U-Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogen

Rutile U-Pb ages and trace elements were determined by the LA-(MC)-ICPMS technique for ultrahigh-pressure (UHP) metamorphic quartzite from the Sulu orogen. The results provide insights into the effect of metamorphic fluid on element transport in the continental subduction factory. Rutile U-Pb dating yields a concordant age of 205 +/- 2 Ma, whereas U-Pb dating of coexisting zircon gives two groups of concordant ages at 243 +/- 4 and 223 +/- 3 Ma, respectively. The Zr-in-rutile thermometry yields temperatures of 600-640 degrees C, consistent with the closure temperatures of 600-650 degrees C for Pb diffusion in the rutile. Although the rutile U-Pb age is significantly younger the younger group of zircon U-Pb ages, the consistency in the two kinds of temperatures indicates that the rutile U-Pb age has dated its growth, a kind of rapid cooling during exhumation. Core-rim profile analyses across a large rutile grain (similar to 15 mm in diameter) yield remarkable trace element zoning, with much lower contents of HFSE such as Nb, Ta, Sb and W but significantly higher U contents in the core than in the rim. The variations in HFSE are ascribed to growth zoning rather than diffusion resetting. Thus, these rutile domains may have grown from different compositions of metamorphic fluid. The significant enrichment of HFSE in the rutile rim suggests its overgrowth from the fluid that is locally enriched in Nb, Ta, Sb and W and thus interpreted as the product of phase separation from a supercritical fluid into aqueous solution and hydrous melt during the exhumation. The rutile rim also exhibits a large variation in Nb/Ta ratios from 19.4 to 29.6 that are variably higher than those of the primitive mantle. This is attributed to the geochemical composition of metamorphic fluid due to breakdown of hydrous minerals such as phengite and biotite during the exhumation. Significant Nb/Ta variations occur in different rutile domains, suggesting that the composition of metamorphic fluid changes during the exhumation. As a consequence, the fluid released from the exhumation dehydration of UHP eclogite-facies rocks tend to exhibit suprachondritic Nb/Ta ratios, whereas subchondritic Nb/Ta rutile would result if its growth is associated with prograde eclogite-facies metamorphism during subduction. (C) 2014 Elsevier Ltd. All rights reserved.