Evidence of a close link between petrology and isotope records: constraints from SEM, EMP, TEM and in situ 40Ar-39Ar laser analyses on multiple generations of white micas (Lanterman Range, Antarctica)

In situ 40Ar-39Ar laserprobe method in conjunction with SEM, EMP and TEM techniques is used to examine the relations between argon isotope records and microtextural, microchemical and microstructural variations in white mica. Gneisses and micaschists belonging to three different tectono-metamorphic complexes of the Lanterman Range (Antarctica) contain multiple generations of potassic white micas. Texturally resolvable white mica generations show microchemical and microstructural (TEM scale) variations that suggest development under different P-T-deformation regimes, ranging from an eclogite-facies stage down to low greenschist-facies conditions. In situ 40Ar-39Ar laserprobe analyses on white mica samples from the three complexes reveal a complex intragrain and intergrain spatial distribution of argon ages which is closely linked to microtextural, microchemical and microstructural variations: texturally, compositionally and microstructurally older generations yield older ages whereas the younger ones yield younger ages. Results show that in the absence of re-crystallisation, white mica preserves argon isotope records pertaining to the high-pressure stage which survived amphibolite retrogression at temperatures of 550-650°C. The texture, petrology and isotope record of white micas in the studied samples preserve a nearly continuous record of P, T and deformation history within the same orogenic cycle.