Indosinian Hydrothermal Activities in Xitieshan Area, North Qaidam: Insight from ⁴⁰Ar/³⁹Ar Dating by In Vacuo Crushing of Eclogitic Garnet and Quartz Vein

One garnet sample from an extensively retrogressed eclogite and a quartz vein located at the boundary between the host gneiss and the eclogite lens from the Xitieshan terrane, North Qaidam, western China, have been analyzed by joint ⁴⁰Ar/³⁹Ar in vacuo crushing and stepwise heating techniques. The results provide significant information to directly constrain the origin and timing of hydrothermal fluid flow activity, as well as the age of the vein-forming. The stepwise heating result could further be used to decipher the hydrothermal alteration history of the ultrahigh pressure (UHP) metamorphic rocks. Garnet sample analyzed by in vacuo crushing forms a falling-down age spectrum and yields a relatively flat age plateaus for the last 7 steps (19 to 25) with a plateau age of 226.3±3.9 Ma (2σ). These data points constitute the plateau age yielding an excellent isochron with an isochron age of 222.6±16.7 Ma and initial ⁴⁰Ar/³⁶Ar ratio of 297±6. The crushed powders of the garnet yields, with further stepwise heating, a flat age spectrum with plateau age of 212.0±9.0 Ma, which defined a good isochron with an age of 212.1±8.1 Ma and initial ⁴⁰Ar/³⁶Ar ratio of 296±1. These results illustrate that the Xitieshan eclogites and their retrogressed equivalents were intensively overprinted by hydrothermal event in the Late Triassic. Quartz samples dated by ⁴⁰Ar/³⁹Ar in vacuo crushing method yield monotonic declining release patterns: anomalously old apparent ages are obtained at the first steps and relative flat age plateaus of 222.8±1.2 Ma over the final several steps. The gases liberated in the final steps are most derived from radiogenic and trapped argon in small primary fluid inclusions and pseudo-secondary fluid inclusions and also atmospheric argon from the crusher. The plateau age is interpreted as the best estimate for the age of quartz veining formation, witnessing a significant aqueous fluid flow episode during the Late Indosinian Period. Microthermometry analyses and ⁴⁰Ar/³⁹Ar in vacuo crushing dating suggest that the secondary inclusions may be originated from the extraneous ⁴⁰Ar rich post-hydrothermal fluid from the depth. In contrast, it hypothesizes that these fluids of primary fluid inclusions mainly derived from meteoric waters transported by regional ductile fault/shearing zones and permeable schistosity. The garnet and quartz vein crushing results also suggest that the structurally-controlled fluid-rock interaction and rock metasomatism, resulting in that eclogites are extensively retrogressed and UHP metamorphic indicator minerals (such as coesite and diamond) are rarely preserved in the Xitieshan terrane.