An 40Ar-39Ar investigation of high-pressure metamorphism and the retrogressive history of mafic eclogites from the Lanterman Range (Antarctica): evidence against a simple temperature control on argon transport in amphibole

Mafic eclogites from a restricted area in the Lanterman Range (Antarctica) retrogressed variably under amphibolite facies metamorphism. Assemblages range from well-preserved eclogite, with minor Na-Ca amphibole, to strongly retrogressed ones with extensive development of Ca amphibole. 40Ar-39Ar furnace step-heating experiments on the different amphiboles yield ages from ca. 498 to 490 Ma, and the greater the amphibolite retrogression, the younger the age. 40Ar-39Ar laser analyses on rock chips from a well-preserved eclogite and a slightly retrogressed one reveal the presence of an excess argon component. Whereas excess argon is invariably present in garnet and clinopyroxene developed under HP stage, it is heterogeneously distributed in amphibole on a millimetre scale. Results indicate that excess argon was incorporated during HP metamorphism; this component was then lost during retrogression, while a change in composition of ambient argon to atmospheric argon occurred. New and previously published data suggest that the oldest Na-Ca amphibole age is reliable and not an artefact due to the incorporation of excess argon. The variably retrogressed eclogites are thought to derive from different parts of the enclosing metasedimentary rocks that were variably invaded by fluids during amphibolite facies metamorphism. Thus the circulation of fluids promoting (re)crystallisation, and not temperature, was the main process controlling the rate of argon transport in the studied eclogites.