The Origin and Tectonic Significance of Ophiolites
Abstract
As defined by the 1972 Penrose Conference, ophiolites consist of a distinctive assemblage of mafic to ultramafic rocks, which, in complete bodies, includes mantle peridotites, gabbros, plagiogranites, sheeted dikes, mafic volcanic rocks and minor sedimentary rocks. Sheeted dyke complexes are taken as evidence of extension in a seafloor-spreading environment. Complete sequences are very rare and most ophiolites consist primarily of peridotite, gabbro and basalt, typically with tectonic contacts. Well-developed sheeted dyke complexes are rare suggesting either a lack of formation or tectonic removal during emplacement. We suggest that those ophiolites with poorly developed dyke complexes formed in an environment dominated by amagmatic extension. For many years ophiolites were thought to have formed at mid-ocean spreading ridges, a misconception fueled by the assumption of large-scale spreading and an abundance of MORB-like lavas. However, most ophiolite lavas have a suprasubduction zone signature, at least in part. The lavas range from MORB to arc tholeiite to boninite and many complexes contain two or more lava types. In some cases, the lavas are highly evolved and include andesites and rhyodacites, as well as basalts. We believe that the different lava types primarily reflect formation in different parts of a suprasubduction zone environment, however, pieces of old ocean lithosphere may also be incorporated in some ophiolites. We define ophiolites as fragments of suprasubduction zone lithosphere formed during subduction rollback that have been emplaced onto continental margins. Some ophiolites may also contain fragments of mid-ocean ridge lithosphere. Although ophiolites are distributed along continental margins, their emplacement does not necessarily reflect final closure of the ocean basin in which they formed.. Thus, several ophiolite belts may occur within a broad suture zone between continental blocks. Thus, ophiolites in the geologic record provide evidence primarily of subduction zone tectonics, not mid-ocean ridge spreading. Although they typically lie between continental blocks, their emplacement may predate final closure of the ocean basin in which they formed by many millions of years.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T52D..02R
- Keywords:
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- 3040 Plate tectonics (8150;
- 8155;
- 8157;
- 8158);
- 3640 Igneous petrology;
- 3670 Minor and trace element composition