Rb-Sr isotopic study of the Chilas Igneous Complex, Kohistan, northern Pakistan

Abstract

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.

Tectonics of the Nanga Parbat syntaxis and the western Himalaya: an introduction

“…The later age is derived from an c. 84 Ma U-Pb zircon age (Zeitler et al 1981) for undeformed gabbro-norites of the Chilas complex, which contain xenoliths of Gilgit Formation gneisses that had been deformed during suturing. The age is compatible with a Rb-Sr age of 87+ 19Ma (Mikoshiba et al 1999) assuming that their sample 92CH60 is included in the regression. After suturing the arc behaved as an Andean-style volcanic arc.…”

Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan

“…Trace and rare‐earth element compositions of Chilas gabbronorites are typical for arc‐related rocks and point to melt generation at rather shallow levels of the mantle wedge. The melts were contaminated by a ‘subduction component’ containing large ion lithophile elements (K, Rb, Sr, Ba), light rare earth elements (Khan et al ., 1993) and increased Sr isotope ratios above mantle values (Mikoshiba et al ., 1999). Hafnium transfer from the slab into the mantle wedge (Hypotheses 1a and 1b, Fig.…”

Tectonic stacking of back-arc formations in the Thelichi section (Indus valley) of the Kohistan arc, northern Pakistan

“…85 Ma ages. A Rb-Sr whole-rock isochron age (111 ± 24 Ma) of gabbro-diorite (Mikoshiba et al, 1999), a Sm-Nd mineral isochron age (107 ± 17 Ma) of pegmatite intrusive to gabbro (Ali et al, 2002), and a Sm-Nd whole-rock-mineral isochron age (102 ± 15 Ma) of gabbro (Jagoutz et al, 2004) suggest that the emplacement process of enormous Chilas complex began in much older than 90 Ma. Late Cretaceous to Miocene felsic to mafic intrusions known as the Kohistan batholith occur in the middle part of the Kohistan arc sequence (e.g.…”