Nd‐Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin

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.