Dr Stephen Read

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PhD abstract

Koettlitz Glacier Alkaline Province: Late Neoproterozoic extensional magmatism in southern Victoria Land, Antarctica (2010)

The Koettlitz Glacier Alkaline Province, in southern Victoria Land, Antarctica, is characterised by Late Neoproterozoic, extension-related plutonism. Stretching 100 kilometres along the Hillary Coast from the Radian Glacier, in the north, to the Mulock Glacier, in the south, the Koettlitz Glacier Alkaline Province is exposed over at least 2000 km².

The oldest intrusions dated during this study are mafic members of the Glee Suite, a suite ranging from 46 wt% SiO₂ alkaline gabbros to 68 wt% SiO₂ quartz monzonites that crop out in the north of the studied area, between the Kempe and Panorama Glaciers. Enriched in Ti, Fe, Mn, K, P & Ba, the Glee Suite comprises borderline subalkaline/alkaline, metaluminous, calc-alkalic/alkali-calcic, transitional basalts and derivatives. Initial epsilon Nd ( -2.58) and initial ⁸⁷Sr/⁸⁶Sr (0.708) values are more primitive than most nearby intrusions, indicating lesser crustal input in the Glee Suite. Zircon ages of 553 ± 5 Ma & 550 ± 5 Ma date emplacement, while titanite ages of 538 ± 2 Ma & 534 ± 1 Ma date subsequent deformation then cooling. Geochemical characteristics of the Glee Suite imply emplacement into a failed intra-continental rift. The alkaline gabbroic components of the Glee Suite are geochemically equivalent to alkaline components of the nearby Panorama Pluton and Dromedary Mafic Complex.

The Skelton Mafic Suite, which crops out on both sides of the Skelton Glacier in the vicinity of Cocks Bluff, ranges from magnesian, calcic, 48 wt% SiO₂ gabbros to ferroan, calc-alkalic, 59 wt% SiO₂ monzonites. This array is a result of fractional crystallisation from a gabbroic end member such as sampled at Breezy Bluff, one kilometre northeast of Cocks Bluff. Zircon and titanite from the Skelton Mafic Suite have been dated at 548 ± 2 Ma and 547 ± 4 Ma, respectively, indicating emplacement at this time. The subalkaline Skelton Mafic Suite is part of a broader mafic association within the Koettlitz Glacier Alkaline Province that contains both subalkaline and alkaline components. This association includes the Panorama Pluton (557 ± 5 Ma) and the Dromedary Mafic Complex. The outcrop characteristics and geochemical nature of these subalkaline-alkaline mafic intrusions indicate emplacement in a transitional compressional / extensional tectonic setting.

The most characteristic suite in the Koettlitz Glacier Alkaline Province is the Hillary Suite that comprises high-silica, A₂ group, A-type granitoids and subordinate syenites. Members of the Hillary Suite are enriched in Si, Fe, Na, K, Ga, Nb, La, Nd, Rb, Y, Pb, Th & U and depleted in Al, Mg, Ca, Ba & Sr. Strong enrichment in REE, particularly the LREE (La(N)/Lu(N)=5-40), and large negative Eu anomalies (Eu/Eu*=0.04-0.53) are apparent in all samples. The Sm-Nd & Rb-Sr isotopic character of the Hillary Suite (initial epsilon Nd = -4.5 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr 0.708-0.710) suggests derivation from crustal rather than mantle source rocks. New Hillary Suite emplacement ages of 550 ± 3 Ma, 548 ± 4 Ma, 546 ± 3 Ma, 546 ± 3 Ma and 542 ± 3 Ma are all Late Neoproterozoic. The geochemical nature, geographic spread and emplacement ages of the Hillary Suite indicate that the Koettlitz Glacier Alkaline Province was the site of broad crustal extension during the Late Neoproterozoic.

The extension and distinctive alkaline magmatism that occurred in the Koettlitz Glacier Alkaline Province from 557-542 Ma is unreported from elsewhere along the Transantarctic Mountains. Outside the Koettlitz Glacier Alkaline Province the oldest dated intrusions are younger, and related to convergence and subduction rather than extension.

Following the broad crustal extension that resulted in the emplacement of the Glee Suite, the Hillary Suite, the Skelton Mafic Suite, and correlatives during the interval 557-542 Ma, there was a period of deformation that resulted in the resetting of many U-Pb geochronometers, particularly in the northern Koettlitz Glacier Alkaline Province. Reset ages range from 538 ± 2 Ma to 534 ± 1 Ma.

Monzodioritic to granodioritic rocks that crop out at Moraine Bluff and Fishtail Point are chemically dissimilar to other intrusions in the studied area and comprise the enclave-rich Moraine Bluff Suite. Geochemical tectonic classification suggests emplacement in a volcanic arc / convergent plate margin but, as the age of the Moraine Bluff Suite is unknown, the significance of this is uncertain.

Post-dating the extension and subsequent deformation, the Hooper Suite is a characteristic group of undeformed, 60-75 wt% SiO₂ , granitoids enriched in Sr, Al, Mg, Ca & Ba and depleted in K, La, Ce, Nd, Y, Zr, Th & HREE. These characteristics, combined with moderately radiogenic Nd & Sr isotopic ratios (initial epsilon Nd = -5.65, -10.95 and initial ⁸⁷Sr/⁸⁶Sr 0.707, 0.711), indicate derivation via water-undersaturated partial melting of underplated basaltic rock beneath thickened crust. The Hooper Suite was emplaced at 533 ± 3 Ma & 524 ± 4 Ma in the southern Skelton Glacier area, at 520 ± 4 Ma at Hooper Crags, in the middle of the Koettlitz Glacier Alkaline Province, and at 515 ± 4 Ma & 511 ± 3 Ma in the north, just south of the Radian Glacier. The ages and locations of these intrusions describe a northward younging of 22 million years over a distance of 80 kilometres.

Emplacement of the Glee Suite, Skelton Mafic Suite, Hillary Suite and Hooper Suite occurred in the Koettlitz Glacier Alkaline Province prior to the onset of voluminous subduction-related Ross Orogeny magmatism at ca. 505 Ma in adjacent areas north of the Koettlitz Glacier Alkaline Province.

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