A High-Grade Evento of ~1100 Ma Preserved within the ~500 Ma Mobile Belt of teh Larsemann Hills, East Antarctica: Further Evidence from ⁴⁰Ar/³⁹Ar Dating

L. Tong^1,2*, C.J.L. Wilson¹ & X. Liu²

¹School of Earth Sciences, The University of Melbourne, Victoria 3010 - Australia
²Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029 - P.R. China

Received 27 December 2001; accepted in revised form 25 September 2002

Abstract- New ⁴⁰Ar-³⁹Ar plateau age of 774±3 Ma yielded from plagioclase in the garnet-plagioclase-bearing amphiboAr-³⁹Ar isotope data for representative single minerals in the structurally early granulite facies rocks from the Larsemann Hills, east Antarctica, provide further evidence for the existence of an early relict ~1100 Ma high-grade event. Orthopyroxenes in an early (syn-D1) garnet-bearing mafic granulite lens within the host paragneiss yield two well-defined plateau ages of 1059±3 Ma and 555±16 Ma. These indicate an important cooling event at ~1060 Ma followed by a thermal disturbance during the early Palaeozoic at ~500 Ma. However, an ⁴⁰Ar-³⁹Ar plateau age of 774±3 Ma yielded from plagioclase in the garnet-plagioclase-bearing amphibolite, in a D2 low-strain zone, suggests a slow cooling of the early high-grade event followed by a later thermal disturbance of the early Palaeozoic event. Syn-D2 biotites in an early garnet-bearing mafic granulite boudin within the paragneiss record an ⁴⁰Ar-³⁹Ar plateau age of 552.5±2.4 Ma. Combined with recent metamorphic observations and other chronological data from the region, these new Ar-Ar isotope data further support the idea that the early high-grade metamorphic event (M1) in the region occurred during the Mesoproterozoic at ~1100 Ma with a slow cooling history, while subsequent separate high-grade events (M2-M4) occurred during the early Palaeozoic at ~500 Ma.
 

*Corresponding author (l.tong@pgrad.unimelb.edu.au)