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Metamorphic and geochronological evolution of the Kemp and MacRobertson Land coast, east Antarctica
thesisposted on 2023-05-26, 06:53 authored by Halpin, JA
Rocks of the Kemp and MacRobertson Land coastline, east Antarctica, expose a semi-continuous profile through the middle- to lower-crustal sections of a Neoproterozoic convergent orogen called the Rayner Complex. Kemp Land exposures are dominated by Archaean felsic orthogneiss interpreted to represent the tectonically reworked margin of a continental fragment called the Napier Complex, whereas MacRobertson Land exposures are dominated by supracrustal successions of probable Proterozoic age and voluminous felsic plutonic rocks. All of these rocks were deformed and metamorphosed at granulite facies conditions during the ca. 1000 ‚Äö- 900 Ma Rayner Structural Episode, which was a consequence of the convergence of crustal blocks that now constitute extensive parts of eastern India and east Antarctica, and formed part of the supercontinent called Rodinia. This thesis examines the geochronological, metamorphic and structural evolution of the Kemp and MacRobertson Land rocks and focuses on two main themes: (1) the Archaean and early Proterozoic history of continental crust before the Rayner Structural Episode as preserved in Kemp Land exposures; and (2) syn-orogenic processes related to the Rayner Structural Episode which affected all rocks along the coast, and implications for the tectonic evolution of the Rayner Complex. Exposures in Kemp Land, which includes parts of both the Napier (Rippon Point) and Rayner (Oygarden Group, Broka and Havstein Islands, Stillwell Hills) Complexes, are dominated by layered Archaean orthogneiss. On the basis of in situ zircon U-Pb and Hf isotopic analysis, precursors to this orthogneiss were emplaced before ca. 3650 ‚Äö- 3500 Ma and involved signifi cant melting of ca. 3900 ‚Äö- 3700 Ma crustal material with input from primary ca. 3600 Ma mantle melts. Plausible protoliths include ca. 3800 Ma orthogneiss exposed in the Tula Mountains in the Napier Complex. Evidence for a pervasive ca. 2470 Ma tectonothermal event is preserved in rocks from most of western Kemp Land and the western Napier Complex. These isotopic data suggest Kemp Land was part of the eastern Napier Complex after ca. 3500 Ma, and part of the greater Napier Craton at, or before, ca. 2470 Ma. Charnockite was emplaced in the Stillwell Hills region prior to ca. 1620 Ma and represents post-Archaean crustal addition to Kemp Land. Source magmas are likely to have involved some juvenile material and the reworking of a ca. 2600 Ma lower crust. The ca. 980 Ma Mawson Charnockite exposed in MacRobertson Land was sourced from ca. 2100 Ma lower crustal protoliths, without significant addition of juvenile magmas. The zircon U-Pb and Hf isotopic data distinguish at least two terranes: a crustal block dominated by Archaean rock in Kemp Land, and a crustal block dominated by comparatively juvenile Proterozoic rock in MacRobertson Land. Much of the Kemp Land terrane represents a part of the Napier Complex that was tectonically reworked during the Neoproterozoic. Rocks to the east of the Stillwell Hills form the MacRobertson Land terrane and represent Proterozoic crust accreted to a complexly deformed Kemp-Napier Archaean craton after ca. 1600 Ma. Any incorporation of juvenile material during orogenesis was minor, indicating that this part of the Rayner Complex consists mainly of ensialic crust. The effects of plate convergence related to the Rayner Structural Episode were first recorded by juvenile metasediments in MacRobertson Land. A shallowly-dipping high-grade gneissosity developed at ca. 995 ‚Äö- 970 Ma, contemporary with peak metamorphic conditions. Well-developed mineral microstructures in metapelitic gneiss from Cape Bruce and the Forbes Glacier along the Mawson Coast record heating to T ‚Äöv¢v† 850 ‚Äö- 920 ¬¨‚àûC at P ‚Äöv¢v† 5.4 ‚Äö- 6.2 kbar. Syn-orogenic felsic granitoids, including voluminous charnockitic magma, were emplaced during continued deformation. This magma flux is interpreted to have maintained elevated temperatures at a regional scale; advection was accompanied by crustal thickening to maximum pressures of P ‚Äöv¢v† 6.0 ‚Äö- 6.8 kbar. Mineral reaction textures in metasedimentary gneiss, formed during this 'anticlockwise' P-T-t path, record near-isobaric cooling and protracted metamorphism over ca. 80 Myrs. Diverse metapelitic assemblages from the Stillwell Hills, Broka and Havstein Islands and the Oygarden Group in Kemp Land record ca. 940 ‚Äö- 930 Ma peak metamorphic conditions that involved T ‚Äöv¢v† 870 ‚Äö- 990 ¬¨‚àûC at P ‚Äöv¢v† 7.4 ‚Äö- 10 kbar, with pressure conditions increasing westward towards the Napier Complex. A penetrative sub-horizontal gneissosity formed in these high-P, high-T rocks some 30 ‚Äö- 60 Myrs after equivalent features formed in lower-P rocks in MacRobertson Land. Mineral reaction textures in Kemp Land rocks define a decompressive-cooling trajectory (dP/dT ‚Äöv¢v† 17 bar/¬¨‚àûC), which is part of a 'clockwise' P-T-t path that documents uplift and cooling of the Napier cratonic margin over ca. 25 Myrs. The contrast in the P-T-t paths recorded by the adjacent terranes, coupled with the spatial variation in the style and intensity of deformation, is likely to reflect the effects of intense strain partitioning and spatially focussed magmatism across the collisional margin. Initial penetrative deformation is inferred to have been accompanied by syn-orogenic magmatic doming in the internal portions of the orogen (MacRobertson Land), while the tectonic reworking of older, anhydrous parts of the crust closer to the orogenic front (Kemp Land) occurred during the waning stages of orogenesis. This later phase of convergence associated with the involvement of the Kemp Land terrane may have made a dynamic contribution to the prevailing tectonic framework, indicated by a change in the style of deformation that occurred across several locations in MacRobertson Land at ca. 940 ‚Äö- 900 Ma. Geological relationships in the Stillwell Hills region, which lies at the eastern margin of tectonically reworked Archaean crust, reflect both the pre- and syn-orogenic evolution of the Rayner Complex. Layered orthogneiss preserves evidence for two high-grade fabrics which developed during at least two deformational events (D1 and D2) prior to ca. 1620 Ma. Correlation with high-grade fabrics in rocks further west suggest that the alignment of early leucocratic segregations (S1) may have occurred at ca. 3490 ‚Äö- 3400 Ma. The correlation of mafic layers within the layered Archaean orthogneiss with ca. 2400 ‚Äö- 2350 Ma mafic dykes in the Napier Complex suggests that the development of the dominant form surface (S2) occurred during the Palaeoproterozoic. D1 and D2 suggest that significant dehydration of the Kemp Land terrane occurred prior to the Rayner Structural Episode. Penetrative deformation at mid- to lower-crustal levels at ca. 945 Ma involved the development of a pervasive sub-horizontal S3 gneissosity and inclined to recumbent F3 folds. North-trending F3 fold axes rotate towards the east in areas of high strain, consistent with strain having accumulated from progressive simple shear. D3 structures are invariably overprinted by D4 and D5 dome and basin-type folding. North to northeast-trending upright to inclined F4 folds are sub-parallel to F3 folds; both fold generations are inferred to have developed during west-directed ductile thrusting of the Napier cratonic margin. D3 and D4 structures were reorientated about east to southeast-trending F5 folds during the waning stages of orogenesis. Late Neoproterozoic to early Palaeoproterozoic D6 mylonitic shear zones cross-cut all ductile structures and reflect north-directed intraplate deformation. Charnockitic magmatism at ca. 1620 Ma in the Stillwell Hills reflects a significant tectonothermal event during the late Palaeoproterozoic in this region of east Antarctica. The correlation of the Kemp and MacRobertson Land rocks with equivalent late Palaeoproterozoic granulite facies rocks in the southern Eastern Ghats Belt (India) suggests that the Kemp-Napier Craton may have collided with the eastern Dharwar Craton at this time. An amalgamated Kemp-Napier-eastern India crustal block presents a plausible continental shield for the accretion of crustal fragments now comprising the Rayner Complex in MacRobertson Land and the central and northern Eastern Ghats Belt during the Meso-Neoproterozoic. This scenario is consistent with an active continental margin setting at ca. 1600 Ma, which may have evolved to continent-continent collision by ca. 1000 Ma. The involvement of the southern and/or eastern (present coordinates) continental block(s) during Neoproterozoic orogenesis remains enigmatic.