Ion microprobe U-Pb ages of the Khunjerab granodiorite and some granitoids from Karakoram, Pakistan

Abstract

The Karakoram block was located at the southern margin of Asian continent before the Late Cretaceous amalgamation of the Kohistan block to Asia. The northward subduction of the ocean under the Asian continent was considered to form Early Cretaceous continental arc magmatism in the Karakoram block. The Karakoram Batholith is a major granitic body in the Karakoram block, extending from Baltoro to Chitral regions. However, several smaller granitic bodies are present to north of the Karakoram Batholith, which comprise the Khunjerab-Tirich Mir granite belt. Ogasawara et al. (1992) investigated several granitoids of this belt in the Khunjerab valley. They reported hornblende and biotite K-Ar ages of the granitoids. As the Karakoram block received continued tectonic activity since the Cretaceous to Cenozoic, the K-Ar ages may not provide clear timing of the primary igneous events. Some U-Pb zircon ages are available for the Karakoram Batholith, however, the U-Pb ages for the granitic rocks in the Khunjerab-Tirich Mir granite belt are limited (Hildebrand et al. 2001). It is important to have precise U-Pb zircon ages of the granitic rocks to understand timing of igneous activities and related tectonism of the region. Thus objectives of this study are (1) to provide precise timing of the igneous activity and (2) to search for the inherited core of the zircon and its age to understand source rock characteristics of the granitoids. The Khunjerab granodiorite occurs around the Khunjerab Pass 60 km north of the Karakoram batholith, mainly along the northern side of the Khunjerab valley. Small outcrop of the granodiorite is found 10 km west of the Khunjerab Pass along the Karakoram Highway, and a sample was collected for the analysis from this outcrop. North Sost pluton is found in a 3.8 km section along the Karakoram Highway in the Khunjerab valley, 8 km north of Sost. Although outcrops of the granitoids are limited in the Khunjerab valley, the granitoids are exposed extensively to the north of the Misgar, west of the Khunjerab valley. This pluton is also evaluated in this study. However, the granitoids contain very small amount of zircon grains. A high resolution ion microprobe (Cameca ims 1270) at the Geological Survey of Japan was used to obtain U-Pb zircon ages. Primary ion beam was about 25 micron diameter with intensity of 2 nA. Mass resolution was set at about 5000. A standard used for the U/Pb calibration is AS-3 from Duluth complex. Zircons were separated with magnetic and heavy liquids methods. Abundant zircon grains were obtained from the Khunjerab granodiorite. Typical size of the zircons from the granodiorite ranges 200 to 400 micron meters in length. Those grains show weak oscillatory zoning, however, inherited core is not common. 206