DEFINITION OF OPHIOLITE ACCRETION

Ophiolites are a distinctive association of allochthonous

rocks interpreted to form in a variety of plate tectonic settings

such as oceanic spreading centers, back arc basins, forearcs,

arcs, and other extensional magmatic settings including

those in association with plumes. A complete ophiolite grades

downward from pelagic sediments into a mafic volcanic complex

that is generally made of mostly pillow basalts, underlain

by a sheeted dike complex. These are underlain by

gabbros exhibiting cumulus textures, then tectonized peridotite,

resting above a thrust fault that marks the contact

with underlying rock sequences. The term ophiolite refers to

this distinctive rock association and should not be used in a

purely generic way to refer to allochthonous oceanic lithosphere

rocks formed at mid-ocean ridges.

Very few complete Phanerozoic-like ophiolite sequences

have been recognized in Archean greenstone belts. However,

the original definition of ophiolites includes “dismembered,”

“partial,” and “metamorphosed” varieties, and many

Archean greenstone belts contain two or more parts of the

full ophiolite sequence. Archean oceanic crust was possibly

thicker than Proterozoic and Phanerozoic counterparts,

resulting in accretion predominantly of the upper section

(basaltic) of oceanic crust. The crustal thickness of Archean

oceanic crust may in fact have resembled modern oceanic

plateaux. If this were the case, complete Phanerozoic-like

MORB (Mid-Ocean Ridge Basalt)-type ophiolite sequences

would have been very unlikely to be accreted or obducted

during Archean orogenies. In contrast, only the upper, pillow

lava-dominated sections would likely be accreted.

Portions of several Archean greenstone belts have been

interpreted to contain dismembered or partial ophiolites.

Accretion of MORB-type ophiolites has been proposed as a

mechanism of continental growth in a number of Archean,

Proterozoic, and Phanerozoic orogens. Several suspected

Archean ophiolites have been particularly well-documented.

One of the most disputed is the circa 3.5 Ga Jamestown ophiolite

in the Barberton greenstone belt of the Kaapvaal craton

of southern Africa—a 1.8-mile (3-km) thick sequence including

a basal peridotite tectonite unit with chemical and textural

affinities to Alpine-type peridotites, overlain by an intrusiveextrusive

igneous sequence, and capped by a chert-shale

sequence. This partial ophiolite is pervasively hydrothermally

altered and shows chemical evidence for interaction with seawater

with high heat and fluid fluxes. SiO2 and MgO alteration

and black smoker-like mineralization is common, with

some hydrothermal vents traceable into banded iron formations,

and subaerial mudpool structures. These features led

Maarten de Wit and others in 1992 to suggest that this ophiolite

formed in a shallow sea and was locally subaerial, analogous

to the Reykjanges ridge of Iceland. In this sense, Archean

oceanic lithosphere may have looked very much like younger

oceanic plateaux lithosphere.

Several partial or dismembered ophiolites have been

described from the Slave Province of northern Canada. A

fault-bounded sequence on Point Lake grades downward

from shales and chemical sediments (umbers) into several

kilometers of pillow lavas intruded by dikes and sills, locally

into multiple dike/sill complexes, then into isotropic and

cumulate-textured layered gabbro. The base of this partial

Archean ophiolite is marked by a one-kilometer thick shear

zone composed predominantly of mafic and ultramafic

mylonites, with less-deformed domains including dunite,

websterite, wherlite, serpentinite, and anorthosite. Synorogenic

conglomerates and sandstones were deposited in several

small foredeep basins and are interbedded with mugearitic

lavas (and associated dikes), all deposited/intruded in a foreland

basin setting.

A complete but dismembered and metamorphosed 2.5-

billion-year-old ophiolite complex has been described from

the North China craton. This ophiolite has structurally complex

pillow lavas, mafic flows, breccia, and chert overlying a

mixed dike and gabbro section that grades down into layered

gabbro, cumulate ultramafics, and mantle peridotites.

High-temperature mantle fabrics and ophiolitic mantle podiform

chromitites have also been documented from the Dongwanzi

ophiolite, and it has ophiolitic mélange intruded by

arc magmas.

Dismembered ophiolites appear to be a widespread component

of greenstone belts in Archean cratons, and many of

these apparently formed as the upper parts of Archean oceanic

crust. Most of these are interpreted to have been accreted

within forearc and intra-arc tectonic settings. The observation

that Archean greenstone belts have such an abundance

of accreted ophiolitic fragments compared with Phanerozoic

orogens suggests that thick, relatively buoyant, young

Archean oceanic lithosphere may have had a rheological

structure favoring delamination of the uppermost parts during

subduction and collisional events.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar