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
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-Na2O-K2O- TiO2-MnO-FeO-MgO-Al2O3-SiO2-H2O are consistent with garnet growth during prograde heating from 540degreesC/7 kbar to 650degreesC/7.5 kbar, and partial resorption during a subsequent P-T decrease to <650degreesC 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 650degreesC/7.0 kbar and a retrograde stage at c. 575degreesC/4.0 kbar.