中国科学院南京地质古生物研究所机构知识库

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 Ar-40/Ar-39 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 Ar-40 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 Ar-40 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 Ar-40/Ar-39 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 Ar-40/Ar-39 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 Ar-40/Ar-36 value. These observations suggest that 1) contrary to common wisdom, excess Ar-40 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 Ar-40/Ar-39 ages could be considered as the timing of andalusite growth in the quartz veins. The younger muscovite Ar-40/Ar-39 ages likely record more recent cooling events. The youngest Ar-40/Ar-39 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 Ar-40/Ar-39 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 Ar-40/Ar-39 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. (C) 2022 Elsevier Ltd. All rights reserved.