Pre-anatectic and anatectic processes in an eclogite from the Erzgebirge, southeastern Germany: Implications for arc magmatism.

摘要

An eclogite body from the Saidenbach Reservoir in the Erzgebirge region of Germany experienced anatexis (partial-melting) during the transition from eclogite facies conditions (35--40 kbar, ∼120--140 km, and 950--1000 ºC) to eclogite-granulite facies conditions (20--25 kbar, ∼70--90 km, and 800--850 ºC). Centimeter-scale granitic veins and segregations formed in many samples, but overall, melts remained stationary on the hand specimen scale. The melt was a strontium-rich granite, and though it is analogous to an adakite, this is the first reported occurrence of such a magma type observed in a metamorphic terrain. The conditions of melting make this occurrence an excellent analogue for adakite genesis in the lower crust of thick continental arcs, such as the Andes. Bulk earth strontium and neodymium isotope values, as well as elevated light rare earth and high field strength element concentrations, indicate that the protolith was an alkali basalt. The alkali-rich protoliths suggest that Saidenbach eclogites were dikes and/or sills intruded into the pre-Variscan crust. The homogeneous N-MORB eclogites throughout the rest of the Erzgebirge likely represent imbricated fragments of former oceanic crust. The most unusual feature of this eclogite is the compositional heterogeneity. Samples range from 48--56 wt % SiO2 and 0.2--2.7 wt % K2O. Variations are not due to melt movement during the observed exhumation-related anatexis. Strong correlations exist between major and trace elements, including large-ion lithophile elements (LILEs), and the magnitude of the europium anomaly in garnet cores. Garnet cores recorded geochemical conditions within the rock at the time of peak metamorphism, meaning that post-peak processes (e.g., anatexis, post-anatectic metasomatism, etc.) cannot be responsible for the variations. Given the isotopic homogeneity, we can conclude that heterogeneity was related to an internal differentiation process during the prograde metamorphic path.