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80 Tethyan ocean between them (Kapp & Cowgill 2001; chap. 3; Schwab et al. in press). In Late Permian/Early Triassic the closure of these ocean basins started and resulted in Triassic/Early Jurassic accretion of the Qiangtang terrane to the southern Eurasian margin. Several slices of oceanic crust and mélange rocks were subducted beneath the Qiangtang block and exhumed in fault-bounded basement domes in the Central Pamirs/Qiangtang block. In the Qiangtang, exhumation of these basement rocks was already in the Mesozoic, whereas in the Central Pamirs in the Tertiary. It is not published if Tertiary central Qiangtang sediments derived Triassic ages probably related to the Mesozoic exhumation of the basement domes. Grain ages of the Palaeogene sediment sample A96K1e cluster around 165 Ma and are comparable with a major age component in Triassic/Jurassic granitoids of the southern margin of the Qiangtang block and may record Rushan Pshart arc activity. (2) Cretaceous magmatic rocks and contact metamorphic hornfelses occur at the northern margin of the Qiangtang block. Sample A96S1b, a subvolcanic latite-andesite, is cropping out in the Saseksu valley (Fig. 4.2); it’s lower intercept U/Pb zircon age is at ~74 Ma (Fig. 3.7k, chap. 3) and the zircon fission track cooling age is about 64 Ma. A similar zircon fission track age was also found at the southern margin of the Qiangtang block; monzogranite 96A10b yielded an apparent zircon fission track age of about 59 Ma. Both the Late Cretaceous magmatic intrusions and their low-T cooling ages thus give evidence for the regional distribution of a Late Cretaceous thermal influence, likely caused by flat-slab subduction of the Shyok suture. If this thermal overprint reached as far as to the northern part of the Qiangtang block and may explain the Late Cretaceous ages determined there, is debatable. Shyok ocean basin subduction and Kohistan-Ladakh arc accretion caused high-grade metamorphism in the Hindu Kush-Karakoram blocks south of the South Pamirs at 80 to 50 Ma (Fraser et al. 2001). As no apatite fission track ages are on hand, it can not be inferred if the Late Cretaceous/Palaeocene cooling in the Qiangtang block led to cooling of the rocks to near surface temperatures. Additionaly, intracontinental deformation caused by the approaching India may have caused renewed lithosphere subduction of the Karakul-Mazar belt beneath the northern Qiangtang margin. This may be evidenced by the geochemical signiture of sample A96S1b close to the Karakul- Mazar substratum. In the light of such Late Cretaceous thermal influences, it would be interesting to determine whether the Early Eocene apatite cooling age of the southernmost sample of the Karakul-Mazar belt is also a result of a Late Cretaceous/Palaeocene heat source. Muzkol and Sares domes, Central Pamirs The Muzkol and Sares domes expose fault bound greenschist to upper amphibolite facies metamorphic rocks of Tertiary age. Monotonous, mainly siliciclastic and mafic rocks with subordinate carbonatic sequences (see chap. 3) are interpreted to characterise the Karakul-Mazar-Sonpan-Ganze accretionary wedge rocks, associated with the subduction of Jinsha oceanic crust (chap. 3, Schwab et al. in press). Whereas these rock associations were formed in Triassic/Early Jurassic times, the exhumation of the domal structure is a Tertiary feature. Metamorphic hornblende and biotite Ar/Ar ages range from 28 to 14 Ma (Appendix A, Tab. A3). Two Rb/Sr whole rock-muscovite
ages from gneissic rocks within the Sares dome are 37 and 30 Ma. Locally, metamorphic grade reached migmatisation and granitoid production. One syenogranodiorite in the Muzkol dome (P15) gave an U/Pb zircon lower intercept age of 31±5 Ma (Appendix A, Tab. A7). The emplacement age of an aplitic dyke of the Sares dome (96A6b) is at 17±8 Ma (U/Pb zircon, Appendix A, Tab. A7); another aplitic dyke (L96A11a) gave a whole rock-muscovite Rb/Sr age of 13.7±0.1 Ma (Appendix A, Tab. A5). Tertiary melt injection is also documented by basalt dykes with Ar/Ar whole rock ages at 20±2 Ma (Appendix A, Tab. A3). Low-T cooling of the dome is constrained by zircon and apatite fission track ages of the above mentioned intrusions (P15, P17) and dykes (96A6b) and range from 20 Ma to 15 Ma. Within the Tertiary intramontane sediment basin along the southern margin of the Muzkol dome, the herein called ‘Murgab section’ (Fig. 4.4), the zircon fission track ages from sandstone samples A96M6b and M96M5a are ~20 Ma and ~19 Ma, respectively. K/Ar sericite ages from the same sedimentary rocks (5 samples) range of 33 to 14 Ma (Fig. 4.4; Appendix C, Tab. C4, C6), whereby the K/Ar ages increase away from the dome (from north to south). Basalt dykes cutting through the sedimentary rocks of the section gave two Ar/Ar biotite ages of about ~20 Ma. As both zircon fission track sandstone samples show only one age population, it can be concluded that either the sandstones are reset, or that only one Miocene source area was delivering around ~19 Ma old grains. From the Geological map 1:200.000 of Tadzhikistan (1968) a Plaeogene stratigraphic age is supposed for the sedimentary rocks. Both samples come from different horizons and the whole section shows changing proximal and distal parts. Considering these stratigraphic and sedimentary aspects, I exclude a single grain age source in the hinterland. More likely, the Miocene rising thermal dome heated the Palaeogene sediment basin to temperatures � the zircon PAZ (190°C to 260°C, Zaun &Wagner 1985) as the zircons from these sediments are interpreted to be reset. The older K/Ar sericite ages (> 25 Ma) are interpreted to be partly reset as the thermal influence of the dome probably decreased from north to south. Preliminary geochemical data of the basalts characterise them as within plate basalts, extruded in an extensional setting in Miocene. Interestingly, in the correlated domal structure (Schwab et al. in press) of the Qiangtang block in Central Tibet Tertiary metamorphism and exhumation are lacking and Mesozoic ages dominate (Kapp et al. 2000). Rushan Pshart arc and Shyok arc The Rushan Pshart zone and the northernmost South Pamirs are strongly deformed by the splays of the dextral Karakoram fault. Triassic/Jurassic arc-type intrusions and a Triassic oceanic-basin-arc sequence in the eastern Pshart and southern Qiangtang constitute the Rushan Pshart arc. Sample L96M25a, a monzogranite, yielded a zircon fission track age of about 69 Ma, suggesting Late Cretaceous low-T cooling. Late Cretaceous granodiorites (P2 and P5) are the southernmost samples dated in this study and are related to the Shyok arc. Apatite fission track ages of these samples are around 11-10 Ma and so far the youngest fission track ages in the Pamirs. This is likely due to Miocene activation along dextral transpressional strike-slip faults, the Karakorum fault splays, leading to low-T cooling and exhumation. 81
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Institut für Geowissenschaften (IF
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Key words: Tien Shan, Pamirs, Tibet
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* Nr. 22 MESCHEDE, M. (1994): Tecto
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* Nr. 55 REINECKER, J. (2000): Stre
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The amalgamation of the Pamirs and
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Wer aber je auf Asiens Erdboden sch
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viii (1971). Black lines with arrow
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x sample locations and age distribu
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xii Swedish expeditions followed. I
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2 postulated that the Safed Khers a
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4 Zusammenfassung Die paläozoische
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6 (d) Oberkreide Spaltspurenalter w
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8 deformation propagate from the pl
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10 source terrains, changes in the
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12 Geochemisches Zentrallabor, Univ
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14 mean UO+/U+ and Pb+/U+ value. Si
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16 apatites and zircons were counte
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18 number of simulation runs and th
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Two Palaeozoic sutures divide the C
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igneous and sedimentary rock sequen
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Triassic and the formation of an is
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27 Lake Karakul The Lake Karakul ar
Page 53: 3.3.2 Major and trace elements Majo
Page 56 and 57: 32 of 26-40 for samples P2, P5, and
Page 58 and 59: 34 some of the zircons suffered Pb
Page 60 and 61: 36 fractions are situated close to
Page 65: 3.4.2 Rb/Sr dating The Rb/Sr minera
Page 69 and 70: 45 The Cretaceous plutons show mode
Page 72 and 73: 48 Thompson et al. (1984), who see
Page 74 and 75: 50 granites and have Sm-Nd model ag
Page 76 and 77: 52 associations in Afghanistan is f
Page 78 and 79: 54 Kara Kunlun or Mazar accretionar
Page 80 and 81: 56 Early Jurassic low-angle normal
Page 82 and 83: 58 Coward et al. (1988) pointed out
Page 84 and 85: 60 oceanic crust for the consumptio
Page 86 and 87: 62 Palaeogene volcanic rocks of cen
Page 88: 64 3.7 Conclusions Based on new own
Page 91 and 92: 4 Cenozoic exhumation history of th
Page 93 and 94: Southern Tien Shan One Permian gran
Page 97 and 98: Thermal modelling The results of th
Page 99 and 100: amphibole, white mica, and biotite
Page 101 and 102: the zircon PAZ. The apparent apatit
Page 103: elt likely operated as an out-of-se
Page 108 and 109: 84 south to 18 Ma in the north; the
Page 110 and 111: 86 Cretaceous plutons that intrude
Page 112 and 113: 88 6 References Allègre C.J., Cout
Page 114 and 115: 90 House, scale 1:2 000 000. Chen F
Page 116 and 117: 92 Karakorum (Sinkiang, China); the
Page 118 and 119: 94 Hurford A.J. and Hammerschmidt K
Page 120 and 121: 96 without double spiking. Geochimi
Page 122 and 123: 98 Geological Society of America, 2
Page 124 and 125: 100 Zamoruyev A., 1995a. Neotectoni
Page 126 and 127: 102 Ann. Rev. of Earth and Planetar
Page 128: 104 this work, especially all membe
Page 132 and 133: II Appendix A Tab. A1 Sample descri
Page 134 and 135: IV Tab. A2 continued Pamir frontal
Page 136 and 137: VI Tab. A2 continued Tertiary exten
Page 138 and 139: VIII Tab. A3 continued Sample Miner
Page 140 and 141: X Tab. A6 Rb/Sr and Sm/Nd isotopic
Page 142 and 143: XII Tab. A7 continued Apparent ages
Page 144 and 145: XIV Tab. A9 U/Pb isotopic ratios fr
Page 146 and 147: XVI Tab. A9 continued 207 Pb/ 206 P
Page 148 and 149: XVIII Tab. A9 continued 207 Pb/ 206
Page 150 and 151: XX Appendix B Plate B1 Cathodolumin
Page 152 and 153: XXII Plate B3 Cathodoluminescence i
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XXIV Plate B5 Cathodoluminescence i
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XXVI Tab. C2 Determination of the
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XXVIII
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XXX
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XXXII
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XXXIV
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XXXVI Tab. C5 continued Sample grai
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XXXVIII Tab. C5 continued Sample gr
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XL Tab. C5 continued Sample grain-n
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XLII Tab. C5 continued Sample grain
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XLIV
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XLVI Tab. C6 Zircon fission track a
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XLVIII
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LII
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LIV Tab. C7 Apatite and zircon fiss
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