Constraints and deception in the isotopic record; The crustal evolution of the west Musgrave Province, central Australia
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The Hf and Nd isotopic evolution of the Musgrave Province, central Australia, is used to constrain the timing of crust formation and lithospheric organisation of Proterozoic Australia. The dataset from this region challenges two widely held tenets of Hf and Nd isotope systematics, namely; that crust formation events can only be identified as periods when crystallisation ages correspond to model ages, and that linear Hf evolution arrays away from depleted mantle (along crustal Lu/Hf or Sm/Nd slopes) reflect reworking of the same source. Hf isotopes in Musgrave Province zircon crystals indicate two major crust formation events at c. 1900. Ma and at 1600-1550. Ma. Although no juvenile rocks or crystals are known from c. 1900. Ma, radiogenic addition into the crust at this time is required to account for consistent Nd and Hf evolution patterns, which show no indication of an initially heterogeneous source. Oxygen isotopes in zircon grains confirm that much of the c. 1900. Ma Hf isotopic signal is not compromised by mixtures. Furthermore, the correspondence between mantle extraction and the commencement of reworking of Archean material supports new crust generation at c. 1900. Ma and a coupling between lower and upper crustal processes. The c. 1900. Ma timing of juvenile addition is dissimilar to that in the Albany-Fraser and Arunta Orogens and may reflect continental arc development on the margin of a southern continent. The general Hf isotopic evolution trend of the Musgrave Province apparently reflects reworking from a dominant c. 1900. Ma source with some additional unradiogenic and radiogenic input through time. However, in the 1220-1050. Ma interval this apparent isotopic evolution contrasts with geological observations that indicate input of voluminous mantle-derived material. Intracontinental rifts and other regions with sustained very-high temperature crustal recycling processes generate magmatic provinces with extreme HFSE-enrichment. This can have a profound influence on isotopic evolution trends, suppressing typical juvenile addition patterns. Isotopic mixture modelling indicates that a significant volume of mantle derived material can be accommodated within HFSE enriched magmas without diverging isotopic signatures from apparent reworking trends. In the Musgrave Province, the crust had become so HFSE enriched during the prolonged Musgrave Orogeny (1220-1150. Ma) that it was insensitive to mantle input, which is estimated to have been as much as 85% during this event. © 2012.
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