Rb-Sr geochronology of Graham Land, Antarctica

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Abstract
Thirty-five new Rb-Sr whole-rock and mineral isochrons are reported for igneous rocks from Graham Land. Magmatism was essentially continuous but peaks of activity occurred in early Jurassic, early Cretaceous, late Cretaceous and Eocene times. The oldest dated granite is Triassic (209 ± 3 Ma). Volcanic rocks were erupted throughout this history and some previously supposed Upper Jurassic volcanic rocks must be mid-Cretaceous or younger. A change in magma type, from uniformly acid to predominantly basic or intermediate, combined with westward migration of the arc, may be partly responsible for previously reported transverse geochemical variations. There is a marked trend of decreasing initial 87 Sr/ 86 Sr ratio of the plutonic rocks with time, from c. 0.707 to 0.704, although volcanic rocks tend to have higher values. This trend is ascribed to the waning influence of an, as yet unidentified, pre-magmatic crustal basement, due to increasing depth of magma generation. Close parallels to the observed age pattern imply continuity of the Andean belt through southern South America into Graham Land back to early Mesozoic times and are not compatible with reconstructions of Gondwana in which the Antarctic Peninsula is located W of South America.