Journal of Petrology Advance Access originally published online on January 28, 2005
Journal of Petrology 2005 46(4):799-827; doi:10.1093/petrology/egi001
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Chronology, Petrology and Isotope Geochemistry of the Erro–Tobbio Peridotites (Ligurian Alps, Italy): Records of Late Palaeozoic Lithospheric Extension
1 DIPARTIMENTO PER LO STUDIO DEL TERRITORIO E DELLE SUE RISORSE, UNIVERSITÀ DI GENOVA, CORSO EUROPA 26, I-16132 GENOVA, ITALY
2 MAX PLANCK INSTITUT FÜR CHEMIE, POSTFACH 3060, D-55020 MAINZ, GERMANY
Mantle peridotites from the Erro–Tobbio (ET) ophiolitic unit (Voltri Massif, Ligurian Alps) record a tectono-metamorphic decompressional evolution, indicated by re-equilibration from spinel- to plagioclase- to amphibole-facies conditions, and progressive deformation from granular to tectonite to mylonite fabrics. The peridotites are considered to represent subcontinental lithospheric mantle that was tectonically denuded during rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the Northern Apennine (External Ligurides) ophiolitic peridotites. We performed chemical and isotopic investigations on selected granular and tectonite spinel peridotites and plagioclase tectonites and mylonites, with the aim of defining the nature of the mantle protoliths, and to date the onset of exhumation of the ET peridotites. Spinel- and plagioclase-bearing tectonites and mylonites exhibit heterogeneous bulk-rock major and trace element composition, despite rather homogeneous mineral chemistry, thus indicating that the ET mantle protoliths record a composite history of partial melting and melt migration by reactive porous flow. The lack of correlation between the observed geochemical heterogeneity and the structural type (granular, tectonite, mylonite) indicates that the inferred reactive porous flow event preceded the exhumation-related lithospheric history of the Erro–Tobbio mantle. The tectono-metamorphic evolution caused systematic chemical changes in minerals: (1) Al decrease in orthopyroxene; (2) Al decrease, and Cr and Ti increase in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc, V and middle to heavy rare earth element increase and development of a negative Eu anomaly in clinopyroxene. The studied samples have Nd isotope compositions consistent with a mid-ocean ridge basalt mantle reservoir. Sm/Nd isotope data on plagioclase and clinopyroxene separates (and corresponding whole rocks) from two plagioclase peridotites, representative of the plagioclase-bearing mylonitic extensional shear zone, have yielded ages of 273 ± 16 Ma and 313 ± 16 Ma, for the plagioclase-facies recrystallization stage, significantly older than the expected Jurassic age. This indicates that the Erro–Tobbio peridotites represent subcontinental lithospheric mantle that was tectonically exhumed from spinel-facies depths to shallower lithospheric levels during Late Carboniferous–Permian times. Our results are consistent with the previously documented evidence for an extensional regime in the Europe–Adria lithosphere during Late Palaeozoic time, and they represent the first record that extensional mechanisms were also active at lithospheric mantle levels.
KEY WORDS: plagioclase-bearing peridotites; subcontinental lithospheric mantle; mantle exhumation; Sm/Nd dating
* Corresponding author. Telephone: 0039 10 3538315. Fax: 0039 10 352169. E-mail: betta{at}dipteris.unige.it
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