Isotopic ages and chemical and isotopic composition of the Archaean Turfloop Batholith, Pietersburg granite-greenstone terrane, Kaapvaal Craton, South Africa

The granodioritic to granitic Turfloop Batholith in the northern Kaapvaal C raton, South Africa, intrudes metamorphosed and deformed greenstone litholo gies of the >2.9 Ga Pietersburg Group and the >2.8 Ga tonalitic to trondhje mtic Baviaanskloof Gneiss. New results presented here include 19 Rb-Sr and 7 Sm-Nd whole-rock isotopic analyses and U-Pb analyses of zircon and titani te from the Batholith. Three U-Pb analyses of zircon define a discordia wit h an upper intercept of 2.78 +/- 0.01 Ga (2 sigma) and a lower intercept of 0.61 +/- 0.13 Ga (2 sigma). The average of two U-Pb analyses of titanite i s concordant at 2.76 +/- 0.015 Ga (2 sigma). A composite Rb-Sr whole-rock e rrorchron (MSWD = 50.6), for 18 new and 11 previously published analyses, y ields an age of 2.78 +/- 0.015 Ga (2 sigma) and initial Sr-87/Sr-86 = 0.701 3 +/- 0.0003 (2 sigma). Pervasive, subsolidus alteration affected the rocks of the Batholith but because isotopic ages show it was early (deuteric), i t contributed little or nothing to the excess scatter of data. Instead the scatter is attributed to variable Sr-87/Sr-86 ratios among the plutons comp rising the Batholith. Whole-rock Sm-Nd isotopic data indicates an age of 2. 72 +/- 0.04 Ga (2 sigma). and initial Nd-143/Nd-144 ratio of 0.50902 +/- 0. 00003 (2 sigma) (epsilon(Nd) = -1.7(2.72 Ga) for depleted mantle). Again, e xcess scatter in these data (MSWD = 5.05) is attributed to variations in in itial Nd-143/Nd-144 ratios of plutons comprising the Batholith. The closene ss of ages derived using four different techniques strongly suggests that t he Turfloop Batholith was emplaced at similar to 2.78 Ga. Isotopic, whole-r ock major-, trace-, and rare-earth element data, the presence of mafic encl aves and emplacement into greenschist to amphibolite-facies rocks suggest t hat the various magmas giving rise to the Turfloop Batholith were generated when a deep-seated heat source, probably mafic magma, partially melted >2. 8 Ga tonalitic and trondhjemitic lower crustal rocks similar in composition to the Baviaanskloof Gneiss, The Turfloop Batholith was emplaced after def ormation, northward thrusting, and low-grade metamorphism of the Pietersbur g granite-greenstone terrane, thus these processes must have occurred prior to similar to 2.78 Ga. The thrusting is possibly related to similarly dire cted thrusting of rocks of the Witwatersrand Supergroup to the south. If em placement of the Turfloop Batholith also post-dates southwestward thrusting in the Southern Marginal Zone of the Limpopo Belt, that thrusting also occ urred prior to similar to 2.78 Ga.