Structural evolution of northern Walcott Bay, South Victoria Land, Antarctica / Sarah Anne Jones.

Jones, Sarah Anne.

Four granitoid plutons, belonging to the DV1a and DV1b plutonic suites, intrude Koettlitz Group metasediments in the Walcott Bay area, South Victoria Land, Antarctica. The plutons display markedly different structural styles, reflecting their emplacement during progressive uplift in the Paleozoic Ross Orogeny. In order of intrusion: 1) Bonney Pluton (530-520 Ma), a regionally extensive hornblende-biotite body, with the southern end mapped in the Walcott Bay area. The pluton syn-kinematically intrudes the metasediments at mid-upper amphibolite fades conditions, 7<>0°C, 5±1 kbars. Fabric development is variable with magmatic flow fabrics at the NE margin and central parts of the body changing to solid-state fabrics along the SW margin Host rock foliations are parallel to pluton margins. 2) Armitage Pluton is a hornblende-biotite DV1a body, chemically distinct from Bonney Pluton, which intrudes central parts of Bonney Pluton at a late stage of the host granite crystallisation. A weak magmatic lineation is present, sub-parallel to magmatic lineations in central parts of Bonney Pluton. 3) Chancellor Orthogneiss is a strongly deformed biotite DV1b body which discordantly intrudes metasediments as randomly orierued dykes and stocks. Solid-state deformation fabrics overprint magmatic fabrics throughout the body indicating post-emplacement ductile deformation. This body was emplaced close to or at the brittle-ductile transition, at least biotite zone, 250-350°C, 10-15 km depth. 4) Hidden Granite is an undeformed DV1b biotite granite which discordantly intrudes metasediments and older granitoid bodies as randomly oriented dykes and stocks. The granite was emplaced into a brittle host rock, above the brittle-ductile tranisition, less than 250-350°C. Hidden Granite is coevally intruded by a swarm of gabbroic to intermediate plugs, dykes and near horizontal· sills (1-40m thick) of the Keyhole Mafic Suite, a genetically related group of mafic bodies with high-K calc-alkaline affinities. Contact relations between the mafic bodies and host granite such as: lobate to crenulate boundaries; enclave swarms; diffuse flame-like contacts; a lack of chilled margins and back-dyking of the mafic bodies indicates the mafic bodies are coeval with Hidden granite. The mafic bodies are divided into two end-member groups (irregular plugs and sheet-like bodies), based on structural features. Contact relations of the irregular plugs and the host granite are generally more variable than the sheet-like bodies. The orientation of the plugs is random whereas the consistent NE-SW orientation of vertical sheet-like bodies indicates a degree of regional structural control on their emplacement. The degree of interaction between mafic and felsic phases generally decreases as the mafic bodies become more sheet-like in form. These changes could reflect the increasing crystallinity of the host granite.Field observations, petrology and geochemical data indicates chemical interaction accompanies physical mixing in the strongly mingled margins to form hybrid zones (at least at a centimetre scale). However, chemical mixing is most likely insignificant at margins of the sheet-like bodies, even though in detail, these boundaries are quite gradational. Vertical NE-SW oriented bodies of the Keyhole Mafic Suite are parallel to a younger swarm of mafic and felsic porphyry dykes which consistently strike 040°. The dyke orientation suggests crusta! extension during dyke emplacement was oriented NW-SE. This extension direction is approximately parallel to the elongation direction of post-emplacement ductile deformation fabrics in Chancellor Orthogneiss and suggests a geometric relationship between the ductile and brittle fabrics. The younger dyke swarm was emplaced towards the end of the Paleozoic Ross Orogeny and forms the dominant NE-SW brittle fabric in the basement rocks. A sequence of brittle fabrics in Hidden Valley indicates repeated reactivation of the inherited crustal anisotropy. The sequence is as follows: 1) Paleozoic NE-SW oriented dykes; 2) hydrothermally altered quartz-filled extensional fractures which occur within or parallel to the dykes; 3) NW-SE dextral faults offset dykes and fractures at right angles and are steeply dipping (to near vertical) with strikes between 110° to 140°; 4) NE-SW oriented structures crosscut young s'urface features and lie parallel to the dykes. These brittle features represent a late Quaternary sinistral reactivation of the pre-existing anisotropy. The late Quaternary faults indicate that this area of South Victoria Land is still tectonically active as indicated by the high topographic relief and active alkaline volcanism in the rift zone.

xiii, 198 p. : ill. (some col.), maps (some col.) ; 30 cm.

1995Jones_S

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