Unraveling the metamorphic history at the crossing of Neoproterozoic orogens, S?r Rondane Mountains, East Antarctica: Constraints from U-Th-Pb geochronology, petrography, and REE geochemistry

摘要

The S?r Rondane Mountains in East Antarctica preserve two stages of tectonothermal events, at c. 650-600. Ma and c. 560-550. Ma, each of which is also widely preserved elsewhere in Gondwana within tectonic suture zones/orogens, although both ages of tectonothermal events are only rarely documented together in these other parts of Gondwana. A geochronological and petrogenetic study of highly retrogressed garnet-sillimanite-biotite-bearing pelitic gneiss, along with three generations of associated sub-concordant to discordant leucocratic felsic veins, which together are exposed in the southern Austkampane area in the central part of the S?r Rondane Mountains, provides important new constraints on the geological and age relationships between these two regional tectonothermal events. U-Pb SHRIMP dating of zircon grains isolated from the host garnet-sillimanite-biotite gneiss yields at least four distinct zircon age populations of c. 2800-720. Ma, c. 700. Ma, 640-630. Ma, and ~500. Ma. Chondrite-normalized REE patterns indicate that a pooled age of 637. ±. 6. Ma represents the timing of recrystallization of metamorphic HREE-depleted zircon, during which the zircon was in equilibrium with garnet. The earliest emplaced leucocratic vein (LV-1), which is partly intermingled with the host garnet-sillimanite-biotite gneiss (and enclosing garnet-bearing pelitic enclaves), contains a population of zircon grains that yielded a crystallization age of 635. ±. 4. Ma, which is almost identical to the age of zircons found within the host pelitic gneiss. The second-generation leucocratic vein (LV-2) occurs as a part of a set of sub-concordant veins/leucosomes comprising mostly biotite and muscovite. Although these particular leucosomes do not contain any zircon, they were found to contain abundant monazite, which yielded U-Pb ages of 640-630. Ma along with some older (relict) ~700. Ma ages. The third-generation leucocratic vein (LV-3) was sampled from a suite of pegmatitic veins that discordantly crosscut both the host pelitic gneisses and the older two sets of leucocratic veins. These third-generation veins contain magmatic zircons that exhibit HREE-enriched patterns typical of magmatic zircon crystallization, and which yielded a crystallization age of 550. ±. 3. Ma. The data imply that a major high-grade metamorphic event (possibly at granulite facies) took place in this region along with a subsequent late-stage re-hydration event, all within a relatively narrow time interval of <10. m.y. at ~635. Ma. A subsequent hydration event that took place much later on is recorded by the intrusion of a suite of discordant pegmatitic veins at ~550. Ma that are observed to crosscut and alter both the pre-existing pelitic gneiss and the other two older sets of felsic veins. These two ages of tectonothermal events in East Antarctica are coeval with the two major stages of Gondwana assembly. Hence, these results provide new insights into the development of metamorphic-fluid regimes within these major continent-transecting Neoproterozoic orogens.