The Chilas Igneous Complex is one of the major geologic units in the Kohistan terrane of the Himalaya of northern Pakistan. The Kohistan terrane is regarded as a tilted island-arc sequence. The Chilas Complex is a 300-km-long, 40-km-wide plutonic body that intrudes the Kamila Amphibolite. The Main facies rocks of the Chilas Complex consist of gabbronorite, diorite, and quartz diorite. Small bodies of ultramafic-mafic association composed mainly of peridotitic-gabbroic cumulates, and a layered gabbronorite body, are also present, probably as xenoliths. A few samples are considered to have been contaminated as a result of assimilation of xenolithic materials and country rocks: Excluding them, 14 whole-rock samples of the Main facies have an Rb-Sr age of 111 ± 24 Ma and an initial 87 Sr/ 86 Sr ratio of 0.70403 ± 0.00006. Gabbroic rocks from the ultramafic-mafic association, from the layered gabbronorite body, and from mafic dikes, have 87 Sr/ 86 Sr ratios between 0.7039 and 0.7044, interpreted to be close to the 87 Sr/ 86 Sr ratios at the time of their generation. The whole-rock isochron age for the Main facies is regarded as the age of intrusion. The initial 87 Sr/ 86 Sr ratio for the Main facies is within the range of typical arc magmas, suggesting that the igneous activity occurred within an island arc or orogenic belt close to a continental margin. The age of intrusion of the Chilas Complex is similar to that of the earliest magmatism in the Kohistan batholith and to the Cretaceous plutonism in Ladakh and Karakorum, indicating large-scale generation of subduction-related magmas in the western Himalayan region and Karakorum during the mid-Cretaceous.
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