Primary Magmas in Continental Arcs and their Differentiated Products: Petrology of a Post-plutonic Dyke Suite in the Tertiary Adamello Batholith (Alps)

Determining the primary compositions of arc magmas is fundamental in retracing the chemical differentiation processes responsible for the formation of juvenile arc crust and the thermal structure of the mantle wedge. We have investigated a series of post-plutonic dykes that intruded the gabbroic to tonalitic southern part of the Tertiary Adamello Batholith in the Alps. The dyke rocks range in composition from primary, hydrous high-Mg basalts to basalts, basaltic andesites, andesites and dacites. Field relationships and high-precision U-Pb dating of titanite and zircon show that the dyke suite ranges in age from 41.67 +/- 0.06 Ma for the high-Mg basalt to 38.62 +/- 0.12 Ma for the youngest dacitic dykes, closely associated with plutonic activity from 42.5 to 39.0 Ma. Andesites and dacites have primitive Sr-87/(86)Sri (0.7032-0.7038) and Nd-143/Nd-144(i) (epsilon Nd-CHUR +3.5-3.2) isotopic signatures strongly limiting the extent of crustal assimilation, whereas some of the high-Mg basalts have selectively assimilated pelitic metasedimentary rocks as shown by high Cs/Rb, Rb/Sr and Rb/Zr ratios, and isotopically more enriched compositions (Sr-87/(86)Sri 0.7039-0.7046; epsilon Nd-CHUR +1.6-0.0). Primitive high-Mg basaltic dykes that escaped assimilation processes are primary mantle partial melts that were extracted from their source at pressures of 2.7 +/- 0.2 GPa and temperatures of 1390 +/- 30 degrees C, conditions corresponding to the spinel-garnet transition in mantle peridotite. Major element modelling constrains the degree of melting to 20 +/- 2% leaving a harzburgite residue, consistent with the trace element chemistry of the high-Mg basalts, which have moderate [Gd/Yb](N) ratios of 1-1.2. Differentiated basaltic andesites and dacites follow experimentally constrained liquid lines of descent for fractional crystallization at mid-to deep crustal levels. The trace element chemistry of amphiboles from basaltic andesite and andesite dykes reveals the coexistence of amphibole with primitive melts, indicating elevated pressures and H2O contents in their parental magmas. Thermobarometric constraints for amphibole phenocrysts result in pressures from 0.65 to 0.78 GPa and temperatures ranging from 930 to >1000 degrees C. The absence of any significant Eu-anomaly in the rare earth element patterns in these amphiboles indicates the late appearance of plagioclase in the crystallization sequence. The crystallization of amphibole drives the differentiated magmas to slightly peraluminous, corundum-normative compositions that are common for tonalites building the major part of the Adamello Batholith. Fractionation models at mid-to lower crustal conditions result in the cumulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to obtain an andesitic composition from a primary, hydrous high-Mg continental arc basalt. Cumulates formed during fractional crystallization at mid- to deep crustal levels are dunites and wehrlites followed by hornblendites and hornblendegabbros. The trace element signatures of basaltic andesites and dacites display low Rb/Zr and Rb/Sr, and are consistent with fractionation-dominated processes within the crust in an active continental margin. Significant crustal assimilation is not required to obtain the trace element signatures of the evolved andesitic magmas.