Intrusive Igneous Rocks, Part 1

Tonalite, Diorite, Gabbro, Norite,

and Anorthosite

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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 a

variety of geologic settings. • One setting in which these rocks are often

associated is a gabbroic layered intrusion. These intrusions include some of the largest plutonic bodies in the world including Bushveld (South Africa), Skaergåard (East Greenland), Duluth Gabbro, Muskox (Northwest Territories, Canada), Great Dike (Zimbabwe), and the Stillwater 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 than normal gabbro

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Hornblende Gabbro

• Contains hornblende in place of the normal cpx

• Hornblende may be green or brown in thin section

• 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 are

plagioclase - note albite twinning

• The black minerals in both pictures are opaque grains of magnetite

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Gabbro Photomicrographs• Olivine gabbro in CN• Brightly colored and fractured

olivine surrounded by plagioclase (upper photo) - small veins in olivine are serpentine

• 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 like greywackes 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 of the ophiolite, on which the oceanic plate was pushed

• From base to top the ophiolite is made of: peridotite, layered gabbro, massive gabbro, dikes, and volcanic rocks

• At Samail this entire sequence is 15 km thick.

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Samail Ophiolite

• The basal peridotite is made of a rock called harzburgite (made mostly of the minerals olivine and enstatite)

• Within the peridotite are many dikes of gabbro and dunite• 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 intrusive igneous rock made mostly of plagioclase and clinopyroxene - augite)

• Sequence is capped by dikes and volcanic rocks (pillow basalts that erupted on the ocean floor)

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Mid-Ocean Ridge Cross-section

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

• Gabbro layer is related, in some way, to the crystallization of the magma chamber (probably with repeated injections of magma)

• Dikes and volcanic rocks are formed by magma in transit to or at the surface

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Gabbro Dikes (Ophiolite)• Oceanic detachment fault,

Troodos ophiolite, Cyprus• Slide shows the first

recognized extensional detachment fault found within 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 dataPhoto: Robert Varga, College of Wooster 22

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 derived from the same type of magma crystallized under different 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 or

andesine)• Essential: A mafic, usually hornblende, or

more rarely, biotite or pyroxene • The name is from the Greek, diorizein, to

distinguish, because the grains are large enough 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 veins

dipping away from each other, and these in turn are cross cut by faults.

• From area in Quebec about 85 miles southwest of Chibougamau

• Photo: Richard Stenstrom

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Quartz Diorite• Intrusive igneous, plutonic • Essential sodic plagioclase (oligoclase or

andesine)• Essential quartz, Q > 5% • Usually, a mafic such as biotite or

hornblende, or rarely pyroxene• Quartz diorite is the intrusive equivalent of

quartz 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, anhedral component, often not visible in hand specimen

• Hornblende is generally green, and may be replacing 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 of the photo on the top are quartz crystals at extinction

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Anorthosite• Intrusive igneous, plutonic • Composed of calcic plagioclase with less

than 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 Garnet mine, Adirondack Mtn. NY

• Photo: R.L.Chase

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