Age and tectonic setting of the Ras Koh ophiolites, Pakistan. | [Acta mineralogica pakistanica • 2000]

Author(s):

1. Edwin Gnos: Mineralogisch-Petrographisches Institute, Baltzerstrasse 1, 3012, Bern, Switzerland

2. Mehrab Khan: Centre of Excellence in Mineralogy, University of Baluchistan, Quetta, Pakistan

3. Khalid Mahmood: Centre of Excellence in Mineralogy, University of Baluchistan, Quetta, Pakistan

4. Abdul Salam Khan: Centre of Excellence in Mineralogy, University of Baluchistan, Quetta, Pakistan

5. Igor Maria Villa: Mineralogisch-Petrographisches Institute, Gruppe fur Isotopengeologie, Erlachstrasse 9a, 3012 Bern, Switzerland

Abstract:

The serpentinized peridotites with preserved subophiolitic metamorphic sole present in the Ras Koh Range, Pakistan, were identified as relics of an ophiolite nappe emplaced onto a Jurassic/Cretaceous island arc sequence. Initial emplacement was dated with hornblendes from the metamorphic soles at 108.7±0.3 Ma to 111.0±1.0 Ma (39Ar/40Ar). The final emplacement onto the island arc sequence occurred between Campanian and Paleocene, and was followed by acretion of the island arc to the Eurasian plates and subaereal exposure of the ophiolite. Both the ophiolite and the Paleocene/Eocene sedimentary cover were intruded by calcalkaline to subalkaline tonalite-diorite-gabbro magmas related to north-directed subduction magmatism, Abiotite and hornblende separate from the main intrusions yielded argon isochrone ages of 46.0±0.5 and 48.4±1.2 Ma. Due to its analogies, the Ras Koh range is viewed as the western, less eroded continuation of the Spin Boldak-Kohistan-LadakhTibet island arc offset by ~800 km along the Chaman-Nushki fault system during the India-Eurasia collision. Uplift of the Ras Koh range started in Late Oligocene/ Miocene and was caused by extrusion of the Helmand Block and shortening in the region west of the Chaman fault. This translated into southward thrusting and ramping of the Ras Koh range, possibly over a hidden micro continental block located beneath the Hamun-I-Maskhel depression.

Page(s): 105-118

DOI: DOI not available

Published: Journal: Acta mineralogica pakistanica, Volume: 11, Issue: 1, Year: 2000

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