Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Chemically, the dominant rock type is slightly silica-

saturated quartz tholeiite more evolved than MORB in

that Fe, Ti, P, and K concentrations are greater and Mg

concentration less. MgO is commonly 5–8 wt.%, Ni

 100 ppm, Mg/(Mg  Fe

2

)  0.65 (Table 13.5; Fig-

ure 13.18).

Origin of Magmas. These attributes indicate substan-

tial crystal fractionation at low P from more primitive

mantle partial melts. Indeed, Cox (1993) argues that

primitive (picritic?) magmas experienced polybaric

olivine removal during ascent, reducing compatible el-

ement concentrations and MgO to 7 wt.%. Sub-

sequently, these fractionated magmas experienced

additional 30–40% removal of olivine, clinopyroxene,

and plagioclase in near-Moho storage chambers, where

they were buoyantly blocked from further ascent. This

fractionated gabbroic assemblage has about the same

composition as the parent magma with regard to many

major elements, and, hence, little indication of its re-

moval may be apparent. However, Ni and Cr contents

are low. Independent evidence for this extensive frac-

tionation, at least in some provinces, lies in the volumi-

nous contemporaneous silicic lavas, which require that

large subterranean magma chambers either fractionate

or supply heat to partially melt country rocks or do

both. Underplating of perhaps 5 km of gabbroic frac-

tionates is implied by the 1 km of permanent post-

Karoo uplift in southern Africa: That is, 5 km of new

crust was added by the magmatic activity.

Normalized trace element patterns for some conti-

nental ﬂood basalts are similar to those of ocean island

tholeiites and E-MORB (compare Figures 13.12 and

13.18), implying a similar plume source. However,

Magmatic Petrotectonic Associations

367

Columbia River

Ethiopia

Deccan

NorthAtlantic

Paraná-Etendeka

Karoo

Central Atlantic

Siberia

Keweenawan

Coppermine

River and

Mackenzie

17-6

(16)

25

(66)

59

138–120

(134–129)

184–179

(183  1)

200  4

(248)

1095  5

1267  2

OLUME

(10

)

0.17

0.35

2.6

6.6

1.5

2.5

2.5

1.3

0.2

HICKNESS

(

)

4

1.7

0.3?

REA

(10

)

0.16

0.75

1.5

1.3

1.2

1.5

2

0.01

OMMENTS

300 Lava ﬂows ranging from 700 to 3000(?) km

Multiple episodes of activity in Cenozoic; upper

part of plateau includes large volume of

peralkaline silicic rocks

100–500 lava ﬂows ranging to 10

; possible

cause of faunal extinctions near K-T

boundary

Contemporaneous alkaline magmatism; late

silicic lava ﬂows

Covered much of southern Africa but now

extensively eroded, exposing abundant sills

and randomly oriented feeder dikes (Figure

13.20), correlative basalts in southern

Australia and Antarctica (Ferrar province); in

southeastern Africa includes voluminous

silicic ﬂows

Mostly sills and dikes in northwestern Africa,

eastern North America, and northeastern

South America; may have caused Triassic-

Jurassic extinctions

Large amounts of intercalated basaltic tuff; not

clearly connected with an active plume; may

have caused Permian-Triassic extinctions

350 ﬂows in Great Lakes region of North

America

Mackenzie radial feeder dike swarm probably

the largest in the world (Figure 9.7a); scat-

tered ﬂows in Coppermine River area

Table 13.4. Continental Flood Basalt Provinces

See Figure 13.16 for locations.

*Peak activity in parentheses.

Data from Sigurdsson et al. (2000), White and McKenzie (1995), and other sources.

other basalts are more enriched in incompatible ele-

ments. A continuing debate is whether this enrichment

stems from contamination by the continental crust or

by partial melting of metasomatically enriched conti-

nental lithospheric mantle (e.g., Wilson, 1989; Lassiter

and De Paolo, 1997). Contamination is an expected

companion to extensive crystal fractionation in crustal

magma chambers and to generation of voluminous

contemporaneous rhyolitic lavas, as in the Karoo and

Paraná plateaus. Some continental ﬂood basalts have

distinct negative Nb-Ta anomalies (Figure 13.18). Such

an anomaly is typical of arc magmas (Figure 11.19; see

also Section 11.4.2) and continental crust built of them,

suggesting contamination by continental crust. Many

368 Igneous and Metamorphic Petrology

13.20 Karoo large igneous province. (a) Distribution of outcrops of basalt (black) and subsurface extent (stippled). Outlined rectangle is area

in (b), showing sills, and (c), showing dikes. (From Marsh et al., 1997.)

0 miles 500

0 km 800

Durban

Lesotho

Swaziland

Botswana

South

Africa

Cape

Town

Namibia

Angola

Zambia

Zimbabwe

bique

Mozam-

(a)

31°

32°

27° 28°

miles

Queenstown

Sills

Dikes

(c)

(b)

ﬂood basalts have greatly elevated

Sr/

Sr and de-

pressed

143

Nd/

144

Nd ratios (Figure 13.19), which are

consistent with extensive contamination with old con-

tinental crust.

13.3.3 Continental Breakup

The upward momentum of the ascending plume to-

gether with the thermally expanding rock in it and the

surrounding mantle domes the lithosphere upward,

creating kilometer-scale uplifts. Extensional stresses

are induced in the domed rigid plate, allowing intru-

sion of radial dike swarms that are thousands of kilo-

meters in diameter (e.g., Figure 9.7). These swarms are

exposed in older large igneous provinces where cover-

ing ﬂood lavas have been removed by erosion. In some

cases, plume activity has caused breakup of a large con-

tinental plate, leaving plateau basalt segments and/or

dike-sill swarms stranded on passive continental mar-

gins adjacent to the opened ocean. Other apparently

plume-related ﬂood volcanism, as in Siberia and in the

Columbia River Plateau (Figure 13.16), did not pro-

duce continental breakup.

Remnants of continental ﬂood lavas on passive con-

tinental margins ﬂanking the Atlantic provide an im-

portant record of plume initiation, continental breakup,

and rifting.

Breakup of Gondwana began in the south Atlantic

during the Cretaceous (Figure 13.21). Most of the

1.5  10

volume of lavas making up the Paraná

and Etendeka ﬂood plateaus (Figure 13.22) bordering

the Atlantic Ocean in eastern South America and

southwestern Africa, respectively, were extruded about

134–129 Ma. These ﬂood lavas are distinctly bimodal

in composition. Most maﬁc rocks, usually aphyric

tholeiite, have 48–59 wt.% SiO

whereas most silicic

rocks have 63–72 wt.% SiO

. Very few unaltered rocks

have 59–63 wt.% SiO

. Two distinct groups of silicic

rocks have different sources. High-Ti porphyritic rhyo-

lites were apparently derived from partial melts of

earlier basalts underplated or lodged in the lower crust

as succeeding basalt magmas supplied heat for melt-

ing. Low-Ti aphyric rhyolites have trace element

and isotopic ratios indicating derivation by AFC—

assimilation of crust into fractionally crystallizing basalt

magmas. The aphyric rhyolite ﬂows occur as remark-

ably homogeneous individual units lacking composi-

tional zonation despite their large volume (commonly

1000 km

). Some ﬂowed distances of 300 km and,

because of unique compositional attributes, have been

correlated on opposite sides of the Atlantic Ocean!

Their low aspect ratios, as low as 1/2000, are unusual

for lava ﬂows, and it has been suggested they were em-

placed as ignimbrites, but the distinctive pyroclastic

fabric is lacking, possibly because it was obliterated

by rheomorphic ﬂow. Alkaline magmatism that was

broadly contemporaneous with Paraná ﬂood volcanism

includes relatively small local occurrences of alkaline

gabbros, syenites, phonolites, and carbonatites.

The North Atlantic igneous province (Figure 13.23)

developed during continental breakup in early Tertiary

time as Greenland and eastern North America drifted

away from northwestern Europe after initial rise of the

Iceland plume.

Magmatic Petrotectonic Associations

369

Table 13.5. Compositions of Select Plateau

Flood Basalts

CEANIC

ONTINENTAL

1 234

SiO

49.25 52.76 49.97 51.10

TiO

1.20 2.325 3.07 1.32

14.00 13.24 8.22 14.45

t 12.69 14.58 12.02 13.26

MnO 0.21 0.219 0.16 0.18

MgO 7.75 4.15 15.52 6.30

CaO 12.35 8.18 7.07 10.61

O 2.05 2.65 1.43 1.96

O 0.14 1.28 2.10 0.53

0.06 0.44 0.45 0.19

Rb 1.32 33 55 7.5

Ba 23.6 503 917 235

Th 0.33

U 0.09

Nb 3.76 13 19 3.7

Ta 0.27

La 3.66 21.80

Ce 10.55 47.34

Sr 153 314 1000 185

Nd 8.26 30.51

Sm 2.67 7.51

Zr 69.3 210 402 117

Eu 0.97 2.34

Gd 3.40 8.56

Dy 3.90 8.60

Y 22.6 53 28 31

Yb 2.28 4.66

Lu 0.34 0.76

Cr 133.0 22 804 156

Ni 98.1 19 827 57

Sc 46.5 39 21

V 314 402 204 285

See also Table 2.1.

1, Tholeiitic basalt, average of 21 samples, Malaita, Ontong Java

plateau. Data from Neal et al. (1997). 2, Grande Ronde basalt, Co-

lumbia River Plateau. Aphyric lava ﬂows like this constitute about

85% of plateau. Data from Hooper (1988). 3, Average of 19 sam-

ples of olivine-rich basalt, Letaba Formation, north Lebombo, Ka-

roo province; represents possible near-primary mantle-derived

magma. Data from Cox (1988). 4, Average of ﬁve samples of

basalt. Transvaal, Karoo province. Data from Cox (1988).

13.4 ARC MAGMATISM: OVERVIEW

Compared to the chieﬂy basaltic petrotectonic associa-

tions of preceding sections, associations in subduction

zones are far more variable. Diverse magmas are gener-

ated in multiple sources and are subsequently sub-

jected to virtually the whole gamut of differentiation

processes.

Arc magmatism at continental and oceanic plate

margins overriding subducting oceanic lithosphere is

second only to oceanic rift magmatism in terms of

volume of magma produced (Figure 1.1), frequency

of eruption, and longevity. Marked by the inclined

Wadati-Benioff zone of earthquake foci, subduction

regimes are the most complex of all global geologic

systems. The complex dynamic interplay between

370 Igneous and Metamorphic Petrology

RGR

Ocean

∼ 40 Ma

∼ 125 Ma

∼ 132 Ma

∼ 145 Ma

Parana´

basalts

Etendeka

basalts

Plume

South America

Africa

lithosphere

Rio Grande

Rise

Walvis

Ridge

Active

plume

tail

Fossil

plume

head

Atlantic

13.21 Plume activity and related breakup of South America and Africa. Left-hand panels are maps of the two conjoined (Gondwana) and then

rifted-apart continents. Right-hand panels are enlarged highly schematic cross sections. Plus sign in both panel sets overlies plume axis.

While conjoined, the two continents both were drifting westward. At about 132 Ma, the decompressing, broadening plume head par-

tially melted, yielding basalt magmas that formed the Paraná-Etendeka ﬂood basalt plateau and local alkaline magma centers (small ﬁlled

circles). At about 125 Ma, the Gondwana protocontinent began to rift apart; the western part of the plume head that was attached to the

South American lithosphere began to separate from the main part of the plume. Subsequently, the decompressing plume tail has gener-

ated basalt magmas that have formed the Walvis Ridge (WR) and the Rio Grande Rise (RGR) as the two continental plates drifted east-

northeast and west-northwest. Since about 40 Ma the plume tail has underlain Tristan da Cunha just east of the Mid-Atlantic Ridge. Seis-

mic studies indicate a 300-km-diameter low-velocity cylinder in the mantle to a depth of about 600 km beneath Brazil that is interpreted

to be a still abnormally hot fossil plume. Most of the new basaltic crust at the Mid-Atlantic Ridge is being added to the South American

plate, causing the ridge and the Nazca-South American plate juncture (Frontispiece) to migrate westward, opening the Atlantic and clos-

ing the Paciﬁc. (Redrawn from Van Decar et al., 1995.)

transfer of matter and energy—chieﬂy gravitational

and thermal—is manifested not only in magmatism

and seismicity, but also in mountain building or

orogeny (forming mountain belts or orogens), crustal

growth, sedimentation, metamorphism, and ore depo-

sition.

Some of the transfer of matter in subduction zones

is unidirectional and irreversible. Thus, arc magma-

tism adds partial melt extracts from the mantle to the

crust, as does ocean ridge and plume activity; such ad-

ditions may have been a signiﬁcant factor in crustal

growth throughout Earth history. Certain unidirec-

tional changes in the composition of the mantle with

time are complementary to crustal growth. Other trans-

fers of matter in subduction zones involve recycling,

the most obvious of which is cycling of volatiles, es-

pecially water. Physically entrapped and chemically

bound water in hydrothermally altered subducting

oceanic crust is liberated into the overlying mantle

wedge. Partial melting generates hydrous magmas,

some of which are extruded from volcanoes. Exsolved

water from erupting magmas is released into the at-

mosphere and condenses as rain and snow that fall to

Earth and collect into rivers that ﬂow back into the

ocean, its original source. Still controversial as to its ex-

tent, is the cycling of oceanic rock. Subducted seaﬂoor

sediment may be partially melted and added to arc

magmas building the crust. Some subducted slabs that

include seaﬂoor maﬁc rock converted to dense eclogite

appear to sink to the bottom of the mantle (Figure 1.4),

where matter may be caught up in ascending plumes

that partially melt near the surface, creating magmas

that are again added to the crust. Erosion of subaerial

crustal rock dumps matter back into the ocean, com-

pleting the mass transfer circuit.

13.5 OCEANIC ISLAND ARCS

It is logical to begin a discussion of arc magmatism with

oceanic island arcs. The effects of the “crustal ﬁlter”

through which mantle-derived magmas ascend toward

the surface are limited relative to continental arcs,

Magmatic Petrotectonic Associations

371

60°

50°

South America

15°

25°

35°

Rhyolite

Basalt

Dikes

Alkaline

rocks

Sills

Luanda

15°E

Rio de

Janeiro

São

Paulo

E T E N D E K A

Africa

800

miles

500

Walvis

Bay

25°55°

Buenos

Aires

15°S

Asuncion

PARANÁ

13.22 Simpliﬁed geologic map of the Paraná and Etendeka ﬂood basalt-rhyolite provinces of South America and Africa restored to their pre-

rift position before about 138 Ma. Paraná basalts in southwest are concealed beneath younger sedimentary rocks. (Redrawn from Peate,

1997.)

372 Igneous and Metamorphic Petrology

Mafic rocks

Onshore

Offshore

Seismic reflection

Plume position

at indicated time

2 km bathymetric

contour

15° W

Rum

intrusion

British

Tertiary

igneous

province

0°

50° N

60° N

NORWAY

Mid-

ICELAND

20 Ma

Atlantic

Ridge

70° N

15° E

0°15° W30°

45°

60 Ma

50 Ma

Skaergaard

intrusion

70 Ma

70° N

GREENLAND

13.23 North Atlantic igneous province. Large areas of maﬁc magmatic rock have been delineated by offshore geophysical surveys and deep sea

drilling. (Redrawn from Saunders et al., 1997.)

arc, is andesite (Gill, 1981). Although rocks with the

same range of SiO

as andesite (57–63 wt.%) occur in

virtually all petrotectonic associations, other element

concentrations differ in these other associations, such

as lower Al and higher Fe in icelandite and higher al-

kalies in benmoreite. Continental crust of intrusive and

metamorphic rock is approximately andesite in average

chemical composition, implying that andesitic magma-

tism has played a role in continental growth.

Arc rocks range widely and continuously in chem-

ical composition on each side of andesite (Table

13.6 and Figure 13.24). Most are subalkaline, silica-

saturated to silica-oversaturated. Rock types typically

include basalt, basaltic andesite, andesite, dacite, and

rhyolite. A large spectrum of K

O concentration is sub-

divided into four rock series in Figures 2.18 and 13.24,

including low-K (tholeiitic), medium- and high-K (calc-

alkaline), and shoshonitic. High-K and shoshonitic

rocks may be alkaline and silica-undersaturated. Com-

plex variations along and across strike and through

time within a single arc as well as variations between

arcs preclude accurate generalizations on a global

scale. Nonetheless, a few tendencies can be cited, as

follows.

Low-K (tholeiitic) arc rocks are mostly basalt,

basaltic andesite, and lesser amounts of andesite, rarely

dacite and rhyolite, that form low mounds and shields

around near-trench vents, chieﬂy in young arcs

founded on thin crust. Lava ﬂows are commonly

which have thicker crust of sialic rather than maﬁc

composition.

Island arcs built on oceanic crust (Plate I) can be

viewed as embryonic continents that grow and mature

as the crustal thickness and area increase over millions

of years with continued magmatic input. Although

rarely exposed, intrusive rocks make up part of these

volcanic ediﬁces. Island arcs are linear groups of volca-

noes several thousands of kilometers long and no more

than 200–300 km wide. The width decreases with in-

creasing dip of the subducting slab. They are bordered

on one side by an ocean trench, as much as 11 km

deep, that is the topographic expression of subducting

lithosphere (Figures 11.16 and 13.17). Scraped-off

ocean crust forms an accretionary prism (or forearc ac-

creted terrane) on the edge of the overriding plate. The

trenchward limit of volcanoes—the volcanic front—is

50–300 km from the trench, depending on slab dip,

but is consistently about 110 km above the top of the

subducting slab (Tatsumi and Eggins, 1995), as deﬁned

by earthquake foci. The volume of extruded magma

decreases away from the volcanic front. In some arcs, a

spreading back arc or marginal basin (Figure 11.16)

lies on the opposite side from the trench.

13.5.1 Rock Associations

Whereas basalt is the typical rock of the ocean ﬂoor, is-

land chains, and plateaus, the most common rock of

convergent plate settings, whether island or continental

aphyric, especially where more maﬁc, but phenocrysts

include olivine, calcic clinopyroxene and plagioclase,

locally Fe-Ti oxides, and subcalcic clinopyroxene (pi-

geonite). In addition, these rocks usually have high

Fe/Mg ratios that justify their designation as tholeiitic

(Figure 2.17).

Medium- and high-K arc rocks of the calc-alkaline

series are chieﬂy porphyritic andesites that contain

phenocrysts of calcic plagioclase, ortho- and clinopy-

roxene, Fe-Ti oxides, and commonly minor amounts

of hornblende and biotite. Basalt, basaltic andesite,

dacite, and rhyolite are generally minor rock types; the

latter two contain the same phenocrysts as andesites,

plus quartz and sanidine in the high-K series. More sili-

cic viscous lavas form steep-sided ediﬁces, including

composite volcanoes, and the greater water content of

Magmatic Petrotectonic Associations

373

Table 13.6. Chemical Compositions of Island Arc Volcanic Rocks

123 4 5 6 7 8 9

SiO

49.49 53.29 65.78 60.48 53.07 58.74 63.89 49.68 48.07

TiO

0.68 0.91 0.65 0.81 0.73 0.23 0.61 0.85 0.73

18.71 17.13 15.53 16.30 17.50 11.22 17.40 19.39 15.64

2.75 3.47 1.63 7.90t 9.30t 2.34

FeO 7.14 7.83 3.39 8.53t 4.21t 9.33t 6.98

MnO 0.18 0.20 0.13 0.19 0.17 0.17 0.08 0.14 0.15

MgO 6.24 4.72 1.76 2.57 3.72 12.09 2.47 5.60 12.68

CaO 12.76 9.87 4.71 6.35 7.76 6.77 5.23 10.97 10.76

O 1.73 2.21 3.82 3.94 3.46 1.69 4.40 2.66 2.14

O 0.28 0.30 0.44 1.25 3.93 0.47 1.52 0.82 0.48

0.05 0.06 0.13 0.21 0.56 0.09 0.19 0.12

Total 100.00 100.00 97.97 100.00 100.20 100.00 100.00 99.44 100.099

Mg/(MgFe) 0.57 0.47 0.44 0.56 0.61 0.72 0.51 0.52 0.74

Cs 0.71 0.49 1.19 0.52 0.06

Rb 5.6 4.9 6.2 18 75 11 30

Ba 94 111 253 419 684 43 485 261 241

Th 0.39 0.36 1.27 1.2 0.40 3.52 1.33 1.42

U 0.49 0.5 0.19 0.99 0.54 0.70

Nb 0.53 0.76 2.4 1.8 2.2 8.3

Ta 0.15 0.07 0.53 0.19 0.09

La 2.3 2.3 39.7 8.8 2.07 17.55 5.99 7.30

Ce 5.7 6.2 31.2 21 4.58 34.65 13.86 18.15

Sr 237 174 184 352 1545 100 869

Nd 27.2 14 2.67 20.14 10.66 11.38

Sm 24 3.8 0.75 3.15 2.77 2.69

Zr 29 39 72 111 71 32 117

Eu 21.1 1.2 0.28 0.97 0.911 0.834

Gd 22.2 1.06 2.25

Dy 18.1 1.35 1.43

Y 13 21 35 26 6.1 9.5

Yb 15 2.5 0.89 0.91 1.91 1.52

Lu 15 0.15 0.15 0.259 0.223

Cr 66 27 22 9 969 54 32 51

Ni 33 16 8 7 223 39 22 272

1, Low-K (tholeiitic) basalt, Kermadec arc. Data from Ewart and Hawkesworth (1987).

2, Low-K (tholeiitic) basaltic andesite, Kermadec arc. Data from Ewart and Hawkesworth (1987).

3, Low-K (tholeiitic) dacite, Kermadec arc. Data from Cole (1982).

4, Medium-K (calc-alkaline) andesite, Mariana arc. Data from Woodhead (1989).

5, Shoshonite, Tavua, Fiji, subaerial Lau Ridge back arc behind Tonga arc. Data from Gill and Whelan (1989).

6, Boninite, average of 134 analyses. Data from Drummond et al. (1996).

7, Adakite, average of 140 analyses. Data from Drummond et al. (1996).

8, High-Al, low-Mg basalt, Aleutian arc. Data from Kay and Kay (1994).

9, High-Mg, low-Al basalt, Aleutian arc. Data from Kay and Kay (1994).

the magmas promotes explosive eruptions, forming

pyroclastic deposits. Calc-alkaline rocks are typical in

more mature arcs and in extrusions farther from the

trench. This tendency implies that thicker crust favors

development of calc-alkaline magmas that are mostly

more evolved with higher alkalies and silica (Section

12.5.2). Apparently where the oceanic crust is still thin,

not much more than average in the open ocean (6–7

km), basaltic magmas can rise and readily extrude. As

the crust thickens from the top by the basaltic extru-

sions and from within by intrusive underplating, man-

tle-derived magmas are more likely to stagnate within

the crust, where they differentiate, mainly by crystal

fractionation, becoming more silicic, potassic, and hy-

drous.

Most arcs, especially the Aleutians, have low- to

medium-K basalts that are highly porphyritic, con-

taining 25–40% phenocrysts of calcic plagioclase,

olivine, and clinopyroxene. Chemically, these high-Al

basalts have 17 wt.% to as much as 22 wt.% Al

and low MgO (6 wt.%) and compatible elements

(Table 13.6). Less abundant high-MgO (9 wt.%)

basalts generally contain only olivine phenocrysts and

have high Ni and Cr but lower Al

Shoshonitic series rocks (Figure 2.18), including ab-

sarokite (essentially a very-high-K basalt), shoshonite

(very-high-K basaltic andesite), and banakite (very-

high-K andesite), are less common than the other rock

series and tend to form in mature arcs that have thick

crust, but some have been found to be related to back-

arc rifting. Plagioclase and two pyroxenes are typical

phenocrysts in an alkali-rich groundmass.

Two less common rock types (Table 13.6) are de-

ﬁned independently of K

O-Si

O relations. Boninite is

a high-MgO (8 wt.%), low-TiO

(0.5 wt.%), and

very incompatible-element-depleted basaltic andesite

or andesite belonging to the low- or medium-K series.

It is commonly glassy, and typical phenocrysts in-

clude both orthorhombic and monoclinic enstatites

(En

), Mg-rich olivine (Fo

), and calcic clinopy-

roxene (Crawford, 1989). Adakite is a dacite-andesite

rock that has high La/Yb ratios relative to those of the

more widespread calc-alkaline rocks (Figure 11.19).

13.5.2 Magma Evolution

The low MgO, Ni, Cr, and Mg/(Mg  Fe) content of

most arc rocks (Table 13.6) indicates that magmas are

not primary and have experienced extensive differenti-

ation after leaving their source. Their evolved compo-

sitions can be appreciated in a CaO-MgO diagram

(Figure 13.25), which shows that only the most Mg-

rich arc rocks might represent relatively primitive mag-

mas generated from hydrous peridotite in the subarc

mantle wedge. The bend in the CaO-MgO variation

trend of arc rocks at about 12 wt.% MgO likely reﬂects

a transition from a fractionating assemblage dominated

374 Igneous and Metamorphic Petrology

Basalt

Basaltic

andesite

Andesite Dacite

45 49 53 57 61 65 69

SiO

(wt. %)

O (wt. %)

Shoshonitic

High-K

(calc-alkaline)

Medium K

(calc alkaline)

Low-K

(tholeiitic)

13.24 Volcanic rocks of the Mariana oceanic arc. Numbers 1–9 are analyses from Table 13.6. Compare with volcanic rocks in the Tonga oceanic

arc that are low-K tholeiitic (Figure 2.18). (Data from Woodhead, 1989 and Bloomer et al., 1989.)

by olivine  orthopyroxene that operates on more

primitive magmas to fractionation of mostly clinopy-

roxene  plagioclase at lower temperatures on previ-

ously olivine fractionated magmas.

Widespread, though generally minor, hornblende

and biotite in arc rocks indicate that magmas contain

at least 3 wt.% or so H

O, implying that this volatile is

an essential component in the evolution of arc mag-

mas. High water fugacity expands the stability ﬁeld

of olivine and reduces plagioclase stability so that Mg,

Fe, Ni, and Cr are reduced and Na and K increased

in residual melts. These factors promote development

of the felsic-rich calc-alkaline trend, rather than the

Skaergaard trend of increasing Fe (Section 12.5.2).

Primitive magmas ascending into more mature arc

crust can also be contaminated by felsic rock, further

accentuating the calc-alkaline tendency.

Differential solubility of incompatible elements in

slab-derived hydrous ﬂuids is believed to produce the

typical spiky arc signature in normalized trace element

diagrams (Figure 11.19).

Boninite and adakite magmas are generated in

young arcs overriding young hot subducting litho-

sphere. Boninite is believed to be generated by partial

melting of hydrated harzburgite in the mantle wedge

and adakite by partial melting of hydrated basaltic

crust in the descending slab (Section 11.4.2).

The origin of the high-Al basalts, whether by partial

melting of the subducted oceanic crust or by fractiona-

tion from parental high-MgO basalt magmas, remains

highly controversial, and both may be factors. High-

MgO basaltic magmas, which may be parental to many

arc rocks, cannot be derived from subducted basaltic

crust and their origin remains problematic.

13.5.3 Back-Arc Basins

Back-arc or marginal basins are areas of spreading

oceanic crust adjacent to many island arcs on the

opposite side from the deep trench (Figure 11.16).

They are especially common in the western Paciﬁc,

such as the Lau Basin behind (west of) the Tonga island

arc (Hawkins, 1995). Crustal extension, conﬁrmed by

Magmatic Petrotectonic Associations

375

Plagioclase

control

Clinopyroxene

20%

30%

Ol control

olivine

Opx control

ISLAND ARC

VOLCANIC

ROCKS

Cpx

Contamination

CENTRAL VOLCANIC

ZONE, ANDES MTNS.

Orthopyroxene

MgO (wt. %)

CaO (wt. %)

13.25 Compositions of island arc volcanic rocks from the west Paciﬁc and Lesser Antilles in the Caribbean (light shading). For comparison,

continental arc rocks from the central Andes are shown in dark shading. Compositions of fractionating crystals indicated by diagonally

ruled boxes with representative control lines. More Mg-rich magmas probably experienced olivine fractionation; less Mg-rich magmas

fractionated by clinopyroxene  plagioclase removal. Filled circles are experimentally produced partial melts (18–38% melt fraction) at

10 kbar and 1200–1350°C of hydrous lherzolite (Hirose and Kawamoto, 1995). Most arc rocks solidify from evolved magmas that have

experienced considerable differentiation, including fractionation, contamination with felsic crustal rocks, and magma mixing. (Redrawn

from Davidson, 1996.)

symmetric linear magnetic anomalies, is paradoxical

and controversial; it may be caused by some sort of

counterﬂow in the subarc mantle wedge or “roll back”

of a gravitationally sinking slab within the overall con-

vergent plate regime. Basalts forming new ocean ﬂoor

along back-arc spreading ridges resemble MORB but

have distinct arc afﬁnities, such as negative Nb-Ta

anomalies and elevated water content. Somehow, aque-

ous ﬂuids liberated from the subducting slab are trans-

ferred into the MORB-like magma source beneath the

back-arc spreading axis.

13.6 OPHIOLITE

Accreted terranes of oceanic rocks emplaced along

margins of continental and oceanic plates overriding

subducting slabs (Figures 13.17 and 11.16) commonly

include a distinctive sequence of rocks called ophiolite.

Most of the hundreds of ophiolite sequences recog-

nized worldwide are dismembered to varying degrees

so that only parts are exposed. Unusually well-exposed,

complete ophiolites include the Semail along the

northeast coast of the Arabian Peninsula in the Sul-

tanate of Oman (see Figure 18.25; Searle and Cox,

1999), the Bay of Islands in western Newfoundland,

and the nearly complete Troodos ophiolite on the is-

land of Cyprus in the Mediterranean (Moores, 1982).

Contact thermal metamorphic effects on country

rocks are limited or absent, even though a high-T

magma is implied by the high content of Mg-rich

olivine in the ultramaﬁc members of the ophiolite se-

quence. Instead, contacts between the ophiolite pack-

age and surrounding rocks, as well as internal lithologic

contacts, are tectonic, marked by breccia and slicken-

sides; hence, ophiolites appear to have been emplaced

as subsolidus masses, commonly along fault zones.

13.6.1 Characteristics

Ophiolite is a distinctive sequence of magmatic,

sedimentary, and metamorphic rocks formed in an

oceanic environment and made up of the oceanic crust

and uppermost mantle rocks (Figure 13.1a; see also

Moores, 1982). (Ophiolite is from the Greek, ophite,

referring to a serpent, in allusion to the widespread

scaly green serpentine in the sequence.) Always vari-

ably deformed, recrystallized, and hydrated, a com-

plete ophiolite sequence consists of the following, from

the top down:

1. Marine sedimentary rock: Thinly layered (centime-

ters-scale) Fe-Mn-rich chert and shale are com-

mon, but deep ocean (pelagic) red limestone occurs

in some. In many ophiolites, volcaniclastic depos-

its intercalated with turbidite sequences indicate

nearby explosive volcanism and development of

deep-sea fans essentially contemporaneous with

underlying magmatic rocks; such deposits are typi-

cal of island arcs rather than the open-ocean, in-

traplate environment where chert, shale, and lime-

stone form. Moores (1982) argues that the uncon-

ventional inclusion of the sedimentary component

in the deﬁnition of ophiolite provides a geologic

criterion for deciding the oceanic environment in

which ophiolites are created.

2. Extrusive magmatic rock, chieﬂy basaltic: Pillow

lavas predominate, but sheet ﬂows and breccias are

common. Apparently arc-related volcanic debris

ﬂows and silicic lavas are found in some ophiolites.

Sills are locally common, as are dikes, which in-

crease downward.

3. Sheeted dike complex: Dikes mostly of basalt

and slightly coarser diabase (dolerite) are generally

1–3 m thick. In the Oman ophiolite, dikes have a

uniform strike over an exposed distance of 400 km!

Dikes intruded into dikes, without any other wall

rock, are convincing proof of formation in actively

extending crust. Anastamosing zones of brecciation

on scales from microscopic to hand sample are

widespread. Their irregular disposition is incom-

patible with a tectonic origin and suggests instead

explosive shattering due to advective entry of cold

seawater into the hot dikes.

4. Massive (isotropic) gabbro: Below the depth of ad-

vective water penetration, magma intrusions cool

more slowly by conduction and convection and so-

lidify in the extending crust by plating crystals onto

the walls, forming bodies of gabbro with isotropic

fabric. Amphibole in local dioritic rocks testiﬁes

to high concentrations of water in the fraction-

ated tops of crustal magma chambers. More felsic

differentiates, which occur as irregularly shaped

masses in diorite and gabbro and as thin dikes in-

truded into basalt, constitute 5%–10% of the plu-

tonic part of ophiolite. These plagiogranite (also

called albite granite, trondhjemite, or granophyre)

differentiates consist of granophyric aggregates of

quartz and strongly zoned oligoclase-andesine

(Table 13.7); K-rich feldspar is notably absent and

minor primary maﬁc minerals are altered to chlorite

and actinolite.

5. Layered ultramaﬁc-maﬁc cumulates: These are ac-

cumulations of fractionated crystals on the ﬂoors of

gabbroic magma chambers. Olivine and pyroxene

cumulates (dunite, peridotite) occur at the base,

succeeded upward by Ol  Cpx  Pl cumulates

(gabbro). Cyclic mineral and phase layering is com-

mon. A general lack of intrusive contacts within the

gabbroic and ultramaﬁc cumulate parts of ophio-

lites may be created by intermittent recharge of

primitive magma into crystallizing magma before

complete solidiﬁcation occurred in the actively ex-

tending oceanic crust.

6. Deformed (“tectonized”) peridotite: This meta-

morphic-textured mantle rock below the magmatic

376 Igneous and Metamorphic Petrology