Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica

Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are (1) consistent development of a discordant, pervasive S 3 axial‐plane foliation; (2) re‐orientation of S 3 axial planes to approximate to the subsequent E‐W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post‐D 3 static thermal overprint; and (4) relatively common development of retrogressive, E‐W‐trending, mylonitic shear zones. Peak metamorphic conditions in excess of 800°C at 900 ± 100 M Pa (9 kbar) were attained at one locality following, but probably close to the time of D 2 folding. D 3 took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 ± 50°C and pressures of 700 MPa (7kbar) were attained in an amphibolite‐facies event, presumably associated with the widespread granulite to amphibolite‐facies metamorphism and intense deformation involved in the formation of the Rayner Complex at about 1100 Ma. The area was subsequently subjected to near‐isothermal uplift. Rb‐Sr isotopic data indicate that the pervasive D 3 fabric developed at about 2400–2500 Ma, and this age can be further refined to 2456 +8 ‐5 Ma by concordant zircon analyses from a syn‐D 3 pegmatite. All zircons were affected by only minor (<7–10%) Pb loss and/or new zircon growth during the Rayner event at about 1100Ma. Thus the 450–850 μg/gU concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite at 522 ± 10 Ma substantially changed the Rb‐Sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions. A good correlation is found between Rb‐Sr total‐rock ages and rock fabric. U‐Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.