MULTIPLE MELTING EVENTS IN POLYCYCLIC HIGH-T TERRANES DEFINED USING GEOCHEMICALLY CONSTRAINED DATING OF ZIRCON AND MONAZITE

The presence or absence of abundant melt significantly effects the behavior of the middle to lower crust, impacting the overall strength and possibly buoyancy during tectonic events. High-grade metamorphic terranes commonly show evidence of partial melting and melt extraction, and many have been involved in more than one orogenic cycle. Therefore, a part of our understanding of how mountain belts evolve relies on better constraining the behavior of older crustal material that may have partially melted during earlier thermal events, limiting the amount of melt that can be generated during subsequent metamorphic events.

To address this issue, a detailed study of granulite facies (~7 kbar, >800 °C) metapelitic migmatites has been carried out on samples from the Larsemann Hills, east Antarctica, where rocks record a polycyclic history of high-grade events. The area experienced at least two cycles of metamorphism: the first, D₁, at ≥900 Ma, followed by D₂ at ~540-510 Ma. Field and petrographic evidence indicates that substantial melting occurred during D₁ and that the terrane was later reworked and possibly re-melted during D₂. Integrated trace element geochemistry on micro-texturally constrained zircon, monazite and garnet demonstrated that the majority of the migmatite structure preserved in the Larsemann Hills formed during the earlier event. However, zircon-garnet REE partitioning suggests that during the ~540-510 Ma event, new melting was focused in previously formed melanosome-leucosome boundaries, where fertility was most likely highest. Melting was also most extensive in cordierite-rich rock types. It is postulated that the degree of melting that was experienced during reworking of the terrane during D₂ may be tied to the volume of melt that was retained within the migmatite following D₁ and therefore underwent hydration of otherwise anhydrous assemblages. In addition, less fertile rock compositions that failed to undergo significant partial melting at the conditions accompanying D₁ may have been susceptible to high degrees of melting during the hotter temperatures that could be achieved during D₂. These detailed petrologically-constrained age data have also provided tighter constraints on the timing of events and thermal evolution of Pan-African reworking of the Larsemann Hills terrane.