MASS AND HEAT TRANSPORT PROCESSES IN MIGMATITE DOMES

Domal structures cored by migmatite are a common feature of orogens. The pressure-temperature-time-deformation record of quartzofeldspathic migmatite (crystallized partially molten crust) and associated rocks (e.g. lenses and layers of metabasalt) provides insights into the driving forces of flow, the magnitude and trajectory of flow, and the thermal-mechanical consequences of mass and heat transport associated with dome formation. Through study of migmatite domes ranging in age from Archean to Cenozoic, we propose a spectrum of domes with respect to the balance of buoyancy (diapirism) vs. isostasy (extension-driven flow) in dome formation. In all cases, there is significant (tens of km) vertical motion of orogenic crust. In isostasy-dominated systems such as the migmatite domes of the northern North American Cordillera (e.g. the Cenozoic Thor-Odin, Okanogan domes) and Variscan (Paleozoic) Europe (e.g. Montagne Noire dome, France), high-grade metamorphic rocks experienced near-isothermal decompression from > 40 km depth to very shallow crustal levels (< 2-5 km); further evidence for major vertical transport of hot rocks from deep to shallow is seen in geo/thermochronology data documenting rapid cooling. In buoyancy-dominated domes (e.g. in the Archean Pilbara craton, Australia), vertical motion involved both upward flow (as fragments of deep crust are dragged into the ascending partially molten crust) and downward flow (as dense upper crustal material is dragged into domes; i.e. sagduction). A “hybrid” type of dome may also exist, in which isostasy and buoyancy were both significant in dome formation (e.g. Entia dome; Paleozoic Alice Springs orogen, Australia). Field-based observations, integrated P-T-t-d paths, and results of numerical modeling illuminate the relative importance of buoyancy forces in the dynamics of partially molten crust and the formation of migmatite domes.