Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam, Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

Retrograde eclogite and garnet amphibolite of the Luliangshan unit of the Shenglikou area, North Qaidam, were studied with emphasis on rutile and titanite. A special focus is on the formation of rutile and its corona of titanite (Ttn1) in retrograde eclogite and on coarse-grained titanite (Ttn2) from the garnet amphibolite. Using zirconium (Zr)-in-rutile and Zr-in-titanite thermometers, the temperatures estimated for the formation of an early generation of rutile are 823-884 degrees C at 2.5-2.8 GPa, while 812-894 degrees C at 1.3-1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite. Therefore, isothermal decompression must have occurred during exhumation, which also has triggered the partial melting of the retrograde eclogite. Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios, indicating that it is newly grown from a Ti, Ca, and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite. LA-ICP MS U-Pb dating yields a lower intercept age of 423 +/- 4 Ma for Ttn2, which is consistent with the granulite-facies metamorphic age of the retrograde eclogite. Moreover, a temperature of 781-823 degrees C at 1.0-1.2 GPa is obtained for Ttn2, which fits the P-T conditions of the HP granulite-facies metamorphic stage (P=1.07-1.24 GPa and T=774-814 degrees C), and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma. The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt. The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2, suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.