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3 Crustal assemblage of the Pamirs: age and origin of magmatic
belts from the southern Tien Shan to the north-eastern Pamirs
The purpose of this chapter is (1) to introduce existing ideas about the formation of
the Pamirs, (2) to describe the geological setting, and (3) to present a new and
compiled set of geochemical and geochronological data of magmatic rocks from
southernmost Tien Shan and the north-eastern Pamirs. Geochemical data characterise
magmatic arc and postcollisional magmatites and therefore assess the nature and
limits of continental margins and linear collisional orogens (e.g. Sengör et al. 1993).
Based on radiometric ages of the different magmatic rock assemblages, the age
zonation of the magmatic belts can be defined. (4) Finally, correlations with magmatic
belts in Tibet and Afghanistan led to a new geodynamic model for the Central Asian
region, which elaborates on previous scenarios.
3.1 Introduction
The Pamirs were formed by accretion of continental fragments of Gondwana to the
southern margin of Eurasia. Magmatic arcs and island arcs of the Palaeotethys and
Tethys oceans and their marginal basins contributed significantly to that process. The
Pamirs were subdivided into a northern, central, and southern part (Fig. 3.1, and 3.3)
based mainly on rock assemblages and their stratigraphic age (e.g., Burtman & Molnar
1993, Zonenshain et al. 1990, Kravchenko 1979). However, it remains unclear how many
of these subduction and collision phases moulded the Pamirs, when they occurred and
how they can be correlated from Afghanistan (W) through the Pamirs into Tibet (E).
Geodynamic models from neighbouring China (e.g., Yin & Nie 1996) show how largescale
strike-slip faults and truncation of structures at the margin of the Tarim basin
hampered along strike correlation of sutures and magmatic belts across the Tibetan
plateau and to the Pamirs. A detailed and quantitative model of the Palaeozoic and
Mesozoic evolution of the Pamirs is basically missing and peculiarities of the Pamirs
compared with neighbouring regions were hitherto poorly envisaged.
3.2 Geological setting
Tien Shan
The Tien Shan orogen is one of the most impressive mountain belts in Central Asia.
It extends east-west for at least 2500 km with peaks exceeding 7400 m (e.g. Peak
Popedy 7439 m). The orogenic belt was formed in late Palaeozoic times (e.g., Burtman
1975, Coleman 1989, Carroll et al. 1990, Windley et al. 1990, Allen et al. 1993, Gao et al.
1995), but the present-day mountain range is a major product of the Tertiary India-Asia
collision (Molnar & Tapponnier 1975, Tapponnier & Molnar 1979). Geological studies
were focused mainly on the Chinese Tien Shan (e.g., Windley et al. 1990, Allen et al.
1993, Carroll et al. 1995, Gao et al. 1998), and the westernmost part of the Tien Shan
(e.g., Brookfield 2000 and Russian literature therein). The whole Tien Shan belt is
segmented by the NW-SE striking Talasso-Ferghana fault, which separates a western
from an eastern part (Fig. 3.2).
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