Lab 6: Oceanic Crust & Ophiolites

Oceanic crustal rocks are composed of mainly what?

Mafic igneous rocks

• basalt at surface

• gabbro at greater depths

How do oceanic crusts form?

• through a process of stretching (extension), rifting, and rise of mafic magma

• as oceanic plates move apart, hot upper mantle rocks partially melt due to decompression melting

How does basaltic lava occur?

Via faults and fractures at and near the ridge axis, some of the magma reaches the seafloor surface where it erupts as basaltic lava

How are pillow lavas formed?

• seawater rapidly cools the basaltic lava as it emerges, quenching it to glass

• at low flow rates, thus results in pillow lavas

How is gabbro formed?

dykes have the composition of basalt lava, but crystallize slow producing the phaneritic mafic igneous rock gabbro

Where is diabase formed?

closer to the surface

How are cumulate deposits produced?

• as the magma crystallizes, the roof of the magma chamber expands downwards with the addition of newly cooled rock

• at the same time the floor expands upwards as crystals of heavy minerals (such as chromite) fall from overlying portions of the chamber and produce cumulate deposits in the lower levels of the magma chamber

The upper cumulate zone - gabbro

• represents upper magma conditions

• composed of phaneritic mafic rock dominated by Ca-rich plagioclase and pyroxene

• This rock is called gabbro

The lower cumulate zone

• underlying to phaneritic mafic rocks (gabbro)

• represents lower magma chamber conditions

• characterized by rocks approaching an ultramafic composition

What underlies the rocks of the crust?

peridotites of the upper (lithospheric) mantle

what is dunite?

a peridotite consisting of more than 90% olivine (or serpentine)

Serpentinized dunite

• Orange/brown weathered surface, matte black fresh surface

• presence of serpentine (altered from ferromagnesian minerals) indicated by waxy lustre

• lack of relict texture suggests that original rock was monomineralic and most likely entirely composed of olivine (and thus would have been massive dunite)

Cumulate gabbro

• Dark grey gabbro showing alternations of pyroxene- (black) and plagioclase- (grey-white) rich layers

• Phaneritic texture indicates slow cooling at depth

• Layering indicates crystal accumulation at the bottom of a magma chamber by episodic gravitational settling of denser crystals (pyroxene) and intervening times of crystallization of less dense crystals (plagioclase)

Leucogabbro

• Medium grey, medium grained, leucogabbro

• mafic phaneritic rock containing 5-40% ferromagnesian minerals (mostly as pyroxene) and dominated by plagioclase (>50% as anorthite)

• The greater proportion of plagioclase relative to clinopyroxene imparts a colour that is lighter than for true gabbro (which tends to be dark green/grey to black)

• Note that the prefix leuco means “white” Probably represents the upper gabbro layer of the ophiolite

Basalt breccia

• Breccia composed of fragments of diabase and assorted aphanitic (nearly glassy) mafic materials in a black basaltic matrix (indicating rapid cooling of lava at the seafloor surface)

• the unsorted and angular character of the fragments suggest the brecciation resulted from steam explosions associated with sea-water at or near the seafloor

Diabase

• Very fine-grained (nearly aphanitic), with tiny, light-coloured, elongated crystals of plagioclase feldspar in a dark green groundmass of ferromagnesian minerals

• Probably formed in the uppermost part of a sheeted diabase dyke unit of an ophiolite complex (close to pillow basalt unit)

Deep sea sediments

• quite fine-grained because far from source

• accumulate slowly

• majority of these sediments are biogenic/biochemical in origin produced by biological processes or chemical precipitation

4 major types of deep sea sediments

1. terrigenous muds

2. calcareous oozes

3. siliceous oozes

4. red clays

Terrigenous muds

• clays that were suspended in the water column and slowly settle to the deep ocean floor

• clay-rich and produce shales and mudstones (fissile - shale, non-fissile - mudstone)

Calcareous oozes

• composed of a mixture of foraminiferal material that rains onto the sea floor and precipitated calcite

• limestones produced from these oozes are micrite and chalk

Siliceous oozes

• sponges produce hard parts containing silica (opal)

• the silica settles to the ocean floor

• Produces chert

Red clays

consist of mixtures of terrigenous sediments and ocean precipitates

Massive sulphides - Black smokers

• pod-like deposits of sulphide minerals associated with black smokers

• grey to brassy yellow on fresh surfaces, greenish grey on weathered surface

• pyrite, chalcopyrite

• direct association with volcanic activity at mid-ocean ridges

Ophiolites

Sections through the oceanic crust that are now exposed on continents

• essentially slices of oceanic lithosphere that have been scraped off during the process of subduction and subsequently been uplifted

• they represent oceanic crust that traveled from the mid-ocean ridge to a subduction zone

General sequence for an ophiolite

Common alteration minerals observed in mafic to ultramafic rocks:

• Serpentinized rock

• Formation of talc

• Epidotized rock

• Amphibolitized peridotites

• Hematite (rust)

Serpentinized rock

• serpentine is a common alteration mineral for high-temp ferromagnesian minerals such as olivine and pyroxene

• a rock containing minerals that have experienced alteration to serpentine is said to be serpentinized

Talc formation

• further alteration of high-temp ferromagnesian minerals can lead to the formation of talc

• can resemble serpentine in colour (green-black) but is much softer and can be scratched with a fingernail

Epidotized rock

• recognizable by pistachio green colour

• commonly occurs as an alteration minerals on surfaces of fractures, weathered surfaces or inside vesicles

• a rock containing minerals that have experienced alteration to epidote is said to be epidotized

Amphibolitized peridotites

• pyroxene can revert to amphibole

• peridotites containing these amphibole pseudomorphs are said to be amphibolitize

Hematite (rust)

• common weathering product of virtually all mafic to ultramafic igneous rocks

• the presence of a yellow/orange/red stain or crust of weathered surface of a rock indicates abundant iron

Amygdules

• infilled vesicles

• a basalt containing amygdules is called an amygdaloidal basalt

Ophiolite sequence positions