Tracing the evolution of calc-alkaline magmas: In-situ Sm-Nd isotope studies of accessory minerals in the Bergell Igneous Complex, Italy

Abstract

The common occurrence of Ca- and Nd-rich accessory minerals titanite, epidote, allanite and apatite in calc-alkaline plutonic suites makes them ideal targets for in-situ tracing of the temporal, chemical and isotopic evolution of tonalitic and granodioritic melts. The Sm-Nd isotope composition of accessory phases from the calc-alkaline Bergell Pluton and the peraluminous Novate leucogranite (central Alps) were investigated using laser ablation MC-ICPMS techniques. Grouping of individual LA-MC-ICPMS analyses produced average εNd values with typical uncertainties of < 0.3 (2σ) epsilon units. SHRIMP dating of magmatic allanite cores of epidote grains from a Bergell gabbro yielded a Th-Pb age of 32.4 ± 0.4 Ma, which provides a new timing constraint on the emplacement of juvenile members within the Bergell intrusive sequence. The Bergell bulk-rock mantle-crust isotopic mixing curve was reproduced, demonstrating that integration of U-Th-rich accessory mineral Nd isotope compositions with crystallisation age is capable of tracing the geochemical evolution of magmatic systems over time. Crucially, the isotopic composition of the mantle end-member was successfully constrained by measurement of magmatic REE-epidote, highlighting the compositional versatility of accessory phases. The removal or addition of feldspar controls the Eu signature of both the bulk-rock and single minerals and therefore presents a unique trace element indicator of magmatic differentiation and assimilation processes in accessory minerals. Therefore the in-situ determination of age, Sm-Nd isotopes and trace elements in accessory minerals permits efficient and accurate reconstruction of complex magmatic processes in calc-alkaline plutonic suites. Sub-grain isotopic heterogeneity in magmatic monazite from the Novate leucogranite was identifiable by LA-MC-ICPMS analysis and emphasizes the additional value of a micro-analytical approach to understanding geological processes.