DEFINITION OF THE WORLD’S OLDEST OPHIOLITE

Definition of The World’s Oldest Ophiolite

Ophiolites are a distinctive association of allochthonous rocks

interpreted to form at oceanic spreading centers in back arc

basins, forearcs, arcs, and in major oceans. A complete ophiolite

grades downward from pelagic sediments into a mafic volcanic

complex comprised mostly of 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, although many

workers interpret the term to mean allochthonous oceanic lithosphere

rocks formed exclusively at mid-ocean ridges.

Prior to 2001, no complete Phanerozoic-like ophiolite sequences

had been recognized in Archean greenstone belts, leading some

workers to the conclusion that no Archean ophiolites or oceanic

crustal fragments are preserved. These ideas were recently challenged

by the discovery of a complete 2.5 billion-year-old ophiolite

sequence in the North China craton. This remarkable rock sequence

includes chert and pillow lava, a sheeted dike complex, gabbro and

layered gabbro, cumulate ultramafic rocks, and a suite of strongly

deformed mantle harzburgite tectonites. The mantle rocks include a

distinctive type of intrusion with metallic chrome nodules called a

podiform chromite deposit, known to form only in oceanic crust.

Well-preserved black smoker chimney structures in metallic

sulfide deposits have also been discovered in some sections of the

Dongwanzi ophiolite belt, and these ancient seafloor hydrothermal

vents are among the oldest known. Deep-sea hydrothermal vents

host the most primitive thermophyllic, chemosynthetic, sulfatereducing

organisms known, believed to be the closest relatives of

the oldest life on Earth, with similar vents having possibly provided

nutrients and protected environments for the first organisms. These

vents are associated with some unusual microscale textures that

may be remnants of early life forms, most likely bacteria. These

ancient fossils provide tantalizing suggestions that early life may

have developed and remained sheltered in deep-sea hydrothermal

vents until surface conditions became favorable for organisms to

inhabit the land.

Archean oceanic crust was possibly thicker than Proterozoic

and Phanerozoic counterparts, resulting in accretion predominantly

of the upper basaltic section 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

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. Remarkably,

Archean greenstone belts contain an abundance of tectonic

slivers of pillow lavas, gabbros, and associated deep-water sedimentary

rocks. 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.

sists of an ultramafic rock known as harzburgite, consisting

of olivine + orthopyroxene (± chromite), often forming

strongly deformed or transposed compositional layering,

forming a distinctive rock known as harzburgite tectonite. In

some ophiolites, harzburgite overlies lherzolite. The harzburgite

is generally interpreted to be the depleted mantle from

which overlying mafic rocks were derived, and the deformation

is related to the overlying lithospheric sequence flowing

away from the ridge along a shear zone within the harzburgite.

The harzburgite sequence may be six miles (10 km) or

more thick in some ophiolites, such as the Semail ophiolite in

Oman and the Bay of Island ophiolite in Newfoundland.

Resting above the harzburgite is a group of rocks that

were crystallized from a magma derived by partial melting of

the harzburgite. The lowest unit of these crustal rocks includes

crystal cumulates of pyroxene and olivine, forming distinctive

layers of pyroxenite, dunite, and other olivine + clinopyroxene

+ orthopyroxene peridotites including wehrlite, websterite, and

pods of chromite + olivine. The boundary between these rocks

(derived by partial melting and crystal fractionation) and those

below from which melts were extracted is one of the most fundamental

boundaries in the crust, known as the Moho, or base

of the crust. It is named after Andrija Mohorovicic, a Yugoslavian

geophysicist who noted a fundamental seismic boundary

beneath the continental crust. In this case, the Moho is a chemical

boundary, without a sharp seismic discontinuity. A seismic

discontinuity occurs about half a kilometer higher than the

chemical Moho in ophiolites.

The layered ultramafic cumulates grade upward into a

transition zone of interlayered pyroxenite and plagioclase-rich

cumulates, then into an approximately half-mile (1-km) thick

unit of strongly layered gabbro. Individual layers within this

thin unit may include gabbro, pyroxenite, and anorthosite.

The layered gabbro is succeeded upward by one to three miles

(2–5 km) of isotropic gabbro, which is generally structureless

but may have a faint layering. The layers within the isotropic

gabbro in some ophiolites define a curving trajectory, interpreted

to represent crystallization along the walls of a paleomagma

chamber. The upper part of the gabbro may contain

many xenoliths of diabase, pods of trondhjemite (plagioclase

plus quartz), and may be cut by diabase dikes.

The next highest unit in a complete, Penrose-style ophiolite

is typically a sheeted dike complex, consisting of a

0.3–1.25-mile (0.5–2-km) thick complex of diabasic, gabbroic,

to silicic dikes that show mutually intrusive relationships with

the underlying gabbro. In ideal cases, each diabase dike

intrudes into the center of the previously intruded dike, forming

a sequence of dikes that have chilled margins developed

only on one side. These dikes are said to exhibit one-way chilling.

In most real ophiolites, examples of one-way chilling may

be found, but statistically the one-way chilling may only show

directional preference in 50–60 percent of cases.

The sheeted dikes represent magma conduits that fed

basaltic flows at the surface. These flows are typically pillowed,

with lobes and tubes of basalt forming bulbous shapes

distinctive of underwater basaltic volcanism. The pillow

basalt section is typically 0.3–0.6-mile (0.5–1-km) thick.

Interstices between the pillows may be filled with chert, and

sulfide minerals are common.

Many ophiolites are overlain by deep-sea sediments,

including chert, red clay, in some cases carbonates, or sulfide

layers. Many variations are possible, depending on tectonic

setting (e.g., conglomerates may form in some settings) and

age (e.g., siliceous biogenic oozes and limestones would not

form in Archean ophiolites, before the life-forms that contribute

their bodies developed).

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