Abstract
We have traced the particle path of high-pressure metasedimentary rocks on Elba Island, Northern Apennines, with the help of a U-Pb-Hf detrital zircon study. One quarter of the analysed zircons are surprisingly young, 41-30 Ma, with a main age peak at ca. 32 Ma, indicating an unexpected early Oligocene maximum deposition age. These Oligocene ages with negative εHf indicate a volcanic source region in the central-southern Alps. Though young by geological means, these zircons record an extraordinary geodynamic history. They originated in a volcanic arc, during the convergence/collision of the the Adria microplate with Europe from ca. 65 to 30 Ma. Thereafter, the Oligocene zircons travelled ca. 400 km southward along the Adria margin and the accretionary prism to present-day Tuscany, where they were subducted to depths of at least 40 km. Shortly thereafter, they were brought to the surface again in the wake of hinge roll back of the Apennine subduction zone and the resulting rapid extensional exhumation. Such a zircon roller coaster requires a microplate that has back-to-back subduction zones with opposing polarities on two sides.
Highlights
The Adria microplate is a Gondwana-derived terrane that drifted northward to eventually collide with Europe
Elba Island forms the innermost part of the Northern Apennines, where the continental Tuscan domain (Adria Plate) and oceanic units (Ligurian domain of the Alpine Tethys) were tectonically juxtaposed and stacked during the Alpine-Apennine subduction-accretion-extension cycle[6,7] (Fig. 1)
In eastern Elba Island, the Tuscan Metamorphic Units record in part subduction to depths of up to 40 km, followed by Miocene exhumation in an eastward retreating subduction framework[2,4,5,8]
Summary
Three samples of the APM underwent a U-Pb + Lu-Hf detrital zircon characterisation. Two samples, EJ50 and EJ67, were collected to the west of the Capo dArco residence in the southern part of the study area, whilst the third sample was collected along the road to Ortano (Fig. 2, Supplementary File 1). SHRIMP U-Pb zircon analyses of samples EJ50, EJ67 and HSE11 provided another 89 concordant single grain age data (Supplementary Files 3b,c). A more robust estimate of the maximum deposition age is the age of the youngest detrital zircon age component (i.e. group of zircons), which, for both LA-ICP-MS and SHRIMP analyses combined, is 31.6 ± 0.5 Ma (2σ) This is the best estimate for the maximum deposition age of the APM, which is much younger than previously assumed[11,13], and makes it coeval with the more felsic foredeep deposits of the unmetamorphosed Macigno or the metamorphic Pseudomacigno units on mainland Tuscany[12]. Their erosional products must have been fed longitudinally from the Alps into the Apennine foredeep basin[32] and reached latitudes equivalent to present-day Elba Island
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