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epresent a slightly younger evolutionary stage of an oceanic basin and the generation
of a primitive island arc than further east – or the section documents probably the
younger back-arc stage. Further geochemical and structural studies have to reveal this.
Continuing east along the central portion of the Altyn Tagh, Sobel & Arnaud (1999)
described two transects across the Altyn Tagh range which include the possible
continuation of the early to middle Palaeozoic sutures described above. The published
oldest rocks in the Altyn Tagh range are considered to be Tarim basement, represented
by the Miran Group of gneiss, granulite and amphibolite. The rocks are dated at 2462
Ma (U/Pb on zircon, Wang et al. 1993) and yielded Sm/Nd ages of 2787±151 Ma and
2792±208 Ma on granulite and amphibolite, respectively (Che et al. 1995). A gabbro
from a mafic magmatic unit in the centre of the range is dated as 1883±110 Ma by the
Rb/Sr method (Wang et al. 1993). Biotite from granitic gneisses of the Miran Group at
Lapeiquan (Fig. 3.12) yielded a WMPA age of 1741±24 Ma (Sobel & Arnaud 1999); this
might reflect a cooling age related to the emplacement of the crosscutting mafic dikes.
In the area just south of the North Altyn Tagh fault, bimodal volcanics, interpreted to
represent a rift, are dated at 1793±270 Ma by Sm/Nd on basalt. The same region
contains a granulite dated at 1704±105 by Sm/Nd. These ages of the Tarim basement
are comparable to the ages found in the Kudi section. Only few Palaeozoic intrusions
are documented across the transects of the Altyn Tagh range; a 424±8 Ma Rb/Sr age
was obtained from an acidic volcanic unit near Lapeiquan (Che et al. 1995). A
metamorphosed granite gneiss yielded an age of 472.8±10.4 Ma, mica schists on the
south side of the range yielded a WMPA of 453.4±8.7 Ma on muscovite, and a foliated
leucogranite a WMPA of 431.5±7.8 Ma on muscovite. Undeformed porphyritic biotite
granite yielded an Rb/Sr age on biotite of 426 Ma (no error given; Xu et al. 1996);
another undeformed granodiorite of the area yielded an WMPA of 413.8±8.0 Ma on
biotite (Sobel & Arnaud 1999). Pegmatites 430 km to the east were dated at 431.6±7.6
Ma on muscovite. Basalt from a dismembered ophiolite suite south from the Altyn Tagh
fault at Mangnai is dated 481±53 Ma by an eight point Sm/Nd isochron (Sobel &
Arnaud 1999). From these data Sobel & Arnaud (1999) suggest the existence of a suture
zone in the Altyn Tagh range, which is most clearly developed near Lapeiquan: an
early Palaeozoic basin with likely oceanic crust closed post-Early Silurian and pre-
Middle Devonian. Basin closure may have resulted in high-pressure metamorphism and
is therefore also interpreted as a suture zone which might be correlated with the
Wuyitake (Oytag)-Kudi-Subasi ophiolite belt and the northern Pamirs belt. Jolivet et al.
(1999) dated a further granite of this region by the U/Pb zircon method at 441±9 Ma. A
five point Rb/Sr isochron age was obtained at 327±7 Ma with an initial 87 Sr/ 86 Sr ratio of
0.7095±0.0036 (±2�).
Correlation of this suture further east can be only tentatively drawn, as available data
cannot reliably distinguish between similar igneous-metamorphic belts recognised in
this area (Sobel & Arnoud 1999). Candidates include the East Kunlun Shan, the
northern margin of the Qaidam basin, and the Bei Shan.
Westward of the Northern Pamirs, Middle Palaeozoic sutures should continue into
Afghanistan (Fig. 3.13). Unfortunately, several years of war hindered scientific research
in this region and therefore only limited geological data are available from Afghanistan.
One of the latest compilations of typology, age and geodynamic setting of plutonic rock
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