Geochemistry of eclogites and metapelites from Trescolmen, Central Alps, as observed from major and trace elements and oxygen isotopes

Eclogites are often the only tangible high-pressure evidence we have from a paleo-subduction zone, and they potentially preserve important geochemical information from the descending slab. Selected Group B/C eclogites and met apelites from the Trescolmen locality in the Adula nappe in the central Swi ss Alps were chosen for a detailed investigation to determine oxygen isotop e ratios and major- and trace-element compositions of the main rock-forming minerals. Complex major-element zonation patterns in garnet porphyroblasts indicate a pre-Alpine, medium-pressure growth history coupled with a high- pressure modification during the Alpine orogeny. Garnet REE patterns are no tably HREE depleted in rim regions. with high overall REE content, particul arly in the cores of grains. Omphacites are relatively homogenous in major elements, and show LREE- acid HREE-depleted patterns, but with overall abun dances of REEs lower than in garnets. These patterns are best explained by partitioning of the HREEs into, garnet and the LREEs into zoisite. Oxygen-i sotope systematics indicate limited fluid flow in eclotgites and surroundin g metapelites. delta O-18 values of quartz and garnet at the interface betw een eclogites and metapelites are indistinguishable and point to fluid exch ange. Oxygen equilibrium conditions among rock-forming minerals. particular ly between quartz and garnet in eclogites and metapelites. were reached, an d fractionation indicates temperatures of similar to 600 degreesC. The delt a O-18 of unaltered eclogites (5.5 to 7.5 parts per thousand) suggests a ba saltic. MORB-type protolith.