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Title: Feldspar microtextures and the cooling histories of high-grade terrains
Author: Cayzer, Nicola
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2002
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Perthitic, antiperthitic and mesoperthitic microtextures were observed in this study representing the wide range of bulk compositions exhibited by ternary feldspars. The mesoperthites were often coarse, but semicoherent, regular intergrowths and often showed fine secondary exsolution. They are likely to have formed by spinodal decomposition below the coherent spinodal. Unusual mesoperthites with two orientations of lamellae may have been the result of deformation of existing mesoperthitic textures. Perthitic textures were characterised be blebs of oligoclase-andesine plagioclase and fine, albitic, secondary lamellae. The blebs are likely to have formed by homogenous nucleation with coarser blebs nucleating earlier by heterogeneous nucleation. The host to the blebs was compositionally homogeneous before the exsolution of the fine secondary lamellae. The orientation of the blebs was sometimes oblique to the lamellae due to the influence of deformation on the blebs as they were forming. Antiperthitic textures were characterised by K-feldspar blebs that had nucleated on the twin boundaries of the host plagioclase. Some samples had compositions with excess K-feldspar for the metamorphic conditions of the terrain and appear to have been altered by replacement involving addition of K-feldspar. Both the perthitic and antiperthitic textures were seen in all of the terrains and similar textures have been reported from other granulite terrains worldwide. The mesoperthitic textures in this study were restricted to the Napier Complex, but are also reported from other granulite terrains with different cooling histories. The feldspar do not show any evidence that the cooling history played a significant part in the development of the texture, but show that bulk composition was the major influence. The bulk composition of the feldspar determined where the coherent ternary solvus and spinodal were intersected and thus the nature of the exsolution. The effects of deformation and later fluid infiltration during the cooling history are, however, recorded by the microtextures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available