Composition and evolution of the pre-oceanic continental lithosphere: evidence from the External Liguride Units
Dipartimento di Scienze della Terra, Università di Parma, Italy
The External Liguride ophiolites from the Northern Apennine are remnants of a Jurassic ocean-continent transition similar to modern magma-poor passive margins. These ophiolites consist of fertile lherzolites, MOR-type basalts and rare gabbroic intrusives, together with lower and upper continental crust bodies exhumed during the Mesozoic rifting phases preceding the oceanization and locally displaying primary relationships with the ophiolites. Both mantle and crustal rocks provide relevant clues about the nature of the pre-oceanic continental lithosphere and preserve a record of the long-lived history of rifting which led to opening of the Jurassic Ligurian Tethys.
Peridotite bodies which did not interact with astenospheric melts during the Ligurian Tethys formation preserve evidence for an early equilibration stage at deep levels of the subcontinental lithosphere. They are spinel and spinel-plagioclase amphibole-bearing lherzolites characterised by garnet-bearing clinopyroxenite to websterite layers. Most garnet pyroxenites were derived from mantle recycling of plagioclase-bearing mafic cumulates, which underwent a partial melting event in the garnet stability field, possibly during their rifting-related exhumation. The continental crust rocks include gabbro-derived mafic granulites, felsic granulites and granitoid rocks of Variscan age, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites characterized by polyphase brittle deformations predating the ophiolitic basalt emplacement .
The mantle rocks record a lithospheric thinning history culminated into mantle exhumation at the ocean-floor and development of a serpentinization-related hydrothermal activity. The low-pressure (< 0.9 GPa) portion of the exhumation path, starting with formation of plagioclase-facies mylonitic shear zones, was related to the Upper Triassic-Lower Jurassic rifting that led to continental break-up. The tectono-metamorphic evolution of the lower crust started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protholits of mafic granulites from MORB-type tholeiitic magmas which suffered extensive crustal contamination. The lower crust exhumation started in Permo-Triassic times and its shallow emplacement was likely accomplished in Upper Trias-Middle Jurassic. This latter period was also characterized by extensive brittle faulting at upper crustal level and formation of extensional allocthons consisting of stretched slices of upper continental crust which were later intruded or covered by post-rift MOR-type basaltic rocks.