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1 Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063, USA
2 Institute for Geophysics and Department of Geological Sciences, University of Texas, 4412 Spicewood Springs Road, Building 600, Austin, Texas 78759-8500, USA, and affiliated with the Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Perth, Australia
The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwana's Pacific margin are recorded in igneous and metamorphic rocks that crop out in Marie Byrd Land, West Antarctica, recognized on geologic and paleomagnetic grounds to compose a discrete crustal block. Widespread metaluminous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites, and granites from the Ruppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ± 3 Ma (age of the oldest granodiorite dated) and 110 ± 1 Ma (the age of the youngest I-type granitoid in the area). The latter, known as the Mount Prince granite, is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent. The dikes have been dated by zircon U-Pb as 101 ± 1 Ma. Thus, the regime along the Ruppert and Hobbs coasts had shifted from subduction-related to rift-related magmatism within an 
9 m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, the calc-alkaline magmatism did not terminate until 96 ± 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ± 3 Ma based on zircons from another. This evidence requires that subduction shut off from west to east, as suggested previously on the basis of geophysical models. No continental separation occurred to the east of Marie Byrd Land. The margins of the Thurston Island and Antarctic Peninsula crustal blocks went directly from convergent to inactive, except at the northernmost tip of the peninsula, where the South Shetlands Island block is actively separating.
With their zircon U-Pb ages clustering around 100 ± 2 Ma, dike-free anorogenic syenites and quartz syenites along the Ruppert and Hobbs coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ± 1 Ma to formation of oceanic crust between Marie Byrd Land and greater New Zealand (Campbell Plateau, Chatham Rise, North Island, and South Island) prior to chron 33o ca. 81 Ma required only 20 m.y. For comparison, this is only two-thirds of the 30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism. The swiftness of the separation between Marie Byrd Land and greater New Zealand demonstrated by our data is consistent with ridge-trench interaction rather than a mantle plume as the primary cause of the breakup, as is the west to east diachroneity in the cessation of subduction.
Exposures of host rocks to the erosion-resistant plutons are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrence in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercepts are as high as 1576 ± 55 Ma). This suggests either that stretched Precambrian basement underlies most of Marie Byrd Land, or that clastic sedimentary sequences with Precambrian detrital zircons underlie much of the margin.
Key Words: geochronology • Marie Byrd Land • tectonic evolution • U-Pb • West Antarctica • zircon
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