- Email: [email protected]
Pb zircon geochronology
EPSL ELSEVIER
Earth and Planetary
Science Letters 151 (1997) 191-203
Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology D.B. Rowley ‘**, F. Xue a, R.D. Tucker b, Z.X. Peng b, J. Baker ‘, A. Davis d a Department of the Geophysical Sciences, The (/nicer&y of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637. USA h Department of Earth and Planetaty Sciences, Washington University, St. Louis, MO 63130. USA ’ Department of Earth Sciences. University of Cambridge, Cambridge, CB2 3EQ. United Kingdom d Enrico Fermi Institute. The University of Chicago, 5640 S. Ellis Avenue, Chicugo. IL 60637. USA Received 21 May 1997: revised 14 July 1997; accepted
14 July 1997
Abstract The Dabie Shan contains two rare geologic features. One, the occurrence of ultra-high pressure metamorphic rocks (peak P > 2.7 GPa = UHP) in the east-central part of this belt, and two, some of these UHP rocks are characterized by anomalously ‘XO-depleted oxygen isotopic compositions. Geochronological data are presented that constrain the ‘age of the UHP metamorphism, the ages of protoliths, and potentially the age of hydrothermal alteration believed to be respdlnsible for the anomalous oxygen isotopic compositions. The age of UHP metamorphism in the eastern Dabie Shan ifi dated at 21 X.5 + 1.7 Ma using U/Pb in zircon separated from host gneisses. This age is identical within uncertainty Cith a less precise age using U/Pb in zircon separated from UHP eclogite (225.5 + 3/ - 6 Ma) and with previously published ages from UHP eclogites of the Dabie Shan. Map relationships strongly support an in-situ, as opposed to exotic. origin of the UHP eclogites in this region. The identical ages of UHP metamorphism in the gneisses and eclogites provide? additional support for this interpretation. The age of the protolith gneisses is dated as 772.5 f 9.5 Ma based on the upper ibtercept of the zircons which accords well with other dates of granitic gneisses in the Qinling belt. The association of mafici and felsic magmatism is interpreted to reflect rifting along the northern margin of the Yangtze block at this time. These upp r intercept ages may date the hydrothermal circulation of ‘*O-depleted meteoric water which gave rise to the anomalously Ixe, -depleted UHP protoliths of the Dabie Shan and Sulu region. The coincidence of these protolith ages with the early Siniarl glaciation of South China is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this legion date from this time. The upper intercept age of the eclogite is imprecisely determined at 447 + 82/ - 79 Ma but is yaunger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions. 0 1997 Elsevier Science;B.V. Kew~wdsr Dahie Mountains: tectonics; earths; rclogite: regional metamorphism
’ Corresponding
author. Tel.: +
plate collision:
geochronology;
U/Pb;
Triassic:
upper Proterozoic;
I 3 12 702 7071. fax: + I 312 702 9505. E-mail: [email protected]
0012-821x/97/$17.00 0 1997 El sevier Science B.V. All rights reserved PII SOOl2-821X(97)00128-3
carhodolumin&cence:
rare
South Dabie & Susong Complexes
D.B. Rowley et al./ Earth and Planetan
one of the critical problems is whether the present close spatial arrangement of these different assemblages records post-tectonic juxtaposition (so-called exotic origin) [3] or differential reequilibration following peak pressure (so-called in situ origin) [4]. In this paper we present results of U/Pb zircon dating of the UHP metamorphism (UHPM) with the intent of addressing the in situ [5,6l versus exotic [7] nature of the UHP-bearing assemblage of the Dabie Shan. The Dabie Shan and Sulu region not only contain UHP-bearing metamorphic assemblages but some of these units are also characterized by some of the most ‘XO-depleted metamorphic rocks in the world [8,9]. This coincidence of UHPM and extreme oxygen isotopic compositions naturally raises questions regarding the potential relationship between these two occurrences. Existing interpretations of the anomalous oxygen isotopic compositions favor a premetamorphic, hydrothermally-driven alteration event completely independent of UHPM [8,9]. Our upper intercept age provides a potential link between the age of the protolith magmatism and the age of glaciation of the South China block during the early Sinian. and thus provides a rationale for their coinci-dence.
2. Regional geology The geology of the eastern Dabie Shan comprises a succession of metamorphic rocks characterized by increasing peak pressures ranging from transitional blueschist-greenschist facies in the south, through amphibolite to high pressure (HP < 2.7 GPa) eclogite and UHP coesite eclogite facies farther north (Fig. 18). These metamorphic rocks are characterized by a variety of granitic to tonalitic gneiss, talc-silicate gneiss. mafic and locally ultramafic eclogite. and clearly supracrustal metapelite, metapsamite, marble and metaphosphorite. These lithologies are interpreted to represent metamorphosed sections of the Yangtze basement and overlying cover succession. To the north and west of the South Dabie Complex is the North Dabie Complex that is dominated by Early Cretaceous felsic to intermediate plutons but with evidence of Proterozoic protoliths in their source area [ 101 as well as limited regions of older metamorphic [I 1] and plutonic rocks [ 101.
Sciencr Letters
151 11’871 IYI--103
193
Detailed mapping in the eastern Dabie Shan [6] has demonstrated that the UHPM terrain is structurally characterized by a series of nearly reclined, tight to isoclinal folds with moderate S-dipping axial surfaces that developed during regionally pervasive amphibolite facies retrogression. These folds deform earlier structures and fabrics. at least some’of which are characterized by associated UHPM minieralogies. Some of the earlier structures that are interpreted to have developed at or near UHPM conditions include folds. boudinage and early, regionally significant S-vergent thrusts [6]. Xue et al. [6] arguedlthat both the eclogitic rocks that still preserve their lrecord of UHPM assemblages as well as the associated, and more voluminous country gneisses. and less voluminous marbles and talc-silicate gneiss all share the same pressure-temperature-time ( PTr ) path as they all share the same deformation history. This detailed mapping also revealed an intact intrusive contact between at least one phase of the tonalitic orthogneiss and a mafic eclogite boudin from an outcrop within IO m of the sample (95029D) whose age is reported here. At least in this specific case it is possible to demonstrate that both the tonalitic orthogneiss and eclogite shared the same P7’r history. Oxygen isotope geochemistry carried out in association with our detailed mapping is reported elsewhere [9]. Baker et al. [9] report the discovery of extremely anomalous (6”O + 5%) oxygen isotope values in some of the eclogites and surrounding cotmtry rock gneisses in the UHP terrain of the eastern Dabie Shan and Yui et al. [8,12] and Rumble et al. [13] reports similar findings in the Sulu regiion. These discoveries raise the additional question as, to the age and origin of this oxygen isotope anomalb. We use the upper intercept ages based on our U/Ph zircon geochronology to date this event.
3. U/Pb
zircon geochronology
In this paper we report ages of zircons derived from three samples of the tonalitic gneisses of the UHPM terrain as well as one UHPM eclogite of the eastern Dabie Shan (Fig. IB). Two splits of the same block (93023 and 93023~) are from east of Wumiao at 30.650”N, 116.359”E in the northern part of the UHPM terrain. Zircon was separated from gray
194
D.B. Rowley et al/Earth
and Planetary Science Letters 15I (1997) 191-203
Fig. 2. Cathodoluminescence images of representative zircons from both the gneissic and eclogitic populations analyzed. (a) Zircon from 95029D with well defined bright, zoned core surrounded by darker mantling rim. Inclusions in the core of this zircon are apatite. (b) Zircon from 93023 lacks a zoned core and instead displays an irregular zonation comparable to that of the rim in (a). (c) Zircon from MW-03 with a ghosted core bounded by dark band in the upper half is surrounded by a rim of zircon with an irregular internal zonation. Larger black oval in lower left part of the grain is an inclusion of omphacite. Each grain is about 200 mm across.
tonalitic gneiss adjacent to eclogites in marbles from which Xu et al. [14] report diamond and others have reported coesite [ 151. The third sample of host gneiss
Notes to Table 1: Notes: a Cardinal number
(95029D)
comes from an within the Shima 17.7 km south of the Wumiao
outcrop (30.497”N. valley located some Zircon sepa-
the number of zircon grains analyzed (e.g. 35 grains); all grains were selected from non-paramagnetic Separator; + 200 = size in mesh (> 75 pm); c = colorless: euhedral; lp = long prismatic; subhedral; from long prisms; t = turbid centers. All grains air-abraded following
separates t-lp = tips
Corrected for 0.0215 mole fraction in the 205Pb-2’5U Calculated Th/U ratio assuming that all “‘Pb in excess of blank, common-Pb, radiogenic (A “?Th = 4.9475 X lo- ” y- ’ ). ’ Measured, uncorrected ratio. f Ratio corrected for fractionation, spike, blank and initial common-Pb (at the determined age from Stacey and Kramers [20]). Pb fractionation correction = O.O94%/amu (+ 0.025% 1o 1; LJ fractionation correction = 0.111 %/amu (+ 0.02% 1(T). U blank = 0.2 pg; Pb blank < 10 pg. Absolute uncertainties (la) in the Pb/U and “‘Pb/ ?“Pb ratios calculated following Ludwig (1980). U and Pb half-lives and isotopic abundance ratios from Jaffey et al. [ 191. s For samples l-3, this is the percent discordance along a chord from 2 18 Ma to 773 Ma; for sample 4, it is the percent discordance along a chord from 226 Ma to 447 Ma. h Independent regression of 93023 and 93023C yield an upper intercept age of 759.8 + 26.3/26.0 Ma, and lower intercept age of 214.0 + 8.7/9.8 Ma. ’ Independent regression of 95029D yields an upper intercept age of 803.0 + 71.4,’ - 73.4 Ma, and lower intercept age of 220.3 + 3.6,’ - 4.8 Ma. ’ Independent regression of MW03 yields an upper intercept age of 446.6 + 8 1,5/ - 78.6 Ma, and lower intercept age of 225.5 + 2.9/- 6.3 Ma.
D.B. Row’ley et al. /Earth
and Planetan,
Science Letters
Q 8
x ti
I51 (1YY 7) 191-203
196
D.B. Rowley et ul./Earth
and Planetav
rated from a sample of the Maowu eclogite [ 1.51 (30.501”N, 116.305”E) was also dated by the isotope dilution technique.
4. Analytical methods U-Pb analyses were performed at the geochronology laboratory at Washington University, St. Louis. Zircon was extracted from l-4 kg samples using standard techniques of density and magnetic separation, and grains were selected for analysis on the basis of size, clarity, color and morphology (Table 1). Fraction sizes varied somewhat depending on U content, grain size and age, but most analyses were performed on fractions containing less than 200 ng of radiogenic Pb. The procedures for zircon digestion, separation of U and Pb, and isotope dilution mass spectrometry using a ‘“‘Pb- ‘35U enriched tracer-solution follow those of Krogh [ 16,171 with modifications detailed in Tucker et al. [18]. During this study, total procedure banks for Pb ranged between 1 and 4 pg, and those for U averaged < 1 pg. Isotope ratios were measured by single-collector peak-jumping using a VG Sector-54 thermal ionization mass spectrometer equipped with a Daly-type detector operating in ioncounting mode. Errors to the atomic and isotopic ratios were calculated with the algorithm of Ludwig (1980) and they are quoted at the I CTconfidence level in Table 1. All ages were calculated using the isotope abundance ratio and decay constants of Jaffey et al. [19]; common-Pb corrections were made by using a model-Pb composition [20] at the determined age of the rock. In all but one case (analysis 6), the uncertainty in the amount and composition of the common-Pb calculated in this manner represents an insignificant contribution to the error in the age calculation. Upper and lower-intercept ages were calculated by a regression technique described by Davis [21]; all ages are quoted at 95% confidence limits. In addition to isotope dilution mass spectrometry we have also performed cathodoluminescence imaging using a Cameca SX-50 electron microprobe and trace and rare earth element geochemistry using an AEI IM-20 ion probe both at the University of Chicago on splits of the separated zircon.
Science Letters 151 (19971 391-203
5. Results
5.1. Host-gneiss
(93023, 93023C. 95029DI
Zircon in all samples was small, clear, equant in shape and multifaceted although one sample, 95029D, had a few long prismatic grains of zircon from which tips and middle-parts were broken and analyzed (Table 1). In general, our strategy was to select grains for analysis that were euhedral, clear and inclusionand fracture-free. Cathodolumines-
Eastern
Dabie Shan zircon
1ooc
ioc
IC
t + 1
La
Ce
Pr
Nd
93023 rim 93023 core
t
93023C rim
-iF
93023C core
t
95029D unzoned
f
95029D unzoned
-k
95029D core
Sm EuGd Tb Dy Ho Tm Lu Y Er Yb
Fig. 3. CI chondrite-normalized REE concentrations of spot analyses of representative zoned cores, rims, and one grain (similar to Fig. 2b) lacking a zoned core of splits of zircon extracted from the three host gneiss samples (93023, 93023C and 95029D). The plot shows that the compositions of the zircons with zoned cores are significantly more enriched in HREE than the surrounding rims from the same grains. Further, it shows that the zircon lacking a zoned core has the same REE distribution as the rims and thus belongs to the same component as the rims.
D.B.
Rowley
rt al. / Earth
und Plmetary
cence imaging of splits of these zircons reveals the presence of two phases of zircon growth (Fig. 2). Older cores are compositionally zoned (Fig. 2a). with zoning commonly truncated at the perimeter of the core. and have younger overgrowths of more homogeneous rims. Some grains (Fig. 2b) lack zoned cores and appear similar to the younger rims. Based on an analysis of more than 50 individual zircon grains the proportion of core relative to mantle and rim, in terms of the area on polished zircon surfaces, varies from about 75% to essentially 0%. We interpret the rims to represent metamorphic overgrowths on inherited, igneous cores. In order to test this interpretation we analyzed trace element and rare earth element concentrations of individual spots within these zircons navigated by the cathodoluminescence images. Fig. 3 plots the CI chondrite-normalized REE distributions from these samples and shows that the zircon cores have higher overall REE contents and more extremely HREE-enriched REE patterns than both the rims and grains lacking cores. All zircon grains analyzed have large positive Ce anomalies as has been reported elswhere [22-241. Fig. 4a shows the variations of U vs. Th/U for the different zircon components and differentiates ion probe based spot and multigrain isotope dilution measurements. Zircon cores lie along a horizontal line on the Th/U vs. U variation diagram (Fig. 4a). This trend is independent of concentrations of either Th or U. We suggest this pattern is controlled by partition of Th and U between zircon and granitic melt. The Th/U in zircon at present is to first order controlled by the following relation. Th [u
1
rlrcml/pre\ent
f DTh 1
1 Th
1
of where D” and DT” are partition coefficients element U and Th in zircon crystallizing from a granitic melt. From [25] Dn = 62-91, D” = 298383. depending upon temperature of magma: and Th/U for granite usually ranges from 3.3 and 5.7 [26,27]. Consequently, igneous zircon should have a Th/U ratio ca. 1.0 + 0.3, identical within error to the observed Th/U ratio in the zircon cores. We
Science
Letters
151 (IYY7J
197
IYI-203
98,
.l t
95029D
93023
93023C
100
U mm) 40
0
0.0
0.2
0.4
0.6
0.x
I .(I
1.3
TNU Fig. 4. U vs. Th/U (a) plot of spot analyses &representative Loned cores. rims, and grains lacking a zoned core of splits of zircon extracted from the three host gneiss aalpples (93023. 93023C and 95029D). Zoned cores plot on a diffe ent trend than do the rims and grains lacking a zoned core. Mul 1 igrain isotope dilution analyses listed in Table 1 are also included and plot new the metamorphic rim compositions. Cores plot abolt the horizontal Th/U = 1 line expected of igneous zircon. (b) plot of discordance vs. multigrain isotope dilution analyses of Th/U ratio for the host-gneiss and eclogite zircon (Table I). Thk relationships suggests that the measured zircon fractions consist :of mixtures of composite grains containing two generations of &on each consisting of distinct zircon components with differend Th/U values. Eclogite zircons, on the other hand. consist of an inherited component dated at c. 447 Ma and with a Th/U of ~c. 0. I, and a metamorphic component dated at c. 225 Ma and {ith a Th/U of c. 0.3 Ma. Lines show the compositions predictediby our mixing models.
198
D.B. Rowley et al./ Earth and Planetav
thus suggest that this supports our contention that the zircon cores represent inheritance derived from an igneous protolith. The compositions of the rims and zircons lacking cores plot along a line with a steep negative slope in the U vs. Th/U variation diagram (Fig. 4a). The compositions of U and Th in this component is not controlled by igneous elemental partitioning, and we infer that these record synmetamorphic growth of zircon. In addition, the observation that U varies by a factor of about 35 while Th shows more limited variation (factor of 5) clearly indicates that the usual geochemical coherence of Th and U must have been broken during crystallization of zircon rims and grains lacking cores. This separation of Th and U can be attributed to immobility of Th in contrast with the enhanced solubility of U in water-rich metamorphic fluids under oxidizing conditions [26,27]. Another possible factor that may have controlled the separation of Th and U is metamorphic reaction buffering, which is controlled by breakdown or growth of phases particularly favoring Th or U at the time of zircon growth. Support for our interpretation of the rims and zircons lacking cores are indeed metamorphic overgrowths derives from our observation of the occurrence of UHP metamorphic mineral inclusions, including omphacite and epidote, and reports of coesite [28] in zircon rims and in zircon without core. Isotope dilution mass spectrometry of U-Pb also demonstrates that two components of zircon are present in all fractions analyzed. Table 1 lists the analytical results, and regressions of our measurements of zircons from the three host gneiss samples, are plotted together on Fig. 5. The fifteen zircon analyses from these samples when combined define a single discordia line extending from c. 773 Ma to c. 2 18 Ma with upper- and lower-intercept errors of about -t 10 Ma and + 2 Ma, respectively. All fractions are discordant, between 35 and 2% (Table 1, Fig. 5), but within each rock the range of discordance is rather small between 19-20% and 6-2% in the case of 93023 and 95029D, respectively, resulting in much larger individual errors in the intercepts (Table 1). There is a strong, positive correlation between discordance and the Th/U ratio of the analyses such that least discordant analyses have the lowest Th/U ratio and most discordant analyses the greatest (Fig.
Science Letters 151 (1997) 191-203
a
00 f-0 ., .07
N, e
HOST GNEISS TO ECLOGITE 450 /
Upper Intercept age 772.5 d-9.5 Ma
J
/ /’
A 400 J / ,,,/’
g CQ
93023,C
.05
Lower Intercept age 218.5 +I-1.7 Ma
.25
.30
.35
.40
.45
.50
.55
.60
Fig. 5. Concordia diagram showng zircon analyses for samples 93023, 93023C and 95029, the host-gneisses to the Dabie Shan eclogites. All analyses conform well to a single discordia with intercept ages of 218.5 + I .7 and 772.5 +9.5 Ma. Ages and errors cited at 95% confidence
limits.
4b). The data suggest, therefore, that age discordance in all analyses is the result of mixing between the two generations of zircon (one dated at c. 773 Ma and the other at c. 2 18 Ma) each having a distinctive Th/U ratio. Discordance is not likely the result of diffusional Pb-loss (at 218 Ma) because such a process will not systematically fractionate thorogenic and uranogenic Pb as is evident in our analyses (Table 1). The Th and U concentrations of the multigrain zircon fractions determined by isotope dilution (Table 1) are also plotted in the U vs. Th/U variation diagram (Fig. 4a). Most of these data fall into the mixing area of the two components that we identified. i.e. the igneous core and metamorphic overgrowth, but plot near the metamorphic overgrowth end. consistent with the U/Pb discordia plot (Fig. 4b). We thus interpret the upper intercept in Fig. 2 to represent the age of the protolith zircons, and the lower intercept to represent the age of the UHPM event. That metamorphic zircons from two widely separated localities regress to a single discordia implies that the Late Triassic metamorphism. as recorded in zircon, occurred at approximately the same time (~218 Ma) over much of the eastern Dabie Shan district, that all three samples share a common-age protolith, and that post-metamorphic
D.B.
Rowley
et al. / Earth
and Planetary
Pb-loss was insignificant in the analyzed grains. This suggests that there is essentially no across-strike (i.e. N-S) diachroneity in the UHPM event in this part of the eastern Dabie Shan. 5.2. Eclogite (MW-03) In support of our previous interpretation [6] that UHP eclogites are in situ with respect to their enclosing host gneiss we report six additional analyses of zircon separated from an eclogite from Maowu. a large mostly ultrramafic pyroxenite in the Shima district. All analyzed fractions of eclogite-derived zircon consist of multigrain composites selected on the basis of optical clarity and common morphological features (Table 1). Cathodoluminescence and back-scattered electron imaging of samples from MW-03 also reveals the presence of two phases of zircon growth (Fig. 2~). Cores are compositionally zoned and are overgrown by more compositionally homogeneous rims that we interpret to represent inherited. igneous cores with metamorphic overgrowths. respectively. All analyses are similarly discordant (9-6%) but, in this case, upper- and lowerintercepts of 447 + 82/ - 79 Ma and 225.5 + 3/ - 6 Ma are resolved (Fig. 6). Although the lower-inter-
>
ECLOGITE
ZIRCON
447 +82/-79 Ma
225.5 +3/-6 Ma
207pb,235U ,245
,250
,255
.260
.265
j
.270
Fig. 6. Concordia diagram showing zircon analyses for sample MW-03, an eclogite from the Dabie Shan region. All analyss conform well to a single discordia with intercept ages of 447 Ma and 225 Ma. Note that the upper intercept age for MW-03 is distinctly younger than that for 93023, 93023C and 95029 (Fig. 5).
Science
Letters
IS1 (1997)
19l-1102
199
cept ages for both the host-gneisses and the eclogite are identical at 95% confidence limits. the’ upper-intercept ages are significantly different. In both cases, a Late Triassic age for the lower-interceet is indicated but, in the case of MW-03, a Palepzoic [29] upper-intercept age is implied. Interestinglg. a plot of Th/U ratios for measured zircons fror$ MW-03 suggests an inverse correlation between discordance and Th/U demonstrating that the UHPM $vent grew metamorphic zircon of a common Th/U ratio, about 0.2 (Fig. 4b).
6. Discussion The ages of zircons reported here cam be compared with those previously reported. Fik. IB and Fig. 7B show the distribution of published U/Pb zircon [30.31] and Sm/Nd and Rb/Sr agps [32-361 of UHPM eclogites and some country rot/k gneisses in the eastern Dabie Shan and Sulu regionn. Ames et al. [30] and Maruyama et al. [31] also interpret their results to record the UHPM event at abotit 218 Ma. Support for the interpretation that the lower intercept ages of accessory zircon represent the agei of UHPM is provided by Sm/Nd and Rb/Sr datbng of the UHPM garnet, omphacite. whole rock ages [31] and garnet and omphacite pairs by 133.35.361: It is quite clear that zircons from both the country rock gneisses and eclogites record identical metamo$hic ageh. providing additional support for our previbus conclusion [6] that the UHPM eclogites are in situ with respect to their enclosing gneisses. Taken together these data establish that UHP metamorphism occurred very close to 220 Ma ago in the elirly Norian [37] stage of the Late Triassic. Comparison of our upper intercept age of 772.5 & 9.S Ma with previously published results (Fig. 7A) also shows good agreement. U/Pb and iPb/Pb zircon ages from the Dabie Shan and Sulu r&$ons [30]. from the northern margin of the North I$abie Complex (sample XT-3 [lo]), as well as farther west in the Tongbai complex [38]. and Wudand Shan [39] (Fig. 1C and Fig. 7A). Chapman et al. [32] also reported a Rb-Sr garnet, omphacite. bhole rock isochron age of 62 1 + 2 1 Ma from Bixili$g, which is not dissimilar to the upper intercept ages ~of eclogites reported by Ames et al. [30].
200
D.B. Rowley et al. / Earth and Phnetary
The upper intercept ages of the country gneisses obtained here, as well as those reviewed above, at about 760 + 20 Ma are interpreted as documenting the age of widespread intrusion of protolith granites within the Qinling-Tongbai-Dabie-Sulu belt. Existing data from the eclogites are not sufficiently good and too few to determine with certainty whether some are the same age or younger. We suggest, as did Ames et al. [30], that the co-occurrence of mafic and felsic igneous activity is reasonably interpreted as occurring within a rift setting. Thus, we suggest that rifting occurred along the northern margin of the South China block during the Late Proterozoic Sinian stage of China. Support for this interpretation is provided by relationships preserved in the alongstrike, lower grade segments of this orogen in the west, in the Qinling of northwestern Hubei and southern Shaanxi provinces (Fig. IA). Basement rocks in this area are extensively intruded by both mafic and felsic dikes, sills and small plutons beneath the unconformably overlying Sinian elastics.
Science Letters ISI (I9971 IYI-203
The Pb/Pb zircon evaporation age of 762 + 0.7 Ma reported by Xue et al. [39] in fact comes from a felsic gneiss in a sliver of the basement just north of the Wudang Shan (Fig. 7A). Sinian elastics in the Wudang and Daba Shan pass stratigraphically upward into younger Sinian, Cambrian, Ordovician and Silurian strata that constitute a 5 to 8 km thick [40], Early Paleozoic passive-type shelf sequence that developed along the northern margin of the South China block. We suggest that the rifting proposed on the basis of the bimodal magmatic record of the Dabie Shan is correlative with that mapped farther west in the Qinling and that together they date at least one phase of passive continental margin formation along the northern margin of the South China block. Our interpretation of the granitic protoliths as dating from Late Proterozoic has further importance as regards the anomalous oxygen isotopic compositions observed in the Dabie Shan and Sulu region. The South China block experienced at least two phases of Late
Sm/Nd isochron: 228.4 + 6 Ma Rb/Sr isochron: 223.9 f 0.9 Ma (6) &
/A217.1I8.7
Ma (5) 1
isochron: 226.3 + 4..5 Ma Rb/Sr isochron: 219.5 LIZ 0.5 Ma (6) Sm/Nd isochrons: 210 f 9 Ma 218 1k4Ma (7) I Ey P
Orogenic
Fig. 7. (A) Map showing the distribution of Late Precambrian ages of granitic rocks, Tongbai. Dabie and Sulu regions of eastern China. (B) Map showing the distribution Dabie and Sulu regions of eastern China. Geochronologic methods: U/Pb, U/Pb evaporation method: Sm/Nd and Rb/Sr isochron, Sm/Nd and Rb/Sr internal mineral This paper, (2) Xue et al. [IO], (3) KrGner et al. [38]. (4) X ue et al. [39]. (5) Ames et Jahn [36].
w E
HPM zlone
L_Ql UHPM zone granitic protoliths. or sources of inheritance in the of dates of ultrahigh-pressure metamorphism in the zircon ages; Pb/Pb. single-zircon 207Pb/2”6Pb isochron methods respectively. Sources of data: (1) al. [30]. (6) Li et al. [33.35] and (7) Chavagnac and
D.B. Rowley et al./
Earth and Planrtuty
Proterozoic glaciation. The earlier phase is represented stratigraphically by the widely distributed Changan tillite while the later phase is represented by the Nantuo tillite [4 1,421. The Changan glaciation occurred within the interval between 800 Ma and 750 Ma, while the Nantuo is believed to have occurred between about 740 Ma and 700 Ma [41,42]. Thus, there is an overlap in age between the Changan glaciation and the rift-related magmatism along the northern margin of the South China block. This magmatism. at least in the Dabie Shan and Sulu region. drove local hydrothermal circulation systems in which high latitude, “O-depleted meteoric water interacted with and variably altered the UHPM protoliths now exposed in the eastern Dabie Shan. Isotopically anomalous oxygen values have been obtained from both the Wumiao and Shima localities (Fig. ICI from which we have obtained the 772.5 A 9.5 Ma protolith ages. Samples of the eclogite intruded by orthogneiss discussed above both show low filxO of about - 5%~ [9] (Fig. 1C) and thus share this premetamorphic alteration event. as well as the UHPM and subsequent nearly pervasive amphibolite facies retrogression. This common history, apparently dating back to at least the Late Proterozoic. allows a further demonstration of the in situ origin of the UHPM eclogites in the eastern Dabie Shan. Thus it appears that the co-occurrence of very anomalous oxygen isotopes and ultrahigh-pressure metamorphic rocks can be reasonably interpreted as a fortuitous coincidence. The upper intercept age of MW-03, even with its large uncertainty. does not overlap with the ages of the granitic host gneisses. Recent oxygen isotope data from the Maowu eclogite [ 131 are low (2-4%) compared with eclogites elsewhere, but are not extreme. Again, to the west along strike in the Qinling there is a second phase of magmatism characterized by abundant mafic volcanism in the Early Silurian [43]. Post-Early Silurian erosion followed by southward onlap of a Devonian to Triassic north-facing passive shelf sequence implies a second phase of rifting along the northern margin of the Yangtze block. The upper intercept age of the Maowu eclogite is compatible with having intruded simultaneously with this second phase of mafic magmatism. The Yangtze block was in low latitude throughout the Paleozoic [44], hence any hydrothermal alteration
Science Letters I51 (I9971
IYI-203
‘0
I
would not be expected to be associated with anomalous oxygen isotopic compositions in accord with the recent data [ 131.
7. Conclusions U/Pb zircon dating of granitic gneisses from the eastern Dabie Shan reveals a three-stage history. The earliest stage. represented by the upper intertcept ages of zircons extracted from orthogneisses ifrom the UHP terrain yield an age of 772.5 + 9.5 #Ma. This age is comparable to upper intercept ages obtained by Ames et al. [30] from the Sulu and Dabie Shan regions, and zircon evaporation ages in thq Tongbai complex [38] and northern Wudang Shan i[39]. The age is also consonant with inference based on stratigraphy and mafic and felsic magmatism fa$ther west in the Qinling that are most reasonably intekpreted as reflecting rifting along the northern mardin of the South China block during the Late Protercizoic. The co-occurrence of depleted ‘“0 values in both orthogneisses and eclogites of the eastern Dbbie Shan and Sulu is interpreted to reflect the development of hydrothermal circulation associated with this magmatism. in which high-latitude. presumably ‘“O-depleted meteoric water interacted with and variably altered the UHPM protoliths. That the madmatism is essentially synchronous with widespread evidence ot early Sinian glaciation on the South ChinU block is taken as further support for this interprbtation. A second phase of rifting is also observed along the northern margin of the South China block during the late Early Paleozoic also characterized by magmatic activity: we interpret the upper intercept ilge of the Maowu eclogite as being associated with this event. The absence of anomalously low 6”O in the Maowu eclogite is compatible with its later intrusibn into the Yangtze basement. Finally, subduction of segments of the northern. attenuated margin of ~the South China block to mantle depths and devel{,pment of UHP metamorphic mineral assemblages ins dated as occurring at 2 18.5 + I .7 Ma. The linearity of discordance among zircon samples from locailities now almost 20 km apart implies that there is dirtually no diachroneity to the age of the UHP metalmorphism. and that our samples share protoliths @ith nearly identical ages. The complete congruity of structural
202
D.B. Row1e.v et al./Earth
and Planetary Science Letters I51 (1997) 191-203
observations [6], oxygen isotopes [9] and identical ages of metamorphic growth of zircons in both eclogites and country rock gneisses demonstrates that all of these rocks share a common history extending from at least the Late Proterozoic through UHPM to the present, and hence demonstrate that at least in this region all the rocks record an in-situ development of UHP metamorphism.
Acknowledgements Helpful reviews by G. Gehrels, J.G. Liou, Olivier Bruguier, R.N. Clayton are gratefully acknowledged. Jin Yugan of the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences provided outstanding logistical support. Ian Steele provided assistance with cathodoluminescence imaging. Support from NSF (EAR-9305 107) to DBR is also gratefully acknowledged. RV
References J.G. Lieu. R.Y. Zhang, E.A. Eide, S. Maruyama. X. Wang. W.G. Ernst, Metamorphism and tectonics of high-P and ultrahigh P belts in Dabie-Sulu Regions, eastern China, in: A. Yin and T.M. Harrison (Eds.), The Tectonic Evolution of Asia, Rubey Vol. IX. Cambridge Univ. Press, 1996. pp. 300-344. 121R.G. Coleman and X. Wang, Ultrahigh Pressure Metamorphism, Cambridge Univ. Press, 1995, 528 pp. 131 D.C. Smith, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics. Nature 3 10 (1984) 641644. of [41 W.L. Griffin and D.A. Carswell, In situ metamorphism Norwegian eclogites: an example, in: D.G. Gee and B.A. Sturt (Eds.), The Caledonide Orogen-Scandanavia and Related Areas, Wiley, New York, 1985, pp. 813-822. coesite[51 X. Wang, J.G. Liou, Regional ultrahigh-pressure bearing eclogitic terrane in central China: Evidence from country rocks, gneiss. marble, and metapelite. Geology 19 (19911 933-936. [‘51 F. Xue, D.B. Rowley, J. Baker, Refolded syn-ultrahigh pressure thrust sheets in the South Dabie complex, China. Field evidence and tectonic implications, Geology 24 (19961 45% 458. 171 B. Cong, M. Zhai. D.A. Carswell. R.N. Wilson, Q. Wang, Z. Zhao, B.F. Windley. Petrogenesis of ultrahigh-pressure rocks and their country rocks at Shuanghe in Dabieshan. Central China, Eur. J. Mineral. 7 (19951 119-138. 181T.Z. Yui, D. Rumble III. C.H. Lo. Unusually low S180
ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China, Geochim. Cosmochim. Acta 59 (19951 28592864. [91 J. Baker. A. Matthews, D. Mattey, D. Rowley, F. Xue. Fluid-rock interactions during ultra-high pressure metamorphism, Dabie Shan, China, Geochim. Cosmochim. Acta 61 (19971 1685-1696. [lOI F. Xue, D.B. Rowley. R.D. Tucker and Z.X. Peng, U-Pb zircon ages of granitoid rocks in the North Dabie Complex. eastern Dabie Shan, China, J. Geol.. in press. [I 11 R.Y. Zhang, J.G. Liou, H. Tsai. Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China, J. Metamorph. Geol. 14 (19961319-333. [I21 T.-F. Yui. D. Rumble III, C-H. Chen, C.-H. Lo. Stable isotope characteristics of eclogites from the ultra-high-pressure metamorphic terrain, east-central China, Chem. Geol. 137 (1997) 135-147. iI31 D. Rumble III, R. Zhang, J.G. Liou and Q.C. Wang, The Qinglongshan oxygen isotope anomaly in coesite-facies eclogites of eastern China, Geol. Sot. Am. Abstr. Programs, 1996. 1141 S.T. Xu. A.I. Okay, S. Ji, A.M.C. Sengor, W. Su, Y. Liu, L. Jiang. Diamond from the Dabie Shan metamorphic rocks and its implications for tectonic setting, Science 256 (19921 80-83. [151 A.I. Okay. Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China, Eur. J. Mineral. 5 ( 19931 659-675. [I61T.E. Krogh, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotope age determinations. Geochim. Cosmochim. Acta 37 (1973) 485-494. [I71 T.E. Krogh, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta 46 (19821 637-649. [I81 R.D. Tucker, T.E. Krogh and A. Raheim. Proterozoic evolution and age-province boundaries in the central part of the Western Gneiss Region, Norway: results of U-Pb dating of accessory minerals from Trondheimsfjord to Geiranger, in: C.F. Gower, T. Rivers and B. Ryan (Eds.). Mid-Proterozoic Laurentia-Baltica, Spec. Pap. 38. Geol. Assoc. Can., 1991, pp. 149- 173. [I91 A.H. Jaffey, K.F. Flynn, L.E. Glendenin, W.C. Bentley, A.M. Essling, Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev. Sect. C. Nucl. Phys. 4 (19711 1889-1906. of terrestrial lead DO1 J.S. Stacey. J.D. Kramers, Approximation isotope evolution by a two-stage model, Earth Planet. Sci. Lett. 26 (1975) 207-22 1. 011 D.W. Davis. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19 (19821 21413149. [22] R.W. Hinton, B.G.J. Upton, The chemistry of zircon: variations within and between large crystals from syenite and alkali basalt xenoliths, Geochim. Cosmochim. Acta 55 (199 1) 3287-3302.
D.B. Rowlq
[23]
A.V.
Murali,
T.M.
et (11./Earth
Parthasarathy. D.M.
and Planeta?
Sankar, Trace ele-
ment characteristics. REE patterns and partition coefficients of rircons from different geological environments-A study on Indian (1983)
zircons. Geochim.
Cosmochim.
case Acta 47
[24] E. Zinner and C. Crozaz.
Ion probe determinations
of the
abundances of rear earth elements in single mineral grains. in: A. Beal (Ed.). SIMS V, Secondary Ion Mass Spectrometry. Springer, Berlin, Hildreth.
Gradients in Temperature,
Chamber
of the Bishop Tuff:
Pressure, and Composition. Ph.D.
Thesis, Univ. Calif.. Berkeley,
1977. New York.
1977, 364 pp. 1271 K.H.
Handbook
of
Geochemistry.
Springer.
S.V. Goryainov.
and 40Ar/3YAr
Area
rocks
and their
in the Dabie
oh ultrahigh-
country
Mountains,
rock.; at
Cenrral
China:
Cooling history and time sequence of the continedtal subduction, ICG Abstracts, Beijing. 0809013. [36] V. Chavagnac,
1996.
B.-m. Jahn. Coeaite-bearing
Complex,
Dabie
Mountains.
eclogites from Chida:
Sm-Nd
ages, geochemical characteristics and tectonic implications. Chem. Geol.
I33 (1996)
29-5 I
Gradstein. F.P. Agterberg. J.G. Ogg. J. Hardenhol.
van Veen. J. Thierry
and Z.H.
P.
Huang. A Triassic. Jurassic Berggren, D.V.
Kent.
M.P. Aubry and J. Hardenbohl (Eda.). Geochronlology Time Stratigraphic
Correlation.
Sediment. Geol.) Spec. Publ., Tulsa.
SlEPM (Sot.
1995. 54. pp. 95-126.
[38] A. Kraner. G. Zhang. Y. Sun. Granulitez, in the Tongbai area.
regions as an unique container of inclusions of diamond.
Qinling belt, China: Geochemistry.
coe\itr
geochronology. and implications for the tectonic cvolutlon of
and coexisting
minerals.
Dokl.
Akad.
Nauk
334
(1994) 488-492.
eastern A\ia. Tectonics 12 (1993)
[29] R.D. Tucker. W.S. McKerrow.
Early Paleozoic chronology: a
review in light of new UP\, zircon ages from Newfoundland and Britain. Can. J. Earth Sci. 32 (1995) [30] L. Ames. G. Zhou. B. Xiong. character of ultrahigh-pressure
368-379.
Geochronology
and isotopic
metamorphism with implica-
tions for collision of the Sino-Korean
and Yangtze Cratona.
central China. Tectonic< 15 (1996) 472-489.
II
metamorphic
Scales and Global
Sobolev, V.S. Shatsky, M.A. Vavilov,
Zircon from ultrahigh-pressure metamorphic rocks of folded
[3
pressure
and Cretaceous Time Scale. in: W.A.
Wedepohl.
Berlin, 1969-1978. [28] N.V.
Rb-Sr 343-3.50.
[35] S. Li. E. Jagoutz and Y. Chen, Geochronology
[37] F.M.
1261 G. Faure, Principles of Isotope Geology. Wylie.
‘03
19/-203
ing of eclogite minerals by Sm-Nd.
the Bixiling
1986. pp. 444-446.
The Magma
151 (1997)
methods, Chem. Geol. 112 (1994)
Shuanghe
2047-2052.
[2S] E.W.
Science Letters
S. Maruyama.
[39] F. Xue. A. Kriinrr.
ling erogenic belt. central China as documented by zircon 409-3
17.
[40] Regional Stratigraphic Tables of Shaanxi Provmce, Peoples [41] S. Liao, Sinian glacial deposits of Guizhou Province. China, in: M.J.
Pleistocene Glacial
1 12-121,
Chapman. J. Baker. M.J. Bickle, .I. Bunbury, A. Matthews
ayhtematics due to ultmhigh
of Nd. Sr. and 0 isotopic
pressure metamorphism of hy--
Hamhrey
China. in: M.J. Hambrey
Goldschmidt Confer., J. Conference Abstr., 101, 1996.
1981. pp. 386-101.
D.L.
Lieu.
Y.Z.
Chen. N.J. Ge. Z.Q.
Harland
Record. Cambridge
(Eds.).
Glacial
and W.B. Record.
Geology
summary). 698 pp.. Geol. Pub]. House. Beijing,
Chem. Geol..
1993.
Excess argon in phengitr from eclogite: Evidence from dat-
Univ.
Press.
[43] Shaanxi Bureau of Geology and Mineral Resources. Regional
Wang. Collision of the North China and Yang&e blocks and
[34] S. Li. S. Wang. Y. Chen. D. Liu, J. Qiu. H. Zhou, Z. Zhang.
Pre10x1.
Harland (Eda.), Earth’s
Cambridge
Zhang. S.S. Sun. B.L. Gong, R.Y. Zhang, S.R. Hart and S.S. formation of coesite-bearing eclogites: timing and processes.
Earth’s
Univ. Pre\s,
pp. 4 I I-423.
Pre-Pleistocene
Xiao.
and W.B.
[42] Y. Wang. S. Lu. Z. Gao. W. Lin and G. Ma. Sin/an tillites 01
drothermally altered eclogites. Dabie Shan. China. 1996 V.M. [33] S.G. Li. Y.L.
Lerch. Paleozoic
ages on granitoid rockh, J. Gcol. Sot. London’ 153 (1996)
hells from central Chind, Island Arc 3 (1994) Characterization
M.F.
magmatism in the Qin-
Republic of China, Geol. Pub]. House. Beijing, 1071, 450 pp.
J.G. Liou. R.Y. Zhang, Tectonic evolution ol
and D. Rowley,
235-255.
T. Reischmann.
pre- and post-collision talc-alkaline
the ultrahigh-pressure (LJHP) and high-pressure metamorphic
[??I H.
petrology. single zircon
of Shaanxi
[44] S. Nie, Paleoclimatic
Province,
(in Chinese
with English 19X9.
and paleomagnetic constiaints on the
Paleozoic reconstructions of south China. north China. and Tarim. Tectonophysics
196 (199 I) 279-70X.
Earth and Planetary
Science Letters 151 (1997) 191-203
Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology D.B. Rowley ‘**, F. Xue a, R.D. Tucker b, Z.X. Peng b, J. Baker ‘, A. Davis d a Department of the Geophysical Sciences, The (/nicer&y of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637. USA h Department of Earth and Planetaty Sciences, Washington University, St. Louis, MO 63130. USA ’ Department of Earth Sciences. University of Cambridge, Cambridge, CB2 3EQ. United Kingdom d Enrico Fermi Institute. The University of Chicago, 5640 S. Ellis Avenue, Chicugo. IL 60637. USA Received 21 May 1997: revised 14 July 1997; accepted
14 July 1997
Abstract The Dabie Shan contains two rare geologic features. One, the occurrence of ultra-high pressure metamorphic rocks (peak P > 2.7 GPa = UHP) in the east-central part of this belt, and two, some of these UHP rocks are characterized by anomalously ‘XO-depleted oxygen isotopic compositions. Geochronological data are presented that constrain the ‘age of the UHP metamorphism, the ages of protoliths, and potentially the age of hydrothermal alteration believed to be respdlnsible for the anomalous oxygen isotopic compositions. The age of UHP metamorphism in the eastern Dabie Shan ifi dated at 21 X.5 + 1.7 Ma using U/Pb in zircon separated from host gneisses. This age is identical within uncertainty Cith a less precise age using U/Pb in zircon separated from UHP eclogite (225.5 + 3/ - 6 Ma) and with previously published ages from UHP eclogites of the Dabie Shan. Map relationships strongly support an in-situ, as opposed to exotic. origin of the UHP eclogites in this region. The identical ages of UHP metamorphism in the gneisses and eclogites provide? additional support for this interpretation. The age of the protolith gneisses is dated as 772.5 f 9.5 Ma based on the upper ibtercept of the zircons which accords well with other dates of granitic gneisses in the Qinling belt. The association of mafici and felsic magmatism is interpreted to reflect rifting along the northern margin of the Yangtze block at this time. These upp r intercept ages may date the hydrothermal circulation of ‘*O-depleted meteoric water which gave rise to the anomalously Ixe, -depleted UHP protoliths of the Dabie Shan and Sulu region. The coincidence of these protolith ages with the early Siniarl glaciation of South China is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this legion date from this time. The upper intercept age of the eclogite is imprecisely determined at 447 + 82/ - 79 Ma but is yaunger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions. 0 1997 Elsevier Science;B.V. Kew~wdsr Dahie Mountains: tectonics; earths; rclogite: regional metamorphism
’ Corresponding
author. Tel.: +
plate collision:
geochronology;
U/Pb;
Triassic:
upper Proterozoic;
I 3 12 702 7071. fax: + I 312 702 9505. E-mail: [email protected]
0012-821x/97/$17.00 0 1997 El sevier Science B.V. All rights reserved PII SOOl2-821X(97)00128-3
carhodolumin&cence:
rare
South Dabie & Susong Complexes
D.B. Rowley et al./ Earth and Planetan
one of the critical problems is whether the present close spatial arrangement of these different assemblages records post-tectonic juxtaposition (so-called exotic origin) [3] or differential reequilibration following peak pressure (so-called in situ origin) [4]. In this paper we present results of U/Pb zircon dating of the UHP metamorphism (UHPM) with the intent of addressing the in situ [5,6l versus exotic [7] nature of the UHP-bearing assemblage of the Dabie Shan. The Dabie Shan and Sulu region not only contain UHP-bearing metamorphic assemblages but some of these units are also characterized by some of the most ‘XO-depleted metamorphic rocks in the world [8,9]. This coincidence of UHPM and extreme oxygen isotopic compositions naturally raises questions regarding the potential relationship between these two occurrences. Existing interpretations of the anomalous oxygen isotopic compositions favor a premetamorphic, hydrothermally-driven alteration event completely independent of UHPM [8,9]. Our upper intercept age provides a potential link between the age of the protolith magmatism and the age of glaciation of the South China block during the early Sinian. and thus provides a rationale for their coinci-dence.
2. Regional geology The geology of the eastern Dabie Shan comprises a succession of metamorphic rocks characterized by increasing peak pressures ranging from transitional blueschist-greenschist facies in the south, through amphibolite to high pressure (HP < 2.7 GPa) eclogite and UHP coesite eclogite facies farther north (Fig. 18). These metamorphic rocks are characterized by a variety of granitic to tonalitic gneiss, talc-silicate gneiss. mafic and locally ultramafic eclogite. and clearly supracrustal metapelite, metapsamite, marble and metaphosphorite. These lithologies are interpreted to represent metamorphosed sections of the Yangtze basement and overlying cover succession. To the north and west of the South Dabie Complex is the North Dabie Complex that is dominated by Early Cretaceous felsic to intermediate plutons but with evidence of Proterozoic protoliths in their source area [ 101 as well as limited regions of older metamorphic [I 1] and plutonic rocks [ 101.
Sciencr Letters
151 11’871 IYI--103
193
Detailed mapping in the eastern Dabie Shan [6] has demonstrated that the UHPM terrain is structurally characterized by a series of nearly reclined, tight to isoclinal folds with moderate S-dipping axial surfaces that developed during regionally pervasive amphibolite facies retrogression. These folds deform earlier structures and fabrics. at least some’of which are characterized by associated UHPM minieralogies. Some of the earlier structures that are interpreted to have developed at or near UHPM conditions include folds. boudinage and early, regionally significant S-vergent thrusts [6]. Xue et al. [6] arguedlthat both the eclogitic rocks that still preserve their lrecord of UHPM assemblages as well as the associated, and more voluminous country gneisses. and less voluminous marbles and talc-silicate gneiss all share the same pressure-temperature-time ( PTr ) path as they all share the same deformation history. This detailed mapping also revealed an intact intrusive contact between at least one phase of the tonalitic orthogneiss and a mafic eclogite boudin from an outcrop within IO m of the sample (95029D) whose age is reported here. At least in this specific case it is possible to demonstrate that both the tonalitic orthogneiss and eclogite shared the same P7’r history. Oxygen isotope geochemistry carried out in association with our detailed mapping is reported elsewhere [9]. Baker et al. [9] report the discovery of extremely anomalous (6”O + 5%) oxygen isotope values in some of the eclogites and surrounding cotmtry rock gneisses in the UHP terrain of the eastern Dabie Shan and Yui et al. [8,12] and Rumble et al. [13] reports similar findings in the Sulu regiion. These discoveries raise the additional question as, to the age and origin of this oxygen isotope anomalb. We use the upper intercept ages based on our U/Ph zircon geochronology to date this event.
3. U/Pb
zircon geochronology
In this paper we report ages of zircons derived from three samples of the tonalitic gneisses of the UHPM terrain as well as one UHPM eclogite of the eastern Dabie Shan (Fig. IB). Two splits of the same block (93023 and 93023~) are from east of Wumiao at 30.650”N, 116.359”E in the northern part of the UHPM terrain. Zircon was separated from gray
194
D.B. Rowley et al/Earth
and Planetary Science Letters 15I (1997) 191-203
Fig. 2. Cathodoluminescence images of representative zircons from both the gneissic and eclogitic populations analyzed. (a) Zircon from 95029D with well defined bright, zoned core surrounded by darker mantling rim. Inclusions in the core of this zircon are apatite. (b) Zircon from 93023 lacks a zoned core and instead displays an irregular zonation comparable to that of the rim in (a). (c) Zircon from MW-03 with a ghosted core bounded by dark band in the upper half is surrounded by a rim of zircon with an irregular internal zonation. Larger black oval in lower left part of the grain is an inclusion of omphacite. Each grain is about 200 mm across.
tonalitic gneiss adjacent to eclogites in marbles from which Xu et al. [14] report diamond and others have reported coesite [ 151. The third sample of host gneiss
Notes to Table 1: Notes: a Cardinal number
(95029D)
comes from an within the Shima 17.7 km south of the Wumiao
outcrop (30.497”N. valley located some Zircon sepa-
the number of zircon grains analyzed (e.g. 35 grains); all grains were selected from non-paramagnetic Separator; + 200 = size in mesh (> 75 pm); c = colorless: euhedral; lp = long prismatic; subhedral; from long prisms; t = turbid centers. All grains air-abraded following
separates t-lp = tips
Corrected for 0.0215 mole fraction in the 205Pb-2’5U Calculated Th/U ratio assuming that all “‘Pb in excess of blank, common-Pb, radiogenic (A “?Th = 4.9475 X lo- ” y- ’ ). ’ Measured, uncorrected ratio. f Ratio corrected for fractionation, spike, blank and initial common-Pb (at the determined age from Stacey and Kramers [20]). Pb fractionation correction = O.O94%/amu (+ 0.025% 1o 1; LJ fractionation correction = 0.111 %/amu (+ 0.02% 1(T). U blank = 0.2 pg; Pb blank < 10 pg. Absolute uncertainties (la) in the Pb/U and “‘Pb/ ?“Pb ratios calculated following Ludwig (1980). U and Pb half-lives and isotopic abundance ratios from Jaffey et al. [ 191. s For samples l-3, this is the percent discordance along a chord from 2 18 Ma to 773 Ma; for sample 4, it is the percent discordance along a chord from 226 Ma to 447 Ma. h Independent regression of 93023 and 93023C yield an upper intercept age of 759.8 + 26.3/26.0 Ma, and lower intercept age of 214.0 + 8.7/9.8 Ma. ’ Independent regression of 95029D yields an upper intercept age of 803.0 + 71.4,’ - 73.4 Ma, and lower intercept age of 220.3 + 3.6,’ - 4.8 Ma. ’ Independent regression of MW03 yields an upper intercept age of 446.6 + 8 1,5/ - 78.6 Ma, and lower intercept age of 225.5 + 2.9/- 6.3 Ma.
D.B. Row’ley et al. /Earth
and Planetan,
Science Letters
Q 8
x ti
I51 (1YY 7) 191-203
196
D.B. Rowley et ul./Earth
and Planetav
rated from a sample of the Maowu eclogite [ 1.51 (30.501”N, 116.305”E) was also dated by the isotope dilution technique.
4. Analytical methods U-Pb analyses were performed at the geochronology laboratory at Washington University, St. Louis. Zircon was extracted from l-4 kg samples using standard techniques of density and magnetic separation, and grains were selected for analysis on the basis of size, clarity, color and morphology (Table 1). Fraction sizes varied somewhat depending on U content, grain size and age, but most analyses were performed on fractions containing less than 200 ng of radiogenic Pb. The procedures for zircon digestion, separation of U and Pb, and isotope dilution mass spectrometry using a ‘“‘Pb- ‘35U enriched tracer-solution follow those of Krogh [ 16,171 with modifications detailed in Tucker et al. [18]. During this study, total procedure banks for Pb ranged between 1 and 4 pg, and those for U averaged < 1 pg. Isotope ratios were measured by single-collector peak-jumping using a VG Sector-54 thermal ionization mass spectrometer equipped with a Daly-type detector operating in ioncounting mode. Errors to the atomic and isotopic ratios were calculated with the algorithm of Ludwig (1980) and they are quoted at the I CTconfidence level in Table 1. All ages were calculated using the isotope abundance ratio and decay constants of Jaffey et al. [19]; common-Pb corrections were made by using a model-Pb composition [20] at the determined age of the rock. In all but one case (analysis 6), the uncertainty in the amount and composition of the common-Pb calculated in this manner represents an insignificant contribution to the error in the age calculation. Upper and lower-intercept ages were calculated by a regression technique described by Davis [21]; all ages are quoted at 95% confidence limits. In addition to isotope dilution mass spectrometry we have also performed cathodoluminescence imaging using a Cameca SX-50 electron microprobe and trace and rare earth element geochemistry using an AEI IM-20 ion probe both at the University of Chicago on splits of the separated zircon.
Science Letters 151 (19971 391-203
5. Results
5.1. Host-gneiss
(93023, 93023C. 95029DI
Zircon in all samples was small, clear, equant in shape and multifaceted although one sample, 95029D, had a few long prismatic grains of zircon from which tips and middle-parts were broken and analyzed (Table 1). In general, our strategy was to select grains for analysis that were euhedral, clear and inclusionand fracture-free. Cathodolumines-
Eastern
Dabie Shan zircon
1ooc
ioc
IC
t + 1
La
Ce
Pr
Nd
93023 rim 93023 core
t
93023C rim
-iF
93023C core
t
95029D unzoned
f
95029D unzoned
-k
95029D core
Sm EuGd Tb Dy Ho Tm Lu Y Er Yb
Fig. 3. CI chondrite-normalized REE concentrations of spot analyses of representative zoned cores, rims, and one grain (similar to Fig. 2b) lacking a zoned core of splits of zircon extracted from the three host gneiss samples (93023, 93023C and 95029D). The plot shows that the compositions of the zircons with zoned cores are significantly more enriched in HREE than the surrounding rims from the same grains. Further, it shows that the zircon lacking a zoned core has the same REE distribution as the rims and thus belongs to the same component as the rims.
D.B.
Rowley
rt al. / Earth
und Plmetary
cence imaging of splits of these zircons reveals the presence of two phases of zircon growth (Fig. 2). Older cores are compositionally zoned (Fig. 2a). with zoning commonly truncated at the perimeter of the core. and have younger overgrowths of more homogeneous rims. Some grains (Fig. 2b) lack zoned cores and appear similar to the younger rims. Based on an analysis of more than 50 individual zircon grains the proportion of core relative to mantle and rim, in terms of the area on polished zircon surfaces, varies from about 75% to essentially 0%. We interpret the rims to represent metamorphic overgrowths on inherited, igneous cores. In order to test this interpretation we analyzed trace element and rare earth element concentrations of individual spots within these zircons navigated by the cathodoluminescence images. Fig. 3 plots the CI chondrite-normalized REE distributions from these samples and shows that the zircon cores have higher overall REE contents and more extremely HREE-enriched REE patterns than both the rims and grains lacking cores. All zircon grains analyzed have large positive Ce anomalies as has been reported elswhere [22-241. Fig. 4a shows the variations of U vs. Th/U for the different zircon components and differentiates ion probe based spot and multigrain isotope dilution measurements. Zircon cores lie along a horizontal line on the Th/U vs. U variation diagram (Fig. 4a). This trend is independent of concentrations of either Th or U. We suggest this pattern is controlled by partition of Th and U between zircon and granitic melt. The Th/U in zircon at present is to first order controlled by the following relation. Th [u
1
rlrcml/pre\ent
f DTh 1
1 Th
1
of where D” and DT” are partition coefficients element U and Th in zircon crystallizing from a granitic melt. From [25] Dn = 62-91, D” = 298383. depending upon temperature of magma: and Th/U for granite usually ranges from 3.3 and 5.7 [26,27]. Consequently, igneous zircon should have a Th/U ratio ca. 1.0 + 0.3, identical within error to the observed Th/U ratio in the zircon cores. We
Science
Letters
151 (IYY7J
197
IYI-203
98,
.l t
95029D
93023
93023C
100
U mm) 40
0
0.0
0.2
0.4
0.6
0.x
I .(I
1.3
TNU Fig. 4. U vs. Th/U (a) plot of spot analyses &representative Loned cores. rims, and grains lacking a zoned core of splits of zircon extracted from the three host gneiss aalpples (93023. 93023C and 95029D). Zoned cores plot on a diffe ent trend than do the rims and grains lacking a zoned core. Mul 1 igrain isotope dilution analyses listed in Table 1 are also included and plot new the metamorphic rim compositions. Cores plot abolt the horizontal Th/U = 1 line expected of igneous zircon. (b) plot of discordance vs. multigrain isotope dilution analyses of Th/U ratio for the host-gneiss and eclogite zircon (Table I). Thk relationships suggests that the measured zircon fractions consist :of mixtures of composite grains containing two generations of &on each consisting of distinct zircon components with differend Th/U values. Eclogite zircons, on the other hand. consist of an inherited component dated at c. 447 Ma and with a Th/U of ~c. 0. I, and a metamorphic component dated at c. 225 Ma and {ith a Th/U of c. 0.3 Ma. Lines show the compositions predictediby our mixing models.
198
D.B. Rowley et al./ Earth and Planetav
thus suggest that this supports our contention that the zircon cores represent inheritance derived from an igneous protolith. The compositions of the rims and zircons lacking cores plot along a line with a steep negative slope in the U vs. Th/U variation diagram (Fig. 4a). The compositions of U and Th in this component is not controlled by igneous elemental partitioning, and we infer that these record synmetamorphic growth of zircon. In addition, the observation that U varies by a factor of about 35 while Th shows more limited variation (factor of 5) clearly indicates that the usual geochemical coherence of Th and U must have been broken during crystallization of zircon rims and grains lacking cores. This separation of Th and U can be attributed to immobility of Th in contrast with the enhanced solubility of U in water-rich metamorphic fluids under oxidizing conditions [26,27]. Another possible factor that may have controlled the separation of Th and U is metamorphic reaction buffering, which is controlled by breakdown or growth of phases particularly favoring Th or U at the time of zircon growth. Support for our interpretation of the rims and zircons lacking cores are indeed metamorphic overgrowths derives from our observation of the occurrence of UHP metamorphic mineral inclusions, including omphacite and epidote, and reports of coesite [28] in zircon rims and in zircon without core. Isotope dilution mass spectrometry of U-Pb also demonstrates that two components of zircon are present in all fractions analyzed. Table 1 lists the analytical results, and regressions of our measurements of zircons from the three host gneiss samples, are plotted together on Fig. 5. The fifteen zircon analyses from these samples when combined define a single discordia line extending from c. 773 Ma to c. 2 18 Ma with upper- and lower-intercept errors of about -t 10 Ma and + 2 Ma, respectively. All fractions are discordant, between 35 and 2% (Table 1, Fig. 5), but within each rock the range of discordance is rather small between 19-20% and 6-2% in the case of 93023 and 95029D, respectively, resulting in much larger individual errors in the intercepts (Table 1). There is a strong, positive correlation between discordance and the Th/U ratio of the analyses such that least discordant analyses have the lowest Th/U ratio and most discordant analyses the greatest (Fig.
Science Letters 151 (1997) 191-203
a
00 f-0 ., .07
N, e
HOST GNEISS TO ECLOGITE 450 /
Upper Intercept age 772.5 d-9.5 Ma
J
/ /’
A 400 J / ,,,/’
g CQ
93023,C
.05
Lower Intercept age 218.5 +I-1.7 Ma
.25
.30
.35
.40
.45
.50
.55
.60
Fig. 5. Concordia diagram showng zircon analyses for samples 93023, 93023C and 95029, the host-gneisses to the Dabie Shan eclogites. All analyses conform well to a single discordia with intercept ages of 218.5 + I .7 and 772.5 +9.5 Ma. Ages and errors cited at 95% confidence
limits.
4b). The data suggest, therefore, that age discordance in all analyses is the result of mixing between the two generations of zircon (one dated at c. 773 Ma and the other at c. 2 18 Ma) each having a distinctive Th/U ratio. Discordance is not likely the result of diffusional Pb-loss (at 218 Ma) because such a process will not systematically fractionate thorogenic and uranogenic Pb as is evident in our analyses (Table 1). The Th and U concentrations of the multigrain zircon fractions determined by isotope dilution (Table 1) are also plotted in the U vs. Th/U variation diagram (Fig. 4a). Most of these data fall into the mixing area of the two components that we identified. i.e. the igneous core and metamorphic overgrowth, but plot near the metamorphic overgrowth end. consistent with the U/Pb discordia plot (Fig. 4b). We thus interpret the upper intercept in Fig. 2 to represent the age of the protolith zircons, and the lower intercept to represent the age of the UHPM event. That metamorphic zircons from two widely separated localities regress to a single discordia implies that the Late Triassic metamorphism. as recorded in zircon, occurred at approximately the same time (~218 Ma) over much of the eastern Dabie Shan district, that all three samples share a common-age protolith, and that post-metamorphic
D.B.
Rowley
et al. / Earth
and Planetary
Pb-loss was insignificant in the analyzed grains. This suggests that there is essentially no across-strike (i.e. N-S) diachroneity in the UHPM event in this part of the eastern Dabie Shan. 5.2. Eclogite (MW-03) In support of our previous interpretation [6] that UHP eclogites are in situ with respect to their enclosing host gneiss we report six additional analyses of zircon separated from an eclogite from Maowu. a large mostly ultrramafic pyroxenite in the Shima district. All analyzed fractions of eclogite-derived zircon consist of multigrain composites selected on the basis of optical clarity and common morphological features (Table 1). Cathodoluminescence and back-scattered electron imaging of samples from MW-03 also reveals the presence of two phases of zircon growth (Fig. 2~). Cores are compositionally zoned and are overgrown by more compositionally homogeneous rims that we interpret to represent inherited. igneous cores with metamorphic overgrowths. respectively. All analyses are similarly discordant (9-6%) but, in this case, upper- and lowerintercepts of 447 + 82/ - 79 Ma and 225.5 + 3/ - 6 Ma are resolved (Fig. 6). Although the lower-inter-
>
ECLOGITE
ZIRCON
447 +82/-79 Ma
225.5 +3/-6 Ma
207pb,235U ,245
,250
,255
.260
.265
j
.270
Fig. 6. Concordia diagram showing zircon analyses for sample MW-03, an eclogite from the Dabie Shan region. All analyss conform well to a single discordia with intercept ages of 447 Ma and 225 Ma. Note that the upper intercept age for MW-03 is distinctly younger than that for 93023, 93023C and 95029 (Fig. 5).
Science
Letters
IS1 (1997)
19l-1102
199
cept ages for both the host-gneisses and the eclogite are identical at 95% confidence limits. the’ upper-intercept ages are significantly different. In both cases, a Late Triassic age for the lower-interceet is indicated but, in the case of MW-03, a Palepzoic [29] upper-intercept age is implied. Interestinglg. a plot of Th/U ratios for measured zircons fror$ MW-03 suggests an inverse correlation between discordance and Th/U demonstrating that the UHPM $vent grew metamorphic zircon of a common Th/U ratio, about 0.2 (Fig. 4b).
6. Discussion The ages of zircons reported here cam be compared with those previously reported. Fik. IB and Fig. 7B show the distribution of published U/Pb zircon [30.31] and Sm/Nd and Rb/Sr agps [32-361 of UHPM eclogites and some country rot/k gneisses in the eastern Dabie Shan and Sulu regionn. Ames et al. [30] and Maruyama et al. [31] also interpret their results to record the UHPM event at abotit 218 Ma. Support for the interpretation that the lower intercept ages of accessory zircon represent the agei of UHPM is provided by Sm/Nd and Rb/Sr datbng of the UHPM garnet, omphacite. whole rock ages [31] and garnet and omphacite pairs by 133.35.361: It is quite clear that zircons from both the country rock gneisses and eclogites record identical metamo$hic ageh. providing additional support for our previbus conclusion [6] that the UHPM eclogites are in situ with respect to their enclosing gneisses. Taken together these data establish that UHP metamorphism occurred very close to 220 Ma ago in the elirly Norian [37] stage of the Late Triassic. Comparison of our upper intercept age of 772.5 & 9.S Ma with previously published results (Fig. 7A) also shows good agreement. U/Pb and iPb/Pb zircon ages from the Dabie Shan and Sulu r&$ons [30]. from the northern margin of the North I$abie Complex (sample XT-3 [lo]), as well as farther west in the Tongbai complex [38]. and Wudand Shan [39] (Fig. 1C and Fig. 7A). Chapman et al. [32] also reported a Rb-Sr garnet, omphacite. bhole rock isochron age of 62 1 + 2 1 Ma from Bixili$g, which is not dissimilar to the upper intercept ages ~of eclogites reported by Ames et al. [30].
200
D.B. Rowley et al. / Earth and Phnetary
The upper intercept ages of the country gneisses obtained here, as well as those reviewed above, at about 760 + 20 Ma are interpreted as documenting the age of widespread intrusion of protolith granites within the Qinling-Tongbai-Dabie-Sulu belt. Existing data from the eclogites are not sufficiently good and too few to determine with certainty whether some are the same age or younger. We suggest, as did Ames et al. [30], that the co-occurrence of mafic and felsic igneous activity is reasonably interpreted as occurring within a rift setting. Thus, we suggest that rifting occurred along the northern margin of the South China block during the Late Proterozoic Sinian stage of China. Support for this interpretation is provided by relationships preserved in the alongstrike, lower grade segments of this orogen in the west, in the Qinling of northwestern Hubei and southern Shaanxi provinces (Fig. IA). Basement rocks in this area are extensively intruded by both mafic and felsic dikes, sills and small plutons beneath the unconformably overlying Sinian elastics.
Science Letters ISI (I9971 IYI-203
The Pb/Pb zircon evaporation age of 762 + 0.7 Ma reported by Xue et al. [39] in fact comes from a felsic gneiss in a sliver of the basement just north of the Wudang Shan (Fig. 7A). Sinian elastics in the Wudang and Daba Shan pass stratigraphically upward into younger Sinian, Cambrian, Ordovician and Silurian strata that constitute a 5 to 8 km thick [40], Early Paleozoic passive-type shelf sequence that developed along the northern margin of the South China block. We suggest that the rifting proposed on the basis of the bimodal magmatic record of the Dabie Shan is correlative with that mapped farther west in the Qinling and that together they date at least one phase of passive continental margin formation along the northern margin of the South China block. Our interpretation of the granitic protoliths as dating from Late Proterozoic has further importance as regards the anomalous oxygen isotopic compositions observed in the Dabie Shan and Sulu region. The South China block experienced at least two phases of Late
Sm/Nd isochron: 228.4 + 6 Ma Rb/Sr isochron: 223.9 f 0.9 Ma (6) &
/A217.1I8.7
Ma (5) 1
isochron: 226.3 + 4..5 Ma Rb/Sr isochron: 219.5 LIZ 0.5 Ma (6) Sm/Nd isochrons: 210 f 9 Ma 218 1k4Ma (7) I Ey P
Orogenic
Fig. 7. (A) Map showing the distribution of Late Precambrian ages of granitic rocks, Tongbai. Dabie and Sulu regions of eastern China. (B) Map showing the distribution Dabie and Sulu regions of eastern China. Geochronologic methods: U/Pb, U/Pb evaporation method: Sm/Nd and Rb/Sr isochron, Sm/Nd and Rb/Sr internal mineral This paper, (2) Xue et al. [IO], (3) KrGner et al. [38]. (4) X ue et al. [39]. (5) Ames et Jahn [36].
w E
HPM zlone
L_Ql UHPM zone granitic protoliths. or sources of inheritance in the of dates of ultrahigh-pressure metamorphism in the zircon ages; Pb/Pb. single-zircon 207Pb/2”6Pb isochron methods respectively. Sources of data: (1) al. [30]. (6) Li et al. [33.35] and (7) Chavagnac and
D.B. Rowley et al./
Earth and Planrtuty
Proterozoic glaciation. The earlier phase is represented stratigraphically by the widely distributed Changan tillite while the later phase is represented by the Nantuo tillite [4 1,421. The Changan glaciation occurred within the interval between 800 Ma and 750 Ma, while the Nantuo is believed to have occurred between about 740 Ma and 700 Ma [41,42]. Thus, there is an overlap in age between the Changan glaciation and the rift-related magmatism along the northern margin of the South China block. This magmatism. at least in the Dabie Shan and Sulu region. drove local hydrothermal circulation systems in which high latitude, “O-depleted meteoric water interacted with and variably altered the UHPM protoliths now exposed in the eastern Dabie Shan. Isotopically anomalous oxygen values have been obtained from both the Wumiao and Shima localities (Fig. ICI from which we have obtained the 772.5 A 9.5 Ma protolith ages. Samples of the eclogite intruded by orthogneiss discussed above both show low filxO of about - 5%~ [9] (Fig. 1C) and thus share this premetamorphic alteration event. as well as the UHPM and subsequent nearly pervasive amphibolite facies retrogression. This common history, apparently dating back to at least the Late Proterozoic. allows a further demonstration of the in situ origin of the UHPM eclogites in the eastern Dabie Shan. Thus it appears that the co-occurrence of very anomalous oxygen isotopes and ultrahigh-pressure metamorphic rocks can be reasonably interpreted as a fortuitous coincidence. The upper intercept age of MW-03, even with its large uncertainty. does not overlap with the ages of the granitic host gneisses. Recent oxygen isotope data from the Maowu eclogite [ 131 are low (2-4%) compared with eclogites elsewhere, but are not extreme. Again, to the west along strike in the Qinling there is a second phase of magmatism characterized by abundant mafic volcanism in the Early Silurian [43]. Post-Early Silurian erosion followed by southward onlap of a Devonian to Triassic north-facing passive shelf sequence implies a second phase of rifting along the northern margin of the Yangtze block. The upper intercept age of the Maowu eclogite is compatible with having intruded simultaneously with this second phase of mafic magmatism. The Yangtze block was in low latitude throughout the Paleozoic [44], hence any hydrothermal alteration
Science Letters I51 (I9971
IYI-203
‘0
I
would not be expected to be associated with anomalous oxygen isotopic compositions in accord with the recent data [ 131.
7. Conclusions U/Pb zircon dating of granitic gneisses from the eastern Dabie Shan reveals a three-stage history. The earliest stage. represented by the upper intertcept ages of zircons extracted from orthogneisses ifrom the UHP terrain yield an age of 772.5 + 9.5 #Ma. This age is comparable to upper intercept ages obtained by Ames et al. [30] from the Sulu and Dabie Shan regions, and zircon evaporation ages in thq Tongbai complex [38] and northern Wudang Shan i[39]. The age is also consonant with inference based on stratigraphy and mafic and felsic magmatism fa$ther west in the Qinling that are most reasonably intekpreted as reflecting rifting along the northern mardin of the South China block during the Late Protercizoic. The co-occurrence of depleted ‘“0 values in both orthogneisses and eclogites of the eastern Dbbie Shan and Sulu is interpreted to reflect the development of hydrothermal circulation associated with this magmatism. in which high-latitude. presumably ‘“O-depleted meteoric water interacted with and variably altered the UHPM protoliths. That the madmatism is essentially synchronous with widespread evidence ot early Sinian glaciation on the South ChinU block is taken as further support for this interprbtation. A second phase of rifting is also observed along the northern margin of the South China block during the late Early Paleozoic also characterized by magmatic activity: we interpret the upper intercept ilge of the Maowu eclogite as being associated with this event. The absence of anomalously low 6”O in the Maowu eclogite is compatible with its later intrusibn into the Yangtze basement. Finally, subduction of segments of the northern. attenuated margin of ~the South China block to mantle depths and devel{,pment of UHP metamorphic mineral assemblages ins dated as occurring at 2 18.5 + I .7 Ma. The linearity of discordance among zircon samples from locailities now almost 20 km apart implies that there is dirtually no diachroneity to the age of the UHP metalmorphism. and that our samples share protoliths @ith nearly identical ages. The complete congruity of structural
202
D.B. Row1e.v et al./Earth
and Planetary Science Letters I51 (1997) 191-203
observations [6], oxygen isotopes [9] and identical ages of metamorphic growth of zircons in both eclogites and country rock gneisses demonstrates that all of these rocks share a common history extending from at least the Late Proterozoic through UHPM to the present, and hence demonstrate that at least in this region all the rocks record an in-situ development of UHP metamorphism.
Acknowledgements Helpful reviews by G. Gehrels, J.G. Liou, Olivier Bruguier, R.N. Clayton are gratefully acknowledged. Jin Yugan of the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences provided outstanding logistical support. Ian Steele provided assistance with cathodoluminescence imaging. Support from NSF (EAR-9305 107) to DBR is also gratefully acknowledged. RV
References J.G. Lieu. R.Y. Zhang, E.A. Eide, S. Maruyama. X. Wang. W.G. Ernst, Metamorphism and tectonics of high-P and ultrahigh P belts in Dabie-Sulu Regions, eastern China, in: A. Yin and T.M. Harrison (Eds.), The Tectonic Evolution of Asia, Rubey Vol. IX. Cambridge Univ. Press, 1996. pp. 300-344. 121R.G. Coleman and X. Wang, Ultrahigh Pressure Metamorphism, Cambridge Univ. Press, 1995, 528 pp. 131 D.C. Smith, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics. Nature 3 10 (1984) 641644. of [41 W.L. Griffin and D.A. Carswell, In situ metamorphism Norwegian eclogites: an example, in: D.G. Gee and B.A. Sturt (Eds.), The Caledonide Orogen-Scandanavia and Related Areas, Wiley, New York, 1985, pp. 813-822. coesite[51 X. Wang, J.G. Liou, Regional ultrahigh-pressure bearing eclogitic terrane in central China: Evidence from country rocks, gneiss. marble, and metapelite. Geology 19 (19911 933-936. [‘51 F. Xue, D.B. Rowley, J. Baker, Refolded syn-ultrahigh pressure thrust sheets in the South Dabie complex, China. Field evidence and tectonic implications, Geology 24 (19961 45% 458. 171 B. Cong, M. Zhai. D.A. Carswell. R.N. Wilson, Q. Wang, Z. Zhao, B.F. Windley. Petrogenesis of ultrahigh-pressure rocks and their country rocks at Shuanghe in Dabieshan. Central China, Eur. J. Mineral. 7 (19951 119-138. 181T.Z. Yui, D. Rumble III. C.H. Lo. Unusually low S180
ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China, Geochim. Cosmochim. Acta 59 (19951 28592864. [91 J. Baker. A. Matthews, D. Mattey, D. Rowley, F. Xue. Fluid-rock interactions during ultra-high pressure metamorphism, Dabie Shan, China, Geochim. Cosmochim. Acta 61 (19971 1685-1696. [lOI F. Xue, D.B. Rowley. R.D. Tucker and Z.X. Peng, U-Pb zircon ages of granitoid rocks in the North Dabie Complex. eastern Dabie Shan, China, J. Geol.. in press. [I 11 R.Y. Zhang, J.G. Liou, H. Tsai. Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China, J. Metamorph. Geol. 14 (19961319-333. [I21 T.-F. Yui. D. Rumble III, C-H. Chen, C.-H. Lo. Stable isotope characteristics of eclogites from the ultra-high-pressure metamorphic terrain, east-central China, Chem. Geol. 137 (1997) 135-147. iI31 D. Rumble III, R. Zhang, J.G. Liou and Q.C. Wang, The Qinglongshan oxygen isotope anomaly in coesite-facies eclogites of eastern China, Geol. Sot. Am. Abstr. Programs, 1996. 1141 S.T. Xu. A.I. Okay, S. Ji, A.M.C. Sengor, W. Su, Y. Liu, L. Jiang. Diamond from the Dabie Shan metamorphic rocks and its implications for tectonic setting, Science 256 (19921 80-83. [151 A.I. Okay. Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China, Eur. J. Mineral. 5 ( 19931 659-675. [I61T.E. Krogh, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotope age determinations. Geochim. Cosmochim. Acta 37 (1973) 485-494. [I71 T.E. Krogh, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta 46 (19821 637-649. [I81 R.D. Tucker, T.E. Krogh and A. Raheim. Proterozoic evolution and age-province boundaries in the central part of the Western Gneiss Region, Norway: results of U-Pb dating of accessory minerals from Trondheimsfjord to Geiranger, in: C.F. Gower, T. Rivers and B. Ryan (Eds.). Mid-Proterozoic Laurentia-Baltica, Spec. Pap. 38. Geol. Assoc. Can., 1991, pp. 149- 173. [I91 A.H. Jaffey, K.F. Flynn, L.E. Glendenin, W.C. Bentley, A.M. Essling, Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev. Sect. C. Nucl. Phys. 4 (19711 1889-1906. of terrestrial lead DO1 J.S. Stacey. J.D. Kramers, Approximation isotope evolution by a two-stage model, Earth Planet. Sci. Lett. 26 (1975) 207-22 1. 011 D.W. Davis. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19 (19821 21413149. [22] R.W. Hinton, B.G.J. Upton, The chemistry of zircon: variations within and between large crystals from syenite and alkali basalt xenoliths, Geochim. Cosmochim. Acta 55 (199 1) 3287-3302.
D.B. Rowlq
[23]
A.V.
Murali,
T.M.
et (11./Earth
Parthasarathy. D.M.
and Planeta?
Sankar, Trace ele-
ment characteristics. REE patterns and partition coefficients of rircons from different geological environments-A study on Indian (1983)
zircons. Geochim.
Cosmochim.
case Acta 47
[24] E. Zinner and C. Crozaz.
Ion probe determinations
of the
abundances of rear earth elements in single mineral grains. in: A. Beal (Ed.). SIMS V, Secondary Ion Mass Spectrometry. Springer, Berlin, Hildreth.
Gradients in Temperature,
Chamber
of the Bishop Tuff:
Pressure, and Composition. Ph.D.
Thesis, Univ. Calif.. Berkeley,
1977. New York.
1977, 364 pp. 1271 K.H.
Handbook
of
Geochemistry.
Springer.
S.V. Goryainov.
and 40Ar/3YAr
Area
rocks
and their
in the Dabie
oh ultrahigh-
country
Mountains,
rock.; at
Cenrral
China:
Cooling history and time sequence of the continedtal subduction, ICG Abstracts, Beijing. 0809013. [36] V. Chavagnac,
1996.
B.-m. Jahn. Coeaite-bearing
Complex,
Dabie
Mountains.
eclogites from Chida:
Sm-Nd
ages, geochemical characteristics and tectonic implications. Chem. Geol.
I33 (1996)
29-5 I
Gradstein. F.P. Agterberg. J.G. Ogg. J. Hardenhol.
van Veen. J. Thierry
and Z.H.
P.
Huang. A Triassic. Jurassic Berggren, D.V.
Kent.
M.P. Aubry and J. Hardenbohl (Eda.). Geochronlology Time Stratigraphic
Correlation.
Sediment. Geol.) Spec. Publ., Tulsa.
SlEPM (Sot.
1995. 54. pp. 95-126.
[38] A. Kraner. G. Zhang. Y. Sun. Granulitez, in the Tongbai area.
regions as an unique container of inclusions of diamond.
Qinling belt, China: Geochemistry.
coe\itr
geochronology. and implications for the tectonic cvolutlon of
and coexisting
minerals.
Dokl.
Akad.
Nauk
334
(1994) 488-492.
eastern A\ia. Tectonics 12 (1993)
[29] R.D. Tucker. W.S. McKerrow.
Early Paleozoic chronology: a
review in light of new UP\, zircon ages from Newfoundland and Britain. Can. J. Earth Sci. 32 (1995) [30] L. Ames. G. Zhou. B. Xiong. character of ultrahigh-pressure
368-379.
Geochronology
and isotopic
metamorphism with implica-
tions for collision of the Sino-Korean
and Yangtze Cratona.
central China. Tectonic< 15 (1996) 472-489.
II
metamorphic
Scales and Global
Sobolev, V.S. Shatsky, M.A. Vavilov,
Zircon from ultrahigh-pressure metamorphic rocks of folded
[3
pressure
and Cretaceous Time Scale. in: W.A.
Wedepohl.
Berlin, 1969-1978. [28] N.V.
Rb-Sr 343-3.50.
[35] S. Li. E. Jagoutz and Y. Chen, Geochronology
[37] F.M.
1261 G. Faure, Principles of Isotope Geology. Wylie.
‘03
19/-203
ing of eclogite minerals by Sm-Nd.
the Bixiling
1986. pp. 444-446.
The Magma
151 (1997)
methods, Chem. Geol. 112 (1994)
Shuanghe
2047-2052.
[2S] E.W.
Science Letters
S. Maruyama.
[39] F. Xue. A. Kriinrr.
ling erogenic belt. central China as documented by zircon 409-3
17.
[40] Regional Stratigraphic Tables of Shaanxi Provmce, Peoples [41] S. Liao, Sinian glacial deposits of Guizhou Province. China, in: M.J.
Pleistocene Glacial
1 12-121,
Chapman. J. Baker. M.J. Bickle, .I. Bunbury, A. Matthews
ayhtematics due to ultmhigh
of Nd. Sr. and 0 isotopic
pressure metamorphism of hy--
Hamhrey
China. in: M.J. Hambrey
Goldschmidt Confer., J. Conference Abstr., 101, 1996.
1981. pp. 386-101.
D.L.
Lieu.
Y.Z.
Chen. N.J. Ge. Z.Q.
Harland
Record. Cambridge
(Eds.).
Glacial
and W.B. Record.
Geology
summary). 698 pp.. Geol. Pub]. House. Beijing,
Chem. Geol..
1993.
Excess argon in phengitr from eclogite: Evidence from dat-
Univ.
Press.
[43] Shaanxi Bureau of Geology and Mineral Resources. Regional
Wang. Collision of the North China and Yang&e blocks and
[34] S. Li. S. Wang. Y. Chen. D. Liu, J. Qiu. H. Zhou, Z. Zhang.
Pre10x1.
Harland (Eda.), Earth’s
Cambridge
Zhang. S.S. Sun. B.L. Gong, R.Y. Zhang, S.R. Hart and S.S. formation of coesite-bearing eclogites: timing and processes.
Earth’s
Univ. Pre\s,
pp. 4 I I-423.
Pre-Pleistocene
Xiao.
and W.B.
[42] Y. Wang. S. Lu. Z. Gao. W. Lin and G. Ma. Sin/an tillites 01
drothermally altered eclogites. Dabie Shan. China. 1996 V.M. [33] S.G. Li. Y.L.
Lerch. Paleozoic
ages on granitoid rockh, J. Gcol. Sot. London’ 153 (1996)
hells from central Chind, Island Arc 3 (1994) Characterization
M.F.
magmatism in the Qin-
Republic of China, Geol. Pub]. House. Beijing, 1071, 450 pp.
J.G. Liou. R.Y. Zhang, Tectonic evolution ol
and D. Rowley,
235-255.
T. Reischmann.
pre- and post-collision talc-alkaline
the ultrahigh-pressure (LJHP) and high-pressure metamorphic
[??I H.
petrology. single zircon
of Shaanxi
[44] S. Nie, Paleoclimatic
Province,
(in Chinese
with English 19X9.
and paleomagnetic constiaints on the
Paleozoic reconstructions of south China. north China. and Tarim. Tectonophysics
196 (199 I) 279-70X.