Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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2. Thoroughly hydrated and extensively carbonated

maﬁc and ultramaﬁc rocks were apparently created

by metasomatism in a subaqueous environment;

this does not necessarily imply reconstitution at

spreading ridges as in modern settings.

3. Metamorphism is commonly polyphase and some

of the multiple episodes are in contact aureoles

around granitoid intrusions.

4. Thermobarometers in mineral assemblages of well

studied belts indicate signiﬁcant regional contrasts

in P and T of mineral equilibration resulting from

variations in tectonic and/or magmatic thickening

and depth of burial, followed by erosional and tec-

tonic uplift.

5. Archean granitoid–greenstone terranes have low to

intermediate P/T metamorphic ﬁeld gradients.

The absence in early Precambrian rocks of high P/T

ﬁeld gradients and associated distinctive minerals, such

as lawsonite, aragonite, jadeite, and so on, has been

recognized for several decades. Possible reasons for

their absence include a lack of preservation resulting

from preferential erosion, subsequent overprinting by

higher-T metamorphism, or unfavorable thermotec-

tonic conditions in the hotter infant Earth (Windley,

1995, p. 324). The oldest known high P/T rocks appear

to be multiply deformed metasedimentary rocks that

are of transitional blueschist–greenschist facies uncon-

formably underlying 800 Ma unmetamorphosed sedi-

mentary rocks in northwestern China (Liou et al.,

1989). For some time, the oldest known subduction-

related high-P eclogites were those that formed at

1000–1100 Ma in a subduction setting near Glen Elg,

Scotland. But Möller et al. (1995) described eclogites

and associated metapelites equilibrated at 18 kbar

and about 750°C in a 2000 Ma paleosubduction zone

bordering an Archean craton in Tanzania.

Mineralization is widespread in greenstone belts

and has provided much of the impetus and funding

for the investigation of their overall geology. Economic

resources include stratiﬁed Fe formations, Au–quartz

lodes, massive and disseminated Cu–Zn–Pb sulﬁdes,

and, especially in ultramaﬁc host rocks, valuable con-

centrations of Ni, Cu, Pt, Pd, Cr, V, and asbestos.

19.2.2 Itsaq Gneiss Complex, West Greenland:

Earliest Record of Crustal Process at 3900–3600 Ma

West Greenland (Figure 19.2) harbors the largest

(3000 km

) and best exposed remnant of oldest crustal

rocks on Earth—the Itsaq Gneiss Complex (Nutman

et al., 1996). In this complex, the Isua supracrustal se-

quence is the earliest record of lithosphere–hydrosphere

interaction on the planet and includes 3850 Ma

carbon-bearing rocks that are claimed by some invest-

igators to represent the earliest record of biological

activity (Myers and Crowley, 2000).

All of the Isua sequence has been metamorphosed

to the granulite and amphibolite facies, which is an

unusually high grade for TTG–greenstone terranes.

Additionally, the sequence is complexly deformed and

variably metasomatized. Appraisal of protoliths has,

accordingly, been equivocal. Calc–silicate rocks that

were formerly believed to be metamorphosed impure

limestones have been reinterpreted by Rosing et al.

(1996) to be the result of carbonation and desilication

of other protoliths. Felsic rocks formerly thought to be

volcaniclastic may instead be metasomatized maﬁc

gneiss. Most workers in West Greenland, however,

concur that about one-half of the protoliths of the Isua

supracrustal sequence were maﬁc and ultramaﬁc

(dunite and peridotite) protoliths. Some of the latter

Precambrian Rock Associations

617

116°

24°S

122°E

300km0

200

Pilbara

Yilgarn

26°

32°

122°E

Ocean

120°

116°

34°

32°

30°

28°

26°

Meekatharra

Wiluna

Mount

Magnet

Southern

Cross

Kalgoorlie

Perth

Indian

miles0

Granitoids, variably

metamorphosed,

2800–2600 Ma

Gneiss 3000 Ma,

granulite facies

Fault

Terrane boundary

Greenstone belts

2750–2600 Ma

Greenstone belts

3000 and 2800–2750 Ma

19.6 Collage of fault-bounded granite–greenstone terranes in the

Archean Yilgarn Craton (shaded) of southern Western Aus-

tralia. Redrawn from Myers and Swagers (1997). Inset in

upper right shows location of the Archean Pilbara and Yilgarn

Cratons in Western Australia. Larger scale map of rectangular

area enclosed by dashed line around Kalgoorlie is Figure 19.7.

have compositional afﬁnities with komatiite (discussed

later) while the maﬁc rocks, which in the least

deformed areas have relict pillow and pillow-breccia

fabric, are geochemically not MORB-like, but have

possible arc afﬁnities. A metamorphosed banded iron

formation that consists of quartz  magnetite  Fe-

rich silicates is an important part of the Isua sequence

and is locally intercalated with layers of metamor-

phosed magmatic rocks. The formation is believed to

have been a chemical precipitate in oceanic basins,

perhaps near seaﬂoor hot springs where Fe was leached

from extruded maﬁc volcanic rocks. Signiﬁcantly,

there appears to be no felsic rock nor any derivative

erosional product from felsic rock in the supracrustal

sequence.

The oldest rocks of the Isua supracrustal sequence,

which formed at about 3900 Ma near the termination

of intense meteorite bombardment on Earth, reveal the

existence of a hydrosphere overlying maﬁc rocks.

Voluminous tonalite, trondhjemite, and granodior-

ite (TTG) magmas were intruded as early as 3870 Ma

into the Isua supracrustal sequence. However, no

coherent greenstone belt as in other Archean terranes is

preserved and the Isua rocks only occur as inclusions,

but some as much as 25 km in length, in TTG ortho-

gneisses. By 3570 Ma, parts of the Itsaq Gneiss

618 Igneous and Metamorphic Petrology

122°E121°30′121°

30°30′

31°S

Kalgoorlie

Lower to middle

greenschist

Middle to upper

amphibolite

Upper greenschist

to lower amphibolite

122°

31°30′

31°S

Kalgoorlie

Granitoids

Conglomerate,

sandstone

Felsic

volcanic

rocks

Basalt,

komatiite

miles

19.7 Archean (2750–2600 Ma) granite–greenstone terrane near Kalgoorlie in the eastern Yilgarn Craton, southern Western Australia. See

Figure 19.6 for location. Note complexly (polyphase) folded and faulted greenstone sequence between less metamorphosed granitoid

plutons. Smaller scale map of same area in lower left depicts character of metamorphic facies in the greenstone sequence. Redrawn from

Myers and Swagers (1997).

Precambrian Rock Associations

619

Kapuskasing uplift

Metasedimentary rocks (accretionary prism)

Granitic plutons (continental arc)

TTG plutons (island arc)

Greenstone belts (island arc)

55°N

50°

100°W90° 80°

0 500km

0 300miles

50°

55°

60°N

80°90°W

Hudson

Bay

2725 Ma

Wabigoon

2695 Ma

2705 Ma

Minnesota

River Valley

gneiss

complex

Lake

Superior

Wawa

Lake

Huron

Lake Michigan

Abitibi

Uchi

Sachigo

James

Bay

SOUTH

2725 Ma MRV

2629 Ma

MRV WA W US

2725 Ma MRV

WA W U S

Minnesota

River Valley

Wabigoon Uchi-

Sachigo

NORTH

Wawa Abitibi

19.8 Archean Superior Province (shaded) of the south central Canadian shield. Redrawn from Hoffman (1989). P, Proterozoic rocks;

Ph, Phanerozoic rocks. Greenstone belts include Uchi-Sachigo, Wabigoon, Wawa, and Abitibi. Lines ornamented with open tri-

angles are major ductile shear zones or thrusts where terranes were sutured together at indicated times (2725, 2705, and 2695 Ma);

triangles are on upper, hanging-wall plate. Lower right is a series of highly schematic N–S cross-sections across western part of province

showing progressive southward closure of ocean basins and suturing of inferred Archean island arcs and continental segments along shear

zones. Shear zone striking southerly from James Bay bounds the upthrusted Kapuskasing section of the Wawa-Abitibi greenstone

sequence.

Complex had experienced granulite facies metamor-

phism at pressures near 10 kbar, indicating the exist-

ence of a crust at least 25 km thick (Nutman et al.,

1996). Also by that time, a variety of granitoids had

been intruded, including 3650 Ma granites geochemic-

ally resembling those in Phanerozoic collision and arc

settings and 3625 Ma granites and ferrodiorites resem-

bling those in anorogenic settings (Section 13.9).

Adding to this lithologic diversity was sediment de-

rived from the granitoids. Swarms of maﬁc dikes, in

part noritic, were intruded beginning about 3500 Ma.

These Ameralik dikes (Figure 14.38) may have accom-

panied rifting of the continental block composed of the

gneiss complex. Parts of it were metamorphosed again

at 3470 and 3140 Ma, the last events recorded in the

rocks.

It is important to put these Precambrian events in a

chronologic perspective. The geologic history recorded

in the Itsaq Gneiss Complex from 3900 to 3140 Ma is

about 200 Ma longer than the entire Phanerozoic

(about 570 Ma)!

19.2.3 Kaapvaal Craton

This and the neighboring Zimbabwe Craton to the north

are important Archean crustal segments in southern

Africa (Figure 19.3). The enormous mineral deposits

of the Kaapvaal have been responsible for extensive

geological investigations.

The oldest rocks are 3540 Ma TTG preserved as

remnants in the oldest greenstone sequences but

metasedimentary rocks contain detrital zircons as old

as 3700 Ma, implying the existence of still older felsic

620 Igneous and Metamorphic Petrology

Unmetamorphosed to weakly

metamorphosed intrusions

Greenschist and lower

amphibolite facies

Amphibolite facies

Upper amphibolite and granulite

facies, locally retrograded

Hudson

Bay

James

Bay

Lake

Superior

Lake

Huron

55°N

50°

100°W 80°W90°

500

300

miles

50°

55°

60°N

80°W

19.9 Metamorphic facies in the Archean Superior Province (shaded) of the south central Canadian shield. Compare with Figure 19.8.

Redrawn from Card (1990).

rocks (Kröner et al., 1996). In the Barberton fold and

thrust greenstone belt (Figure 19.4), the stratigraphic

sequence begins at the base with mostly komatiitic

and tholeiitic basaltic rocks overlain by lesser felsic

volcaniclastic rocks and chert and clastic sedimentary

rocks (Figure 19.5). TTG magmas locally intruded this

sequence. This predominantly oceanic sequence is

interpreted to have formed in relatively shallow water

where it was extensively hydrated and metasomatized

by advecting seawater (de Wit et al., 1992). Boundaries

between adjacent lithotectonic terranes that have dis-

tinctive rock units and deformational histories and

styles are all tectonic and display thrust, strike-slip, and

oblique movement. This tectonic overprint is believed

to have resulted from imbricate stacking of slices of

hydrated, hot oceanic crust, perhaps plateaus, that were

too buoyant to subduct. This convergent tectonism is

envisaged to have been not unlike that in the western

Paciﬁc and Caribbean during the Mesozoic and Ceno-

zoic (Section 13.3.1 and Figure 13.17), except that a

thick buttressing continental margin was nowhere

present. Thickening of these stacks of maﬁc oceanic

rocks, including possible local island arcs (Figure

19.10), permitted dehydration partial melting in deeper

parts of the stack, generating TTG magmas (Figure

11.23). Intrusion of these granitoid magmas into the

overlying greenstone sequence was instrumental in

creating continental nuclei that had coalesced by about

3100 Ma into the proto-Kaapvaal Craton. Himalayan-

type collision of this cratonic assemblage with the Zim-

babwe Craton along the Limpopo Orogen, or “mobile

belt,” at 2680 Ma, resulted in signiﬁcant crustal thick-

ening and ﬁnal craton stabilization at about 2600 Ma.

Mantle-derived xenoliths in numerous kimberlite

and orangeite pipes (Figure 13.42) provide tantalizing

additional insights into the evolution of the Kaapvaal

Craton (Carlson et al., 2000). The oxygen isotope

composition of many Archean eclogite xenoliths is con-

sistent with derivation from basalt that was oxidized

under seaﬂoor conditions before subduction into the

mantle. Archean harzburgitic and dunitic xenoliths

derived from depths of 200–250 km in the abnormally

cool, 350-km-thick lithospheric root beneath the cra-

ton are highly depleted in major compatible elements

(Ca, Al, Fe) and indicate extraction of appreciable

melt, possibly of komatiitic composition. The low Fe

and low modal garnet (resulting from low Al) makes

this residual mantle less dense than fertile mantle under

the same P–T conditions so that it more buoyant,

adding to the long-term stability of the craton. Re–Os

isotopes in the xenoliths indicate the growth of the

lithospheric root coincided with the main period of

crustal growth and thickening. A mantle plume is pos-

tulated to have played an important role in providing

maﬁc and ultramaﬁc magmas.

19.2.4 Yilgarn Craton

Archean TTG–greenstone cratons in Western Australia

(Figure 19.6) include the 3600–2800 Ma Pilbara in the

north and the younger Yilgarn in the south. Although

the Pilbara is well exposed (Figure 9.14) and has a

relatively complete rock record it is not as well docu-

mented as the Yilgarn (Myers and Swagers, 1997).

The Yilgarn Craton consists mostly of 3000–2600 Ma

granitoid plutons and greenstone sequences meta-

morphosed at low- to intermediate-grade; local high-

grade terranes occur along the western margin (Figure

19.6). In the northwestern Narryer terrane, the oldest

rocks are heterogeneous gneisses derived from 3680–

3600 Ma granite and 3730 Ma tonalite and layered

anorthosite, gabbro, and ultramaﬁc rocks. After defor-

mation and metamorphism, more granite magma was

intruded at 3400–3300 Ma followed by migmatization

at 3300 Ma. Sedimentary rocks—quartzite, shale, and

banded iron formation—were deposited on the older

rocks between 3100 and 2700; some of these contain

detrital zircons as old as 4270 Ma—the oldest known

fragments of terrestrial rocks. Between 2780 and 2630 Ma

still more granite magma was intruded, all rocks

were subjected to granulite facies metamorphism and

intense deformation accompanied stacking of thrust

sheets.

Precambrian Rock Associations

621

Submarine

basaltic volcanism

Depleted

mantle

TTG

magma

Intra-oceanic obduction

“ﬂake” tectonics

19.10 “Flake” tectonics and granite–greenstone belts. Cartoon speculating that maﬁc–ultramaﬁc oceanic crust produced by submarine

magmatism above a mantle plume or at a spreading ridge becomes hydrated and buoyant, moves laterally, and overthrusts onto a

thickening stack of oceanic rock, forming a volcanic greenstone sequence. The stack becomes thick enough so that dehydration melting

occurs at the base, generating TTG partial melts that diapirically intrude the overlying pile. Compare de Wit et al. (1992), Hoffman and

Ranelli (1988), and Davies (1992).

The stratigraphic sequence in the greenstone belt

around Kalgoorlie (Figure 19.7) resembles that in the

Barberton belt. Intense deformation manifest in tight

polyphase folding in this belt is particularly noteworthy.

Rock sequences and structures with histories that

range from 3730 to 2550 Ma (twice the duration of the

Phanerozoic) are superﬁcially similar over much of the

Yilgarn Craton. Remarkably, some komatiitic–basaltic

sequences extend along strike for hundreds of kilo-

meters despite individual layers being 50 m thick.

Detailed work has revealed numerous distinct litho-

stratigraphic terranes separated by major fault zones.

Some zones are shallow dipping and extend to depths

of 25–35 km (based on seismic reﬂection data) and are

interpreted to be crustal-scale sutures. Regardless of

age, all terranes experienced intense tectonic, volcanic,

plutonic, and metamorphic activity between 2780 and

2630 Ma that created the present craton. This activity

amalgamated many continental nuclei, including what

are interpreted to have been—in a plate tectonic

view—volcanic arcs and back-arc basins.

Careful ﬁeld work at two exceptional exposures

by Fripp and Jones (1997) has revealed a previously

unrecognized 2700 Ma ophiolite that they believe was

created in an oceanic spreading environment. In an

upper crustal section, marine sedimentary rock is under-

lain by Mg-rich maﬁc–ultramaﬁc rocks consisting of

devitriﬁed hyaloclastic breccia, pillow lavas, sheeted

amygdaloidal dikes and possible sills. All experienced

moderate-T alteration that created widespread actinolite

–tremolite and minor chlorite, serpentine; silica and

sulﬁdes were introduced. In an apparent lower oceanic

crust–mantle transition, sheeted dikes with moderate-T

alteration are underlain by isotropic and local mylo-

nitic gabbro underlain by peridotite that has been

subjected to high-T metamorphism.

19.2.5 Superior Province

Almost one-fourth of the exposed Archean crust in the

world lies in the Superior Province of Canada (Fig-

ure 19.8). It shares signiﬁcant similarities in the char-

acter of rocks and in evolution to that of the Yilgarn,

Kaapvaal, and other Archean cratons in consisting of

numerous terranes of differing histories and ages

(3300–2550 Ma) sutured together along crustal-scale

fault zones (Hoffman, 1989; Card, 1990; Stott, 1997).

In the Superior Province, greenstone belts resemble

modern island arc associations, terranes of metasedi-

mentary rocks resemble those in near-trench accre-

tionary prisms, and granitoid batholiths resemble those

in Mesozoic continental margin arcs. Protoliths in

accretionary prisms were mostly clastic sedimentary

rocks deposited in submarine fans with sources from

nearby volcanic islands. Interior parts of these

metasedimentary terranes were metamorphosed to

higher grade than margins. Continental arc granitoids

consist of early TTG and younger, less deformed, more

potassic granites; S-type granites are common. In addi-

tion, local high-grade gneiss terranes are thought to

represent more deeply eroded equivalents of the three

arc-related terranes. The oldest of these high-grade

terranes consist of granulite-facies orthogneisses and

amphibolites exposed in the Minnesota River Valley in

the southwestern part of the province. The gneisses

were plutons of tonalite and granodiorite intruded at

3660 Ma into older rocks and repeatedly metamor-

phosed and deformed at 3600, 3000–2800, and about

2700 Ma.

Abitibi Granitoid–Greenstone Belt: An Archean Island

Arc Association. The Abitibi terrane in the south-

eastern part of the Superior Province (Figure 19.8) is

not only the largest contiguous area of supracrustal

Archean rocks in the world but also one of the richest

mining areas; production during the twentieth century

exceeded 10

ounces of Au plus major amounts of Cu,

Zn, Ag, and Fe. Obviously, this well exposed terrane

has received considerable study.

Dominantly volcanic supracrustal sequences de-

posited from about 2750 to 2700 Ma consist of tholeiitic

basalts and lesser interlayered komatiites, which dis-

appear upwards, overlain by diverse tholeiitic and calc–

alkaline maﬁc–intermediate–felsic rocks, and capped

by marine sedimentary rocks. Local, late shoshonitic

and alkalic rocks unconformably overlie the calc–

alkaline sequence. The volcanic rocks were deposited

in two main environments (Dimroth et al., 1985):

1. Submarine lava plains made of basalt and lesser

komatiite lava ﬂows erupted from ﬁssures onto

nearly ﬂat seaﬂoor. Proximal to vents the ﬂows are

massive but distally become more pillowed. Pillow-

and ﬂow-breccias and hyaloclastic deposits are

common near ﬂow tops and toes. Vesicularity in-

creases upwards, suggesting shallow water condi-

tions. Some large volume basaltic ﬂows (100 km

)

covering hundreds of km

are believed to have

been extruded at high discharge rates; thicknesses

of hundreds of meters allowed development of dia-

basic and gabbroic interiors. Thin beds of marine

chert and shale were deposited among the ﬂows.

Flow sequences, whose aggregate thicknesses is

5–10 km, resemble modern oceanic plateaus.

2. Mostly younger central volcanic complexes 15–30

km in diameter made of compositionally diverse

rock types and emplaced as pyroclastic deposits,

voluminous volcanic debris ﬂows, and comagmatic

intrusions. Most central ediﬁces began subaque-

ously but some grew into emergent volcanic islands.

Plutonic rocks include local layered gabbro–

anorthosite complexes intruding the lower tholeiitic

sequence, but more widespread are more or less

622 Igneous and Metamorphic Petrology

synvolcanic, and some comagmatic, TTG plutons and

younger (2700–2670 Ma) granodiorite, granite, and

syenite intruding the volcanic pile. Elongate domal

structures in the greenstone belts are cored by large

plutons and separated by synforms of thick volcanic

sequences. Younger plutons have higher-grade contact

aureoles than the generally prehnite–pumpellyite and

greenschist facies characterizing the regional green-

stone sequences (Figure 19.9). Metamorphism began

with seaﬂoor alteration and was followed by meta-

somatism related to creation of massive volcanogenic

Cu–Zn sulﬁde concentrations and then by burial meta-

morphism; a culminating regional metamorphic event

included ductile deformation at about 2700 Ma.

Separating the Abitibi terrane and the similar

Wawa terrane to the west is the Kapuskasing uplift

(Figure 19.8). This linear terrane is the deeply eroded

hanging wall of a southeast directed overthrust of late

Proterozoic age that exposes a thick crustal section of

Wawa–Abitibi rocks (Figure 19.11).

Amalgamation of the Superior Craton was accom-

plished by convergence and suturing of arclike terranes

progressively from north to south along major, north-

dipping crustal-scale thrusts that are marked by ductile

shear zones from 2725 to 2695 Ma (Figure 19.8). A

component of lateral displacement along the shears

indicates oblique convergence of lithospheric plates.

The long-term geologic stability of the craton is related

to its unusually thick lithospheric keel (400 km based

on seismic data). Multiple swarms of Proterozoic basaltic

dikes (Figure 19.12) manifest repeated generation of

copious volumes of maﬁc magma. The depleted mantle

source in the lithosphere adds to the buoyancy of the

stable craton.

19.3 ARCHEAN VOLCANIC ROCKS

19.3.1 Komatiite

Although ultramaﬁc cumulates of pyroxene and olivine

crystals in basal sequences of large differentiated

basaltic intrusions (Section 12.4.2) have been recog-

nized for many decades, the absence of ultramaﬁc vol-

canic (aphanitic or glassy) rocks in the geologic record

had convinced petrologists that corresponding magmas

were nonexistent. However, this dogma was overturned

when Viljoen and Viljoen (1969) described exactly

such rocks near the Komati River in the Barberton

Greenstone Belt in southern Africa (Figure 19.4). Soon

after, similar ultramaﬁc lava ﬂows were described in the

Yilgarn block of Western Australia (Nesbitt, 1971) and

in the Superior Province of Canada (Pyke et al., 1973).

Precambrian Rock Associations

623

Depth (km)

TTG

Amphibolite

faciesGranulite facies

Greenschist

facies

Meta-

sedimentary

rocks

Metavolcanic rocks

Granite

Paragneiss

Maﬁc gneiss

Tonalitic gneiss

Anorthosite

19.11 Schematic crustal section through the Abitibi Granite–Greenstone Belt as exposed in the Kapuskasing uplift. Redrawn from Hofmann

(1989; original source is J. A. Percival in an unavailable NATO workshop publication). See Figure 19.8 for location of Kapuskasing

uplift.

Komatiites have now been found on every continent

except Antarctica (see summaries in Arndt and Nisbet,

1982; Arndt, 1994; Arndt et al., 1997). Because ko-

matiite constitutes 5% of the total volcanic rocks in

most greenstone belts and in many belts is absent, it is

not surprising that recognition took so long, especially

because of the universal metamorphism of greenstone

belts.

Although most komatiites are Archean, many are

Proterozoic. The youngest, and apparently only

Phanerozoic, occurrence is on Gorgona Island, Colom-

bia (Section 13.3.1) that is part of a late Jurassic–

Cretaceous accreted oceanic plateau. Because so little

is known of nonaccreted plateaus in their oceanic en-

vironment, it is uncertain whether there may be other

Phanerozoic komatiites. The reason for the decrease in

komatiite occurrences since the Archean is debat-

able but the prevailing opinion is that conditions for

generation of ultramaﬁc magmas were more favorable

in the hotter Archean mantle.

Petrography. Komatiites have a simple primary min-

eral composition of olivine and clinopyroxene plus

accessory chromite or Cr-rich magnetite. Even the

freshest rocks are metamorphosed to some degree so

that original widespread glass is invariably devitriﬁed

and magnesian olivines (near Fo

) are always replaced

to some degree by talc, serpentine, and minor mag-

netite. Magnesian, Al-rich clinopyroxenes are variably

altered to chlorite, tremolite, and epidote.

In contrast to their simple primary mineralogy, the

fabric of komatiites is complex. Subaqueous lava ﬂows

that are generally only a few meters thick are pillowed,

ﬂow tops are brecciated and glassy and have relict vesi-

cles. Locally, komatiite occurs as volcaniclastic deposits

and as intrusive sheets (sills) that may be locally

624 Igneous and Metamorphic Petrology

90°W

60°N

80°

Hudson

Bay

70°

55°

50°

45°

90°W

80°

70°

45°N

50°

60°

55°

Matachewan

2470

Mackenzie

1267

Molson

1884

Lake

Winni-

peg

Hearst

2450

Keeweenawan 1100

Lake

Huron

Sudbury 1238, 575

Lake Michigan

Abitibi 1141

Grenville

Front

Preissic

0 400km

0 250miles

19.12 Basalt dike swarms in the Superior Province, Canada (shaded). Age of swarm in Ma. Redrawn from Card (1990).

19.13 Relict ocelli in Archean tholeiitic basalt metamorphosed under

greenschist facies conditions. This locally pillowed basalt is in

the Komati Formation in the Barberton Greenstone Belt,

South Africa (Figures 19.4 and 19.5). Pale gray centers of

ocelli are mostly plagioclase, whereas thin darker rims are

mostly amphibole and chlorite. Camera lens-cap for scale.

Photograph courtesy of Maarten de Wit.

pillowed. Komatiites and kindred basalts are generally

aphyric and many have spherulitic aggregates called

varioles or ocelli (Figure 19.13; see also de Wit and

Ashwal, 1997, p. xvii). These radial bundles of inter-

grown needles of clinopyroxene (replaced by secondary

amphiboles) and plagioclase are commonly coalesced

balls that range from 0.5 to 50 cm in diameter. Whether

they formed as immiscible melt segregations or by

some other means is controversial but the aphyric

nature of their host rock and common occurrence with

devitriﬁed glass suggests an origin by rapid cooling

of superliquidus melts (compare similar spherulites of

felsic minerals in Figure 6.15d).

Widespread and locally thick volcaniclastic deposits

of komatiite have been found in the Barberton green-

stone belt (Lowe and Byerly, 1999). All have been ex-

tensively metasomatized, mostly to chert (about 90%

SiO

), and were previously interpreted to have felsic

protoliths; however, the absence of relict felsic minerals,

Precambrian Rock Associations

625

their repeated occurrence at the tops of layers in a

komatiite sequence, and their immobile trace element

concentrations, ratios, and patterns indicate a koma-

tiite ancestry. These deposits are believed to have res-

ulted from hydromagmatic eruptions in water depths

of 700 m.

The most distinctive fabric of komatiite is spinifex

texture, named after a spiky Australian grass. “Spinifex

is a texture characterized by large, skeletal, platy,

bladed or acicular (needle-like) grains of olivine or pyro-

xene, found in the upper parts of komatiitic ﬂows, or,

less commonly, at the margins of sills and dikes. The

texture is believed to form during relatively rapid,

in situ crystallization of ultramaﬁc or highly maﬁc

liquids” (Arndt, 1994). Spinifex consists of arrays of

criss-crossing sheafs of subparallel blade-like olivines

that can be as long as 80 cm but 1 cm thick or of

clinopyroxenes (Figure 19.14b). Between such crystals,

and comprising about half of the spinifex zone of the

ﬂow, is a ﬁner-grained aggregate of devitriﬁed glass,

skeletal to feathery clinopyroxenes, and skeletal

chromites (Figure 19.14c). Olivine crystals are largest

at the base of the spinifex zone and are oriented sub-

perpendicular to ﬂow margins; they grew normal to the

cooling surface in the direction of maximum supersat-

uration (compare comb layering, Figure 7.47).

Not all the details of spinifex texture have been ac-

counted for, but a basic picture has emerged. Komati-

ite magmas have very low viscosities (0.1–1 Pa s; Figure

6.2). Because of the scarcity of phenocrysts, magmas

appear to have been near liquidus temperatures or

even superheated when extruded. If so, homogeneous

nucleation of crystals could have been inhibited (Sec-

tion 6.5.2). As the unusually hot melt loses heat upon

extrusion or very shallow intrusion in subaqueous

environments, heterogeneous nucleation on quenched

margins of the melt may have dominated. In the low

viscosity, non-polymerized melt structure, rapid diffu-

sion permitted rapid growth of platy, skeletal olivines,

clinopyroxenes, and chromites.

The origin of layering (Figure 19.14a) in horizontal

sheets of komatiite, whether lava ﬂow or sill, has been

controversial. The original proposal of Pyke et al.

(1973) was that any olivine phenocrysts present in the

sheet as it was emplaced, or that grew immediately

thereafter, gravitated to the base in the very low vis-

cosity melt, creating an olivine-rich cumulate; concur-

rently, spinifex crystals grew downward from the top.

Chemical Composition and Origin of Magmas. Chemic-

ally, komatiites are deﬁned as having very low alkalies,

MgO  18 wt.%, TiO

 1 wt.%, and SiO

 53 wt.%

(Le Maitre, 1989). Chemical variations resulting from

olivine fractionation within a particular body far exceed

the effects of variable partial melting conditions in pos-

sibly variable mantle sources, of crustal contamination,

(b)(a)

Overlying ﬂow

Brecciated ﬂow top with

ﬁlling of tremolite

 chlorite  serpentine

Spinifex zone

Aligned skeletal olivines

Medium- to ﬁne-grained

komatiite with downward

enrichment in gravity-settled

olivines; also contains

skeletal clinopyroxenes

and relict glass

Underlying ﬂow

Feathery

clinopyroxene

Olivine

0 0.5mm 0 0.15mm

19.14 Fabric of komatiite lava ﬂows. (a) Idealized cross-section through a layered lava ﬂow. Redrawn from Pyke et al. (1973). (b) Coarse

spinifex texture enhanced by weathering of Archean lava ﬂow in the Superior Province, Canada. Photograph courtesy of N. T. Arndt.

(c) Microspinifex texture under plane polarized light. Devitriﬁed glass and feathery clinopyroxenes between platy skeletal olivines that

are 1–2 mm long. (d) Plane polarized light photomicrograph showing skeletal clinopyroxenes that are embedded in devitriﬁed glass

(black) and lie between platy olivines (not shown). (c) and (d) are of the late Mesozoic Gorgona Island, Colombia, komatiite (Section

13.3.1). Photographs courtesy of Lina M. Echeverria.

of alteration, or of any other perturbing process.

Plots of compatible and incompatible elements deﬁne

olivine-control lines that are useful in determining

what elements were mobile during metamorphism of a

626 Igneous and Metamorphic Petrology

particular komatiite. MgO is commonly plotted on two-

element variation diagrams because of its substantial

range, as much as 16–50 wt.% in extremely fraction-

ated sheets. Compatible Ni and Mg co-vary whereas