Metamorphic studies in the Vredefort Dome, South Africa
Date
2010-09-08
Authors
Ogilvie, Paula
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Abstract
Metasedimentary granulites in the core of the Vredefort Dome present textural and
chemical evidence for three discrete metamorphic events. These include a peak
Archaean anatectic event (M1), shock metamorphism (M2) with impact at 2.02 Ga
and post-shock metamorphism (M3) of the target rocks related to Dome formation
and non-adiabatic loading of the crust.
Regional granulite facies metamorphism (M1) occurred between 3.10 Ga and 3.08
Ga with tectonomagmatic thickening of the crust attributable to easterly- to
northeasterly-directed subduction of an oceanic slab beneath the Kaapvaal craton.
Phase equilibria modelling in THERMOCALC of highly restitic pelitic granulites
constrains peak conditions of M1 metamorphism at 870 - 885 ºC and 7.1 - 7.7 kbar.
Slightly lower peak conditions of 858 °C and 7.1 kbar were obtained for a more
melt-rich granulite, reflecting back-reaction with a melt on the suprasolidus
retrograde path. The prograde up-pressure trajectory is dominated by heating from
6.5 kbar, 700 °C to 7.5 kbar, 850 °C. Phase equilibria constraints on the prograde
and suprasolidus retrograde evolution are consistent with a clockwise Archaean P-T
path for the M1 event.
Overprinting the M1 peak assemblage are shock-induced, extreme disequilibrium
deformation features (irregular shock fractures, planar fractures, planar deformation
features, isotropization and shock melting) that formed instantaneously during
meteorite impact at 2.02 Ga (M2). Reconstruction of the shock pressure and postshock
temperature distribution across the central core of the Vredefort Dome from
observed shock effects in component phases from the pelitic granulites required an
experimental study to constrain shock effects in an analogous, complex,
polymineralic, pelitic granulite from the Etivé aureole, with a significant proportion
of hydrous and ferromagnesian minerals. Shock experiments were performed at
12.5, 25, 34, 40 and 56 GPa at 25 °C, and at 18 and 25 GPa at 400 °C to investigate
the roles of both increasing shock pressure and pre-shock temperature on shock
deformation features in major minerals. Both the shock experiments and Vredefort
granulites are characterised by heterogeneous distribution of shock effects in
minerals on intragranular and intergranular scales. Shock heterogeneity
compromises estimates of absolute shock pressures based exclusively on observed shock effects in minerals. Independent constraints on shock pressures are obtained
from post-shock metamorphic conditions and range from > 35 GPa to > 40 GPa at 8
and 5 km from the centre of the Dome, respectively.
The Vredefort granulites underwent unusually rapid and highly variable M3 heating,
exhumation and cooling associated with the 2.02 Ga meteorite impact event. The
short-lived nature of the thermal event and restitic bulk rock compositions owing to
melt loss during the Archaean M1 event, led to diffusion-controlled reaction and the
growth of coronas around garnet. Coronas display a strongly sectoral development
indicative of highly localized compositional domains. Grain size, sectoral
complexity and thickness of coronas all increase toward the centre of the Dome,
indicating strong temperature control on the extent of reaction. This sectoral
complexity is unique to Vredefort coronas compared to coronas reported from
regional and contact metamorphic terranes and affords the opportunity to evaluate
controls on extent of corona development and degree of equilibration. Minimum
peak M3 temperatures were 980 °C at 2.5 – 3.0 kbar, between 8 and 5 km from the
centre of the Dome.
Open-system diffusion and phase equilibria modelling of the Vredefort coronas has
established a relationship between equilibration in granulites at the micrometre-scale
as a function of temperature and melt fertility of the corona bulk composition.
Higher melt modes and solidus depression in fertile corona bulk compositions
enhance component diffusion and equalization of chemical potential gradients
throughout the equilibration volume. Coronas are characterized by non-linear opensystem
metasomatic exchange of components with adjacent domains. Selective and
variable open-system metasomatic exchange of components with the matrix or with
contiguous domains is required to reproduce observed mineral modes and
compositions. Reaction may be induced in chemically inert corona domains through
open-system diffusive communication with a hydrous matrix, thereby fluxing the
solidus and elevating melt modes. A better understanding of the textural and
compositional evolution of coronas requires a shift from closed-system or linear
phase equilibria modelling to non-linear, open-system modelling.