Abstract:

The Larsemann Hills are located in Prydz Bay, East Antarctica, and consist mainly of gneisses, which underwent low pressure granulite facies metamorphism, with minor undeformed granites. The syenogranite in the Larsemann Hills is derived from late partial melting. Although in Broknes Peninsula a dyke of this granite crosscuts gneisses, in northern Mirror Peninsula the syenogranite is transitional to less foliated gneisses. The biotite samples from the syenogranite (12520) and a granite pegmatite (13101) which has intruded the syenogranite are unaltered. The samples studied were packed with aluminum foils, and loaded into small aluminium cylinders. These cylinders were vertically sealed into a glass container with dimensions of 30～35 diameter and 35mm height. The glass container was covered with a sheet of Cadmium (thickness 0.5mm), then it was placed into an Al can that could be placed in the reactor. These samples were irradiated in the core of irradiation facility H 4 of 49-2 Reactor in Chinese Academy of Atomic Energy Sciences. The can is rotated twice per minute to decrease horizontal change of neutron flux. Immediate fast neutron flux is 5.8×10 12 n/cm 2·s. Irradiation duration is 59.7 hours and accumulative fast neutron flux 1.2×10 18 n/cm 2. K-Ar standard biotite ZBH-25 (Fangshan of Beijing;133.5Ma in age) was used as "flux monitor". When J value is corrected for the position of the sample in the reactor, the relative variance can be about ±1%. From irradiated optical pure CaF 2 and K 2SO 4 (G.R),we obtained interfering isotopic factors:C 2=( 36 Ar/ 37 Ar)Ca=2.40×10 -4 ;C 4=( 39 Ar/ 37 Ar)Ca=8.06×10 -4 ;C 3=( 40 Ar/ 39 Ar)K=4.78×10 -3 . Incremental heating experiments were performed on the mass spectrometer MM-1200, where Argon isotope was analysed in static. We measured sample biotite 12520 and biotite 13101 with 40 Ar/ 39 Ar technique. Results of the experiments yield 40 Ar/ 39 Ar age spectrogram ,which is approximately a line , and yield plateau age=485.84±0.58Ma, isochron age=486.34±4.31Ma, and total age=486.08±1.17Ma for sample biotite 13101;and yiels plateau age=494.39±1.26Ma,isochron age=495.38±4.63Ma,total age=491.05±10.36Ma for sample biotite 12520.The plateau ages may be considered as the late stage of Pan African thermal event,whose effects were regionally important.