Constraints on the origin of Damaran granites by Rb/Sr and δ18O data |
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| Authors: | U Haack J Hoefs E Gohn |
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| Institution: | (1) Geochemisches Institut der Universität, Goldschmidtstr. 1, D-3400 Göttingen, Federal Republic of Germany |
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| Abstract: | The mean (87Sr/86Sr) and mean (87Rb/86Sr) ratios of the intrusive granites from the North and South 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 passed 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 center, on the other hand, the Sr isotope ratios are more radiogenic and derivation from common Damaran metasediments is a distinct possibility.The total rock 
18O values show an unusual 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. A plot of
18O vs. initial Sr isotope ratios of the granites from the center which could have been derived from Damaran metasediments 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 a mixture of volcanogenic and metasedimentary rocks as the source for the younger group. The high
18O values show that the granites are crustal remelts. |
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