The Frio Peak area, upper Walcott Glacier region, southern Victoria Land, Antarctica : a detailed study of rocks comprising Frio shear zone

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Beckett, Sally Ann.

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Abstract:

The Frio Peak area provides a complete section through Frio Shear Zone (FSZ) the area dividing the Dry Valleys Block and KoettlitzGlacier Alkaline Province (KGAP), southern Victoria Land, Antarctica and is located in the upper Walcott Glacier region extending from McConchie Ridge ip. the SW to the Bonney Pluton contact on Salient Ridge in the NE. The Frio Peak area consists of a 5 km wide zone of SSE-SSW dipping and SE plunging amphibolite facies Neoproterozoic Skelton Group metasediments of biotite, biotite-amphibole and calc-silicate schist and lesser marble, amphibolite, pelitic and psammitic schist. The metasediment is intruded by Bonney Pluton, late-stage Chancellor Orthogneiss dykes, rare muscovite leucogranite dykes and finally felsic porphyry dykes during the Cambro-Ordovician Ross Orogeny related to westward subduction of the paleo-Pacific margin beneath the East Antarctic Craton. Chancellor Orthogneiss is a weakly peraluminous biotite granite with adakitic characteristics, depleted in Y, Nb, Rb and enriched in Sr and intrudes predominantly as late-stage Chancellor Orthogneiss dykes in the Frio Peak area. The muscovite leucogranite dykes are weakly to strongly peraluminous with normative corundum ranging from 0.8 to 3.97 and aluminium saturation index of between 1.06 and 1.36 and display no adakitic characteristics. Variably deformed late-stage Chancellor Orthogneiss and rare muscovite leucogranite dykes intrude pre-, syn- and post the youngest phase of ductile deformation c. 490 Ma under greenschist facies conditions, generally intruding and foliated parallel to the metasediment foliation. Skelton Group metasediment across the entire Frio Peak area is amphibolite facies, indicated by phase relations, the presence of sillimanite, migmatites, feldspar composition, garnet-biotite and tourmaline-biotite Fe-Mg exchange. A metamorphic gradient exists ranging from lower to middle amphibolite facies on McConchie Ridge (580-600°C) to upper amphibolite facies in the northern Frio Peak area (600-670°C). A gradual zone characterised by three different subparallel fabrics was identified across FSZ in the Frio Peak area. These fabrics reflect the three main phases of deformation along FSZ that increase in strain and metamorphic grade from the SW to the NE. The age of the dominant foliation is well constrained at the northern and southern extent of FSZ in the Frio Peak area. There is evidence of a lower to mid amphibolite facies c. 530 Ma foliation at the McConchie Ridge (SW) end of the Frio Peak area related to Walcott Block deformation; an upper amphibolite facies c. 500 Ma foliation at the Bonney Pluton (NE) end of the Frio Peak area related to Dry Valleys Block Bonney Pluton intrusion; and a greenschist facies c. 490 Ma foliation throughout the entire Frio Peak area unique to FSZ associated with the intrusion of the late-stage Chancellor Orthogneiss dykes. These three phases of deformation reflect the long-lived complex history of deformation along FSZ. Ductile deformation continued along FSZ during the main phase of DVlb magmatism 495-490 Ma once it had ceased in the Dry Valleys Block and the Walcott Block. This distinct phase of ductile deformation along FSZ is at least 5- 10 Ma younger than ductile deformation to the north and up to 40 Ma younger than ductile deformation to the south in the Walcott Block. Chancellor Orthogneiss, due to its intrusion along FSZ, is the only DVlb adakitic intrusion from the major DVlb phase to be strongly foliated with solid-state fabrics overprinting magmatic fabrics. Detailed study across FSZ in the Frio Peak area suggests FSZ is a 'subtle' NW-trending crusta! structure dividing the Koettlitz Glacier Alkaline Province (KGAP) and the Dry Valleys Block in the Walcott Glacier area. The FSZ rocks are no more highly deformed than throughout the Dry Valleys Block and many parts of the W alcott Block. The main evidence for FSZ are regional structural trends, the distinct magmatic and tectonic histories of the Dry Valleys Block and the Walcott Block (northern KGAP), the juxtaposition of two foliations of different ages and the fact that a unique phase of ductile deformation c. 490 Ma occurs along this zone. FSZ was active over a period of approximately 40 Ma during the main phase of the Cambro-Ordovician Ross Orogeny from c. 530 - 490 Ma and is present due to its location at the northern extent of a bend in the orogen that resulted in major differences in tectonics and magmatism to the north in the Dry Valleys Block and to the south in the Walcott Block (northern KGAP). The extent of FSZ is marked by the presence of Chancellor Orthogneiss and the c. 490 Ma phase of predominantly NW-trending ductile deformation. This places the northern boundary of FSZ at the Bonney Pluton contact while the southern boundary lies under the Walcott Glacier. The three phases of deformation that occurred along FSZ correlate with the regional tectonics related to changes in the direction of plate movement and cessation of subduction followed by uplift marking the final stages of the Ross Orogeny. Dextral deformation occurred along FSZ c. 530 Ma causing the rotation of the Walcott Block foliation to become parallel to FSZ while extension was occurring in the KGAP and transcurrent deformation in the Dry Valleys Block. During Bonney Pluton intrusion c. 500 Ma where subduction and DVla calc-alkaline magmatism dominate the Dry Valleys Block and transcurrent deformation and relative quiescence prevails in the KGAP, FSZ controls the SW end of Bonney Pluton resulting in two SE-trending lobes and a change in the orientation of the pluton from NNW to NW. Sinistral-normal deformation occurred along FSZ c. 490 Ma during late-stage Chancellor Orthogneiss dyke intrusion when adakitic magmatism and post-subduction uplift is occurring in the Dry Valleys Block and the KGAP.

Thesis description:

x, 176 leaves : ill. (some col. and folded), maps (some col., 1 folded) ; 30 cm.

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OU geology Identifier:

2003Beckett

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POLYGON ((163.022564627118641 -78.147383235657955,162.991206052597988 -78.165951912336396,162.895352855257215 -78.163890073400111,162.85734593822491 -78.140299685467298,162.852807266778569 -78.137476057670156,162.959443128887642 -78.126662520950532,162.984449706579966 -78.124118098136591,163.033807554479552 -78.119086319661449,163.063186531592663 -78.123238660257485,163.061024222737956 -78.124526467389586,163.053724016813476 -78.128872101176455,163.022564627118641 -78.147383235657955))

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http://download.otagogeology.org.nz/temp/Abstracts/2003Beckett.pdf

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Citation

Beckett, Sally Ann., “The Frio Peak area, upper Walcott Glacier region, southern Victoria Land, Antarctica : a detailed study of rocks comprising Frio shear zone,” Otago Geology Theses, accessed February 13, 2025, https://theses.otagogeology.org.nz/items/show/404.

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