tübinger geowissenschaftliche arbeiten (tga) - TOBIAS-lib ...

60
oceanic crust for the consumption of the DNB basin. The arc is likely represented by
Jurassic volcanic rocks north and south-west of the Amdo massif (Pearce & Mei 1988,
Pearce & Deng 1988). This model accounts for the zonation of rock types such as forearc
ophiolites, volcanic arc rocks, and back-arc amphibolites from south to north. So far,
igneous rocks north of the BNZ are scarce and not much determined. The description
and recognition of the mid-Jurassic Rushan Pshart arc along the southern margin of the
Central Pamirs/Qiangtang on base of the herein presented geochemical and
geochronological data is therefore exceptional.
The correlation of the Karakul-Mazar, Qiangtang and Rushan Pshart zone to
Afghanistan is difficult, due to the strong alpine overprint. Furthermore, there are only
limited radiometric data, mainly Rb/Sr and K/Ar data, from these regions available (e.g.
Debon et al. 1987). Most of them yield Tertiary ages and the truly emplacement ages
are obscured. Correlation is more related to the rock associations than to geochemical
and geochronological data and the most likely candidates are the Band-e Bayan, Farah
Rod, Khas Rod and Safed Khers domains (Fig. 3.1, 3.13).
The Band-e Bayan area is a very poorly known area; it is made up of metamorphic,
sedimentary and volcanic formations ranging in age from (?)Precambrian to Cretaceous
(Debon et al. 1987). It can only be speculated if these rock associations may be
correlatives to the Karakul-Mazar belt in the Central Pamirs. The Band-e Bayan
contains small plutonic outcrops. The most common rock types are granodiorites,
granites and adamellites, and to a lesser extend trondhjemites. The general character
is peraluminous and light, dark or two mica types. In the eastern part, the rocks are
transitional to metaluminous types. Most of the rocks are calc-alkaline or sub-alkaline.
Like in the Central Pamirs most of the rocks obtained Tertiary and subordinate Lower
Cretaceous ages by Rb/Sr and K/Ar dating. Unfortunately no U/Pb ages are available to
constrain the crystallisation ages of these intrusions.
The Farah Rod and Khas Rod domains are quite different and tectonically separated by
the Khas Rod fault (Fig. 3.13). The Farah Rod domain is probably made up of
continental basement overlain by Upper Mesozoic sediments and by dominantly
Eocene-Oligocene volcanic rocks. Numerous stocks and small plutons, often associated
with volcanic rocks occur. The dominant intrusive types are metaluminous and
peraluminous granodiorites, granites and adamellites of subalkaline character. Like in
the Central Pamirs there occur transitional to alkaline intrusions with quartz syenites
or quartz monzonites. The Khas Rod is an ophiolitic complex associated with spilitediabas
stocks of assumed Early Cretaceous age. Several authors (Karapetov et al. 1975,
Bassoullet et al. 1980, Vachard 1980, Mattauer et al. 1981, Tapponnier et al. 1981,
Debon et al. 1987) have suggested that the Khas Rod domain is probably the
continuation of the Rushan Pshart zone. According to Blaise et al. (1978), the Khas Rod
domain represents remnants of a Triassic/Jurassic oceanic basin which closed in Early
Cretaceous. Subduction direction was probably to the south, beneath the Central
Afghanistan mountains. Like in the Band-e Bayan only Tertiary and Lower Cretaceous
K/Ar ages were determined (Debon et al. 1987).
The Safed Khers domain (Fig. 3.1, 3.13) is bounded in the north-west by western
Badakhshan and to the south-east by the Axial batholith zone of Pakistan and by the
Wakhan zone. Precambrian gneisses and metasedimentary formations (black slates) of