The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications

Chemical Geology 178 Ž2001. 143–158 www.elsevier.comrlocaterchemgeo

The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications Bor-ming Jahn a,) , Renaud Caby b, Patrick Monie c a Geosciences Rennes, UMR-6118-CNRS, UniÕersite´ de Rennes 1, 35042 Rennes Cedex, France ´ Laboratoire de Tectonophysique, UMR 5568 CNRS, UniÕersite´ Montpellier II, Place Eugene ` Bataillon, 34095 Montpellier Cedex 5, France Laboratoire de Geophysique, Tectonique et Sedimentologie, UMR 5573 CNRS, UniÕersite´ Montpellier II, Place Eugene ´ ´ ´ Bataillon, 34095 Montpellier Cedex 5, France b

c

Received 11 May 2000; accepted 29 January 2001

Abstract Coesite-bearing eclogitic rocks from the Pan-African belt in northern Mali were dated by multichronological methods ŽRb–Sr, Sm–Nd and Ar–Ar.. Rb–Sr and Sm–Nd isotope analyses on whole rock and mineral separates of an omphacite– kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 Ma. This is interpreted as the time of the eclogitisation, which represents the oldest ultrahigh-pressure ŽUHP. metamorphic event so far recorded in the continental crust. The nearly identical Rb–Sr and Sm–Nd ages suggest very rapid exhumation as observed in many Phanerozoic UHP rocks. 40Ar– 39Ar dating of phengites from two UHP micaschists gave much older ages of 1045 and 760 Ma. Phengite from a quartzite collected from the same site yielded a plateau date of 623 " 3 Ma, which is in agreement with the Rb–Sr and Sm–Nd dates. The two 40Ar– 39Ar ages, older than 625 Ma, testify once more the recurrent problem of excess Ar in ultrahigh-pressure phengites. The occurrence of the Mali UHP eclogites indicates that by the late Precambrian, the Earth has cooled enough to sustain the formation and preservation of deeply subducted UHP metamorphic rocks. The new ages of ca. 620 Ma also constrain the amalgamation in NW Gondwana to have occurred in the latest Proterozoic, but not in the Middle Cambrian. This is supported by several recent chronological data for the southern segment of the major suture zone in the Dahomeyide orogenic belt. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ar–Ar dating; Mali; Pan-African; Rb–Sr dating; Sm–Nd isotopes; UHP eclogite

1. Introduction

)

Corresponding author. Tel.: q33-2-99-28-60-83; fax: q33-299-28-1499. E-mail address: [email protected] ŽB. Jahn..

Ultrahigh-pressure ŽUHP. metamorphic rocks of continental origin provide indisputable evidence for subduction of the continental lithosphere to mantle

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 1 . 0 0 2 6 4 - 9

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B. Jahn et al.r Chemical Geology 178 (2001) 143–158

depths over 100 km. UHP eclogitic rocks and highpressure ŽHP. blueschists mainly occur in Phanerozoic orogenic belts, such as in the Caledonides ŽNorwegian Western Gneiss Region., Uralides, Hercynides ŽBohemian Massif., Indosinides ŽDabie-Sulu. and Alpides ŽDora Maira., but are rarely found in Precambrian terranes ŽMaruyama et al., 1996;

Maruyama and Liou, 1998.. The oldest blueschist belt occurs along the Neo-Proterozoic Jiangnan belt of SE China Žca. 900 Ma, Shu and Charvet, 1996., and the oldest UHP eclogites were reported to occur in northern Mali in West Africa ŽCaby, 1994., but the age of this UHP metamorphism remains poorly constrained until the present. Other Neoproterozoic

Fig. 1. Geological maps of Ža. the Trans-Saharan segment of the Pan-African belt, and Žb. the Gourma area showing the UHP eclogites and related litho-tectonic units.

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

145

kyanite eclogites have also been found in Togo ŽMenot and Seddoh, 1985; Bernard-Griffiths et al., 1991. and Zambia ŽCosi et al., 1992., but no UHP mineralogy was documented from these occurrences. In this paper, we report new Sm–Nd, Rb–Sr and 40 Ar– 39Ar geochronological results on the UHP eclogites and associated rocks from northern Mali, and hence establish the world’s oldest UHP metamorphic event. A small set of geochemical and Sr–Nd isotopic data will be used to constrain the nature of protoliths and in the end, we discuss tectonic implications of the Pan-African orogeny in the Trans-Saharan segment.

without biotite in metapelites and by kyanite in aluminous quartzites. Regional EW-trending stretching and mineral lineation indicate a westward tectonic transport and the nappes root towards the suture zone ŽCaby, 1979, 1994.. The northern part of the internal nappes consists of coarse-grained eclogitic micaschists, quartzites and eclogites Žs the Eclogite unit; Fig. 1b.. The Eclogite unit forms a flat, less than 3-km-thick imbricate assembly, dipping westward and underlying phyllites of the external nappes. All samples studied by Caby Ž1994. and those used in the present investigation come from the In Edem Well.

2. General geologic setting

3. Sample description

Coesite-bearing eclogites occur in the nappes of the Gourma area in northern Mali, which is in the southern Saharan segment of the Pan-African belt in northwestern Africa ŽFig. 1a.. The G 1000-km-wide Pan-African orogen was developed along a N–Strending zone east of the West African craton. Evidence for the closure of the Pan-African ocean and ensuing continental collision prior to 500–600 Ma has been documented ŽCaby et al., 1981.. The orogen comprises in the Tuareg shield a package of terranes including Neoprotezoic juvenile crust and reworked basement ŽBlack et al., 1979; Caby, 1987.. The western part includes, in northern Mali, an active continental margin in the east, and a passive continental margin in the west, separated by the Pan-African suture zone ŽCaby, 1994.. The passive continental margin of the West African craton is well characterized in the Gourma basin ŽFig. 1b.. The basin is filled with shelf carbonates grading eastwards into turbiditic slope facies overlain by a thick terrigeneous turbidite sequence. The inner part of the basins is in direct contact with the Gourma fold-and-thrust belt that forms a convex arc of about 1208 curvature ŽCaby, 1979.. The external nappes, affected by greenschist facies metamorphism, are mainly composed of phyllites similar to those of the easternmost part of the basin. By contrast, the internal nappes comprise micaschists and quatzites displaying a refolded recumbent foliation. Their syn-kinematic high-pressure metamorphism is represented by phengite–garnet–rutile assemblages

Forty samples were collected in a small area Ž200 = 300 m2 . between the In Edem Well and the Niger River. The petrographic characteristics have been described by Caby Ž1994.. The principal rock types include metaquartzite Žquartz-garnet-omphacite-kyanite-phengite-carbonate-zoisite-rutile., eclogitic marble and mafic eclogite. All rock types are complexly interlayered. Marble layers contain omphacitic schlieren and coesite-bearing omphacitite nodules. Metaquartzite seems to represent the most abundant exposed rock type. Micaschist occurs as layers of a few meters thick, containing thin quartzite layers of a few centimeters wide. Some «impure» quartzite layers show zoisite-rich or garnet-rich bands of a few millimeters to a few centimeters thick. Thicker garnet–quartz layers suffered some mobilization in the form of contorted quartz veins, which contain omphacite, rutile andror garnet. Due to the poor exposure between sand dunes, no map showing detailed lithological relationship, even on the outcrop scale, was produced. Mafic ecologites form two layers about 5-m thick, intercalated with quartz-rich micaschists and quartzites. A sharp contact with enclosing micaschists was observed at the margin of one ecologite layer, suggesting that the two eclogite layers may represent mafic sills. However, some cryptic compositional layering defined by variable abundances of quartz and ferroan dolomite Ž XCa s 0.49, X Mg s 0.45, X Fe s 0.06; Caby, 1994., as well as leucocratic nodules with quartz and zoisite equally suggest that

146

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they do not represent meta-igenous rocks; instead, they could have derived from Ca–Fe–Mg layers or lenses Ži.e. calc-silicates. within the sedimentary piles as commonly observed in Proterozoic metasedimentary sequences. Three samples were selected for Rb–Sr and Sm– Nd dating and bulk chemical analyses. S504 and S506-3 are mafic eclogites containing omphacite, garnet, rutile as major phases, and quartz, phengite, ferroan dolomite, zoisite and secondary amphiboles as accessory minerals. The linear fabric of the ecologites is defined by acicular omphacite, and is cut by centimeter-thick veins made up of coarsergrained omphacite and garnet Ž0.5 to 1 cm.. The veins showing no preferred orientation or fabric, are interpreted as syn-metamorphic segregations. Sigmoidal radiating fractures in garnet are present in the mafic eclogites, and this feature suggests syn-kinematic inversion of coesite ŽCaby, 1994.. Coesite is not observed in the present samples, but was observed in an omphacitic nodule enclosed in marble ŽCaby, 1994.. Sample S520 is a quartz-poor, kyanite–omphacite micaschist. It contains G 70% of phengitic mica and subordinate amounts of kyanite, garnet and omphacite. Quartz is extremely rare. The micaschist forms a distinct layer within quartz– micaschist and impure quarzite in which coarsegrained omphacite and phengite segregates are also. No plagioclase or biotite is observed in the analysed samples, though these occasionally appear in a few retrogressed samples. Actinolite and albite also occur as late kinematic thin veinlets within retrogressed mafic eclogites. For the 40Ar– 39Ar dating, phengite was separated from the same micaschist ŽS520. plus two or more samples, a micaschist Žsample a1219. and a quartzite Žsample aIC1031., from the same region. Chemical data from microprobe analyses on the constituent minerals were used by Caby Ž1994. to estimate the metamorphic conditions. Using the Fe– Mg partitioning between garnet and omphacite and the geothermometer of Ellis and Green Ž1979., a temperature of 6908C to 7508C Žat 25 kbar. was obtained for the kyanite–omphacite micaschist Žsample S520.. The occurrence of coesite in omphacitite nodule and quartz pseudomorph after coesite in this sample and other mafic eclogites argues for equilibrium pressure in excess of 27 kbar. This P–T condi-

tion is comparable to that of the Dabie terrane in China ŽWang et al., 1990; Okay, 1994; Cong et al., 1995; Zhang et al., 1995; Liou et al., 1996, 1998; Carswell et al., 1997..

4. Analytical procedures 4.1. Rb–Sr and Sm–Nd analyses The analytical procedures for isotopic analyses are the same as reported in Jahn et al. Ž1996.. Analytical precisions, Sr–Nd isotope standard and normalization values, and blank levels can be found in the footnotes of Table 1. The decay constants Ž l. used in age computation are: 87 Rb s 0.0142 Gay1 and 147 Sm s 0.00654 Gay1 . Sm–Nd model ages are calculated assuming a linear Nd isotopic growth of the depleted mantle reservoir from ´ Nd s 0 at 4.56 Ga to ´ Nd s q10 at the present.

½

T DM I s Ž 1rl . ln 1 q r

Ž 143 Ndr144 Nd. s y 0.51315

Ž 147 Smr144 Nd. s y 0.2137

5,

where s s sample and l s decay constant of 147 Sm Ž0.00654 Gay1 .. Rb–Sr and Sm–Nd isochron calculations were done using the regression programs of IsoplotrEx version 2.06 ŽLudwig, 1999.. Input errors used in age computations are: 147 Smr 144 Nd s 0.2%, 143 Ndr144 Nd s 0.005%; 87 Rbr86 Sr s 1.5%, and 87 Srr86 Sr s 0.005%. Analytical precision of isotope ratio measurement is given as "2 standard errors Ž2 sm ., whereas the quoted errors in age and initial isotopic ratios represent "2 standard deviations Ž2 s .. 4.2. 4 0Ar r39Ar analyses During this study, phengites were analysed using both 40Arr39Ar step-heating of bulk separates and laser probe dating of single grains. Detailed analytical procedures have been previously described ŽMcdougall and Harrison, 1988; Monie´ et al., 1994, 1997.. Briefly, for the bulk separate analyses, the samples were irradiated together with different flux

Table 1 Rb–Sr and Sm–Nd isotope data for rocks and minerals from the Gourma UHP terrane, northern Mali, West Africa wRbx wSrx Žppm. Žppm.

WR 6.29 WR 1.27 garnet cpx-1 cpx-2 Micaschist WR Ž1. 107.5 WR Ž2. 106.5 phengite 420.9 garnet cpx 8.85

87 86

Rbr Sr

87 86

Srr Sr

"2 sm ISr T UR Ž600 Ma. ŽGa.

S504 Eclogite S506.3 Eclogite

533.0 0.034 46.66 0.079

0.705702 8 0.707405 8

0.70541 0.70673

S520

316.0 312.8 774.1

0.984 0.985 1.574

0.721400 8 0.721394 7 0.726714 6

0.71298 0.71297 0.71325

170.8

0.150

0.714130 8

0.71285

wSmx wNdx Žppm. Žppm.

y1.34 3.798 13.77 y27.77 1.879 4.82 2.183 2.024 0.500 1.557 0.500 1.530 1.30 9.777 58.71 1.30 58.72 0.923 5.36 3.228 5.88 2.040 7.03

147 144

Smr Nd

0.1668 0.2355 0.6521 0.1941 0.1975 0.1007 0.1042 0.3317 0.1752

143 144

Ndr Nd

"2 sm ´ Nd Ž0. ´ Nd ŽT . f SmrNd TDM Ž600 Ma. ŽGa.

0.512635 4 0.512887 4 0.514501 4 0.512640 4 0.512710 5 0.511454 4 0.511437 4 0.511461 13 0.512395 6 0.511746 5

y0.1 4.9 36.3 0.0 1.4 y23.1 y23.4 y23.0 y4.7 y17.4

2.2 1.9 1.5 0.2 1.3 y15.7

y0.15 1.67 0.20 y1.85 2.31 y0.01 0.00 y0.49 2.28

y15.9 y15.1 y15.8

y0.47 0.69 y0.11

143 Ndr144 Nd ratios have been corrected for mass fractionation relative to 146 Ndr144 Nd s 0.7219 and are reported relative to the La Jolla Nd standards 0.511860 or Ames Nd standards 0.511960. 87 Srr86 Sr ratios have been corrected for mass fractionation relative to 86 Srr88 Sr s 0.1194 and are reported relative to the NBS-987 Sr standards 0.710250. Chondritic uniform reservoir ŽCHUR.: 147 Smr144 Nd s 0.1967; 143 Ndr144 Nd s 0.512638. Used in model age calculation, Uniform Reservoir ŽUR.: 87 Rbr86 Sr s 0.087; 87 Srr86 Sr s 0.7047. Used in model age calculation, DM Ždepleted mantle.: 147 Smr144 Nd s 0.2137; 143 Ndr144 Nd s 0.51315. Blanks: Rbs 30 pg, Sr s 400 pg, Sm s 40 pg, Nd s100 pg.

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

Sample Rock type WR or number mineral

147

148

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

monitors including MMHb-1 Ž520.4 " 1.7 Ma. in the Grenoble nuclear reactor in France. For this reactor, the following correction factors for argon nuclear interferences were applied: Ž36Arr37Ar. Ca s 0.000289; Ž39Arr37Ar. Ca s 0.000676; Ž40Arr39Ar. k s 0.0307. Heating was achieved using an HF generator coupling on a Mo crucible containing about 50 mg of mica. The samples used for the laser probe dating have been irradiated in the 5C position of the McMaster nuclear reactor ŽCanada. for 70 h. The correction factors were the following: Ž36Arr37Ar. Ca s 0.000254; Ž39Arr37Ar. Ca s 0.000651; Ž40Arr39Ar. k s 0.0156. Laser analyses were conducted on separated single grains using an argon laser probe operating in the continuous mode. The analytical device consists of: Ža. a multiline continuous 6-W argon-ion laser with two main wavelengths of 488 and 514 nm; Žb. a beam shutter for selection of exposure times, typically 30 s for each heating step; Žc. optical lenses to obtain a laser beam diameter at least twice the size of the mineral being dated; Žd. a small inlet line for the extraction and purification of gases, Že. a MAP 215-50 noble gas mass spectrometer equipped with a Nier source and a Johnston MM1 electron multiplier. Each analysis involved 5 min for gas extraction and cleaning and 15 min for data acquisition. System blanks were evaluated after every three experiments and they ranged from 3 = 10y1 2 cm3 for 40Ar to 6 = 10y1 4 cm3 for 36Ar. Only the errors on total ages and plateau dates include the uncertainty on the monitor age and its 40Arr39Ar ratio.

5. Results and discussion 5.1. New geochronological data 5.1.1. Rb–Sr and Sm–Nd ages The results of Rb–Sr and Sm–Nd isotope analyses on whole rock and mineral separates of two eclogites and one micaschist samples are given in Table 1 and further displayed in three isochron diagrams. Fig. 2a is a three- or four-point Rb–Sr isochron for micaschist S520 using a phengite and two WR Žwhole rock. and a clinopyroxene data. The isochron gives an age of 617 " 7 Ma with initial 87 Srr86 Sr ratio ŽI Sr . of 0.71281 " 0.00005. Note that the WR sample has been duplicated Žrenewed disso-

Fig. 2. Ža to c. Rb–Sr and Sm–Nd isochron diagrams for a micaschist ŽS520. and a mafic eclogite ŽS506.3.. Isochron ages were calculated using IsoplotrEx of Ludwig Ž1999.. Input errors for age computation: 1.5% for 87 Rbr86 Sr and 0.005% for 87 Rbr86 Sr; 0.2% for 147 Smr144 Nd and 0.005% for 143 Ndr144 Nd.

lution and spiking. and the two results are nearly completely superposed. Inclusion of one or two WR data does not change the calculated age. Fig. 2b is a Sm–Nd isochron diagram for the same micaschist

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

sample. Four data points define an age of 624 " 20 Ma with ´ Nd ŽT . s y15.3 " 0.6. The Sm–Nd data of mafic eclogite S506.3 do not define a unique isochron for all the data points ŽFig. 2c.. The two garnet and omphacite tie-lines yield ages of 620 " 13 Ma Žgt and cpx-1. and 601 " 13 Ma Žgt and cpx-2.. The WR data point lies above both tie-lines. The non-linearity of the four data points most likely represent a result of isotope disequilibrium between peak and retrograde metamorphic minerals contained in whole rock samples. This phenomenon has been commonly observed in UHP or HP eclogites Že.g., Jagoutz, 1988; Thoni ¨ and Jagoutz, 1992; Miller and Thoni, ¨ 1995; Li et al., 2000.. In the present case, the clean omphacite fraction are considered to be in closer equilibrium with garnet than the whole rock, which contains a secondary mineral assemblage whose effect on the isotope budget is difficult to evaluate. In any case, the Sm–Nd ages for micaschist and mafic eclogite are considered identical within the error limits. They also overlap with the Rb–Sr age of the micaschist. Besides, the fact that the WR sample of micaschist has the lowest 147 Smr144 Nd ratio suggests that a substantial proportion of Nd resides in other non-analyzed accessory minerals with very low SmrNd ratios. The same problem of mass balance in eclogites has recently been investigated by Bocchio et al. Ž2000.. The WR data point of another mafic eclogite, sample S504, is shown for reference ŽFig. 2c.. It lies above the gt-cpx tie-line of sample S506-3. No individual minerals of S504 were analyzed because the size of sample collected was too small Žca. 100 g. for adequate minerals separation. Nevertheless, in view of the above results, we interpret the age of ca. 620 Ž"20. Ma as the time of the UHP metamorphism. The present resolution is not fine enough to separate second-order events such as the age of peak or retrograde metamorphism. However, the indistinguishable phengite Rb–Sr and garnet Sm–Nd ages suggest a very high cooling rate from a temperature regime of f 7508C Žgarnet Sm–Nd; Hensen and Zhou, 1995; Zhou and Hensen, 1995. to f 5008C Žphengite Rb–Sr; Cliff, 1985.. 5.1.2. 4 0Ar r39Ar ages 40 Arr39Ar isotope investigation was carried out on phengite from three samples using both bulk separate

149

analyses Žsample S520. and laser probe dating of single grains Žsamples 1219 and IC1031.. The data are given in Table 2 and further displayed as age spectra in Fig. 3. Phengite of sample S520 Žmicaschist. yields a well-defined plateau age of 1045 " 9 Ma for about 98% of the 39Ar released. A similar age of 1046 " 12 Ma is obtained in an 36Arr40Ar vs. 39Arr40Ar correlation plot with an initial 40Arr36Ar ratio of 308 " 87 ŽMSWDs 2.0.. These dates are older than the Rb–Sr and Sm–Nd ages by about 400 Ma, and thus suggest a presence of excess 40Ar in the phengite, despite the good concordance of the plateau and intercept ages and an apparent initial 40Arr36Ar ratio Ž308 " 87. close to the atmospheric value of 296 ŽFig. 3.. A phengite single grain from a similar micaschist Žsample 1219. displays a more discordant age pattern than that of the bulk separate. It gives apparent ages ranging from 770 to 745 Ma and a total age of 758 " 5 Ma ŽTable 2; Fig. 3.. In an 36Arr40Ar vs. 39 Arr40Ar correlation diagram, the data points show a poor linear relationship ŽMSWDs 14. with an intercept age of 754 " 8 Ma and initial 40Arr36Ar ratio of 398 " 128 ŽFig. 3.. By contrast, step-heating of a phengite single grain from a quartzite sample ŽIC1031. yielded a well-defined plateau date of 623 " 3 Ma and an identical intercept age of 622 " 2 Ma Ž40Arr36Ar s 322 " 33; MSWDs 0.2.. This age is concordant with the Rb–Sr and Sm–Nd ages reported above, thus precluding excess argon contamination in this sample. These new 40Arr39Ar results point again to the complexity of argon isotopic behaviour in UHP metamorphism. The range of ages between 1045 and 623 Ma for the three dated phengites and the comparison with the Rb–Sr and Sm–Nd dates reveal that isotopic disequilibrium prevails in these phengites due to the presence of excess argon. Contamination of UHP minerals by excess argon has been well documented in coesite-bearing terranes from the Western Alps in Dora Maira ŽTilton et al., 1989, 1991; Arnaud and Kelley, 1995; Scaillet, 1996. and from the Dabeishan ŽLi et al., 1994; Hacker and Wang, 1995.. A detailed study using both step-heating and spot mapping laser probe techniques suggests that under UHP conditions, 40Ar is transported only over short distances ŽScaillet, 1996., possibly resulting in the incorporation of excess argon in the

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

150

Table 2 Ar–Ar isotope analyses of phengites from the Gourma UHP terrane of Mali 39

Arr40Ar =100

37

%

S520 Phengite (bulk separate) J s 0.011041 1 55.90 0.2952 2 61.57 0.0251 3 69.72 0.0546 4 71.24 0.0813 5 71.58 0.0387 6 71.35 0.0458 7 71.51 0.1095 8 70.80 0.0658 9 71.31 0.0407 10 70.83 0.0419 11 72.06 0.0423 12 70.80 0.0792 12 71.93 0.0433 14 70.68 0.0174 15 71.88 0.0368 Total age:

1.65 1.62 1.42 1.38 1.39 1.39 1.36 1.39 1.39 1.40 1.38 1.39 1.38 1.42 1.39

0.07 0.11 0.06 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.07 0.03

0.15 0.39 0.88 1.97 4.13 8.29 15.00 43.06 63.37 82.64 84.08 94.31 97.06 98.38 100.00

8.93 0.76 1.66 2.46 1.17 1.39 3.31 1.99 1.23 1.27 1.28 2.40 1.31 0.52 1.12

867.3 935.7 1030.0 1046.9 1050.7 1048.1 1049.9 1042.0 1047.8 1042.3 1056.0 1042.0 1054.6 1040.7 1054.0 1044.4 " 8.9

7.6 7.3 8.0 8.1 8.0 8.0 8.2 8.1 8.0 8.0 8.6 8.1 8.8 9.5 8.1

1219 Phengite (single grain) J s 0.016741 1 32.71 0.7260 2 30.97 0.4948 3 30.74 0.2769 4 31.57 0.1926 5 31.46 0.0995 6 31.86 0.2948 7 30.56 0.1056 8 31.57 0.0902 9 31.46 0.0536 10 31.01 0.0888 11 30.96 0.0544 Total age:

2.42 2.78 3.01 3.01 3.11 2.89 3.20 3.11 3.15 3.17 3.20

8.07 1.44 1.99 0.33 0.01 0.09 0.00 0.00 0.01 1.74 0.04

0.35 1.98 3.53 4.91 5.84 8.52 32.38 66.16 84.14 87.49 100.00

21.37 14.53 8.15 5.71 2.97 8.65 3.14 2.68 1.58 2.60 1.63

787.9 753.6 749.0 765.5 763.4 771.2 745.5 765.4 763.3 754.4 753.5 758.2 " 4.9

68.7 18.5 19.1 26.5 15.2 8.5 1.7 1.6 2.6 9.1 2.2

IC1031 Phengite (single grain) J s 0.016741 1 27.81 1.0976 2 24.70 0.1634 3 24.70 0.1634 4 24.61 0.0444 5 24.61 0.0404 6 24.61 0.0444 7 24.73 0.0162 8 24.67 0.0163 Total age:

2.45 3.89 3.89 4.04 4.04 4.04 4.05 4.07

0.78 0.07 0.06 0.01 0.01 0.11 0.34 0.05

1.51 11.22 30.65 64.15 80.91 93.47 95.91 100.00

32.29 4.83 4.83 1.31 1.30 1.32 0.59 0.50

689.7 624.4 624.4 622.4 622.5 622.4 625.0 623.7 624.1 " 4.2

27.9 3.0 2.5 1.8 2.0 2.6 4.8 3.9

Analysis number

40

Ar ) r39Ar

36

Arr40Ar =1000

minerals formed at high pressure. Local argon transport during HP metamorphism has also been demonstrated by Maurel et al. Ž2000.. A combined 40Arr39 Ar and U–Pb study led these authors to conclude that 520-Ma-old magmatic biotite from a Variscan eclogitized meta-granite has remained closed to argon diffusion during subduction and subsequent ex-

Arr39Ar

39

Ar

%

40

Ar atm.

Age ŽMa.

"2 s

humation in the absence of penetrative deformation and fluid interaction. Consequently, we think that the excess argon in phengites from the Mali UHP micaschists is a locally derived component from the protolith that has been trapped during their metamorphic recrystallisation at great depth. The lack of large-scale Ar diffusion at such depth is probably due to a

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

Fig. 3. Ar–Ar age spectra and 36Arr40Ar vs. 39Arr40Ar correlation diagrams for three phengite samples from micaschists ŽS520 and 1219. and quartzite ŽIC1031..

151

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

152

Table 3 Chemical compositions of UHP rocks from Mali No. CRPG Sample No. Rock type Locality

9910424 S504 Eclogite Mali

9910425 S506-3 Eclogite Mali

9910426 S520 Mica schist Mali

SiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2O TiO 2 P2 O5 LOI Total Cs Rb Sr Ba Be Nb Ta Th U Pb Zr Hf Y V Co Ni Cr Mo W Cu Zn Cd Ga In Ge Sn As Sb Bi La Ce Pr Nd Sm Eu Gd Tb Dy

47.74 18.47 11.81 0.16 4.64 12.84 2.26 0.12 1.32 0.11 0.43 99.90 0.4 7.1 512.4 41.9 0.35 4.1 0.31 1.3 0.5 4.0 61 1.6 18.9 338 40.1 45.8 24.7 0.1 0.1 138 113 0.20 24 0.10 1.7 0.7 0.3 0.04 0.07 7.06 18.53 2.79 12.23 3.69 1.37 3.46 0.53 3.14

46.71 17.24 14.83 0.20 5.14 11.59 2.60 0.04 1.29 0.08 0.17 99.89 0.2 1.9 47.1 10.8 0.65 3.4 0.26 0.2 0.0 0.5 65 1.6 22.1 452 50.4 53.3 22.0 0.2 0.1 88 113 0.20 22 0.08 1.7 0.8 0.2 0.01 y0.01 2.03 4.81 0.83 4.53 1.89 0.75 2.53 0.46 3.27

62.03 16.20 5.03 0.05 4.71 4.33 2.32 2.35 0.87 0.18 1.86 99.93 6.4 113.0 308.1 776.0 1.66 18.6 1.80 27.1 2.8 7.5 127 3.3 25.3 72 14.7 156.9 126.3 0.4 1.8 13 61 0.03 22 0.09 2.5 3.1 0.6 y0.01 0.14 67.59 126.46 15.50 55.30 10.11 1.75 6.04 0.84 4.40

S504 S506.3 ŽAnalyzed in Montpellier. 49.70 18.75 11.42 0.14 4.68 11.28 2.20 0.17 1.10 0.12 0.26 99.82

48.75 17.50 13.50 0.18 4.85 10.30 2.72 0.04 1.11 0.10 0.02 99.07

BJ93-15 BJ93-29 Eclogite hosted in marble Dabieshan 45.51 15.76 13.11 0.20 5.78 13.03 1.44 0.67 1.49 0.14 1.69 98.82 19 169 340 16.8

45.24 13.19 10.71 0.14 6.96 12.23 3.75 0.80 0.73 0.04 5.11 98.90 4 260 274 4.4

-1

-1

19 121

26 66

35 248 26 14 77

19 242 33 64 157

25 97

56 91

17

6

24.91 9.26

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

153

Table 3 Ž continued . No. CRPG Sample No. Rock type Locality

9910424 S504 Eclogite Mali

9910425 S506-3 Eclogite Mali

9910426 S520 Mica schist Mali

Ho Er Tm Yb Lu EurEu )

0.62 1.75 0.22 1.65 0.24 1.19

0.74 1.78 0.25 1.88 0.23 1.07

0.84 2.43 0.33 2.41 0.38 0.69

S504 S506.3 ŽAnalyzed in Montpellier.

BJ93-15 BJ93-29 Eclogite hosted in marble Dabieshan

Major and trace element abundances of two eclogite and one micaschist were analyzed by ICP-AES and ICP-MS, respectively, at the CRPG-Nancy. Lithium borate flux was added to the powdered samples for total fusion and followed by complete dissolution in HNO 3 .

pressure effect on the fluid activity and argon diffusion in the eclogitized rocks. On the other hand, phengite IC1031 appears uncontaminated by excess argon as argued from the concordance of the Ar–Ar with the Rb–Sr and Sm–Nd ages. In this sample, the quartz-rich composition of the protolith has probably promoted more efficiently argon circulation along grain boundaries due to the high ductility of quartz. The arrangement between Rb–Sr, Sm–Nd and 40 Arr39Ar ages indicates that a state of isotopic equilibrium had been reached as the rock was exhumed from depth. Once again, the concordant ages obtained from the three chronometers with blocking temperatures ranging from f 7508C ŽSm–Nd garnet. to f 4008C ŽAr–Ar phengite; assumed to be similar to K–Ar muscovite, Kirschner et al., 1996. tend to argue for a high rate of cooling in this temperature interval. 5.2. Protolith characteristics The results of chemical analyses are presented in Table 3. Pertinent arguments for the protolith characterization are presented below. 5.2.1. Mafic eclogites The high Al 2 O 3 contents Ž18.5 and 17.2 wt.%. and low Ni–Cr concentrations Ž46–53 and 25–22 ppm. of two mafic eclogites invite a comparison with a calc-alkaline or high-Al basalt from subduction zones. However, the eclogites occur as discontinuous lenses of a few meters thick within quartzite, showing both sharp and gradational contacts. Caby Ž1994. interpreted the protolith of the eclogitic lenses

as basaltic sills, but this interpretation may be subject to change. Mafic eclogites occurring as lenses within or interbeded with marble and quartzite layers are also found in several places in Dabie UHP metamorphic terrane Že.g., Taolichung and Xindian.. It has been suggested that these eclogites could have been derived from marl or calcareous silicate sedimentary layers ŽJahn, 1998, p. 228.. A comparison of major element abundances between the Mali and Dabie eclogites reveals a striking similarity except higher Al 2 O 3 in the Mali rocks ŽTable 3.. A sedimentary origin may be supported by the following arguments: Ž1. the overall chemical compositions cannot be matched by the commonly known magmatic rocks; Ž2. for the Mali eclogites, despite their similarity in major element compositions, the 10- to 4-fold difference in some trace element abundances, such as Sr Ž510 vs. 47 ppm., Ba Ž42 vs. 11., Th Ž1.4 vs. 0.2., and U Ž0.5 vs. 0.04., cannot be explained by a normal magmatic differentiation, but it can be easily accounted for by different amounts of accessory minerals incorporated in the sediments; Ž3. occurrence of ferroan dolomite is not compatible with magmatic rocks. The field relationship and the difference in REE patterns ŽFig. 4a. and spidegrams ŽFig. 4b. may not be strong arguments, but they are not at variance with a sedimentary origin.

5.2.2. Micaschist Geochemically, the micaschist has roughly a tonalitic to granodioritic bulk composition ŽTable 3. and a typical granitoid REE pattern ŽFig. 4a. as well as negative anomalies in Nb, P, Zr and Ti in the

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whereas the Sm–Nd data yielded a TDM of 2.3 Ga ŽTable 1..

Fig. 4. Ža. Chondrite-normalized REE patterns and Žb. primitivemantle ŽPM.-normalized spidergrams. ŽChondrite values from Masuda et al. Ž1973. divided by 1.2; PM from Sun and McDonough, 1989..

spidegram ŽFig. 4b.. Its Rb, Sr, and Ba contents Ž113, 308, and 776 ppm. and RbrSr ratio Ž0.37. are also typical of granitic rocks. However, the unusually high MgO Ž4.71%. and Ni–Cr contents Ž157, 126 ppm. prevent a direct comparison between the micaschist and a common granitoid protolith. In fact, their peraluminous nature ŽArCNK s 1.14. and abundant white mica in the mode suggest that the micaschist is most likely derived from terrigenous sediments. This is in agreement with the field relationship showing lateral passage towards quartzite and calcareous metasediments. High Ni contents Ž100–500 ppm. are not uncommon for volcaniclastic sediments ŽTaylor and McLennan, 1985; Condie, 1993.. Regarding the possible age of the ultimate source, its Rb–Sr isotope compositions gave a uniform mantle reservoir model age ŽTUR . of 1.3 Ga,

5.2.3. Isotope constraints The initial ´ Nd ŽT . values and 87 Srr86 Sr ratios at 600 Ma for the eclogites of the Pan-African belt are shown in Fig. 5. Also shown for reference are the UHP eclogites from the Dabie Orogen of central China. The Chinese data are exclusively for the eclogites that occur as enclaves or interlayers in granitic gneisses and marbles ŽJahn, 1998, 1999.. They have a clear AcontinentalB affinity in terms of the Nd–Sr isotope characteristics. By contrast, the Mali mafic eclogites have positive ´ Nd ŽT . values as are the eclogites from Togo ŽBernard-Griffiths et al., 1991.. The eclogites of Togo ŽMount Lato. were thought to have derived from continental rift basalts of Neoproterozoic age Žca. 0.8 Ga. based on chemical and isotopic compositions as well as U–Pb zircon upper intercept age ŽBernard-Griffiths et al., 1991.. However, the protolith age of the Mali eclogites cannot be easily estimated from the present isotopic information. The two TDM model ages are conflicting Ž1.67 and y1.85 Ga; Table 1. and not meaningful as they are calculated with f SmrNd values Žy0.15 and q0.20., which are not favorable for normal model age calculation. The positive ´ Nd ŽT . values may favor the interpretation for a mafic magma extracted from a depleted mantle source. We have not reached a satisfactory explanation for the protoliths of the mafic eclogites. By contrast, the data point of micaschist Ž ´ Nd s y16, 87 Srr86 Sr s 0.713. is completely separated from the field of eclogites ŽFig. 5. and it plots in the most radiogenic Sr end of the Dabie field. This is indicative of a very old protolith of continental origin, which is supported by its TDM model age of 2.28 Ga ŽTable 1.. 5.3. Tectonic implications Maruyama et al. Ž1996., and later Maruyama and Liou Ž1998., made an extensive review on the world’s blueschist and eclogites belts. They made three important conclusions: Ž1. the HPrUHP belts can be classified into two types based on their protolith characters. Type A Žor collision type. belt includes passive margin protoliths characterized by granitic gneiss basement rocks, platform-type carbonates, bimodal volcanics and peraluminous sediments;

B. Jahn et al.r Chemical Geology 178 (2001) 143–158

155

Fig. 5. ´ Nd ŽT . vs. ISr diagram for the Mali and other Pan-African eclogites from Togo and Hoggar. UHP eclogites of the Dabie orogen from central China are shown for comparison. The Dabie eclogites were formed at ca. 220 Ma. A direction of shift is indicated if they were metamorphosed at 600 Ma as the Pan-African eclogites. The positive ´ Nd ŽT . field of the Pan-African eclogites is distinguished from the negative ´ Nd ŽT . field or AcontinentalB affinity of the Dabie eclogites. The Mali micaschist shows a clear old continental signature ŽData sources for Dabie & Su-Lu: Jahn, 1998, 1999; Mali: present study; Togo and Hoggar: Bernard-Griffiths et al., 1991..

whereas type B Žor cordilleran type. belt consists of active margin protoliths formed in an accretionary complex, and are characterized by bedded cherts, MORB, ocean–island basalts with or without capped reef limestones, and greywackesrturbidites. Ž2. Most type A belts are found in Europe and the Tethyan domains and in intra-continental collisional belts in Eurasia. The metamorphic pressure conditions may attain 45 kbar. By contrast, type B belts occur mainly in the Circum-Pacific and intracontinental orogens in Asia, with the pressure regime F 12 kbar. All known ultrahigh-pressure metamorphic rocks occur in type A belts. Ž3. HPrUHP metamorphic belts rarely occur in Precambrian terranes. Two rare Precambrian blueschists are now known to occur near Aksu along the northwestern margin of the Tarim Craton in NW China Žca. 700 Ma; Liou et al., 1989., and in NE Jiangxi of southern China Žca. 900 Ma; Shu and Charvet, 1996.. In northern Mali, the protolith assemblage of the UHP belt evidently belongs to type A of Maruyama et al. Ž1996.. The Gourma basin belongs to the passive continental margin, as represented by shelf

carbonates grading into turbidites. The external nappes comprise mainly phyllites and the internal nappes are made up of micaschists and quartzites. The protolith for the micaschist Žterrigenous sediment. was most probably derived from the platform series of the Paleoproterozoic rocks of the West African craton, whereas that for the mafic eclogites Žbasalt or calcareous sediments. is more difficult to determine with certainty. Nevertheless, the new age data of ca. 620 Ma establish the coesite-bearing eclogites of Mali as the oldest UHP metamorphic rocks so far identified. Older eclogites have been reported to occur in Tanzania Žca. 2 Ga; Moller et ¨ al., 1995., in the eastern Grenville Province ŽMolson Lake terrane, 1.4 to Ga; Indares, 1993., and in the Churchill Province ŽStriding-Athabasca mylonite zone, 2.6 Ga; Snoeyenbos et al., 1994., but these eclogites were not metamorphosed in the UHP conditions and their eclogite facies metamorphic ages have not been well determined Žsee Maruyama et al., 1996, for further discussion.. The rarity of HPrUHP rocks in the Precambrian is often ascribed to the higher heat production and

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heat flow, which prevented the formation and survival of deeply subducted cold plates. However, the occurrence of blueschist belts at ca. 700 and 900 Ma in China, UHP eclogites at ca. 620 Ma in Mali, and other contemporaneous non-UHP eclogites in the eastern margin of the West African Craton, such as the Aleksod eclogites in the Hoggar and the Mount Lato and Mount Kabie´ eclogites in Togo ŽBernardGriffiths et al., 1991., as well as the Neoptroterozoic kyanite eclogites and associated whiteschists from Zambia Žcentral Africa, Cosi et al., 1992., all suggest that by the late Proterozoic subduction of lithospheric plates was able to produce HPrUHP assemblages, and that the exhumation style was similar to that in Phanerozoic orogens, which require a very short time interval. As Maruyama et al. Ž1996. put it, the late Proterozoic may signify a drastic change of P–T conditions or heat flow patterns within the lithosphere. Finally, in their assessment of periodic formation of HPrUHP metamorphic belts, Maruyama et al. Ž1996. noted that active periods of blueschist andror eclogite metamorphism correspond to world-wide marine transgression and to faster sea-floor spreading. Due to the lack of reliable age information, they were wrong in pointing out that the periods of 300–200 Ma and ca. 600 Ma were the time of reduced blueschist formation, but they were probably correct in the correlation of active HPrUHP metamorphism and rapid sea-floor spreading. At 600–800 Ma, the world’s oceans had a greatly reduced 87 Srr86 Sr and enhanced 143 Ndr144 Nd ratios ŽDerry and Jacobsen, 1988.. This implies that abundant newly created oceanic crust Žfast spreading. had significantly contributed the mantle Sr–Nd isotope composition to the seawater through active hydrothermal activities. This period appears to correlate with the active period of eclogite formation, at least in West Africa. It has been proposed that Gondwana was assembled from various continental fragments derived from the breakup of the Neoproterozoic supercontinent Rodinia during the Pan-African orogenic events ŽHoffman, 1991; Dalziel, 1991.. The old West African Craton was probably incorporated into northwest Gondwana through suturing with other continental fragments. The suturing along its eastern margin resulted in the formation of ) 2000-km-long

orogenic beltŽs. with numerous nappe complexes from the Sahara to the Gulf of Benin ´ ŽCaby, 1987.. The northern part of this Pan-African orogen is exposed in the Trans-Saharan belt, to which the present study area belongs, whereas the southwestern segment comprises the Dahomeyide orogen Žsee Fig. 1a for location; Attoh et al., 1997.. Current hypotheses predict that the earliest deformation events Žorogenies. took place after ca. 600 Ma ŽHoffman, 1991. and that the final amalgamation may have occurred as late as the Middle Cambrian ŽPowell et al., 1993.. The newly obtained age data for the Mali eclogites ŽT f 620 Ma. and the available chronological data of about 600 Ma for the Dahomeyides Žzircon U–Pb, phengite Rb–Sr ages from Bernard-Griffiths et al., 1991; muscovite and hornblende 40Arr39Ar ages from Attoh et al., 1997; U–Pb zircon age from Attoh, 1998; Pb–Pb zircon ages from Affaton et al., 2000. clearly indicate that the final amalgamation in NW Gondwana occurred in the latest Proterozoic, but not in the Middle Cambrian.

Acknowledgements Rb–Sr and Sm–Nd isotopic analyses in Rennes were assisted by Nicole Morin and Joel ¨ Mace. ´ Bulkrock chemical compositions were analyzed using ICP-AES and ICP-MS at the Analytical Center of the CRPG-Nancy. J.G. Liou and M.A.H. Maboko reviewed the manuscript. We particularly thank J.G. Liou for his insightful comments. Financial support of an INSU Programme AInterieur de la TerreB ´ Ž1997. to BmJ is acknowledged. This is INSU Contribution no. 260.

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