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Geochronology and temperature history of the Nanga Parbat–Haramosh Massif, Pakistan

By
Peter K. Zeitler
Peter K. Zeitler
Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia
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John F. Sutter
John F. Sutter
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Ian S. Williams
Ian S. Williams
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Robert Zartman
Robert Zartman
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R.A.K. Tahirkheli
R.A.K. Tahirkheli
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Published:
January 01, 1989

The gneisses of the Nanga Parbat–Haramosh Massif (NPHM), Pakistan, experienced peak metamorphic temperatures in the interval from 25 to 30 Ma, as revealed by 40Ar/39Ar cooling ages of hornblende and the ages of the youngest intrusions of the Kohistan batholith located immediately adjacent to the NPHM. 40Ar/39Ar and fission-track mineral ages indicate that the postmetamorphic cooling history of the NPHM has been controlled over the past 5 to 10 m.y. by active tectonism associated with the Raikhot Fault, although passive uplift and erosion in response to overthrusting of the NPHM by the Kohistan Arc has been underway as well. Net cooling rates for NPHM gneisses exposed today along the Indus River at low elevations have accelerated, from 20°C/m.y. at ∼ 20 Ma to 300°C/m.y. at 0 to 0.4 Ma. Following emplacement of aplite dikes at about 30 to 35 Ma, portions of the Kohistan Batholith adjacent to the NPHM experienced cooling rates similar to the NPHM of about 20°C/m.y. over the period 25 to 10 Ma, but the net cooling rates for the batholith of ∼30°C/m.y. over the past 10 m.y. have been much lower than those experienced within the NPHM. Ion microprobe and conventional U/Pb analyses of zircon show that the protoliths for the Iskere Gneiss and the structurally lower Shengus Gneiss of the NPHM are, respectively, ∼1850 Ma and 400 to 500 Ma in age. Zircons from the Iskere Gneiss have thin, relatively high U rims that yield ages from 2.3 to 11 Ma. These rims indicate that metamorphism of the NPHM gneisses is Tertiary, not Precambrian, in age. The ages and Concordia systematics of analyses of Shengus Gneiss zircons suggest that this gneiss may be a metamorphosed equivalent of the Mansehra Granite and other Paleozoic S-type granites found throughout the Himalaya.

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GSA Special Papers

Tectonics of the western Himalayas

Lawrence L. Malinconico, Jr.
Lawrence L. Malinconico, Jr.
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Robert J. Lillie
Robert J. Lillie
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Geological Society of America
Volume
232
ISBN print:
9780813722320
Publication date:
January 01, 1989

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