Geochemistry of eclogites and metapelites from the Adula nappe, Central Alps, Switzerland

Exhumed “crustal” eclogites are important recorders of physical and chemical conditions in subduction zones and allow, under favorable circumstances, evaluation of processes such as dehydration reactions, migration of fluids, mobility of elements and metasomatism. Selected group- B/C eclogites and metapelites from Trescolmen, Adula nappe, Switzerland, have been chosen for a detailed study to explore the petrogenesis and the nature and extent of fluid-rock interaction in this polymetamorphic terrane. To accomplish this objective, besides careful petrographic characterization of samples, oxygenisotope fractionations in quartz, garnet, omphacite, kyanite, and rutile, and major— and trace-elements of the main rock-forming minerals are utilized to infer the type and magnitude of fluid-rock interaction at different scales at Trescolmen. Major-element zonation in selected garnet porphyroblasts indicates a pre-Alpine, medium pressure growth history that has been modified under eclogite— and amphibolitefacies conditions during the Alpine orogeny. Trace-element patterns in porphyroblastic garnets show a depletion of HREE’s in the rim region. Omphacites are relatively homogeneous in major elements and exhibit a LREE- and HREE-depletion. These REE-patterns can be explained by preferential partitioning of LREE into zoisite and HREE into garnet during recrystallization. In corroboration with 5130“,], data from eclogites (~5 .5 to ~9.5%), trace element patterns from whole rock analyses point to a MORE-type source as a possible protolith for the eclogites. Furthermore, the variation seen in the 5‘30WR values in eclogites are an indication of low temperature, hydrothermal alteration in the upper oceanic crust, experienced prior to the Alpine orogeny.