Upper Greenstones

Most of the greenstone successions of the Zimbabwe Craton were formed at c. 2.7 Ga and are referred to as the Upper Greenstones (Wilson 1979), or Upper Bulawayan Group (Wilson et al. 1995). Amongst these, two distinctly different greenstone groupings may be recognized in different parts of the craton, which broadly correspond to Wilson s (1979) eastern and western successions (Fig. 2a). 

Fig. 2. Crustal growth between 2.72 and 2.64 Ma. (a) Volcanic rocks, sedimentary rocks and base metal mineralization, (b) Emplacement of Chingezi Suite granitoids, (c) Tentative terrane interpretation. Dashed line indicates the position of the boundary between the western and eastern succession of Upper Greenstones of Wilson (1979).

Fig. 1. The Belingwe greenstone belt, Zimbabwe, showing sample locations. The thin Manjeri Fm (including Spring Valley, Shavi, Rubweruchena and Jimmy Members, not shown here) outcrops along the base of the main (Upper Greenstones) syncline.

The BIF of the greenstone belts of Southern Africa are very old (older than 3.3- yr). Beukes (1973) mentions that in the lower part of the section of the Hooggenoeg formation of this belt, carbonaceous cherts completely devoid of iron appear in a thick sequence of metamorphosed tholeiitic basalts with intercalations of ultrabasic and acid lavas. The first signs of iron appear in the upper part of the section in a banded chert directly underlying the BIF. In this chert, small segregations of iron-rich material are closely associated with black bands of carbonaceous chert or replace them. 

Wellman, P. 2000. Upper crust of the Pilbara Craton, Australia 3D geometry of a granite/greenstone terrain. Precambrian Research, 104, 175-186. 

The type stratigraphy of the Upper Bulawayan Group is in the Belingwe greenstone belt (Wilson... 

Of these, late Archaean sedimentary basins are the most "modern" in their form and can be clearly recognized as upper crust. Similarly, the relatively low metamorphic grade of many Archaean greenstone belts indicates that they too represent upper Archaean crust. In contrast, Archaean granite-gneiss terrains more commonly belong to the deeper continental crust and variously represent middle to lower continental crust. There are only a few places... 

FIGURE 3.25 The secular cooling curve for the potential temperature of the Earth s mantle (from Richter, 1988) and calculated upper mantle potential temperatures based on basalt chemistry (Abbott et al., 1994 - shaded field). Also shown are the potential temperatures for komatiites from Barberton Greenstone Belt (Barb) and the Belingwe Greenstone Belt (Bel), estimated komatiite temperatures from Grove and Parman (2004) and the calculated temperature of the subcontinental lithosphere from beneath the Kaapvaal Craton (Richardson et al., 1984). 

This site is located in a side valley of the Rhone catchment, in an approximately 2-km-wide zone of greenstones (i.e. metamorphic gabbros and basalts) at an altitude of about 2000 m (Figs. 1 and 9A,B). This zone represents a 120 million-years-old ocean floor. About 50% of the area is covered by local till material, on top of which a half meter thick podzol developed (Derron, 1999). The soil profile lies in a pasture area close to the upper tree limit (Lac Bleu area) and comprises four different horizons (A, E, B and C Fig. 9D). 

Precambtian Seviletta Formation, Upper Metadastic Series, Lower Metadastic Series, Greenstone complex intrusives 5500+ Metamorphosed sedimentary and pyrodastic rock and intmsives, and granites and pegmatites Low permeability, very low porosity all water in these formations occurs in cracks and joints... 

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