Transcript Document

Order of crystallisation & enclaves
• It has long been common to
work out the sequence in
which the minerals in an
igneous rock began and/or
ceased to crystallise.
• Melting experiments show
that in many rocks there is a
sequence in which the
minerals begin and in a few
cases cease to crystallise.
If it could be shown that
Plagioclase came out before
Hornblende it would indicate
Low pressure.
Published order
of crystallisation
• Note that authors
“know” the order of
starting and finishing!
• Even though K-feld.
is last, it will start
when there is 60%
melt in most granites.
Criteria for order of crystallisation.
• SIZE:Commonly
believed that early are
large, late are small.
Very very dangerous!
Experiments show that
K-feldspar is the last to
crystallise from granite
but commonly they are
very large. For a
porphyritic rock the
phenocrysts will have
formed before any
minerals just in the
groundmass.
Inclusion relationships
• In general the inclusion of one mineral in
another is also dangerous BUT “reaction
relationships” where one mineral is
replaced by another that forms a rim can
be very reliable.
• In many rocks the first minerals to form
are anhydrous (e.g.pyroxene) which stops
when the residual melt is sufficiently
water-rich to have a hydrous mineral (e.g.
hornblende) form instead. The hornblende
commonly forms a rim around all the
pyroxene grains and must be later. It only
works if ALL pyroxene is surrounded by
hornblende
Ophitic microstructure
is formed by
simultaneous
crystallisation of
pyroxene & plag. But
with poor nucleation of
the pyrox. And ready
heterogeneous
nucleation of the plag.
How well formed the crystals are
• Shape: polyhedral
early and moulding
or irregular late.
Again very
dangerous! Most
crystals will only
come in contact long
after they both start.
Orbicular Granites & Comb Layering
• These are very unusual outcrops, generally only a couple per
continent. The orbs seem to be made of rings of radiating
minerals (the normal granite minerals but in layers). Ron Vernon
argued these formed when the melt had been superheated and had
no nucleii except for fragments of solid rock. The superheating
may be the result of water being added. Comb Layering is the
same but with the nucleation being on a wall of the magma
chamber.
XENOLITHS & ENCLAVES
• Most granitic rocks have
fine grained inclusions
that are mineralogically
similar to the granite but
more slightly more mafic
and with an igneous
microstructure. These are
now commonly known as
“microgranitoid enclaves”
(MGE).
• Xenoliths are pieces of
foreign rock, generally
metamorphic.
Quenched Magma Globules
• In a few localities the
microgranitoid
enclaves have
quenched ultra-fine
grained margins that
indicate clearly they
represent quenched
magma globules.
Syn-Plutonic dykes and sills
• A “syn-plutonic dyke” in
granite. The irregular
margin suggests both were
soft at the time. The
“blebs” may be rod-like in
the vertical plane.
• R. Wiebe has described
“basaltic flows” that are
emplaced along the
surface of the crystal mush
at the bottom of a granitic
magma chamber in Maine.
Origins of MGE
• Microgranitoid enclaves (MGE) have a medium grainsize igneous
microstructure. Three models:
• mingled magma globules being more mafic (higher temperature)
and quench in the granite (99% believe this) OR
• pieces of rock from the source region that did not melt enough for
the melt to separate (a few Australians believe this)OR
• are large crystal clusters formed when the magma lost water as
the roof fractured and pressure quenched (I could be the only
person who believes this).
Many MGE are only slightly
more mafic than host granite
• Fine grainsize
makes MGE
look darker
than the host
granite.
• Note absence
of chilled
margins.
Quench growth of hornblende in
MGE forms spikes
Phenocrystas in MGE are the
minerals in the host granite.
• Magma
globule
proponents
argue this is
due to
magma
mingling.
MGE Hornblende the same as the host
• Hornblende prefers Na over K. Na/K in most granites the ratio
in the granite is half that in the hornblende. The MGE have
hornblende that could not have come from a melt of the
composition of the MGE
• Dropping the
PH2O increases
the range of
magmas
crystallising plag.
And reduces the
range that
crystallise K-feld.
• One explanation of
rapakivi (magma
mixing the other)
• One explanation of
K-feldspar poor
MGE.
Pressure Quench
MGE Pressure Quench Model
• It is agreed that all
granitic magmas will
saturate with H2O.
• & released H2O
vapour will exert a
pressure sufficient to
crack the roof.
• Loss of H2O causes
quench
Quenched Icelandic basalt has
crystal clusters
• Olivine,
plagioclase
and pyroxene
aggregate in
glassy basalt.
Conclusions
• Determination of the order of crystallisation
in plutonic igneous rocks is not easy and
may not be possible!
• Enclaves in granitic rocks are in small part
quenched magma globules.
• The mineralogical and isotopic similarity of
many enclaves to the host suggests
formation from similar magmas.
• MGE have more of the high temp minerals
and could be crystal clusters formed during
pressure quench events.