Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

ophiolite complexes

ophiolites (Mohorovičić discontinuity) in Gros Morne National Park, NewfoundlandOphiolite complexes, or ophiolites (for 'snake stones') are rock sequences interpreted as being uplifted sections of oceanic crust and subjacent upper mantle that have become emplaced within rocks of the continental crustal. The age of ophiolite formation is often quite close to the age of their emplacement into the continental crust.

(left - click to route to larger image - Ophiolites (Mohorovičić discontinuity) in Gros Morne National Park, Newfoundland.)

Similar ophiolite sequences are also associated with spreading centers at mid-oceanic ridges in the oceanic crust proper.

Stratigraphic components of ophiolite complexes from uppermost layer to base:
● frequently, oceanic sedimentary rocks (pelagic, Flysch sediments) such as bedded chert, mudstone, limestone, and graywacke sandstone
pillow basalts – formed when hot magma is extruded onto the ocean floor. These basalts are subject to extensive low temperature/low pressure alteration (prehnite-pumpellyite facies metamorphism) of existing minerals and precipitation of new minerals when lavas and crustal and mantle fluids interact with seawater. This circulation reaches depths of 3 km and provides for chemical interactions that release metal sulphides, water, methane, and carbon dioxide into the ocean, and entrain water and metal oxides into the crust. The circulation causes metamorphism and serpentinization of the basalts and gabbros.
● sheeted mafic dikes and sills – feeders to the subaqueous pillow basalts that typically intruded consecutively into one another before cooling was complete
gabbro (frequently layered) – the upper layer of the gabbro is typically not stratified; the basal gabbro layer often comprises cumulate layers, which were the first formed crystals that sank to the base of the chamber.
__● for ophiolite sequences at spreading oceanic ridges, the base of the gabbro, where the cumulate gabbro passes into an ultramafic cumulate, marks the geophysical base of the crust (the Moho, where the density contrast between causes a marked attenuation in seismic velocity)
peridotite layer, or
__● for oceanic crust, where this layer marks the petrological contact between the Earth's crust and the mantle, the 'depleted' harzburgite layer at the top of the mantle is composed only of orthopyroxene and olivine, and lacks the typical clinopyroxene and spinel of the underlying fertile mantle rocks (the lherzolite).

Ophiolite complexes are remnants of ocean crust and underlying oceanic mantle, which have been embedded in continental crust. As such, they represent the relicts of earlier ocean basins with spreading zones, which closed up following a reversal of the plate’s movement from rifting to accreting, with mountain building in the collision zone. Depending on the relative geotectonic positions of basins to oceanic crusts prior to orogenies, geologist can differentiate between “mid-ocean ridge”, “supra-subduction”, and “back arc basin” types of ophiolites.

Most continental crust ophiolites are assigned to either the Tethyan or Cordilleran groups, which have different modes of emplacement yet are both SSZ in origin. The episodic emplacement of ophiolites throughout geological history suggests that the complexes formed and emplaced at times of super-continent break-up and dispersal when large ocean basins adjacent to super-continents subducted as rifting progressed.

Tethyan ophiolites are named for the ancient Tethys ocean, and are found in the eastern Mediterranean areas ( Troodos in Cyprus, Semail in Oman). These are relatively complete classic ophiolite assemblages that were emplaced intact onto a passive continental margin.

Cordilleran ophiolites are named for the Cordillera mountain belts of western North America. These ophiolites are not associated with a passive continental margin and sit on subduction zone accretionary complexes (subduction complexes). Cordilleran ophiolites include the Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America.

Ophiolite assemblages in collisional mountain belts, such as the Alps, represent incipient ocean crust at thinned continental margins (formed during rifting and continental drift) that has been emplaced into the collision zone.

The stratigraphic sequences observed in some ophiolites suggest origins in lithosphere-forming, spreading centers at mid-oceanic ridges. However, supra-subduction zone (SSZ) ophiolites are more closely related to island arcs than to ocean ridges, and are formed by rapid extension of fore-arc crust during subduction initiation followed by rebound of the continental crust carrying forearc lithosphere (ophiolite) atop it. The occurrence of ophiolite complexes within orogenic belts documents the former existence of ocean basins now consumed by subduction, so providing supporting evidence for plate tectonics.

subduction zone magmas

[images: maps: global distribution of ophiolite complexes, Newfoundland, location of Bay of Islands ophiolite, Geological map, Bay of Islands ophiolite; Josephine Ophiolate; Cyprus; photos: red jasper cherts and silicified turbidites overlying ophiolite at Nippers Harbour, Newfoundland, associated gallery, pillow lavas of ophiolite complex at Green Gardens Trail, pillow lavas Bottle Cove, wp ; Troodos Ophiolite, 92 Ma section of oceanic crust created in Tethys Ocean, wp, Troodos Ophiolite; Oman ophiolite and the Hawasina sedimentary units, Oman Ophiolite, Oman LandSat, hi-res, 3D; Semail ASTER, wp#16; Troodos Ophiolite, ASTER, wp#17; Unst ophiolite, Scotland, wp; lithography: Felsic intrusion of "Kennack Gneiss" into mafic oceanic crust at the base of the Lizard Ophiolite, injections of felsic magma into ultramafic mantle; chevron folds in radiolarian chert, folded bedding in radiolarian chert, ultramafic rocks, intensely sheared serpentinite, hanzburgite, dunite and pyroxenite, serpentinite and alteration to asbestos Del Puerto Canyon; model, Semail Ophiolite pillow lavas, Oman, wp, Ophiolite d'Oman, pillow, pillows, dike, layered gabbro, layered gabbro, gabbro lens in dunites, Moho, harzburgites, gallery; close-ups; Troodos; veinlets with fibrous serpentine at Amiandos, Cyprus ; Leka, Norway; plagiogranite intrusive in serpentinites, Le Chenaillet, Alps, wp, red radiolarites from the Gondran cirque, Chenaillet ophiolite; ophiolite complex, Sierra del Cenvento ophiolite mountain interpreted as an allocthonous serpentinite body thrusting over Cretaceous metavolcanite of Purial complex: ophiolite mélange, eastern Cuba; websites: L'Ophiolite de Chamrousse (English); Troodos Ophiolite; Cyprus Rocks - Ophiolite; Oman ophiolites, Oman Mountains; Mantle - Ocean crust rocks; Josephine ophiolite; radiolarian-bearing strata; diagrams: SSZ, ophiolite sequence]

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