Igneous Rock Associations 9. Ophiolites 1: From the Beginning to Modern Times

This paper on ophiolites is the first of four that will appear in Geoscience Canada; it reviews a fascinating history dating back to the very first, glorious days of field mapping in the western Alps in the 1800s. Originally used to describe serpentinites, the term ophiolite progressively evolved to designate a field association (rock clan) of peridotites, gabbros, and radiolarites. Early on, the deep ocean origin of this association was recognized. Recognition of the ophiolitic rock clan in North America was late, due to conflicts among field geologists, experimental petrologists, and marine geologists. The advent of plate tectonic theory helped put the ophiolitic rock clan in the right geodynamic context with respect to mountain belts. Also, the clan was expanded to include pillow lavas, sheeted dikes, abyssal sedimentary rocks, and other metamorphic units. Starting in 1957, it was clear that the basal peridotites were partly derived from the upper mantle and some others derived by crystal fractionation processes, as defined by Bowen. This basal peridotite and the rest of the rock clan constituted the so-called Penrose-type stratigraphy as defined in 1972. Since then, countless studies have shown that ophiolites are transported sheets, a concept fitting plate tectonic theory and the Wilson cycle. Still debated is the geodynamic significance of ophiolites. Are they produced in large ocean basins at mid-ocean ridges or in more confined settings like forearcs, arcs or back-arc basins close to continental margins? Decades of research have shown that Penrose-type ophiolitic sequences can be produced in a variety of geodynamic settings and with complex geochemical attributes. This new approach led to the possibility that ophiolites could be present throughout the geological column and particularly in Precambrian terrains. Originally thought to begin in Neoproterozoic time, ophiolite sequences are now thought to be found in the Archean but those older than 1 Ga have different stratigraphy and geo-chemical compositions. However, these differences are currently matters of debate. The new concept of Precambrian ophiolites might shed light on the early mechanisms controlling the mobile parts of the Earth. SOMMAIRE Dans ce premier article de quatre des « Igneous Rock Associations 9 »de Geo-science Canada sur les ophiolites on decouvre une histoire fascinante qui remonte aux temps glorieux des premiers leves geologiques dans les Alpes occidentales au XIXeme siecle. D’abord utilise pour decrire principalement les serpentinites, le terme ophiolite a progressivement evolue pour designer une association geologique de terrain comprenant des peridotites, des gabbros et des radiolarites dont on a percu tres tot leur origine marine profonde. Le terme ophiolite a mis beau-coup de temps avant d’etre introduit dans la litterature nord-americaine pour cause de mesentente ideologique entre les geologues de terrain, les experimentalistes et les geologues marins. La croissance rapide des donnees marines qui ont conduit a l’enonce de la theorie des plaques a permis de replacer geodynamiquement cet ensemble lithologique auquel on a associe depuis les laves en coussins, les complexes filoniens, les sediments abyssaux et autres lithologies metamorphiques derivees. Il devenait clair a partir de 1957 que les peridotites basales originaient du manteau. La partie crustale superieure des ophiolites equivalait a une sequence se formant selon les regles de la cristallisation fractionnee enoncees par Bowen et surmontee de laves sous-marines elles-memes coiffees de sediments abyssaux. C’est la stratigraphie de type Penrose definie en 1972. Depuis, les nombreux travaux de recherche sur les ophiolites ont montre que ces sequences etaient transportees appuyant du meme coup la theorie de la tectonique des plaques et le fameux cycle de Wilson. Les ophiolites sont toujours sujettes a controverses quant a leur signification geodynamique. Proviennent-elles de grands bassins oceaniques et formees en contexte de dorsales en expansion ou bien, de bassins plus restreints en marge des continents ou des dispositifs en expansion comme les avant-arcs, les arcs et les bassins arriere-arcs peuvent generer une stratigraphie de type Penrose et expliquer leur complexe composition geochimique? Cette vision plus large de la formation des ophiolites a permis que les ophiolites soient reconnues dans des ensembles lithologiques de plus en plus anciens du Proterozoique superieur puis progressivement jusqu’a l’Archeen. Les sequences plus anciennes que 1 Ga sont distinctes en ce qui a trait a leur stratigraphie interne et leur composition geochimique. La signification de ces differences fait l’objet d’un debat contemporain. Les series ophiolitiques precambriennes pour-raient nous en apprendre beaucoup sur les mecanismes et le fonctionnement des premieres parties mobiles terrestres.

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