RB-SR ISOTOPIC STUDY OF THE HWACHEON GRANITE IN NORTHERN GYEONGGI MASSIF, KOREA: A CASE OF SPURIOUS RB-SR WHOLE ROCK AGE

Abstract

The Yonghwasan pluton of the Hwacheon granite intruding the Precambrian metamorphic rocks of the northern Gyeonggi massif gives two apparent Rb-Sr whole rock isochrons depending on lithology: 177±11 Ma with an initial87Sr/86Sr of 0.71538±0.00031 for mica granite and 134±10 Ma with an initial87Sr/86Sr of 0.72183±0.00096 for garnet two-mica granite. However, the whole rock age for the garnet two-mica granite is much younger than mineral ages: Rb-Sr biotite (146.2±2.2 Ma) and muscovite (160.2±1.6 Ma), and reported CHIME monazite and K−Ar muscovite. Circumstantial evidences suggest that the young whole rock isochron may be the result from fortuitous alignment of data due to assimilation of country rocks, and thus meaningless, rather than resetting of whole rock Sr isotope ratios by later events such as post-emplacement heating and fluid infiltration. Considering the above, the best Rb-Sr whole rock age of the Yonghwasan pluton is estimated to be 172.0±6.8 Ma. The isotopic ages of this and previous studies for the garnet two-mica granite form a near linear cooling curve with a cooling rate of about 14.5°C/Ma.