U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

Abstract

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons (c. 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

The Gerola Valley site (Orobic Basin, Northern Italy): A key for understanding late Early Permian tetrapod ichnofaunas

“…288 Ma (Cadel et al, 1987), ca. 280 Ma (Philippe et al, 1987), 280 ± 2.5 Ma and 270 ± 2 Ma (Berra et al, 2014).…”

“…While the ages of Cadel (1986) and Cadel et al (1987) are affected by hydrothermal alteration so they could show older values (Cadel et al, 1996), the youngest age indicated by Berra et al (2014) could be due to "a complex mixing of antecrystic and autocrystic zircon populations in the analysed samples" (Berra et al, 2014). Thus, since both the end-members' ages (i.e the early Artinskian ones -ca.…”

“…Radiometric ages obtained by U-Pb analyses of zircons from the underlying Vulcanite del Cabianca formation suggest a late Early to Middle Permian (Artinskian-Roadian) age: ca. 288-270 Ma (Cadel, 1986;Cadel et al, 1987;Philippe et al, 1987;Berra et al, 2014); while the tetrapod ichnoassociation of the Pizzo del Diavolo formation suggests a Kungurian age (Marchetti et al, 2013a;Marchetti, 2014;Voigt and Haubold, 2015).…”

An unusual Amphisauropus trackway and its implication for understanding seymouriamorph locomotion

“…The Pizzo del Diavolo formation is dated late Early Permian (Cisuralian) to early Middle Permian (Guadalupian) by radiometric ages on the underlying Vulcanite del Cabianca (Berra et al . ), while the tetrapod ichnoassociation, recently revised by Marchetti et al . (, ) and Marchetti (), is most probably Late Cisuralian (Kungurian) in age.…”

New occurrences of tetrapod ichnotaxa from the Permian Orobic Basin (Northern Italy) and critical discussion of the age of the ichnoassociation

“…), while radiometric dating (Berra et al . ) indicates a late Kungurian or Guadalupian age. Therefore, the assemblage covers a gap in the Permian footprint record.…”