The dynamics of migmatite domes in extending orogens.

Abstract

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 ~ 720-750 ºC 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 (~13 km) calculated conditions of metamorphism at relatively shallow crustal levels (~13 km) 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.