Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating

• Published in: Journal of Petrology
• Main Authors: ZEH, A., MILLAR, I. L., HORSTWOOD, M. S. A.
• Format: Text
• Language: English
• Published: Oxford University Press 2004
• Subjects: ARTICLES; Lord Nunatak; Meade; Meade Nunatak; Shackleton; Shackleton Range; Antarc*; Antarctica
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/45/5/949; https://doi.org/10.1093/petrology/egg117

Metapelitic rock samples from the NE Shackleton Range, Antarctica, include garnet with contrasting zonation patterns and two age spectra. Garnet porphyroblasts in K-rich kyanite–sillimanite– staurolite–garnet–muscovite–biotite schists from Lord Nunatak show prograde growth zonation, and give Sm–Nd garnet, U–Pb monazite and Rb–Sr muscovite ages of 518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively. Geothermobarometry and P – T pseudo-section calculations in the model system CaO–Na 2 O–K 2 O– TiO 2 –MnO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O are consistent with garnet growth during prograde heating from 540°C/7 kbar to 650°C/7·5 kbar, and partial resorption during a subsequent P – T decrease to <650°C at <6 kbar. All data indicate that rocks from Lord Nunatak were affected by a single orogenic cycle. In contrast, garnet porphyroblasts in K-poor kyanite–sillimanite– staurolite–garnet–cordierite–biotite-schists from Meade Nunatak show two growth stages and diffusion-controlled zonation. Two distinct age groups were obtained. Laser ablation plasma ionization multicollector mass spectrometry in situ analyses of monazite, completely enclosed by a first garnet generation, yield ages of c . 1700 Ma, whereas monazite grains in open garnet fractures and in most matrix domains give c . 500 Ma. Both age groups are also obtained by U–Pb thermal ionization mass spectrometry analyses of matrix monazite and zircon, which fall on a discordia with lower and upper intercepts at 502 ± 1 and 1686 ± 2 Ma, respectively. Sm–Nd garnet dating yields an age of 1571 ± 40 Ma and Rb–Sr biotite analyses give an age of 504 ± 1 Ma. Integrated geochronological and petrological data provide evidence that rocks from Meade Nunatak underwent a polymetamorphic Barrovian-type metamorphism: (1) garnet 1 growth and subsequent diffusive garnet annealing between 1700 and 1570 Ma; (2) garnet 2 growth during the Ross Orogeny at c . 500 Ma. During the final orogenic event the rocks experienced peak P – T conditions of about 650°C/7·0 kbar and a retrograde stage at ...