Abstract:
The Chilas Complex in northern Pakistan stretches for about 300 km from Astor to western Dir and attains a width of 40 km in the middle. It was probably emplaced during the early stages of development of the Cretaceous Kohistan island arc. Field observations, supported by petrography and mineral chemistry, suggest that the complex is composed of two groups of rocks. Group A consists predominantly of noritic rocks with subordinate pyroxenites, anorthosites and hypersthenequartz diorites. These cover most of the complex, are generally strongly foliated and display some layering. The mineral composition of these rocks varies as follows : orthopyroxene En75 to En51; clinopyroxene Mg=43 to 27, Fe=, 9 to 22, Ca=42 to 50 ; plagioclase An62 to An43. Group B rocks occur in the form of lenses (upto 5 km) and smaller bodies showing complex contact relations, ranging from concordant to discordant, with their host (group A) rocks. These comprise dunite, peridotites, pyroxenites, tractolites, norite, anorthosite, and pyroxene pegmatites; of these the former two are the most abundant. The principal outcrops of these rocks are found in Chilas area but isolated, small bodies occur as far to the west as Swat. The range in the mineral composition of these rocks is :olivine Fo88 to Fo73; orthopyroxene En90 to En62; clinopyroxene Mig.46 to 37, Fe=5 to 17, Ca=49 to 45; plagioclase An98 to AN77. Characteristic features of this group of rocks are the development of excellent layering and corona structure. The group A association may have evolved from a high-alumina basalt (calc-alkaline} magma. It is not clear whether the group B rocks represent magmatic cumulates of this magma or younger intrusions of a more basic (picritic)magma. Both groups were probably metamorphosed under pyroxene granulite facies, to be followed by amphibolite dykes and hornblende_plogioclose pegmatites.
Page(s):
153-169
DOI:
DOI not available
Published:
Journal: Geological bulletin (university of peshawar), Volume: 17, Issue: , Year: 1984