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A thermodynamic model for the deposition of oxide facies of microbanded precambrian banded iron formations
145 GEOCHEMICAL HETEROGENEITY OF THE BIMODAL SUITE, SWAZILAND: IMPLICATIONS FOR CRUSTAL GROWTH
ZIRCONS OLDER THAN 4 Ga, INDICATED BY STEPWISE PbEVAPORATION FRON SINGLE GRAINS OF A JACK HILLS NETACOHGLOMERATH ( ~ S T E R N AUSTRALIA).
D.R. HUNTER Department o f Geology, University of Natal,
Pietermaritzburg,
South A f r i c a .
Geochemical parameters allow recognition of four groups of f e l s i c gneisses in the Bimodal Suite (BMS). Group I is characterized by fractionated l i g h t REE patterns, Rb, Ta and Nb + Y contents similar to Phanerozoic subduction-related granites but d i f f e r with respect to t h e i r heavy REE depletion and low Th/Ba r a t i o s . Group 2 is distinguished by very strongly fractionated REE patterns with small to large Eu p o s i t i v e anomalies, low Th/Ba r a t i o s and Sr 500 ppm. Group 3 gneisses show enrichment of Th, l i g h t REE and high f i e l d strength elements t y p i c a l of Phanerozoic within plate granitoids. K-rich gneisses c o n s t i t u t e a minor component interlayered in the BMS and have geochemical parameters similar to Group 3. Interlayered amphibolites have major element abundances similar to those of Dasaitic komatiite,
Mg-tholeiite
and
Ee-tholeiite.
ine
former have REE patterns with gentle slopes from 20x chondrite f o r La to 5x chondrite for Yb. Some Mg-tholei i t e s have similar gently slopi ng patterns, or f l a t patterns ( 10x c h o n d r i t e ) . The Fe-rich group has f l a t REE patterns 20 - 3Ox chondrite. Mg-tholeiites normalized to tholeiitic MORB have patterns broadly simi far to modern island arc basalts but Fe-rich amphibolites have a "humped" pattern with only Y showing no enrichment r e l a t i v e to MORB. The geochemical signature of Group 1 gneisses is a t t r i b u t e o to p a r t i a l melting of garnet e c l o g i t e or amphibolitic source rocks. Group 3 f e l s i c gneisses with whicil Fe-rich amphiboiites and locally K-rich gnei sses are spatially assoclateo oisplay features akin to w i t h i n - p i a t e granitoiOs. Present evioence suggests a v a r i e t y of palaeotectonic environments f o r the evolution of %he early Archaean s i a l i c crust. A Yi~l~lEDYl~I~CM~JI~ FORTI~ IJl~3srrIoNOF ~ HICI~I~ED PP,~A~RIAN BArBeD ~ FORMATIONS.
FACIES CF
B. l ~ l ~ 0 b ~ and E. l ~ (Lahmcatoire de P~trologie de la surface, Oniversit6 de Poitiers, 40 Avenue du Recteur ~ u , 86022 Poitiers cedem).
The interpretations concernn~ the origin of the Banded Iron Fc{mations (B.I.F, alternating layers of i r ~ mnerals and chert) of the Precambrian are numerous. The majerity of authors ilxiicates B.I.F are mazlne ot~igln under very low (~elgen ccoditic~s, hi@a fO3z mxl temperature ~ l y higher than today. Our purpese is to precise the pmmary iron mineralogy and the m e r c h m d i ~ . Usir~ thermdynamcs data p~owded in the literature, stability diagrams are Ixdlt at different teI~Deraares in FeO-Fe2OJ-SiOz-ODz-BzO system. /_~G~fFe~* bearing minerals values differ comsiderably from one to another (1~uble I). The effect of /kG°f valueson the stability fields of iron o ~ i - h ! ~ d e s are very sensitive. Table I : 1 : N A ~ et ad (1971); 2 : ~0BIE et eL1 (1978); 3 : TARDY et ~ (1985). ~t 25~C 1 2 3 /kG°f Fea* (kJ.m01e-*) -17.87 -4.6 -4.6 Z~Gof @oethite(kJ.mele-* ) -490.36 -488.55 -474~30 /kGof hematite{kJ.mole-1 ) -742.41 -742.68 -71/.11 AG~f maQnetite(kJ.mole-l -1015.4 -1012.6 / For 1 and 2, magnetite is more stable than hematite or goethite when fOz is less than i0-v° Aim and for 3, less than 10-sa'2° Arm. This last value seems high considerng fOz value eucotmtered in actual esvn-onments where ma~tite dses not appear. Stability diagrams built in FeO-Fe~OJ-SiOz-COzq~O system exhibit precipitation of ~oethite (1) or hematite (2) when fOz increases and this can explain formation of primary iron o x i ~ d e s in Precambrian Sea. Micrchandi~ of B.I.F results from both inereasi~ of temperature and fOz. Goethite, hematite and a m o ~ silica solubilities inc~easo with temperature. H A [ ~ et all data 25° 45° 65°C Coethite loq KT -7.42 -7.00 -6.62 Amorphous silica log KT -2.70 -2.47 -2.24 Acoerding to equilibrium equation of ~oethite, l~1[Fez*]/[B'] z= icgKT 0.251o~JfOz 1.5[HaO]. when IcgK"r and fOz Increase, goethite preclpitates{in aglqeement with presence of pr/~itive orgy). When icgKT decrea~, amorphous silica precipitates.
H.T. PIDGEON, B. KOHHR, and H.J. LIPPOLT. (Curtin University, Perth, Australia, and L a b o r a t o r y Geochronology, University Heidelberg, F,H. Germany). U/Pb-analyse8 on the ion microprobe ("SBRINP") of the Australian National University discovered zircons with ages > 4 Ga in quartz]tee from the Narryer area (FROUDE et al., 1983), and in a metaconglomerate from the Jack Hills (COMPSTON & PIDGEON 1966) in the northern Yilgarn Block of Western Australia. The existence of these ancient zircons so far could not be confirmed by wetchemical graln-by-graln analyses of zircons from the Narryer detrital population (SCH~RER & ALLEGRE 1985). Now their occurrence has been substantiated by slngle-zircon analyses on 42 grains from the Jack Hills metaconglomerate using stepwise Pbevaporation direct from the grain in a doublefilament thermal ionization mass spectrometer (KOBHR 1987, KOBEH et el., in prep.). All the characteristics of the Archean age patterns of the evaporation data agree well with those previously reported using the SHRIMP. The detrital population is complex. Inheritance from different Archean sources can be assumed. The data sets demonstrate Pb isotope variations within single individuals. These patterns can be discussed by Pb loss phenomena, and by asssoclation of zircon phases with different Archean ages in the respective crystals. COMYSTON, W. & PIDGRON, R.T.: Nature 321, 766-769, (1986) FROUDE, D.C. et el.: Nature 304, 616-618 (1983) KOBEH, B.: Contrib. Nineral. Petrol. 96, 63-71, (1987) KDBER, B, PIDGEON, H.T. & LIPPOLT, H.J., submitted to Earth. Planet. Sci. Lett. SCHXHER, U. & ALLEGRE, C.: Nature 315,52-55, (1985)
LEAD ISOTOPE DATA FROM THE ZIMBABWE ARCHAEAN: AGESAND CRATONIC RIDDLES. S. MOORBATH, P.N. TAYLOR, 3.D. KRAMERS, J.F. WILSON, and J.L. ORPEN. (Dept, of Earth Sciences, University of Oxford, Oxford, England). Lead isotope data From 5 supracrustsl and 7 i n t r a crustal sample suites From the Zimbabwe Arcbaean Craton are reported. Pb-Pb ages of the intracrusLals range from 2580 to 3470 Ma, and are generally in agreement with existing Rb-Sr whole rock ages and with Nd isotope data. The 5600 Ma old 5habani Gneiss constitutes an exception by yielding a 5090 Ma Pb-Pb age which is coupled with a very high model PI value of 9. The discrepancy could be caused by U loss in some samples shortly after petrogenesis. In the other suites, model PI values vary from 7.8 to 8.5, which is a larger range than has been Found in Arehaean intracrustals so far. Among the supraerustal suites, a precise result of 2650 + 40 Ma, in agreement with existing constraints, has been obtained For a felsic volcanic suite From the Harare greenstone belt; the model PI value is high at 8.6. The suites of mafic greenstone bell volcanics yielded mostly poorly defined apparent ages, more frequently too old than too young, with generally low model Pl values around 7.8. A striking Feature in the older intracrustal suites in particular is the variation in Th/U ratios From about 0.5 to 50 reflected by Pb isotope dale, with some units (e.g. the 5habani Gneiss) yielding mainly high values, and others (e.g. the Chingezi tonal]re) mainly low ones. All supracrustals conform much more to average crustal Th/U values, in spite of the low grade metamorphism which affected them in the late Archaean. Anomalous Th/U ratios going back to the time of petrogenesis are not uncommon in intraerustals of other Arehaean eratons, and are difficult to explain in a purely magmatic context. Models of crustal genesis are discussed in relation to this observation.
ZIRCONS OLDER THAN 4 Ga, INDICATED BY STEPWISE PbEVAPORATION FRON SINGLE GRAINS OF A JACK HILLS NETACOHGLOMERATH ( ~ S T E R N AUSTRALIA).
D.R. HUNTER Department o f Geology, University of Natal,
Pietermaritzburg,
South A f r i c a .
Geochemical parameters allow recognition of four groups of f e l s i c gneisses in the Bimodal Suite (BMS). Group I is characterized by fractionated l i g h t REE patterns, Rb, Ta and Nb + Y contents similar to Phanerozoic subduction-related granites but d i f f e r with respect to t h e i r heavy REE depletion and low Th/Ba r a t i o s . Group 2 is distinguished by very strongly fractionated REE patterns with small to large Eu p o s i t i v e anomalies, low Th/Ba r a t i o s and Sr 500 ppm. Group 3 gneisses show enrichment of Th, l i g h t REE and high f i e l d strength elements t y p i c a l of Phanerozoic within plate granitoids. K-rich gneisses c o n s t i t u t e a minor component interlayered in the BMS and have geochemical parameters similar to Group 3. Interlayered amphibolites have major element abundances similar to those of Dasaitic komatiite,
Mg-tholeiite
and
Ee-tholeiite.
ine
former have REE patterns with gentle slopes from 20x chondrite f o r La to 5x chondrite for Yb. Some Mg-tholei i t e s have similar gently slopi ng patterns, or f l a t patterns ( 10x c h o n d r i t e ) . The Fe-rich group has f l a t REE patterns 20 - 3Ox chondrite. Mg-tholeiites normalized to tholeiitic MORB have patterns broadly simi far to modern island arc basalts but Fe-rich amphibolites have a "humped" pattern with only Y showing no enrichment r e l a t i v e to MORB. The geochemical signature of Group 1 gneisses is a t t r i b u t e o to p a r t i a l melting of garnet e c l o g i t e or amphibolitic source rocks. Group 3 f e l s i c gneisses with whicil Fe-rich amphiboiites and locally K-rich gnei sses are spatially assoclateo oisplay features akin to w i t h i n - p i a t e granitoiOs. Present evioence suggests a v a r i e t y of palaeotectonic environments f o r the evolution of %he early Archaean s i a l i c crust. A Yi~l~lEDYl~I~CM~JI~ FORTI~ IJl~3srrIoNOF ~ HICI~I~ED PP,~A~RIAN BArBeD ~ FORMATIONS.
FACIES CF
B. l ~ l ~ 0 b ~ and E. l ~ (Lahmcatoire de P~trologie de la surface, Oniversit6 de Poitiers, 40 Avenue du Recteur ~ u , 86022 Poitiers cedem).
The interpretations concernn~ the origin of the Banded Iron Fc{mations (B.I.F, alternating layers of i r ~ mnerals and chert) of the Precambrian are numerous. The majerity of authors ilxiicates B.I.F are mazlne ot~igln under very low (~elgen ccoditic~s, hi@a fO3z mxl temperature ~ l y higher than today. Our purpese is to precise the pmmary iron mineralogy and the m e r c h m d i ~ . Usir~ thermdynamcs data p~owded in the literature, stability diagrams are Ixdlt at different teI~Deraares in FeO-Fe2OJ-SiOz-ODz-BzO system. /_~G~fFe~* bearing minerals values differ comsiderably from one to another (1~uble I). The effect of /kG°f valueson the stability fields of iron o ~ i - h ! ~ d e s are very sensitive. Table I : 1 : N A ~ et ad (1971); 2 : ~0BIE et eL1 (1978); 3 : TARDY et ~ (1985). ~t 25~C 1 2 3 /kG°f Fea* (kJ.m01e-*) -17.87 -4.6 -4.6 Z~Gof @oethite(kJ.mele-* ) -490.36 -488.55 -474~30 /kGof hematite{kJ.mole-1 ) -742.41 -742.68 -71/.11 AG~f maQnetite(kJ.mole-l -1015.4 -1012.6 / For 1 and 2, magnetite is more stable than hematite or goethite when fOz is less than i0-v° Aim and for 3, less than 10-sa'2° Arm. This last value seems high considerng fOz value eucotmtered in actual esvn-onments where ma~tite dses not appear. Stability diagrams built in FeO-Fe~OJ-SiOz-COzq~O system exhibit precipitation of ~oethite (1) or hematite (2) when fOz increases and this can explain formation of primary iron o x i ~ d e s in Precambrian Sea. Micrchandi~ of B.I.F results from both inereasi~ of temperature and fOz. Goethite, hematite and a m o ~ silica solubilities inc~easo with temperature. H A [ ~ et all data 25° 45° 65°C Coethite loq KT -7.42 -7.00 -6.62 Amorphous silica log KT -2.70 -2.47 -2.24 Acoerding to equilibrium equation of ~oethite, l~1[Fez*]/[B'] z= icgKT 0.251o~JfOz 1.5[HaO]. when IcgK"r and fOz Increase, goethite preclpitates{in aglqeement with presence of pr/~itive orgy). When icgKT decrea~, amorphous silica precipitates.
H.T. PIDGEON, B. KOHHR, and H.J. LIPPOLT. (Curtin University, Perth, Australia, and L a b o r a t o r y Geochronology, University Heidelberg, F,H. Germany). U/Pb-analyse8 on the ion microprobe ("SBRINP") of the Australian National University discovered zircons with ages > 4 Ga in quartz]tee from the Narryer area (FROUDE et al., 1983), and in a metaconglomerate from the Jack Hills (COMPSTON & PIDGEON 1966) in the northern Yilgarn Block of Western Australia. The existence of these ancient zircons so far could not be confirmed by wetchemical graln-by-graln analyses of zircons from the Narryer detrital population (SCH~RER & ALLEGRE 1985). Now their occurrence has been substantiated by slngle-zircon analyses on 42 grains from the Jack Hills metaconglomerate using stepwise Pbevaporation direct from the grain in a doublefilament thermal ionization mass spectrometer (KOBHR 1987, KOBEH et el., in prep.). All the characteristics of the Archean age patterns of the evaporation data agree well with those previously reported using the SHRIMP. The detrital population is complex. Inheritance from different Archean sources can be assumed. The data sets demonstrate Pb isotope variations within single individuals. These patterns can be discussed by Pb loss phenomena, and by asssoclation of zircon phases with different Archean ages in the respective crystals. COMYSTON, W. & PIDGRON, R.T.: Nature 321, 766-769, (1986) FROUDE, D.C. et el.: Nature 304, 616-618 (1983) KOBEH, B.: Contrib. Nineral. Petrol. 96, 63-71, (1987) KDBER, B, PIDGEON, H.T. & LIPPOLT, H.J., submitted to Earth. Planet. Sci. Lett. SCHXHER, U. & ALLEGRE, C.: Nature 315,52-55, (1985)
LEAD ISOTOPE DATA FROM THE ZIMBABWE ARCHAEAN: AGESAND CRATONIC RIDDLES. S. MOORBATH, P.N. TAYLOR, 3.D. KRAMERS, J.F. WILSON, and J.L. ORPEN. (Dept, of Earth Sciences, University of Oxford, Oxford, England). Lead isotope data From 5 supracrustsl and 7 i n t r a crustal sample suites From the Zimbabwe Arcbaean Craton are reported. Pb-Pb ages of the intracrusLals range from 2580 to 3470 Ma, and are generally in agreement with existing Rb-Sr whole rock ages and with Nd isotope data. The 5600 Ma old 5habani Gneiss constitutes an exception by yielding a 5090 Ma Pb-Pb age which is coupled with a very high model PI value of 9. The discrepancy could be caused by U loss in some samples shortly after petrogenesis. In the other suites, model PI values vary from 7.8 to 8.5, which is a larger range than has been Found in Arehaean intracrustals so far. Among the supraerustal suites, a precise result of 2650 + 40 Ma, in agreement with existing constraints, has been obtained For a felsic volcanic suite From the Harare greenstone belt; the model PI value is high at 8.6. The suites of mafic greenstone bell volcanics yielded mostly poorly defined apparent ages, more frequently too old than too young, with generally low model Pl values around 7.8. A striking Feature in the older intracrustal suites in particular is the variation in Th/U ratios From about 0.5 to 50 reflected by Pb isotope dale, with some units (e.g. the 5habani Gneiss) yielding mainly high values, and others (e.g. the Chingezi tonal]re) mainly low ones. All supracrustals conform much more to average crustal Th/U values, in spite of the low grade metamorphism which affected them in the late Archaean. Anomalous Th/U ratios going back to the time of petrogenesis are not uncommon in intraerustals of other Arehaean eratons, and are difficult to explain in a purely magmatic context. Models of crustal genesis are discussed in relation to this observation.