1 research outputs found
Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy)
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 EHf(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