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Abstract. An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian gabbro intrusion. The underplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm 3, respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and •-800øC and ended at 0.85 GPa and 600øC and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust-to-mantle section was exhumed at the Adria margin of the Tethys ocean.

Among the Periadriatic Tertiary calc‐alkaline intrusions in northern Italy, the Adamello batholith displays the most convincing evidence for a mafic parentage. This is particularly the case for the southern Adamello, where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons. Isotopic analyses are presented for the Val Fredda and Blumone ultramafic‐gabbroic to tonalitic‐leucoquartzdioritic suites, for the more felsic rock types of the spatially predominant Re di Castello group, as well as for a lamprophyre. This dike of picrobasaltic composition is thought to represent the parental magma. Field relations and a large body of petrographic, geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism. This is compatible with the isotopic results for the Blumone series and the mafic rocks of Val Fredda with εNd = +3.8 to +1.5 and εSr = −9.1 to −4.6. A common parental magma similar to the lamprophyre (εNd = +3.1, εSr = −4.4) also seems plausible. The isotopically homogeneous felsic lithologies of Val Fredda (εNd = +0.1 to −0.3, εSr = +2.9 to +0.7), structurally intimately associated with their mafic counterpart, indicate crustal contamination incurred prior to their differentiation. For the Re di Castello group, progressing crustal affinity of isotopic compositions (εNd of −1.9 to −4.9 and εSr of +16.5 to +40.9) with chemical differentiation implies large‐scale interaction with a granitoid crustal component. Attempts at quantifying this process reveal individual characteristics and evolution for each pluton. The data indicate extraction of this component from a source in the lower crust with time‐integrated low Rb/Sr, εSr approximating +50 and εN close to −6 corresponding to a 1000 Ma crustal residence age. If a two‐stage evolution is assumed, formation of a gabbroic reservoir from the above parental melt would require uptake of some 4–12 wt % of granitoid component by combined assimilation/fractional crystallization, whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40–70 wt % of granitoid partial melt. Assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions, when the mafic magma is ponded near the mantle‐crust boundary and where homogenization prior to resuming buoyant ascent is assured. Precise locations and more detailed petrography of the samples is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document 91B‐002; $2.50. Payment must accompany order.

Weathering of bedrock and pedogenic processes can result in elevated heavy metal concentrations in the soil. Small-scale variations in bedrock composition can therefore cause local variations in the metal content of the soil. Such a case was found in the remote alpine area of the Swiss National Park. Soil profiles were sampled at an altitude of about 2,400 m, representing soils developed above different bedrocks. The concentration of lead in the profiles was found to be strongly dependent on the metal content in the bedrock underlying the soil and was strongly enriched in the top 10 cm. The dolomitic bedrock in the study area contains elevated lead concentrations compared with other dolomites. Dissolution of dolomite and accumulation of weathering residues during soil formation resulted in high lead concentrations throughout the soil profile. The enrichment of lead in the topsoil, however, is largely attributed to atmospheric input. The isotopic signature of the lead clearly indicates that it is mainly of natural origin and that atmospheric deposition of anthropogenic lead contributed to about 20 to 40% to the lead concentration in the topsoil on the bedrock with elevated lead concentrations. In the soils on bedrock with normal lead concentrations, the anthropogenic contribution is estimated to be about 75%. Also, zinc was very strongly enriched in the topsoil. This enrichment was closely correlated with the organic matter distribution in the profiles, suggesting that recycling through plant uptake and litter deposition was a dominant process in the long-term retention of this metal in the soil.

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