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Silicate‐oxide mineral chemistry of mafic–ultramafic rocks as an indicator of the roots of an island arc: The Chilas Complex, Kohistan (Pakistan)
Author(s) -
Bilqees Rubina,
Qasim Jan M.,
Asif Khan M.,
Windley Brian F.
Publication year - 2016
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12130
Subject(s) - geology , geochemistry , ultramafic rock , mafic , olivine , island arc , granulite , partial melting , igneous rock , diorite , chromite , pyroxene , metamorphism , gabbro , petrology , basalt , facies , zircon , subduction , tectonics , geomorphology , paleontology , structural basin
Abstract The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km 2 , it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km 2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.