锡石<sup>40</sup>Ar/<sup>39</sup>Ar 法直接定年探讨

Abstract

The aim of this study is to investigate the possibility of dating fluid inclusions in cassiterite using the 40Ar/39Ar technique by crushing in vacuo . Paragenous muscovite and cassiterite in a piece of tungsten ore from the Piaotang tungsten deposit in southern Jiangxi Province were selected. Laser stepwise heating of the muscovite yields a flat age spectrum with a plateau age of 159.3±1.5 Ma. The 40Ar/39Ar dating result of cassiterite by crushing forms a declining age spectrum marked by the highest apparent age of 3954 Ma at the 1st stage, then shows an apparent decline step-by-step, and finally a plateau appears with a plateau age of 159.1±1.8 Ma in the last 12 stages (from 7 to 18, or about 83% of the total 39Ar). The data points of cassiterite clearly distribute into two groups corresponding to secondary and primary fluid inclusions, and their mixtures. The secondary ones contain excess 40Ar and their data points scatter on the inverse isochron diagram of 36Ar/40Ar vs. 39Ar/40Ar, while the primary ones do not contain excess 40Ar and their data points yield an excellent isochron line corresponding to an age of 158.6±2.9 Ma. The 40Ar/39Ar isochron age of the cassiterite by crushing is well concordant with the age of the paragenous muscovite by 40Ar/39Ar stepwise heating, indicating that mineralization occurred at 159 Ma. This study reveals that cassiterite is a suitable dating target for 40Ar/39Ar stepwise crushing. Moreover, the isochron age of the primary inclusions represent the mineralization age of the Piaotang tungsten deposit.