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Milne, Alastair John
(1990).
DOI: https://doi.org/10.21954/ou.ro.0000fc4d
Abstract
The exposed rocks of Graham Land, northern Antarctic Peninsula are almost entirely Mesozoic or Cenozoic in age. The existence of a pre-Mesozoic basement, however, has long been advocated and is of particular importance when considering reconstructions of the Antarctic Peninsula within Gondwana. Detailed field mapping combined with geochronological studies have identified for the first time the pre-Mesozoic basement of Graham Land. It comprises:
1. Leppard Glacier Granite Gneiss: Silurian calc-alkaline granite gneiss cut by basaltic amphibolite sheets. Isochron ages obtained from these rocks of 426±12 and 10±15 Ma are the oldest known from Graham Land.
2 . Target Hill Banded Gneiss: Banded orthogneiss and paragneiss interpreted as a metamorphosed succession of sedimentary and volcanic rocks.
3 . Bildad Peak Amphibolite: Strongly sheared amphibolites and minor metapelite.
4 . Scar Inlet Metaplutonic Rock: Carboniferous plutonic rock predominantly of granitic composition.
Trace element geochemistry of these units suggests that subduction beneath Graham Land occurred not only during the Mesozoic and Cenozoic but also during some of the Palaeozoic. Two geochemically distinct groups of calc-alkaline granite gneiss are present. Group I granite gneiss has a geochemistry similar to Mesozoic granites of Graham Land whereas low concentrations of certain minor and trace elements distinguish group II granite gneiss. Amphibolites in the Target Hill Banded Gneiss and the Leppard Glacier Granite Gneiss are relatively primitive and have MORB-like trace element characteristics. This indicates the influence of extensional processes during the mid-Palaeozoic.
During the Carboniferous, the mid-Palaeozoic lithologies underwent amphibolite facies metamorphism which resulted in anatexis and granite emplacement. The timing of this metamorphism is constrained by Sm-Nd and Rb-Sr geochronology. A garnet-whole-rock age of 321±8 Ma for garnet cores dates the onset of metamorphism. A mixed gamet-whole-rock isochron age of 311±8 Ma is interpreted as the peak of metamorphism and a muscovite, plagioclase, whole-rock Rb-Sr isochron of 297±3 Ma dates cooling below c. 500°C. A Rb-Sr errorchron age of 325±10 Ma for a garnet-muscovite-bearing metagranite intrusion indicates its emplacement during this metamorphic episode. Trace element and isotope geochemistry of this pluton is consistent with its formation by partial melting of older units of the basement.
The presence of Palaeozoic granites in Graham Land allows magmatic belts to be traced along much of the Pacific margin of Gondwana from South America, through West Antarctica and into Australasia. This emphasises the importance that accretionary processes have played in the Palaeozoic development of the Pacific margin of Gondwana.
Isotopic studies of the basement and of Mesozoic magmatic rocks of Graham Land have identified two periods of crustal growth (at c. 400 Ma and < 50 Ma) separated by a period of crustal reworking which was most intense during the Middle Jurassic (c. 175 Ma). Modelling of the contribution of pre-Mesozoic basement in granitoids of Graham Land shows that during the late Mesozoic and Cenozoic, the mantle contribution to granitoid genesis increased
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