Download Differentiation of Igneous Rocks: Crystal Fractionation and Layered

Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring
2009
Differentiation of Igneous Rocks:
Crystal Fractionation and Layered Mafic Intrusions
Igneous Rocks are grouped into Suites
Rocks in a Suite might come from the same volcano (Kilauea), a group of
island volcanoes (Hawaii, the Galapagos), a single intrusion (the Skaergaard
intrusion, Greenland), a chain of volcanoes (the Cascades)
Different magmas (rocks) in a Suite must be related by some process
•
•
•
•
•
Parental magma - the one from which others are descended
highest liquidus temperature
most primitive composition (hi MgO, low SiO2, low incompatible elements)
large volume erupted
Daughters, Differentiates, Derivatives - different names for the descendants
Changes are displayed on Harker Diagrams (Metal Oxide vs. SiO2)
Trends on AFM Diagrams
Some Differentiation Processes that can change magma composition
•
•
Crystal fractionation (We will talk mainly about this.)
Magma mixing
•
Assimilation of country rocks
Crystal Fractionation
•
•
Crystals are removed from the liquid in which they formed
Commonly by settling under the influence of gravity
ρ (olivine) = 3.22 g/cm3 (Mg) - 4.30 g/cm3 (Fe)
ρ (cpx) = 2.96-3.52 g/cm3
ρ (opx) = 3.21-3.96 g/cm3
ρ (plag) = 2.63-2.76 g/cm3
ρ (magmas) = 2.4 - 2.8 g/cm3 (calculated)
Norman L. Bowen popularized Crystal Fractionation
•
•
•
He thought all igneous rocks came from a basaltic parent; mainly by crystal
fractionation
His idea was too extreme, but very important as a starting point
This is the origin of Bowen’s Reaction Series - see handout from before
Bowen's Reaction Series (BRS) is inadequate for generating most granite
•
•
•
Amount of basalt in crust is approximately equal to the amount of granite
Bowen’s reaction series could only produce about 1/20 as much granite as the
initial volume of basalt
Where are all the fractionated mafic minerals? (there would have to be a huge
amount of ultramafic cumulate rocks hiding at the base of the continental crust)
Layered Mafic Intrusions are the best examples of Crystal Fractionation
•
•
•
Palisades Sill along Hudson River in NJ (see textbook)
Bushveld Intrusion in South Africa, pC, colossal!! 320 km in diameter
Skaergaard, Tertiary, E. Greenland**
•
•
•
•
Muskox, northern Canada
Great Dike, Zimbabwe
Duke Island Complex, SE Alaska
Stillwater, pC, Montana
We did an imitation of a Layered Mafic Intrusion with the M&M® Magma
Chamber
Skaergaard in E. Greenland
•
•
•
•
Perhaps the most studied rock body on Earth
Best example of an igneous body that has fractionated to an extreme degree
through crystal fractionation (Bowen’s idea)
Most of its thickness is exposed
Explored in 1930s, 1950s and 1970s
Upper Zone Layered Series (Wager 1930s photo)
Evidence for Crystal Settling
•
•
•
•
•
•
Cumulus mineral textures, euhedral-subhedral grains piled up as if they settled in
a liquid
Sedimentary-like structures
layering
graded bedding
cross-bedding
slump structures, etc.
Cumulus Mineral Texture
Layering (variation in mineral proportions and sizes)
Graded Bedding (coarsest at bottom)
Cross-bedding
Trough banding
Skaergaard is an asymmetric lopolith - see handout
Crystallization
What is Sandwich Horizon?
There are two kinds of Layering in the Layered Series
•
•
Rhythmic Layering - changes in the identity and proportion of minerals
Cryptic Layering - changes in chemical composition of minerals upwards through
the layers (hidden, you can’t see it, must have chemical analyses of minerals)
Original Skaergaard Magma was a Tholeiitic Basalt
•
Layering and compositional changes mainly resulted from crystal fractionation by
gravity settling (fractional crystallization).
Current Exposure (E-W cross section)
The Layered Series - see handout
Why does Olivine disappear in Middle Zone?
Explained by Fo-En-SiO2 diagram - see previous handout
Olivine reappears in Upper Zone; Fe-rich Olivine is OK in SiO2-rich liquid
Ab-An Diagram explains why plagioclase composition changes from bottom to
top of Skaergaard
Note other cumulus minerals
Remember, there are two kinds of Layering in the Layered Series
•
•
Rhythmic Layering - changes in the identity and proportion of minerals
Cryptic Layering - changes in mineral compositions upwards through the layers
Crystal Fractionation to an Extreme Degree
•
•
•
Mafic minerals are all Fe-richer toward the top of layered series (Fe-end members
have lower melting/crystallization temperatures)
Plagioclase is more Na-rich toward the top (Na-plag crystallizes later and at lower
temperature than Ca-plag)
Quartz and micropegmatite represent the little bit of "granite" that can result from
crystal fractionation of a tholeiitic basalt