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High-pressure Eo-Alpine parageneses developed in magnesian metagabbros, Gruppo di Voltri, Western Liguria, Italy

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Abstract

Aluminous, magnesian metagabbroic layers and pods occur in the Beigua serpentinite of Western Liguria, where they are associated with more voluminous (Fe+Ti)-rich lithologies. Both types were subjected to localized early rodingitization. All rocks show the effects of a pervasive, essentially isochemical, high-pressure Early Alpine metamorphism for which a sequence of mineral assemblages is partially preserved: eclogite → glaucophanic eclogite → Na+Al actinolitic amphibolite → greenschist. The nature of the eo-Alpine high-pressure recrystallization, and the origin of the protoliths are the subjects of this paper.

Eighteen new bulk rock XRF analyses for the magnesian metagabbros, combined with previously published major element analytical data for the ferroan metagabbros, demonstrate that a spectrum of compositions exist in the Gruppo di Voltri. This chemical range is comparable to differentiated oceanic tholeiites and to plutonic ophiolite suites cropping out elsewhere in the Northern Apennines and Western Alps. Both (Al+Mg)- and (Fe+Ti)-rich protoliths are regarded as the products of crystal fractionation of an original basaltic melt which intruded the Beigua mantle during the opening of Mesozoic Tethys.

Textural observations show that local reactions along grain boundaries produced the garnet+sodic clinopyroxene assemblages from low-grade precursor phase compatibilities. Garnet coronas and zoned blue → blue-green → green amphiboles attest to the prevalence of domain equilibrium. Summarization of approximately 170 new mineral analyses documents this paragenesis. Assuming an attendant load pressure of about 10 Kb during formation of the eclogites, nominal temperatures of Fe-Mg partitioning between garnet and omphacite range from 483 to 544° C and average 508° C, in good agreement with petrologic constraints provided by the regional metamorphic assemblages in the enclosing antigoritic serpentinite and in adjacent parts of Western Liguria.

The early stage of rodingitization and serpentinization is thought to reflect metasomatism and hydration in the vicinity of a Tethyan spreading center or fracture zone near the surface of the sea floor. In contrast, the subsequent high-pressure production of eclogites probably was a consequence of the profound subduction of oceanic crust and its mantle underpinnings during eo-Alpine convergence. Only the culmination of this prograde event has been preserved in the Gruppo di Voltri, but buoyant return towards upper crustal levels following decoupling is recorded in the eclogitized magnesian metagabbros by a decompression paragenetic series, successive stages of which are represented by glaucophanic eclogite, Na+Al actinolitic amphibolite and prasinite, respectively.

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Authors and Affiliations

  1. Istituto di Petrografia, Universitä di Genova, 16132, Genova, Italy

    B. Messiga

  2. Istituto di Mineralogia, Petrografia e Geochimica, Universitá di Firenze, 50121, Firenze, Italy

    G. B. Piccardo

  3. Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, 90024, Los Angeles, California, USA

    W. G. Ernst

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  1. B. Messiga
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  2. G. B. Piccardo
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  3. W. G. Ernst
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Institute of Geophysics and Planetary Physics Publication No. 2361

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Messiga, B., Piccardo, G.B. & Ernst, W.G. High-pressure Eo-Alpine parageneses developed in magnesian metagabbros, Gruppo di Voltri, Western Liguria, Italy. Contr. Mineral. and Petrol. 83, 1–15 (1983). https://doi.org/10.1007/BF00373074

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