PRECAMBRIAN OPHIOLITES IN COLLISIONAL AND ACCRETIONARY OROGENS: INCREASING EVIDENCE TO CONSTRAIN AN EVOLVING CONCEPT

When the term ophiolite was defined by Anonymous in 1972, ophiolites were considered to be an important component in collisional orogens. However, now it is realised that ophiolites also occur in accretionary orogens, and in their modern tectonic-equivalent environment, the accretionary wedges in circum-Pacific orogens. Today many examples are known of Precambrian ophiolites in both collisional and accretionary orogens, ranging from ca. 3.80 Ga to ca. 540 Ma, providing us with key information, not only of plate tectonic processes in the Precambrian, but also of early crustal growth processes. Study of Phanerozoic and Precambrian ophiolites is closely connected with a) the recognition of oceanic plateaux and thus of plume tectonics in the geological record, and b) the occurrence of ultrahigh-pressure metamorphic rocks, which provide information on subduction and exhumation of material to/from mantle depths. Thus, study of Precambrian ophiolites reveals a wealth of information that is essential for understanding crust-mantle growth processes, and plate tectonic versus plume tectonic processes with time.

To help understand Precambrian ophiolites, we will discuss several aspects of Cenozoic ophiolites, as well as the following factors: the presence or absence of associated high-pressure and ultrahigh-pressure rocks, geochemical parameters such as Dupal-type ophiolites, the trace element and REE geochemistry of Archean and Early Proterozoic ophiolites, the mode of occurrence of ophiolites in Phanerozoic and Archean orogens, the differences between modern and Archean ocean-ridge metamorphism, the recognition of plateaus in ophiolites with implications for plume and superplume origins, the style of accretionary processes during the Archean early crustal growth stage, changes in stratigraphy and crustal thickness with time, and the supra-subduction zone origin of ophiolites. Finally, we emphasize the importance of unravelling late-stage overprinting by consuming plate boundary processes, before it is possible to understand the origin and emplacement of ophiolites, past and present.