MELTARC

Modelling and geochemical implications of intra-arc melting: consequences for the composition of the continental crust

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 Obiettivo del progetto (Objective)

'Arcs are expected to be the fundamental brick of the Continental Crust (CC). The main problem with this hypothesis is that the bulk CC has an andesitic composition while most arcs are formed by successive inputs of basaltic magmas. If island arcs accretion is indeed the main mechanism of crustal growth, one must explain how arcs can reach an intermediate composition and if these processes are effective in the nature. Intra-arc differentiation takes place in their lower crustal section. Removal of magmatic cumulates is not a sufficient process to drive the bulk composition of island arcs towards intermediate ones. Because melting and dehydration lead to the production of dense iron-rich melting and/or dehydration residues, it could causes delamination of mafic-ultramafic rocks into the mantle due to gravitational instability in the lowermost arc crust. Melting at the basis of arcs is most probably a necessary conditions to evolve towards andesitic bulk composition. The mechanisms of arc differentiation and their consequences are still poorly understood; this is due to the scarcity of exhumed section of arcs that have preserved evidences for lower crustal melting and/or dehydration. This project will focus on selected samples from well-constrained Precambrian and Phanerozoic exhumed arc sections. The main objectives are to: (i) quantify the geochemical differentiation induced during partial melting by directly analysing and modelling the composition of melts and residues sampled in exhumed arc roots; (ii) model the P-T-X conditions of intra-arc differentiation and to establish the mechanisms responsible for such a process; (iii) evaluate the causes and consequences of melting on the bulk structure, composition and stability of arcs by numerical modelling. The results of this study will be used to evaluate if delamination and foundering of dense residues at the base of arcs is a viable mechanism to change their bulk composition towards the one of continental crust.'

Introduzione (Teaser)

EU-funded scientists focused on the boundaries of tectonic plates to shed light into the roots of mountain ranges.

Descrizione progetto (Article)

The uppermost layer of the mantle together with the crust form the rigid lithosphere of Earth, which is split up into plates moving relative to each other. The boundaries of tectonic plates are places where most of Earth's surface activity is concentrated. This is where the most active volcanoes are found and where, during volcanic eruptions, molten rock and gases that formerly lay deep within the mantle are forced skyward. But material that composes the plate moving under another is also recycled in the Earth's interior.

Volcanic activity not only forms new islands, lava plains and mountain ranges, but also provided scientists working on the MELTARC project a brief glimpse into the dynamic processes that shape our planet. They had set out to examine the building blocks of exhumed island arcs at the Kohistan complex, near the continental margin of Asia and the Amalaoulaou complex in the Western Africa.

At these two regions, the Asian and African continent are expected to be composed of rocks made of sediment laid on the ocean floor. Collisions between tectonic plates caused accretion of these deposits at the edges of the continents. Specifically, the MELTARC scientists described how compression, folding and faulting between the oceanic and continental plate created mountain ranges.

On the other hand, investigation of the chemistry composition of the exposed roots of island arcs and the continental crust allowed them to establish several element and isotope ratios. These served as proxies for the processes operating within subduction zones. For example, friction between continental plates results in hot plumes of magma moving from the Earth's mantle upward, causing seafloor sediments to melt and mix up with mantle-derived magma.

Scientists are still a long way from fully understanding volcano arcs or their role in shaping our planet and its atmosphere. But the MELTRAC project contributed to the body of knowledge that helps to define how the chemical exchange between the crust and the mantle influences the growth of continental crust.