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

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

Rongguo Hu, Huaning Qiu*, Jan Wijbrans, Fraukje Brouwer, Min Wang

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

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.

Translated title of the contribution40Ar/39Ar geochronological study and the genesis of extraneous 40Ar in Yuka HP/UHP phengite, North Qaidam, NW China
Original languageChinese
Pages (from-to)216-227
Number of pages12
JournalEarth Science Frontiers
Volume21
Issue number1
Publication statusPublished - Jan 2014

Keywords

  • Extraneous Ar
  • Inherited Ar
  • Phengite
  • UHP metamorphic rock
  • Yuka terrane

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