THE GENERATION OF CONTINENTAL-CRUST - AN INTEGRATED STUDY OF CRUST-FORMING PROCESSES IN THE ARCHEAN OF ZIMBABWE

The Archaean craton of Zimbabwe includes two major episodes of crust g eneration at 3.5 and 2.9 Ga recorded in the emplacement of tonalite-gn eiss granitoids. A total of 180 samples of representative gneisses and massive tonalites and sills has been collected from three areas in th e southern part of the craton, at Mashaba, Chingezi, and Shabani. Thes e rocks have been analysed for major, trace, and rare earth elements t o evaluate the effects of the fractional crystallization and partial m elting processes in the generation of this segment of Archaean crust. Three groups are distinguished on the basis of their major and trace e lement contents, and they follow two main trends of differentiation: t he sodic and the calc-alkaline (sensu stricto) trends. Group I samples are tonalitic in composition and follow a sodic trend characterized b y decreasing CaO/Na2O ratios. Y and Sr behave as compatible elements a nd are negatively correlated with Rb. REE patterns are moderately frac tionated with La/Yb(n)=4-23.5. The characteristics of this group have been described only in the Archaean craton from Swaziland. Group II is an intermediate Group with a marked decrease in Na2O/K2O with increas ing differentiation, similar to the Archaean tonalite-trondhjemite-gra nodiorite suites from Finland or the Pilbara Block, Australia. Samples display biotite tonalite and trondhjemite compositions, and Y, Sr, an d Rb are all incompatible. The REE patterns are strongly fractionated, with La/Yb(n)=23-44, and with small positive or negative Eu anomalies , as observed in other Archaean tonalite-trondhjemites. Group III is c omposed mainly of trondhjemites and granites similar to many post-Arch aean granitoids: they follow a calc-alkaline trend (sensu stricto) wit h decreasing CaO/Na2O and Na2O/K2O. Sr and Y are incompatible, whereas Rb increases with differentiation. REE patterns are variably fraction ated, with La/Yb(n) = 6-36, with high REE contents, and marked negativ e Eu anomalies. The above geochemical features are explained in a thre e-stage petrogenetic model. The first stage consists of 6-20% melting of upper-mantle peridotite and the generation of tholeiitic basalts, a s observed in the associated greenstone belts. The second stage involv es 4-25% partial melting of metamorphosed basalts with a Gt amphibolit e (15-45% Pl + 30-50% Hb + 2-35% Cpx + 3-15 % Gt) residue resulting in the Group I samples, under water-unsaturated conditions at intermedia te pressure (approximately 16 kbar), or with an eclogite residue to ge nerate the parental magmas for the Group II rocks. The third stage is low-pressure fractional crystallization (< 8 kbar) of liquids generate d during this second stage, leaving a 19-20% Qtz + 36-42% P1 +/- 0-2% Hb +/- Mt cumulate for the more evolved Group II samples, and 55% frac tional crystallization of a 14% Qtz + 37.6% P1 (An26) +/- 3.3% Bt + 0. 1 % Ilm +/- 0.8% Mt cumulate for Group III samples. The highly fractio nated REE patterns of the Group II rocks are inherited from the second stage of partial melting of the metamorphosed basalt source rocks wit h an eclogite residue. Thus Group II and III initial liquids were gene rated through partial melting of eclogite and Gt amphibolite, respecti vely. The genetic relationships between Group I sodic and Group III ca lc-alkaline suites are evaluated, with the latter resulting from vario us stages of fractional crystallization processes of parental magmas w ithin the sodic suite.