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

Abstract

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.

The sources of granitic melt in Deep Hot Zones

“…Since the incubation period for the residual melt is shorter than for partial melt, the first melts to be generated are residual from the mafic magma and would carry a mantle signature, assuming that mantle and crust isotope signatures are discernable. The Adamello batholith, Italy, provides an example in which geochemical and isotopic data indicate that the oldest rocks were generated by basalt fractional crystallisation and younger granites exhibit an increasing crustal component (Kagami et al 1991). The duration of the period of pure residual melt is limited if the crust is fertile and if the mafic magma emplacement rate is high.…”

Tertiary-Quaternary subduction processes and related magmatism in the Alpine-Mediterranean region

“…They continue eastward in the Pannonian Basin along the Balaton line Benedek 2002) and south-eastwards in the Dinarides (Pamić et al 2002). These igneous rocks have a bimodal character (granodioritic-tonalitic intrusions and basaltic dykes ;Exner 1976;Cortecci et al 1979;Bellieni et al 1981;Dupuy et al 1982;Beccaluva et al 1983;Ulmer et al 1983;Kagami et al 1991;Müller et al 1992;von Blanckenburg & Davies 1995;Berger et al 1996). In addition, calc-alkaline andesites, shoshonites and ultrapotassic rocks (lamproites) also occur in subvolcanic facies (Deutsch 1984;Venturelli et al 1984b, Altherr et al 1995.…”

“…Laubscher 1983), and gradual slab detachment (von Blanckenburg & Davies 1995;von Blanckenburg et al 1998). The source regions of the primary magmas of the Periadriatic line are inferred to be in the lithospheric mantle (Venturelli et al 1984b;Kagami et al 1991;von Blanckenburg 1992). Mafic melts could have subsequently mixed with silicic magmas generated in the lower crust.…”

“…Suggested models for the origin of these igneous rocks include subduction (e.g. Tollmann 1987;Kagami et al 1991;Waibel 1993), extension (e.g. Laubscher 1983), and gradual slab detachment (von Blanckenburg & Davies 1995;von Blanckenburg et al 1998).…”

“…Most of the volcanic series define a continuous trend suggesting a common origin in terms of two-component mixing between a mantle component and an enriched component. Assimilation of crustal material by mantle-derived magma has been suggested for the Periadriatic magmas Juteau et al 1986;Kagami et al 1991;von Blanckenburg et al 1998) and for the East Carpathians calc-alkaline magmatism . However, such trends could imply also derivation of magmas from a strongly heterogeneous upper mantle without significant upper crustal assimilation, as has been proposed for the Central Italian magmas (e.g.…”