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Thompson et al. (1984), who see calc-alkaline lamprophyres as melts of enriched subcontinental
lithosphere, frequently associated with orogenic belts, but post-dating the
peak of compressional deformation and rather associated with varying degrees of
contemporary extension. On the other hand, there are also lamprophyres known in an
Andean-type continental margin setting (Carlier & Lorand 1997). In the Central Tien
Shan, in the Chinese sector, a high volume of granite intrusions is exposed, which
increases to the west. Hu et al. (1986) obtained a variety of ages from 350 Ma to 250 Ma
by both U/Pb and K/Ar methods. Hopson et al. (1989) recorded four phases of plutonic
magmatism in the Tien Shan: from 415 to 398 Ma (Late Silurian/Early Devonian), ~365
Ma (Late Devonian), ~334 Ma (Early Carboniferous), and ~261 Ma (Permian). A section
of the western part of the Chinese Tien Shan (Fig. 3.11), described by Chen et al.
(1999), confirms a north-facing passive continental margin along the northern Tarim
margin during the Early Palaeozoic, separating the Tarim block from the Central Tien
Shan (e.g. Yili block). Along the northern margin of this south Tien Shan oceanic basin
was an active continental margin established, at least from the Silurian to the Middle
Devonian, subducting northward beneath the Yili microcontinent. Isotopic ages of the
south Tien Shan ophiolites and island arc granites show a considerable variation along
strike (Chen et al. 1999): in the eastern segment of the South Tien Shan suture, oblique
collision is documented for the Late Devonian/Early Carboniferous, leaving a remnant
oceanic basin to the west. Subduction ceased in the western segment in the Late
Carboniferous/Early Permian, creating an Early Permian magmatic arc along the
northern Tarim margin and probably in the Kyrgyz South Tien Shan. This magmatic arc
existed for only a short time during the Early Permian.
Here, it is concluded that the obtained Early Permian Ar/Ar ages of the southernmost
Tien Shan can either be interpreted as denudational cooling ages close to the
crystallisation age, and therefore as evidence for final Early Permian subduction of this
remnant basin or that the obtained ages reflect cooling related to an Early Permian
extensional event, documented in some regions along the northern Tarim margin. In
the area of the northern Tarim basin a Late Permian/Early Triassic peripheral foreland
basin and a foreland deformation belt developed (e.g., Bazhenov et al. 1993, Caroll et al.
1995) (Fig. 3.11).
North Pamirs-Altyndara metavolcanic rocks
The studied magmatic rocks from the Trans-Alay range (Fig. 3.3-3.4) of the
northernmost Pamirs are mafic to acid, greenschist grade and deformed metavolcanic
rocks. The age of the volcanic rocks is ~370-320 Ma and likely extends into the Triassic.
Based on major, trace, and isotope geochemistry two different geotectonic
environments could be distinguished: the southernmost part of a N-S section in the
Altyndara valley consists of tholeiitic basalts which are interpreted to represent either
oceanic crust or primitive island-arc basalts. Northward, more evolved calc-alkaline
basalts, andesites and rhyolites of early arc stages are exposed. Pospelov (1987),
Budanov & Pashkov (1988) and Leven (1981) recorded magmatic rocks belonging to an
oceanic environment north of the Lake Karakul area, which are herein interpreted as
likely correlatives of the Altyndara section. Furthermore, in the western part of the
Northern Pamirs is the lower part of sections also composed of oceanic volcanics, but