Petrogenesis of leucosome sheets in migmatitic UHP eclogites—Evolution from silicate-rich supercritical fluid to hydrous melt

Abstract

Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UHP metamorphic conditions. Here, we investigate decimeter- to meter-thick leucosome sheets in retrogressed and migmatitic UHP eclogite in the central Sulu belt. The leucosomes have variable mineral modes and deformation fabrics; some are composite with marginal and interior facies, whereas others are not. They range from granite to trondhjemite, with high abundances of SiO2, K2O + Na2O and Al2O3, similar to hydrous melts, and have major oxide and trace element variations consistent with modal changes in phengite and albite + epidote sensu lato. The leucosomes are enriched in large ion lithophile elements relative to high field strength elements and have rare earth element (REE) patterns enriched in light REE relative to heavy REE, generally with small positive or negative Eu anomalies. Newly-crystallized zircon in the leucosomes records weighted mean ages of c. 223–218 Ma. Early-formed leucosome has Sr-Nd isotope compositions similar to those of nearby unretrogressed UHP eclogites, whereas younger leucosome has Sr-Nd isotope compositions intermediate between the eclogites and surrounding gneisses. Ti-in-zircon thermometry combined with Si-in-phengite barometry indicates crystallization of the leucosomes between ~850 and ~770 °C, over a wide range of pressure (P) from 3.5 to 2.2 GPa, which correlates with age from older (higher P) to younger (lower P). At the metamorphic peak, which may have exceeded 5.5 GPa, the source rocks were likely fluid deficient or fluid absent. During exhumation from UHP conditions, we posit that exsolution of water stored in nominally anhydrous minerals formed a silicate-rich supercritical fluid in eclogite that evolved to a denser, more viscous and more polymerized hydrous melt. Phengite barometry indicates that the early-formed leucosomes crystallized at pressures close to the critical line for the basalt plus water system, possibly by diffusive loss of water to the host eclogite. Infiltration of solute-rich supercritical fluid from the surrounding gneisses blended with melt in the eclogites generating variable Sr-Nd isotope composition intermediate between these end-members, as recorded by leucosomes that crystallized at lower pressures. The youngest leucosomes crystallized at pressures near the critical line for the Ca-granite plus water system creating a low volume of aqueous fluid that precipitated quartz veins associated with the leucosome sheets, particularly at their margins. Subsequently, limited phengite-breakdown melting in the leucosomes is recorded by aggregates of plagioclase + biotite around phengite and thin films and cuspate veinlets and patches of K-feldspar along grain boundaries. Phase equilibrium modelling indicates that this late stage melting occurred at P–T conditions near the transition from HP eclogite to amphibolite facies, with final subsolidus equilibration at 1.0–0.9 GPa and T < 640 °C.