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Early crustal evolution as viewed from archean basic rocks of China
141 THE SIGNIFICANCE OF 300Q Ma MAFIC DYKES IN TIE CENTRAL ZONE OF THE LI~POPO BELT, SOUTHERE AFRICA
EARLY CRUSTAL EVOLUTION AS VIEWED FROM ARCHEAN BASIC ROCKS OF CHINA
J. M. BARTON Jr. and D. D. VAN REENEN (Department of Geology, Rand Afrikaans University, Johannesburg 2000, South Africa)
BOR-MING JAHN (CAESS-CNRS, Universit6 de Rennes, Institut de G6ologie, 35042 Rennes, France).
A suite of approximately 3000 Ma gabbroic dykes are exposed throughout the Central Zone of the Limpopo Belt as far west as the Tshane complex in Botswana. Many dykes have bern metamorphosed at granulite facies conditions. They cross cut the main fabric in thc area defined both by compositional layering and mineral orientation and formed in a brittle regime. These facts indicate that the dykes were emplaced into high crustal levels after a major deformational event. The nature of this eve6t is controversial. The Central Zone and adjacent terranes were involved in the Limpopo Orogeny about 2700 Ma ago. However, Pb-isotopic data indicate that the pre-2600 Ma history of the Central Zone was unrelated to that of the adjacent terranes. For this reason, a pre-2700 Ma tectonic event in the Central Zone may not have manifestations in the rocks of the adjacent marginal and transitional zones. Granulite facies parageneses in the Central Zone and the high grade portions of the adjacent terranes yield similar P-T paths and in the Southern Marginal Zone, these paths arc re lated to the Limpopo Orogeny. However, in most published models for the evolution of the Limpopo Belt, the granulite facies parageneses in the Central Zone are attributed to an approximately 3150 Ma pre-Limpopo event. Three interpretations of the data arc possible: I) there have been two different granulite facies events at different times in the Central Zone; 2) granulite facies metamorphism took place in the Central Zone about 3150 Ma ago and about 2700 Ma ago in the marginal zones and 3) granulite facies metamorphism occurred during the Limpopo Orogeny in both areas and the pre-dyke metamorphism in the Central Zone is of unkno~m intensity.
Like in other early Precambrian terrains of the world, Archean basic rocks in China occur in close association with TTG granitoids and late K-rich granites. Most basic and associated supracrustal rocks have been metamorphosed to high grade amphibolite/granulite facies. Recent age studies have established that basic magmatisms in China spanned over one Ga since 3.5 Ga a~o. Tholeiitic and high-Fe basalts predominate, and basalttc komatiites show restricted occurrences, mainly from greenstone belts of NE China. Nd isotopic data suggest that most known early Precambrian basic magmas of China and elswehere were derived from long-term depleted mantle sources. However, several geochemical characteristics may be used to argue for a significant role of recycled continental material in the generation of Archeanbasic magmas from China : (1) low Ti/Zr ratio (30-1OO), (2) in many cases, high CaO/TiO 2 & AI,O3/TiO 2 ratios (20-60), (3) generally high (La/Sm)N ratios varying from < 1.O to 3.3, with the majority (85%) > 1.O, and (4) negative Nb anomalies in most cases. These features are most compatible with magma generation in subduction zone related environments, and are not at variance with the arguments based on Nd isotopic data. Temporal and genetic relationships between T r G gneisses and associated amphibolites are probably the most controversial issue regarding the early development of continental crust. Available age data do not always allow unequivocal interpretations. Geochemical modelling commonly requires presence of pre-existing mafic crust for the generation of q ~ G magmas, but the basic assemblages are not necessarily direct parents of the associated q~FG rocks. Nd isotopic evidence also suggests that sizable "older" sialic crust probably have returned to mantle which eventually served as "modified" sources for later basic and komatiitic magmas (source contamination by recycled continental material). Alternatively, mantle-derived melts could have been contaminated by continental rocks during magma ascension (melt contamination).
CON MICROPROBEANALYSIS OF ZIRCONS IN RECOGNITION OF THE EARLY HISTORY OF THE EARTH (EARLY ABCHEAN ON THE USSRTERRYTORY)
LEAD ISOTOPIC COMPOSITION OF KOMATIITE-HOSTED NICKEL ORES AT KAMBALDA : EVIDENCE FOR METAMORPHIC DISTURBANCE
E.V. BIBIKOVA, I. WILLIAMS, W. COMPSTON
PauZ Browninq, A l i s t a i r Cowden and David I. Groves. (Department of Earth Sciences, University of Cambridge, U.K.)
(V.I. Vernadsky Institute Analytical Chemistry USSR
of Geochemistry and Academy of Sciences,
Moscow, USSR). One of the most reliable method for dating geological events in Early Precambrian is U-Th-Pb isotopic dating of accessory zircons. The recent advance in the U-Th-Rb dating was demonstrated on sensitive, high mass-resolution ion microprobe SHRIMP, designed in Australian National University. (Compston et al., 1984, Williams et al., 1984). We have investigated by SHRIMP d i f f e r e n t zircon generations from ancient high-grade metamorphic rocks of Ukrainian and BaLtic shields, Anabar and Omolon crystalline massifs. All zircon fractions were originally analyzed by conventional U-Pb technique. SHRIMP analysis have demonstrated real advantages of this method is deciphering the primary age and metamorphic history of high-grade metamorphic terrains. The possibility to analyze small ( - 3 0 p m) area gives chance to discover the best preserved zones inside grains with undisturbed U-Th-Pb isotopic system. The best material for U-Th-Pb dating turned to be unzoned, non fractured grains with moderate concentrations of radioactive elements. The reverse discordance is observed mainly for zones highly enriched in uranium. Comparing the data obtained on SHRIMP for the ancient high-metamorphic rocks on the USSR territory with those of conventional U-Pb technique it is possible to state that SHRIMP data either confirm them or approach closer to the primary a~e of protolith.
Fe-Ni-Cu sulphides f r o m komatiite-associated nickel deposits and adjacent metasedimentary units in 2.7 Ga greenstones from Kambalda, Western Australia show a range of Pb isotopic compositions. Sulphide phases and textural types sampled on a metre scale, and identified as structurally early (and p o t e n t i a l l y r e l i c t magmatic), display Pb isotope compositions l e s s radiogenic than those demonstrably related to structural and metamorphic events that occurred shortly (probably less than 30 Ma) after greenstone belt volcanism. The range in Pb i s o t o p i c composition of sulphides from the Kambalda nickel deposits is bracketed by galenas related to gold mineralization (interpreted to be broadly related to regional metamorphism) and by K-feldspars from syn- and post-kinematic granitoids; all are dispersed along a 2.7 Ga isochron. This array represents the minimum range in Pb isotopic compositions of mantle and crustal sources that were available to the regional metamorphic (and gold-deposit-forming) f l u i d at 2.7 Ga. I t is this or a similar fluid that was most likely responsible for the introduction of radiogenic Pb into the nickel ore bodies. Testing of ground melting models at Kambalda by Pb isotope methods alone fails because both mafic and ultramafic metavolcanic rocks and interflow metasedimentary rocks share the same i n i t i a l Pb isotopic composition; the metasediments are largely local in t h e i r d e r i v a t i o n . The Pb i s o t o p i c compositions of komatiite-hosted nickel sulphides cannot be simply equated with that of the Archaean mantle; only when s u b s t a n t i a l l y older crustal materials are absent from a granite-greenstone terrain is i t possible that they may preserve an ~unmodified Archaean mantle signature.
EARLY CRUSTAL EVOLUTION AS VIEWED FROM ARCHEAN BASIC ROCKS OF CHINA
J. M. BARTON Jr. and D. D. VAN REENEN (Department of Geology, Rand Afrikaans University, Johannesburg 2000, South Africa)
BOR-MING JAHN (CAESS-CNRS, Universit6 de Rennes, Institut de G6ologie, 35042 Rennes, France).
A suite of approximately 3000 Ma gabbroic dykes are exposed throughout the Central Zone of the Limpopo Belt as far west as the Tshane complex in Botswana. Many dykes have bern metamorphosed at granulite facies conditions. They cross cut the main fabric in thc area defined both by compositional layering and mineral orientation and formed in a brittle regime. These facts indicate that the dykes were emplaced into high crustal levels after a major deformational event. The nature of this eve6t is controversial. The Central Zone and adjacent terranes were involved in the Limpopo Orogeny about 2700 Ma ago. However, Pb-isotopic data indicate that the pre-2600 Ma history of the Central Zone was unrelated to that of the adjacent terranes. For this reason, a pre-2700 Ma tectonic event in the Central Zone may not have manifestations in the rocks of the adjacent marginal and transitional zones. Granulite facies parageneses in the Central Zone and the high grade portions of the adjacent terranes yield similar P-T paths and in the Southern Marginal Zone, these paths arc re lated to the Limpopo Orogeny. However, in most published models for the evolution of the Limpopo Belt, the granulite facies parageneses in the Central Zone are attributed to an approximately 3150 Ma pre-Limpopo event. Three interpretations of the data arc possible: I) there have been two different granulite facies events at different times in the Central Zone; 2) granulite facies metamorphism took place in the Central Zone about 3150 Ma ago and about 2700 Ma ago in the marginal zones and 3) granulite facies metamorphism occurred during the Limpopo Orogeny in both areas and the pre-dyke metamorphism in the Central Zone is of unkno~m intensity.
Like in other early Precambrian terrains of the world, Archean basic rocks in China occur in close association with TTG granitoids and late K-rich granites. Most basic and associated supracrustal rocks have been metamorphosed to high grade amphibolite/granulite facies. Recent age studies have established that basic magmatisms in China spanned over one Ga since 3.5 Ga a~o. Tholeiitic and high-Fe basalts predominate, and basalttc komatiites show restricted occurrences, mainly from greenstone belts of NE China. Nd isotopic data suggest that most known early Precambrian basic magmas of China and elswehere were derived from long-term depleted mantle sources. However, several geochemical characteristics may be used to argue for a significant role of recycled continental material in the generation of Archeanbasic magmas from China : (1) low Ti/Zr ratio (30-1OO), (2) in many cases, high CaO/TiO 2 & AI,O3/TiO 2 ratios (20-60), (3) generally high (La/Sm)N ratios varying from < 1.O to 3.3, with the majority (85%) > 1.O, and (4) negative Nb anomalies in most cases. These features are most compatible with magma generation in subduction zone related environments, and are not at variance with the arguments based on Nd isotopic data. Temporal and genetic relationships between T r G gneisses and associated amphibolites are probably the most controversial issue regarding the early development of continental crust. Available age data do not always allow unequivocal interpretations. Geochemical modelling commonly requires presence of pre-existing mafic crust for the generation of q ~ G magmas, but the basic assemblages are not necessarily direct parents of the associated q~FG rocks. Nd isotopic evidence also suggests that sizable "older" sialic crust probably have returned to mantle which eventually served as "modified" sources for later basic and komatiitic magmas (source contamination by recycled continental material). Alternatively, mantle-derived melts could have been contaminated by continental rocks during magma ascension (melt contamination).
CON MICROPROBEANALYSIS OF ZIRCONS IN RECOGNITION OF THE EARLY HISTORY OF THE EARTH (EARLY ABCHEAN ON THE USSRTERRYTORY)
LEAD ISOTOPIC COMPOSITION OF KOMATIITE-HOSTED NICKEL ORES AT KAMBALDA : EVIDENCE FOR METAMORPHIC DISTURBANCE
E.V. BIBIKOVA, I. WILLIAMS, W. COMPSTON
PauZ Browninq, A l i s t a i r Cowden and David I. Groves. (Department of Earth Sciences, University of Cambridge, U.K.)
(V.I. Vernadsky Institute Analytical Chemistry USSR
of Geochemistry and Academy of Sciences,
Moscow, USSR). One of the most reliable method for dating geological events in Early Precambrian is U-Th-Pb isotopic dating of accessory zircons. The recent advance in the U-Th-Rb dating was demonstrated on sensitive, high mass-resolution ion microprobe SHRIMP, designed in Australian National University. (Compston et al., 1984, Williams et al., 1984). We have investigated by SHRIMP d i f f e r e n t zircon generations from ancient high-grade metamorphic rocks of Ukrainian and BaLtic shields, Anabar and Omolon crystalline massifs. All zircon fractions were originally analyzed by conventional U-Pb technique. SHRIMP analysis have demonstrated real advantages of this method is deciphering the primary age and metamorphic history of high-grade metamorphic terrains. The possibility to analyze small ( - 3 0 p m) area gives chance to discover the best preserved zones inside grains with undisturbed U-Th-Pb isotopic system. The best material for U-Th-Pb dating turned to be unzoned, non fractured grains with moderate concentrations of radioactive elements. The reverse discordance is observed mainly for zones highly enriched in uranium. Comparing the data obtained on SHRIMP for the ancient high-metamorphic rocks on the USSR territory with those of conventional U-Pb technique it is possible to state that SHRIMP data either confirm them or approach closer to the primary a~e of protolith.
Fe-Ni-Cu sulphides f r o m komatiite-associated nickel deposits and adjacent metasedimentary units in 2.7 Ga greenstones from Kambalda, Western Australia show a range of Pb isotopic compositions. Sulphide phases and textural types sampled on a metre scale, and identified as structurally early (and p o t e n t i a l l y r e l i c t magmatic), display Pb isotope compositions l e s s radiogenic than those demonstrably related to structural and metamorphic events that occurred shortly (probably less than 30 Ma) after greenstone belt volcanism. The range in Pb i s o t o p i c composition of sulphides from the Kambalda nickel deposits is bracketed by galenas related to gold mineralization (interpreted to be broadly related to regional metamorphism) and by K-feldspars from syn- and post-kinematic granitoids; all are dispersed along a 2.7 Ga isochron. This array represents the minimum range in Pb isotopic compositions of mantle and crustal sources that were available to the regional metamorphic (and gold-deposit-forming) f l u i d at 2.7 Ga. I t is this or a similar fluid that was most likely responsible for the introduction of radiogenic Pb into the nickel ore bodies. Testing of ground melting models at Kambalda by Pb isotope methods alone fails because both mafic and ultramafic metavolcanic rocks and interflow metasedimentary rocks share the same i n i t i a l Pb isotopic composition; the metasediments are largely local in t h e i r d e r i v a t i o n . The Pb i s o t o p i c compositions of komatiite-hosted nickel sulphides cannot be simply equated with that of the Archaean mantle; only when s u b s t a n t i a l l y older crustal materials are absent from a granite-greenstone terrain is i t possible that they may preserve an ~unmodified Archaean mantle signature.