Abstract
Corundum-rich (up to 55 vol.%) felsic dykes formed with albite, ±K-feldspar, ±hercynite and ±biotite-siderophyllite cut the lower crustal rocks exposed in the Ivrea–Verbano Zone (NW Italy). Zircon is an abundant accessory mineral and its investigation through laser ablation-inductively coupled plasma (multi-collector)-mass spectrometer (LA-ICP-(MC)MS) has allowed results to directly constrain the timing of emplacement, as well as petrology and geochemistry of parental melts. Zircons are characterized by very large concentration in rare earth elements (REE), Th, U, Nb and Ta, and negative Eu anomaly. U–Pb analysis points to Norian emplacement ages (223 ± 7 Ma and 224 ± 6 Ma), whereas large positive εHf(t) values (+13 on average) indicate a derivation from depleted to mildly enriched mantle source. The mantle signature and the corundum oversaturation were preserved thanks to limited crustal contamination of the host, high-temperature refractory granulites and mafic intrusives. According to the geochemical data and to the evidence of the development of violent explosions into the conduits, it is proposed that dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism. This work provides robust evidence about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian time, linked to a regional uprising of the asthenosphere and change of tectonic regime.
Highlights
The petrogenesis of corundum (α-Al2 O3 ) has been deeply investigated in the last decades either because corundum crystallization can provide valuable insights into the development of complex petrological processes [1] or for its importance in prospecting and mining gemstone deposits, e.g., [2].Geosciences 2020, 10, 281; doi:10.3390/geosciences10080281 www.mdpi.com/journal/geosciencesCorundum may be formed under very different P-T conditions as a result of magmatic and metamorphic processes [3].Experimentally determined liquidus phase relationships in the CaO-MgO-Al2 O3 -SiO2 (CMAS)system indicate that at high pressure (2 GPa; [4]), the stability field of corundum is large, and it can be in equilibrium with quartz and sapphirine
System indicate that at high pressure (2 GPa; [4]), the stability field of corundum is large, and it can be in equilibrium with quartz and sapphirine
SB09 dyke is located at Alpe Campo (Figure 1a). These samples were studied in four thin sections (SB08, SB09A, SB09B and SB09C); zircon grains were studied directly in thin section and as separates
Summary
Corundum may be formed under very different P-T conditions as a result of magmatic and metamorphic processes [3]. System indicate that at high pressure (2 GPa; [4]), the stability field of corundum is large, and it can be in equilibrium with quartz and sapphirine. At lower pressure, typical of continental crust (1 GPa; [5]), corundum stability is restricted to high aluminous basalt compositions and at very high temperatures. The spectrum of the metamorphic processes and rocks involving the segregation of corundum is huge. It includes near-closed system, iso-chemical metamorphic reactions in silica-poor alumina-rich rocks and open-system, metasomatic introduction of fluids/melts in contact rocks leading fluids/melts and/or the host rock towards silica-poor alumina-rich composition. A special case of metamorphic segregation is represented by the anatexis of metapelites, during which silica enters the melt first, and corundum can crystallize in the aluminum-rich residue
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