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Comparative petrology of the mantle-crust transition zone: examples from the Western Alps and the Atlantic Ridge
BOLOU, Lionel, BERTRAND, Jean
BOLOU, Lionel, BERTRAND, Jean. Comparative petrology of the mantle-crust transition zone:
examples from the Western Alps and the Atlantic Ridge. Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 1996, vol. 141, p. 74
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COMPARATIVE PETROLOGY OF THE MANTLE-CRUST TRANSITION ZONE:
EXAMPLES FROM THE WESTERN ALPS AND THE ATLANTIC RIDGE L. BOLOU and J. BERTRAND
Departement de mineralogie, Universite de Geneve, 13 rue des Mara1chers, CH-1211 Geneve 4, Suisse
The mantle-crust transition zone of the oceanic lithosphere is one of the most active part of the oceanic accretion system not only in term of magmatic accumulation, impregnation, and percolation, and subsequent chemical interactions, but also in terms of geodynam ic processes.
A way to get a better understanding of such phenomena is a comparative approach between present-day oceanic lithosphere material and their fossil equivalents from alpine ophiolitic successions.
Mantle tectonites and ultrabasic-basic plutonic rocks coming from the equatorial Atlantic Verna and Romanche fracture zones and from the liguro-piemontais Montgenevre ophiolitic complex (MOC) are involved in this study.
Both environments are interpreted as slow spreading ridges and the MOC has largely escaped the alpine metamorph ic effects. This allows a significant comparison to precise how the mantle-crust transition zone, in such a dynamic geotectonic context, acquires its petrological features.
From both types of localities, mantle tectonites, impregnated or not, members of a strongly differenciated cumulate gabbroic sequence, such as troctolites, cpx- gabbros, Fe-Ti gabbros, are implied.
The main primary mineral phases, often as relics only, are spinet, olivine, orthopyroxene and clinopyroxene, plagioclase, Fe-Ti oxides. The most significant secondary minerals consist of serpentinite minerals, a suite of amphiboles mainly developed after pyroxenes, and plagioclase.
Petrographical and geochemical aspects are presented. The chemical interactions between the ascending mantle extracted magmas and previously crystallized rocks, as well as the textural features of the concerned lithologies, are discussed.
Numerous analogies in terms of primary mineral assemblages, textural characteristics, and mode of development of secondary parageneses are demonstrated.
All the collected data supply evidences on the timing of the recrystallizations and deformations suffered by these rocks. The discrimination between the respective contributions of both oceanic and alpine orogenic metamorphisms is significantly improved.
