SHRIMP TRACE ELEMENT INVESTIGATION OF PROTOLITH, UHP, AND AMPHIBOLITE-FACIES ZIRCON GROWTH, SOUTH ALTYN TERRANE, WESTERN CHINA

Eclogite lenses in host ortho- and paragneisses near Jianggalesayi in the western part of the South Altyn terrane yield peak conditions of 28–30 kbar, 730–850°C, (consistent with the presence of coesite pseudomorphs), and retrograde conditions of 11–14 kbar, 670–800°C, and 6.3–9.5 kbar, 619–738°C.

SHRIMP-RG zircon U-Pb geochronology, Ti-thermometry, and trace element analyses constrain the age of the protoliths, HP/UHP metamorphism, and amphibolite-facies retrogression. Eclogite zircons contain CL-dark zircon cores, with high U, Th, and REE concentrations, and negative Eu-anomalies, consistent with magmatic zircon growth. Ages are discordant, and suggest minimum protolith ages of 800-900 Ma. Ti-in-zircon thermometry of one sample yields well-clustered, 750-780°C results.

CL-bright zircon rims and whole grains contain low U, Th/U, and REE concentrations, lack negative Eu-anomalies, and HREE are depleted relative to cores, suggesting plagioclase-absent, garnet-present growth in the eclogite-facies. Ages and Ti-temperatures (pressure-corrected to 30 kbar) of the five samples are: 496 ± 7 Ma, 700-800°C; 492 ± 7 Ma, 770-815°C; 486 ± 5 Ma, 760-820°C; 486 ± 4 Ma, 700-750°C; 482 ± 5 Ma, 775-825°C.

The felsic host gneiss contains CL-medium zircon grains, with moderate Th/U and U concentrations, negative Eu-anomalies, and depleted HREE, suggesting growth in the presence of both garnet and plagioclase during amphibolite-facies retrogression. The age is 450 ± 6 Ma, and Ti-temperatures are tightly clustered at 720-740°C. Analysis of one of the sparse, CL-bright cores yields a discordant, 1.4 Ga age which reflects the source for the sedimentary protolith of the gneiss.

These results indicate a 14 ± 9 m.y. duration of eclogite-facies zircon growth. Amphibolite-facies overprinting occurred 30-40 m.y. after eclogite-facies metamorphism. Further investigation is required to determine if this represents extremely slow exhumation, long residence at amphibolite-facies conditions, or a separate tectonic event. Ages determined in this study are similar to 480-495 Ma metamorphic ages and ~750-800 Ma protolith ages determined for eclogites in the North Qaidam terrane to the southeast, supporting correlation of these two terranes across the Altyn Tagh fault.