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 generation at 3.5 and 2.9 Ga recorded in the emplacement of tonalite-gneiss granitoids. A total of 180 samples of representative gneisses and massive tonalites and sills has been collected from three areas in the southern part of the craton, at Mashaba, Chingezi, and Shabani. These rocks have been analysed for major, trace, and rare earth elements to evaluate the effects of the fractional crystallization and partial melting processes in the generation of this segment of Archaean crust.

Three groups are distinguished on the basis of their major and trace element contents, and they follow two main trends of differentiation: the sodic and the calc-alkaline (sensu stricto) trends. Group I samples are tonalitic in composition and follow a sodic trend characterized by decreasing CaO/Na2O ratios. Y and Sr behave as compatible elements and are negatively correlated with Rb. REE patterns are moderately fractionated 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 increasing differentiation, similar to the Archaean tonalite-trondhjemite-granodiorite suites from Finland or the Pilbara Block, Australia. Samples display biotite tonalite and trondhjemite compositions, and Y, Sr, and 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 composed mainly of trondhjemites and granites similar to many post-Archaean granitoids: they follow a calc-alkaline trend (sensu stricto) with decreasing CaO/Na2O and Na2O/K2O. Sr and Y are incompatible, whereas Rb increases with differentiation. REE patterns are variably fractionated, with La/Yb(n) = 6-36, with high REE contents, and marked negative Eu anomalies.

The above geochemical features are explained in a three-stage petrogenetic model. The first stage consists of 6-20% melting of upper-mantle peridotite and the generation of tholeiitic basalts, as observed in the associated greenstone belts. The second stage involves 4-25% partial melting of metamorphosed basalts with a Gt amphibolite (15-45% Pl + 30-50% Hb + 2-35% Cpx + 3-15 % Gt) residue resulting in the Group I samples, under water-unsaturated conditions at intermediate pressure (approximately 16 kbar), or with an eclogite residue to generate the parental magmas for the Group II rocks. The third stage is low-pressure fractional crystallization (< 8 kbar) of liquids generated 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% fractional 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 fractionated REE patterns of the Group II rocks are inherited from the second stage of partial melting of the metamorphosed basalt source rocks with an eclogite residue. Thus Group II and III initial liquids were generated through partial melting of eclogite and Gt amphibolite, respectively. The genetic relationships between Group I sodic and Group III calc-alkaline suites are evaluated, with the latter resulting from various stages of fractional crystallization processes of parental magmas within the sodic suite.