Journal of Petrology
Marlina A. Elburg
ABSTRACT
The Violet Town Volcanics (Lachlan Fold Belt, Australia) is an S-type ignimbrite
suite containing microgranitoid enclaves, basaltic andesite enclaves and enclaves
of high-silica rhyolite. The microgranitoid enclaves are similar to those in
peraluminous granites. They typically have lower initial 87Sr/86Sr and higher
eNd than the host, and represent globules of a mafic, mantle-derived magma,
which was hybridised by mixing and diffusional exchange with the host magma.
The basaltic andesite enclaves were incorporated into the ignimbrite as xenoliths,
but their parental magma may have been similar to that of the microgranitoid
enclaves. They are isotopically less depleted than other mantle-derived rocks
from the Lachlan Fold Belt, reflecting contamination by crustal material, or
derivation from less depleted mantle sources. The high-silica rhyolite enclaves,
previously interpreted to be related to the ignimbrite by crystal fractionation,
have eNd values up to 3 units higher than their host, and cannot be related
to their host by crystal fractionation or AFC processes.
The coexistence of S-type magmas and mantle-derived magmas suggests that the
latter may have played a role in the Palaeozoic magmatism of the Lachlan Fold
Belt, acting as a heat source for melting and perhaps also contributing chemical
components to the crustally derived magmas.
Keywords:
enclaves / magma mingling / magma mixing / S-type
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