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A granite pebble from a ?Miocene conglomerate in the Pshart valley (96P4e) gave a
Late Jurassic 40 Ar/ 39 Ar age of 151.4 ± 1.7 Ma (Fig. 3.9k). 40 Ar/ 39 Ar dating of muscovite
from a basement rock north of Murgab (A96M18h) yielded an age of 166.2 ± 0.5 Ma
(Fig. 3.9l).
Cretaceous intrusions of the Central Pamirs and RPZ gave late Early Cretaceous
40 Ar/ 39 Ar cooling ages of ~105 and ~101 Ma (samples M96M7 and P7, respectively; Fig.
3.9m,n) and 40 Ar/ 39 Ar cooling ages from two intrusions of the northernmost South
Pamirs spread from ~98 Ma (P2) to ~70 Ma (P5) (Fig. 3.9o,p).
3.5 Sr and Nd isotope compositions of the granitoids
Correlated isotopic compositions of the Sm/Nd and Rb/Sr systems have been used
to obtain information about the magma sources and to determine crustal formation
ages. Twelve igneous samples have been analysed (Appendix A, Tab. A6) from the
Northern and Central Pamirs and the Rushan Pshart Zone. In general, model ages
assess the time at which a rock separated from its mantle source region. For igneous
and meta-igneous rocks this is a good estimate of the crust formation age.
Most of the Pamiran granitoids were derived by the melting of older continental crust
(S-type granites). Therefore model ages will be indicative of the age of the crustal
source. This is possible because the intracrustal fractionation process does not greatly
disturb the Sm/Nd ratio of the source. Often, however, granites are a mixture of crustal
and mantle sources. When this is the case, calculated model ages may give very
misleading results (Arndt & Goldstein 1987). In this study, model ages were calculated
with the equation of Liew & Hofmann (1988) with a two component reservoir of
continental crust (CC) and depleted mantle (DM).
Initial Sr-ratios are lowest for the Carboniferous metavolcanic rocks of the Northern
Pamirs (Altyndara valley) with ( 87 Sr/ 86 Sr) i = 0.705264 to 0.705642 and for the Triassic
leucogabbro (( 87 Sr/ 86 Sr) i = 0.705311). All other Sr-initials from Triassic/Jurassic
granitoids range from 0.706974 to 0.709310, indicating crustal contamination. The
Cretaceous granitoids have initial Sr-ratios between 0.709745 and 0.711105, implying
even more crustal involvement. Such increasing contribution of crustal material in
Cretaceous granites correlates with the low initial Nd-values of 0.512001 to 0.511820.
The highest ( 143 Nd/ 144 Nd) i values obtained the metavolcanic rocks from the Altyndara
valley, which range from 0.512191 to 0.512508. The leucogabbro shows also a high
( 143 Nd/ 144 Nd) i value of 0.512477, whereas the rocks of the Lake Karakul batholith fall into
a very narrow range between 0.512148 and 0.512159. The Triassic/Jurassic samples
have slightly lower values around 0.512050. Cretaceous granites show the lowest
( 143 Nd/ 144 Nd) i values . The �Nd (0) values of the Cretaceous granites fall within the range
of -11 to -12, those of the Triassic/Jurassic granitoids of the Central Pamirs and Rushan
Pshart zone around -8 with the exception of the leucogabbro with an �Nd (0) value of -
0.3. �Nd (0) values of the Karakul lake batholith are -6 to -7. The metavolcanic rocks show
a wider spread of �Nd (0) values from +4.5 to -3.4. Model ages for these rocks mirror this
variety with ages increasing from 0.6 Ga for the southern samples to 1.1 Ga for the
northern samples. The model ages of the Lake Karakul batholith are about 1.3 Ga,
whereas the Triassic/Jurassic plutons are very consistent in the range of 1.4 Ga.