Genesis of Damaran Granites in the Light of Rb/Sr and δ18O Data
The mean (87Sr/86Sr) and mean (87Rb/86Sr) ratios of the intrusive granites from the northern and southern parts of the Orogen’s Central Zone plot on straight lines. These are interpreted as areal isochrons indicating the time of last Sr isotope homogenization 526 and 571 Ma ago in the respective source rocks.
Initial (87Sr/86Sr) and mean (87Rb/86Sr) ratios of approximately coeval granites of the main magmatic pulses in the North (~470 Ma) and in the South (~520 Ma) line up along isochrons suggesting that in both cases ~60 Ma elapsed after the homogenization in the protolith before large scale intrusions took place.
The data require that the intrusions have preserved the Rb/Sr ratios of their source rocks permitting only very little assimilation or fractionation.
The source rocks in the North and South had rather unradiogenic Sr 526 and 571 Ma ago, respectively. At Ri ≤ 0.7066 all presently known Damaran metasediments and metavolcanics as well as the basement must be excluded as the protoliths. A hypothetical source with a large proportion of low (87Sr/86Sr) volcanic material is required. In the centre, on the other hand, the Sr isotope ratios are more radiogenic and derivation from common Damaran metasediments is a distinct possibility.
Suitable protoliths for the alaskites are neither the granitic basement nor the arkosic Etusis Formation nor metasediments of the Kuiseb Formation. Derivation from Khan Formation metasediments or acidic lavas like those of the Naaupoort type is suggested by the Rb/Sr data. Two red granites could also stem from such sources whereas a third one was perhaps produced from Etusis metasediments.
The total rock δ18O values show an usual spread from 7.1 to 15.2% , the majority being very heavy. This excludes granulites and requires sediments or heavily altered volcanics as source rocks. Granites from the centre could have been derived from Damaran metasediments. A plot of their δ18O vs. initial Sr isotope ratios has a very clear negative slope. No trend is visible for the southern granites. Of the northern granites the older group shows a negative, the younger group a positive correlation. This is interpreted as indicating mainly altered volcanics (perhaps spilites) for the older and mixture of volcanogenic and metasedimentary rocks as the source for the younger group. The high δ18O values show that the granites are crustal remelts. The plot for the non-granitic intrusives which are all older than 550 Ma has a positive slope with δ18O between 6.5 and 10.5%, indicating a mixture between unaltered mantle derived and crustal material.
KeywordsSource Rock Central Zone Regional Metamorphism Southern Central Leucocratic Granite
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