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fractions are situated close to the lower intercept, the Early Cretaceous age is
interpreted as formation age of the zircons with older inherited components. This is
confirmed by the CL images, which show zircons with large cores. Sometimes in the
cores a second, older core is included. The cores are either metamict or have a
magmatic zonation. The older core type is well rounded, whereas the younger core
generation shows only abrasion at the corners. Some zircons seem to reveal a younger
rim. SHRIMP analyses (Fig. 3.7l) of 24 grains aimed to probe the core types and rims
(Appendix B, plate B4). Three different core ages have been detected: one spot yielded
2187 � 13 Ma, four spots of rounded metamict cores and one weakly zoned inner grain
spot range from 250 to 214 Ma (weighted mean 221 � 24 Ma), and four rounded,
metamict (one weakly zone) cores range from 182 to 156 Ma (weighted mean 162 � 18
Ma). The majority of spots probed spectacularly zoned idiomorphic grains with spot
ages ranging from 130 to 104 Ma (weighted mean of 17 spots 113 � 3.4 Ma); four spots
of idiomorphic, well-zoned grains, and outermost idiomorphic rims of grains with cores
yielded even younger ages (94 to 77 Ma, weighted mean 90 � 10 Ma).
96A10b: The sample yielded three zircon fractions for conventional U/Pb dating (Fig.
3.7m). Two of the fractions (F1 and F2) plot on the concordia. As fraction F3 has a very
small analytical uncertainty but plots below the concordia, Pb loss can be assumed.
This might be also true for fraction F1. No older Pb component seems to be inherited.
Only one intercept could be calculated which is interpreted as the crystallisation age at
126 � 50 Ma.
A96S1b: Six fractions were selected from this subvolcanic latite-andesite (Fig. 3.7n). All
fractions consist of small zircons (~63-80 �m). One half is long prismatic, transparent
without inclusions. The other half shows transparent whitish to yellowish tabular
zircons. Many zircons are broken. Zircons plot into the field of calc-alkaline and
subalkaline magma series in the Pupin-diagram. Two fractions (F6 and F7) plot close
but above the concordia and fractions F2 and F3 close but below the concordia. These
four fractions are interpreted to approximate the crystallisation age, whereas fraction
F8 might include inherited cores. Fraction F5 might have lost Pb. The lower intercept
age is 74.2 � 0.7 Ma.
Northernmost South Pamirs
P2: As only one zircon fraction was analysed, which is discordant, no age
interpretation could be made (Fig. 3.7o). SHRIMP analyses on 15 zircons (Fig. 3.7p)
reveal ages on well rounded cores (Appendix B, plate B4) of about 1624 Ma, 902 Ma,
417 Ma, 355 Ma, and 196 Ma; one 158 Ma spot is from a nodular inner grain portion. 15
spots from rims and multi-faced, idiomorphic grains range from 126 to 102 Ma with a
weighted mean age of 117.9 � 3.7 Ma; one grain is as young as 78 � 5 Ma.
P5: The two analysed zircon fractions plot close to the lower discordia interception
(Fig. 3.7o). Therefore one could choose a simple model of crystallisation with some
minor inheritance. Assuming no subsequent Pb loss, the crystallisation age is 74.5 � 8.8
Ma with Precambrian zircon inheritance (i.e. ~710 Ma). The two fractions of sample P5