Records of Metamorphism and Deformation at Maximum Depth in Gneiss Domes

Abstract

In exhumed orogens, much of the accessible material is quartzofeldspathic and records conditions of late-stage emplacement in the shallow crust at the high-temperature, low-pressure (HT-LP) conditions at which they equilibrated. Orogenic material may, however, be more deeply sourced than indicated by the recorded conditions of felsic rocks. To understand the magnitude of transport of heat and mass taking place during orogenic cycles, it is important to determine the depths from which material is sourced, and how this material is mobilized within the crust via lateral channelized flow and vertical flow that results in exhumation of former deep crust in gneiss dome structures. Mafic rocks are chemically and structurally refractory. As such, they have greater potential to preserve a record of petrogenesis and metamorphism at or near the maximum depth (peak-P) of metamorphism. Many gneiss domes contain mafic rocks as layers and lenses within the dominant felsic materials; e.g., the Montagne Noire (French Massif Central) and the Entia (central Australia) gneiss domes. In the Montagne Noire, scarce eclogites formed at HP conditions record metamorphic processes that occurred at or near the maximum-depth (peak-P) of metamorphism during lateral crustal flow. In the Entia, mafic granulites and amphibolites (± garnet) record progressive deformation and metamorphism primarily during exhumation. Geochemically and texturally distinct eclogites from the core and margin of the Montagne Noire dome retain geochemical and temporal records of deep-crustal processes. Results of in situ zircon and rutile U-Pb petrochronology show that both eclogites formed in the deep crust at HP (> 15 kbar, ~ 50 km) in association with doming at c. 320–310 Ma but had different protoliths and source regions within a pre-orogenic Cambro-Ordovician continental crustal package. In situ O-isotope analyses of zircon and garnet reveal that eclogite in the dome core traveled greater lateral distances and interacted more extensively with the surrounding gneisses during crustal flow relative to eclogite in the margin of the dome. These results show that a multi-method, multi-systems approach to studying eclogite in migmatite domes can be used to evaluate the relative magnitude and trajectory of deep orogenic crustal flow. In the Entia dome, two geochemically distinct groups of mafic rocks are identified based on their major element geochemistry but nevertheless share a common igneous protolith emplacement history marked by crustal contamination and enrichment in a continental arc-like setting. The least-deformed, dry mafic granulites preserve evidence for mid/HP metamorphism (~8–12 kbar), whereas moderately deformed garnet amphibolites record peak-T (~800ºC); the most deformed amphibolites record late retrogressive metamorphism in the mid crust during cooling. Magnetic fabric analysis reveals that the highest-T and plane to constrictional strains are preserved in the core of the Entia dome, representing a transposition of material deformed in compression in the lower- to mid-crust as material enters the central exhumation channel of the dome. In contrast, lower-T and predominantly plane to flattening strains are preserved along the margins of the dome and represent deformation of material flowing towards the dome apex and outward along the margins of the domal structure in the shallow crust in an overall extensional regime. These results are consistent with numerical model predictions of the internal strain patterns of gneiss domes formed during exhumation. Although the Montagne Noire and Entia domes are in different orogens and tectonic settings, they both provide field-based insight about the temporal, geochemical and structural records of crustal flow dynamics inherent to orogenic cores prior to their collapse. The Montagne Noire eclogites reveal that at least parts of the dome are deeply sourced, and that rocks exhumed in the core of domes may have a more extensive and protracted history of deep-crustal flow than those exhumed at the margin. In addition, mafic rocks in both domes provide a set of geochemical tools to distinguish multiple sources and origins of protolith material incorporated in domes. The Entia dome in particular reveals that the latest and potentially deepest-sourced material may be preferentially preserved in dome cores and record constrictional fabrics associated with deformation in the deep crust. These early records of metamorphism and processes animating the mid- to deep crust of orogens are effectively preserved in mafic rocks that can be successfully investigated using high-resolution in situ petrochronological, geochemical, structural and geothermobarometric tools. The integration of structural and metamorphic analyses of refractory lithologies in the Montagne Noire and Entia domes provides the most comprehensive P–T–t–d–X records of internal orogenic recycling and constraints on the magnitudes, rates and volumes of mass and heat transfer that contribute to the long-term stabilization of continents over time during crustal flow.