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

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.; Major-element zonation in selected garnet porphyroblasts from eclogites indicates a pre-Alpine, medium pressure growth history that has been modified under eclogite- and amphibolite-facies conditions. Trace-element patterns in garnet (HREE-depletion in rims) and omphacite (LREE- and HREE depletion) can be explained by preferential partioning of LREE into clinozoisite/zoisite and HREE into garnet during recrystallization. In support of δ 18O_WR data from eclogites (5.5–9.5‰), trace-element patterns from whole-rock analyses point to a MORB-type source as a possible protolith of the eclogites. Furthermore, the variations seen in the δ 18O_WR values in eclogites are an indication of low temperature, hydrothermal alteration in the upper oceanic crust, experienced prior to the Alpine orogeny.; Gradients in δ18O of minerals in traverses across eclogite-metapelite contacts imply isotopic exchange between the two disparate rock types. The shape of these δ18O profiles points to diffusive interchange in an at least temporarily present grain-boundary fluid. Oxygen-isotope fractionation among Qtz-Grt, Qtz-Ky, and Qtz-Rt pairs yield apparent temperatures between 550 to 750°C. Although some mineral pairs, particularly from fine-grained samples, approach equilibrium fractionations, disequilibrium due to retention of pre-Alpine δ18O values of refractory minerals such as garnet is common. Average δ18 O values of quartz (∼10.6‰) from Trescolmen compare well with values of quartz from other pre-Mesozoic basement rocks in the lower Penninic nappes. This is taken as evidence that no exotic fluids, out of isotopic equilibrium, infiltrated into the Trescolmen area during Alpine metamorphism. Local δ18O variations in the Trescolmen area are the result of isotopic exchange during the Alpine orogeny. This may also explain the δ18O enrichment of all measured smaller (∼1m) eclogite boudins by interaction with a fluid derived from metapelites during eclogite-facies metamorphism.