Granite Xenolith Disaggregation in Magma at Point Brown, St Peter Suite, South Australia

 

Granite Disaggregation

 

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This page is part of a group of pages on the magmatic aspects of the St Peter Suite. Outcrops exposed in Point Brown are characterized by a red, K-feldspar-rich, coarse granite, intruded by multiple pulses of a medium-to-coarse grained granodiorite with varying proportions of mafic microdioritic enclaves. Enclaves are distributed heterogeneously at outcrop scale, marking a combination of differential fractionation, and different intrusive pulses with gradational boundaries due to co-mingling. The most dramatic feature of these outcrops is the erosion and disaggregation of pre-existing magmatic rocks, particularly the oldest red granite, by new pulses. The process leads to xenoliths and xenocrysts giving rise to heterogeneous hybrids. There are thus two processes recorded that leads to magma differentiation: fractionation of mmes and disaggregation of xenoliths. This page records the interaction between the intrusions, with particular focus on the disaggregation,

 

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a) Dyke of grey granodiorite with mafic enclaves intruding and disrupting a coarse, K-feldspar-rich, red granite. Width of main dyke is ~5m.	b) Detail of a (a) width of main square xenolith is ~2m.
c) Xenolith of red granite, ~1.5 m wide, within a 5m wide dyke of hybrid magma.	d) Xenolith of red granite in hybrid granodiorite.
e) Coarse red granite xenoliths in grey granodiorite with a few mafic enclaves, showing different degrees of disaggregation down to individual K-feldspar phenocrysts.	f) Same.
g) Dyke of grey granodiorite intruding the contact between a coarse, K-feldspar-rich, red granite, above, and a finer, red granite below, both with small and few mmes. Dyke has more small mafic enclaves, as well as xenoliths of the coarse, red granite from above, disaggregate to a heterogeneous mass.	h) Detail of a coarse, K-feldspar-rich, red granite xenolith in enclave-rich grey granodiorite, in contact with a finer granite. Note that the contact is irregular at grain scale, and granodiorite is rich in red K-feldspar phenocrysts.
i) Lense of granite in between lenses of hybrid magmatic rock. The contact between the two is irregular and gradational over one centimetre.	j) Same.
k) Intrusive grey granodiorite with irregular contact with the surrounding granite and eroding individual grains.	l) Intrusive grey granodiorite with enclaves with irregular contact against granite including granite xenoliths as well as K-feldspar xenocrysts. This shows the full complexity of the interaction between the intrusive hybrid magma and the resident magmatic rock.

 

 

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Multiple intrusive pulses

 

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a) Intrusion of enclave-rich grey granodiorite, cutting a sharp contact between a coarse, K-feldspar-rich, red granite and a grey, enclave-poor granodiorite.	b) Enclave-free yellowish-grey granite into enclave-rich granodiorite. Contacts are irregular and ill-defined suggesting that intrusion was syn-magmatic.
c) Irregular, pillowed margins between a decametric gabbroic intrusion and red, coarse granite. Towards the margin the gabbro disaggregates into pillows surrounding by a grey granodiorite matrix (shown), and this hybrid interacts with the external granite. The contact between the hybrid granodiorite and the red external granite is gradational, suggesting hybridization.	d) Deatails of the margin where a mafic dyke has xenoliths of the surrounding granite resulting from erosion and disaggregation of the granite, and the host granite, contains rare mmes, presumably related to a previous process of hybridization.
e) Large block of red granite, with a tail and a rim of mafic magmatic rock, with a sharp boundary against a grey granodiorite with rare enclaves.

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