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Though Munksgaard (1988) found that chemically a mixture of subequal amounts of high-grade metapelite and metapsammite could produce the Cooma Granodiorite, our work has shown that the first (syn-D3) melting occurs only in the
calcium-poor metapelites, producing plagioclase-free magmas with
compositions unlike that of the granodiorite. The fact that the incipient
intrusions of granitic material (metapsammite leucosome) resembling the
Cooma Granodiorite have disaggregated leucosomes in Snake Creek, implies
that this granitic material post-dates the partial melting responsible for
the leucosomes. In contrast to the metapelite leucosomes, this leucosome
is rich in plagioclase, as is the Cooma Granodiorite. If this late
intrusive material can be related to the Cooma Granodiorite (requiring
more mineralogical and chemical work to be certain of the correlation),
the Cooma Granodiorite cannot be regarded as a product of in situ partial
melting of the adjacent metapelitic rocks. Though in situ melting of
quartzofeldspathic rocks appears to have produced some leucosome of
potentially suitable composition, the locally abundant volumes of Cooma
Granodiorite appear to support the suggestion of Vallance (1969, p. 185)
that the granodiorite magma has moved away from its source-rocks, even if
only for a small distance.
Munksgaard (1988) suggested that
water-rich fluid was involved in the formation of the Cooma Granodiorite,
because it has slightly, but consistently lower d18O values than the adjacent metasediments. This is supported by evidence of water access into the rocks of Snake Creek, producing coarse-grained, strongly foliated, retrograde mica schists and pegmatite. It remains to be determined whether or not this water correlates with the formation of plagioclase-bearing leucosome and intrusion of the Cooma Granodiorite late in the history of the Cooma Complex.
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