US EPA

7464223 

Journal Article 

Mantle dynamics during Permo-Mesozoic extension of the Europe-Adria lithosphere: insights from the Ligurian ophiolites 

Rampone, E 

2004 

1 

Periodico di Mineralogia
ISSN: 0369-8963 

73 

SPEC. ISSUE 1 

215-230 

English 

WOS:000208523000011 

Petrologie and isotope investigations on the Ligurian ophiolites have provided evidence that they do not resemble oceanic lithosphere formed at a mid-ocean ridge setting, rather, they represent peculiar and atypical sectors of oceanic lithosphere composed by older lithospheric mantle peridotites (Proterozoic and Permian) intruded by younger MORB-type magmas (Jurassic and late-Jurassic). The Ligurian ophiolites thus reflect a lithologic association which develops in response to passive lithosphere extension and slow-spreading oceanization, and which characterizes embryonic stages of evolution of an oceanic basin. Mantle peridotites from the External Liguride (EL, Northern Apennines) and the Erro-Tobbio (ET, Ligurian Alps) ophiolitic units both record a tectono-metamorphic subsolidus evolution characterized by progressive recrystallization from spinel- to plagioclase- to amphibole-bearing assemblages and deformation from granular to tectonite- to mylonite-types. Sr, Nd and Os isotope investigations have indicated that the EL lherzolites were accreted to the subcontinental lithospheric mantle since Proterozoic times. Sm-Nd dating on the plagioclase-facies recrystallization stage have yielded 273-313 Ma in the ET peridotites and 165 Ma in the EL lherzolites. The ET and EL peridotites thus represent different pieces of subcontinental lithospheric mantle which experienced tectonic exhumation during distinct stages of extension of the Europe-Adria continental lithosphere, leading to the formation of the Jurassic Ligurian Tethys ocean. Results on the ET peridotites point that the decompressional evolution of lithospheric mantle was already active since Late-Carboniferous-Permian times. A striking feature of oceanic basins developed by passive lithosphere extension is therefore the tectonic sea-floor exposure of large sectors of subcontinental lithospheric mantle. This is consistent with the results of petrologie and structural investigations on mantle peridotites from modern oceanic analogues (embryonic oceans and passive continental margins). Another peculiar feature of the Ligurian ophiolites is the predominant lack of a mantle-crust cogenetic link, as it would be expected in mid-ocean ridge type oceanic lithosphere. Both the EL and ET peridotites represent subcontinental lithospheric mantle whose tectonic exhumation was even completely unrelated to mantle melting and melt production. Sm/Nd isotope studies on the depleted mantle peridotites from the Internal Liguride (Northern Apennine) ophiolitic units provided Permian (275 Ma) DM model age of depletion. Associated gabbroic rocks have been dated to 164 ± 14 Ma. Thus, even in the IL ophiolitic sequences, residual mantle and associated crustal rocks are not cogenetic and coheval. Sm/Nd isotope data on the depleted ophiolitic peridotites from Mt.Maggiore (Corsica) have furnished Jurassic (165 Ma) DM model age of depletion. Associated gabbroic rocks have been dated to 162 + 10 Ma. The Mt. Maggiore gabbro-peridotite association thus constitutes the first record of the attainment of a mature oceanic stage of the Ligurian Tethys ocean, where residual peridotites and associated magmatic rocks are in isotopie equilibrium. 

Mantle peridotites; ophiolites; Sm-Nd isotopes; mantle exhumation