Lack of Fluid During Ultrahigh-P Metamorphism in the Dabie-Sulu Region, Eastern China

摘要

A general lack of fluid during ultruhigh-P (UHP) metamorphism of supracruslal rocks at mantle depths for Dabiu-Sulu III11 rocks is suggested based on the following petrological and genehemieal observations: (I) no coeval ealc-alkaline magmatic are was identified in the Dabie-Sulu terraiie nor in other recognized UUP regions of the world: (2) existence of intcrgranular coesite grains in some IIIII1 rocks from the Stilu region supports the absence of fluid in the rock matrix, and at least partial preservation of 111 IP minerals during rapid exhumation; (3) retention of premeiamoiphie gabhroie minerals (elinopyroxene, orthopyroxene, and biotile) and textures and persistence of metastable albite-bearing assemblages together with eoesite-bearing assemblages in a single block 30 m across resulted from sluggish reactions due to lack of aqueous fluid unending both prograde and retrograde P-T paths; (4) unusually negative 8I8O isotopic values (-11 to -9‰) for garnet, omphaeite. and kyanite of eclogite. and quartz, kyanite. and garnel from interlayered quarl/ilc suggested llial these stiprucrustal rocks have been isolated from iliO during both subducliou and exhuinalioii; and (5) no fluid inclusions have been positively identilled for the peak inelainorphic stage in UUP eclogilic rocks. At least two possible causes are responsible lor the luck of lluid during the UUP event: (I) the protoliths for the UUP rocks were old, cold, dry snpraaustul rocks that contained little or no lluid prior to subduetion to mantle depths, ami (2) minor volatiles were liberated at shallow subduetion depths, with the remaining fluids stored in both hydrous and carbonate phases (e.g.. epiclote. zoisite. phengile, talc, magnesile. dolomite, and Ti-uiinohumile), and at even greater depths, in dense hydrous magnesian silicates. In environments where continental collision involved the profound subduetion and UUP metamorphism of l-rieh sialic crust, both a massive calc-alkaline arc would be formed, and the deeply buried, juicy crust would backreact to lower- assemblages on exhumation. Thus, the only UUP tracts even partially preserved are those which were initially and continuously sensibly anhydrous.