Abstract
Granite enclaves. Three main types of enclaves can be recognised
in granitic rocks. Some, though not abundant, are metamorphic
rocks (quartzites and gneisses). Some, that can be locally
abundant, are extremely fine grained igneous rocks, have chilled
margins, and are, with little doubt, mafic magma sheets or globules
quenched in the felsic magma. The majority, are microgranitoid
enclaves (MGE), that have medium grainsize igneous
microstructures, have no chilled margins and virtually all are as
mafic or more mafic than the host. While outcrops that record
the sites of mafic magma injection and quenching have been
well-documented, many enclaves of this type are commonly found
in outcrop in association with the other two types as disordered
accumulations. The site of enclave accumulation (that can include
all three types) appears unrelated to the position in the magma
chamber where they formed, and are therefore analogous to palaeontological
‘‘death assemblages.’’
MGE as cumulates from felsic magmas. Observations relevant
to the origin of MGE include: MGE in S-type granites are generally
S-type; in general 87Sr/86Sr initial ratios are similar to the
host; generally the MGE have the same minerals as the host but
in different proportions; some have minerals that could not have
crystallised from a magma of the bulk composition of the MGE;
many MGE have epsilon 176Hf/177Hf values lower than the host
[2]; some have abundant phenocrysts that are the same size and
composition as those of the host or are minerals that might be
expected to be near-liquidus phases in a magma of that composition.
The recognition that many granitic plutons have significant
isotopic variation, and that in at least one zoned pluton, it is the
felsic core that is the most isotopically primitive leads to the suggestion
that primitive isotopic ratios do not necessarily mean mafic
compositions. It is argued that many MGE are not thermally
quenched mafic magma globules but are cumulates formed near
the roof of the magma chamber as rafts of crystals that heterogeneously
nucleated on phenocrysts during pressure quench events.
in granitic rocks. Some, though not abundant, are metamorphic
rocks (quartzites and gneisses). Some, that can be locally
abundant, are extremely fine grained igneous rocks, have chilled
margins, and are, with little doubt, mafic magma sheets or globules
quenched in the felsic magma. The majority, are microgranitoid
enclaves (MGE), that have medium grainsize igneous
microstructures, have no chilled margins and virtually all are as
mafic or more mafic than the host. While outcrops that record
the sites of mafic magma injection and quenching have been
well-documented, many enclaves of this type are commonly found
in outcrop in association with the other two types as disordered
accumulations. The site of enclave accumulation (that can include
all three types) appears unrelated to the position in the magma
chamber where they formed, and are therefore analogous to palaeontological
‘‘death assemblages.’’
MGE as cumulates from felsic magmas. Observations relevant
to the origin of MGE include: MGE in S-type granites are generally
S-type; in general 87Sr/86Sr initial ratios are similar to the
host; generally the MGE have the same minerals as the host but
in different proportions; some have minerals that could not have
crystallised from a magma of the bulk composition of the MGE;
many MGE have epsilon 176Hf/177Hf values lower than the host
[2]; some have abundant phenocrysts that are the same size and
composition as those of the host or are minerals that might be
expected to be near-liquidus phases in a magma of that composition.
The recognition that many granitic plutons have significant
isotopic variation, and that in at least one zoned pluton, it is the
felsic core that is the most isotopically primitive leads to the suggestion
that primitive isotopic ratios do not necessarily mean mafic
compositions. It is argued that many MGE are not thermally
quenched mafic magma globules but are cumulates formed near
the roof of the magma chamber as rafts of crystals that heterogeneously
nucleated on phenocrysts during pressure quench events.
| Original language | English |
|---|---|
| Pages (from-to) | A178-A178 |
| Number of pages | 1 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 70 |
| Issue number | 18 supplement |
| DOIs | |
| Publication status | Published - 2006 |
| Event | Goldschmidt Conference (16th : 2006) - Melbourne, Australia Duration: 27 Aug 2006 → 1 Sep 2006 |