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

摘要

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 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-realted to rift-related magmatism within an approx9 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.