Igneous & Metamorphic
Petrology Lecture Notes
By David T. Allison
Earth’s Internal Layers
2 Criteria
– Composition (i.e. mineralogy and geochemistry)
– Seismic (mechanical behavior)
Earth’s Layering by Composition
Depth Thick Layer
7-50km Basalt (3.0) Diorite (2.7)
Crust
650km Upper mantle Peridotite (3.3)
700km
2200km Lower mantle Garnet Lherzolite (3.5-5.5)
2900km
2200km Outer core Ni-Fe-S Alloy (10-12)
5100km
1300km Inner core Ni-Fe Alloy (13)
6400km
Earth’s Layering by Seismic
Properties
Depth Thick Layer
70-150km Brittle
Lithosphere
550km Asthenosphere Ductile
700km
2200km Mesosphere Brittle
2900km
2200km Outer core Liquid (very ductile)
5100km
1300km Inner core Brittle
6400km
Plate Tectonics and Petrology
Plate Boundaries
– Divergent: plates move directly away from
boundary
– Convergent: plates move directly toward
boundary
– Transform: plates move parallel to boundary
– Collisonal: continental lithosphere in each plate
collide, therefore, subduction is terminated
Seafloor Spreading
Alfred Wegener (1915): continental drift
theory
Arthur Holmes (1931): mantle convection
Harry Hess (1962): “conveyor belt” model
for seafloor
Vine and Mathews (1963): paleomagnetism
discovery definitively proved that the
seafloor was spreading away from ocean
ridges
Petrology of Paleomagnetism
Paleomagnetism is the preservation of the
Earth’s magnetic field magnitude, orientation
and polarity in minerals
- Magnetite crystal
+
Paleomagnetic Reversals
Periodically the Earth’s magnetic field
reverses polarity, an event that is recorded
in igneous magnetic minerals
Reversal of
polarity
+ - + -
S - + - + N
- + - +
Paleomagnetic Seafloor “Stripes”
Each section of ocean lithosphere inherits
the Earth’s magnetic polarity
Global Paleomagnetic Patterns
Width of paleomagnetic stripes indicates the
relative rate of spreading
Ophiolite Suite
1. Sediment layer (1-100m)
– Radiolarian chert, Fe oxides, turbidites
2. Pillow basalt layer (1 km)
– Submarine extrusion of basaltic lava
3. Sheeted dike layer (2 km)
– Diabase dikes
4. Gabbro layer (4 km)
– Batholiths of gabbro with layering
controlled by fractional crystallization
5. Peridotite layer (63 km)
– Ol+Opx+Cpx mantle
Formation of Ophiolites
Formation of ophiolites
is driven by magma
chamber under
divergent ocean ridge
systems
Basalts produced at
ocean ridge divergent
boundaries have a
geochemical signature
that is termed tholeiitic
(MORB)
Continental Rift Zones
Initial developing phase of a divergent plate boundary
Magma is enriched in alkali (K, Na, Rb) because of high
pressure source area (Alkali basalt, Nepheline syenite;
Bimodal volcanics)
Transition from continental to ocean lithosphere is termed
a passive continental margin
Convergent Plate Boundaries
Always marked by ocean trench physiographic features
One of the converging plates, always ocean lithosphere, is
consumed by subduction
Volcanic and magmatic arcs are associated with
subduction
Characteristics of Convergent
Boundaries Volcanic Arc Back-
Accretionary Fore-arc basin arc
Trench Prism (eugeocline) basin
Melange
1200 deg C
isotherm Greenschist
Blueschist (miogeocline)
Amphibolite
Granulite
Benioff Zone seismic
Focal points
Calc-
Alkaline
Eclogite magma
Magma source
region
Transform Boundaries
Fault action produces a type of metamorphic
fault-zone rock termed mylonite
“Leaky transforms” erupt alkali basalt similar
to continental rift zones
Example: Death Valley along the
San Andreas fault system
Collisional Zones
Igneous activity associated with
convergence ceases
Seismic activity becomes intense
Eclogites may become exposed
Extreme topographic relief
Ophiolites are preserved in suture zones
between colliding continents/Island Arcs
