⁴⁰Ar/³⁹Ar geochronological study and the genesis of extraneous ⁴⁰Ar in Yuka HP/UHP phengite, North Qaidam, NW China

Six phengites from a restrict area in the Yuka terrane of the north Qaidam high/ultrahigh pressure (HP/UHP) metamorphic belt were dated using the ⁴⁰Ar/³⁹Ar method by laser stepwise heating, in order to investigate their geochronological significances and decipher the genesis of extraneous ⁴⁰Ar in phengite. New ⁴⁰Ar/³⁹Ar age results show that the extraneous ⁴⁰Ar exists in UHP metamorphic rocks (eclogite and amphibolite) phengite but not in their country rocks (gneiss and schist) phengite. High-Si and Mg-rich phengites from eclogite and phengite-bearing amphibolite yield disturbed age spectra, anomalously old apparent ages and a large variation in total gas ages (708-534 Ma) and isochron ages (681-513 Ma), indicating the existence of heterogeneous extraneous ⁴⁰Ar. In contrast, relative low-Si and Fe-rich phengites from granitic gneiss and schist yield flat age spectra and geologically significant plateau ages (454.4 Ma and 418.0 Ma). The data points constituting the age plateaux yield isochrons with concordantly intercept ages of 456.2±2.3 and 418.1±2.2 Ma, corresponding to initial ⁴⁰Ar/³⁶Ar ratios of 177.3±151.2 and 287.1±16.9, respectively, implying that there is no extraneous ⁴⁰Ar inside the minerals. The plateau age of 454.4 Ma recorded by phengite from granitic gneiss is interpreted as the age of cooling through its retention temperature for argon (ca. 400°C), and responding to the time of the Yuka metamorphic rocks exhumed from upper-mantle depth to middle-lower crustal levels (ca. 10 km in depth) after deep subduction. In contrast, the plateau age of 418 Ma from the schist should correspond to a Later Silurian ductile shearing deformation event. Considering the protoliths characteristics of the eclogite and amphibolite, the extremely high p-T metamorphic condition and aqueous fluid shortage environment during eclogite formation, we would suggest that the extraneous ⁴⁰Ar^*(not produced by in situ radiogenic decay of ⁴⁰K) in phengites from Yuka is a locally derived component from the protolith rather than from a late infiltrating fluid.