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3.4 Geochronology
3.4.1 U/Pb isotope dilution, SHRIMP U/Pb and Pb-evaporation zircon ages
The results of U/Pb isotope dilution analyses of 50 single zircon fractions from 15
magmatic rock samples are given in Appendix A, Tab. A7. In average 5-10 zircon
grains were analysed per fraction. The radiogenic 207 Pb/ 206 Pb evaporation method was
conducted on 2 single zircon grains of sample 96M9a (Appendix A, Tab. A8). Appendix
A, Tab. A9 shows the isotopic ratios of single zircon SHRIMP analyses performed on
zircons of 5 different samples. Concordia diagrams with the U/Pb isotopic data, the
histogram of the radiogenic 207 Pb/ 206 Pb ratios obtained by the evaporation method, and
weighted mean age diagrams of SHRIMP data are shown in Fig. 3.7.
Northern Pamirs/Altyndara valley
AD2a: From AD2a, the Altyndara metarhyolite, three zircon fractions were analysed.
The zircons show elongated magmatic habitus with longer (fraction F1) and smaller
prisms (F3 and F5). The zircons are < 200 �m in size, corners are sometimes rounded
and many grains are broken. Most grains show magmatic zonation (cathodoluminescence
images, Appendix B, plate B1), but some are also metamict with diffuse
internal parts. In some cases embayments can be seen at the outer margin but also at
margins of cores. Frequently sector zoning is observable. Based on the external
morphology of the zircons, the grains plot in the calc-alkaline and sub-alkaline fields of
a Pupin diagram (Pupin 1980). Two data points are concordant at 329 � 5 Ma, which is
interpreted as crystallisation age, and one data point plots above the concordia (Fig.
3.7a).
AD6c: An andesite-dacite sample from the metavolcanic unit of the Altyndara valley
has only few and small (80-150 �m) zircons and only one fraction (F2) was dated. The
U/Pb isotopic ratios were plotted into the same diagram as sample AD2a (Fig. 3.7a),
due to its close association within the same Palaeozoic metavolcano-sedimentary
succession. Fraction 2 (AD6c) plots slightly below the concordia close to the fractions
of sample AD2a, and it is concluded that AD6c has the same late Early Carboniferous
age than AD2a. All fractions from both samples have identical 207 Pb/ 235 U ages. In
cathodoluminescence images (Appendix B, plate B1) these zircons reveal magmatic
zoning with low-angle truncations dominating over high-angle truncations. In
comparison with sample AD2a, AD6c zircons have only small or no cores. In some
zircons magmatic or fluid resorption is visible.
Northern Pamirs/Karakul lake
P26: From the P26 granite three zircon fractions were analysed with the U/Pb isotope
dilution method (Fig. 3.7b). This sample yielded ambiguous results. Data are neither
concordant nor colinear. One fraction (2b) plots above the concordia, possibly
suggesting an analytical problem due to U loss during wet-chemical treatment.
Embedding of more common lead into the lattice during zircon alteration could be an
alternative explanation. A minimum U/Pb age of about 216 ± 2.0 Ma is suggested. To
constrain the age of this sample more precise SHRIMP analyses were performed on 10
single zircon grains. The weighted mean 206 Pb/ 238 U age is 227.0 ± 4.1 Ma. Most likely
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