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arrangement with a lower intercept at 169 +6/-8 Ma and an upper intercept at 828 � 95
Ma. The lower intercept age is interpreted as minimum age for magmatic
crystallisation.
The radiogenic 207 Pb/ 206 Pb evaporation method was applied on two zircon grains of this
sample (Appendix A, Tab. A8, Fig. 3.7g). Both grains yielded identical ages with 231 �
10 Ma (grain 1, 226 ratios) and 228 � 10 Ma (grain 2, 260 ratios). Grain 1 was
evaporated in two temperature steps and grain 2 in one evaporation step.
Central Pamirs/Rushan Pshart Zone
L96A9: This sample offered five datable zircon fractions (Fig. 3.7h). Fractions F1, F3,
and F5 contain clear pinkish zircon grains of different size, whereas F2 and F4 are
composed of huge euhedral prismatic grains with a brownish, unclear colour due to
internal fractures. All fractions plot below the concordia and no discordia can be drawn
through all fractions. In combination with the SHRIMP data (Fig. 3.7i) a meaningful age
interpretation is possible by drawing a discordia through the fractions F1, F3 and F5 to
get a maximum age estimate for a lower intercept age which is 205 � 1.9 Ma. A
discordia through fractions F1, F2 and F4 results in a minimum age estimate for the
lower intercept age which is 179 ± 44 Ma. The latter two fractions show very low
206 204
Pb/ Pb ratios of 176.9 (F2) and 129.0 (F4) and therefore might be less reliable for age
interpretation. SHRIMP analyses on 12 grains probed core and rim regions and zoning
patterns (Appendix B, plate B3). Three cores cover 194-202 Ma and the corresponding
rims are between 154-171 Ma. Assuming that an older group of spot ages represents
ages typified by these cores, the weighted mean of 13 measurements of this group is
201 ± 4.4 Ma. A younger group, possibly typifying rim ages, is 170 ± 6 Ma; the
youngest rim is ~146 Ma, the oldest spot ~217 Ma. In the Pupin-diagram most of the
zircon grains plot into the alkaline and peralkaline mantle differentiates, and a minority
into the field of hybrid calc-alkaline magmas. The CL images reveal two different types
of zircon grains. Some show complete magmatic zoning, sometimes with a resorption
phase, others have large inherited cores which are mainly metamict.
L96M25a: The four fractions from this sample plot below the concordia (Fig. 3.7j). A
heterogeneous source for this zircons is concluded, followed from the distribution and
non linear arrangement of the data points in the concordia plot. The emplacement
occurred approximately between 167 and 235 Ma; the lower estimated age was
determined with a discordia through fractions F2 and F4 and the upper estimated age
with a discordia through fractions F4 and F5. In the Pupin-diagram the fractions plot
into the calc-alkaline to subalkaline area. The cathodoluminescence (Appendix B, plate
B3) depict long prismatic grains and huge inherited, mainly metamict cores. The
zircons have a magmatic zonation which is interrupted by a well rounded resorption
rim in most of the grains.
M96A7: The zircons from this sample are most complex ones. Five zircon fractions were
determined by the conventional U/Pb dilution method (Fig. 3.7k). Owing to a lack of
concordance and colinearity, this sample yielded ambiguous results. Most reliable is a
lower intercept age at 131.1 � 1.6 Ma connecting fractions F2, F3, F4, and F5. As the
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