Intraplate volcanism in a convergent zone?: Origin and Tempo delineated by REE and 40Ar/39Ar

Veneto Volcanic Province

by Ralph MT

Different tectonic settings have its own geochemical signatures that tells a tell-tale sign on the processes that occurred in the geological past. High temperature geochemistry which involves geochemical affinity and geochronology provides a window to these processes. One such example is the Veneto Volcanic Province, which is a Cenozoic intraplate volcanic province located in the Southeastern Alps, Northwest of Italy. The Veneto volcanic province is closely associated with the formation of the Alpine Belts that was formed out from the convergence of the European plate and Adria microplate that initiated about a 100 million years ago. 

Geochemical signatures of the subduction event between the European plate and Adriatic microplate have been noted from the presence of calcalkaline to shoshonitic affinities of intrusive bodies and basaltic and andesitic dikes. Calc-alkaline volcanism is expected to be constrained in the tectonic setting of convergent margins. However, the Veneto Volcanic Province, located in the Adriatic microplate, has a clear and strong affinity with intraplate volcanism, characterized by alkaline rocks with LREE enrichments, negative K–Rb, Zr, Hf, and Ti anomalies which is clearly distinct from the calc-alkaline and sub-alkaline products with arc signatures of the Periadriatic Central Alps. Now, how do we explain this anomaly of intraplate volcanism in a subduction zone tectonic setting?

Schematic diagram for the magmatism of the VVP from Brombin et. al. (2019)

In this study by Brombin et. al. (2019), analytical methods such as whole rock and trace elements analysis were identified by X-Ray Fluorescence Spectrometry, while REEs were determined by Inductively Coupled Plasma-Mass Spectrometry. Data were plotted into the chondrite-normalized REE spidergram and primitive mantle-normalized extended spidergram along with previously published results and trace elements of the Periadriatic Central Alps. These methods established the various partial melting trace-element modelling in evaluating the mantle source(s) of the alkaline rocks from the Veneto Volcanic Province. Additionally, geochronological analysis of 40Ar/39Ar were implored to the determine ages of the magmatic products of the Veneto volcanic province.

Results showed that the Veneto volcanic province magmas were derived by the partial melting of carbonated phlogopite-bearing garnet lherzolite as source for these Ocean Island Basalts-like signatures. Improved seismic tomography conducted by other authors with numerical models and 3D experiments showed that the intraplate volcanism was associated with a slab rollback mechanism that generated a complex mantle flow that risen and incorporating carbonates in the subducted slab. Decompressional melting then followed which induced the eruption of the volcanic province. 40Ar/39Ar revealed that the magmatic activity of the province went on intermittently with the first one 45 to 39 million years ago, followed by another eruption at 32 Ma and then lastly at 23 Ma, all in the different locations in the Adria microplate. It was further discussed that the eruptions were associated with phases of crustal extension in the Eastern Alps. 

Reference article:

Brombin, Valentina & Bonadiman, Costanza & Jourdan, Fred & Roghi, Guido & Coltorti, Massimo & Webb, L. & Callegaro, Sara & Bellieni, Giuliano & Vecchi, Giampaolo & Sedea, Roberto & Marzoli, Andrea. (2019). Intraplate magmatism at a convergent plate boundary: The case of the Cenozoic northern Adria magmatism. Earth-Science Reviews. 192. 355-378. https://doi.org/10.1016/j.earscirev.2019.03.016