PETROGRAPHIC CHARACTERISTICS AND ZIRCON U-PB GEOCHRONOLOGY OF GRANITOID ROCKS IN THE SOUTHERN BÊN GIANG, QUNG NAM PROVINCE

The early Palaeozoic calc-alkaline granitoid association in south Nam Giang town is forming large area reaching hundreds of km2, along the east – west ductile deformation zones, which is studied in detail in diffirent geologic maps scales by the geologists (Huynh Trung and Nguyen Xuan Bao, 1981 at 1:500,000 scale or Nguyen Van Trang et al, 1986, at 1:200,000 scale). The six samples were studied in details, which are composed mainly quartz diorite and granodiorite. The samples were crushed and large zircons were extracted. The laser ablation microprobe-inductively coupled plasma mass spectrometer (LA-ICP MS) U-Pb zircon dating was carried out by standard methods at CODES, University of Tasmania-Australia. In-situ zircon U–Pb geochronology was conducted on six samples of age between 306 Ma and 278 Ma. A total of seventytwo zircons were analysed and the results exist a narrow spread in individual zircon ages between ca. 269 and 313 Ma, with significant form a coherent single age suite of 295 Ma (early Permian). As the six samples are all assigned to the same granitic suite, I have combined the individual isotopic data yields a weighted mean age from 72 analyses of 294 Ma, which represents the minimum age for the crystallisation of the south Ben Giang granite.