БОРЗОНГИЙН ГОВЬ-50 ТӨСЛИЙН ТАЛБАЙН ХАЛЗАН-УУЛ БҮРДЛИЙН U-PB (SHRIMP IIe) НАСНЫ СУДАЛГАА

Abstract

The Khalzan-Uul granite complex occurs in the western part of the Tsagaan Uul block, South Mongolia, intruding the Permian Lugiingol Formation, which is mainly composed of terrigenous sedimentary rocks. The granite complex occupies an area of 60 km2 with an oval outline and consists mainly of leucocratic granite, biotite granite, and alkali granite. The main constituent minerals of the granitic rocks are K-feldspar, quartz, plagioclase, and mica, and accessory phases are titanite, apatite, zircon and opaque minerals. Sericite, pelite, albite, chlorite, and epidote are secondary phases. A granite sample is selected for the SHRIMP IIe zircon U-Pb age dating. The zircon sample was photographed in SEM images, back-scattered (BSE) and cathodoluminescence (CL) images that are used to examine the internal texture of the analyzed zircons and with petrographic observations guided the selection of analytical spots, using a JEOL JSM-6610LV scanning electron microscope at KBSI (Korea Basic Science Institute). High-precision data on U and Th contents and Th/U ratios of zircon were obtained using a SHRIMP IIe housed at KBSI.

A total of 24 spot analyses were made on 20 zircon grains. 15 spots of them analysed yield a weighted mean 206Pb/238U age of 223.3±2.3 Ma (MSDW=3.3) which correspond to the late Trassic Т3 (Norian). The igneous origin zircons have Th/ U>0.30 ratios (Hoskin and Schaltegger, 2003), and the zircon SHRIMP U–Pb dating results are listed in Table 1, and related concordant diagrams are shown in Fig. 5. The sample images on the CL have an oscillatory zone that is narrow. It indicates that zircons were formed in the felsic rocks.