Depth and time constraints on monzogranite emplacement and cooling in the Northern Tyrrhenian Sea: implications for the exhumation history of the Zuccale Fault (Island of Elba, Italy)

Abstract

Plutons in the Northern Apennines are generally thought to have been emplaced under low-angle normal faults (LANFs). However, the limited knowledge of their emplacement depth hinders a full understanding of their exhumation history and the tectonic setting in which they emplaced remains heavily debated. Indeed, granites in the Tuscan Archipelago are loosely constrained to P < 2 kbar, roughly corresponding to a maximum emplacement depth of 7-8 km. When considering that faults interpreted as LANFs in the area only show limited total displacements of a few km, the lack of constraints on the minimum depth of emplacement make it impossible to reconstruct the exhumation history of plutons in the area based on geochronology and low-temperature thermochronology only. Here we present the first precise constraints on the depth of magma emplacement on the Island of Elba, in the hinterland zone of the Northern Apennines. We investigated a spotted schist from the Terranera area of Eastern Elba, located in the footwall of the Zuccale Fault. The schist displays the garnet + cordierite metamorphic assemblage, which is very uncommon in the low-pressure Elban aureoles. We performed a detailed microstructural and petrographic study coupled with the analysis of mineral and bulk-rock chemistry through the Electron Microprobe and X-ray Fluorescence. We modelled the sample with the phase equilibrium modelling approach using the PerpleX software, constraining the co-stability of garnet and cordierite to the narrow P interval of 0.3 – 1.2 kbar, corresponding to 1.1 – 4.4 km depth. Considering the limited horizontal throw of the Zuccale Fault, of just ~6 km and its current subhorizontal attitude, it is unrealistic that this fault contributed significantly to the exhumation of igneous rocks in eastern Elba. Rather, the estimated pressure is consistent with the thickness of the Northern Apennines orogenic nappe pile exposed on the island. Nevertheless, we acknowledge the need to obtain precise P constraints also from the Zuccale Fault footwall and we discuss P constraints derived from metamorphic assemblages therein. We also provide the first U/Pb monazite ages of the young aureole which constrain peak metamorphic temperature to 6.7 ± 0.2 Ma. Considering the published radiometric constraints from the area, we suggest that Eastern Elba experienced a long-lived pluton-related thermal anomaly that persisted for ~0.8 – 1.2 Ma before cooling down to ‘normal’ upper crustal conditions, similar to the nearby, younger Larderello geothermal field. This work was carried out as part of the MIGRATE project, funded by the SNFS.