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Ig Rx

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IUGS Intrusive Igneous Rock

Chart - General

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IUGS Intrusive Igneous Rock

Chart

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Plutonic Igneous Rocks

Grain size is larger than extrusive rocks

Grains are visible, usually identifiable

Volume percent abundance is an important

means of rock classification

Total lack of glass - slow cooling allows

matter to achieve the lowest energy state,

which is as crystalline matter

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Plutonic Igneous Rocks

Continued Generally, intrusive rock field names are

more reliable than the extrusive rock names

because of the larger, more visible crystals Accurate determination of rock name

demands a careful examination of thin

sections of selected specimens

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Gabbroic Layered Intrusions

The rocks in this laboratory may occur in avariety of geologic settings.

One setting in which these rocks are oftenassociated is a gabbroic layered intrusion. These

intrusions include some of the largest plutonicbodies in the world including Bushveld (SouthAfrica), Skaergard (East Greenland), DuluthGabbro, Muskox (Northwest Territories,Canada), Great Dike (Zimbabwe), and theStillwater Complex (Montana).

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Gabbroic Layered Intrusions Gabbroic layered intrusions, as the name

indicates, show distinct layers

Layers may be quite complex

Origin of layering is often at least partially

due to fractional crystallization

Early formed crystals segregate themselves

from the main magma body by sinking (mafics)

or rising (calcic plagioclase)

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Layering Sequence in Gabbro

Intrusions A typical sequence might be:

Anorthosite

Norite, feldspathic pyroxenites Harzburgite, Dunite (ultramafic rocks)

Peridotite

Fine-grained norite

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Gabbro

Intrusive igneous, plutonic to hypabyssal

A phaneritic mafic rock with medium to

coarse grains

Gabbro is the intrusive equivalent of basalt

The name may come from the gabbro

region, Tuscany, Italy

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Gabbro Mineralogy

Essential: Mid to calcic plagioclase,commonly labradorite to bytownite

(occasionally anorthite)

Essential: Clinopyroxene, usually augite Accessory: Olivine and/or orthopyroxene

Plagioclase grains range from equant to lath-

shaped, and are almost always well-twinned Zoning is limited to the edge of the

plagioclase grains, if present at all

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Gabbro Mineralogy Continued

Clinopyroxene is augite or diopside

Some of these may be brown due to titanium

and/or ferric iron content

Twinning is often present, but zoning is

very rare

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Gabbro: IUGS

Q = 0-5% Q

P/(A + P) >90

pl/(pl + px + ol) is 10 - 90

Plagioclase composition > An50

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Olivine Gabbro

Contains olivine in addition to plagioclase

and cpx

Olivine gabbro is often richer in mafics thannormal gabbro

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Hornblende Gabbro

Contains hornblende in place of the normal

cpx

Hornblende may be green or brown in thinsection

It may occur as independent prisms or as

crusts on the pyroxene

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Gabbro Photomicrographs

CN views of gabbros The brightly colored minerals

are clinopyroxene

The white to gray minerals areplagioclase - note albite

twinning

The black minerals in both

pictures are opaque grains ofmagnetite

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Gabbro Photomicrographs

Olivine gabbro in CN Brightly colored and fractured

olivine surrounded by

plagioclase (upper photo) -

small veins in olivine areserpentine

Gabbro (lower photo) with

cumulate texture, plagioclase

poikilitically enclosed by

clinopyroxene

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Ophiolites Pieces of oceanic plate that have been thrust

(obducted) onto the edge of continental plates

Provide information about processes at mid-

ocean ridges Composed of an assemblage of mafic and

ultramafic lavas and hypabyssal rocks found in

association with sedimentary rocks likegreywackes and cherts

Found in areas that have complex structure

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Ophiolite Obduction

Ophiolites have been found in

Cyprus, New Guinea,

Newfoundland, California, and

Oman

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Typical Ophiolite Sequence Ophiolites are characterized by a

classic sequence of rocks This sequence is well exposed at the

Samail ophiolite

Base of the sequence is sedimentary

rocks of the Arabian shield, not part ofthe ophiolite, on which the oceanicplate was pushed

From base to top the ophiolite is made

of: peridotite, layered gabbro, massivegabbro, dikes, and volcanic rocks

At Samail this entire sequence is 15 kmthick.

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Samail Ophiolite

The basal peridotite is made of a rock calledharzburgite (made mostly of the minerals olivine andenstatite)

Within the peridotite are many dikes of gabbro anddunite

The peridotite is deformed and is overlain by dunite(an intrusive igneous rock made mostly of the mineral

olivine) that grades upward to gabbro (an intrusiveigneous rock made mostly of plagioclase andclinopyroxene - augite)

Sequence is capped by dikes and volcanic rocks

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Mid-Ocean Ridge Cross-section

From a tectonic perspective, peridotite is depletedmantle that was under the magma chamber at the mid-ocean ridge crest

Gabbro layer is related, in some way, to thecrystallization of the magma chamber (probably withrepeated injections of magma)

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Gabbro Dikes (Ophiolite) Oceanic detachment fault,

Troodos ophiolite, Cyprus Slide shows the first

recognized extensionaldetachment fault foundwithin an ophiolite

Detachment separates sheeted

dikes (lower part of oceanic

Layer 2) from the underlying gabbro of the plutonic complex (oceanic

Layer 3)Dikes above the detachment are presently near-horizontal and were

tilted by normal faulting to this position from originally vertical

attitudes, as shown by paleomagnetic data

Photo: Robert Varga, College of Wooster 22

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Norite Intrusive igneous, plutonic to hypabyssal

A gabbro with predominantly orthorhombic

pyroxene (enstatite or hypersthene) rather

than clinopyroxenes

Opx may sometimes be identified in hand

specimen by the presence of Schiller luster

The name is for Norway, the original

locality where it was first identified

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Norite, IUGS

Opx/(opx + cpx) > 95

Q = 0-5%

P/(A + P) > 90

Pl/(pl + px + ol) is 10 - 90

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Origin of Gabbro and Norite Found in sills, dikes, stocks, lopoliths, and other

bodies.

Mineralogy and texture of gabbros, norites, diabases,and basalts indicates that they are probably derivedfrom the same type of magma crystallized underdifferent conditions

Diabase and gabbro are often found together

Gabbroic layered intrusions may show considerable

gradation in rock types between layers

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Diorite Intrusive igneous, plutonic

Essential: sodic plagioclase (oligoclase orandesine)

Essential: A mafic, usually hornblende, ormore rarely, biotite or pyroxene

The name is from the Greek, diorizein, to

distinguish, because the grains are largeenough to be recognized in hand specimen

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Diorite Continued

These rocks are found in small bodies such

as satellite stocks or batholiths, of the type

associated with subduction zones Diorite is the intrusive equivalent of

andesite

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Diorite Photo

Diorite cut by two veinsdipping away from eachother, and these in turn

are cross cut by faults. From area in Quebec

about 85 milessouthwest of

Chibougamau Photo: Richard

Stenstrom

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Quartz Diorite Intrusive igneous, plutonic

Essential sodic plagioclase (oligoclase orandesine)

Essential quartz, Q > 5%

Usually, a mafic such as biotite orhornblende, or rarely pyroxene

Quartz diorite is the intrusive equivalent ofquartz andesite

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Quartz Diorite, IUGS

Q = 5 - 20

P/(A + P) >90

Plagioclase composition < An50

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Quartz Diorite Photo

Handspecimen of

quartz diorite, location

unknown

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Diorite IUGS

Q= 0 - 5

P/(A + P) >90

Plagioclase composition < An50

Mafics are generally 10-40% of the rock

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Diorite Mineralogy

Quartz is present as an interstitial, anhedralcomponent, often not visible in hand

specimen

Hornblende is generally green, and may bereplacing pyroxene (uralite)

Biotite is very commonly found with the

hornblende and is generally brown

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Diorite Photomicrographs Upper photo, CN; lower, PP

Large, twinned plagioclase crystals

Hornblende grains

Porphyritic texture

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Tonalite Intrusive igneous, plutonic

Occurrence: Batholiths

Sometimes mistakenly used as synonymous

with quartz diorite

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Tonalite Mineralogy: Essential

Essential: Quartz, Q > 20

Quartz is almost always anhedral

Essential: Plagioclase feldspar, usually

andesine

Zoning in plagioclase is common and often

very strong

Zoning may be oscillatory

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Tonalite Mineralogy: Accessory

Accessory : K-spar - if K-feldspar is present it

should be less than 5% of the rock

Typically it will be orthoclase or perthitic orthoclase Accessory: Mafics such as hornblende, biotite,

and/or pyroxene

Hornblende is the characteristic mafic mineral

Biotite is usually brown to brownish-green

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Tonalite, IUGS

Q = 20 - 60

P/(A + P) >90

If M < 10, the rock may be called

Trondhjemite (after Trondhjem, Norway,

the type locality)

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Tonalite Photomicrographs

Upper photo, CN; Lower, PP Large grain of zoned, altered

plagioclase in the lower right

(partially altered to sericite and

epidote) Large pleochroic crystals of biotite.

The extinct (dark) areas in the upper

left corner and lower left corner ofthe photo on the top are quartz

crystals at extinction

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Anorthosite Intrusive igneous, plutonic

Composed of calcic plagioclase with lessthan 10% ferro-magnesium minerals -

plagioclase may be labradorite, bytownite,or anorthite

The name is from anorthose, an old name

for triclinic feldspars Anorthosite is often associated with gabbro

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Anorthosite: IUGS

Q =0-5%

P/(A + P) >90

M < 10 (M = Mafics)

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Anorthosite Photo

Garnets in a gabbroic

anorthosite

Location : Gore

Mountain Garnetmine, Adirondack

Mtn. NY

Photo: R.L.Chase