Megacrystic zircon with planar fractures in miaskite-type nepheline pegmatites formed at high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland)

Abstract

Trace element, Hf, and O isotopic composition and U-Pb geochronological data are reported for zircon megacrysts found in miaskitic (zircon, biotite, plagioclase-bearing) nepheline syenite pegmatites from the Finero complex in the Northeastern part of the Ivrea-Verbano Zone, southern Alps. Zircon from these pegmatites was reported to reach up to 9 cm in length and is characterized by ~100 μm spaced planar fractures in different directions. Small volumes of these highly evolved alkaline melts intruded into the lower crust and were emplaced within amphibole peridotites and gabbros between 212.5 and 190 Ma. A zircon crystal of 1.5 cm size records a systematic core-to-rim younging of 4.5 Ma found by high-precision CA-ID-TIMS 206Pb/238U dating of fragments, and of 8.7 Ma detected by laser ablation ICP-MS spot dating. Volume diffusion at high temperatures was found to be insufficient to explain the observed within-grain scatter in dates, despite the fact that the planar fractures would act as fast diffusion pathways and thus reduce effective diffusion radii to 50 μm. The U-Pb system of zircon is therefore interpreted to reflect an episodic protracted growth history. These high-pressure miaskites probably formed by episodic, low-degree decompression melting of a metasomatically enriched mantle source and subsequent crystallization in the lower crust at volatile saturation with explosive volatile release, evidenced by their brecciated texture in the field and by the occurrence of planar fractures in zircon. They point to the existence of a long-lived period of heat advection in the deep crust by highly differentiated melts from enriched, lithospheric mantl

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U–Pb dating of rutile from the Ivrea–Verbano Zone, Italy

“…7 of Ewing et al 2013). The sporadic influx of fluids at 220-200 Ma is consistent with the occurrence of episodic mantle-derived magmatism and associated metasomatism in the northern IVZ from ~220 to 190 Ma (Grieco et al 2001;Morishita et al 2008;Schaltegger et al 2015;Stähle et al 1990), as described in detail in Sect. 2.…”

“…Episodic Triassic to Jurassic mantle-derived magmatism in the northernmost IVZ is recorded by alkaline dikes and pegmatoids intruding the Finero peridotite, with zircon UPb ages ranging between ~190 and 225 ± 13 Ma for these localised intrusions (Schaltegger et al 2015;Stähle et al 1990). Associated Late Triassic metasomatic events are also recorded in this area.…”

“…However, over the past two decades, increasing evidence has mounted against this view, favouring an episodic thermal evolution with background conductive cooling punctuated (at least locally) by pulses of magmatism, fluid influx, and heating throughout the Triassic and Jurassic (e.g. Grieco et al 2001;Lu et al 1997;Mayer et al 2000;Mazzucchelli et al 2010;Morishita et al 2008;Schaltegger et al 2015;Stähle et al 1990;Zanetti et al 2013, and see Sect. 2).…”

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…In the northeastern and southwestern parts of the Ivrea Zone, some samples show an age generation < 220 Ma (Zanetti et al 2013;Schaltegger et al 2015;Langone et al 2017) we have found no evidence for that in Val Strona di Omegna (Fig. S9 Online Resource 3).…”

“…The main intrusive phase has been dated on zircon by Peressini et al (2007) at 288 ± 4 Ma, with locally earlier magmatic activity at ~ 295 Ma (Sinigoi et al 2011) and 314 ± 5 Ma (Klötzli et al 2014). In the northern part of the Ivrea Zone, the Finero mafic-ultramafic body is characterised by a range of Permian U-Pb zircon dates as well as younger zircon intrusion ages of 232 ± 3 Ma and late fluid-related recrystallisation between ~ 220 and 200 Ma (Zanetti et al 2013;Schaltegger et al 2015;Langone et al 2017).…”