Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions

The Paleozoic and Mesozoic development and subsequent fragmentation of Gond wana's Pacific margin are recorded in igneous and metamorphic rocks that cr op out in Marie Byrd Land, West Antarctica, recognized on geologic and pale omagnetic grounds to compose a discrete crustal block. Widespread metalumin ous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of th is margin. Dates for granodiorites, monzogranites, and granites from the Ru ppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 +/- 3 M a (age of the oldest granodiorite dated) and 110 +/- 1 Ma (the age of the y oungest I-type granitoid in the area), The latter, known as the Mount Princ e granite, is intruded by swarms of mafic and intermediate dikes believed t o record the onset of rifting that led to separation of the New Zealand mic rocontinent, 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 similar to 9 m.y. period. In t he Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, t he 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 zir cons from another, This evidence requires that subduction shut off from wes t to east, as suggested previously on the basis of geophysical models. No c ontinental 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 peninsu la, where the South Shetlands Island block is actively separating. With their zircon U-Pb ages clustering around 100 +/- 2 Ma, dike-free anoro genic 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 Zeala nd (Campbell Plateau, Chatham Rise, North Island, and South Island) prior t o chron 33o ca, 81 Ma required only 20 m.y. For comparison, this is only tw o-thirds of the similar to 30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism, The swiftness of the separation betw een Marie Byrd Land and greater New Zealand demonstrated by our data Is con sistent with ridge-trench interaction rather than a mantle plume as the pri mary cause of the breakup, as is the west to east diachroneity in the cessa tion of subduction. Exposures of host rocks to the erosion-resistant pluton s are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrenc e in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercep ts are as high as 1576 +/- 55 Ma). This suggests either that stretched Prec ambrian basement underlies most of Marie Byrd Land, or that elastic sedimen tary sequences with Precambrian detrital zircons underlie much of the margi n.