Layered/Mafic Intrusions Flashcards
(17 cards)
What is a layered basic/mafic intrusion and where does it form?
Coarse grained layered rocks, dominated by olivine, pyroxene, plagioclase and various oxides.
Forms within continental crust.
What does a layered intrusion represent?
Accumulations of minerals that have fractionated from a basaltic/mafic magma
Where are layered intrusions commonly found?
In thick sequences of flood basalt lavas.
What is a major source of Platinum group elements e.g. Pt, Ir, Os?
Layered basic and mafic intrusions
Give five examples of names, ages, locations and areas of layered intrusions
Bushveld, Precambrian, S. Africa, 66,000km2
Dufex, Jurassic, Antartica, 50,000km2
Duluth, Precambrian, Minnesota, 4,700km2
Stillwater, Precambrian, Montana, 4,400km2
Muskox, Precambrian, Canada, 3,500km2
What is a cumulate?
Minerals which have ‘accumulated’ to
produce layers e.g. olivine cumulate, pyroxene-plag cumulate, gabbro cumulate
What is layering?
Sheet-like unit distinguishable from others by texture or composition
Describe modal layering
Characterised by variation in relative proportions of constituent minerals. Can be uniform or graded.
What characterises phase layering?
Presence or absence on constituent minerals
What is cryptic layering?
Systematic (or non-systematic) variation in the chemical compositions of certain minerals with stratigraphic/structural height in a layered sequence
What is meant by rhythmic layering?
Layers repeat, macro - ms, micro - mms
Describe intermittent layering
Less regular patterns e.g. graded rhythmic layers with occasional uniform layers
How can rhythmic and modal layers be explained?
Crystal settling interrupted by periodic large scale convective overturns of magma body
What are the problems with crystal settling in Stokes’ Law?
Size factor is more important than density factor in Stokes’ Law;
Size grading is rare;
Dense olivine occurs in the UBS;
‘Buoyant’ plagioclase occurs low down in the LS;
Inverted cryptic layering in the UBS suggests flotation of dense minerals;
MBS show near-vertical layering;
Basaltic magmas develop a significant yield strength just below their liquidus temperatures which will retard crystal magma segregation;
An alternative theory is that, nucleation and growth of minerals in a thin, stagnant boundary layer around the margins of a cooling body of magma, is this possible?
– Fractionation can still be achieved by differential motion of crystals and magma;
– Dominant motion is migration of depleted magma away from site at which crystals grow;
– Crystal may settle or float a short distance within such a boundary layer as the melt migrates away.
What is compositional convection?
Occurs in systems with gradients in two or more properties, e.g. thermal and compositional involving different rates of diffusion
Gradients may cancel or enhance
Explain density current factors
Cooler, more dense crystal laden liquids may descend within a chamber and move across the floor. Dense crystals (e.g. olivine) may stay in agitation Light crystals (e.g. plag) may be trapped and carried down.
