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older 230-200 Ma magamtic arc along the northern margin of the Qiangtang block
(Tanymas-Jinsha suture). The Sr-initials of the Triassic/Jurassic intrusions are in the
range of 0.706974 to 0.709310 and therefore indicate only limited crustal contamination
in comparison to the Cretaceous granites (see below) and thus only limited crustal
stacking. The rock types are mainly biotite granites or two mica granites into Triassic-
(?)Jurassic siliciclastic host rocks. As the samples L96M25a, L96A9, M96A7, 96P4e, and
A96M18h do all show mid-Jurassic lower intercept ages or SHRIMP spot ages and only
in case of the first two samples also Triassic age components, these samples are
interpreted to belong to mid-Jurassic basin closure along the Rushan Pshart zone.
Rushan Pshart Zone
During most of the Permo-Triassic, the southern margin of the Qiangtang terrane
was under extension. The Rushan-Pshart zone, a transitional zone between the Central
Pamirs and the South Pamirs, is built of Upper Palaeozoic to Jurassic volcanosedimentary
successions probably representing a rift-margin and/or an oceanic basin –
(?)arc sequence. Some earlier models postulated the existence of a vast Permo-Triassic
ocean, closed during the Jurassic (e.g. Sengör 1990). Leven (1995) is more arguing for a
narrow sea-way, which separated the West Pshart from the East Pshart instead of
interpreting the Rushan Pshart zone as a continuous suture zone between the Central
and South Pamirs.
Across the Tibetan plateau, the Bangong-Nujiang zone (BNZ) separates the Qiangtang
block in the north from the Lhasa block in the south. Several ophiolite-bearing basins
are aligned along the BNZ, which are of Mesozoic age (Yu et al. 1991). The average
width is between 10 to 20 km, but can locally reach up to 200 km. The interpretation of
the Bangong-Nujiang zone is very controversial. Most authors interpreted the BNZ as a
collisional suture (e.g. Pan & Zheng 1983, Zheng et al. 1984, Coulon et al. 1986).
Girardeau et al. (1985) considered Late Jurassic to Early Cretaceous ophiolite
obduction associated with long-range thrusting of >100 km. Xu et al. (1985) assumed a
mid-Jurassic collision. Pearce & Deng (1988) and Hsü et al. (1995) suggested that
different ophiolite units of the BNZ may represent different types of lithosphere.
However, according to Schneider et al. (2003) the views of what may be forearc or
backarc lithosphere are inconsistent. As discussed earlier, Gaetani (1997) deny a
correlation of the Waser-Panjao ocean (opened in Central Afghanistan north of the
Helmand block), the Rushan Pshart zone, and the Bangong-Nujiang ocean as the
Waser-Panjao and Bangong-Nujiang oceans closed mainly in Jurassic time whereas he
suggests, that the Rushan Pshart oceanic basin closed already in Late Triassic. In the
central part of the SE Pamirs, as well as in the Karakorum, no major episodes of
deformation are recordable at the end of the Triassic although terrigenous, and
sometimes even continental sediments rest on older marine successions. This is also
true for the Central Pamirs, where predominantly terrigenous rocks including
conglomerates of Late Triassic to Early Jurassic age are found (Gaetani 1997). As a
consequence, Gaetani (1997) follows an interpretation of the BNZ as a back-arc basin
and connects the BNZ to the Shyok back-arc, and links them both to the Kohistan and
Ladakh-Gangdese oceanic arcs.
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