Autometasomatic Rb enrichments in highly evolved granites causing lowered Rb-Sr isochron intercepts

Details Autometasomatic Rb enrichments in highly evolved granites causing lowered Rb-Sr isochron intercepts Autometasomatic Rb enrichments in highly evolved granites causing lowered Rb-Sr isochron intercepts

May 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..93...65g&link_type=abstract

Earth and Planetary Science Letters, Volume 93, Issue 1, p. 65-75.

Physics

7

Scientific paper

Whole-rock samples of the younger members of a suite of the Erzgebirge granites yield Rb-Sr isochrons with remarkably low intercepts (0.689-0.702) whereas the older granites yield initial Sr isotope ratios which are higher and rather uniform (0.70677 +/- 22).
A model is proposed to explain this observation by a two-stage evolution of the younger granites. After a time span t1 following the intrusion, autometasomatic processes caused by residual magmatic solutions increase the Rb contents of the younger granites by more or less the same amount. This Rb addition lowers the intercepts of the isochrons but it leaves their slopes undisturbed. The Rb amounts added during autometasomatosis can be derived by a petrogenetic model of fractional crystallization. The time t1 may be calculated approximately on the basis that the true initial ratio is at least as high as those of the older granite suite. t1 is in the order of 10 Ma and agrees well with the difference in age between whole-rock data and the Rb-Sr mineral ages of metasomatically formed dark micas in the rocks (10-20 Ma) and with K-Ar data.
It is concluded that decreased granite isochron intercepts may be more common than hitherto expected.

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