Geodynamic cycles of subcontinental lithosphere in the Central Alps and the Arami enigma


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Geodynamic cycles of subcontinental lithosphere in the Central Alps and the Arami enigma
Journal of Geodynamics
Trommsdorff V., Hermann J., Muntener O., Pfiffner M., Risold A.C.
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Major masses of metamorphosed ultramafic rocks in the Central Alps,
Western Alps and Liguria are former subcontinental lithospheric mantle.
It occurs in two, tectonically distinct ophiolitic zones which
paleogeographically correspond to the Jurassic, South Penninic,
Piemontese ocean belt and to the more external, Cretaceous, North
Penninic, Valais ocean belt, respectively. In both belts, the
geodynamic cycle of pieces of subcontinental lithospheric mantle
started with its exhumation during continental rifting and resulted in
the exposure of denuded mantle in the Tethyan ocean basins. With the
onset of oceanic closure, these mantle rocks were subducted and, after
continental collision, integrated in the Alpine mountain chain and
again exhumed. Combined evidence from two key areas of the Central Alps
is presented. At Val Malenco, a stable continental crust-mantle
transition (1.0-0.9 GPa; 800-600 degrees C) welded by a 270 Ma gabbro
intrusion existed into the Triassic. During Jurassic rifting, Malenco
mantle with attached pieces of lower crust was rapidly exhumed and
exposed in the Piemontese ocean basin near its Adriatic margin. There,
an `Alpine type ophiolite suite' developed, containing serpentinized
ex-subcontinental mantle, rodingitized Permian gabbro, ophicarbonate
rocks and MOR basalts. During Cretaceous convergence, the rifted margin
of Val Malenco was only moderately subducted (similar to 450 degrees C,
0.6 Cpa) as it remained in a supra-subduction wedge. In the Western
Alps and Liguria, rifted margins of the Piemontese belt were subducted
to eclogite conditions (similar to 3 GPa, 650 degrees C), but never
beyond the stability of Antigorite. In the North-Penninic Adula-Cima
Lunga nappe of the Central Alps, however, a suite analogous to Val
Malenco was subducted to much higher metamorphic conditions (similar to
3 GPa, 830 degrees C) during Eocene collision. Successive regional
metamorphic prograde stages are proven by overgrowth textures in mafic
rocks up to kyanite eclogite and in ultramafic rocks up to garnet
peridotite. The inferred ultradeep garnet Iherzolite of Alpe Arami
forms part of the prograde Alpine suite of Adula-Cima Lunga. Ilmenite
rods in olivine, as at Arami, invoked as `proof for an ultradeep orinin
have been detected in over 10 localities. They have been shown to form
from planar, Ti-bearing, (OH)-defects at conditions below 3 GPa. No
direct evidence for an ultradeep origin for Alpe Arami has been
presented so far. (C) 2000 Elsevier Science Ltd. All rights reserved.
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