Igneous and metamorphic petrology Flashcards
What are the three ways to cause the mantle to melt?
Stretching
* Extensional stresses on a plate at a constructive boundary causes it to thin
* Reduced thickness pulls up mantle - too fast to cool by conduction (adiabatic)
* Geotherm moves upwards to intersect solidus
Hot spots
* Unsteady convection in mantle creates hot plumes
* Plumes shift geotherm to the right, causing melting at depth
Addition of water
* Seawater penetrates upper few km of oceanic crust, minerals hydrated e.g. pyroxene
* Subduction of crust reverses hydration
* Water released into overlying mantle
* Wet mantle melts at a lower temperature, hence solidus shifts to left to intersect geotherm
What happens after the mantle melts?
- Mantle (peridotite) melts, rises due to buoyancy and loses olivine to form basalt
- Magma rises to its level of neutral buoyancy (LNB)
- If LNB at base of crust, hot magma will melt crust, forming silica-rich magma
- If LNB in crust, magma stalls and forms a magma chamber
- Cools by conduction and convection, fractional crystallisation occurs
- Mafic rocks crystallise first - liquid magma Si content increases
What are three different types of igneous intrusions, and how are they formed?
- Batholith
* Large bodies of magma (10-1000km scale) emplaced in Earth’s crust, revealed by substantial uplift and erosion
* Granite plutons - magma rises along fractures and congregates in inflating chambers
* Xenolith - Fragments of rock from another source found in the batholith
* Can be derived from country rock or earlier parts of the intrusion - Dyke
* Steep dipping bodies formed by magma filling fractures and cooling
* Discordant (cut bedding planes) - Sill
* Sheet like intrusions between bedding planes (concordant)
Characteristics of volcanoes in intraplate settings (constructive plate boundary/hotspot)
- Basaltic
- Low viscosity, runny magma
- Loses volatiles easily, no explosive eruptions
- Pahoehoe - fast lava does not crystallise during flow, runny and wrinkly on top
- A’a - slow lava nucleates crystals, increasing viscosity, upper mass of rubble
- Shield volcanoes (wide and flat)
Characteristics of underwater eruptions
- Pillow basalts with glassy chill margins
- Explosive if water enters the vent - boils, steam expands and shatters magma
- Produces tuffs (glassy material)
Characteristics of volcanoes at destructive plate margins
- Most explosive eruptions at boundary involving continental plate
- Magma is high in Si (fractional crystallisation, melting country rock, wet) so viscous (Si tetrahedra join)
- Typically andesitic
- High gas content (wet mantle source, country rock melting)
- Lava dome builds, gases decompress and expand, explosion shatters lava into rock and glass
- Volcanic column can project ash into stratosphere
- Volcano can collapse
- Steep-sided volcanoes
What is the structure of the ocean floor?
- Sediments
- Pillow basalts
- Sheeted dyke complex
- Intrusive rocks
- Mantle material
What does hydrothermal circulation do?
- Water circulates in the crust at spreading ridges
- Cooling causes ridge to subside faster than conduction alone
- Adds water to initially dry basalt
What is an accretionary prism?
- A series of low-angle faults in seafloor sedimentary rocks at subduction zone
- Intense deformation, high pressure metamorphic rocks (eclogite and blueschist)
What is slab pull?
- Deep in the mantle, olivine transforms to wadsleyite
- Wadsleyite is denser, so acts to pull the slab down
What is metamorphism?
- Process whereby new minerals/textures form in pre-existing rock
- Occurs when old assemblage not stable
- Depends on bulk composition, pressure and temperature
- Metamorphic rocks at the surface are metastable (need water + high activation energy to revert)
What are the three main types of metamorphism and where do they occur?
Contact metamorphism
* High T, low P, low timescale (fine grain)
* Rocks intruded by plutons, hot at the contact
* Low P - old fabric retained, minerals grow with random orientations over fabric
High pressure, low temperature
* Requires subduction zone- subducting cool slab
* Formation of blueschist
* Strong foliation due to shearing
Regional metamorphism
* High P, high T
* Requires heating and burial - continental collision (orogeny)
* Burial through faulting and overthrusting
* Heat from radioactive decay of isotopes in granite, erosion of mountains making geotherm shallower
What is the regional metamorphic sequence of a mudstone?
- Mudstone
- Slate (fine-grained, cleavage along bedding)
- Phyllite (coarser-grained, shiny)
- Schist (med to coarse grain, foliation, clear crystals, layers of platy micas are planes of weakness)
- Gneiss (med to coarse, banding of crystals, no platy micas)
Which rocks form from regional metamorphism of basalts and gabbros?
Amphibolite
* Dominated by elongate amphibole
* Plagioclase also present
* Formed in high-grade regional metamorphism
Eclogite
* Contains garnet and pyroxene
* Forms by direct conversion (plag reacts with px to form garnet and different px) or dehydration of amphibole
* Higher P than amphibolite
