RECONSTRUCTION OF REGIONAL-SCALE FLOW PATHS OF PARTIALLY- MOLTEN CRUST AS RECORDED BY MIGMATITES AND ECLOGITES

Anatectic migmatites flow in response to tectonic or gravitational forces: from micro-scale deformation of crystals and associated displacement of melt to regional flow of partially-molten crust. Significant parameters for understanding migmatite systems include driving forces, duration, rate, magnitude, and trajectory of flow. The evolution of fabrics derived from migmatite, in addition to P-T-time data, informs the geometry and kinematics of crustal flow; e.g., during formation of migmatite domes. Geologic and numerical modeling studies indicate that partially-molten crust may flow rapidly (cms/yr), particularly in extension/transtension systems in which upper-crust normal faulting solicits low-viscosity deep (near-Moho) material. The result of rapid flow from deep to shallow is formation of hot migmatite bodies that record low-P (LP)/ high-T (HT) conditions of emplacement, which may be tens of km less than the source depth.

In eclogite inclusions in the Montagne Noire migmatite dome, French Massif Central (FMC), mineral and rock composition (bulk, mineral major/trace elements; O isotopes in zircon and garnet; bulk Sm-Nd, Lu-Hf, Rb-Sr), textures (inclusions, symplectites), extent of zircon recrystallization, and deformation fabrics (flattening/constriction at eclogite conditions) allow reconstruction of the flow paths of eclogite and therefore the provenance and flow history of the host migmatite, which no longer records this information. For example, eclogite in the core of the dome is isotopically similar to eclogite and other HP mafic rocks exposed in other FMC domes, whereas eclogite at the dome-margin is distinct. This is consistent with interpretation of zircon textures (highly recrystallized in dome-core eclogite; minimal HP zircon growth in dome-margin) and other observations consistent with the interpretation that dome-core eclogite experienced protracted HT metamorphism and deformation during flow in the deep crust, whereas dome-margin eclogite was more locally-sourced. Variation of paths in different domains of a migmatite complex shows how partially-molten crust flows laterally and vertically on a regional scale and informs early (HP-HT) and later (LP-HT) geotherms during orogenic evolution.