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 i sotope measurements on a stromatic migmatite in the Phikwe Complex (Botswan a) 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 t his area present a very good layering of migmatitic components. Leucocratic layers were segregated from mafic layers, dominated either by hornblende o r biotite, on a centimeter to millimeter scale. Accessory minerals such as monazite, apatite and zircon were not identified in hornblende-rich and qua rtzo-feldspathic layers. The combination of petrography and trace element c oncentrations in all layers indicates that the migmatite may have been form ed either by partial melting according to biotite and/or hornblende dehydra tion 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 result s of the quartzo-feldspathic layers match those observed via metamorphic se gregation at subsolidus conditions. The epsilon(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 iso tope data are interpreted as reflecting the proportional amount of each min eral in the different migmatitic components. The positive epsilon(Nd) value of hornblende-rich layers suggests hornblende crystallization from a mantl e-derived source and Sm/Nd fractionation at protolith formation age. Appare nt Nd model ages ranging from 2.3 to 3.1 Ga, do not reflect the crustal evo lution in Southern Africa in Archean to Proterozoic time. Based on the geoc hemical features and the Pb-Pb WR isochron at 2.0 Ga, mass conservation dur ing migmatization can be assumed. Therefore, chemical and Nd isotopic compo sitions of the protolith prior to migmatization can be calculated. The resu lts indicate a Si and K poor diorite as protolith, Moreover, calculated pro tolith 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 constra in the timing of an Archean crust formation episode in the Phikwe Complex. (C) 1999 Elsevier Science B.V. All rights reserved.