Fluid inclusion 40Ar/39Ar geochronology of andalusite from syn-tectonic quartz veins: New perspectives on dating deformation and metamorphism in low-pressure metamorphic belts

Syn-tectonic quartz veins are widely present in low- to medium-grade metapelitic belts, however, dating these veins is challenging due to the lack of suitable target minerals for most geochronological methods. Andalusite is a common K-poor aluminosilicate mineral in these quartz veins. In this study, we demonstrate the feasibility of dating fluid inclusions in andalusite using the 40Ar/39Ar gentle stepwise crushing technique. Three andalusite samples in syn-tectonic quartz veins from the Chinese Altai, Central Asia yielded similar gas release patterns. The first crushing steps generated significant excess 40Ar which likely originated from secondary gas inclusions. These steps gave anomalously old and quickly declining apparent ages. The following crushing steps generated less significant excess 40Ar and yielded concordant and well-defined Early Permian ages of 282–277 Ma. These steps were likely dominated by the degassing of intra-crystal liquid-rich fluid inclusion planes. Two of the crushed powder samples from the stepwise crushing experiments were selected for additional 40Ar/39Ar stepwise heating using a furnace and yielded a much younger age of 216 Ma. Laser stepwise heating of muscovite flakes in two of the investigated andalusite samples and one chloridized biotite sample from a different andalusite-bearing quartz vein yielded 40Ar/39Ar ages of 243–241 Ma and 214 Ma, respectively. Compared with gases released from the crushing experiments, gases extracted from the heating experiments showed distinct Cl–K–Ca compositional characteristics and younger ages with near atmospheric initial 40Ar/36Ar value. These observations suggest that 1) contrary to common wisdom, excess 40Ar in K-poor nesosilicates are preferably preserved in fluid inclusions rather than mineral lattices, and 2) K (Ar) contamination from K-bearing solid phases is negligible during gentle stepwise crushing. As such, the 282–277 Ma fluid inclusion 40Ar/39Ar ages could be considered as the timing of andalusite growth in the quartz veins. The younger muscovite 40Ar/39Ar ages likely record more recent cooling events. The youngest 40Ar/39Ar ages of the crushed andalusite powder and the biotite sample could reflect the timing of sericitization and chloritization of the samples, which is evidenced by petrologic observations and x-ray K mapping results. Furthermore, andalusite 40Ar/39Ar ages agree with the monazite/zircon U–Pb ages (280–273 Ma) of syn-tectonic leucogranite and pegmatite dykes and metamorphic ages (299–262 Ma) for the low-pressure metamorphic rocks of the region. These results suggest that fluid inclusion 40Ar/39Ar geochronology of andalusite can be used to constrain the timing of deformation and associated metamorphism in low-pressure metapelitic terranes, which can also provide more complete pictures of the tectono-thermal evolution in combination with traditional 40Ar/39Ar stepwise heating geochronology.