ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract In the Larsemann Hills, East Antarctica, there are two important types of zoned segregations that formed during a Proterozoic, granulite facies metamorphic event: (1) segregations with a large garnet porphyroblast in the core, potassium feldspar in the mantle, and biotite, cordierite and plagioclase in the matrix; and (2) segregations with a large orthopyroxene porphyroblast in the core, potassium feldspar in the mantle, and biotite and quartz in the matrix. The main difference between these and zoned metamorphic segregations with leucocratic mantles described by previous authors is that the mantle phases are not a depleted subset of the matrix assemblage, but part of the product assemblage. The concentrical arrangement in the garnet segregations can be explained by the sequence in which a number of garnet- and potassium feldspar-producing reactions were met during temperature increase, with internal buffering of the activity of H2O. These reactions are modelled in the system, K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). The main reaction responsible for the garnet porphyroblast-bearing segregations is a KFMASH divariant equilibrium, quartz+biotite+cordierite = garnet+potassium feldspar+melt, taking place in a quartz+biotite+ cordierite+garnet assemblage. Potassium feldspar and melt produced by this reaction are associated spatially with garnet as a consequence of the sparse nucleation of garnet, and the chemical potential gradients generated prior to the interception of this equilibrium. In order to account for the mineral assemblage of the mantles, and the preservation of the porphyroblasts during retrogression, some melt loss from the segregation is necessary. The orthopyroxene segregations can be explained in an analogous manner. Within this model, the PT conditions in which the two types of segregations formed are consistent with the M1 pressures and temperatures suggested by previous work: about 4.5 kbar and 750° C.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01041757
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