PRE-RODINIAN OPHIOLITES; THEIR TECTONIC AND ENVIRONMENTAL IMPLICATIONS

A global survey of deformed belts (suture belts) that predate the assembly of the supercontinent Rodinia has yielded evidence for approximately 30 designated and possible ophiolite complexes, exposed in diverse cratons throughout the world. Ophiolitic complexes older than 1000 Ma mostly lack mantle tectonites below the magmatic rocks, supporting a previously suggested hypothesis for abrupt oceanic crustal thinning at about 1000 Ma. Geochemically, pre-Rodinian ophiolites display a mix of supra-subduction (SSZ), mid ocean ridge (MORB), and ocean island (OIB) compositions, with an accompanying increase of komatiite in complexes older than 1.6 Ga. Meso-Paleoproterozoic complexes tend to cluster in time intervals of 1000-1400 Ga and 1700-2100 Ga. Mesozoic oceanic large igneous provinces, such as the Caribbean, may provide an analogue to pre 1000 Ma oceanic crust. A modified ophiolite definition to accommodate Pre-Rodinian complexes includes a "Penrose-type" definition together with a thick accumulation of interlayered sediments and volcanic deposits, as well as high-level intrusive complexes and plutons Increased magmatic activity to produce thicker oceanic crust may have offset the need for faster spreading in the Proterozoic. A few examples suggest spreading processes occurred in Archean time. Thinning of oceanic crust, together with the end of the anorthosite-anorgenic granite "event' at 1000 Ma suggest discontinuous magmatic evolutionary processes in earth history, similar to other planets. Possible mechanisms include exhaustion of more fertile mantle together with decreased heat production, decline in heat from the core perhaps by thickening of the D'' layer, or stabilization of the "hot abyssal layer" of the lower mantle. The stratigraphic and sea level consequences of this thinning may have affected Neoproterozoic environments and atmospheric compositions. Thus one of the great transition periods of Earth environmental/evolutionary history may have had an internal origin.