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Book Chapter

The Chilas Mafic-Ultramafic Igneous Complex; The root of the Kohistan Island Arc in the Himalaya of northern Pakistan

By
M. Asif Khan
M. Asif Khan
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M. Qasim Jan
M. Qasim Jan
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Brian F. Windley
Brian F. Windley
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John Tarney
John Tarney
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Matthew F. Thirlwall
Matthew F. Thirlwall
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Published:
January 01, 1989

The Chilas Complex is a large mafic-ultramafic body closely associated with the Kohistan Arc sequence in the western Himalaya of northern Pakistan. The arc and the Chilas Complex occupy an area of 36,000 km2, bounded on the north and south by major sutures. The arc formed close to the margin of Eurasia in response to the northward subduction of neo-Tethyan ocean lithosphere in Late Jurassic to middle Cretaceous time, and consists of intra-arc sediments, calc-alkaline volcanics, and diorite-tonalite-granite plutons. At its base is the Chilas Complex, which extends for more than 300 km and which has a maximum width of 40 km. Most of the complex consists of massive (although locally layered) gabbro-norites, which comprise variable amounts of plagioclase (An64-40), orthopyroxene (En76-48), clinopyroxene (mg = 75-55), magnetite, ilmenite, ±quartz, ±K-feldspar, ±hornblende, ±biotite, ±rare scapolite. In the central part of the complex, near the base, there are minor discordant dikes and intrusive bodies as large as 5 km2 of a dunite-peridotite-troctolite-gabbronorite-pyroxenite-anorthosite association that displays excellent layering, graded bedding, slump breccias, and syndepositional faults. These rocks contain olivine (Fo94-71), relatively Mg-rich orthopyroxene (En91-65), clinopyroxene (mg = 85-67), and calcic plagioclase (An98-83), ±hornblende, ±chrome spinel, and ±pleonaste, and represent a more primitive magma batch emplaced into the base of the gabbro-norite magma chamber.

The mafic complex is not an ophiolite. Rocks of the complex have more petrographic and compositional similarities with plutonic blocks from island arcs and with other major mafic complexes such as the Border Ranges Complex of Alaska and those from the Ivrea Zone in the Alps. Trace-element patterns of the gabbro-norites have marked negative Nb anomalies, positive Sr, Ba, and P anomalies, and high K/Rb ratios, features consistent with melting of a hornblende-bearing sub-arc mantle source. The Chilas Complex either represents the root zone magma chamber of the Kohistan island arc, or magma generated by diapirism in the early stages of intra-arc rifting during formation of a back-arc basin.

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Contents

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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