Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

Abstract

Abstract. An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian gabbro intrusion. The underplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm 3, respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and •-800øC and ended at 0.85 GPa and 600øC and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust-to-mantle section was exhumed at the Adria margin of the Tethys ocean.

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

“…As a consequence, the important question arises as to whether the dated mafics and granitoids, which are part of the Punta Rossa Complex, represent a piece of Variscan basement exhibiting pre-Alpine deformation and metamorphism, or alternatively, postVariscan magmatic rocks of Permian age, deformed and metamorphosed during the Alpine cycle. In the light of our stratigraphic and structural data, as well as very detailed mapping (Dalla Torre 1998), we prefer the second option that would indicate bimodal magmatism typical for Permian times and found in many other parts of the Alps that expose lower crustal rocks, later exhumed near an ocean-continent transition (Rubatto et al 1999;Handy et al1999;Hermann et al 1997;Froitzheim and Rubatto 1998;Manatschal et al 2006).…”

Garnet-bearing tonalitic porphyry from East Kunlun, Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

“…The MASH zone therefore becomes a key factor for understanding magmatic evolution and crustal growth. Fossil arc sections and maficultramafic xenoliths have been investigated to understand the magmatic and geochemical processes in the MASH zone, but these samples generally represent restites or cumulates of magma (Hermann et al 1997;Greene et al 2006;Berger et al 2007;Garrido et al 2006;Bryant et al 2007), and samples of residual melt from the MASH zone are still rare.…”

Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite-Granite Linkage: Inferences from Granulitic Xenoliths

“…The question of whether granulitic tenanes are representative of the lower crust has been debated fr equently (Downes, I Some high-grade tenanes are probably more rep resentative of midclle-cI1l stal levels, as is the case for the granulite tenaJles in central Spain (Barbero, 1995) or the Agly massifin the Pyrenees (Pin, 1989). Other exposed granulite tenanes fr om deeper cI1lstal levels have been described in the Pyrenees (Saleix massif, Vielzeuf, 198,1), Calabria (Maccanone et al, 1983) and northem Italy (Pin & Sills, 1986;Hermann et al , 1997), and are undoubteclly representative of the lower continental cIl.1st. Data fr om these granulite fa cies tenanes indicate a larger con tribution of mantle-derived material, as in the Ivrea Zone (Pin, 1990;Voshage et al , 1990), the Calabrian massif (Maccarrone et al, 1983) ami the Pyrenees (Pin, 1989).…”