GEOCHRONOLOGY AND TECTONIC CORRELATIONS, NORTH QAIDAM UHP TERRANE, NORTHWEST CHINA

Amphibolite-facies felsic gneisses near Dulan, at the southest end of the North Qaidam Mountains, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism. Field relations, and coesite inclusions in zircons from paragneiss suggest felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U/Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 +/- 7 Ma and 921 +/- 7 Ma, identical to a 925 +/- 25 Ma age from a Bashiwake orthogneiss, and previously reported orthogneiss magmatic ages near Da Qaidam. A 2.5 Ga age from an inherited core suggests that the granites intruded the surrounding paragneiss. Zircon cores from paragneiss yield 2.0 - 2.55 Ga ages, and are probably of detrital origin, but zircon rims contain inclusions, Th/U, U, and REE indicating eclogite-facies growth at 426 +/- 4 Ma. Eclogite zircon cores (partial overlap on rims) yield high Th/U and U, and ages up to 475 Ma, which places a minimum age on eclogite protolith crystallization; albite and Th + REE-rich epidote inclusions, and garnet cores contain blocky polycrystalline clinozoisite probably pseudomorphic after lawsonite, indicating a cold, prograde subduction path. The rims contain inclusions, Th/U, U, and REE indicating eclogite-facies growth at 449 +/- 3 Ma, 440 +/- 4 Ma, and 422 +/- 4 Ma. Two amphibolite gneisses yield discordant analyses with 444 Ma & 1.9 Ga and 453 Ma & 2.4 Ga intercepts. The spread in metamorphic ages (449 - 422 Ma) may represent either protracted zircon growth at eclogite-facies conditions, or discrete events prior to tectonic juxtaposition. The association near Dulan of metamorphosed Middle Proterozoic granites with paragneisses containing Early to Middle Proterozoic detrital cores is very similar to HP/UHP occurrences 400 km NE, near Da Qaidam (ca. 486 Ma metamorphism), and on the NE side of the Altyn Tagh fault, in the south Altyn Mountains near Bashiwake (ca. 495 Ma metamorphism), supporting the proposed correlation of these localities in a HP/UHP belt. The Early Proterozoic to Late Archean ages recorded in paragneiss require an old, cratonal sediment source for the paragneiss protolith, and together with the 920 - 930 Ma granitoids may indicate an affinity with the North China or Tarim cratons.