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3.4.2 Rb/Sr dating
The Rb/Sr mineral ages of granites and basement rocks were determined by isotope
dilution on whole rock samples and muscovites. The data are listed in Appendix A,
Tab. A5 and the two-point isochron plots are presented in Fig. 3.8. For sample locations
see Fig. 3.3.
The granite sample M96A7 has a late Early Cretaceous Rb/Sr mineral age of 103.6 ± 1.1
Ma (Fig. 3.8a). Also sample P7 yielded a Cretaceous Rb/Sr mineral age of 111.2 ± 1.1
Ma (Fig. 3.8b). Sample 96P4e, the granite pebble from a ?Miocene conglomerate of a
Central Pamiran Tertiary intramontane molasse basin (southerly adjacent to the Muzkol
dome) yielded a Rb/Sr mineral age of 188.4 ± 1.9 Ma (late Early Jurassic) (Fig. 3.8c).
3.4.3 Ar/Ar dating
The results of Ar/Ar dating are listed in Appendix A, Tab. A3 and the age spectra
are depicted in Fig. 3.9. For sample locations see Fig. 3.3.
To assess the age of the STS magmatic rocks 40 Ar/ 39 Ar biotite cooling ages of samples
TS12b and TS20a were obtained (Fig. 3.9a,b). The K/Ca ratio and age spectra of sample
TS12b suggest the existence of a composite mineral, eventually by the appearance of
chlorite. Both samples gave identical Early Permian ages of 277 Ma, although they are
separated by around 100 km in an E-W direction.
From the Northern Pamirs two hornblende samples from the Altyndara metaandesites
gave Early Carboniferous ages. Sample AD2e has excess Ar in the mid-T steps and
shows Ar loss in the low-T steps; the probable age is 350 ± 20 Ma (Fig. 3.9c). From the
hornblende-rich metaandesite AD6b (Fig. 3.9d), a total fusion age (TFA) of 319.9 ± 4.6
Ma was calculated; the highest-T step gives 354.7 ± 2.2 Ma, compatible with the age of
hornblende from sample AD2e.
Samples from the Karakul lake area give very consistent Late Triassic/Early Jurassic
muscovite and biotite 40 Ar/ 39 Ar cooling ages. The age spectra show no disturbance and
the calculated ages range from 207 to 191 Ma (Fig. 3.9e-j).
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