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Behaviour of the Sm-Nd isotopie system during metamorphism : Examples from the HT-LP metamorphic terrane of the Limpopo Belt, South Africa and the UHP metamorphic terrane of Dabieshan, Central China.

Abstract : The aims of this Ph.D. Thesis were to decipher whether the use of Nd model ages are reliable constraints for the crustal evolution on Proterozoic migmatized terrane (example of the Limpopo Belt in South Africa) and to examine the consistency between Sm-Nd gamet and U-Pb zircon ages on eclogites (ex ample of the UHPM Dabieshan in Central China). The Central Zone of the Limpopo Belt which is mai nI y composed of orthogneisses and paragneisses underwent granulite facies metamorphism followed by decompression metamorphic conditions (clockwise P-T loop) at 2.0 Ga. We have performed Sm-Nd and V-Pb work associated with major and trace element analyses on three examples of migmatites generated at 2.0 Ga to study the chemical and isotopic fractionation which may occur during migmatization. Partial melting following the Bt dehydration reaction did lead to different behaviour of Sm-Nd systematics for metagreywacke and metapelite. In the first case, chemical equilibrium and full Nd isotope exchanges were not reached due to the incrongruent Pl melting and to the effect of accessory mineraIs (Mnz and Ap) on the compositions of the partial melt. Monazite entered the melt in prefarence to apatite, either by dissolution or by entrainment, and its unradiogenic Nd isotope signature dominates the leucosome. The Nd model ages on partial melt are up to 400 Ma oIder than those on paleosomes. The Nd model ages of these rocks cannot be used in a simple way to cons train the evolution of Southem Africa. In the second case, incongruent Pl melting influenced the distribution of major, LILE and also REE between the partial melts and the paleosomes. Full Nd isotope exchange was nearly achieved while Pb-Pb systematics were not fully resetted. Incomplete exchange for both Nd and Pb for two leucosomes is also argued. It appears that the accessory minerals were chemically and isotopically equilibrated with the partial melt and the paleosome. The Nd model ages can be used to constrain the crustal growth of the Limpopo Central Zone. Finally, in migmatites formed by metamorphic segregation at subsolidus conditions on orthogneiss, chemical equilibrium and Nd isotope exchange were not reached during migmatization. The distributions of the elements are related to the proportion al amounts of each mineraI in the migmatitic components. However, full Pb isotope exchange is reached here as highlighted by a Pb-Pb WR isochron yielding 2.0 Ga. In addition, the geochemical characteristics combined with REE modelling provided arguments for closed system behaviour of the whole rock during migmatization. The isotopic compositions of the protolith obtained via mass balance calculations can be used to constrain crustal evolution. The UHPM Dabieshan is characterized by the occurrence of coesite and its quartz pseudomorph as inclusions in eclogites of basaltic composition but also of undoubted metasedimentary origin. Sm.-Nd gamet-omphacite-whole rock isochrons on the coesite-bearing ec10gites of the Bixiling Complex gave ages at 210 ± 9 to 218 ± 4 Ma which are similar to 218.4 ± 1.8 Ma and 218.4 ± 2.5 Ma ages obtained by V-Pb zircon data. At the Shuanghe locality, Sm-Nd garnet-kyanite-whole rock isochron on a retrogressed eclogite gave an age at 231 ± 35 Ma in line with the V-Pb zircon data at 233 ± 21 Ma (lower intercept in the concordia diagram). In addition, Ar-Ar and Rb-Sr phengite ages performed on the ec10gite host gneisses (Bixilingg locality) yield 198 ± 4 Ma to 212 ± 2 Ma ages, overlapping the previous Sm-Nd and V-Pb zircon ages on the coesite-bearing eclogites. Consequently, the Sm-Nd gamet dating provide ages in agreement with V-Pb zircon ages and the quartzofeldspathic gneisses underwent an event coeval to VRP on ec1ogites. Nd isotope equilibration at the VRP metamorphism appears to have been complete.
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Submitted on : Wednesday, September 7, 2011 - 10:09:15 AM
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Valérie Chavagnac. Behaviour of the Sm-Nd isotopie system during metamorphism : Examples from the HT-LP metamorphic terrane of the Limpopo Belt, South Africa and the UHP metamorphic terrane of Dabieshan, Central China.. Applied geology. Université Rennes 1, 1998. English. ⟨tel-00620022⟩



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