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

Abstract

Thegranodioritic to granitic Turfloop Batholith in the northern Kaapvaal Craton, South Africa, intrudes metamorphosed and deformed greenstone lithologies ofthe >2.9 Ga Pietersburg Group and the >2.8 Ga tonalitic to trondhjemtic BaviaanskloofGneiss. New results presented here include 19 Rb—Sr and 7 Sm—Nd whole-rock isotopic analyses and U—Pb analyses of zircon and titanite from the Batholith. Three U—Pb analyses of zircon define a discordia with an upper intercept of 2.78 ±0.01 Ga (2o) and a lower intercept of 0.61 ±0.13 Ga (2σ).The average of two U—Pb analyses of titanite is concordant at 2.76 ±0.015 Ga(2σ). A composite Rb—Sr whole-rock errorchron (MSWD = 50.6), for 18 new and 11 previously published analyses, yields an age of 2.78 ±0.015 Ga (2σ) and initial 87Sr/86Sr = 0.7013 ±0.0003 (2σ).Pervasive, sub-solidus alteration affected the rocks of the Batholith but because isotopic ages show it was early (deuteric), it contributed little or nothing to the excess scatter of data. Instead the scatter is attributed to variable 87Sr/86Sr ratios among the plutons comprising the Batholith. Whole-rock Sm—Nd isotopic data indicates an age of 2.72 ±0.04 Ga (2a), and initial 43Nd/144Nd ratio of 0.50902 ±0.00003 (2σ) (eNd = –172.72 Ga for depleted mantle). Again, excess scatter in these data (MSWD = 5.05) is attributed to variations in initial 43Nd/144Nd ratios of plutons comprising the Batholith. The closeness of ages derived using four different techniques strongly suggests that the Turfloop Batholith was emplaced at ~2.78 Ga. Isotopic, whole-rock major-, trace-, and rare-earth element data, the presence of mafic enclaves and emplacement into greenschist to amphibolite-facies rocks suggest that 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 BaviaanskloofGneiss. The Turfloop Batholith was emplaced after deformation, northward thrusting, and low-grade metamorphism of the Pietersburggranite—greenstone terrane, thus these processes must have occurred prior to ~.2.78 Ga. The thrusting is possibly related to similarly directed thrusting of rocks of the Witwatersrand Supergroup to the south. Ifemplacement of theTurfloop Batholith also post-dates southwestward thrusting in the Southern Marginal Zone of the Limpopo Belt, that thrusting also occurred prior to ~2.78 Ga.