Nd-, Sr-, O-isotopic and chemical evidence for a two-stage contamination history of mantle magma in the Central-Alpine Bergell intrusion

Two different contamination processes have been identified as having been operative in the genesis of a plutonic suite: initial contamination of a mantle source, and subsequent crustal contamination of uprising partial melts from the mantle. These processes are indicated by a detailed analyses of Nd, Sr, and oxygen isotopes together with major-and trace-elements of the 32–30 Ma calc-alkaline Bergell intrusion. This intrusion is located at the suture of the Alpine continental collision zone and contains rock types capable of discriminating between mantle and intracrustal processes. A range from basaltic-andesitic dykes in the surrounding country rocks, cumulitic hornblendites, gabbros, tonalite, granodiorite and lamprophyres, to pegmatites and aplites, is exposed in this single intrusion. The results of REE modelling and isotopic compositions of the basic members suggest that the cumulates were fractionated from a picrobasaltic liquid originating by partial melting of enriched subcontinental mantle (_Nd=+4). Increases in ⁸⁷Sr/⁸⁶Sr (0.7055) and ¹⁸O(+6.7) in these samples relative to the mantle array and compositions of other Periadriatic intrusions are most likely the result of an initial contamination of the mantle source by dehydration or partial melting of altered subducted oceanic crust. Slight differentiation of such a picrobasaltic liquid produced the basaltic-andesitic dykes. Simultaneous fractional crystallization and contamination of the uprising magma by continental crust produced crustal isotopic signatures which increase with acidity to values of (_Nd=-7.6), ⁸⁷Sr/⁸⁶Sr=0.716 and ¹⁸O=+10. The crustal imprint and LREE enrichment in the dominating tonalite increase with decreasing crystallization depth which indicates that the tonalites were emplaced in several distinct batches with different degrees of contamination. Shoshonitic lamprophyres, which intruded into the partly solidified granodiorite, were generated in a deep, strongly contaminated mantle source. The posttectonic 26 Ma Novate leucogranite is not cogenetic with the main Bergell body, but rather formed from a predominantly crustal source. If the described features are indeed due to mantle source contamination processes, which are well known for volcanic arcs, it must be concluded that these may also play a significant role in the genesis of calcalkaline plutonic suites.