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

Abstract

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 ( I ) consistent development of a discordant, pervasive S, axial-plane foliation; (2) re-orientation of S, axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D, static thermal overprint; and (4) relatively common development of retrogressive, E-Wtrending, mylonitic shear zones.Peak metamorphic conditions in excess of 800°C at 900 100 MPa (9 kbar) were attained at one locality following, but probably close to the time of D, folding. D, 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 f 50°C and pressures of 700MPa (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 a t about 1100 Ma. The area was subsequently subjected to near-isothermal uplift.Rb-Sr isotopic data indicate that the pervasive D, fabric developed at about 240&2500Ma, and this age can be further refined to 2456': Ma by concordant zircon analyses from a syn-D, c) 1983 Blackwell Scientific Pubhcdtions 02634929/83/1200-0277 $02.00 pegmatite. All zircons were affected by only minor (<7-10%) Pb loss and/or new zircon growth during the Rayner event at about 1100 Ma. Thus the 45&8SOpg/g U 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 a t 522 f 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the I100 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.

High-temperature Metamorphism and Crustal Anatexis

“…Locally developed reaction textures in granulites affected by D3 and associated shearing also indicate IBC subsequent to those events (Harley 1985a;Sandiford 1985a;Black et al 1983b;Sheraton et al , 1987. Evidence for IBC in high-grade metapelites is provided by the occasional development of coronas where one or more uni-and divariant equilibria are crossed.…”

“…considered here to be later than 2900 Ma) and prior to the imposition of the third deformation (D3) and associated high-pressure granulite to amphibolite facies metamorphism at 2460 Ma (Black et al 1983b). considered here to be later than 2900 Ma) and prior to the imposition of the third deformation (D3) and associated high-pressure granulite to amphibolite facies metamorphism at 2460 Ma (Black et al 1983b).…”

“…The deformational and metamorphic features of mainly east-west-trending, steep, ductile shear zones which dissect the Napier Complex in the Khmara Bay region have been described by Black et al (1983b), Sandiford (1985b), and Harley (1985b). Initial movements on these appear to have been strike-slip in nature (Black et al 1983b, Sandiford 1985b, and are inferred to have occurred at c. 1100-1000 Ma (Black et al 1983b.…”

“…Dominantly dextral strike-slip with some dip-slip component or trans pression in these zones would be consistent with west-directed thrusting of the Rayner Complex, interpreted from deformation patterns observed in areas further east (Clarke 1987; see Section 12.7) and with the recorded P-T-t paths in the shear zones. These ductile zones were reworked by lower-grade mylonite zones preserving steeply plunging lineations, which are thought to be related to north-directed thrusting and local pegmatite emplacement at c. 500-550 Ma (Grew 1981a, Black et al 1983b, Sandiford 1985b, Clarke 1987. The Proterozoic metamorphic histories of the shear zones are weIl defined by mineral and reaction textures developed within overprinted metapelites.…”

“…An important feature of these shear zones and the coeval or later magmatism localized within them is the geochemical (Sandiford 1985b) and isotopie (Black et al 1983b) evidence that some of the crust underlying the Napier Complex granulites must be relatively hydrous and undepleted in heat-producing and LIL elements, in contrast to the Napier Complex granulites themselves. Grew (1981a) reports the occurrence of boron-bearing and beryllian phases (beryl, tourmaline, and dumortierite) in late pegmatites (520 Ma) cutting the shear zones.…”