2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 9:00 AM


ISHIHARA, Shunso, Geol Survey of Japan, AIST Central 7, Tsukuba, 305-8567, Japan, ROBB, Laurence J., ANHAEUSSER, Carl R. and IMAI, Akira, s-ishihara@aist.go.jp

Granitoid rocks represent an integrated part of the continental lithosphere, hydrosphere and also the upper mantle throughout Earth history. The magnetite-series/ ilmenite-series classification of granitoid rocks should provide an indication of the redox state of the early continental crust. Thus, the Archean granitoid series rocks of the Barberton and Johannesburg Dome areas of the Kaapvaal Craton, South Africa were examined. The granitoid rocks in the Barberton region comprise: (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) plutons, the latter closely associated with metavolcanic komatiite and komatiitic basalt greenstone belt lithologies. These include the older Theespruit and Stolzburg trondhjemite gneiss plutons (c. 3450 Ma) and the younger Kaap Valley tonalite and Nelshoogte trondhjemite gneiss plutons (c. 3230 Ma); (2) late-to post-tectonic, calc-alkaline granitoid intrusives, including the Nelspruit and Mpuluzi batholiths (c. 3105 Ma); and (3) post-tectonic low- and high-Ca granitoid plutons (c. 2700 Ma). Most of the early-stage syntectonic TTG granitoids have low magnetic susceptibilities with all the measured samples yielding less than 3 x 10-3 SI units, and corresponding to ilmenite-series granitoids. The TTG granitoids commonly display indications suggesting that they assimilated much reduced greenstone material. The Kaap Valley tonalite gneiss pluton contains sporadically distributed higher magnetic susceptibility values (greater than 3 x 10-3 SI units), which are less than one-third the magnetic susceptibility values obtained from typical magnetite-series TTG granitoid rocks of the Japanese Island Arc (intermediate series granitoids). Similar TTG suites (c. 3340 Ma) and calc-alkaline granitoids (c. 3120 Ma) of the Johannesburg Dome are also largely composed of ilmenite series and some intermediate series granitoids. The late- to post- tectonic c. 3105 Ma granodiorite-adamellite batholithic complexes mostly belong to the magnetite series suggesting that relatively oxidized continental crust existed by this time in the Barberton region. Post-tectonic granitic plutons emplaced between c. 2900 and 2700 Ma can be subdivided into low-Ca ilmenite-series and high-Ca magnetite-series granitoids, which may correspond to ilmenite- and magnetie-series granitoids of Phanerozoic age.