Migmatization by metamorphic segregation at subsolidus conditions: Implications for Nd-Pb isotope exchange

We have carried out major and trace element analyses as well as Nd and Pb isotope measurements on a stromatic migmatite in the Phikwe Complex (Botswana) in order to assess the effect of high-grade metamorphic event leading to migmatization on apparent Nd model ages. The Phikwe Complex, considered as an entity of the Central Zone of the Limpopo Belt, underwent a high-grade metamorphic event at 2.0 Ga leading to migmatite formation. Migmatites in this area present a very good layering of migmatitic components. Leucocratic layers were segregated from mafic layers, dominated either by hornblende or biotite, on a centimeter to millimeter scale. Accessory minerals such as monazite, apatite and zircon were not identified in hornblende-rich and quartzo-feldspathic layers. The combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and/or hornblende dehydration reactions, or by metamorphic segregation at subsolidus conditions. REE modeling, applying batch, fractional and disequilibrium melting equations, allows to discriminate between these two processes. The modeled REE results of the quartzo-feldspathic layers match those observed via metamorphic segregation at subsolidus conditions. The ε_Nd values at 2.0 Ga range from 0.3 to -8.9 with lower values in the REE-rich migmatitic layers. Thus, the distribution of major and trace elements as well as the scatter Nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components. The positive ε_Nd value of hornblende-rich layers suggests hornblende crystallization from a mantle-derived source and Sm/Nd fractionation at protolith formation age. Apparent Nd model ages ranging from 2.3 to 3.1 Ga, do not reflect the crustal evolution in Southern Africa in Archean to Proterozoic time. Based on the geochemical features and the Pb-Pb WR isochron at 2.0 Ga, mass conservation during migmatization can be assumed. Therefore, chemical and Nd isotopic compositions of the protolith prior to migmatization can be calculated. The results indicate a Si and K poor diorite as protolith. Moreover, calculated protolith compositions yield a two-point Sm-Nd 'isochron' indicating an age of 2.7 Ga, and very similar Nd model ages of 2.73-2.75 Ga. Those ages constrain the timing of an Archean crust formation episode in the Phikwe Complex.