Magma Mingling and Mobilization, St Peter Suite, South Australia

 

Magma Mingling and Mobilization at Point Brown
This page is part of a group of pages on the magmatic aspects of the St Peter Suite. Here, the focus is the various processes related to mingling in intrusive irregular sheets of hybrid magma with mafic enclaves. These preserve many interesting aspects of flow and mingling. Further, as they intrude into a red granite there is evidence for local hybridization and that the hybridized matrix of the intrusive sheets is more mobile than the enclaves so that enclave swarms are left behind, and the mobile matrix intrudes further into the surrounding as enclave-free sheets.

 

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Intrusion of mingled magma pulses

 

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a) Intrusive irregular sheet of hybrid magma comprising a grey granodiorite matrix surrounding a densely packed mass of mafic microdioritic enclaves. Hybrid sheet intrudes coarse granite. Point Brown.	b) Same.
c) Same as (b) showing an irregular, pillow-shaped margin with mullion-shaped intrusions of the granodiorite matrix into the surrounding granite. Point Brown.	d) Detail of (c). Notice heterogeneous distribution of K-feldspar phenocrysts in grey granodiorite, presumably inherited from the disaggregation of the surrounding granite.
e) Flow differences marked by shapes of mafic enclaves. At the strain shadow of a granite protrusion, mmes are pillow shaped and undeformed whereas at the margin, away from the strain shadow mmes are strongly elliptic, indicating higher strains. Point Brown.	f) Detail of (e) showing pillows.
g) Variation in the packing of mmes. Note a faint boundary between a mme-rich band (closer to the viewer) and a mme-poor band (away from the viewer) coinciding roughly with the position of the two xenoliths of granitic material. This boundary might indicate the boundary between two co-magmatic pulses of hybrid magma. Point Brown.	h) 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. Point Brown.
i) Lateral gradation in the packing of mmes, suggesting an effective process of sorting. Notice parallel gradation in the nature of the felsic matrix, from mafic-rich on the left to mafic-poor on the right, with increasing number of K-feldspar phenocrysts (to the right). Point Brown.	j) Enclave-rich margin of a wide dyke with margins parallel to the pen on the upper part. The long axes of the enclaves are oriented at an angle to that margin and sheared locally into parallelism with it, indicating flow and strain localization. Point Brown.

 

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Mobilization of interstitial hybrid magma from enclave and in situ hybridization

 

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a) Enclave-rich intrusion feeding further intrusions that are enclave free, suggesting mobility of interstitial, intermediate hybrid magma, and filter pressing of enclaves. Point Brown.	b) Branching dykes of intermediate hybrid magma, essentially enclave free, except for a few small enclaves are found where the dykes merge and are at their narrowest. Interpretation: enclaves were filtered out. Point Brown.
c) Hybrid magma intruding granite. Point Brown.	d) A dyke branching out of a larger intrusion, with small mafic enclaves, and irregular erosional contact with the surrounding granite further hybridizing the intrusive magma. Point Brown.
e) Irregular intrusive sheets with enclaves in their more central parts. These are linked to a larger enclave rich intrusive body. Point Brown.	f) Irregular intrusive magma, interacting with the immediate surrounding creating gradational contacts. Point Brown.

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