Magma Evolution: Mingling-Mixing, Disaggregation-Assimilation, Flow and Fractionation, Saint Peter Suite

 

The Saint Peter Suite shows a series of features indicative of a number parallel processes that affect magma differentiation that are recorded and partially preserved at emplacement level. These are detailed over four separate pages and summarised here. Most observations are based on the observations at Point Brown where we have the intrusion of a hybrid granodioritic magma loaded with mafic enclaves, into a coarse-grained red granite. This intrusive disaggregates and assimilates granite, and also older pegmatites. This is visible by preservation of a number of steps of disaggregation of xenoliths of granite and pegmatite in the hybrid, and erosion of the contact leading to gradational contacts between the hybrid granodiorite and granite. This leads to further hybridization, forming a magmatic rock with heterogenously distributed red K-feldspar xenocrysts and presumably also xenocrysts of quartz (not recognized in the field). This hybrid intrusive with enclaves form irregular dykes where the mafic enclaves mmes) tend to be concentrated towards the centre and the hybrid matrix magma at the margins from where it intrudes the surrouding granite, without enclaves. This feature, together with the tight packing of enclaves suggests that the enclaves might have been solid and were filter pressed out and that the hybrid magma remained mobile and continued to migrate. The intrusive hybrid matrix commonly records flow banding interpreted to represent flow sorting against rigid margins, forming marginal schlieren, and banding characterized by biotite enrichment along the margins and gradation inwards, channel erosion and infill, and cross-bedding.

 

This page is the general summary of all of these features, and is divided into four parts which are detailed in the these four linked pages:

Granite Disruption

Pegmatite Disaggregation

Magma mingling and Mobilization

Flow Banding

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Granite Disaggregation

 

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a) Xenolith of red granite, ~1.5 m wide, within a 5m wide dyke of hybrid magma.	b) Xenolith of red granite in hybrid granodiorite.
c) Coarse red granite xenoliths in grey granodiorite with a few mafic enclaves, showing different degrees of disaggregation down to individual K-feldspar phenocrysts.	d) Same.
e) Intrusion of enclave-rich grey granodiorite, cutting a sharp contact between a coarse, K-feldspar-rich, red granite and a grey, enclave-poor granodiorite.	f) 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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Pegmatite Disaggregation

 

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a) Early pegmatite disrupted by a grey granodiorite with numerous mafic microdioritic enclaves.	b) Large and rounded pegmatitic K-feldspar and quartz single grains isolated in hybrid magma..
c) Small pocket of hybrid magma with small mafic enclaves and pegmatitic K-feldspar single grains.	d) Large and rounded pegmatitic K-feldspar single grains isolated in hybrid magma.

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Mingling Features and Mobilization of Hybrid Magma

 

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a) Hybrid magmas with tightly packed mafic pillows with granodiorite matrix which intrudes the surrounding granite. Point Brown.	b) 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.
c) 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.	d) 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.
e) 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.
f) Hybrid magma intruding granite. Point Brown.	g) 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.

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Flow Banding

 

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a) Rythmic vertical flow banding in granite at Point Brown.	b) Flow banding: a top layer cuts across and truncates previously formed, curved flow banding below. This is interpreted as an erosional channel. Point Brown.
c) Irregular flow banding.	d) Flow banding folded in between two xenoliths of red coarse granite.

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