Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Magmatic Petrotectonic Associations

357

W. Molokai (WM) 2.1

E. Molokai (EM) 1.9

West Maui (WI) 1.8

Lanai (L) 1.6

Haleakala (Ha) 1.4

Kahoolawe (K)

Mahukona (M) 1.1

Kohala (Ko) 0.8

Hualalai (H) 0.75

Mauna Kea (MK) 0.65

Mauna Loa (ML) 0.4

Kilauea (Kl) 0.25

Loihi (Lo) 0.0 (now?)

160°W

158° 156° 154°

Kauai

22°N

21°

20°

19°

18°

Hawaii

Maui

Oahu

Volcano

Niihau (N)

Kauai (Ki)

Waianae (W)

Koolau (Ku) 2.8

Molokai

Lanai

Kahoolawe

5.6

5.25

4.0

Niihau

Age(Ma)

13.9 Hawaiian Islands and shield volcanoes on the Hawaiian Ridge. Age is approximate inception of tholeiitic shield building stage. A hypo-

thetical volcano (Ke, Keikikea) is anticipated to emerge in the future off the southeast coast of Hawaii. Depth contours in kilometers.

(Redrawn from Clague and Dalrymple, 1987; Moore and Clague, 1992.)

tridymite) in the groundmass and in vesicles. Tholeiites

generally lack hydrous phases, but increasingly evolved

alkaline rocks are more likely to contain Ti-rich amphi-

bole (kaersutite) and phlogopite-biotite, either in the

groundmass or as phenocrysts.

Much of the difference in bulk chemical composi-

tion between tholeiitic and alkaline basalts lies in com-

positional contrasts in the major pyroxene phase. Ca-

rich clinopyroxene occurs in both basalt types, but its

normative composition mirrors that of its host rock.

Thus, clinopyroxene in alkaline basaltic rocks is silica-

undersaturated because of relatively high concentra-

tions of Ti and Al that replace Si. Titaniferous clinopy-

roxenes are slightly darker brown than those in

tholeiites or are pale lavender-brown and have anom-

alous extinction colors under cross-polarized light in

thin section. Some clinopyroxenes in alkaline rocks

are enriched in Na, as the aegirine end member

(NaFe

3

), and these are greenish. Many Ca-rich

clinopyroxenes are conspicuously zoned to brown (Ti-

rich) or green (Na) rims. Alkaline rocks never contain

orthopyroxene. However, it is common in tholeiites of

oceanic islands, whereas rare in MORB. Many tholei-

itic rocks also contain a third pyroxene phase—pi-

geonite (Ca-poor clinopyroxene; Figure 5.28).

Alkaline maﬁc rocks locally contain a variety of

mantle-derived inclusions (Section 11.2.1) that are en-

tirely lacking in tholeiites.

Olivine-rich picrite and olivine-pyroxene-rich an-

karamite, some of which may have originated by accu-

mulation of phenocrysts and may be ultramaﬁc, are ei-

ther alkaline or subalkaline.

Tholeiitic and alkaline parental magmas evolve

into contrasting derivatives. Tholeiitic basalt magmas

evolve through low-P crystal-melt fractionation into

Fe-rich derivative ferrobasalt, ferroandesite, and so on.

Mildly alkaline basaltic magmas in oceanic islands typ-

ically differentiate into the sodic lineage of hawaiite-

mugearite-benmoreite-trachyte (Table 13.2; Figure

13.10) by multiphase fractional crystallization in crustal

358 Igneous and Metamorphic Petrology

13.10 IUGS classiﬁcation of average Hawaiian rocks from Table 13.2. Compare Figures 2.12 and 2.16, from which rock type names and thin

curved dividing line between ﬁelds of alkaline and subalkaline rocks are taken. Filled square is preshield basanite. Shaded area encom-

passes 200 analyses of shield-forming tholeiitic basalt and large  is the average. Postshield alkaline suite evolved from parental alkaline

basalt by multiphase fractionation denoted by ﬁlled circles. Posterosion, highly-alkaline maﬁc suite denoted by open triangles.

53 57

Tholeiitic

basalt

SiO

(wt. %)

SHIELD

Mugearite

Benmoreite

Trachyte

Ha-

waiite

Alkaline

basalt

Ankaramite

PRE-

SHIELD

Basanite

Nephelinite

Melilite nephelinite

O + K

O (wt. %)

POST-SHIELD

Table 13.2. Major Oxide Compositions of Hawaiian Volcanic Rocks

RESHIELD

HIELD

OSTSHIELD

OSTEROSION

TAGE

1 2 34567891011

SiO

45.7 49.4 45.4 44.1 47.9 51.6 57.1 61.7 44.1 39.7 36.0

TiO

3.37 2.5 3.0 2.7 3.4 2.4 1.2 0.5 2.6 2.8 2.8

13.2 13.9 14.7 12.1 15.9 16.9 17.6 18.0 12.7 11.4 10.0

3.50 3.0 4.1 3.2 4.9 4.2 4.8 3.3 3.6 5.3 5.7

FeO 8.55 8.5 9.2 9.6 7.6 6.1 3.0 1.5 9.1 8.2 8.9

MnO 0.16 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1

MgO 7.08 8.4 7.8 13.0 4.8 3.3 1.6 0.4 11.2 12.1 12.0

CaO 12.00 10.3 10.5 11.5 8.0 6.1 3.5 1.2 10.6 12.8 13.0

O 3.13 2.2 3.0 1.9 4.2 5.4 5.9 7.4 3.6 3.8 4.1

O 0.99 0.4 1.0 0.7 1.5 2.1 2.8 4.2 1.0 1.2 1.0

0.45 0.3 0.4 0.3 0.7 1.1 0.7 0.2 0.5 0.9 1.1

1, Alkaline basalt; 2, tholeiitic basalt (200); 3, alkaline basalt (35); 4, ankaramite (9); 5, hawaiite (62); 6, mugearite (23); 7, benmoreite (5); 8,

sodic trachyte (5); 9, basanite (11); 10, nephelinite (10); 11, melilite nephelinite (7).

Column 1, from Frey and Clague (1983), columns 2–11, averages (number of samples in parentheses) from Macdonald (1968).

chambers, rather than the potassic lineage of trachy-

basalt-trachyandesite-latite (Figure 2.12). Some mildly

alkaline magmas, or those transitional between tholei-

itic and alkaline, evolve into silica-oversaturated peral-

kaline trachyte and rhyolite. More alkaline maﬁc

magmas, such as basanite, can differentiate into silica-

undersaturated, Ne-normative tephrite and phonolite,

as on Tristan da Cunha (Figure 2.16). Another highly

alkaline oceanic island association, not necessarily

related via differentiation, is basanite-nephelinite-

melilitite. (Nephelinite consists of nepheline  clino-

pyroxene  olivine  Fe-Ti oxides; melilitite is the

same except that it includes melilite instead of

nepheline.)

13.2.2 Hawaiian Islands: Tholeiitic and

Alkaline Associations

owth of Hawaiian Shield Volcanoes. Hawaiian shield

volcanoes grow from the ocean ﬂoor, or on the ﬂank of

an older shield, through three stages, lasting altogether

about 0.6 My; a fourth evolutionary stage may follow

after a period of inactivity and erosion. These stages,

generalized in Figure 13.11, have been established

principally by studies of the island of Hawaii (Figures

10.11 and 13.9; see also Moore and Clague, 1992). A

summary follows:

1. Because the submarine parts of the island volca-

noes are poorly known, the earliest, preshield stage

of growth is highly speculative but is believed to be

one of infrequent small-volume extrusions of alka-

line basalt. The still submerged (about 1 km below

sea level), active Loihi volcano is believed to be in

transition from the preshield to shield stage of

growth because its submarine rocks include both

alkaline and tholeiitic lavas. In the preshield and

early shield stages, magmas are extruded onto the

ocean ﬂoor, forming submarine lava ﬂows, com-

monly pillowed, and hydroclastic deposits formed

by magma-water interaction (Figure 10.11c).

2. The main shield building stage is characterized by fre-

quent, large-volume extrusions of tholeiitic basalt. As

activity continues from the preshield stage, the vol-

cano emerges above sea level and subaerial ﬂows of

thin pahoehoe and thicker aa lavas erupt from sum-

mit vents and from rift-controlled ﬁssures (Figures

9.8 and 10.11). Pyroclastic deposits constitute 1%

of the subaerial shield. A network of magma cham-

bers and conduits within the volcano solidiﬁes as

coarser-textured, locally phaneritic gabbro in dikes

and sills. Ultramaﬁc cumulates and small volumes of

evolved magmas are derived by crystal fractionation.

Summit lavas are generally olivine-controlled (Figure

12.3). Less magnesian rift-zone lavas extruded on the

shield ﬂanks have experienced low-P multiphase

fractionation, mainly of clinopyroxene and plagio-

clase, and mixing with more primitive magma

recharging the fractionating chambers. As the shield

grows, it sinks isostatically and massive landslides

slough off gravitationally unstable ﬂanks (Figure

10.11b, c). Mauna Loa is in shield-building decline,

whereas Kilauea shield activity is waxing stronger.

3. The transition to the postshield stage is marked

by waning tholeiitic activity and inception of in-

frequent, small-volume alkaline basalt extrusions; al-

kaline and tholeiitic lavas are commonly intercalated.

Smaller shields have little or no postshield alkaline

lava. Larger shields fed from robust magma generat-

ing plume systems have well developed alkaline ac-

tivity (hawaiite-mugearite-benmoreite-trachyte) be-

ginning before the culmination of shield building.

4. After a period of volcanic dormancy (as little as

0.3 My on Kauai to about 2 My on Oahu) and

canyon cutting, the posterosion stage of activity in

most shields involves commonly explosive extru-

sion of very small volumes of highly alkaline maﬁc

magma. Rock types include alkaline basalt, basan-

ite, nephelinite, and melilite nephelinite (Table

13.2; Figure 13.10).

Some postshield and posterosion magmas carry

dense mantle-derived inclusions, testifying to their

rapid ascent. However, other postshield magmas stall

Magmatic Petrotectonic Associations

359

500 km

PRE-SHIELD

STAGE

Alkaline

basalt

100

200

300

500

600

700

SHIELD

STAGE

Mauna

Loa

Kilauea

Loihi

Keikikea

850 km

25 km

Mauna

Kea

POST-EROSION

STAGE; ≤ 1My

duration

Erosion; continued subsidence; 0.3−2My

POST-SHIELD

STAGE

Alkaline basalt,

basanite, neph-

elinite, melilitite

Hawaiite,

mugearite

Alkaline and

tholeiitic

basalt

32,000 km

Tholeiitic

basalt

Volcanic growth rate (10

/y)

Time (10

13.11 Schematic evolution of Hawaiian shield volcanos. Heavy-line,

bell-shaped curve shows estimated variation in growth rate

through time in the four stages of volcano evolution. A similar

curve would represent the variation in degree of partial melting

of the mantle source through time. Shaded rectangles show esti-

mated volumes of rock. Rock types characterizing each stage

listed on right in italics. Presumed stage status of some other

Hawaiian volcanoes is indicated on the left, as follows: Mauna

Kea has apparently just ended its postshield stage (last erupted

about 4000 years ago). Kilauea activity is waxing in the shield-

building stage (roughly, an eruption every year but some con-

tinue for 1 y); the activity of Mauna Loa, though waning (an

eruption every 3.4 y on average), is still in the shield building

stage; their shield stage is estimated to terminate about 300,000

and 150,000 years in the future, respectively. Lo hi may be near

the end of its preshield stage. Keikikea, a hypothesized volcano,

has yet to be born. (Redrawn from Frey et al., 1990.)

within the crust, where they differentiate. More volu-

minous, rapidly erupting shield-stage tholeiitic magmas

experience limited differentiation before eruption.

Trace Element and Isotopic Characteristics of Hawaiian

Magmas. Besides the differences through time in the

four stages of Hawaiian volcanism, variations in the

trace element concentrations and isotopic ratios of the

magmas reﬂect the degree of melting and character of

their sources, respectively (Table 13.3).

Compared with MORB at similar MgO, Hawaiian

shield-building tholeiites are enriched in TiO

, K

and other incompatible elements (Figure 13.2).

Alkaline basaltic rocks are even more enriched than

tholeiites (Figure 13.12). Nearly all Hawaiian rocks

have higher

Sr/

Sr and lower

143

Nd/

144

Nd than

N-MORB (Figure 13.13). On Haleakala (east Maui) vol-

cano (Figure 13.9), lavas of the three latest stages show

differences in isotopic ratios. Shield tholeiites have the

highest

Sr/

Sr and lowest

143

Nd/

144

Nd whereas pos-

terosion alkaline lavas have the opposite ratios and

partly overlap N-MORB. Postshield alkaline lavas lie

between these two. Preshield lavas are impossible to

sample on Haleakala, but on the submarine Loihi vol-

cano, alkaline lavas from this early stage have Nd-Sr

isotopic compositions between those of the tholeiitic

and alkaline lavas of Haleakala (Figure 13.13). This

ﬁnding leads to the unexpected conclusion that Rb/Sr

and

Sr/

Sr are higher in the sources of the tholeiites

than in the alkaline lava sources. And Sm/Nd and

143

Nd/

144

Nd are lower in tholeiite than alkaline

sources. In other words, tholeiitic basalts are derived

from more enriched sources than those of incom-

patible-element-enriched alkaline lavas.

Geochemists try to explain this paradox in terms of

different degrees of partial melting in different mantle

sources. Shield tholeiites are largely generated from rel-

atively enriched mantle near bulk Earth composition but

mixed with depleted upper mantle similar to that of the

MORB source. The relatively low Rb/Sr ratios and in-

compatible element concentrations in tholeiites indicate

relatively high degrees of partial melting. Extensive melt-

ing dilutes the more incompatible Rb (and Nd) relative

to Sr (Sm) while retaining the higher

Sr/

Sr (lower

143

Nd/

144

Nd) of the enriched source in the melt. The

generation of alkaline magmas is just the opposite. Small

degrees of partial melting creates higher Rb/Sr ratios but

from a source that is depleted in incompatible elements

and is more like that of MORB. Posterosion lavas must

also have a MORB-like source, since their isotopic ratios

are like those of MORB, but are generated by even lower

degrees of melting, so their incompatible element con-

centrations are highest; this source might lie in a meta-

somatically enriched lithosphere (Section 11.5.1).

Dynamics of Hawaiian Magma Generation: A Moving

Plate above an Ascending Plume. A long-standing

problem concerning petrogenesis of Hawaiian magmas

has been explaining the transitions between alkaline

and tholeiitic magmas and back again that occur dur-

ing the four stages of volcanic evolution. These se-

quential transitions are not readily explained by differ-

entiation processes. To account for them by magma

generation in a static mantle source there would have

to be corresponding changes in degrees of melting,

perhaps at different depths and volatile conditions, for

which no reasonable independent explanation exists.

However, the mantle beneath the Hawaiian Islands is

not a static source, as Wilson (1963) ﬁrst postulated,

but is rather a rising plume beneath the moving Hawai-

ian lithosphere. This dynamic mantle source that is

coupled to the moving overriding lithospheric plate ex-

plains time-dependent variations in eruption rate and

magma composition (Figure 13.14). In such a ther-

mally zoned plume, potential sources include the

plume itself of enriched mantle, a sheath of entrained

shallow and depleted asthenosphere through which it

360 Igneous and Metamorphic Petrology

Table 13.3. Trace Element Compositions of Select

Hawaiian Volcanic Rocks

123

Rb 18.6 16

Ba 50 304 791

Th 0.4 1.8 4.4

U 0.2 1.1

Nb 30.3 52

Ta 0.5 1.9 3

La 8.6 24.7 36.3

Ce 23.5 58.7 62.7

Sr 250 693 660

Nd 19 33.3 37.9

Sm 4.85 7.98 7.9

Zr 150 244 156

Hf 3.3 5.94 3.5

Eu 1.75 2.74 2.2

Gd 9.2

Tb 0.79 1.07 0.8

Y25

Ho 1.2 0.8

Tm 0.3

Yb 2.03 2.21 1.5

Lu 0.28 0.29 0.05

Cr 260 576

Ni 125 402

Sc 30 13

V 282 309

1, Shield tholeiitic basalt, Mauna Loa volcano, Hawaii; 2, postshield

alkaline basalt, Haleakala, volcano Maui; 3, posterosion nephelinite,

Kauai.

Data from references in Watson (1993).

Magmatic Petrotectonic Associations

361

Post-erosion nephelinite, Kauai

Post-shield alkaline basalt, Haleakala

Late-shield tholeiite basalt, Haleakala

Shield tholeiitic basalt, Mauna Loa

Pre-shield alkaline basalt, Loihi

100

200

Rock/primitive mantle

Ba U

LuTh

13.12 Normalized trace element diagram of four stages of Hawaiian volcanic rocks. (Data from references in Watson, 1993.)

0.702

0.5126

0.5128

0.5130

0.5132

0.703 0.704 0.705

Sr/

143

144

N-MORB

Haleakala

Post-erosion

alkaline lavas

Post-shield

alkaline lavas

Shield tholeiitic

basalt

0.51264

0.7052

Bulk

earth

All Hawaiian lavas

Loihi pre-shield

alkaline lavas

13.13 Isotopic ratios in Hawaiian lavas (shaded ﬁeld) compared with Paciﬁc N-MORB. (Redrawn from Chen and Frey, 1985.)

moved, and heated overlying depleted lithosphere. Iso-

topically, the latter two correspond to the normal

MORB source.

Blichert-Toft et al. (1999) ﬁnd signatures of old

crust and marine sediments in Pb, O, Hf, and Os iso-

topes in Hawaiian lavas, suggesting the plume source

has somehow entrained old subducted crustal compo-

nents, perhaps from the slab “graveyard” atop the core

(Figure 1.4). They furthermore contend that Hf-Nd

isotope trends preclude melting of depleted upper

mantle sources. Obviously, much has yet to be learned

about the origin of Hawaiian magmas, global recycling

of the differentiated crust, and the process through

which the mantle convects.

13.2.3 Highly Alkaline Rocks on Other

Oceanic Islands

Unlike the Hawaiian Islands, most intraplate oceanic

islands have little, if any, exposed tholeiitic basalt.

Whether the dominance of alkaline rocks is simply the

result of their concealing the underlying largely tholei-

itic shield, as in the older Hawaiian Islands, or whether

volcanic islands other than the Hawaiian are domi-

nantly alkaline throughout is difﬁcult to say because

of lack of subsurface information. However, in the

plume model, degrees of magma generation and over-

all magma production rate are lower where the

lithosphere is thicker because the ascending decom-

pressing mantle encounters the rigid, nonconvecting

362 Igneous and Metamorphic Petrology

North-west

PRE-SHIELD

ALKALINE

10, 4

100

150

200

250

miles

Sea level

SHIELD

THOLEIITE

POST-SHIELD

ALKALINE

POST-EROSION

ALKALINE

South-east

Moho

Spinel

Very

low

Garnet

melts

Low

High

Depth (km)

∼10 cm/y

∼30 cm/y

Metasomatic

1500°C

1300°C

Low

Depleted

Primitive

1300°C

13.14 Schematic cross section through the southeast end of the Hawaiian Islands and underlying plume (see also Figure 1.5 and Watson

and McKenzie, 1991). Note expanded vertical scale above ocean ﬂoor to show island topographical features. Horizontal scale for

entire cross section is same as deeper vertical scale. Lithosphere lies above upper 1300°C isotherm. Asymmetry of plume is due to

viscous drag of overlying lithosphere. Deep mantle part of plume inside line of small open circles has bulk Earth isotopic ratios;

outside is depleted shallow mantle. Transition from spinel- to garnet-peridotite occurs at about 60- to 80-km depth but perhaps

at 85–95 km in the hotter core of the plume. Preshield alkaline magmas (represented by a youthful Loihi volcano) are generated

by relatively low degrees of partial melting (dashed-line circle) of depleted, decompressing shallow asthenospheric mantle en-

trained along margin of the plume. Relatively high degrees of partial melting (dashed-line ellipses), mainly in the hotter core of

the enriched plume, generate tholeiitic magma in the shield-building stage; frequent eruptions occur mostly after storage, re-

charge, and fractionation in crustal magma chambers above the Moho (black treelike ornament beneath KI and ML). Peak magma

production rates from the core of the plume whose T is 1500°C to 1300°C yield a layer of magma about 25 km thick—the

Hawaiian crust (see path B in Figure 13.7). Mauna Kea (MK) apparently ﬁnished its postshield activity about 4000 years ago.

Postshield alkaline magmas are again generated in depleted mantle by low degrees of partial melting. During this stage, the

rate of magma production has dropped signiﬁcantly so that some small crustal magma chambers cool and fractionate without fre-

quent recharge, forming hawaiite, mugearite, and locally more fractionated daughter magmas. After a hiatus of as much as 2 My and

about 160 km “downstream” of shield-building activity, a very low degree of partial melting of depleted MORB-like mantle that has per-

haps been metasomatically enriched may locally segregate and ascend from near the base of the lithosphere. These posterosion

nephelinitic magmas are most like MORB isotopically of any stage and reﬂect a decline in heat input from the passed-by plume.

lithosphere before much partial melting can occur

(Figure 13.15). This model suggests that oceanic is-

lands founded on thicker lithosphere, such as the

Canaries, have no major tholeiitic underpinnings and

are predominantly alkaline basaltic rocks. Iceland, at

the thin end of the lithospheric thickness spectrum, is

predominantly tholeiitic.

On Gran Canaria, the largest of the seven volcanic

islands in the Canary chain off the northwestern coast

of Africa in the Atlantic Ocean, late Cenozoic volcan-

ism created only sparse tholeiite basalt but widespread

nephelinite, peralkaline trachyte and rhyolite, phono-

lite, and less common picrite, basanite, tephrite, alka-

line basalt, hawaiite, and melilite nephelinite.

The Azores are another group of presumed plume-

generated islands just east of the Mid-Atlantic Ridge

and the triple junction of the North American, African,

and Eurasian plates. The islands delineate the African-

Magmatic Petrotectonic Associations

363

Iceland

(crust)

Hawaii

Canary

20 40

80 100

120

0.1

0.2

0.3

Melt production rate (km

/y)

Canary Islands

Lithosphere

Astheno-

sphere

Partially

melted

120

Lithosphere thickness (km)

Depth (km)

(b)

(a)

Iceland crust

13.15 Partial melt production in a decompressing mantle plume decreases with increasing thickness of the overlying cooler mechanical lid

(lithosphere). In the Canary Islands located on lithosphere about 110 km thick, tholeiitic basalt is scarce and the predominantly alkaline

rocks represent magmas generated by small degrees of partial melting in a plume that could not decompress much above solidus tem-

peratures. In contrast, the plume beneath Iceland experiences far more decompression before encountering thin lithosphere (essentially

all crust), with the result that continuously high degrees of partial melting occur, generating copious amounts of tholeiitic magma. See

also Watson and McKenzie (1991) and White (1993).

Eurasian plate juncture, which is a slow, obliquely

spreading rift. Terceira, near the middle of the group,

exposes mildly silica-undersaturated to -oversaturated

basalt, hawaiite, mugearite, benmorite, and peralkaline

trachyte lava ﬂows and pyroclastic deposits making up

50% of the island.

Last erupted in 1961, Tristan da Cunha is the larg-

est of a group of three islands in the South Atlantic

about 430 km east of the Mid-Atlantic Ridge at the

southwest end of the aseismic Walvis Ridge—a pre-

sumed plume track feature. The island is built of highly

silica undersaturated basanite and lesser amounts of

phonotephrite, tephriphonolite, and phonolite (Figure

2.16). Most of the evolved alkaline magmas—phono-

lites, trachytes, benmoreites, and others—in these is-

lands originated through fractionation of plagioclase

and maﬁc phases in parental maﬁc magmas.

13.3 PLUME HEADS AND BASALT

FLOOD PLATEAU LAVAS

Prodigious ﬂoods of mainly basaltic lava have been ex-

truded during relatively brief episodes throughout

most of the history of the Earth. These large igneous

provinces (Mahoney and Cofﬁn, 1997) occur as sub-

marine oceanic plateaus as well as continental ﬂood

basalt plateaus (Figure 13.16).

13.3.1 Oceanic Plateaus

Oceanic plateaus are broad topographic prominences

rising several hundred meters above the surrounding

seaﬂoor. Little is known about these submerged fea-

tures, which are estimated to cover about 3% of the

seaﬂoor, because they are so inaccessible. Shipboard

drilling, dredging off the seaﬂoor, and sampling of rare,

and sometimes questionable exposures on continental

margins reveal that they are made mostly of thick se-

quences of basaltic lava ﬂows. Some plateaus are un-

derlain by crust as much as 40 km-thick—at least ﬁve

times the thickness of normal oceanic crust.

The apparent magma production rate in the larg-

est oceanic plateau—the Ontong-Java of the western

Paciﬁc—may be compared to that of oceanic ridge

systems and oceanic island chains. The Alaska-size

Ontong-Java is 30–43 km thick and has a volume of

about 6  10

. If it was produced in about 6 My,

as available chronologic data suggest, the magma pro-

duction rate was about 10 km

/y, compared to the

21-km

/y rate of the entire global ridge system (Figure

1.1) and 100 times the peak rate of a Hawaiian shield

volcano (Figure 13.11).

One possible phenomenon that could produce such

localized, brief bursts of tholeiitic magma production is

very large-scale decompression melting of the head of a

hot mantle plume. Plumes probably begin their ascent

as Rayleigh-Taylor instabilities in the buoyant D layer

364 Igneous and Metamorphic Petrology

Columbia

River

Wrangellia

Atlantic

North

Caribbean

Parana'

Coccos

Galap-

agos

Rio

Grande

Walvis

Etendeka

Karoo

Mascarene

Broken

Ridge

Kerguelen

Ninety east

Deccan

Rajmahal

Siberia

Ontong

Java

13.16 Large igneous provinces, chieﬂy basaltic. (See also Table 13.4.) Position of some currently active plumes indicated by open circles. Plume-

caused ﬂood basalt plateaus and tail-related island chains or ridge tracks can accompany opening of oceans. Examples include the fol-

lowing: (1) North Atlantic province (60 Ma) related to currently active Iceland plume. (2) Northward opening of the South Atlantic

about the Tristan da Cunha plume (T) since about 135 Ma; the Walvis Ridge tracks to the Etendeka province in Africa and the Rio

Grande Plateau lies on the way to the Paraná Plateau in South America. (3) The Réunion (R) hotspot in the western Indian Ocean tracks

northward along the Mascarene Plateau, thence along the Chagos-Maldive-Laccadive oceanic ridges to the Deccan Plateau (66 Ma) in

western India. (4) The Kerguelen (K) hotspot formed the Kerguelen Plateau and Broken Ridge and Ninety east Ridge and the Rajmahal

Plateau in eastern India (117 Ma). The Indian Ocean tracks have been modiﬁed by recent seaﬂoor spreading. The huge Mackenzie basalt

dike swarm in northern Canada (M; see also Figure 9.7a) formed above a 1.27-Ga plume head. The Central Atlantic igneous province,

now mostly dikes and sills (C; see also Figure 9.7b), formed above a plume head at about 200 Ma near the Triassic-Jurassic boundary just

preceding drifting apart of the American continents from Africa and Europe. (Redrawn from Cofﬁn and Eldholm, 1994.)

of the lowermost mantle just above the core (Figure

1.5). Laboratory model experiments in tanks of viscous

ﬂuids reveal that a buoyant plume rises with a large-

diameter head and a smaller-diameter tail. Large

amounts of mantle wall rock are viscously drawn into

the ascending plume and heated by it, particularly in

the head so that temperatures there are perhaps

1350–1400°C compared to 1550°C in the tail (Hill

et al., 1992). As the plume head nears the rigid litho-

sphere, it ﬂattens to as much as 2500 km in diameter,

compared to the order-of-magnitude-smaller-diameter

tail. Plume tails are thought to produce magma at a

lower rate than plume heads and for tens of millions of

years, creating linear volcanic island chains like the

Hawaiian on moving lithospheric plates.

Because of their thickness and buoyancy relative

to the mantle, oceanic plateaus riding on plates

converging into island arcs or continental margins can-

not wholly subduct. Consequently, parts are sliced

off and tectonically emplaced, or accreted, onto the

overriding plate. The Wrangellia terrane of coastal

British Columbia and southern Alaska (Figure 13.16)

is one such segment of an oceanic plateau formed

in the Paciﬁc and accreted onto the North Ameri-

can plate margin during the Mesozoic. More complete

sections through a late Jurassic-Cretaceous ocean

plateau are represented by widespread thick expo-

sures of tholeiitic basalt and ultramaﬁc rocks in

the Caribbean area and western Colombia (Figure

13.17). Exposures on the small island of Gorgona

off the west coast of Colombia are especially note-

worthy as they are the only known Phanerozoic (post-

Precambrian) occurrence of ultramaﬁc lava ﬂows

called komatiite.

The high MgO (18 wt.%) of komatiites and the

general absence of phenocrysts, which precludes a cu-

mulate origin, imply relatively high degrees of partial

melting of a peridotite source. The necessarily high T

for komatiite generation is consistent with derivation

from a plume. So, too, is the somewhat enriched trace

element pattern for the tholeiitic basalt lavas that ap-

parently constitute most of oceanic plateaus (Figure

13.18; Table 13.5). In terms of their isotopic ratios,

these lavas lie closer to bulk Earth than MORB and

Hawaiian lavas (Figures 13.19), again consistently with

a plume origin.

13.3.2 Continental Flood Basalt Plateaus

The best known ﬂood basalts on continents are Meso-

zoic and Cenozoic, but some are Proterozoic (Figure

13.16; Table 13.4). Swarms of basalt sills and feeder

dikes are associated with lava ﬂows, particularly in the

older, more deeply eroded plateaus (Figure 13.20). In a

few locales, unusually large volumes of basalt magma

were trapped within the crust and differentiated to

form major layered intrusions (Table 12.5).

Magmatic Petrotectonic Associations

365

Sediment

Pillow lavas,

intrusive sheets

Komatiite, picrite,

basalt lavas

Isotropic gabbro

Layered gabbro

Layered dunite,

peridotite, pyrox-

enite cumulates

Oceanic plateau

Continental

crust

Trench

migration

Deformed and

accreted

oceanic

terrane

13.17 Highly schematic accretion of an oceanic plateau onto a continental margin in a subduction zone. This is one form of continental growth.

Expanded stratigraphic section through the Caribbean Plateau on the left from Kerr et al. (1997). The lack of intercalated sediment in

the sequence of extrusive rocks suggests rapid accumulation. The lack of sheeted feeder dikes, typical of the MORB association seen in

ophiolite (Figure 13.1a), indicates no concurrent crustal extension occurred during extrusions of magma.

Flood basalt plateaus consist of large numbers

of superposed, ﬁssure-fed, sheetlike ﬂoods of lava

(Figures 10.12–10.14), at least some of which were

extruded in a compound manner as successive lobes.

The enormous volume of ﬂood plateaus (generally

10

) and short duration of peak activity ( a few

million years and apparently 1 My for well-dated

ones) are compatible with copious magma generation

in the head of a hot decompressing plume.

Lavas are chieﬂy tholeiitic basalt, but more evolved,

Fe-rich lavas occur in some places and, in the Karoo,

Ethiopia, and Paraná plateaus, substantial amounts of

silicic lavas were also extruded. Picrites are uncommon

except in the Karoo and Deccan. Basalts are commonly

aphyric; porphyritic ones have plagioclase as the usual

phenocryst. The groundmass typically contains plagio-

clase, Ca-rich and Ca-poor clinopyroxenes, relatively

abundant Fe-Ti oxides, local glass, and, rarely, olivine.

366 Igneous and Metamorphic Petrology

Siberia

Columbia

River

E-MORB

Ontong Java

N-MORB

Ba U Ta La

Sr Nd

Rb Th

YLu

100

Rock/primitive mantle

13.18 Normalized trace element diagram for ﬂood plateau basalts compared with E-MORB. See also Figure 13.2. The Columbia River sample

(Table 2.1) characterizes the more enriched continental ﬂood basalt plateaus; note strong negative Nb-Ta anomaly. Nine Siberia samples

(shaded; from Sharma, 1997) range to a less enriched pattern similar to E-MORB. Ontong Java (Table 13.5) is less enriched in more in-

compatible elements than some other oceanic plateau rocks.

MORB

Hawaiian

Islands

Oceanic

plateaus

Columbia River

Karoo

Deccan

0.714 0.7180.7100.7060.702

Sr/

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

143

144

13.19 Sr and Nd isotopic ratios in ﬂood basalts compared to MORB (dark shade) and Hawaiian Islands (Figure 13.8). Oceanic plateaus (light

shade) are from Kerr et al. (1997). Continental Columbia River plateau from Hooper (1997), Karoo from Cox (1988), and Deccan from

Mahoney (1988). Compare Figure 2.27. BE, bulk silicate Earth.