Gneiss Geology: A structural perspective of foliated granitoids and their host rocks in the Wright Valley, South Victoria Land, Antarctica.
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The Lower Paleozoic 'Bonney Pluton' is a coarse-grained, variably megacrystic, monzodioritegranite body which crops out in the Wright Valley, southern Victoria Land, Antarctica. It contains the assemblage quartz- plagioclase (An20-40)- K-feldspar- hornblende- biotite +/- clinopyroxene, with accessory phases titanite, magnetite, ilmenite, zircon, allanite and apatite. Although this calc-alkaline pluton contains numerous northwest-southeast striking mesoscopic compositional variations, the body is unzoned at a macroscopic scale.
Two end-member fabrics are developed in the Bonney Pluton: 'pre-full crystallisation fabric' is characteristic of the central granitoid; while at the margins of the pluton 'crystal plastic strain fabric' variably overprints pre-full crystallisation textures parallel to the pluton/host rock contacts. Quantitative strain analysis suggests X/Z ~ 1.5 for crystal plastic strain. Crystal alignments are either S fabric (planar) or L fabric (linear). Foliation of the pluton generally strikes northwest-southeast. Lineation, due to both crystal plastic strain and pre-full crystallisation magmatic flow, is parallel in the eastern margin of the pluton plunging 20° to the northwest. Prefull crystallisation 'flow' towards the northwest occurred in the eastern margin, parallel to the pluton/host rock contact.
The Bonney Pluton intruded Koettlitz Group metasediments and interlayered orthogneisses. Three phases of deformation, which were accompanied by amphibolite facies metamorphism, are recognised in Koettlitz Group rocks. The first two phases were syn-migmatitic. Fe-Mg garnet-biotite geothermometry suggests a peak metamorphic temperature of 680°± 40°C.
Intense folding and transposition during the first phase of deformation (D1) resulted in a pervasive foliation parallel to lithologic layering. During the second phase of deformation, D1 structures were folded both macroscopically and mesoscopically, and subjected to continued deformation. Macroscopic F2 folds vary in tightness, and axes vary in plunge by 40° within a plane striking 135°. Maximum D2 extension directions, deduced from the apparent rotation of linear features, differ significantly in the host rocks either side of the Bonney Pluton. East of the pluton, extension occurred about an axis of 320/40° with compression about an axis of 050/0°. West of the pluton, extension occurred about an axis of 300/40° with compression about an axis of 090/50°.
Three orthogneiss lithologies are present in a variety of structural positions suggesting intrusion into the Koettlitz Group prior, during, and after D2. Orthogneisses can be difficult to distinguish from paragneisses of the Koettlitz Group, particularly where both have been intensely migmatised and deformed, but a number of textural and chemical features indicate a plutonic origin. Orthogneisses typically contain significantly less normative quartz than quartzofeldspathic paragneiss, and have higher Sr values (>325ppm) and have lower Cr and Ni contents than Koettlitz Group rocks containing the same amount of Si02.
On the basis of hand-specimen characteristics and geochemistry, hornblende+biotite orthogneisses are indistinguishable from the granitoid of the Bonney Pluton. Although they are in different structural positions, the two granitoids are considered to represent a single continuum of intrusion.
Ballooning of the 'proto-pluton' resulted in intrusion-related flattening perpendicular to the margins of the Bonney Pluton. In response to flattening, macroscopic D2 structures were developed in the host rocks (metasediments and precursor 'orthogneiss' intrusions), crystal plastic strain fabrics in the pluton margins; and pre-full crystallisation fabrics in the more central granitoid. Late-stage D3 structures developed along tectonically reactivated pluton/host rock contacts with continued intrusion.
Published data suggest the Bonney Pluton crops out over an area greater than 1000km2. The body is elongate with a northwest-southeast trending long axis, parallel to strongly developed stretching lineations and maximum extension directions in the host rocks. Emplacement of the pluton may have been accompanied by a regional deformation event.
Two end-member fabrics are developed in the Bonney Pluton: 'pre-full crystallisation fabric' is characteristic of the central granitoid; while at the margins of the pluton 'crystal plastic strain fabric' variably overprints pre-full crystallisation textures parallel to the pluton/host rock contacts. Quantitative strain analysis suggests X/Z ~ 1.5 for crystal plastic strain. Crystal alignments are either S fabric (planar) or L fabric (linear). Foliation of the pluton generally strikes northwest-southeast. Lineation, due to both crystal plastic strain and pre-full crystallisation magmatic flow, is parallel in the eastern margin of the pluton plunging 20° to the northwest. Prefull crystallisation 'flow' towards the northwest occurred in the eastern margin, parallel to the pluton/host rock contact.
The Bonney Pluton intruded Koettlitz Group metasediments and interlayered orthogneisses. Three phases of deformation, which were accompanied by amphibolite facies metamorphism, are recognised in Koettlitz Group rocks. The first two phases were syn-migmatitic. Fe-Mg garnet-biotite geothermometry suggests a peak metamorphic temperature of 680°± 40°C.
Intense folding and transposition during the first phase of deformation (D1) resulted in a pervasive foliation parallel to lithologic layering. During the second phase of deformation, D1 structures were folded both macroscopically and mesoscopically, and subjected to continued deformation. Macroscopic F2 folds vary in tightness, and axes vary in plunge by 40° within a plane striking 135°. Maximum D2 extension directions, deduced from the apparent rotation of linear features, differ significantly in the host rocks either side of the Bonney Pluton. East of the pluton, extension occurred about an axis of 320/40° with compression about an axis of 050/0°. West of the pluton, extension occurred about an axis of 300/40° with compression about an axis of 090/50°.
Three orthogneiss lithologies are present in a variety of structural positions suggesting intrusion into the Koettlitz Group prior, during, and after D2. Orthogneisses can be difficult to distinguish from paragneisses of the Koettlitz Group, particularly where both have been intensely migmatised and deformed, but a number of textural and chemical features indicate a plutonic origin. Orthogneisses typically contain significantly less normative quartz than quartzofeldspathic paragneiss, and have higher Sr values (>325ppm) and have lower Cr and Ni contents than Koettlitz Group rocks containing the same amount of Si02.
On the basis of hand-specimen characteristics and geochemistry, hornblende+biotite orthogneisses are indistinguishable from the granitoid of the Bonney Pluton. Although they are in different structural positions, the two granitoids are considered to represent a single continuum of intrusion.
Ballooning of the 'proto-pluton' resulted in intrusion-related flattening perpendicular to the margins of the Bonney Pluton. In response to flattening, macroscopic D2 structures were developed in the host rocks (metasediments and precursor 'orthogneiss' intrusions), crystal plastic strain fabrics in the pluton margins; and pre-full crystallisation fabrics in the more central granitoid. Late-stage D3 structures developed along tectonically reactivated pluton/host rock contacts with continued intrusion.
Published data suggest the Bonney Pluton crops out over an area greater than 1000km2. The body is elongate with a northwest-southeast trending long axis, parallel to strongly developed stretching lineations and maximum extension directions in the host rocks. Emplacement of the pluton may have been accompanied by a regional deformation event.
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ix., 185 p., ill., Maps in pocket folded., 30cm.
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1989Cox
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Cox, SC, “Gneiss Geology: A structural perspective of foliated granitoids and their host rocks in the Wright Valley, South Victoria Land, Antarctica.,” Otago Geology Theses, accessed March 19, 2025, https://theses.otagogeology.org.nz/items/show/228.