P– T– t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica

Abstract

Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P– T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P– T– t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd. The low-pressure granulite-facies to upper-amphibolite-facies stage is dated by six nearly concordant U–Pb monazite ages of 484–494 Ma from three migmatite samples and correlates to the late Pan-African Ross Orogeny in Cambro-Ordovician times. The age of the medium-pressure granulite-facies assemblages is not constrained by geochronological data. Either they form relics of the Precambrian Antarctic Craton, or they represent an early metamorphic stage of the Ross Orogeny. Rb–Sr and K–Ar dating on biotites yielded 470, 468, 470 Ma and 473, 469, 470 Ma (±5 Ma each), indicating the time of cooling to 450–300°C. This is confirmed by 40Ar– 39Ar plateau ages of 476±2, 472±3 and 470±2 Ma for these biotites. A late tectonic pegmatite yielded a concordant U–Pb monazite age of 489±3 Ma, while slightly discordant U–Pb data of two zircon fractions are explained by recent minor lead loss of ca 490 Ma old zircons. Cooling to ca 500–350°C is dated to 472±2 Ma by concordant 40Ar– 39Ar plateau ages of two muscovite fractions. The cooling history of the basement from high-grade conditions to the blocking temperature of Rb–Sr and K–Ar in micas took place within ca 15–20 Ma. Cooling rates of 18–25°C Ma −1 can be derived, if continuous cooling is assumed. U–Pb data points of zircons as well as Sm–Nd whole rock model ages between 1.8 and 1.9 Ga indicate that at least part of the migmatites derive from Early Proterozoic crustal protoliths. Comparing the new P– T– t data from the northern Wilson Terrane with those from the southern Wilson Terrane, a common tectono-metamorphic history becomes evident for a 600 km long sector of the Ross Orogenic belt at the Pacific end of the Transantarctic Mountains, at least since the granulite-facies metamorphic event.