GEOL 3000
IGNEOUS ROCK TEXTURES
f (rate of cooling = crustal environment)
Plutonic---Volcanic
1 mm
Degree of Undercooling
1 mm
Magma Creation
by Partial Melting
Melting the mantle makes
MAFIC Magma
Melting the crust
makes FELSIC Magma
Igneous rocks
crystallize over a
range of temperatures
Map Attributes of Volcanic and
Volcaniclastic Rocks
Volcanic Rocks- crystallized from lava flows
Volcaniclastic Rocks – air-fall accumulations of lava, ash and preexisting volcanic rock ; Rock types: tuff, lahar, agglomerates)
Areal distribution more limited than sedimentary units
Map unit shapes controlled by gravity, landscape, volume of
material erupted, explosiveness of eruption, proximity to vent,
and fluidity (viscosity) of lava
Tabular shapes – distal areas of volcaniclastic deposits, fluid
lava flows
Lensoid/Triangular/Fan/Irregular shapes – proximal areas of
volcaniclastic deposits and lava flows, especially
viscous lavas and low volume eruptions
Massive Ash-fall Events from Caldera Collapse
Tabular Lava Flows
Typical of Low Viscosity
Magmas
Up Indicators in Volcanic and
Volcaniclastic Rocks
 Pillow structures
 Vesicular/Amygdaloidal upper zones
 Distinctive Flow Tops
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Brecciation (AA)
Billowing, ropey tops (Pahoehoe)
Oxidation
Sediment infiltration
Pipe vesicles/amygdules at base
 Pipe vesicles/amygdules at base
 Graded bedding in volcaniclastic deposits and
clastic dikes
Pillowed Basalts
Submarine Eruptions
BASALT FLOW SURFACES
AA
AA
Pahoehoe
HAWAII
Pahoehoe
NORTH SHORE
Other Up Indicators
Pipe Amygdules
Oxidized Pahoehoe surface
Amygdule Cylinders
Graded bedding of tuff
Bedded
siltstone in
a clastic
dike
Map Attributes of Intrusive Rocks
 Shape not controlled by gravity, but often by weaknesses and
stresses in the crust
 As such, may be discordant or locally concordant with
structures in country rock
 For large intrusions, will create a zone of thermal
metamorphism (and local melting) in surrounding country
rock
 Commonly shows internal structure/zonation related to the
shape of the magma chamber
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Chilled margins
Layering* (phase, textural, modal, cryptic)
Igneous Foliation* (alignment of tabular minerals)
Jointing (orthogonal to the cooling surfaces or internal structure of
the magma body)
* These features are commonly controlled by gravity
 Multiple (composite) intrusions are common because magmas
are prone to reuse the same plumbing system
Intrusion Shapes
Generally Concordant
Sill
Laccolith
Lopolith
Phacolith
Generally Disconcordant
Dike
Dike Swarm (co-parallel or radial)
Ring Dike/Cone Sheet
Volcanic Neck
Sheets
Stock/Boss
Batholith
Indicators of Style of Magma Emplacement
• Shape of intrusion
• Shape and distribution of Inclusions (Xenoliths)
• Internal zoning and structure
• Primary and secondary structures in the country rock
• Large-scale structures in the intrusion (folds and faults)
Emplacement Styles:
Forceful – Magma forces entry by deforming and displacing the country rock
Permissive – the country rock, under stress, dilates or moves apart as magma wells
into the space
Stoping – rising magma detaches blocks of country rock, which sinks into it
Timing of Emplacement (relative to deformation):
Pre-kinematic– Intrusion is deformed in the same way as the rocks it intrudes
Syn-kinematic – Intrusion shape conforms to regional structures
Post-kinematic – Intrusion shape is independent or cross cuts deformational fabric of
country rock
Intrusions of Northern Minnesota
Post-Kinematic
Pre/Syn-Kinematic
Post-Kinematic
Discriminating a Sill From a Flow
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• Typically lacks amygdules
• Never pillowed
• May contain inclusions of
hanging wall and footwall rocks
•Thermal metamorphism of hanging
wall and footwall rocks
• Zoning &/or layering common,
especially in larger intrusions
• Upper amygdaloid zone
• May be pillowed
• May contain inclusions only of
footwall rocks
• May cause weak contact
metamorphism of footwall rocks
• May show zoning, but rarely
displays layering
Mafic Layered Intrusions
Skaergaard
Intrusion
East Greenland
Bushveld
Complex
South Africa
Fractional
Crystallization-driven
Magma
Differentiation
Phase Layering In Mafic
Layered Intrusions
Duluth Complex
The 2nd Largest Mafic Complex in the World
Layered Series at Duluth
Internal
Structure in
the DLS
Modal Layering
Modal Layering
Igneous Foliation
Lithostratigraphic Classification for Keweenawan Rocks of NE Minnesota
Miller, Severson, and Green, 2002