Paleomagnetic data and U‐Pb isotopic age determinations from Coats Land, Antarctica: Implications for late Proterozoic plate reconstructions

Abstract

Paleomagnetic results and isotopic age determinations for granophyre and rhyolite from small, isolated nunataks in southern Coats Land, Antarctica, are used to evaluate late Proterozoic plate reconstructions. U‐Pb zircon dates for the two rock types indicate coeval crystallization at 1112±4 Ma. A concordant 1106±3 Ma titanite date from the granophyre overlaps the crystallization age, implying rapid cooling, and is consistent with field and petrographic evidence of no subsequent penetrative deformation, metamorphism, or hydrothermal disturbance. The mean direction of magnetization of the rhyolite at Littlewood Nunataks is statistically indistinguishable from the mean directions of five sites in the granophyre and crosscutting rhyolite dikes at Bertrab Nunataks. The group mean virtual geomagnetic pole of 22.9°N, 80.3°E (N=6, A95=6.8°) compares favorably with the only other extant Precambrian paleomagnetic poles for the East Antarctic craton, two poles from western Dronning Maud Land. The East Antarctic and Laurentian poles of 1.1 Ga do not coincide after restoration of the continents to a position suggested by the SWEAT hypothesis juxtaposing the Pacific margins of East Antarctica‐Australia and Laurentia, indicating either that the hypothesis is incorrect or that Coats Land and parts of western Dronning Maud Land (herein the CMG province) were not part of the East Antarctic craton at 1.1 Ga. In support of the latter, there is reasonable agreement of the 1.1 Ga CMG poles and approximately coeval poles from the Kalahari craton of West Gondwana when the CMG is restored to a position adjacent the Kalahari craton. Such a reconstruction places the CMG in West Gondwana rather than East Gondwana, as originally implied in Rodinia reconstructions, and is consistent with previously recognized links between the geology of the Kalahari craton and western Dronning Maud Land. It further implies that the CMG did not become part of East Antarctica until latest Precambrian to Cambrian time. A new reconstruction places a partially assembled West Gondwana off the present southeastern margin of Laurentia at 1.1 Ga such that poles of the CMG and Kalahari fall on the Laurentian polar wander path for this time period.