Transcript Intrusive Igneous Rocks, part 1

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 (top to
bottom):
 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 lathshaped, 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 midocean 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 midocean 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 21
transit to or at the surface
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 data
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Photo: Robert Varga, College of Wooster
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
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•
•
•
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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