Petrogenesis of post – orogenic Lower Permian andesites in southern Europe: insights into the collapse of the Variscan range

Abstract

In the post-Variscan Early Permian deposits of southwestern Europe, andesites belong to successions characterized by the constant occurrence of a basal rhyolite ignimbrite, followed by andesites, and by abundant dacite-rhyodacite products, interbedded with lacustrine sediments. The subalkaline andesite to K-andesite volcanism developed within intramontane basins following the collapse of the Variscan orogen. The compositional features, including trace element ratios and initial Sr (and Nd) of post-Variscan intermediate products allow excluding continental or island arc settings. The andesite composition of Lower Permian lavas was reproduced starting from two potential primary magmas: picrobasaltic and enriched MORB compositions. In particular, the picrobasalt was modified, by adding incremental amounts of a felsic granulite, from the lower continental crust. Mixing and fractional crystallization (MFC) were modelled by the MELTS software in a closed system under isobaric, isenthalpic conditions (P = 0.3 GPa). The computed residual liquid evolves to match closely the composition of Lower Permian andesite lavas after ~34 % crystallization by adding ~27 % of contaminant to the parental magma.

U–Pb zircon geochronological and petrographic constraints on late to post‐collisional Variscan magmatism and metamorphism in the Ligurian Alps, Italy

“…Subsequent collapse of the orogenic belt and the associated adiabatic decompression, recorded by the eclogite–granulite transition (Cortesogno et al ., ), could have caused partial melting in the lithospheric mantle and at the lower crust–mantle interface. In the Ligurian Alps, the initial (290 Ma) 144 Nd/ 143 Nd and 87 Sr/ 86 Sr values for the andesite lavas fit with a composition explained by the following assimilation–fractional crystallization model: a subcontinental, lithospheric‐mantle‐type parental magma generated at a pressure of 3 GPa and a temperature of ≈1400 °C (i.e., a picrobasaltic melt) is combined with an additional ~80 wt.% of a crustal metasedimentary component (Buzzi and Gaggero, , ).…”

Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

“…Indeed, the persistence of high geotherms in the Variscan belt (e.g., Fig. 2d, e) due to widespread partial melting and accumulation of magmas in the middle crust after lithospheric delamination (Gutiérrez- Alonso et al, 2004;Buzzi & Gaggero, 2008;Laurent et al, 2017), as exemplified in most of the massifs (Fig. 11b), favored global strain partitioning and triggered extreme mobility of the continental crust.…”

Structure of lamprophyres: a discriminant marker for Variscan and Alpine tectonics in the Argentera-Mercantour Massif, Maritime Alps

“…Both the calc-alkaline and the subsequent alkaline cycles, comprising transitional to tholeiitic mafic rocks, have been intepreted either as a result of Variscan collapse, or crustal delamination, or post-orogenic extension (e.g. Lardeaux and Spalla, 1991;Bussy et al, 2000;Cannic et al, 2002;Cocherie et al, 2005;Bussien et al, 2008;Buzzi and Gaggero, 2008;Dallagiovanna et al, 2009;Spalla et al, 2014;Rossi et al, 2015). Numerical models performed to explore the transition from Variscan convergence to Permian-Triassic extension indicate that forced extension (at least 2 cm/yr) is necessary to produce successive pulses of mantle-derived magma in Permian-Triassic times: such a magmatic activity could not result from the gravitational evolution characterising the end of the Variscan subduction-collision (e.g.…”

“…In the Pelvoux Massif, minor swarms of alkaline mafic dykes intersect Carboniferous granites and calc-alkaline diorite dykes, whereas alkaline volcaniclastics are interlayered in the Triassic sedimentary sequences (Vatin-Pérignon et al, 1972;Vatin-Pérignon et al, 1974;Buffet and Aumaitre, 1979). In the Ligurian basement, calc-alkaline rhyolites, andesites, and rhyodacites of 286 to 272 Ma are post-dated by alkaline rhyolites dated at 258.5 ± 2.8 Ma (Buzzi and Gaggero, 2008; U-Pb zircon dating, Dallagiovanna et al, 2009). Permian and Triassic calc-alkaline to alkaline intrusives and volcanics are also abundant in the Southern Alps (Giobbi Origoni et al, 1988;Sloman, 1989;Rottura et al, 1998;Garuti et al, 2001;Cassinis et al, 2007;Schaltegger and Brack, 2007;Locmelis et al, 2016;Casetta et al, 2018;Zanoni and Spalla, 2018;Storck et al, 2019).…”