The petrogenesis of the Nelshoogte pluton: The youngest and most compositionally variable TTG pluton in the Barberton Granite-Greenstone Terrain

[摘要] ENGLISH ABSTRACT:The compositions of tonalite-trondhjemite-granodiorite (TTG)-series rocks from theBarberton Granite-Greenstone Terrain (BGGT) of Southern Africa vary in terms of majorelement concentrations (leuco-trondhjemites to tonalites), trace elements (e.g. Sr, Rb, REE)and degree of HREE depletion [(La/Yb)N ~ 0.50 - 121]. These geochemical parameters havebeen interpreted to have significance for the geodynamic processes that produced TTGmagmas, particularly the depth of the source. In the Nelshoogte pluton, trondhjemiteemplacement occurred prior to and during (D2)-tectonism, and hornblende-bearingtonalites/granodiorites occur as intrusive plugs within the trondhjemite. The geochemicalrange portrayed by the ca. 3230 Ma Nelshoogte pluton covers almost the whole rangeportrayed by all the TTG rocks of the BGGT, formed in the time interval 3550 to 3230 Ma.U-Pb zircon dating reveals no significant differences in apparent ages between thetrondhjemites and tonalites of the Nelshoogte pluton; all lie between ca. 3240 and 3220 Ma.A/CNK- and Ti-maficity correlations within the Nelshoogte pluton show behaviour typical ofI-type granitic rocks, confirming the metamafic character of the source and the involvementof a peritectic garnet and clinopyroxene component in the magmas. The REE and traceelementpatterns resemble those of both ca. 3450 Ma TTG plutons and other ~ 3230 Ma TTGplutons in the BGGT. This suggests that these different generations of TTG magmas weregenerated from similar sources and by similar processes. The geochemical details areconsistent with a metamafic source which underwent melting at high pressure. This TTGsource was fundamentally different from the more potassic source/s that produced theyounger 3100 Ma granites. Additionally, high Zr/Sm and low Nb/Ta ratios in the BarbertonTTG rocks suggest that the Nelshoogte pluton originated as a group of chemically distinctmagmas, in a similar way to other ~ 3200 Ma TTGs. The protoliths of ~ 3450 Ma plutons and~ 3200 Ma plutons might typically be rutile-bearing eclogite and/or 10 to 30 % garnetbearing amphibolite.The geochemical features of the Barberton TTGs suggest similar source compositions, butdifferent pressures of partial melting. For the ca. 3230 Ma plutons, the rocks of the Badplaaspluton and the Nelshoogte trondhjemites/tonalites correspond to the high-pressure magmas,as reflected by high Sr content, as well as high Sr/Y and (La/Yb)N ratios. In contrast, theKaap Valley TTGs and Nelshoogte granodiorites reflect the melting of a similar source atmoderate pressure. Furthermore, the Hf isotope data;Hft (+ 0.1 to + 1.9) and TDM model ages (3330 - 3230 Ma), suggest that the source rocks are ~ 20 to 100 Myr older than thepluton.

[发布机构] Stellenbosch University