ABSTRACT

Characterization and in vacuo crushing ⁴⁰Ar/³⁹Ar dating have been performed on four quartz veins within ultrahigh pressure (UHP) metamorphic rocks from the Yuka terrane, North Qaidam Orogen, western China. The data are used to decipher the nature, origin and age of the vein-forming fluids during the exhumation of UHP rocks. By using petrography and microthermometry analyses, three principal types of fluid inclusions have been identified: (1) hypersaline inclusions (type-a), (2) intermediate to high-salinity inclusions (type-b), and (3) low-salinity aqueous inclusions (type-c). The type-a and partly type-b inclusions occur as isolated, suggesting a primary origin. Most of the type-b inclusions occur as cluster in crystal micro-fracture, but never crosscut the crystal boundaries between individual quartz grains, indicating they are pseudo-secondary inclusions. The type-c inclusions commonly distributed in trails along the micro-fracture, suggesting a secondary origin. 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 (429–411 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. Consequently, the plateau ages are concluded as the best estimate for the ages of quartz veining during the UHP rocks exhumation. Meanwhile, we ascribe the extremely high initial apparent ages to the most easily crushed and excess ⁴⁰Ar (⁴⁰Ar_E) dominated secondary fluid inclusions, because they generally distributed along crystal fractures. On the plots of ⁴⁰Ar^*/³⁹Ar_K vs. ³⁸Ar_Cl/³⁹Ar_K, the early data points yield well-defined isochrons and with two group ages: 392 Ma and ∼344 Ma, representing two episodes of post-collisional fluid flow activities. Microthermometry analyses and ⁴⁰Ar/³⁹Ar in vacuo crushing dating suggest that the secondary inclusions may be originated from the excess ⁴⁰Ar rich post-hydrothermal fluid from the depth. In contrast, the primary inclusions have more complex origins, including meteoric waters transported by fault/shearing zones, retrograde decomposition of hydrous minerals and partial melt of HP/UHP rocks.