial average Pb for common Pb correc-tion has been tested for data from sample Fjb3 byregressing uncorrected 207Pbr206 Pb ratios against

204 206 Ž .measured Pbr Pb ratios Fig. 6a . This exerciseŽ .yields a regression line MSWDs3.2 intersecting

the modern terrestrial 204 Pbr206 Pb of ;0.0543 at a207Pbr206 Pb of 0.87"0.30, which clearly encom-

207 206 Žpasses the expected Pbr Pb of 0.83 this inter-section and the regression line MSWD change, re-spectively, to 0.78"0.30 and 1.7 using selectedpoints as described below, still within error of the

. 207expected value . In Table 1, we also present Pb-corrected concordia model ages which are calculatedby projecting from the assumed common Pb compo-sition through the data point onto concordia. The

( )J. Petersson et al.rChemical Geology 175 2001 703–712 709

Table 1Ion microprobe U–Pb data for xenotimes of sample Fjb3

Ž .Grain a Isotope ratios Ages Ma238 206 a 207 206 b 206 204 c 206 238 dŽ . Ž .Ur Pb "s % Pbr Pb "s % Pbr Pb f % Pbr U 207-corr.206

9 27.11 7.2 0.05041 1.7 31,358 0.06 234"17 234"1744 23.23 2.7 0.05678 0.7 1,612 1.16 272"7 270"747 26.25 2.6 0.05092 0.8 27,563 0.07 241"6 241"656 27.42 4.1 0.06106 1.8 1,758 1.06 231"9 228"962a 25.42 2.6 0.05225 0.9 14,771 0.13 249"6 248"662b 24.70 2.6 0.05237 0.8 17,150 0.11 256"7 256"767 24.41 4.1 0.05270 2.6 4,148 0.45 259"10 258"1074 41.68 6.2 0.05461 1.5 7,788 0.24 153"9 152"981a 11.71 5.4 0.04928 1.4 5,760 0.32 528"28 534"2881b 7.04 4.6 0.05082 1.0 14,857 0.13 856"37 874"3985 173.88 4.7 0.05262 1.2 11,221 0.17 37"2 37"287 46.71 4.2 0.05596 1.0 4,796 0.39 137"6 135"690 25.96 2.7 0.05788 0.9 1,694 1.10 244"6 242"6100a 24.78 2.6 0.05293 1.0 8,026 0.23 255"6 254"7100b 28.44 2.9 0.05374 1.3 5,851 0.32 223"6 222"6100c 23.96 2.7 0.05433 0.8 7,117 0.26 264"7 263"7104 21.40 2.7 0.05695 0.8 2,565 0.73 294"8 293"8113 25.42 2.7 0.05398 0.6 4,097 0.46 249"7 248"7115 23.72 8.8 0.05546 1.5 4,926 0.38 266"23 265"23124 23.38 2.7 0.05738 0.7 1,890 0.99 270"7 268"7125a 27.16 2.6 0.05410 1.3 6,414 0.29 233"6 232"6125b 23.26 2.7 0.05272 0.6 8,467 0.22 271"7 271"7127a 25.78 2.8 0.05246 3.1 7,645 0.24 245"7 245"7127b 23.71 2.8 0.05254 0.9 25,667 0.07 266"7 266"7

6132 26.43 4.1 0.05172 2.9 )10 0.00 239"10 239"10

Errors on ages are 1s .a Error includes external variability in standard.bObserved analytical errors.c 206 Ž .Percentage of unsupported Pb common Pb .dAssumes concordance of true radiogenic ratios.

difference between these ages and reported 206 Pbr238U ages is generally less than ;1 Ma, showingthat common Pb does not unduly influence the inter-pretation of geochronological data from this sample.

All data from Fjb3 are presented, uncorrected forcommon Pb, in an inverse concordia diagramŽ207 206 238 206 .Pbr Pb vs. Ur Pb in Fig. 6b. These dataclearly comprise a main group of 20 points extend-ing above concordia at ;250 Ma. Regression ofthese points with the assumed common Pb composi-tion yields a concordia intercept age of 255"12 MaŽ . 2072s , MSWDs5.5 . A weighted average of Pb-corrected ages yields a robust, outlier independent

Ž .age of 252"8 Ma 95% confidence using theŽ .Tukey-biweight algorithm of Isoplot Ludwig, 1999 .

The degree of scatter in the regression is somewhathigher than would be expected from analytical errors

alone. Two factors that might affect this scatter aresuggested by the outliers in the data set, which wereomitted from the regression. Three of these outliersŽ .analyses 74, 85 and 87 plot to the right of the maingroup in Fig. 6b and are assumed to be affected byrecent Pb-loss, consistent with the observation that

Ž .these were slightly porous crystals see Fig. 3b . TheŽother two outlying data points analyses 81a and

.81b plot below concordia to the left of the maingroup, and yield much older concordia model ages;207Pbr206 Pb ratios of these two points are similar tothose of the main group and it may be possible to

Ž .explain their position by Pb-gain andror U-loss ,although we can offer no independent evidence forsuch a process occurring. Alternatively, they mightsimply represent an analytical problem related toincorrect mass calibration of 238U although, since the

( )J. Petersson et al.rChemical Geology 175 2001 703–712710

Ž . 207 206Fig. 6. a Bivariate plot of uncorrected Pbr Pb vs.204 Pbr206 Pb for xenotime from Fjb3. Regression of this data

Ž .yields a line MSWDs3.2 intersecting the modern terrestrial204 Pbr206 Pb of ;0.0543 at a 207Pbr206 Pb of 0.87"0.30, whichis within error of the expected 207Pbr206 Pb of 0.83. Error bars are

Ž .1s . b Inverse concordia diagram showing analytical data forFjb3 xenotimes. The majority of the data are dispersed along amixing line between assumed common Pb composition and aconcordia intersection at ;250 Ma. Regression of the main group

Ž .of 20 analyses shaded circles yields a concordia intercept age ofŽ .255"12 Ma 2s , MSWDs5.5 . Error bars are 1s , and concor-

dia tick marks are at 40 Ma.

two analyses are from the same crystal and themagnet is calibrated before each analysis during highmass-resolution running, the latter seems unlikely.The U dependent matrix effect, as suggested by both

Ž .the data of FC-1 and McNaughton et al. 1999 , canbe assumed to be insignificant for Fjb3, having

approximately the same U content as reference stan-dard 88102-5. However, we cannot exclude the exis-tence of minor Th andror REE dependent matrixeffects.

Despite the outliers and evident scatter in the dataset, we consider the 252"8 Ma robust average as areliable estimate of the age of the xenotime in sam-ple Fjb3. This corresponds well with the ages of the

Žepigenetic mineralizations within the Oslo Rift ;

200–250 Ma; Rohrman et al., 1994 and references.therein . It therefore seems reasonable to infer that

the thermal anomaly associated with the Permo-Carboniferous crustal extension triggered the fluidcirculation and thereby inducing an alteration pro-cess. The rift-related dykes in the region are, how-

Ž Ž .ever, older 275"12 Ma 2s ; Sundvoll and Larsen,.1993 and their thermal influence was probably neg-

ligible since most of them are too thin to develophydrothermal circulation before complete coolingŽ .Delaney, 1988 . This view is strongly supported bythe unaltered nature of the dyke wall rock and theabsence of direct spatial relationship between dykesand episyenites.

Judging from textural relationships, xenotimeprecipitation appears to be coeval with the illite–hematite–ankerite–anatase assemblage, which char-

Žacterize the second alteration stage Eliasson and.Petersson, 1997 . Depending on whether the episyen-

ite system evolved continuously or if the two alter-ation stages are temporally distinct, the Late Permianage can either be taken to represent the time ofepisyenite formation or a later, separate period ofreactivation. It is widely accepted that the quartzdissolution resulting in episyenites occurs at temper-

Žatures between 3508 and 4508C Cathelineau, 1986;.El Jarray et al., 1994; Recio et al., 1997 ; thermal

conditions which seem highly unlikely in hydrother-Žmal systems flanking the actual rift area cf. Ihlen,

.1986; Rohrman et al., 1994 . This is further sup-ported by the low-temperature assemblages charac-terizing other rift-margin alterations, comprised byepidotization, chloritization as well as sericitic andargillitic alteration, mainly associated with small epi-genetic vein deposits carrying Fe-oxides, base metal

Ž .sulphides and native silver e.g. Ihlen, 1986 . Thisinformation suggests that the episyenite system wasreactivated during the rift-related tectono-thermalregime. Thus, the initial episyenitization must be

( )J. Petersson et al.rChemical Geology 175 2001 703–712 711

ascribed to an earlier high temperature event, proba-bly the post-magmatic cooling of the host granite.

On the basis of Rb–Sr isochron ages, severalworkers have proposed that the episyenites of theSierra del Guadarrama in the Iberian Hercynian beltŽ .Caballero et al., 1993; Gonzalez-Casado et al., 1996´and the Hercynian Saint-Sylvestre granitic complex

Ž .in the French Massif Central Turpin et al., 1990formed during extensional regimes, disconnectedfrom the cooling history of the host granites, contem-poraneously with the emplacement of dykes. How-ever, such a genetic association seems implausiblebecause the thermal input from the dyke injection orcrustal thinning is far too low to reach the necessarytemperatures for episyenite formation. Moreover,since most episyenite systems have experienced asecond imprinting stage of intense alteration androrvug-filling, it is often impossible to unravel thenature of the initial stage, mainly characterized byquartz dissolution. Thus, a more likely interpretationis that the Rb–Sr ages, as the U–Pb age in thepresent study, record a second episode of superim-posed hydrothermal interaction, genetically unrelatedwith the preceding episyenitization.

6. Conclusions

Ion microprobe U–Pb chronometry of xenotimehas great potential for providing isotopic data fromcomplex intergrowths, being common in hydrother-mal deposits. Unlike more well-established methodsŽ .i.e. the Rb–Sr, Ar–Ar and K–Ar systems , it en-ables direct dating of an ore-stage mineral, which ishighly resistant to later alteration. The 88102-5 xeno-times used as a reference standard in our work aregood enough to obtain meaningful and reasonablyprecise isotope data. The technique is, however, stillin its infancy and a rigorous assessment of, forexample, possible matrix effects and hydride inter-ference must remain a goal for further investigations.

Twenty of 25 ion microprobe analyses of xeno-time crystals from an episyenite in the Bohus graniteyielded a Permian age. Estimates of the temperature

Ž .conditions during episyenitization e.g. 3508–4508Csuggests that a second superimposed stage of alter-ation resulted from the thermal pulse associated withthe crustal stretching which generated the Oslo Rift.

This gives support to the idea of pre-existingepisyenite structures acting as a focus for hydrother-mal activity during later, independent thermal events,in this case, up to ;670 m.y. after the initialepisyenite formation.

Acknowledgements

The authors are deeply grateful to J. Vestin and T.Sunde for analytical assistance during the sessionson the NORDSIM facility. We would also like tothank K. Helge for his invaluable help with samplepreparation. H. Harryson kindly performed the EMPwork. We are also indebted to R.R. Parrish, A.Schersten, H. Lokrantz for critical review on earlier´versions of this paper. Constructive journal reviewsby D. Rubatto and M. Wiedenbeck are greatly appre-ciated. The NORDSIM ion microprobe facility isjointly financed by the research councils in the Nordic

Ž .countries Denmark, Finland, Norway and Sweden ,as well as Knut and Alice Wallenberg Foundation.This study was supported in part by grants from Carl

Ž .Trygger Foundation No. 98:258 to J.P., andŽ .NUTEK No. P5431-1 to T.E. This is NORDSIM

laboratory contribution No. 29.

References

Caballero, J.M., Casquest, C., Galindo, C., Gonzalez-Casado,J.M., Pankhurst, R., Tornos, F., 1993. Geocronologia por elmetodo Rb–Sr de las episienitas de la Sierra del Guadarrama,´S.C.E., Espana. Geogaceta 13, 16–18.˜

Casquet, C., Caballero, J.M., Galindo, C., Tornos, F., 1992. Arevised model for the formation of dequartzified and alkalin-

Ž .ized granites episyenites . In: Kharaka, Y.K., Maest, A.S.Ž .Eds. , Water–Rock Interaction. Balkema, Rotterdam, pp.1481–1484.

Cathelineau, M., 1986. The hydrothermal alkali metasomatismeffects on granitic rocks: quartz dissolution and related sub-solidus changes. J. Petrol. 27, 945–965.

Cathelineau, M., 1987. U–Th–REE mobility during albitizationand quartz dissolution in granitoids: evidence from south-eastFrench Massif Central. Bull. Mineral. 110, 249–259.´

Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M.,´1995. Two Carboniferous ages: a comparison of SHRIMPzircon dating with conventional zircon ages and 40Arr39Aranalysis. In: Berggren, W.A., Kent, D.V., Aubry, M.P., Hard-

Ž .enbol, J. Eds. , Geochronology, Time Scales and GlobalStratigraphic Correlation. Society for Sedimentary GeologyŽ .SEPM , Spec. Publ., vol. 54, pp. 3–21.

( )J. Petersson et al.rChemical Geology 175 2001 703–712712

Compston, D.M., Matthai, S.K., 1994. U–Pb age constraints onearly Proterozoic gold deposits, Pine Creek Inlier, northernAustralia, by hydrothermal zircon, xenotime, and monazite.

Ž .U.S. Geol. Surv. Circ. 1107, 65 Abstr. .Delaney, P.T., 1988. Heat transfer during emplacement and cool-

Ž .ing of mafic dykes. In: Hall, H.C., Fahrig, W.F. Eds. , MaficDyke Swarms. Geol. Assoc. Can., Spec. Pap., vol. 34, pp.31–46.

Dempsey, C.S., Meighan, I.G., Fallick, A.E., 1990. Desilication ofCaledonian granites in the Barnesmore complex, Co. Donegal:the origin and significance of metasomatic syenite bodies.Geol. J. 25, 371–380.

Eliasson, T., 1992. Magma genesis and emplacement character-istics of the peraluminous Sveconorwegian Bohus granite, SWSweden. Geol. Foren. Stockholm Forh. 114, 452–455.¨ ¨

Eliasson, T., Schoberg, H., 1991. U–Pb dating of the post-kine-¨Ž .matic Sveconorwegian Grenvillian Bohus granite, SW Swe-

den: evidence of restitic zircon. Precambrian Res. 51, 337–350.El Jarray, A., Boiron, M.-C., Cathelineau, M., 1994. Percolation

microfissurale de vapeurs aqueuses dans le granite de Peny´Ž .Massif de Saint Sylvestre, Massif Central : relation avec la

Ž .dissolution du quartz. C.R. Acad. Sci. Paris 318 serie II ,1095–1102.

Gonzalez-Casado, J.M., Caballero, J.M., Casquet, C., Galindo, C.,´Tornos, F., 1996. Paleostresses and geotectonic interpretationof the Alpine Cycle onset in the Sierra del GuadarramaŽ .eastern Iberian Central System , based on evidence fromepisyenites. Tectonophysics 262, 213–229.

Hecht, L., Thuro, K., Plinninger, R., Cuney, M., 1999. Mineralog-ical and geochemical characteristics of hydrothermal alterationand episyenitization in the Konigshain granites, northern Bo-¨hemian Massif, Germany. Int. J. Earth Sci. 88, 236–252.

Ihlen, P.M., 1986. The geological evolution and metallogeny ofŽ .the Oslo Paleorift. In: Olerud, S., Ihlen, P.M Eds. , Metal-

logeny Associated with the Oslo Paleorift. Sver. Geol. Unders.59, pp. 6–17.

Kamber, B.S., Frei, R., Gibb, A.J., 1998. Pitfalls and new ap-proaches in granulite chronometry: an example from theLimpopo Belt, Zimbabwe. Precambrian Res. 91, 269–285.

Ludwig, K.R., 1999. IsoplotrEx — a geochronological toolkit forMicrosoft Excel. Berkeley Geochronol. Center Spec. Publ. 1a.

McNaughton, N.J., Rasmussen, B., Fletcher, I.R., 1999. Datingsiliciclastic sedimentary rocks by SHRIMP U–Pb geochronol-ogy of early diagenetic xenotime. Science 285, 78–80.

Parrish, R.R., 1990. U–Pb dating of monazite and its applicationto geological problems. Can. J. Earth Sci. 27, 1431–1450.

Patrier, P., Beaufort, D., Bril, H., Bonhomme, M., Fouillac, A.M.,Aumaitre, R., 1997. Alteration–mineralization at the Bernar-ˆ

Ž .dan U deposit Western Marche, France : the contribution ofalteration petrology and crystal chemistry of secondary phasesto a new genetic model. Econ. Geol. 92, 448–467.

Petersson, J., Eliasson, T., 1997. Mineral evolution and elementŽ .mobility during episyenitization dequartzification and albiti-

zation in the post-kinematic Bohus granite, SW Sweden. Lithos42, 123–146.

Recio, C., Fallick, A.E., Ugidos, J.M., Stephens, W.E., 1997.Characterization of multiple fluid–granite interaction pro-cesses in the episyenites of Avila-Bejar, Central Iberian Mas-´sif, Spain. Chem. Geol. 143, 127–144.

Rohrman, M., van der Beek, P., Andriessen, P., 1994. Syn-riftthermal structure and post-rift evolution of the Oslo RiftŽ .southeast Norway : new constraints from fission track ther-mochronology. Earth Planet. Sci. Lett. 127, 39–54.

Sano, Y., Oyama, T., Terada, K., Hidaka, H., 1999. Ion micro-probe U–Pb dating of apatite. Chem. Geol. 153, 249–258.

Schuhmacher, M., de Chambost, E., McKeegan, K.D., Harrison,T.M., Migeon, H., 1994. In situ UrPb dating of zircon withthe CAMECA ims 1270. In: Benninghoven, A., Nihei, Y.,

Ž .Shimizu, R., Werner, H.W. Eds. , Secondary Ion Mass Spec-trometry — SIMS IX. Wiley, Chichester, pp. 919–922.

Sevigny, J.H., Hanson, G.H., 1995. Late-Taconian and pre-Acadian history of the New England Appalachians of south-western Connecticut. Geol. Soc. Am. Bull. 107, 487–498.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestriallead isotope evolution by a two-stage model. Earth Planet. Sci.Lett. 26, 207–221.

Steiger, R.H., Jager, E., 1977. Subcommission on geochronology:¨convention on the use of decay constants in geo- and cos-mochronology. Earth Planet. Sci. Lett. 36, 359–362.

Sundvoll, B., Larsen, B.T., 1993. Rb–Sr and Sm–Nd relationshipsin dyke and sill intrusions in the Oslo Rift and related areas.Norg. Geol. Unders. Bull. 425, 25–42.

Titterington, D.M., Halliday, A.N., 1979. On the fitting of parallelisochrons and the method of maximum likelihood. Chem.Geol. 26, 183–195.

Turpin, L., Leroy, J.L., Sheppard, S.M.F., 1990. Isotopic system-Ž .atics O, H, C, Sr, Nd of superimposed barren and U-bearing

hydrothermal systems in a Hercynian granite, Massif Central,France. Chem. Geol. 88, 85–98.

Whitehouse, M.J., Claesson, S., Sunde, T., Vestin, J., 1997a. Ionmicroprobe U–Pb zircon geochronology and correlation ofArchaean gneisses from the Lewisian Complex of GruinardBay, northwestern Scotland. Geochim. Cosmochim. Acta 61,4429–4438.

Whitehouse, M.J., Claesson, S., Sunde, T., Vestin, J., 1997b.U–Pb calibration of zircons analysed by SIMS-Observations

Žfrom the Nordic Cameca IMS1270. Geol. Assoc. Can. Ottawa. Ž .97 22, A157 abstr. .

Whitehouse, M.J., Kamber, B., Moorbath, S., 1999. Age signifi-cance of U–Th–Pb zircon data from early Archaean rocks ofwest Greenland — a reassessment based on combined ion-mi-croprobe and imaging studies. Chem. Geol. 160, 201–224.

Williams, I.S., Buick, I.S., Cartwright, I., 1996. An extendedepisode of early Mesoproterozoic metamorphic fluid flow inthe Reynolds Range, central Australia. J. Metamorph. Geol.14, 29–47.

Zhu, X.K., O’Nions, R.K., Belshaw, N.S., Gibb, A.J., 1997.Lewisian crustal history from in situ SIMS mineral chronome-try and related metamorphic textures. Chem. Geol. 136, 205–218.