Granulite-facies metamorphism and partial melting associated with UHP rocks, North Qaidam terrane, NW China

In the southeastern North Qaidam terrane, near Dulan, high-P granulites (grt + cpx + pl ± ky ± czo, with variably developed amphibolite-facies overprinting) occur near coesite-eclogite and UHP paragneiss. The granulites lack textural evidence that they represent overprinted UHP rocks, suggesting they may represent a separate tectonic unit. SHRIMP-RG zircon U-Pb geochronology and trace element analyses constrain the ages of metamorphism and partial melting of the granulites. A garnet-rich mafic granulite with abundant czo contains oscillatory- and sector-zoned zircons that yield a weighted mean age of 449 ± 3 Ma. These zircons contain a small negative Eu anomaly, consistent with the presence of plagioclase, but HREE patterns are steep, unexpected for this garnet-rich rock. Ti-in-zrn temperatures are mainly 660-760^oC. An intermediate granulite contains zircon with oscillatory- and sector-zoned cores, and CL-dark, oscillatory-zoned or unzoned rims. Cores and rims yield indistinguishable ages, with a weighted mean of 418 ± 2 Ma. HREE patterns are shallow, consistent with the presence of garnet, but an Eu-anomaly is absent despite the presence of plagioclase, possibly due to oxidized conditions during metamorphism. Cores contain low Th and Y, and yield Ti-in-zrn temperatures of 750-800^oC. Rims contain higher Th and Y, and yield Ti-in-zrn temperatures of 860-940^oC. A biotite tonalite that cuts the intermediate granulite contains oscillatory- and sector-zoned zircons that yield a weighted mean age of 419 ± 3 Ma, indistinguishable from the surrounding intermediate granulite. HREE patterns are steep, with a small Eu-anomaly, and Ti-in-zrn temperatures range from 640-800^oC, consistent with magmatic growth. These samples document a temperature increase from ca. 700^oC at 449 Ma to ca. 900^oC at 418 Ma. The biotite tonalite probably represents a partial melt formed during high-T metamorphism. This history contrasts sharply with that of the nearby UHP rocks: (U)HP metamorphism (650-750^oC, >27 kbar) was underway by 449 Ma, continued until 422 Ma, and was followed by decompression to amphibolite-facies conditions (<700^oC), without high-temperature overprinting. The presence of strongly deformed garnet amphibolite around the margins of the HP granulite body may indicate that the granulite and UHP rocks were juxtaposed under amphibolite-facies conditions, consistent with the P-T-t history of both rocks.