The dynamics of migmatite domes in extending orogens

Two migmatite domes, the Okanogan dome (Washington state) and the Naxos dome (Cyclades, Greece) record the dynamics of migmatite dome formation in extending orogens. Both domes are exposed in cordilleran-style metamorphic core complexes that developed during extension of previously thickened crust and contain a core of anatectic migmatite mantled by metasedimentary rocks and exposed below crustal detachments. Structural observations in both domes indicate that deformation in dome migmatites took place in the presence of melt and flow within the dome migmatites was coeval with detachment faulting, upper crustal extension, and migmatite dome formation.In the Okanogan dome, the timing of partial melting is constrained by U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of zircon, monazite, and titanite. Results of the U-Pb analysis document Paleocene-Eocene ages of partial melting and migmatite crystallization in the Okanogan dome based on a range of 206Pb/238U ages from ca. 61-49 Ma for zircon and monazite in migmatite leucosomes of various structural settings, interpreted as the timing of migmatite crystallization. Similar ages of migmatite crystallization are reported in other domes of the northern Cordillera, suggesting that partial melting was widespread in this region during Paleocene to Eocene time.The conditions of metamorphism in the Okanogan dome associated with migmatization are constrained by mineral assemblages and reaction textures preserved in sapphirine- and corundum-bearing orthoamphibole-cordierite gneiss layers that structurally overlie the Okanogan migmatite domain. Petrologic and pseudosection analyses of these Mg-Al-rich layers suggest conditions of metamorphism reached &sim 720-750°C at moderate pressure (ca. 4 kbar). An earlier, high-pressure metamorphic history is recorded by relict kyanite pseudomorphed by symplectite and coronal reaction textures. A minimum of 4 kbar of decompression is suggested by the calculated conditions of metamorphism at relatively shallow crustal levels (&sim13 km) subsequent to conditions of metamorphism that were within the stability field of kyanite. High grade metamorphism and crystallization of the Okanogan dome migmatites completed largely by ca. 49 Ma (age of youngest 206Pb/238U ages of zircon and monazite attributable to migmatite crystallization). Rapid cooling of the migmatites through &sim325°C by ca. 48-47 Ma is evidenced by U-Pb SHRIMP analyses of titanite and 40Ar-39Ar analyses of biotite.A continuous structural section exposed on the western margin of the Okanogan dome documents that melt-present deformation fabrics developed in the structurally lowest domain of migmatite are systematically reoriented and deformed at higher structural levels at a rheological interface between the flowing partially molten layer at depth and subsolidus deformation associated with the Okanogan detachment. These structural features record consistent strain and kinematics under a single deformational regime in which flow in the migmatite domain was likely mechanically coupled to upper crustal extension.In the Naxos dome, the geodynamic context in which the migmatites formed is well constrained by previous geochronology and metamorphic petrology research. Partial melting in the Naxos dome was coeval with dome formation associated with regional NNE-SSW-directed extension. The internal structure of the Naxos dome contrasts that of the Okanogan dome and is characterized by a well-developed migmatitic foliation that defines a complex internal geometry with numerous subdomes. Flow directions in the Naxos migmatites were obtained through application of the anisotropy of magnetic susceptibility (AMS) technique, since linear fabrics in the migmatitic core of the Naxos dome are largely lacking. Magnetic fabrics in the migmatites are well correlated with the migmatitic foliation (and field lineation where observed) suggesting that the AMS is a reliable indicator of the fabric in migmatites. Over a range of scales, AMS yields consistent directions of flow in the migmatites, suggesting that the apparent complexity of migmatites observed in the field masks a simpler pattern of deformation developed during viscous flow. At the scale of the Naxos dome, lineations obtained by AMS are highly variable in plunge with steeply plunging lineations commonly associated with the cores of subdomes or adjacent synforms and more shallowly plunging lineations elsewhere. The combination of structural and magnetic observations in the Naxos dome suggest that the combined effects of upwelling of low-viscosity migmatites under dominantly NNE-SSW extension, E-W compression, and top-to-the-north shearing likely contributed to the structural pattern recorded in the Naxos dome migmatites. The presence of subdomes within the migmatitic core of the Naxos dome, and discordant structural relationships between the migmatitic foliation and that in the mantling units at some localities in the dome, suggest that the internal dynamics of the migmatites (i.e., buoyancy, convection) were likely significant in the formation of the Naxos dome and subdomes.Multidisciplinary study of the Okanogan and Naxos domes emphasize the role of partial melting and flow of migmatites in the formation of domes developed during extension of previously thickened crust. Observations in both domes suggest that partial melting and flow within the migmatites was kinematically consistent with regional patterns of extension reflecting a component of lateral flow during their formation. The results of metamorphic petrology in the Okanogan dome, and the combined structural and magnetic analysis of the Naxos dome, further suggest that vertical flow of partially molten crust is also an important process in the formation of migmatite domes (and associated subdomes) in extending orogens.