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cumulates, locally in sharp contact with them, is

variably depleted in basaltic components. The de-

formed peridotite ranges from lherzolite to

harzburgite to dunite with increasing degrees of de-

pletion. This component of ophiolite is generally

the most prominent, in some locales cropping out

over thousands of square kilometers. Because of

their occurrence in orogenic belts, such as the Alps

(Figures 13.26 and 18.24), they have been referred

to as alpine peridotite. They are variably hydrated,

or serpentinized, form serpentinite rock, and show

effects of pervasive, grain-scale deformation.

13.6.2 Origin and Emplacement

The initial formation of ophiolite in an oceanic exten-

sional environment is unquestioned. But, in what

kind—ocean ridge, spreading back-arc basin, or possi-

bly an early extensional episode in island arc evolution?

The overwhelming extent of ocean ridges worldwide

no doubt led to the early opinion in the 1960s that they

were the environment from which ophiolites formed.

However, in the mid-1970s A. Miyashiro noted that

some rocks in the classic Troodos ophiolite have arclike

chemical afﬁnities rather than normal MORB charac-

ter. Criticism was immediately raised that the wide-

spread and locally intense metamorphism likely invali-

dated any conclusions based on chemical composition

of the rocks. Nonetheless, fresh volcanic glass subse-

quently found in the Troodos has arc attributes, and

rocks from other ophiolites have also been found to

have typical arc signatures in relatively immobile ele-

ments Th, Nb, Ta, and REEs.

A dwindling proportion of ophiolites, if any, ap-

pears to be of ocean ridge origin. This has signiﬁcant

implications for reconstructions of continental evolu-

tion by accretion of oceanic terranes.

The second basic question regarding ophiolites is

how variably dismembered slices of dense oceanic

lithosphere are accreted (tectonically emplaced) onto

margins of less dense continents and onto island arcs in

subduction zones, rather than being subducted. In one

possible model, oceanic mantle lithosphere overlying

subducting slabs is more buoyant because of wide-

spread serpentinization from released water, compared

to dry ocean-ridge mantle. (Density of olivine Fo

3.3 g/cm

versus serpentine 2.6 g/cm

.) Testifying to

the buoyancy of hydrated ultramaﬁc rock are serpenti-

nite diapirs that have breached the ocean ﬂoor to cre-

ate seamounts in the Mariana forearc (O’Hanley, 1996,

p. 229). If ophiolites are emplaced within several mil-

lion years of their creation, as some chronologic data

seem to indicate, then the still relatively hot young

oceanic lithosphere would have additional buoyancy.

Lithosphere made buoyant by its youth and serpen-

tinization may allow nearby converging continental

margins of approximately similar, or slightly greater,

density (about 2.7 g/cm

) to be underthrust. Cool,

nonhydrated, ocean-ridge lithosphere may only be very

locally scraped off, if at all. (See Sections 18.5 and 18.6.)

13.7 CALC-ALKALINE CONTINENTAL

MARGIN MAGMATIC ARCS

During much of the Mesozoic and Cenozoic, the west-

ern coasts of the American continents were sites of arc

magmatism as oceanic lithosphere subducted beneath

them. Arc magmatism is also found on smaller conti-

nents, such as New Zealand and Japan, and islands in

the Caribbean and Mediterranean.

The same rock types (basalt to rhyolite) and rock

suites (low-, medium-, and high-K and shoshonitic) can

be found in both island and continental arcs. How-

ever, in continental arcs there is a greater proportion

of silicic calc-alkaline rocks—dacite, rhyolite (Figure

12.26b), and their plutonic granodiorite and granite

counterparts. Quartz and alkali feldspar are wide-

spread in silicic continental rocks but rare in island arc

rocks. Vast sheets of silicic ignimbrite and huge com-

posite granitic batholiths are unique to continents.

Concentrations of large-ion lithophile elements, in-

cluding K, Rb, Ba, Th, and U, are greater in conti-

nental rocks at a given silica content. In addition,

they generally have higher

Sr/

Sr and lower

143

Nd/

144

Nd ratios (Figure 12.31). These compositional con-

trasts are basically a result of the thicker and more

sialic crust in continental arcs into which primitive

mantle-derived magmas are intruded and experience

differentiation.

Magmatic Petrotectonic Associations

377

Table 13.7. Composition of Plagiogranite from the

Bay of Islands Ophiolite, Newfoundland

SiO

76.99 Co 4.4

TiO

0.13 Cr 43

13.07 Ni 16

1.12t Sc 2.06

MnO 0.01 Ba 92

MgO 0.39 Th 4.67

CaO 1.99 Ta 0.90

O 5.53 La 21

O 0.12 Ce 47.3

0.00 Sr 82

Total 99.35 Sm 6.79

Zr 182

Hf 7.43

Eu 1.12

Tb 1.24

Y62

Yb 6.89

Lu 1.04

Data from Elthon (1991).

378 Igneous and Metamorphic Petrology

miles

0 400

0 250

miles

0 400

0 250

New

York

City

Atlantic Ocean

St.

John's

New-

foundland

MOUNTAINS

APPALACHIAN

Montreal

Mediterranean

Rome

Sea

Sarajevo

Skopje

HELLENIDES

Athens

DINARDES

Vienna

Zurich

Milan

ALPS

13.26 Ophiolite in the northern Mediterranean and Appalachian regions. Ophiolite shown in black and areas containing ophiolite or deep

oceanic sedimentary rock are stippled. (Redrawn from Moores, 1982.)

13.7.1 Volcanic Arcs on Continental Margins

Four areas in the circum-Paciﬁc provide a glimpse of

the nature and the variety of continental volcanic arcs.

North Island of New Zealand. This continental seg-

ment of the Tonga–Kermadec–New Zealand arc in

the southwest Paciﬁc Ocean was previously cited in

Section 12.5.1 to demonstrate the contrast between

oceanic island and continental arc associations. Most of

the volcanic rocks in the Taupo volcanic zone of the

North Island of New Zealand (Figure 13.27) are calc-

alkaline andesite and rhyolite; basalt and dacite are

rare. The volcanic zone is a complex volcano-tectonic

depression created by crustal extension and ﬁlled with

as much as 3 km of volcanic rock. Eruption of poly-

genetic andesite magmas since about 2 Ma continues

today at composite volcanoes north and south of the

central rhyolite ignimbrite ﬁeld. This is the most fre-

quently active and productive Quaternary rhyolite ﬁeld

on Earth, where an estimated 16,000 km

of rhyolite

magma has been explosively erupted in the past 1.6 My

in 34 caldera-forming ignimbrite eruptions whose vol-

umes range from 30 to 300 km

(Graham et al.,

1995).

Andes of Western South America. This is the classic

example of continental arc magmatism, consisting of

thousands of Mesozoic-Cenozoic volcanoes as well as

local deeply eroded plutonic roots. The overall rela-

tively shallow dip (30°) of the subducting oceanic

lithosphere along the arc is attributed to its relatively

rapid rate of convergence, 10 cm/y, and youthful

buoyancy. Numerous contrasts along the arc are sig-

niﬁcant (Figure 13.28). Active Quaternary volcanoes

are concentrated in three segments—northern, central,

and southern—of differing crustal thickness, age, and

composition that lie above lithosphere subducting

at angles of 20°–30°. Between these three segments,

along-arc ﬂexures in the subducting slab allow dips

of only 5°–10°. These nearly horizontally subducting

slabs appear to be buoyed up by thick aseismic ocean

ridges, causing uplift of the overlying crust and ero-

sional unrooﬁng of older plutonic rocks. Because there

is little or no subarc mantle wedge that could otherwise

generate magma, no active volcanism occurs.

Virtually the whole spectrum of volcanic ediﬁces

(Chapter 10) are developed along the Andean arc, but

andesitic composite volcanoes dominate. Single vol-

canic debris avalanches cover as much as 100 km

Magmatic Petrotectonic Associations

379

However, in the central volcanic zone, where the crust

is oldest (includes Precambrian metamorphic rocks)

and thickest (as much as 70 km), late Cenozoic rocks

are predominantly silicic ignimbrites exposed over an

area of about 5  10

. The topographically high

central Andes, which include the Altiplano-Puna

(somewhat smaller than the Tibetan plateau in central

Asia), is exceptionally arid, barren, and unpopulated.

Consequently, very little is known of this remote cen-

tral region, other than from a few reconnaisance stud-

ies (see de Silva and Francis, 1991). Huge resurgent

calderas, as much as 35  60 km, are related to volu-

minous (1000 km

) dacite ash-ﬂow eruptions appar-

ently spawned by crustal thickening and maﬁc magma

underplating since the late Miocene.

Southwestern North America Ignimbrite Pr

ovinces.

remarkable facet of Cenozoic volcanism in southwest-

ern North America was an ignimbrite ﬂareup during

the Oligocene (Best et al., 1989). In 10 My, at least

of ignimbrite blanketed parts of Nevada,

Utah, Colorado, Arizona, New Mexico, and Mexico

White Is.

Indian

plate

Auck-

land

Well-

ington

miles

TAUPO

VOLCANIC

1.6 −

0.9

ignimbrite

<0.3

Whale

Island

ZONE

Bay of

Plenty

Silicic dome

38°S

39°S

Rhyolite

<0.9

<0.2

<0.3

Lake

Taupo

Rhyolite ignimbrite

Pacific plate

mm/y

Basalt

Caldera

Composite

andesite volcanoes

13.27 Taupo volcanic zone (shaded) on the North Island of New Zealand. See also Figure 12.26a. White and Whale Islands are active andesite

composite volcanoes in the Bay of Plenty northeast of the caldera-ignimbrite ﬁeld. Exposed metamorphosed sedimentary rocks are stip-

pled on small-scale map lower right. Time of caldera-forming eruptions indicated in millions of years (e.g., 0.3, 1.6–0.9). (Redrawn from

Graham et al., 1995.)

380 Igneous and Metamorphic Petrology

(Figures 13.29 and 13.30). Although most of the hun-

dreds of caldera-forming ash-ﬂow eruptions that cre-

ated this ignimbrite blanket were of rhyolite magma

derived from compositionally zoned crustal chambers,

the very largest were of crystal-rich dacite magma. Af-

ter disgorging thousands of cubic kilometers of dacite

ejecta to depths of as much as 500 m, huge, 50–70 km-

diameter calderas formed above probably slablike

chambers as similar volumes of ejecta continued to be

erupted into the subsiding depression.

It may be surprising that subduction-related ig-

nimbrites having typical arc trace element signatures

occur so far inland from the North American plate

margin, as far as southwestern Colorado. However,

east-west crustal extension since the ignimbrite ﬂareup

has increased this distance by perhaps 200–300 km in

13.28 Western South America magmatic arc. Coastal Peruvian batholith (black) is Cretaceous next to coast and Cenozoic inland. Triangles,

Holocene (active) volcanoes. Heavy dashed line, position of 150 km-depth contour on top of Wadati-Benioff seismic zone showing two

segments of arc where slab subducts only 5–10°, precluding active volcanism. Shaded, area of average elevation 3 km and to as much

as about 7 km, including Altiplano in Bolivia and southern Peru and Puna in northern Chile and Argentina. The notes along the right

side contrast the three active volcanic arc segments. S, shoshonitic volcanic centers. (Redrawn from Wilson, 1989; Jordan et al., 1983.)

0 800

0 500

miles

40° S

30°

20°

10° S

10° N

Ecuador

90° W

70° W

Juan Fernandez

Ridge

∼ 10 cm/y

Nazca Ridge

Trench

Bolivia

150 km

Argentina

Peru

Columbia

90° W

Batholith

Chile

50° S

Volcanic zone

Crustal thickness (km)

Age of crust

Dominant

volcanic rocks

Southern

30−35

Meso-Cenozoic

Basalt ± andesite

Central

50−70

Precambrian-Paleozoic

Andesite + dacite +

Northern

Late Meso-Cenozoic

Andesite

1.4−2.2

6.3−7.7

0.7036−0.7046

O (wt.%)

Sr/

0.4−2.8

5.2−6.8

0.7037−0.7044

± dacite

rhyolite

1.4−5.4

6.8−14.0

0.7054−0.7149

13.29 Some major Cenozoic volcanic provinces in the southwestern part of the North America plate and current plate boundaries. Cenozoic

volcanic ﬁelds to the north of this map are shown in Figure 13.31. Plate conﬁguration at 30 Ma in small-scale inset lower left shows that

a subduction zone extended continuously along the western margin of North America, and the eastern edge of the “viable” subducting

slab (approximately located by dotted line) extended far inland. (Redrawn from Severinghaus and Atwater, 1990.) In most of California

this convergent plate boundary has now been replaced by the San Andreas transform fault. Shaded areas are major ignimbrite ﬁelds em-

placed during the middle Tertiary. Hundreds of source calderas in these ash-ﬂow ﬁelds are not shown. Extent of the Sierra Madre Occi-

dental ignimbrite plateau in Mexico (Figure 13.30) is highly generalized because so little is known of this region. A thin veneer of basalt

ﬂows erupted 17 Ma in the Snake River Plain covers thick rhyolite lava and ash ﬂows that emerge from beneath the veneer at currently

active Yellowstone (Y) (Hildreth et al., 1991). Rhyolite calderas monotonically decrease in age to the northeast, possibly because of the

southwestward passage of the continental plate over a mantle plume, the head of which may have created the Columbia River Plateau

off the northwest corner of this map (Figure 13.31). Late Cenozoic normal faults (lines ornamented with ticks) have formed rift grabens

in the western part of the Mexican Volcanic Belt.

0 miles 200

0 km 300

115°

20° Ν

25°

30°

120°

Los

Angeles

San

Fran-

cisco

PACIFIC

PLATE

San Andreas

Fault

Sierra Nevada

Batholith

Basin

Range

Phoenix

Arizona

Nevada

California

120° 110° W

Snake

Plain

Idaho

Utah

Wyoming

Salt Lake

City

Colorado

Denver

San

Juan

New

Mexico

Mogollon

Datil

Texas

35°

110°

Mazatlan

Guadalajara

Mexican volcanic belt

Mexico

City

COCOS PLATE

100° W

Sierra

Madre Occidental

RIVERA

PLATE

25° Ν

MEXICO

and

Northern

382 Igneous and Metamorphic Petrology

places. Also, plate reconstructions for the Tertiary of

western North America disclose a rapidly converging,

northeastward-moving, low-dip oceanic lithosphere

that advanced far inland (Figure 13.29, inset). This

low-dip slab is reminiscent of the post-late Miocene

character of the central Andes. Indeed, the composi-

tions of Oligocene volcanic rocks in the northern Basin

and Range province are most like those of the central

Andes of any continental arcs and suggest that this

Oligocene ignimbrite province was a high plateau un-

derlain by crust thickened during Mesozoic compres-

sional orogeny and probably by middle Tertiary maﬁc

underplating.

Cascade Range. The linear arc of composite Qua-

ternary volcanoes from southern British Columbia

through Washington and Oregon and into northern

California (Figure 13.31) is one of the better known ac-

tive volcanic arcs in the world. Included in the arc are,

from north to south, the recently active Mount Saint

Helens (Figures 10.1 and 10.20), Crater Lake and an-

cestral Mount Mazama (Figure 10.38), Mount Shasta

(Figure 10.2), and Lassen Peak (Figure 10.18). Several

attributes of the Cascades should dispel any notions

that this arc is like all others and that all continental

margin volcanic arcs are the same.

The subducting Juan de Fuca oceanic plate beneath

the arc is relatively thin and warm because the ocean

spreading ridge is near the trench. Volumetrically mi-

nor composite Quaternary volcanoes have been built

within the past 1 My on a platform of older basaltic and

andesitic rocks produced by more diffuse and inter-

mittent volcanism during the Cenozoic. Deep erosion

has exposed the plutonic roots of some of these magma

systems (Figure 9.5).

While focused magma systems were creating

andesite-dacite composite volcanoes, numerous sur-

rounding vents were erupting mainly basaltic lavas,

forming monogenetic cinder cones, lava ﬂows, and

shields. Many of these basaltic lavas in southern Wash-

ington (Leeman et al., 1990) are primitive, ranging

from calc-alkaline through tholeiitic to alkaline. Signif-

icantly, only a few have typical arc signatures. Instead,

most have trace element and isotopic compositions of

oceanic island basalts, a feature noted in some rocks of

other arcs. In the Cascades, the subducting young and

still warm Juan de Fuca slab may have been virtually

dehydrated before reaching conditions at which typical

arc magmas could be generated in the subarc mantle

wedge. Mount Saint Helens, for example, has erupted

adakite magmas generated in the basaltic subducted

slab. This postulated “dry” wedge, together with con-

tamination by accreted Mesozoic and early Cenozoic

oceanic and island-arc lithosphere, may explain the di-

verse and primitive basalt magma compositions. Qua-

ternary composite volcanoes at the northern and par-

ticularly the southern end of the Cascades have a

tendency to erupt magma of more silicic composition.

This may reﬂect their construction on older continen-

tal crust, as opposed to the more oceanic character of

the young maﬁc crust beneath the central Cascades.

13.7.2 Plutonic Arcs on Continental Margins:

Granitic Batholiths

Felsic plutonic and volcanic rock-forming systems were

once considered to be genetically separate. However,

many observations and arguments clearly indicate that

volcanoes have plutonic roots; however, the opposite is

not necessarily true because not all intrusive magma

vents to the surface. The volcanic-plutonic connection

follows from the reasonable inference that erupting

magma chambers do not completely empty themselves

and is conﬁrmed by the continuities in exposure between

volcanic and plutonic bodies at some locales (e.g., Figure

9.5; also Figure 13.36). Along strike of the Cascade vol-

canic arc (Figure 13.31) and the Andean volcanic arc

(Figure 13.28) locally deep erosion has stripped off the

volcanic cover, exposing plutons of much the same age

and compositions as the volcanic rocks.

Batholiths (Section 9.4.2) made predominantly of

tonalite and granodiorite, but ranging from granite to

gabbro, are a typical feature of every orogenic belt as-

sociated with subduction of oceanic lithosphere be-

neath a continental margin. In the western North

American Cordilleran orogen (Figure 9.16), these

mostly Mesozoic batholiths include the Coast Range,

Idaho, Sierra Nevada, and Southern California–Penin-

sular Ranges. Additional Mesozoic-Cenozoic bath-

oliths occur along the west coasts of Central and South

America (Kay and Rapela, 1990).

Sierra Nevada Batholith, California. As no two batho-

liths are exactly the same, any description of one omits

13.30 Panoramic view of the Sierra Madre Occidental ignimbrite

plateau east of Mazatlan, Mexico. The entire rock sequence is

rhyolitic ignimbrite; individual layers are as much as hundreds

of meters thick. (Photograph courtesy of Gary J. Axen.)

Magmatic Petrotectonic Associations

383

features of others. However, because the central one-

fourth of the Sierra Nevada Batholith (Bateman, 1992)

is well-documented, it will be used as an example. Em-

placement of the approximately 120 separate mapped

plutons began in the late Triassic at about 210 Ma and

continued episodically into the late Cretaceous until

about 85 Ma (Figure 13.32). For uncertain reason,

much of the topographic uplift of this mountain range

did not occur until the late Cenozoic. Pluton host rocks

are strongly deformed and mildly metamorphosed

Paleozoic-Mesozoic volcanic and sedimentary rocks; in

the west they include accreted oceanic terranes.

Most plutons in the Sierra are tonalite, granodiorite,

and granite (Figures 2.6 and 12.30). Among maﬁc

minerals, biotite and Fe-Ti oxides are ubiquitous;

hornblende and titanite (sphene) are common. Many

plutons are composite or zoned in fabric and/or com-

position and constitute comagmatic suites. Zones can

be gradational from one to another or in sharp contact

with one another, implying some cooling before the

next intrusion of magma. The Tuolumne Intrusive Se-

ries (Figure 9.18; Table 13.8) is a classic example of a

concentrically zoned pluton that has zones of younger

and progressively more felsic rock that crystallized at

lower T inward from the pluton margin. This comag-

matic sequence, which appears to have been emplaced

within a few million years near 87 Ma, spans the en-

tire spectrum of rock types constituting the whole

batholith, with the exception of the most maﬁc rock

types; hence, understanding its origin can provide

insight into the evolution of the entire Mesozoic

batholith.

Competing models for zoned plutons include the

following:

1. Contamination of the outermost part of the magma

body with maﬁc wall rocks

2. Multiple intrusion

3. Convective melt fractionation driven by sidewall

crystallization (e.g., Bateman and Nokleberg, 1978;

Sawka et al., 1990; Bateman, 1992, pp. 67–82; see

also Section 12.2.1)

This model has received widespread acceptance as

the probable mechanism of differentiation of

13.31 Selected Cenozoic magmatic rocks of southern British Columbia, Washington, Oregon, Idaho, and northern California. Juan de Fuca

oceanic plate subducts beneath the continental margin. For clarity, names of most Quaternary composite volcanoes are placed off coast.

Hundreds of known exposed feeder dikes in Columbia River Plateau are schematic.

Mt.

Garibaldi

British Columbia

Washington Idaho

Mt.

Adams

Newberry

volcano

AMERICAN

Oregon

PLATE

miles

1000

1600

California

Mt. Shasta

Lassen Peak

40° N

Ocean

PACIFIC

PLATE

Mt. Jefferson

Mt. Hood

Mt. St. Helens

Mt. Rainier

Glacier Peak

Mt. Baker

Three Sisters

130° 120° W

45°

Columbia River flood

basalt plateau

Tertiary diorite and

granodiorite plutons

Quaternary composite

volcano

Quaternary andesite

and basalt

NORTH

Crater

Lake

JUAN DE FUCA

PLATE

Pacific

384 Igneous and Metamorphic Petrology

andesite-dacite-rhyolite magma chambers that erupt

well before total crystallization to produce composi-

tionally zoned pyroclastic deposits (Figure 9.38).

Crystallization of higher-T maﬁc minerals and pla-

gioclase along the cooled margin of the magma

chamber allows buoyant residual melts enriched in

water and more felsic components to escape, collect-

ing into the interior and eventually rising to the top

of the magma chamber. Because of limited vertical

exposure, few plutons can be examined to verify this

model (but see Cornejo and Mahood, 1997).

For the Tuolumne series, Kistler et al. (1986) ﬁnd el-

emental and isotopic evidence that mantle-derived

basalt magmas stagnated in the lower crust, fraction-

ated, and mixed with variable amounts of more felsic

and radiogenic anatectic melts generated by their heat.

These magmas created by AFC were emplaced at the

present level of exposure in at least four surges (Figure

9.18), each more felsic than the preceding one. Once

emplaced, the magmas experienced limited convective

melt fractionation as hornblende and plagioclase crys-

tallized.

13.32 (a) Generalized geologic map across the central Sierra Nevada batholith. Shown are 18 outlined intrusive suites of multiple plutons of

similar character that are believed to have originated in a distinct magmatic episode. Note space-time relations, including the predomi-

nantly Cretaceous eastward-migrating magmatism that split the older Jurassic arc. More detailed map of the Tuolumne Intrusive Series

is shown in Figure 9.18. Late Cenozoic extensional faulting dropped valley-forming grabens, such as the one in which the town of Bishop

is located, separating the main part of the batholith on the west from an eastern segment. Blank areas within batholith were unmapped

(as of 1981). (Redrawn from Stern et al., 1981.) (b) Variation in modal percentages of alkali feldspar and

Sr/

Sr across the batholith

along the east-northeast trending line through Bishop shown in (a). Redrawn from Bateman (1992).

Tuolumne

Intrusive Series

120° W

119° W 118° W

Post-batholith

deposits

Country

rocks

Ultramafic

rocks

My ago

100

150

200

Cretaceous

Jurassic

Triassic

37° N

38° N

Bishop

0.710

0.706

0.702

Sr/

West East

Modal % alkali

feldspar

(a)

(b)

Magmatic Petrotectonic Associations

385

A similar sort of interaction with the preexisting

crust, but on a larger scale, is manifested in a regu-

lar compositional variation across the Sierra Nevada

batholith (Figure 13.32). This pattern is mostly inde-

pendent of pluton age. Tonalite is the most common

rock type in the west, granodiorite along the batholith

axis, and granite in the east. Alkali feldspar increases

in modal proportion from about 4% in the west to

about 30% in the east. Whereas SiO

is fairly con-

stant, large-ion-lithophile elements (K, Rb, Ba, U, Th)

and light REEs increase eastward, as does

Sr/

Sr,

whereas

143

Nd/

144

Nd, CaO, MgO, and total Fe de-

crease. Tonalites on the west likely formed from

fractionated mantle-derived magmas, possibly contam-

inated only by young maﬁc crust. Progressively east-

ward, these primitive magmas were increasingly conta-

minated and mixed with partial melts of older felsic

continental crust.

Table 13.8. Compositions of Granitic Rocks from the Sierra Nevada Batholith

1 2345 6

SiO

(61.52) 58.59 65.61 67.83 69.76 71.65

TiO

(0.73) 0.90 0.54 0.49 0.37 0.24

(16.48) 16.93 15.44 15.44 15.49 14.87

2.26 1.76 1.55 1.24 0.84

FeO 6.18t 4.31 2.38 1.44 0.95 0.81

MnO 0.11 0.11 0.80 0.06 0.05 0.04

MgO (2.80) 3.26 1.80 1.03 0.66 0.38

CaO (5.42) 6.25 4.10 3.22 2.52 1.87

O 3.30 3.53 3.62 4.02 4.33 3.98

O 2.41 2.38 3.11 3.65 3.72 4.19

(0.25) 0.22 0.16 0.17 0.13 0.08



(1.04) 1.01 0.78 0.61 0.45 0.58



(0.2) 0.14 0.14 0.1 0.09 0.17

Total 100.44 99.89 99.52 99.61 99.76 99.70

Rb 79.8 108 138 129 137 158

Ba 715 720 515 905 740 1170

Th 6.37 16.6 21.8 18 18.4 16.8

U 2.89 4.2 7.4 5.2 8.8 3.8

Nb 8 7 7 8 9

Ta 1.27

La 30.2 21 23 25 29 26

Ce 53.9 47 38 44 49 49

Sr 452 574 451 658 621 484

Nd 21.7 18 11 13 21 15

Sm 6.12 4 2.5 3 3 2

Zr 241 126 116 119 126 138

Hf 5.23

Eu 1.52 1 0.7 0.9 0.8 0.6

Tb 0.98 0.7

Dy 3.41

Y189878

Ho 8 5.5 5 6.6 5.2

Yb 1.77 1.3 1.3 0.6 0.6 0.6

Lu 0.31 0.2 0.17 0.1 0.1 0.1

Cr 22.8 17 9 6 1 1

Ni 11 11 5 2 2 2

Sc 9.66 16 8 4 3 2

Columns 2–6 from the Tuolumne Intrusive Series represent the ﬁve intrusive stages of the zoned intrusion shown in Figure 9.18a–d. (From

Bateman and Chappell, 1979; Frey et al., 1978.)

1, Bass Lake Tonalite. Data from Dodge et al. (1982). Values in parentheses are average tonalite. From Le Maitre (1976). 2, Quartz diorite north

of May Lake. 3, Equigranular Half Dome Granodiorite. 4, Porphyritic Half Dome Granodiorite. 5, Cathedral Peak Granodiorite. 6, Johnson

Granite Porphyry.

386 Igneous and Metamorphic Petrology

A belt of widely scattered Mesozoic plutons extends

several hundred kilometers inland of the North Amer-

ica batholith arc (Miller and Barton, 1990). Magmas

producing these mostly Cretaceous plutons had an in-

creasing crustal component through time. In some ar-

eas, latest Cretaceous plutons are strongly peralumi-

nous two-mica granites that were probably derived

entirely by anatexis of ancient crust. Unlike the

S-type granites of eastern Australia, the North Ameri-

can peraluminous granites contain no cordierite.

Nonetheless, high

Sr/

Sr (to as much as 0.737) and



O (to 13%o), and low

143

Nd/

144

Nd (to 0.5118) in-

dicate signiﬁcant amounts of magma from a metasedi-

mentary source. Many petrologists have hypothesized

that crustal anatexis was energized by internal heating

effects in the crust thickened by orogenic contraction

(Section 11.1.1). Even if the thickened crust did not

itself provide sufﬁcient internal heat for anatexis, it

would promote more contamination of ascending mag-

mas because of the greater path length and residence

time of ascending more primitive magmas.

13.8 GRANITES IN CONTINENT-

CONTINENT COLLISION ZONES

Ancient examples of continent-continent collisions are

the Paleozoic Hercynian orogen in southern Europe

and the Proterozoic Grenville orogen in eastern

Canada. In more recent geologic time, since about

55 Ma, the Indian and Asian continental plates have

converged, forming the highest mountains on Earth—

the Himalaya. An estimated 1000–1500 km of crustal

shortening and doubling of crustal thickness to about

80 km have taken place in the collision zone that in-

cludes the Himalaya Mountains and the Tibetan

Plateau farther north in the Asian plate. A volcanic arc

formed before continent-continent collision but was

extinguished as subduction of oceanic crust stopped.

Consequently, no volcanic rocks are directly associated

with this collision. However, highly potassic maﬁc lavas

and local peraluminous rhyolites  13 Ma old occur in

the central Tibetan Plateau that may be related to local

extensional tectonism in the gravitationally unstable

high landmass.

Early Miocene (20 Ma) magmatism in the

Himalaya orogen is represented by generally sheet-

like granitic plutons and swarms of innumerable

smaller dikes, sills, and irregular intrusive bodies.

They form a chain of intrusions for about 2000 km

along the crest of the High Himalaya (Figure 13.33).

Unlike in other granite associations where a range

of rock types coexist, the Himalayan rocks are rela-

tively uniform S-type leucogranites composed of a per-

aluminous assemblage of quartz (about 31 modal %)

13.33 Generalized geologic map and cross section of the Himalaya Orogen. Note three belts of plutonic magmatic rocks: (1) The northernmost

is the Gangdese batholith (dark shade) emplaced from the late Cretaceous to Eocene and generated during subduction of the oceanic

Indian plate before continental collision. The North and High Himalayan granites were emplaced in thickened continental crust in the

early Miocene after continental collision. The suture between the Eurasian continent (light shade) and the northward-colliding Indian

continent is marked by lenticular slices of seaﬂoor ophiolite scraped off the Indian oceanic lithosphere. The crustal thickening and north-

south shortening are produced by an imbricate stack of thick thrust sheets that moved along the Main Central Thrust (MCT) and Main

Boundary Thrust (MBT). SDT, Southern Tibetan Detachment, a low-angle normal fault that formed late in the topographically elevated

orogen. (Redrawn from France-Lanord and Le Fort, 1988.)

MCT

MBT

Gangdese

batholith

suture

90° E

30° N

Mt.

Everest

Eurasian

plate

Kathmandu

MCT

MBT

Moho?

South

North

28°N

80°E

400

miles

Sea level

Indian

plate

1600

100

miles

Ophiolite

Metamorphosed

sedimentary

rocks

North Himalayan granites

High Himalayan leuco-

granite

Gneiss, migmatite

Sedimentary rocks

STD

Indian Ocean