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The
following summary of the geology of basalts is modified from the material
presented on a web page for an Igneous Petrology course at
University College (London University).
The original material was written by Ruth Siddall of UCL. The first
half of this summary (to the end of the section entitled Ophiolites), is
the most relevant. |
Basalts occur in every tectonic environment, so we
need to know a little more about them before we can successfully use them as
indicators of tectonic and magmatic processes. Four main environments where
basalts occur are defined;
at mid-ocean
ridges
on ocean islands
on oceanic or
continental crust above subduction zones
on continental
crust away from subduction zones.
There are also four broad classifications of basalts
to cover their compositional ranges;
·
MORB (mid-ocean ridge basalt) - the most abundant volcanic rock
type, it makes up much of the upper oceanic crust. The dominant minerals are
olivine and Ca-plagioclase, clinopyroxene is rare.
Characterized by low concentrations of incompatible elements. (a.k.a.
Olivine tholeiites)
·
OIB (ocean island basalts) - typical of the Hawaiian and Icelandic
volcanoes, these are olivine-tholeiites, having Ca-plagioclase, pyroxenes and ±
olivine. They are enriched relative to MORB in volatiles, alkali and
incompatible elements.
·
Island arc and continental tholeiites - are less mafic and more
siliceous than MORB. Associated with flood basalts. They contain plagioclase and
clinopyroxene, with less common olivine. Continental tholeiites are enriched in
alkalis relative to island arc tholeiites.
·
Alkali basalts - enriched in alkali elements, contain nepheline or
analcite; alkali-olivine basalts, basanites.
EXTENSIONAL PLATE TECTONIC
SETTINGS
Mid-Ocean
Ridges (Oceanic Rifts - spreading centres)
Over the last 200 m.y. (ie. since the Jurassic)
spreading at ocean ridges has created the entirety of the Earth’s oceanic
crust, almost 70% of the total Earth’s surface.
All older oceanic crust has been consumed by subduction.
Ridges spread at rates of around 5-10 cm/yr. They are the main zones of
heat loss for our planet, and ridge-push is the dominant force, over slab-pull,
for continuing the motion of the plates.
Ridges have a definite geomorphology, forming a
chain of undersea mountains 2-3 km higher than the surrounding abyssal plains
(most of the abyssal plains are 4-5 km deep). Along the centre of the chain
there is a rift valley, usually around 1 -1.5 km deep and 30 - 50 km wide.
The depth and width of these rift valleys is mainly dependent on the rate of
spreading of the ridge. Fast spreading ridges like the East Pacific Rise have
narrow, shallow or even no median rifts and are characterized by a smooth
topography, whereas slow spreading ridges like the Mid Atlantic Ridge have
broad, deep median rifts and are characterized by very rugged topography. The
water depth makes the features of mid-ocean ridges difficult to observe and
therefore it is only recently that we have begun to learn about them.
The igneous rocks are characterized by basalts,
specifically MORB, petrologically olivine-tholeiites. These have a strong mantle
derived geochemical signature and they are actually sourced from the anomalously
high-level mantle upwelling beneath the rift. These sea-floor basalts are
erupted as sheet flows and pillows. Texturally they are characterized by glassy,
rapidly cooled margins and a fine grain size (aphanitic - cryptocrystalline)
also indicative of rapid cooling. Phenocrysts of plagioclase and olivine and
occasionally, but rarely clinopyroxenes occur, but are not common.
Geochemically, MORBs are fairly uniform.
Pyroclastic rocks are rare, but one type is
relatively common. It is a breccia of angular, wedge-shaped fragments of pillow
lavas called a hyaloclastic breccia. The pillows will break up along their
radial cooling cracks to form this material.
The lavas are fed sporadically from a magma chamber
lying only a kilometre or so beneath the crust. The lava is channeled to the
surface by dykes and so technically fissure eruptions will occur, although these
are very different from the fissure eruptions we see on land, i.e. in Iceland.
Evidence for permanent axial magma chambers is sketchy. There is no way a single
long and thin magma chamber can exist along an entire ridge system. In actual
fact, a series of magma chambers with an elongate, sausage shape occur along the
ridge, about 1-2 km wide and 4-5 km long. These expand and deflate with pulse of
magma. As the ridge is spreads, magma chambers become successively intruded by
new magma chambers, and the old ones get pushed away from the heat source and
solidify as gabbros. As magma is constantly intruding pre-existing chambers,
contact relationships are rarely seen. There is a gradation between one gabbro
intrusion and the next.
Much of our knowledge about the magmatic processes
active beneath MORs comes from geophysical surveys, especially gravity. Further
information has been gleaned from ophiolite complexes (see below).
Alteration
The marine environment makes sea-floor basalts incredibly susceptible to
alteration. Alteration occurs as a result of continual contact with seawater and
also the circulation of hydrothermal fluids sourced from the shallow magma
chambers. This later case is often called seafloor metamorphism.
Common alteration by seawater transforms olivine
tholeiite basalts into spilites. These have undergone an ion-exchange reaction
with the seawater, whereby the Ca from the plagioclases is replaced by Na (from
sodium chloride, sea salt). The effect is that the basalts now have an albite-rich
rather than anorthite-rich composition to the plagioclases. The lost calcium is
frequently seen, infilling vesicles and fractures with calcite.
Seafloor metamorphism reduces basalts to greenschist
mineral assemblages. This will be further explained when we look at
metamorphism, but new minerals (green minerals) are developed, namely chlorite,
epidote and actinolite (an amphibole). Ferro-magnesian minerals like olivine and
pyroxenes will be altered to serpentine and associated clay minerals.
Serpentinite the rock is not monomineralic, but an intimate mixture of
serpentine-family clays, iron oxides, iron-rich clays and other minerals like
talc and fibrous amphiboles. These phases are generally too fine grained to
discern with an optical microscope.
Ophiolites
Evidence of oceanic crust prior to the Jurassic comes mostly in the form of
ophiolites. These are slices of oceanic crust that have been emplaced on land
during collision of plates. As a
result we often see cross sections through the seafloor, the fossil magma
chambers and the mantle below. Such sequences are called ophiolite complexes.
Occasionally the phrase “Alpine ultramafic complexes” is used because
these rock bodies were first described in areas affected by the Alpine Orogeny.
The classic example is the Troodos Ophiolite in Cyprus. As these rock bodies are
tectonically emplaced, deformation within them disrupts the structure so that no
two ophiolites are the same, many missing key features. The process by which
oceanic crust is emplaced on continental crust is called obduction. The Lizard
Complex in Cornwall and the Ballantrae Complex in the Southern Uplands of
Scotland are the two ophiolitic complexes in the UK. In both cases, the
sequences are incomplete.
Ophiolite complexes are divided into six layers, all of which may or may not be
observed. The top layer representing the fossil sea-floor is characterised by
deep sea sediments like chert. Below that is a layer of pillow basalts, and
below that the sheeted dyke layer. These latter represent the conduits which
would have fed the basalts from the magma chamber to the sea floor. As the ridge
spreads, dykes intrude dykes (at a rate of ~ 1 dyke every 50 years) so no
country rock exists, just dykes. Below the sheeted dyke swarm lies the relicts
of the magma chambers. These are gabbros. Towards the top of the chamber they
are homogenous, but towards the bottom, they become rich in cumulates of
ultramafic minerals like olivine and pyroxene. It is within these cumulate zones
that the Seismic Moho is defined. At this point there is a marked increase in
seismic wave velocity. Beneath the gabbros lies the true mantle, which in
ophiolites is developed as ultramafic, ultrabasic rocks like dunites (ol
peridotite), lherzolites (ol + opx + cpx peridotite) and harzburgites (ol + opx
peridotite). These are almost always transformed to serpentinite, and fresh
peridotite is rare. The transition from crustal plagioclase bearing rocks to
plagioclase-free olivine and pyroxene rocks is called the Petrological Moho.
Thereby the crust-mantle boundary is described slightly differently depending on
whether you are a petrologist or geophysicist. As it is better to be good at
both, both Mohos should be acknowledged and recognised.
Continental rift zones are discrete regions of
extension in the continental crust which may or may not become sufficiently thin
to cause continents to split and form new ocean crust. The Atlantic Ocean formed
as a direct result of continental rifting in the supercontinent Pangea in the
early Triassic. Other rifts fail, they extend a little, thinning the continental
crust and then the forces are removed and extension ceases. Such features are
called aulacogens.
The mode of extension in continental rift zones also
varies. As a result rifts have been divided into two categories, active and
passive. Characteristically active rifts are associated with large amounts of
magmatic activity and passive rifts with little or no magmatic activity.
The best-studied example of a present day active
rift is the East African Rift (EAR), part of the Afro-Arabian Rift System (the
Great African Rift). In fact this system shows the entire process of early
continental rifting grading into the formation of new ocean crust and a
mid-ocean ridge in the Red Sea and Gulf of Aden. Other examples of active rifts
are the Rio Grande Rift of New Mexico, USA, and the Benue Trough, Cameroon.
Passive rifts include The Rhone-Rhine Grabens and North Sea system (N. Europe)
and the Baikal Rift, Siberia.
The magmatism associated with continental rift zones
is varied but often expressed as explosive volcanism with pyroclastic products
dominating, associated with trachytic, rhyolitic and phonolitic lavas.
Chemically, the rocks range from alkali-basalts to SiO2-undersaturated basanites,
nephelinites and leucitites (essentially they tend o be enriched in the alkali
elements sodium, Na, potassium, K and even calcium, Ca) and is therefore bimodal
in chemistry, basaltic-alkaline. As a rule of thumb, the rocks become more
basaltic with increasing crustal extension.
Because the East African Rift is the best present day example, we shall use it
as a case study to illustrate the character of magmatism associated with
continental extension.
The East African Rift - some facts
The
volcanoes of the East and west African Rifts are typically stratovolcanoes like
Oldoinyo Lengai and Mt Kenya, with a few large caldera structures (Ngorongoro
Crater).
The volcanism associated with the western rift is
highly potassic lavas, those rich in leucite. The Gregory Rift is characterized
by nepheline rich magmas.
The southern parts of the Rift in Tanzania and
Malawi (both Western and Gregory Rifts) are well known for their carbonatites.
Although these are not voluminous magmas they are worth looking into because
they are extremely unusual. Oldoinyo Lengai has recently erupted carbonatite and
nephelinite lavas and pyroclastic rocks. Carbonatites are composed dominantly of
the mineral calcite (CaCO3) and contain virtually no silica at all. Although
recognized in the rock record, none were seen to erupt until 1960, thus proving
that they were really carbonate melts and not metamorphically melted limestone
as had hitherto been assumed - the presence of non-silicate melts being
generally disbelieved by petrologists! Analyses of strontium isotopes from
the lavas gave indisputable proof that they were derived from the mantle and not
from limestones. Also, trace elements distributions in carbonatites and
limestones are entirely different.
Carbonatite rocks - either intrusive or extrusive
are always associated with a nephelinite lava (or the intrusive equivalent,
ijolite). Another characteristic rock is fenite which is a contact metamorphism-metasomatic
alteration of country rocks which is very widespread around carbonatite centres.
Geochemistry
The alkaline magmas must be derived from mantle-derived basaltic melts which
have had residence in the crust, evolving through AFC (assimilation and
fractional crystallisation) processes. As a result they are significantly,
and characteristically enriched in volatiles (particularly halogens and CO2)
LILEs (large ion lithophile elements - K, Ba, Rb and large Rare Earth Elements,
LREEs). However, the areas of greater crustal extension and more basaltic magmas
have a (weak) MORB signature (depleted mantle source). Consequently we see a
transition from lithospheric to asthenospheric mantle source regions.
Low volcanicity rifts, like Baikal and the Rhine
Graben, are characterized by small volumes of highly alkalic magmas (nephelinites,
basanites, leucitites) associated with small amounts of crustal extension.
The processes that lead to Himalayan-style
continent-continent collision are driven by a convergence stage which involves
the subduction of oceanic crust beneath a continental margin, or beneath another
oceanic plate to close an intervening ocean basin. The former process will bring
about the formation of a cordillera mountain belt and the later will form an
island arc. Both these features may be incorporated into and overprinted by
continent-continent collision.
The majority of the igneous rocks associated with
subduction in both cases are of the calc-alkaline suite. That is to say they are
intermediate with respect to silica content; extrusive andesites and intrusive
granites, granodiorites and tonalites often in the form of large batholiths. The
characteristic andesites typically contain plagioclase, pyroxenes, hornblende,
biotite ± olivine or quartz, and are commonly porphyritic containing one or
more of these phases as phenocrysts.
Island
Arcs
Oceanic island arcs represent the site where oceanic lithosphere is subducted
under oceanic plates. They are characterized by linear or arcuate chains of
islands, like the Aleutians of the North Pacific, and the Indonesian
archipelagos.
Volcanoes on island arcs can be divided into two
main subgroups. These are; i). basalt - basaltic andesite volcanoes and ii).
andesite-dacite volcanoes. Obviously, each volcano has an architecture dependent
on itís eruptive materials; shield volcanoes for the basaltic melts and
stratovolcanoes from andesitic magmas, and these contrasting types of volcano
can exist only a few tens of kilometres apart. Eruptions from both
varieties tend to be explosive both from phreatic interaction and from the
anomalously high volatile content of the magmas.
The magmas themselves are generated from the
descending slab beneath the continental lithosphere. As the slab is heated, it
dehydrates and the arising fluids pass up into the overriding lithosphere
initiating partial melting. The island is built up of successive additions to
the volcanic pile, obviously originating as seamounts on a basement of oceanic
crust. As the crust thickens due to continued activity, the seamount becomes
subaerial and forms and island. This thickening of the crust slows the ascent of
magma allowing it to reside in magma chambers where it will undergo fractional
crystallisation and form andesitic melts.
Active
Continental Margins
Subduction
of oceanic crust beneath a continental margin will produce a cordillera range of
mountains such as that extending from Alaska to Tierra del Fuego in southern
Argentina, the entire length the west coast of the Americas. This range
comprises the Andes of South America and the Cascade Range of North America.
Both ranges have a great number of active volcanoes and form the eastern
perimeter of the Pacific Ring of Fire. Other examples of subduction beneath
continental crust from the eastern Pacific are New Zealand, Japan and Kamchatka
(Russia).
Volcanism at active continental margins is characterised by large, long lived
stratovolcanoes. Eruptions are typically explosive, with pyroclastic rocks
predominating over lavas. Consequently eruptions tend to be Vulcanian to
Ultra-Plinian in style.
The 10, 000 km long Andes volcanic arc was developed
on Precambrian and Palaeozoic crust during the Triassic and has been continually
active ever since. It is the longest subaerial mountain chain on Earth and has
some of the worlds highest peaks. As it is essentially a simple system involving
only subduction (the North American Continental margin is far more complicated,
involving terrane accretion and strike slip movement, as well as the subduction
of the East Pacific Rise mid-ocean ridge) it has been extensively studied and
has given us the name andesite to describe intermediate composition lavas.
The nature and distribution of the igneous activity
in the overriding plate is governed by the geometry of descending subducting
slab. Volcanoes occur approximately 200 - 300 km away from the trench but this
distance varies depending on the steepness of dip of the Benioff Zone of the
descending slab. Beneath the Andes, the slab dips at an average 30°C although
it is slightly shallower beneath the central regions of the chain.
The composition of the igneous rocks is in fact wide
ranging from basic through intermediate to acid (rhyolites - not found in island
arc settings) and even alkaline rocks occur, although the intermediate rocks
dominate. There is a zoning related to chemistry of the magmas. The andesitic
(and basic) magmatism is generally on the seaward side of the belt, the acidic
on thickened crust in the axis of the belt and the alkaline magmatism towards
the landward side of the belt.
The petrogenesis of these calc-alkaline suites is complicated. Like island arcs, dehydration is initiated in the descending slab and fluids rise into the overriding lithosphere and crust lowering the melting temperature there. At depths the slab itself may melt, and also sediments that are carried down with it. The ascending melts will interact with the crust as they pass through it, residing temporarily in a series of magma chambers. Whilst residing in magma chambers, melts may be injected by new melts and magma mixing will ensue, altering the composition further. The thicker the crust, the more opportunity the magmas have to undergo AFC (assimilation and fractional crystallisation). Consequently those melts moving through thin crust will evolve not far from the basaltic end of the spectrum, and those under thick continental crust will evolve towards alkaline magmas. The majority, in the middle, will have a calc-alkaline affinity.
Continent-Continent
Collision
Collision of two continental masses is a complex
process involving the closure of an ocean basin as a result of subduction,
or a series of subduction events, which may squash ocean islands, small
continents and island arcs between the converging continents. Therefore the
igneous signature may be the result of a wide combination of environments, often
with cordilleras and island arcs dominating. The actual collision is often
accompanied by the emplacement of ophiolite complexes, and so ironically these
expressions of oceanic extensional regimes are often associated with collision
belts. Compressional tectonics like thrust faulting and folding thicken the
crust substantially, lowering its base into regions where temperatures may be
sufficient to cause melting. In this case ëlateí orogenic granite batholiths
may be intruded. The Cornubian Batholith of Cornwall was intruded in this way at
the end of the Palaeozoic Variscan Orogeny. The Himalayas show a sequence of
events revealing a history of northward dipping subduction beneath Tibet,
initiating the intrusion of the trans- Himalayan Batholith along a cordilleran
margin. Later, an island arc, the Kohistan-Ladakh terrane was accreted to the
Tibetan margin and final collision of the Indian subcontinent provided immense
crustal thickening of the Himalayas and the Tibetan Plateau. We have yet to
erode the Himalayas down to a suitable level to observe any late orogenic
granites!
The photograph shows Nanga Parbat at the western
end of the Himalaya. Nanga Parbat is over 8 km high (one of the worlds highest
mountains) and has one of the highest geotherms in the world (away from active
volcanoes) in places a massive 150°C per kilometre! This means that
temperatures sufficiently high to produce granitic melts at depths not too far
below sea level.
The appearance of volcanism within the centres of
plates are something of a mystery, existing as it does away from the plate
margins with which we normally associate igneous activity. Volcanism in the
middle of plates has been attributed to hotspots, probably mantle plumes which
are stationary with respect to the overriding plate. Consequently the trails
left by volcanic islands as the plate moves them away from the heat source can
reveal a great deal about plate motion. This is particularly so of the hotspot
trails in the Pacific which show a dog-leg indicating a change in the direction
of plate movement at 40 Ma and the trails in the Atlantic which clearly display
its opening, complementing magnetic anomalies on the seafloor. Intraplate
hotspots on oceanic crust characteristically feature ocean island basalt (OIB -
olivine-tholeiites, with Ca-plagioclase, pyroxenes and ± olivine) and those on
continental crust erupt floods of continental-tholeiites.
Plumes
Plumes are generally held to be responsible for the occurrence of hotspots.
Morphologically they are upwelling diapiric pods which develop a mushroom shape
formed by convection within the upwelling hot material, capable of generating
large volumes of melt. They are originated at boundaries within the structure of
the Earth, the largest (ësuperplumesí) probably originate at the core-mantle
boundary, whereas smaller ones probably originate at the boundary between the
depleted (asthenospheric MORB source mantle) and enriched (OIB source mantle)
which lies at around 700 km depth. The OIB source mantle is a mixture of
primordial mantle plus recycled crust from subducted slabs. The plume is of
course fixed, and during its lifetime causes partial melting at its head which
generates the melt to feed intraplate volcanoes. The initiation mechanism of
plumes is poorly understood, however, the largest were apparently responsible
for the break-up of the supercontinent Pangaea during the Mesozoic producing
enormous outpourings of continental flood basalts.
Oceanic Hotspots
The classic examples of intraplate oceanic hotspots are the islands of Hawaii
and Iceland. Both are believed to overly plumes. The Hawaiian islands,
specifically the active Mauna Loa, is a small island in comparison to Iceland,
and is probably fed by a small plume sitting happily in the middle of the
Pacific Ocean. Iceland sits on the remains of the superplume which aided
break-up of the North Atlantic 60 million years ago.
Hawaii, the currently volcanically active island
lying at the end of the chain has three active volcanoes, Hualalai, Mauna Loa
and Kilauea, and the extinct Mauna Kea. A new seamount, Loihi is emerging,
continuing the chain to the SW of the island. Mauna Loa and Mauna Kea are the
tallest mountains on Earth rising 9 km from the ocean floor, although by far the
greatest volume is under water. The volcanoes erupt almost exclusively basalts;
predominantly tholeiites (90%) and alkali olivine basalts, including localised
picrites. Also, small volumes of andesitic lavas (hawaiite - phenocrysts of
andesine plagioclase and olivine), trachytes and rare dacites occur.
The Hawaiian lavas are deeply sourced, probably from
about 80 km depth, thus corresponding with the position of the inferred plume
head. The sub-volcano plumbing has been imaged to depths of ~15 km. The main
feeders appear to be vertical conduits with some magma chambers along their
length and a final large magma chamber, the summit reservoir crown beneath the
crater. Therefore there are opportunities for the magmas to undergo limited AFC
during their ascent.
Iceland, in the North Atlantic in its position
astride the Mid Atlantic Ridge is rather anomalous, because although it lies on
plate boundaries, its evolution is distinct from that normally observed at
mid-ocean ridges. The enormous quantities of basalt erupted have caused the
oceanic crust to thicken, therefore becoming more buoyant and produce a
subaerial setting with shield and fissure volcanoes. Although Iceland is
currently active, itís legacy is evident in a broad ridge lying E-W across the
North Atlantic between Greenland and the Faeroe Islands. It is becoming accepted
that Iceland lies on top of a plume which now coincidently lies on top of a
mid-ocean ridge. This plume was partially responsible for the opening of the
North Atlantic in the Tertiary and the generation of flood basalts in Northern
Ireland and Scotland (Antrim Plateau Basalts) and the offshore Thulean Flood
Basalt Province west of Greenland.
The main active volcanoes are Hekla, Askja, Krafla
and a number of fissure eruption sites, the most famous of which is Laki. The
chemistry of the Icelandic basalts is more characteristic of ocean island basalt
(e-MORB; sourced from the enriched mantle) rather than normal mid-ocean ridge
basalt (n-MORB sourced from the depleted asthenosphere), so this provides
furrther proof that the island is related to a plume.
Continental Hotspots
Continental Flood Basalt Provinces such as the Columbia River Plateau, USA, the
Parana in Brazil and the Deccan Traps of India are characterized by huge volumes
of tholeiitic basalt erupted over a short period of time. The Columbia River
Plateau basalts occupy a volume of 174, 300 km3 and cover an area of 163, 700
km2 and most of the basalt was erupted in a 2 million year period.
These basalts are erupted onto continental crust
through fissures rather than typical volcanic edifices. Flows spread out as
sheet flows. The variation in chemistry is minimal. The flows all seem to be
quartz tholeiites; i.e. oversaturated with respect to silica.
Such large amounts of basaltic melt in a continental
area could only have been produced by mantle plumes. They must have passed
through the crust fairly rapidly through feeder dykes only assimilating enough
material to increase the silica content. It has been postulated that if the
plume was large enough and long-lived enough it may have been responsible for
the cause of continental break-up ... or did continental break-up cause the
plume?