ELEMENT TRANSFER DURING RETROGRADE METAMORPHISM OF A UHP TERRANE: PRELIMINARY DATA FROM THE ULLA GNEISS, WESTERN GNEISS REGION, NORWAY

Mass transfer within subduction zones remains poorly understood, with limited knowledge of the P–T conditions over which element recycling occurs. Many field-based studies have focused on exhumed UHP terranes to better understand the reactions and products involved with recycling processes. However, the retrograde reactions that transpire during UHP exhumation from mantle depths have not been significantly studied despite the fact that peak temperatures generally occur along the retrograde path and devolatilization and melting reactions are common. In order to better understand the retrograde element-transfer processes, we collected samples on a transect that spans a transition from the fresh core to the retrogressed rind of an eclogite pod and into the adjacent host gneiss in the Ulla Gneiss of the Western Gneiss Region UHP terrane. Whole-rock geochemical composition and split-stream zircon geochronology data (U-Pb and trace-element) were collected. All samples reveal metamorphic Scandian U–Pb zircon rim dates (415–405 Ma), and the trace-element profiles yield a high-pressure signature, with flat Eu anomalies and HREE patterns, from the eclogite and amphibolite samples. Moreover, dates from the leucocratic samples appear to be metamorphic rather than documenting Scandian melt crystallization. For example, the texturally latest leucosome, exposed within shear zones, reveals a crystallization date of ca. 900 Ma, with no Scandian zircon growth. Thus, these rocks were significantly affected by Scandian UHP metamorphism, but did not appear to undergo melting during this event. Parts of the whole-rock geochemistry dataset record evidence for element transfer during exhumation of these rocks from mantle depths, and this most likely occurred post-zircon growth at ca. 415 Ma. Retrogressed eclogite, including amphibolite interpreted to represent completely retrogressed eclogite, exhibit depletion of Th, Pb, and the LREE. Amphibolite also shows enrichment in fluid-mobile elements (Sr, Ba) and Ni and depletion in all middle and heavy REE elements. All samples show enrichment in LILE and LREE compared to average N-MORB. These preliminary results indicate that incompatible elements may be removed from the system via fluids during retrogression of the eclogites and the host gneiss.