Giorgio Dallagiovanna

U–Pb zircon ages for post-Variscan volcanism in the Ligurian Alps(Northern Italy)

Abstract: Ages of Permian volcanic rocks from the Ligurian Briançonnais domain (Western Italian Alps) have been determined by laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon. Three major volcanic units yielded zircons that were dated: calc-alkaline rhyolites (285.6 ± 2.6 Ma), andesites (with inherited cores yielded ages c. 476 Ma and older) and voluminous rhyodacites–rhyolites (272.7 ± 2.2 Ma). Following an amagmatic, sediment-starved time gap of c. 14 Ma, alkaline volcanic activity is recorded, at the top of the sequence, by K-alkaline rhyolites dated at 258.5 ± 2.8 Ma. The Ligurian segment of the Southern Variscan belt records transtensional and then extensional tectonics associated with the volcanic activity. The switch from calc-alkaline to alkaline activity corresponds to the transition from a post-orogenic to an anorogenic setting in the Southern Variscides; it may represent progressive and increasing delamination of the continental lithosphere, accompanied by partial melting of the lithospheric mantle. Supplementary material: Technical and data acquisition parameters, U–Th–Pb isotope analyses and calculated ages of zircons from samples and standard, and trace element compositions of selected zircons are available at: http://www.geolsoc.org.uk/SUP18322.

Large sheath folds in the Briançonnais of the Ligurian Alps reconstructed by analysis of minor structures and stratigraphic mapping

This study presents a geometric and structural analysis of curvilinear sheath folds exposed in blueschist rocks of the Ligurian Alps. Field data are presented in a geological map of the structural synthesis with related geological sections (at the 1:10,000 scale) that illustrate the relationships and evolution of large-scale, sheath folds within metamorphic rocks. We based our analyses on the geometric parameters of more than 40 minor folds, as shape (hinge-Lm1 angle, main axial plane/S1 angle, interlimb angle and hinge curvature), asymmetry, fold hinge/stretching lineation obliquity and structural facing pattern. The summary of the whole data depicts a coherent 3D structure showing several orders of minor folds. Sense of asymmetry of minor folds and stratigraphic order has been used to reconstruct a reliable large-scale structure, and to define the sheath fold shape.