The Generation of Continental Crust: An Integrated Study of Crust-Forming Processes in the Archaean of Zimbabwe

The Archaean craton of Zimbabwe includes two major episodesof crust generation at 3.5 and 2.9 Ga recorded in the emplacementof tonalite-gneiss granitoids. A total of 180 samples of representativegneisses and massive tonalites and sills has been collectedfrom 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 thefractional crystallization and partial melting processes inthe generation of this segment of Archaean crust. Three groups are distinguished on the basis of their major andtrace element contents, and they follow two main trends of differentiation:the sodic and the calc-alkaline (sensu stricto) trends. GroupI samples are tonalitic in composition and follow a sodic trendcharacterized by decreasing CaO/Na₂O ratios. Y and Sr behaveas compatible elements and are negatively correlated with Rb.REE patterns are moderately fractionated with La/Ybn=4–23.5.The characteristics of this group have been described only inthe Archaean craton from Swaziland. Group II is an intermediateGroup with a marked decrease in Na₂O/K₂O with increasing differentiation,similar to the Archaean tonalite-trondhjemite-granodiorite suitesfrom Finland or the Pilbara Block, Australia. Samples displaybiotite tonalite and trondhjemite compositions, and Y, Sr, andRb are all incompatible. The REE patterns are strongly fractionated,with La/Ybn=23–44, and with small positive or negativeEu anomalies, as observed in other Archaean tonalite-trondhjemites.Group III is composed mainly of trondhjemites and granites similarto many post-Archaean granitoids: they follow a calc-alkalinetrend (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/Ybn=6–36,with high REE contents, and marked negative Eu anomalies. The above geochemical features are explained in a three-stagepetrogenetic model. The first stage consists of 6–20%melting of upper-mantle peridotite and the generation of tholeiiticbasalts, as observed in the associated greenstone belts. Thesecond stage involves 4–25% partial melting of metamorphosedbasalts with a Gt amphibolite (15–45% Pl + 30–50%Hb+2–35% Cpx+3– 15% Gt) residue resulting in theGroup I samples, under water-unsaturated conditions at intermediatepressure (16 kbar), or with an eclogite residue to generatethe parental magmas for the Group II rocks. The third stageis lowpressure fractional crystallization (<8 kbar) of liquidsgenerated during this second stage, leaving a 19–20% Qtz+36–42%Pl0–2% HbMt cumulate for the more evolved Group II samples,and 55% fractional crystallization of a 14% Qtz+37.6% Pl (An₂₆)3.3%Bt+0.1% Ilm0.8% Mt cumulate for Group III samples. The highlyfractionated REE patterns of the Group II rocks are inheritedfrom the second stage of partial melting of the metamorphosedbasalt source rocks with an eclogite residue. Thus Group IIand III initial liquids were generated through partial meltingof eclogite and Gt amphibolite, respectively. The genetic relationshipsbetween Group I sodic and Group III calc-alkaline suites areevaluated, with the latter resulting from various stages offractional crystallization processes of parental magmas withinthe sodic suite.